INTRODUCTION
1.1 INTRODUCTION
As the name of the project is antenna positioning control through C++,in order
to understand this project we must go through all these devices PCB,
transformer, stepper motor, optocoupler, power supply, antenna, infrared eye
and interfacing circuit.
In computer we make a C++ program by which we control the data on the
parallel ports and through this parallel port data will be given to interface circuit
which provides protection for the motherboard and for interfacing, we use
transistor circuit. An antenna is mounted on a stepper motor which provides the
rotation of 360 degree. Antenna is a infra red receiver which detects the infra red
signal and steps rotating at the position where the antenna gets the signal.
The project has various modules:
Parallel port
Opto coupler
Stepper motor
Radar
Transformer
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Various modules of the project are
1.2.1 PARALLEL PORT
Our circuit is connected to the parallel port of the PC. A parallel port contains a
set of signal lines that the CPU sends or receive data with other components. We
use ports to communicate via modem, printer, keyboard, mouse etc. In signaling,
open signal are 1 and close signal are 0 so it is like binary system. A parallel port
sends 8 bits and receives 5 bits at a time. The serial port RS 232 sends only 1 bit
at a time but it is multidirectional so it can be send 1 bit and receive 1 bit at a
time. In the MS DOS operative system three parallel ports. Called LPT1, LPT2
AND LPT3 are supported. So we can find three addresses dedicated to these
ports in the memory map of the PC. Lets study the addresses dedicated to LPT1
first. Each parallel ports uses three addresses of the I/O map. For LPT1 these
addresses are 378H, 379H, and 37AH.
1.2.1 STEPPER MOTOR
A stepper motor is an electromechanical device which converts electrical pulses
into discrete mechanical movements. The shaft or spindle of a stepper motor
rotates in discrete step increments when electrical command pulses are applied
to it in the proper sequence. The motors rotation has several direct relationships
to these applied input pulses. The sequence of the applied pulses is directly
related to the direction of motor shafts rotation. The speed of the motor shafts
rotation is directly related to the frequency of the input pulses and the length of
rotation is directly related to the number of input pulses applied.
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1.2.3 OPTOCOUPLER
In electronics, an opto-isolator (or optical isolator, optical coupling
device, optocoupler, photo coupler, or photo MOS) is a device that uses a
short optical transmission path to transfer an electronic signal between elements
of a circuit, typically a transmitter and a receiver, while keeping them electrically
isolated—since the electrical signal is converted to a light beam, transferred, then
converted back to an electrical signal, there is no need for electrical connection
between the source and destination circuits.
The opto-isolator is simply a package that contains both an infrared light-emitting
diode (LED) and a photo detector such as a photosensitive silicon diode,
transistor Darlington pair, or silicon controlled rectifier (SCR). The wave-length
responses of the two devices are tailored to be as identical as possible to permit
the highest measure of coupling possible. Other circuitry—for example an output
amplifier—may be integrated into the package. An opto-isolator is usually thought
of as a single integrated package, but opto-isolation can also be achieved by
using separate devices.
1.2.4 RADAR
Radar is an object detection system that uses electromagnetic waves to identify
the range, altitude, direction, or speed of both moving and fixed objects such
as aircraft, ships, motor vehicles, weather formations, and terrain. The
term RADAR was coined in 1940 by the U.S. Navy as an
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acronym for RAdio Detection And Ranging. A radar system has a transmitter that
emits radio waves. When they come into contact with an object they
are scattered in all directions. The signal is thus partly reflected back and it has a
slight change of wavelength (and thus frequency) if the target is moving. The
receiver is usually, but not always, in the same location as the transmitter.
Although the signal returned is usually very weak, the signal can be amplified
through use of electronic techniques in the receiver and in the antenna
configuration. This enables radar to detect objects at ranges where other
emissions, such as sound or visible light, would be too weak to detect. Radar
uses include meteorological detection of precipitation, measuring ocean surface
waves, air traffic control, police detection of speeding traffic, military applications,
or to simply determine the speed of a baseball.
1.2.5 TRANSFORMER
A transformer is a device that transfers electrical energy from one circuit to
another through inductively coupled conductors—the transformer's coils. A
varying current in the first or primary winding creates a varying magnetic flux in
the transformer's core, and thus a varying magnetic field through
the secondary winding. This varying magnetic field induces a
varying electromotive force (EMF) or "voltage" in the secondary winding. This
effect is called mutual induction.
If a load is connected to the secondary, an electric current will flow in the
secondary winding and electrical energy will be transferred from the primary
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circuit through the transformer to the load. In an ideal transformer, the induced
voltage in the secondary winding (VS) is in proportion to the primary voltage (VP),
and is given by the ratio of the number of turns in the secondary (NS) to the
number of turns in the primary (NP) as follows:
By appropriate selection of the ratio of turns, a transformer thus allows
an alternating current (AC) voltage to be "stepped up" by making NSgreater
than NP, or "stepped down" by making NS less than NP.
1.3 SIMPLE LED DRIVING CIRCUITS
You can make simple circuit for driving a small led through PC parallel port. The
only components needed are one LED and one 470 ohm resistors. You simply
connect the diode and resistor in series. The resistors is needed to limit the
current taken from parallel port to a value which light up acceptably normal LEDs
and is still safe value (not overloading the parallel port chip). In practical case the
output current will be few milliampres for the LED, which will cause a typical LED
to somewhat light up visibly, but not get the full brigtness.
5
Fig 1.1
Then you connect the circuit to the parallel port so that one end of the circuit
goes to one data pin (that one you with to use for controlling that LED) and
another one goes to any of the ground pins. Be sure to fit the circuit so that the
LED positive lead (the longer one) goes to the data pin. If you put the led in the
wrong way, it will not light in any condition. You can connect one circuit to each
of the parallel port data pins. In this way you get eight software controllable
LEDs.
The software controlling is easy. When you send out 1 to the data pin, where the
LED is connected that LED will light. When you send 0 to that same pin, the LED
will no longer light.
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1.4 CIRCUIT USED IN THE PROJECT
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Fig 1.2
CHAPTER 2
8
PARALLEL PORT
2.1 INTRODUCTION
Our circuit is connected to the parallel port of the PC. A parallel port contains a
set of signal lines that the CPU sends or receive data with other components. We
use ports to communicate via modem, printer, keyboard, mouse etc. In signaling,
open signal are 1 and close signal are 0 so it is like binary system. A parallel port
sends 8 bits and receives 5 bits at a time. The serial port RS 232 sends only 1
bit at a time but it is multidirectional so it can be send 1 bit and receive 1 bit at a
time. In the MS DOS operative system three parallel ports. Called LPT1, LPT2
AND LPT3 are supported. So we can find three addresses dedicated to these
ports in the memory map of the PC. Lets study the addresses dedicated to LPT1
first. Each parallel ports uses three addresses of the I/O map. For LPT1 these
addresses are 378H, 379H, and 37AH.
P378H PORT
In this addresses the CPU writes the data to be sent to the printer. It is an
OUTPUT port. The eight data bits ( D0- D7) are latched to appear in the
output connector.
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379h PORT
This is an input port. These signals are used by the CPU to know the state
of the printer. The location of the bits is listed in table.
37AH PORT
In this port the computer writes the signals that controls the printer
therefore, it is an output port see table.
2.2 PARALLEL PORT CONFIGURATION
SIGNAL BIT PIN DIRECTION
-STROBE C0 1 OUTPUT
+DATA BIT 0D0 2 OUTPUT
+DATA BIT1 D1 3 OUTPUT
+DATA BIT2 D2 4 OUTPUT
+DATA BIT3 D3 5 OUTPUT
+DATA BIT4 D4 6 OUTPUT
+DATA BIT5 D5 7 OUTPUT
+DATA BIT6 D6 8 OUTPUT
+DATA BIT7 D7 9 OUTPUT
-ACKNOWLE S6 10 INPUT
+BUSY -S7 11 INPUT
+PAPER END S5 12 INPUT
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+SELECT IN S4 13 INPUT
-AUTO FEED -C1 14 OUTPUT
-ERROR S3 15 INPUT
-INITIALIZE C2 16 OUTPUT
-SELECT -C3 17 OUTPUT
GROUND 18-25 GROUND.
Simple circuit and program to show how to use PC parallel port output
capabilities. PC parallel port can be very useful I/O channel for connecting your
own circuits to PC. The port is very easy to use when you first understand some
basic tricks. This document tries to show those tricks in easy to understand way.
2.3 HOW TO CONNECT CIRCUITS TO PARALLEL PORT
PC parallel port is 25 pin D-shaped female connector in the back of the
computer. It is normally used for connecting computer to printer, but many other
types of hardware for that port is available today.
Not all 25 are needed always. Usually you can easily do with only 8 output pins
(data lines) and signal ground. I have presented those pins in the table below.
Those output pins are adequate for many purposes.
pin function
2 D0
3 D1
4 D2
11
5 D3
6 D4
7 D5
8 D6
9 D7
Pins 18,19,20,21,22,23,24 and 25 are all ground pins.
Those data pins are TTL level output pins. This means that they put out ideally
0V when they are in low logic level (0) and +5V when they are in high logic level
(1). In real world the voltages can be something different from ideal when the
circuit is loaded. The output current capacity of the parallel port is limited to only
few mill amperes.
Dn Out ------+
|+
Sourcing Load (up to 2.6 mA @ 2.4 v)
2.4 CONTROL PROGRAM
The following program is an example how to control parallel port LPT1 data pins
from your software. This example directly controls the parallel port registers, so it
does not work under some multitasking operating system which does not allow
that. It works nicely under MSDOS. You can look the Borland Pascal 7.0 code
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(should compile also with earlier versions also) and then download the compiled
program
Program lpt1_output;
Uses Dos;
Var
addr: word;
data: byte;
e:integer;
Begin
addr:= MemW[$0040:$0008];
Val(ParamStr(1),data,e);
Port[addr]:=data;
End.
HOW TO USE THE PROGRAM
LPTOUT.EXE is very easy to use program. The program takes one parameter,
which is the data value to send to the parallel port. That value must be integer in
decimal format (for example 255). Hexadecimal numbers can also be used, but
they must be preceded by $ mark (for example $FF). The program hoes not have
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any type of error checking to keep it simple. If your number is not in correct
format, the program will send some strange value to the port.
EXAMPLE:
LPTOUT 0
Set all datapins to low level.
LPTOUT 255
Set all datapins to high level.
LPTOUT 1
Set datapin D0 to high level and all other datapins to low level.
HOW TO CALCULATE YOUR OWN VALUES TO SEND TO PROGRAM
You have to think the value you give to the program as a binary number. Every
bit of the binary number control one output bit. The following table describes the
relation of the bits, parallel port output pins and the value of those bits.
Pin 2 3 4 5 6 7 8 9
Bit D0 D1 D2 D3 D4 D5 D6 D7
Value 1 2 4 8 16 32 64 128
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For example if you want to set pins 2 and 3 to logic 1 (led on) then you have to
output value 1+2=3. If you want to set on pins 3,5 and 6 then you need to output
value 2+8+16=26. In this way you can calculate the value for any bit combination
you want to output.
MAKING CHANGES TO SOURCE CODE
You can easily change to parallel port number int. the source code by just
changing the memory address where the program read the parallel port address.
For more information, check the following table.
Format of BIOS Data Segment at segment 40h:
Offset Size Description
08h WORD Base I/O address of 1st parallel I/O port, zero if none
0Ah WORD Base I/O address of 2nd parallel I/O port, zero if none
0Ch WORD Base I/O address of 3rd parallel I/O port, zero if none
0Eh WORD [non-PS] Base I/O address of 4th parallel I/O port, zero if none
For example change the line addr: = MemW[$0040:$0008]; in the source code to
addr:=MemW[$0040:$000A]; if you want to output to LPT2.
2.5 ALTERNATIVE CONTROL PROGRAM: DOS DEBUG
DOS Debug is a byte editor that enables files to be viewed and modified at the
byte level. It is a standard feature of many modern DOS versions (for example
MD-DOS version 7). While MS-DOS is not used commonly today, it still can be
accessed from Windows 95, Windows 98 or Windows NT by clicking Start / Run
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and typing command or CMD in Windows NT / 2000. You can try Debug under
those (although the success rate may depend on the system you use).
You can start Debug from DOS command line (true DOS or MS-DOS window) by
writing DEBUG and pressing enter. Once Debug has been called, the somewhat
cryptic "Debug prompt", a hyphen (-), is displayed. Write the following command
to turn the parallel port output pins high:
o 0378 ff
And you can write the following command to turn all output pins low again.
o 0378 00
In the command line the first command "o" means port byte output. The next
number "0378" is the parallel port I/O address in hexadecimal format (change it if
your port is in another address). The last number ("ff" or "00") is the byte sent to
the parallel port data pins in hexadecimal format.
DOS debug can be used as a very basic tool to experiment with PC parallel port.
2.6 USING OTHER LANGUAGES
The following examples are short code examples how to write to I/O ports using
different languages. In the examples I have used I/O address 378h which is one
of the addresses where parallel port can be. The following examples are useful in
DOS.
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Assembler
MOV DX,0378H
MOV AL,n
OUT DX,AL
Where n is the data you want to output.
BASIC
OUT &H378, N
Where N is the number you want to output.
C
outp(0x378,n);
or
outportb(0x378,n);
Where N is the data you want to output. The actual I/O port controlling command
varies from compiler to compiler because it is not part of standardized C libraries.
Here is an example source code for Borland C++ 3.1 compiler:
#include <stdio.h>
#include <dos.h>
#include <conio.h>
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/********************************************/
/*This program set the parallel port outputs*/
/********************************************/
void main (void)
{
clrscr(); /* clear screen */
outportb(0x378,0xff); /* output the data to parallel port */
getch(); /* wait for key press before exiting */
}
PARALLEL PORT CONTROLLING IN WINDOWS PROGRAMS
Direct parallel port controlling in possible under Windows 3x and Windows 95
directly from 16 bit application programs and DLL libraries. So you can use the C
example above in Windows 3x and Windows 95 if you make your program 16 bit
application. If you want to control parallel port from Visual Basic or Delphi then
take a look at the libraries at Parallel Port Central at
http://www.lvr.com/parport.htm.
Direct port controlling from application is not possible under Windows NT and to
be ale to control the parallel port directly you will need to write some kind of
device driver to do this. You can find also this kind of drivers from Parallel Port
Central.
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PARALLEL PORT CONTROLLING IN LINUX
Linux will allow access to any port using the ioperm syscall. Here is some code
parts for Linux to write 255 to printer port:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <asm/io.h>
#define base 0x378 /* printer port base address */
#define value 255 /* numeric value to send to printer port */
main(int argc, char **argv)
{
if (ioperm(base,1,1))
fprintf(stderr, "Couldn't get the port at %x\n", base), exit(1);
outb(value, base);
}
Save the source code to file lpt_test.c and compile it with command:
gcc -O lpt_test.c -o lpt_test.c
The user has to have the privileges to have access to the ports for the program
to run, so you have to be root to be able to run this kind of programs without
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access problems. If you want to make a program which can be run by anybody
then you have to first set the owner of the program to be root (for example do
compilation when you are root), give the users rights to execute the program and
then set the program to be always executed with owner (root) rights instead of
the right of the user who runs it. You can set the program to be run on owner
rights by using following command:
chmod +s lpt_test.c
If you want a more useful program, then download my lptout.c parallel port
controlling program source code. That program works so that you can give the
data to send to parallel port as the command line argument (both decimal and
hexadecimal numbers supported) to that program and it will then output that
value to parallel port. You can compile the source code to lptout.c command
using the following line to do the compilation:
gcc -O lptout.c -o lptout
After you have compiled the program you can run it easily. For example
running ./lptout 0xFF will turn all data pins to 1 and running ./lptout 0x00 will turn
all data pins to 0.
BUILDING YOUR OWN RELAY CONTROLLING CIRCUIT
The following circuit is the simples interface you can use to control relay from
parallel port:
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Parallel Port >ground
Ground
The circuit can handle relays which take currents up to 100 mA and operate at
24V or less. The circuit need external power supply which has the output voltage
which is right for controlling the relay (5..24V depending on relay). The transistor
does the switching of current and the diode prevent spikes from the relay coil
form damaging your computer (if you leave the diode out, then the transistor and
your computer can be damaged).
Since coils (solenoids and relay coils) have a large amount of inductance, when
they are released (when the current is cut off) they generate a very large voltage
spike. Most designs have a diode or crowbar circuit designed to block that
voltage spike from hitting the rest of the circuit. If that diode is bad, then the
voltage spike might be destroying your "sink" transistor or even your I/O card
over a period of time. The mode of failure for the sink transistor might be short
circuit, and consequently you would have the solenoid tap shorted to ground
indefinitely.
The circuit can be also used for controlling other small loads like powerful LEDS,
lamps and small DC motors. Keep in mind that those devices you plan to control
directly from the transistor must take less than 100 mA current.
WARNING: Check and double check the circuit before connecting it to your PC.
Using wrong type or damaged components can cause you parallel port get
damaged. Mistakes in making the circuit can result that you damage your parallel
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port and need to buy a new multi-io card. The 1N4002 diode in parallel with the
relay is an essential protection component and it should not be left out in acu
case, or a damage of the parallel port can occur because of high voltage
inductive kickback from the relay coil (that diode stops that spike from occuring),
SAFER NEW DESIGN
The circuit example above works well and when transistor is of correct type and
working properly. If for some reason B and C should be shorted together and you
are suing more than +5V in the relay side, the circuit can push that higher voltage
to the parallel port to damage it. The following circuit uses two 1N4148 diodes to
protect parallel port against higher than +5V signals and also against wrong
polarity signals (power on the circuit is accidentally at wrong polarity.
Adding even more safety idea: Replace the 1N4148 diode connected to ground
with 5.1V zener diode. That diode will then protect against overvoltage spikes
and negative voltage at the same time.
2.7 READING THE INPUT PINS IN PARALLEL PORT
PC parallel port has 5 input pins. The input pins can be read from the I/O address
LPT port base address + 1.
The meaning of the buts in byte you read from that I/O port:
D0: state not specified
D1: state not specified
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D2: state not specified
D3: state of pin 15 (ERROR) inverted
D4: state of pin 13 (SELECTED)
D5: state of pin 12 (PAPER OUT)
D6: state of pin 10 (ACK)
D7: state of pin 11 (BUSY) inverted
Here are some code snippets to read LPT port:
ASSEMBLER
MOV DX,0379H
IN AL,DX
You get the result to read from AL register
BASIC
N = INP(&H379);
Where N is the numerical value you read.
C
in = inportb(0x379);
or
in = inp(0x379);
Where N is the data you want to output. The actual I/O port controlling command
varies from compiler to compiler because it is not part of standardized C libraries.
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CHAPTER 3
STEPPER MOTOR
3.1 INTRODUCTION
A stepper motor is an electromechanical device which converts electrical pulses
into discrete mechanical movements. The shaft or spindle of a stepper motor
rotates in discrete step increments when electrical command pulses are applied
to it in the proper sequence. The motors rotation has several direct relationships
to these applied input pulses. The sequence of the applied pulses is directly
related to the direction of motor shafts rotation. The speed of the motor shafts
rotation is directly related to the frequency of the input pulses and the length of
rotation is directly related to the number of input pulses applied.
ADVANTAGES
The rotation angle of the motor is proportional to the input pulse.
The motor has full torque at standstill(if the windings are energized)
Precise positioning and repeatability of movement since good
stepper motors have an accuracy of 3 – 5% of a step and this error is
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non cumulative from one step to the next.
Excellent response to starting/stopping/reversing.
Very reliable since there are no contact brushes in the motor. Therefore
the life of the motor is simply dependant on the life of the bearing.
The motors response to digital input pulses provides open-loop control,
making the motor simpler and less costly to control.
It is possible to achieve very low speed synchronous rotation with a load
that is directly coupled to the shaft.
A wide range of rotational speeds can be realized as the speed is
proportional to the frequency of the input pulses.
DISADVANTAGES
Resonances can occur if not properly controlled.
Not easy to operate at extremely high speeds.
Open Loop Operation- one of the most significant advantages of a stepper
motor is its ability to be accurately controlled in an open loop system.
Open loop control means no feedback information about position is
needed. This type of control eliminates the need for expensive sensing
and feedback devices such as optical encoders. Your position is known
simply by keeping track of the input step pulses.
3.2 STEPPER MOTOR TYPES
There are three basic stepper motor types. They are:
Variable-reluctance
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Permanent-magnet
Hybrid
3.2.1 VARIABLE-RELUCTANCE (VR)
This type of stepper motor has been around for a long time. It is probably the
easiest to understand from a structural point of view. This type of motor consists
of a soft iron multi-toothed rotor and a wound stator. When the stator windings
are energized with DC current the poles become magnetized. Rotation occurs
when the rotor teeth are attracted to the energized stator poles.
Unlike unipolar stepper motors, Bipolar units require more complex driver
circuitry. Bipolar motorsare known for their excellent size/torque ratio, and
provide more torque for their size than unipolar motors. Bipolar motors are
designed with separate coils that need to be driven in either direction (the polarity
needs to be reversed during operation) for proper stepping to occur. This
presents a driver challenge. Bipolar stepper motors use the same binary drive
pattern as a unipolar motor, only the '0' and '1' signals correspond to the polarity
of the voltage applied to the coils, not simply 'on-off' signals. Figure 5.1 shows a
basic 4-phase bipolar motor's coil setup and drive sequence.
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Fig 3.1 - Bipolar stepper motor coil setup (left) and drive pattern (right).
A circuit known as an "H-bridge" (shown below) is used to drive Bipolar
stepper motors. Each coil of the stepper motor needs its own H-bridge
driver circuit. Typical bipolar steppers have 4 leads, connected to two
isolated coils in the motor. ICs specifically designed to drive bipolar
steppers (or DC motors) are available (Popular are the L297/298 series
from ST Microelectronics, and the LMD18T245 from National
Semiconductor). Usually these IC modules only contain a single H-bridge
circuit inside of them, so two of them are required for driving a single
bipolar motor. One problem with the basic (transistor) H-bridge circuit is
that with a certain combination of input values (both '1's) the result is that
the power supply feeding the motor becomes shorted by the transistors.
This could cause a situation where the transistors and/or power supply
may be destroyed. A small XOR logic circuit was added in figure 6.1 to
keep both inputs from being seen as '1's by the transistors.
Another characteristic of H-bridge circuits is that they have electrical "brakes"
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that can be applied to slow or even stop the motor from spinning freely when not
moving under control by the driver circuit. This is accomplished by essentially
shorting the coil(s) of the motor together, causing any voltage produced in the
coils by during rotation to "fold back" on itself and make the shaft difficult to
turn. The faster the shaft is made to turn, the more the electrical "brakes" tighten.
3.2.2 PERMANENT MAGNET (PM)
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Often referred to as a “tin can” or “can stock” motor the permanent magnet step
motor is a low cost and low resolution type motor with typical step angles of 7.5
degree to 15degree. (48 – 24steps/revolution) PM motors as the Industrial
Circuits Application.The rotor no longer has teeth as with the VR motor. Instead
the rotor is magnetized with alternating north and south poles situated in a
straight line parallel to the rotor shaft. These magnetized rotor poles provide an
increased magnetic flux intensity and because of this the PM motor exhibits
improved torque characteristics when compared with the VR type.
Unipolar motors are relatively easy to control. A simple 1-of-'n' counter circuit
can generate the proper stepping sequence, and drivers as simple as 1 transistor
per winding are possible with unipolar motors. Unipolar stepper motors are
characterized by their center-tapped windings. A common wiring scheme is to
take all the taps of the center-tapped windings and feed them +MV (Motor
voltage). The driver circuit would then ground each winding to energize it.
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Fig 3.3 - A typical unipolar stepper motor driver circuit. Note the 4 back EMF protection
diodes.
Unipolar stepper motors are recognized by their center-tapped windings. The
number of phases is twice the number of coils, since each coil is divided in two.
So the diagram below (Figure 3.1), which has two center-tapped coils, represents
the connection of a 4-phase unipolar stepper motor.
Fig 3.4 - Unipolar stepper motor coil setup (left) and 1-phase drive pattern (right).
In addition to the standard drive sequence, high-torque and half-step drive
sequences are also possible. In the high-torque sequence, two windings
are active at a time for each motor step. This two-winding combination
yields around 1.5 times more torque than the standard sequence, but it
draws twice the current. Half-stepping is achieved by combining the two
sequences. First, one of the windings is activated, then two, then one, etc.
This effectively doubles the number of steps the motor will advance for
each revolution of the shaft, and it cuts the number of degrees per step in
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half.
Fig 3.5 - Two-phase stepping sequence (left) and half-step sequence (right)
3.2.3 HYBRID (HB)
The hybrid stepper motor is more expensive than the PM stepper motor but
provides better performance with respect to step resolution, torque and speed.
Typical step angles for the HB stepper motor ranges from 3.6 degree to
0.9degree (100 – 400 steps per revolution). The hybrid stepper motor combines
the best features of both the PM and VR type stepper motors. The rotor is multi-
toothed like the VR motor and contains an axially magnetized concentric magnet
around its shaft. The teeth on the rotor provide an even better path which helps
guide the magnetic flux to preferred locations in the airgap. This further increases
the detent, holding and dynamic torque characteristics of the motor when
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compared with both the VR and PM types. The two most commonly used types
of stepper motors are the permanent magnet and the hybrid types. If a designer
is not sure which type will best fit his applications requirements he should first
evaluate the PM type as it is normally several times less expensive. If not then
the hybrid motor may be the right choice. There also exist some special stepper
motor designs. One is the disc magnet motor. Here the rotor is designed sa a
disc with rare earth magnets. This motor type has some advantages such as very
low inertia and a optimized magnetic flow path with no coupling between the two
stator windings. These qualities are essential in some applications.
Sometimes referred to as Hybrid motors, variable reluctance stepper motors are
the simplest to control over other types of stepper motors. Their drive sequence
is simply to energize each of the windings in order, one after the other (see drive
pattern table below) This type of stepper motor will often have only one lead,
which is the common lead for all the other leads. This type of motor feels like a DC
motor when the shaft is spun by hand; it turns freely and you cannot feel the steps. This
type of stepper motor is not permanently magnetized like its unipolar and bipolar
counterparts.
Fig 3.6 - Variable reluctance stepper motor coil setup (left) and drive pattern (right).
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EXAMPLE
Figure illustrates the simplest solution to generating a one-phase drive sequence. For
unipolar stepper motors, the circuit in Figure 2.1, or for bipolar stepper motors, the circuit
in Figure 6.1 can be connected to the 4 outputs of this circuit to provide a complete
translator + driver solution. This circuit is limited in that it cannot reverse the direction of
the motor. This circuit would be most useful in applications where the motor does not
need to change directions.
Fig 3.7 - A simple, single direction, single phase drive translator.
I have seen this circuit many places, but I believe it originated from The Robot
Builders' Bonanza book, by Gordon McComb. I have used this circuit in the past and
seem to recall that it had a problem. This may not be the case but I think when you
33
reverse direction and continue stepping, the motor will advance 1 more step in the
previous direction it was going before responding. As always, prototype this circuit to be
sure it will work for your application before you build anything with it.
Figure 3.8 - A simple, bidirectional, two-phase drive stepper motor translator circuit.
There are several standard stepper motor translation circuits which use discrete logic ICs.
Below you will find yet another one of these. The circuit in Figure 10.1 has not been
tested but theoretically should work without problems.
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3.3 CHARACTERISTICS OF STEPPER MOTOR
3.3.1 SIZE AND POWER
In addition to being classified by their step angle stepper motors are also
classified according to frame sizes which correspond to the diameter of the body
of the motor. For instance a size 11 stepper motor has a body diameter of
approximately 1.1 inches. Likewise a size 23 stepper motor has a body diameter
of 2.3 inches (58 mm), etc. The body length may however, vary from motor to
motor within the same frame size classification. As a general rule the available
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torque output from a motor of a particular frame size will increase with increased
body length.Power levels for IC-driven stepper motors typically range from below
a watt for very small motors up to 10 – 20 watts for larger motors. The maximum
power dissipation level or thermal limits of the motor are seldom clearly stated in
the motor manufacturer’s data.
To determine this we must apply the relationship
PÊ=V ÊI.
For example, a size 23 step motor may be rated at 6V and 1A per phase.
Therefore, with two phases energized the motor has a rated power dissipation of
12 watts. It is normal practice to rate a stepper motor at the power dissipation
level where the motor case rises 65degree C above the ambient in still air.
Therefore, if the motor can be mounted to a heatsink it is often possible to
increase the allowable power dissipation level. This is important as the motor is
designed to be and should be used at its maximum power dissipation, to be
efficient from a size/output power/cost point of view.
3.3.2 THE ROTATING MAGNETIC FIELD
When a phase winding of a stepper motor is energized with current a magnetic
flux is developed in the stator. The direction of this flux is determined by the
“Right Hand Rule” which states:
“If the coil is grasped in the right hand with the fingers pointing in the direction of
the current in the winding (the thumb is extended at a 90 degree angle to the
fingers), then the thumb will point in the direction of the magnetic field.”The rotor
then aligns itself so that the flux opposition is minimized. In this case the motor
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would rotate clockwise so that its south pole aligns with the north pole of the
stator B at position 2 and its north pole aligns with the South Pole of stator B at
position 6. To get the motor to rotate we can now see that we must provide a
sequence of energizing the stator windings in such a fashion that provides a
rotating magnetic flux field which the rotor follows due to magnetic attraction.
3.3.3 TORQUE GENERATION
The torque produced by a stepper motor depends on several factors.
• The step rate
• The drive current in the windings
• The drive design or type
In a stepper motor a torque is developed when the magnetic fluxes of the rotor
and stator are displaced from each other. The stator is made up of a high
permeability magnetic material. The presence of this high permeability material
causes the magnetic flux to be confined for the most part to the paths defined by
the stator structure in the same fashion that currents are confined to the
conductors of an electronic circuit. This serves to concentrate the flux at the
stator poles. The torque output produced by the motor is proportional to the
intensity of the magnetic flux generated when the winding is energized.
The basic relationship which defines the intensity of the magnetic flux is defined
by:
H = (N i) l
Where:
N = the number of winding turns
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i = current
H = Magnetic field intensity
l = Magnetic flux path length
This relationship shows that the magnetic flux intensity and consequently the
torque is proportional to the number of winding turns and the current and
inversely proportional to the length of the magnetic flux path. From this basic
relationship one can see that the same frame size stepper motor could have very
different torque output capabilities simply by changing the winding parameters.
3.3.4 PHASES, POLES AND STEPPING ANGLES
Usually stepper motors have two phases, but three- and five-phase motors also
exist. A bipolar motor with two phases has one winding/phase and a unipolar
motor has one winding, with a center tap per phase. Sometimes the unipolar
Stepper motor is referred to as a “four phase motor”, even though it only has two
phases. Motors that have two separate windings per phase also exist—these can
be driven in either bipolar or unipolar mode. A pole can be defined as one of the
regions in a magnetized body where the magnetic flux density is concentrated.
Both the rotor and the stator of a step motor have poles.In reality several more
poles are added to both the rotor and stator structure in order to increase the
number of steps per revolution of the motor, or in other words to provide a
smaller basic (full step) stepping angle. The permanent magnet stepper motor
contains an equal number of rotor and stator pole pairs. Typically the PM motor
38
has 12 pole pairs. The stator has 12 pole pairs per phase. The hybrid type
stepper motor has a rotor with teeth. The rotor is split into two parts, separated
by a permanent magnet—making half of the teeth south poles and half north
poles. The number of pole pairs is equal to the number of teeth on one of the
rotor halves. The stator of a hybrid motor also has teeth to build up a higher
number of equivalent poles (smaller pole pitch, number of equivalent poles =
360/teeth pitch) compared to the main poles, on which the winding coils are
wound. Usually 4 main poles are used for 3.6 hybrids and 8 for 1.8- and 0.9-
degree types. It is the relationship between the number of rotor poles and the
equivalent stator poles, and the number the number of phases that determines
the full-step angle of a stepper motor.
Step angle=360 (NPh Ph)=360/N
NPh = Number of equivalent poles per phase = number of
Rotor poles
Ph = Number of phases
N = Total number of poles for all phases together
If the rotor and stator tooth pitch is unequal, a more-complicated relationship
exists.
3.3.5 STEPPING MODES
The following are the most common drive modes.
• Wave Drive (1 phase on)
• Full Step Drive (2 phases on)
39
• Half Step Drive (1 & 2 phases on)
• Microstepping (Continuously varying motor currents)
In Wave Drive only one winding is energized at any given time. The stator is
energized according to the sequence A B A B and the rotor steps from
position 8 2 4 6. For unipolar and bipolar wound IB
Phase A
Phase B
Stator A
Stator B
Rotor motors with the same winding parameters this excitation mode would result
in the same mechanical position. The disadvantage of this drive mode is that in
the unipolar wound motor you are only using 25% and in the bipolar motor only
50% of the total motor winding at any given time. In Full Step Drive you are
energizing two phases at any given time. The stator is energized according to the
sequence AB AB AB AB and the rotor steps from position 1 3 5
7 . Full step mode results in the same angular movement as 1 phase on drive
but the mechanical position is offset by one half of a full step. The torque output
of the unipolar wound motor is lower than the bipolar motor (for motors with the
same winding parameters) since the unipolar motor uses only 50% of the
available winding while the bipolar motor uses the entire winding.Half Step Drive
combines both wave and full step (1&2 phases on) drive modes. Every second
step only one phase is energized and during the other steps one phase on each
stator.The stator is energized according to the sequence AB B AB A
40
AB B AB A and the rotor steps from position 1 2 3 4 5
6 7 8. This results in angular movements that are half of those in 1- or
2-phases-on drive modes. Half stepping can reduce a phenomena referred to as
resonance which can be experienced in 1- or 2- phases-on drive modes.The
displacement angle is determined by the following relationship:
X = (Z 2) sin (Ta Th) , where:
Z = rotor tooth pitch
Ta = Load torque
Th = Motors rated holding torque
X = Displacement angle.
Increasing the holding torque for a constant load causes a shift in the lag angle
from Q2 to Q1.
STEP ANGLE ACCURACY
One reason why the stepper motor has achieved such popularity as a positioning
device is its accuracy and repeatability. Typically stepper motors will have a step
angle accuracy of 3 – 5% of one step. This error is also noncumulative from step
to step. The accuracy of the stepper motor is mainly a function of the mechanical
precision of its parts and assembly.
STEP POSITION ERROR
The maximum positive or negative position error caused when the motor has
rotated one step from the previous holding position.
41
Step position error = measured step angle - theoretical angle
3.3.6 TORQUE VS ANGLE CHARACTERISTICS
The torque vs angle characteristics of a stepper motor are the relationship
between the displacement of the rotor and the torque which applied to the rotor
shaft when the stepper motor is energized at its rated voltage. When you apply
an external force Ta to the motor shaft you in essence create an angular
displacement, a. This angular displacement, a, is referred to as a lead or lag
angle depending on weather the motor is actively accelerating or decelerating.
When the rotor stops with an applied load it will come to rest at the position
defined by this displacement angle. The motor develops a torque, Ta, in
opposition to the applied external force in order to balance the load. As the load
is increased the displacement angle also increases until it reaches the maximum
holding torque, Th, of the motor. Once Th is exceeded the motor enters an
unstable region. In this region a torque is the opposite direction is created and
the rotor jumps over the unstable point to the next stable point.
3.3.7 TORQUE VS SPEED CHARACTERISTICS
The torque vs. speed characteristics are the key to selecting the right motor and
drive method for a specific application. These characteristics are dependent upon
(change with) the motor, excitation mode and type of driver or drive method.
To get a better understanding of this curve it is useful to define the different
aspect of this curve.
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3.4 MECHANICAL PARAMETERS-LOAD, FRICTION, INERTIA
The performance of a stepper motor system (driver and motor) is also highly
dependent on the mechanical parameters of the load. The load is defined as
what the motor drives. It is typically frictional, inertial or a combination of the
two.Friction is the resistance to motion due to the unevenness of surfaces which
rub together. Friction is constant with velocity. A minimum torque level is required
throughout the step in over to overcome this friction (at least equal to the friction).
Increasing a frictional load lowers the top speed, lowers the acceleration and
increases the positional error. The converse is true if the frictional load is
lowered.Inertia is the resistance to changes in speed. A high inertial load requires
a high inertial starting torque and the same would apply for braking. Increasing
an inertial load will increase speed stability, increase the amount of time it takes
to reach a desired speed and decrease the maximum self start pulse rate. The
converse is again true if the inertia is decreased.The rotor oscillations of a
stepper motor will vary with the amount of friction and inertia load. Because of
this relationship unwanted rotor oscillations can be reduced by mechanical
damping means however it is more often simpler to reduce these unwanted
oscillations by electrical damping methods such as switch from full step drive to
half step drive.
3.4.1 HOLDING TORQUE
The maximum torque produced by the motor at standstill.
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Pull-In Curve
The pull-in curve defines an area referred to as the start stop region. This is the
maximum frequency at which the motor can start/stop instantaneously, with a
load applied, without loss of synchronism.
Maximum Start Rate
The maximum starting step frequency with no load applied.
Pull-Out Curve
The pull-out curve defines an area referred to as the slew region. It defines the
maximum frequency at which the motor can operate without losing synchronism.
Since this region is outside the pull-in area the motor must ramped (accelerated
or decelerated) into this region.
Maximum Slew Rate
The maximum operating frequency of the motor with no load applied. The pull-in
characteristics vary also depending on the load. The larger the load inertia, the
smaller the pull-in area. We can see from the shape of the curve that the step
rate affects the torque output capability of stepper motor. The decreasing torque
output as the speed increases is caused by the fact that at high speeds the
inductance of the motor is the dominant circuit element.
3.5 HOW STEPPER MOTORS WORK
Stepper motors consist of a permanent magnet rotating shaft, called the rotor,
and electromagnets on the stationary portion that surrounds the motor, called the
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stator. Figure 1 illustrates one complete rotation of a stepper motor. At position 1,
we can see that the rotor is beginning at the upper electromagnet, which is
currently active (has voltage applied to it). To move the rotor clockwise (CW), the
upper electromagnet is deactivated and the right electromagnet is activated,
causing the rotor to move 90 degrees CW, aligning itself with the active magnet.
This process is repeated in the same manner at the south and west
electromagnets until we once again reach the starting position.
In the above example, we used a motor with a resolution of 90 degrees or
demonstration purposes. In reality, this would not be a very practical motor for
most applications. The average stepper motor's resolution -- the amount of
degrees rotated per pulse -- is much higher than this. For example, a motor with
a resolution of 5 degrees would move its rotor 5 degrees per step, thereby
requiring 72 pulses (steps) to complete a full 360 degree rotation.
45
You may double the resolution of some motors by a process known as "half-
stepping". Instead of switching the next electromagnet in the rotation on one at a
time, with half stepping you turn on both electromagnets, causing an equal
attraction between, thereby doubling the resolution. As you can see in Figure 2,
in the first position only the upper electromagnet is active, and the rotor is drawn
completely to it. In position 2, both the top and right electromagnets are active,
causing the rotor to position itself between the two active poles. Finally, in
position 3, the top magnet is deactivated and the rotor is drawn all the way right.
This process can then be repeated for the entire rotation.
There are several types of stepper motors. 4-wire stepper motors contain only
two electro magnets, however the operation is more complicated than those with
three or four magnets, because the driving circuit must be able to reverse the
current after each step. For our purposes, we will be using a 6-wire motor.
Unlike our example motors which rotated 90 degrees per step, real-world motors
employ a series of mini-poles on the stator and rotor to increase resolution.
Although this may seem to add more complexity to the process of driving the
46
motors, the operation is identical to the simple 90 degree motor we used in our
example.
An example of a multipole motor can be seen in Figure 3. In position 1, the north
pole of the rotor's permanent magnet is aligned with the south pole of the stator's
electromagnet. Note that multiple positions are aligned at once. In position 2, the
upper electromagnet is deactivated and the next one to its immediate left is
activated, causing the rotor to rotate a precise amount of degrees. In this
example, after eight steps the sequence repeats.
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The specific stepper motor we are using for our experiments (ST-02: 5VDC, 5
degrees per step) has 6 wires coming out of the casing. If we follow Figure 5, the
electrical equivalent of the stepper motor, we can see that 3 wires go to each half
of the coils, and that the coil windings are connected in pairs.
However, if you do not have an equivalent diagram for the motor you want to
use, you can make a resistance chart to decipher the mystery connections.
48
There is a 13 ohm resistance between the centre-tap wire and each end lead,
and 26 ohms between the two end leads. Wires originating from separate coils
are not connected, and therefore would not read on the ohm meter.
FIRST STEPPER CIRCUIT
The PIC's output lines are first buffered by a 4050 hex buffer chip, and are then
connected to an NPN transistor. The transistor used, TIP120, is actually a NPN
Darlington (it is shown as a standard NPN). The TIP120's act like switches,
activating one stepper motor coil at a time.
Due to a inductive surge created when a coil is toggled, a standard 1N4001
diode is usually placed across each transistor as shown in the figure, providing a
safe way of dispersing the reverse current without damaging the transistor,
49
sometimes called a snubbing diode. The TIP120 transistors do not need an
external snubbing diode because they have a built in diode. So the diodes shown
in the drawing are the internal diodes in the TIP120 transistors.
The simplest way to operate a stepper motor with a PIC is with the full step
pattern shown in Table 1. Each part of the sequence turns on only one transistor
at a time, one after the other. After the sequence is completed, it repeats infinitly
until power is removed.
Q1 Q2 Q3 Q4
+ - - -
- + - -
- - + -
- - - +
I purposely made this first program as small as possible; simply to demonstrate
how easy it is to control a stepper motor. Also note the use of high and low
commands to control the output lines, rather than peek and poke routines. For
our purposes, high and low are sufficient.
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3.6 SOURCES
Stepper motors can be found in almost any piece of electro-mechanical
equipment. From my personal experiences, good sources for stepper motors
include:
SURPLUS DOT-MATRIX PRINTERS
If you find one of these at a swap meet, surplus store, or garage sale for a good
price, buy it! They usually contain at least 2 motors, sometimes with optical shaft
encoders attached to the motors! Also a good source for matching gears and
toothed belts. As a general rule, larger printers will have larger, more powerful
stepper motors in them.
OLD FLOPPY DISK DRIVES
These usually contain at least 1 stepper motor, and if you're fortunate, possibly a
driver IC that can be salvaged and re-used in your own projects. Along with the
motor you will get some optical interrupter units used by the drive to sense the
state of the write-protect tabs and to index the disk.
SURPLUS STORES
These places buy surplus from others and sell it to the public, often at great
prices. The average price for a small to medium stepper motor is usually around
$5.00.
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Stepper motors are not just rated by voltage. The following elements characterize
a given stepper motor:
VOLTAGE
Stepper motors usually have a voltage rating. This is either printed directly
on the unit, or is specified in the motor's datasheet. Exceeding the rated
voltage is sometimes necessary to obtain the desired torque from a given
motor, but doing so may produce excessive heat and/or shorten the life of
the motor.
RESISTANCE
Resistance-per-winding is another characteristic of a stepper motor. This
resistance will determine current draw of the motor, as well as affect the
motor's torque curve and maximum operating speed.
DEGREES PER STEP
This is often the most important factor in choosing a stepper motor for a
given application. This factor specifies the number of degrees the shaft will
rotate for each full step. Half step operation of the motor will double the
number of steps/revolution, and cut the degrees-per-step in half. For
unmarked motors, it is often possible to carefully count, by hand, the
number of steps per revolution of the motor. The degrees per step can be
calculated by dividing 360 by the number of steps in 1 complete revolution
52
Common degree/step numbers include: 0.72, 1.8, 3.6, 7.5, 15, and even
90. Degrees per step is often referred to as the resolution of the motor. As
in the case of an unmarked motor, if a motor has only the number of
steps/revolution printed on it, dividing 360 by this number will yield the
degree/step value.
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CHAPTER 4
OPTO-ISOLATOR
4.1 INTRODUCTION
In electronics, an opto-isolator (or optical isolator, optical coupling
device, optocoupler, photo coupler, or photo MOS) is a device that uses a
short optical transmission path to transfer an electronic signal between elements
of a circuit, typically a transmitter and a receiver, while keeping them electrically
isolated—since the electrical signal is converted to a light beam, transferred, then
converted back to an electrical signal, there is no need for electrical connection
between the source and destination circuits.
The opto-isolator is simply a package that contains both an infrared light-emitting
diode (LED) and a photo detector such as a photosensitive silicon diode,
transistor Darlington pair, or silicon controlled rectifier (SCR). The wave-length
responses of the two devices are tailored to be as identical as possible to permit
the highest measure of coupling possible. Other circuitry—for example an output
amplifier—may be integrated into the package. An opto-isolator is usually thought
of as a single integrated package, but opto-isolation can also be achieved by
using separate devices.
Digital opto-isolators change the state of their output when the input state
changes; analog isolators produce an analog signal which reproduces the input.
Optical couplers, also referred to as optocouplers, are well known devices used to
54
direct light from one light source to a light receiving member. Optical couplers are the
heart of an optical communication network. Optical fiber technology is used in a
variety of applications such as telecommunication, computer, and medical
applications. In the past, optical fiber communication technology and optical fiber
communication elements mainly were used on backbone networks. These days they
are widely used in metropolitan optical communication networks. An important aspect
of optical fiber technology is the coupling of an optical fiber to an optoelectronic device
for transmitting information conducted by the optical fiber. Fiber optic systems have a
number of features that make them superior to systems that use traditional copper
cables. For example, fiber optic systems can have a much larger information-carrying
capacity and are not subject to electrical interference. Signals transmitted over long-
distance optic fibers need less amplification than do signals transmitted over copper
cables of equal length. Optical fibers have by far the greatest transmission bandwidth
of any conventional transmission medium, and therefore optical fibers provide an
excellent transmission medium. An optical fiber is a thin filament of drawn or extruded
glass or plastic having a central core and a surrounding cladding of lower index
material to promote internal reflection. Optical fibers are typically arranged in a bundle
and protected by a sheath. Such a bundle of optical fibers is often referred to as an
optical cable. The light receiving and emitting ends of the optical fibers are housed in
fiber ferrules. The fiber ferrule at the light receiving end of the bundle is coupled to a
light emitting device via an optical interface unit. Likewise, the fiber ferrule at the light
emitting end of the bundle is coupled to a light detecting device via an optical
interface.
55
An optical coupler is a passive device for branching or coupling an optical signal.
Generally, a coupler is centralized by joining the two fibers together so that the light
can pass from the sender unit to the two receivers, or else it can be made by
juxtaposing the two "receiver" fibers which will then be aligned and positioned so as to
be facing the "sender" fiber. The function of branching or coupling an optical signal in
optical communications can be simply performed by various photomechanical
connections, similar to branching or coupling in electric communications. However,
the optical signal cannot be simply realized because of the characteristics of the
optical fiber, so that a special optical coupler is employed as a light branching and
coupling device. Optocouplers are used to electrically isolate an input signal from a
corresponding output signal. Optocouplers, also referred to as optically coupled
isolator devices or optical coupler circuits; provide isolation between different circuit
portions which operate at vastly different voltages. The optocoupler offers an
advantage of providing electrical isolation between the two circuits, thus reducing
interface problems. Optocouplers have been used for electrical isolation in systems
such as computers, power supplies , telecommunications, and controllers. For
example, optocouplers are used in applications such as telecommunications
equipment, programmable controllers, direct current (DC) to DC converters,
alternating current (AC) to DC converters and battery chargers. Optocouplers are
commonly used in switched-mode power supplies and other analog circuits to provide
an analog feedback control signal across the isolation barrier. Optocouplers are also
commonly used in circuits as indicators, or as control devices, wherein the light-
responsive transistors are energized to control other circuits. Optocouplers usually
56
include a light-emitting diode (LED) and a light-responsive transistor (light sensor)
such as a phototransistor or a photodiode. Electrical isolation occurs because
information is transmitted using light emitted by the LED and received by the light-
responsive transistor. When the current driving the LED is changed, the amount of
light that is emits also changes proportionally and consequently also the electrical
resistance of the photoresistor.
Optical couplers are key components in optical networks. Optical couplers, optical
switches, and optical power splitters are needed in many optical applications. In fiber
optical transmission systems the light beams travelling in two or more fibers must
often be combined into a single fiber, a device which accomplishes this is called a
combiner or multiplexer. Similarly, in such systems one beam must frequently be split
into two or more beams, a device which accomplishes this is called a splitter or
divider. The optical fiber coupler, also called optical fiber splitter, is an essential
element to implement Fiber-To-The-Home (FTTH). Optical couplers are optical
transmission system components used to connect planar arrangements of
waveguides. Optical couplers are used for routing signals from one waveguide to
another and/or for splitting optical signals into two independent signals at a
predetermined power ratio to be transmitted over two different waveguides. Optical
couplers are typically utilized to separate or combine an optic signal, such as an optic
signal in a fiber optic cable. An optocoupler is generally used for causing outgoing
light from an end face of an optical fiber to fall on another optical fiber. It is important
for the optocoupler to be able to cause outgoing light from an optical fiber to fall on
57
another optical fiber without loss and generation of optical noises. Optical fiber
couplers have a coupling section which connects a plurality of optical fibers and, at
the coupling section, separate or combine the light within the optical fibers. Typical
optical couplers are comprised of a plurality of fiber optic cables. Where an optic
signal is to be combined, the number of inputting signals is greater than the number of
outputting signals. Where an optic signal is to be split, the number of cables inputting
a signal is less than the number of cables outputting a signal. The optical fiber coupler
usually is fabricated through a fused biconical tapered fiber coupling technique. The
optical fiber passive device thus made costs less and has excellent optical
characteristics.
4.2 CIRCUIT WITH OPTOISOLATION
If you want to have a very good protection of the parallel port you might consider
optoisolation using the following type of circuit:
V+ (12V)
External Circuit Ground
Typical optoisolator pin out (CNY 17 and 4N25):
The opto-isolator is there to protect the port. Note that there are no connections
between the port's electrical contacts. The circuit is powered from external power
supply which is not connected to PC if there is no need for that. This arrangement
prevents any currents on the external circuits from damaging the parallel port.
The opto-isolator's input is a light emitting diode.R1 is used to limit the current when
58
the output from the port is on. That 1kohm resistor limits the current to around 3 mA,
which is well sufficient for that output transistor driving.
The output side of the opto-isolator is just like a transistor, with the collector at the top
of the circuit and the emitter at the bottom. When the output is turned on (by the input
light from the internal LED in the opto-coupler), current flows through the resistor and
into the transistor, turning it on. This allows current to flow into the relay.
Turning the input on the parallel port off causes the output of the opto-isolator to turn
off, so no current flows through it into the transistor and the transistor turns off. When
transistor is off no current flows into the relay, so it switches off. The diode provides
an outlet for the energy stored in the coil, preventing the relay from back feeding the
circuit in an undesired manner.
The circuit can be used for controlling output loads to maximum of around 100 mA
(depends somewhat on components and operation voltage used). The external power
supply can be in 5V to 24V range.
4.2.1 OPTOISOLATED HIGHER POWER CIRCUIT
Here is a higher power version of the circuit described above:
External Circuit Ground
In this circuit Q1 is used for controlling the base current of Q2 which controls the
actual current. You can select almost any general purpose power transistor for this
circuit which matches your current and voltage controlling needs. Some example
59
alternatives are for example TIP41C (6A 100V) or 2N3055 (100V 15A). Depending
your amplification factor inherent to the transistor Q2 you might not Hough be able to
use the full current capability of the output device T2 before there will be excessive
losses (heating) in that transistor.
This circuit is basically very simple modification of the original optoisolator circuit with
one transistor. The difference in this circuit is that here T2 controls the load current
and Q1 acts as a current amplifier for T2 base control current. Optoisolator, R1, R2,
Q1, D1 work exactly in the same way as in one transistor circuit described earlier in
this documents. R3 acts like an extra resistor which guarantees that T2 does not
conduct when there is no signal fed to the optoisolator (small possible current leaking
on optoisolator output does not make T1 and T2 to conduct).
4.3 CONFIGURATIONS
Fig 4.1Schematic diagram of a very simple opto-isolator with an LED and
phototransistor. The dashed line represents the isolation barrier, over which there is no electrical contact.
A common implementation is a LED and a phototransistor in a light-tight housing
to exclude ambient light and without common electrical connection, positioned so
that light from the LED will impinge on the photodetector. When an electrical
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signal is applied to the input of the opto-isolator, its LED lights and illuminates the
photodetector, producing a corresponding electrical signal in the output circuit.
Unlike a transformer the opto-isolator allows DC coupling and can provide any
desired degree of electrical isolation and protection from serious over voltage
conditions in one circuit affecting the other. A higher transmission ratio can be
obtained by using a Darlington instead of a simple phototransistor, at the cost of
reduced noise immunity and higher delay.
With a photodiode as the detector, the output current is proportional to the
intensity of incident light supplied by the emitter. The diode can be used in
a photovoltaic mode or a photoconductive mode. In photovoltaic mode, the diode
acts as a current source in parallel with a forward-biased diode. The output
current and voltage are dependent on the load impedance and light intensity. In
photoconductive mode, the diode is connected to a supply voltage, and the
magnitude of the current conducted is directly proportional to the intensity of light.
This optocoupler type is significantly faster than photo transistor type, but the
transmission ratio is very low; it is common to integrate an output amplifier circuit
into the same package.
The optical path may be air or a dielectric waveguide. When high noise immunity
is required an optical conductive shield can be integrated into the optical path.
The transmitting and receiving elements of an optical isolator may be contained
within a single compact module, for mounting, for example, on a circuit board; in
this case, the module is often called an optoisolator or opto-isolator.This device
may in turn operate a power relay or contactor.
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Analog optoisolators often have two independent, closely matched output
phototransistors, one of which is used to linearize the response using negative
feedback.
CHAPTER 5
RADAR
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5.1 INTRODUCTION
Radar is an object detection system that uses electromagnetic waves to identify
the range, altitude, direction, or speed of both moving and fixed objects such
as aircraft, ships, motor vehicles, weather formations, and terrain. The
term RADAR was coined in 1940 by the U.S. Navy as an
acronym for RAdio Detection And Ranging. The term has since entered the
English language as a standard word, radar, losing the capitalization. Radar was
originally called RDF (Range and Direction Finding) in the United Kingdom, using
the same acronym as Radio Direction Finding to preserve the secrecy of its
ranging capability.
A radar system has a transmitter that emits radio waves. When they come into
contact with an object they are scattered in all directions. The signal is thus partly
reflected back and it has a slight change of wavelength (and thus frequency) if
the target is moving. The receiver is usually, but not always, in the same location
as the transmitter. Although the signal returned is usually very weak, the signal
can be amplified through use of electronic techniques in the receiver and in the
antenna configuration. This enables radar to detect objects at ranges where
other emissions, such as sound or visible light, would be too weak to detect.
Radar uses include meteorological detection of precipitation, measuring ocean
surface waves, air traffic control, police detection
of speeding traffic, military applications, or to simply determine the speed of a
baseball.
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5.2 APPLICATIONS OF RADAR
The information provided by radar includes the bearing and range (and therefore
position) of the object from the radar scanner. It is thus used in many different
fields where the need for such positioning is crucial. The first use of radar was for
military purposes; to locate air, ground and sea targets. This has evolved in the
civilian field into applications for aircraft, ships and roads.
In aviation, aircraft are equipped with radar devices that warn of obstacles in or
approaching their path and give accurate altitude readings. They can land in fog
at airports equipped with radar-assisted ground-controlled approach (GCA)
systems, in which the plane's flight is observed on radar screens while operators
radio landing directions to the pilot.
Marine radars are used to measure the bearing and distance of ships to prevent
collision with other ships, to navigate and to fix their position at sea when within
range of shore or other fixed references such as islands, buoys, and lightships. In
port or in harbour, Vessel traffic service radar systems are used to monitor and
regulate ship movements in busy waters. Police forces use radar guns to monitor
vehicle speeds on the roads.
Radar has invaded many other fields. Meteorologists use radar to
monitor precipitation. It has become the primary tool for short-term weather
forecasting and to watch for severe weather such
as thunderstorms, tornadoes, winter storms precipitation types,
etc... Geologists use specialised ground-penetrating radars to map the
composition of the Earth crust. The list is getting longer all the time.
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PRINCIPLES
The radar dish, or antenna, transmits pulses of radio waves or microwaves which
bounce off any object in their path. The object returns a tiny part of the wave's
energy to a dish or antenna which is usually located at the same site as the
transmitter. The time it takes for the reflected waves to return to the dish enables
a computer to calculate how far away the object is, its radial velocity and other
characteristics.
REFLECTION
Fig 5.1
Brightness can indicate reflectivity as in this 1960 weather radar image
(of Hurricane Abby). The radar's frequency, pulse form, polarization, signal
processing, and antenna determine what it can observe.
Electromagnetic waves reflect (scatter) from any large change in the dielectric
constant or diamagnetic constants. This means that a solid object in air or
a vacuum, or other significant change in atomic density between the object and
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what is surrounding it, will usually scatter radar (radio) waves. This is particularly
true for electrically conductive materials, such as metal and carbon fiber, making
radar particularly well suited to the detection of aircraft and ships. Radar
absorbing material, containing resistive and sometimes magnetic substances, is
used on military vehicles to reduce radar reflection. This is the radio equivalent of
painting something a dark color so that it cannot be seen through normal means.
Radar waves scatter in a variety of ways depending on the size (wavelength) of
the radio wave and the shape of the target. If the wavelength is much shorter
than the target's size, the wave will bounce off in a way similar to the way light is
reflected by a mirror. If the wavelength is much longer than the size of the target,
the wave is polarized (positive and negative charges are separated), like a dipole
antenna. This is described by Rayleigh scattering, an effect that creates the
Earth's blue sky and red sunsets. When the two length scales are comparable,
there may be resonances. Early radars used very long wavelengths that were
larger than the targets and received a vague signal, whereas some modern
systems use shorter wavelengths (a few centimeters or shorter) that can image
objects as small as a loaf of bread.
Short radio waves reflect from curves and corners, in a way similar to glint from a
rounded piece of glass. The most reflective targets for short wavelengths have
90° angles between the reflective surfaces. A structure consisting of three flat
surfaces meeting at a single corner, like the corner on a box, will always reflect
waves entering its opening directly back at the source. These so-called corner
reflectors are commonly used as radar reflectors to make otherwise difficult-to-
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detect objects easier to detect, and are often found on boats in order to improve
their detection in a rescue situation and to reduce collisions.
For similar reasons, objects attempting to avoid detection will angle their surfaces
in a way to eliminate inside corners and avoid surfaces and edges perpendicular
to likely detection directions, which leads to "odd" looking stealth aircraft. These
precautions do not completely eliminate reflection because of diffraction,
especially at longer wavelengths. Half wavelength long wires or strips of
conducting material, such as chaff, are very reflective but do not direct the
scattered energy back toward the source. The extent to which an object reflects
or scatters radio waves is called its radar cross section.
5.3 RADAR EQUATION
The power Pr returning to the receiving antenna is given by the radar
equation:
Where,
Pt = transmitter power
Gt = gain of the transmitting antenna
Ar = effective aperture (area) of the receiving antenna
σ = radar cross section, or scattering coefficient, of the target
F = pattern propagation factor
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Rt = distance from the transmitter to the target
Rr = distance from the target to the receiver.
In the common case where the transmitter and the receiver are at the same
location, Rt = Rr and the term Rt² Rr² can be replaced by R4, where R is the range,
This shows that the received power declines as the fourth power of the range,
which means that the reflected power from distant targets is very, very small.
The equation above with F = 1 is a simplification for vacuum without interference.
The propagation factor accounts for the effects of multi path and shadowing and
depends on the details of the environment. In a real-world situation, path
loss effects should also be considered.
POLARIZATION
In the transmitted radar signal, the electric field is perpendicular to the direction
of propagation, and this direction of the electric field is the polarization of the
wave. Radars use horizontal, vertical, linear and circular polarization to detect
different types of reflections. For example, circular polarization is used to
minimize the interference caused by rain. Linear polarization returns usually
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indicate metal surfaces. Random polarization returns usually indicate
a fractal surface, such as rocks or soil, and are used by navigation radars.
INTERFERENCE
Radar systems must overcome unwanted signals in order to focus only on the
actual targets of interest. These unwanted signals may originate from internal
and external sources, both passive and active. The ability of the radar system to
overcome these unwanted signals defines its signal-to-noise ratio (SNR). SNR is
defined as the ratio of a signal power to the noise power within the desired
signal.
In less technical terms, SNR compares the level of a desired signal (such as
targets) to the level of background noise. The higher a system's SNR, the better
it is in isolating actual targets from the surrounding noise signals.
NOISE
Signal noise is an internal source of random variations in the signal, which is
generated by all electronic components. Noise typically appears as random
variations superimposed on the desired echo signal received in the radar
receiver. The lower the power of the desired signal, the more difficult it is to
discern it from the noise (similar to trying to hear a whisper while standing near a
busy road). Noise figure is a measure of the noise produced by a receiver
compared to an ideal receiver, and this needs to be minimized.
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Noise is also generated by external sources, most importantly the natural thermal
radiation of the background scene surrounding the target of interest. In modern
radar systems, due to the high performance of their receivers, the internal noise
is typically about equal to or lower than the external scene noise. An exception is
if the radar is aimed upwards at clear sky, where the scene is so "cold" that it
generates very little thermal noise.
There will be also flicker noise due to electrons transit, but depending on 1/f, will
be much lower than thermal noise when the frequency is high. Hence, in pulse
radar, the system will be always heterodyne. See intermediate frequency.
CLUTTER
Clutter refers to radio frequency (RF) echoes returned from targets which are
uninteresting to the radar operators. Such targets include natural objects such as
ground, sea, precipitation(such as rain, snow or hail), sand storms, animals
(especially birds), atmospheric turbulence, and other atmospheric effects, such
as ionosphere reflections, meteor trails, and three body scatter spike. Clutter may
also be returned from man-made objects such as buildings and, intentionally, by
radar countermeasures such as chaff.
Some clutter may also be caused by a long radar waveguide between the radar
transceiver and the antenna. In a typical plan position indicator (PPI) radar with a
rotating antenna, this will usually be seen as a "sun" or "sunburst" in the centre of
the display as the receiver responds to echoes from dust particles and misguided
RF in the waveguide. Adjusting the timing between when the transmitter sends a
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pulse and when the receiver stage is enabled will generally reduce the sunburst
without affecting the accuracy of the range, since most sunburst is caused by a
diffused transmit pulse reflected before it leaves the antenna.
While some clutter sources may be undesirable for some radar applications
(such as storm clouds for air-defence radars), they may be desirable for others
(meteorological radars in this example). Clutter is considered a passive
interference source, since it only appears in response to radar signals sent by the
radar.
There are several methods of detecting and neutralizing clutter. Many of these
methods rely on the fact that clutter tends to appear static between radar scans.
Therefore, when comparing subsequent scans echoes, desirable targets will
appear to move and all stationary echoes can be eliminated. Sea clutter can be
reduced by using horizontal polarization, while rain is reduced with circular
polarization (note that meteorological radars wish for the opposite effect,
therefore using linear polarization the better to detect precipitation). Other
methods attempt to increase the signal-to-clutter ratio.
Constant False Alarm Rate (CFAR, a form of Automatic Gain Control, or AGC) s
a method relying on the fact that clutter returns far outnumber echoes from
targets of interest. The receiver's gain is automatically adjusted to maintain a
constant level of overall visible clutter. While this does not help detect targets
masked by stronger surrounding clutter, it does help to distinguish strong target
sources. In the past, radar AGC was electronically controlled and affected the
gain of the entire radar receiver. As radars evolved, AGC became computer-
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software controlled, and affected the gain with greater granularity, in specific
detection cells.
Radar multipath echoes from a target cause ghosts to appear.
Clutter may also originate from multipath echoes from valid targets due to ground
reflection, atmospheric ducting or ionospheric reflection/refraction. This clutter
type is especially bothersome, since it appears to move and behave like other
normal (point) targets of interest, thereby creating a ghost. In a typical scenario,
an aircraft echo is multipath-reflected from the ground below, appearing to the
receiver as an identical target below the correct one. The radar may try to unify
the targets, reporting the target at an incorrect height, or - worse - eliminating it
on the basis of jitter or a physical impossibility. These problems can be overcome
by incorporating a ground map of the radar's surroundings and eliminating all
echoes which appear to originate below ground or above a certain height. In
newer Air Traffic Control (ATC) radar equipment, algorithms are used to identify
the false targets by comparing the current pulse returns, to those adjacent, as
well as calculating return improbabilities due to calculated height, distance, and
radar timing.
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JAMMING
Radar jamming refers to radio frequency signals originating from sources outside
the radar, transmitting in the radar's frequency and thereby masking targets of
interest. Jamming may be intentional, as with an electronic warfare (EW) tactic,
or unintentional, as with friendly forces operating equipment that transmits using
the same frequency range. Jamming is considered an active interference source,
since it is initiated by elements outside the radar and in general unrelated to the
radar signals.
Jamming is problematic to radar since the jamming signal only needs to travel
one-way (from the jammer to the radar receiver) whereas the radar echoes travel
two-ways (radar-target-radar) and are therefore significantly reduced in power by
the time they return to the radar receiver. Jammers therefore can be much less
powerful than their jammed radars and still effectively mask targets along the line
of sight from the jammer to the radar (Mainlobe Jamming). Jammers have an
added effect of affecting radars along other lines of sight, due to the radar
receiver's sidelobes (Sidelobe Jamming).
Mainlobe jamming can generally only be reduced by narrowing the
mainlobe solid angle, and can never fully be eliminated when directly facing a
jammer which uses the same frequency and polarization as the radar. Sidelobe
jamming can be overcome by reducing receiving sidelobes in the radar antenna
design and by using an omnidirectional antenna to detect and disregard non-
mainlobe signals. Other anti-jamming techniques are frequency
hopping and polarization. Interference has recently become a problem for C-
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band (5.66 GHz) meteorological radars with the proliferation of 5.4 GHz
band WiFi equipment.
Pulse radar system
Fig 5.2
SONAR RADAR
One way to measure the distance to an object is to transmit a short pulse of radio
signal (electromagnetic radiation), and measure the time it takes for the reflection
to return. The distance is one-half the product of the round trip time (because the
signal has to travel to the target and then back to the receiver) and the speed of
the signal. Since radio waves travel at the speed of light (186,000 miles per
second or 300,000,000 meters per second), accurate distance measurement
requires high-performance electronics.
In most cases, the receiver does not detect the return while the signal is being
transmitted. Through the use of a device called a duplexer, the radar switches
between transmitting and receiving at a predetermined rate. The minimum range
is calculated by measuring the length of the pulse multiplied by the speed of light,
divided by two. In order to detect closer targets one must use a shorter pulse
length.
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A similar effect imposes a maximum range as well. If the return from the target
comes in when the next pulse is being sent out, once again the receiver cannot
tell the difference. In order to maximize range, longer times between pulses
should be used, referred to as a pulse repetition time (PRT), or it’s reciprocal,
pulse repetition frequency (PRF).
These two effects tend to be at odds with each other, and it is not easy to
combine both good short range and good long range in a single radar. This is
because the short pulses needed for a good minimum range broadcast have less
total energy, making the returns much smaller and the target harder to detect.
This could be offset by using more pulses, but this would shorten the maximum
range again. So each radar uses a particular type of signal. Long-range radars
tend to use long pulses with long delays between them, and short range radars
use smaller pulses with less time between them. This pattern of pulses and
pauses is known as the pulse repetition frequency (or PRF), and is one of the
main ways to characterize a radar. As electronics have improved many types of
radar now can change their PRF thereby changing their range. The newest
radars fire 2 pulses during one cell, one for short range 10 km / 6 miles and a
separate signal for longer ranges 100 km /60 miles.
The distance resolution and the characteristics of the received signal as
compared to noise depend heavily on the shape of the pulse. The pulse is
often modulated to achieve better performance using a technique known
as pulse compression.
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Distance may also be measured as a function of time. The radar mile is the
amount of time it takes for a radar pulse to travel one nautical mile, reflect off a
target, and return to the radar antenna. Since a nautical mile is defined
as exactly 1,852 meters, then dividing this distance by the speed of light
(exactly 299,792,458 meters per second), and then multiplying the result by 2
(round trip = twice the distance), yields a result of approximately 12.36
microseconds in duration.
5.4 FREQUENCY MODULATION
Another form of distance measuring radar is based on frequency modulation.
Frequency comparison between two signals is considerably more accurate, even
with older electronics, than timing the signal. By changing the frequency of the
returned signal and comparing that with the original, the difference can be easily
measured.
This technique can be used in continuous wave radar, and is often found in
aircraft radar altimeters. In these systems a "carrier" radar signal is frequency
modulated in a predictable way, typically varying up and down with a sine
wave or sawtooth pattern at audio frequencies. The signal is then sent out from
one antenna and received on another, typically located on the bottom of the
aircraft, and the signal can be continuously compared using a simple beat
frequency modulator that produces an audio frequency tone from the returned
signal and a portion of the transmitted signal.
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Since the signal frequency is changing, by the time the signal returns to the
aircraft the broadcast has shifted to some other frequency. The amount of that
shift is greater over longer times, so greater frequency differences mean a longer
distance, the exact amount being the "ramp speed" selected by the electronics.
The amount of shift is therefore directly related to the distance travelled, and can
be displayed on an instrument. This signal processing is similar to that used in
speed detecting Doppler radar. Example systems using this approach are
AZUSA, MISTRAM, and UDOP.
A further advantage is that the radar can operate effectively at relatively low
frequencies, comparable to that used by UHF television. This was important in
the early development of this type when high frequency signal generation was
difficult or expensive.
New terrestrial radar uses low-power FM signals that cover a larger frequency
range. The multiple reflections are analyzed mathematically for pattern changes
with multiple passes creating a computerized synthetic image. Doppler effects
are not utilized which allows slow moving objects to be detected as well as
largely eliminating "noise" from the surfaces of bodies of water. Used primarily for
detection of intruders approaching in small boats or intruders crawling on the
ground toward an objective.
5.5 SPEED MEASUREMENT
Speed is the change in distance to an object with respect to time. Thus the
existing system for measuring distance, combined with a memory capacity to see
where the target last was, is enough to measure speed. At one time the memory
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consisted of a user making grease-pencil marks on the radar screen, and then
calculating the speed using a slide rule. Modern radar systems perform the
equivalent operation faster and more accurately using computers.
However, if the transmitter's output is coherent (phase synchronized), there is
another effect that can be used to make almost instant speed measurements (no
memory is required), known as the Doppler Effect. Most modern radar systems
use this principle in the pulse-Doppler radar system. Return signals from targets
are shifted away from this base frequency via the Doppler Effect enabling the
calculation of the speed of the object relative to the radar. The Doppler Effect is
only able to determine the relative speed of the target along the line of sight from
the radar to the target. Any component of target velocity perpendicular to the line
of sight cannot be determined by using the Doppler Effect alone, but it can be
determined by tracking the target's azimuth over time. Additional information of
the nature of the Doppler returns may be found in the radar signal
characteristics article.
It is also possible to make a radar without any pulsing, known as a continuous-
wave radar (CW radar), by sending out a very pure signal of a known frequency.
CW radar is ideal for determining the radial component of a target's velocity, but
it cannot determine the target's range. CW radar is typically used by traffic
enforcement to measure vehicle speed quickly and accurately where range is not
important.
Other mathematical developments in radar signal processing include time-
frequency analysis (Weyl Heisenberg or wavelet), as well as the chirplet
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transform which makes use of the fact that radar returns from moving targets
typically "chirp" (change their frequency as a function of time, as does the sound
of a bird or bat).
REDUCTION OF INTERFERENCE EFFECTS
Signal processing is employed in radar systems to reduce the radar interference
effects. Signal processing techniques include moving target
indication (MTI), pulse doppler, moving target detection (MTD) processors,
correlation with secondary surveillance radar (SSR) targets, space-time adaptive
processing (STAP), and track-before-detect (TBD). Constant false alarm rate
(CFAR) and digital terrain model (DTM) processing are also used in clutter
environments.
PLOT AND TRACK EXTRACTION
Radar video returns on aircraft can be subjected to a plot extraction process
whereby spurious and interfering signals are discarded. A sequence of target
returns can be monitored through a device known as a plot extractor. The non
relevant real time returns can be removed from the displayed information and a
single plot displayed. In some radar systems, or alternatively in the command
and control system to which the radar is connected, a radar tracker is used to
associate the sequence of plots belonging to individual targets and estimate the
targets' headings and speeds.
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5.6 RADAR COMPONENTS
Fig 5.3
A radar’s component are: A transmitter that generates the radio signal with an
oscillator such as a klystron or a magnetron and controls its duration by
a modulator. A waveguide that links the transmitter and the antenna.
A duplexer that serves as a switch between the antenna and the transmitter or
the receiver for the signal when the antenna is used in both situations.Knowing
the shape of the desired received signal (a pulse), an optimal receiver can be
designed using a matched filter.An electronic section that controls all those
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devices and the antenna to perform the radar scan ordered by a software.A link
to end users.
5.6.1 ANTENNA DESIGN
Radio signals broadcast from a single antenna will spread out in all directions,
and likewise a single antenna will receive signals equally from all directions. This
leaves the radar with the problem of deciding where the target object is located.
Early systems tended to use omni-directional broadcast antennas, with
directional receiver antennas which were pointed in various directions. For
instance the first system to be deployed, Chain Home, used two straight
antennas at right angles for reception, each on a different display. The maximum
return would be detected with an antenna at right angles to the target, and a
minimum with the antenna pointed directly at it (end on). The operator could
determine the direction to a target by rotating the antenna so one display showed
a maximum while the other shows a minimum.
One serious limitation with this type of solution is that the broadcast is sent out in
all directions, so the amount of energy in the direction being examined is a small
part of that transmitted. To get a reasonable amount of power on the "target", the
transmitting aerial should also be directional.
5.6.2 PARABOLIC REFLECTOR
More modern systems use a steerable parabolic "dish" to create a tight
broadcast beam, typically using the same dish as the receiver. Such systems
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often combine two radar frequencies in the same antenna in order to allow
automatic steering, or radar lock.
Parabolic reflectors can be either symmetric parabolas or spoiled parabolas:
Symmetric parabolic antennas produce a narrow "pencil" beam in both the X and
Y dimensions and consequently have a higher gain. The NEXRAD Pulse-
Doppler weather radar uses a symmetric antenna to perform detailed volumetric
scans of the atmosphere.
Fig 5.4 Pulse radar system
Spoiled parabolic antennas produce a narrow beam in one dimension and a
relatively wide beam in the other. This feature is useful if target detection over a
wide range of angles is more important than target location in three dimensions.
Most 2D surveillance radars use a spoiled parabolic antenna with a narrow
azimuthal beamwidth and wide vertical beamwidth. This beam configuration
allows the radar operator to detect an aircraft at a specific azimuth but at an
indeterminate height. Conversely, so-called "nodder" height finding radars use a
dish with a narrow vertical beamwidth and wide azimuthal beamwidth to detect
an aircraft at a specific height but with low azimuthal precision.
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TYPES OF SCAN
Primary Scan: A scanning technique where the main antenna aerial is moved to
produce a scanning beam, examples include circular scan, sector scan etc
Secondary Scan: A scanning technique where the antenna feed is moved to
produce a scanning beam, examples include conical scan, unidirectional sector
scan, lobe switching etc.
Palmer Scan: A scanning technique that produces a scanning beam by moving
the main antenna and its feed. A Palmer Scan is a combination of a Primary
Scan and a Secondary Scan.
5.7 FREQUENCY BANDS
The traditional band names originated as code-names during World War II and
are still in military and aviation use throughout the world in the 21st century. They
have been adopted in the United States by the IEEE, and internationally by
the ITU. Most countries have additional regulations to control which parts of each
band are available for civilian or military use.
Other users of the radio spectrum, such as the broadcasting and electronic
countermeasures (ECM) industries, have replaced the traditional military
designations with their own systems.
RADAR FREQUENCY BANDS
Band name
Frequency range
Wavelength range
Notes
HF 3–30 MHz 10–100 m coastal radar systems, over-the-horizon
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radar (OTH) radars; 'high frequency'
P< 300 MHz
1 m+'P' for 'previous', applied retrospectively to early radar systems
VHF30–330 MHz
0.9–6 mVery long range, ground penetrating; 'very high frequency'
UHF300–1000 MHz
0.3–1 mVery long range (e.g. ballistic missile early warning), ground penetrating, foliage penetrating; 'ultra high frequency'
L 1–2 GHz 15–30 cmLong range air traffic control and surveillance; 'L' for 'long'
S 2–4 GHz 7.5–15 cmTerminal air traffic control, long-range weather, marine radar; 'S' for 'short'
C 4–8 GHz 3.75–7.5 cmSatellite transponders; a compromise (hence 'C') between X and S bands; weather
X 8–12 GHz 2.5–3.75 cm
Missile guidance, marine radar, weather, medium-resolution mapping and ground surveillance; in the USA the narrow range 10.525 GHz ±25 MHz is used for airport radar. Named X band because the frequency was a secret during WW2.
Ku12–18 GHz
1.67–2.5 cm high-resolution
K18–24 GHz
1.11–1.67 cm
from German kurz, meaning 'short'; limited use due to absorption by water vapour, so Ku and Ka were used instead for surveillance. K-band is used for detecting clouds by meteorologists, and by police for detecting speeding motorists. K-band radar guns operate at 24.150 ± 0.100 GHz.
Ka24–40 GHz
0.75–1.11 cm
mapping, short range, airport surveillance; frequency just above K band (hence 'a') Photo radar, used to trigger cameras which take pictures of license plates of cars running red lights, operates at 34.300 ± 0.100 GHz.
mm 40–300 GHz
7.5 mm – 1 mm
millimetre band, subdivided as below. The frequency ranges depend on waveguide size. Multiple letters are assigned to these bands by different
84
groups. These are from Baytron, a now defunct company that made test equipment.
Q40–60 GHz
7.5 mm – 5 mm
Used for Military communication.
V50–75 GHz
6.0–4 mmVery strongly absorbed by atmospheric oxygen, which resonates at 60 GHz.
E60–90 GHz
6.0–3.33 mm
W75–110 GHz
2.7 – 4.0 mm
used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.
UWB1.6–10.5 GHz
18.75 cm – 2.8 cm
used for through-the-wall radar and imaging systems.
Table 5.1
RADAR MODULATORS
Modulators act to provide the short pulses of power to the magnetron, a special
type of vacuum tube that converts DC (usually pulsed) into microwaves. This
technology is known as Pulsed power. In this way, the transmitted pulse of RF
radiation is kept to a defined, and usually, very short duration. Modulators consist
of a high voltage pulse generator formed from an HV supply, a pulse forming
network, and a high voltage switch such as a thyratron.
A klystron tube may also be used as a modulator because it is an amplifier, so it
can be modulated by its low power input signal.
5.8 HOW RADAR WORKS
When people use radar, they are usually trying to accomplish one of three things:
Detect the presence of an object at a distance - Usually the "something" is
moving, like an airplane, but radar can also be used to detect stationary
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objects buried underground. In some cases, radar can identify an object
as well; for example, it can identify the type of aircraft it has detected.
Detect the speed of an object - This is the reason why police use radar.
Map something - The space shuttle and orbiting satellites use something
called Synthetic Aperture Radar to create detailed topographic maps of
the surface of planets and moons.
All three of these activities can be accomplished using two things you may be
familiar with from everyday life: echo and Doppler shift. These two concepts are
easy to understand in the realm of sound because your ears hear echo and
Doppler shift every day. Radar makes use of the same techniques using radio
waves.
ECHO AND DOPPLER SHIFT
Echo is something you experience all the time. If
you shout into a well or a canyon, the echo comes
back a moment later. The echo occurs because
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Fig 5.5When you shout into a well, the sound of your shout travels down the well and is reflected (echoes) off the surface of the water at the bottom of the well. If you measure the time it takes for the echo to return and if you know the speed of sound, you can calculate the depth of the well fairly accurately.
some of the sound waves in your shout reflect off of a surface (either the water at
the bottom of the well or the canyon wall on the far side) and travel back to your
ears. The length of time between the moment you shout and the moment that
you hear the echo is determined by the distance between you and the surface
that creates the echo.
Doppler shift is also common. You probably experience it daily (often without
realizing it). Doppler shift occurs when sound is generated by, or reflected off of,
a moving object. Doppler shift in the extreme creates sonic booms. Here's how to
understand Doppler shift (you may also want to try this experiment in an empty
parking lot). Let's say there is a car coming toward you at 60 miles per hour
(mph) and its horn is blaring. You will hear the horn playing one "note" as the car
approaches, but when the car passes you the sound of the horn will suddenly
shift to a lower note. It's the same horn making the same sound the whole time.
The change you hear is caused by Doppler shift.
Here's what happens. The speed of sound through the air in the parking lot is
fixed. For simplicity of calculation, let's say it's 600 mph (the exact speed is
determined by the air's pressure, temperature and humidity). Imagine that the car
is standing still, it is exactly 1 mile away from you and it toots its horn for exactly
one minute. The sound waves from the horn will propagate from the car toward
you at a rate of 600 mph. What you will hear is a six-second delay (while the
sound travels 1 mile at 600 mph) followed by exactly one minute's worth of
sound.
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Fig 5.6Doppler shift: The person behind the car hears a lower tone than the driver because the car is moving away. The person in front of the car hears a higher tone than the driver because the car is approaching.
Now let's say that the car is moving toward you at 60 mph. It starts from a mile
away and toots its horn for exactly one minute. You will still hear the six-second
delay. However, the sound will only play for 54 seconds. That's because the car
will be right next to you after one minute, and the sound at the end of the minute
gets to you instantaneously. The car (from the driver's perspective) is still blaring
its horn for one minute. Because the car is moving, however, the minute's worth
of sound gets packed into 54 seconds from your perspective. The same numbers
of sound waves are packed into a smaller amount of time. Therefore, their
frequency is increased, and the horn's tone sounds higher to you. As the car
passes you and moves away, the process is reversed and the sound expands to
fill more time. Therefore, the tone is lower.
You can combine echo and doppler shift in the following way. Say you send out a
loud sound toward a car moving toward you. Some of the sound waves will
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bounce off the car (an echo). Because the car is moving toward you, however,
the sound waves will be compressed. Therefore, the sound of the echo will have
a higher pitch than the original sound you sent. If you measure the pitch of the
echo, you can determine how fast the car is going.
CHAPTER 6
TRANSFORMER
6.1 INTRODUCTION
A transformer is a device that transfers electrical energy from one circuit to
another through inductively coupled conductors—the transformer's coils. A
varying current in the first or primary winding creates a varying magnetic flux in
the transformer's core, and thus a varying magnetic field through
the secondary winding. This varying magnetic field induces a
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varying electromotive force (EMF) or "voltage" in the secondary winding. This
effect is called mutual induction.
If a load is connected to the secondary, an electric current will flow in the
secondary winding and electrical energy will be transferred from the primary
circuit through the transformer to the load. In an ideal transformer, the induced
voltage in the secondary winding (VS) is in proportion to the primary voltage (VP),
and is given by the ratio of the number of turns in the secondary (NS) to the
number of turns in the primary (NP) as follows:
By appropriate selection of the ratio of turns, a transformer thus allows
an alternating current (AC) voltage to be "stepped up" by making NSgreater
than NP, or "stepped down" by making NS less than NP.
In the vast majority of transformers, the windings are coils wound around
a ferromagnetic core, air-core transformers being a notable exception.
Transformers range in size from a thumbnail-sized coupling transformer hidden
inside a stage microphone to huge units weighing hundreds of tons used to
interconnect portions of power grids. All operate with the same basic principles,
although the range of designs is wide. While new technologies have eliminated
the need for transformers in some electronic circuits, transformers are still found
in nearly all electronic devices designed for household ("mains") voltage.
Transformers are essential for high voltage power transmission, which makes
long distance transmission economically practical.
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6.2 INDUCTION COILS
The first type of transformer to see wide use was the induction coil, invented by
Rev. Nicholas Callan of Maynooth College, Ireland in 1836. He was one of the
first researchers to realize that the more turns the secondary winding has in
relation to the primary winding, the larger is the increase in EMF. Induction coils
evolved from scientists' and inventors' efforts to get higher voltages from
batteries. Since batteries produce direct current (DC) rather than alternating
current (AC), induction coils relied upon vibrating electrical contacts that regularly
interrupted the current in the primary to create the flux changes necessary for
induction. Between the 1830s and the 1870s, efforts to build better induction
coils, mostly by trial and error, slowly revealed the basic principles of
transformers.
In 1876, Russian engineer Pavel Yablochkov invented a lighting system based
on a set of induction coils where the primary windings were connected to a
source of alternating current and the secondary windings could be connected to
several "electric candles" (arc lamps) of his own design.[5][6] The coils Yablochkov
employed functioned essentially as transformers.[5]
Induction coils with open magnetic circuits are inefficient for transfer of power
to loads. Until about 1880 the paradigm for AC power transmission from a high
voltage supply to a low voltage load was a series circuit. Open-core transformers
with a ratio near 1:1 were connected with their primaries in series to allow use of
a high voltage for transmission while presenting a low voltage to the lamps. The
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inherent flaw in this method was that turning off a single lamp affected the
voltage supplied to all others on the same circuit. Many adjustable transformer
designs were introduced to compensate for this problematic characteristic of the
series circuit, including those employing methods of adjusting the core or
bypassing the magnetic flux around part of a coil.[7]
In 1878, the Ganz Company in Hungary began manufacturing equipment for
electric lighting, and by 1883 had installed over fifty systems in Austria-Hungary.
Their systems used alternating current exclusively, and included those
comprising both arc and incandescent lamps, along with generators and other
equipment.
Lucien Gaulard and John Dixon Gibbs first exhibited a device with an open iron
core called a "secondary generator" in London in 1882, then sold the idea to
the Westinghouse company in the United States.[9] They also exhibited the
invention in Turin, Italy in 1884, where it was adopted for an electric lighting
system. However, the efficiency of their open-core bipolar apparatus remained
low.
Efficient, practical transformer designs did not appear until the 1880s, but within
a decade the transformer would be instrumental in the "War of Currents", and in
seeing AC distribution systems triumph over their DC counterparts, a position in
which they have remained dominant ever since.
CLOSED-CORE LIGHTING TRANSFORMERS
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Fig 6.1 Drawing of Ganz Company's 1885 prototype
Fig 6.2 Prototypes of the world's first high-efficiency transformers, the so-
called Ganz"ZBD" (Museum of Applied Arts, Budapest)
Between 1884 and 1885, Ganz Company engineers Károly Zipernowsky, Ottó
Bláthy and Miksa Déri had determined that open-core devices were
impracticable, as they were incapable of reliably regulating voltage. In their joint
patent application for the "Z.B.D." transformers, they described the design of two
with no poles: the "closed-core" and the "shell-core" transformers. In the closed-
core type, the primary and secondary windings were wound around a closed iron
ring; in the shell type, the windings were passed through the iron core. In both
designs, the magnetic flux linking the primary and secondary windings travelled
almost entirely within the iron core, with no intentional path through air. When
employed in electric distribution systems, this revolutionary design concept would
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finally make it technically and economically feasible to provide electric power for
lighting in homes, businesses and public spaces. Bláthy had suggested the use
of closed-cores, Zipernowsky the use of shunt connections, and Déri had
performed the experiments. Bláthy also discovered the transformer formula,
Vs/Vp = Ns/Np, and electrical and electronic systems the world over continue to
rely on the principles of the original Z.B.D. transformers. The inventors also
popularized the word "transformer" to describe a device for altering the EMF of
an electric current, although the term had already been in use by 1882. In 1886,
the Ganz Company installed the world's first power station that used
AC generators to power a parallel-connected common electrical network, the
steam-powered Rome-Cerchi power plant.
Fig 6.3 Stanley's 1886 design for adjustable gap open-core induction coils
George Westinghouse had bought Gaulard and Gibbs' patents in 1885, and had
purchased an option on the Z.B.D. design. He entrusted engineer William
Stanley with the building of a device for commercial use. Stanley's first patented
design was for induction coils with single cores of soft iron and adjustable gaps to
94
regulate the EMF present in the secondary winding. (See drawing at left.) This
design was first used commercially in 1886.
6.3 BASIC PRINCIPLES
The transformer is based on two principles: firstly, that an electric current can
produce a magnetic field (electromagnetism) and secondly that a changing
magnetic field within a coil of wire induces a voltage across the ends of the coil
(electromagnetic induction). Changing the current in the primary coil changes the
magnetic flux that is developed. The changing magnetic flux induces a voltage in
the secondary coil.
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Fig 6.5 An ideal transformer
INDUCTION LAW
The voltage induced across the secondary coil may be calculated from Faraday's
law of induction, which states that:
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Where VS is the instantaneous voltage, NS is the number of turns in the
secondary coil and Φ equals the magnetic flux through one turn of the coil. If the
turns of the coil are oriented perpendicular to the magnetic field lines, the flux is
the product of the magnetic flux density B and the area A through which it cuts.
The area is constant, being equal to the cross-sectional area of the transformer
core, whereas the magnetic field varies with time according to the excitation of
the primary. Since the same magnetic flux passes through both the primary and
secondary coils in an ideal transformer, the instantaneous voltage across the
primary winding equals
Taking the ratio of the two equations for VS and VP gives the basic equation for
stepping up or stepping down the voltage
Ideal power equation
Fig 6.6 The ideal transformer as a circuit elementIf the secondary coil is attached to a load that allows current to flow, electrical
power is transmitted from the primary circuit to the secondary circuit. Ideally, the
transformer is perfectly efficient; all the incoming energy is transformed from the
97
primary circuit to the magnetic field and into the secondary circuit. If this condition
is met, the incoming electric power must equal the outgoing power.
giving the ideal transformer equation
Transformers normally have high efficiency, so this formula is a reasonable
approximation.
If the voltage is increased, then the current is decreased by the same factor. The
impedance in one circuit is transformed by the square of the turns ratio.[26] For
example, if an impedance ZS is attached across the terminals of the secondary
coil, it appears to the primary circuit to have an impedance of . This
relationship is reciprocal, so that the impedance ZP of the primary circuit appears
to the secondary to be .
6.4 DETAILED OPERATION
The simplified description above neglects several practical factors, in particular
the primary current required to establish a magnetic field in the core, and the
contribution to the field due to current in the secondary circuit.
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Pincoming = IPVP = Poutgoing = ISVS
Models of an ideal transformer typically assume a core of
negligible reluctance with two windings of zero resistance. When a voltage is
applied to the primary winding, a small current flows, driving flux around
the magnetic circuit of the core. The current required to create the flux is termed
the magnetizing current; since the ideal core has been assumed to have near-
zero reluctance, the magnetizing current is negligible, although still required to
create the magnetic field.
The changing magnetic field induces an electromotive force (EMF) across each
winding. Since the ideal windings have no impedance, they have no associated
voltage drop, and so the voltages VP and VS measured at the terminals of the
transformer, are equal to the corresponding EMFs. The primary EMF, acting as it
does in opposition to the primary voltage, is sometimes termed the "back
EMF". This is due to Lenz's law which states that the induction of EMF would
always be such that it will oppose development of any such change in magnetic
field.
6.4.1 LEAKAGE FLUX
Fig 6.7 Leakage flux of a transformer
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The ideal transformer model assumes that all flux generated by the primary
winding links all the turns of every winding, including itself. In practice, some flux
traverses paths that take it outside the windings.[31] Such flux is termed leakage
flux, and results in leakage inductance in series with the mutually coupled
transformer windings. Leakage results in energy being alternately stored in and
discharged from the magnetic fields with each cycle of the power supply. It is not
directly a power loss, but results in inferior voltage regulation, causing the
secondary voltage to fail to be directly proportional to the primary, particularly
under heavy load. Transformers are therefore normally designed to have very
low leakage inductance.
However, in some applications, leakage can be a desirable property, and long
magnetic paths, air gaps, or magnetic bypass shunts may be deliberately
introduced to a transformer's design to limit the short-circuit current it will
supply. Leaky transformers may be used to supply loads that exhibit negative
resistance, such as electric arcs, mercury vapour lamps, and neon signs; or for
safely handling loads that become periodically short-circuited such as electric arc
welders. Air gaps are also used to keep a transformer from saturating, especially
audio-frequency transformers in circuits that have a direct current flowing through
the windings.
Leakage inductance is also helpful when transformers are operated in parallel. It
can be shown that if the "per-unit" inductance of two transformers is the same (a
typical value is 5%), they will automatically split power "correctly" (e.g. 500 kVA
units in parallel with 1000 kVA unit, the larger one will carry twice the current).
100
6.4.2 EFFECT OF FREQUENCY
The time-derivative term in Faraday's Law shows that the flux in the core is
the integral with respect to time of the applied voltage. Hypothetically an ideal
transformer would work with direct-current excitation, with the core flux
increasing linearly with time. In practice, the flux would rise to the point
where magnetic saturation of the core occurs, causing a huge increase in the
magnetizing current and overheating the transformer. All practical transformers
must therefore operate with alternating (or pulsed) current.
6.5 TRANSFORMER UNIVERSAL EMF EQUATION
If the flux in the core is purely sinusoidal, the relationship for either winding
between its rms voltage Erms of the winding , and the supply frequency f, number
of turns N, core cross-sectional area a and peak magnetic flux density B is given
by the universal EMF equation:
If the flux does not contain even harmonics the following equation can be used
for half-cycle average voltage Eavg of any wave shape:
The EMF of a transformer at a given flux density increases with frequency. By
operating at higher frequencies, transformers can be physically more compact
101
because a given core is able to transfer more power without reaching saturation,
and fewer turns are needed to achieve the same impedance. However properties
such as core loss and conductor skin effect also increase with frequency.
Operation of a transformer at its designed voltage but at a higher frequency than
intended will lead to reduced magnetizing current; at lower frequency, the
magnetizing current will increase. Operation of a transformer at other than its
design frequency may require assessment of voltages, losses, and cooling to
establish if safe operation is practical. For example, transformers may need to be
equipped with "volts per hertz" over-excitation relays to protect the transformer
from over voltage at higher than rated frequency.
Knowledge of natural frequencies of transformer windings is of importance for the
determination of the transient response of the windings to impulse and switching
surge voltages.
6.6 LOSSES
An ideal transformer would have no energy losses, and would be 100% efficient.
In practical transformers energy is dissipated in the windings, core, and
surrounding structures. Larger transformers are generally more efficient, and
those rated for electricity distribution usually perform better than 98%.
Experimental transformers using superconducting windings achieve efficiencies
of 99.85%. While the increase in efficiency is small, when applied to large heavily
loaded transformers the annual savings in energy losses are significant.
A small transformer, such as a plug-in "wall wart" power adapter commonly used
for low-power consumer electronics devices, may be as low as 20% efficient,
102
with considerable energy loss even when not supplying any power to the device.
Though individual losses may be only a few watts, it has been estimated that the
cumulative loss from such transformers in the United States alone exceeded 32
billion kilowatt-hours (kWh) in 2002.
The losses vary with load current, and may be expressed as "no-load" or "full-
load" loss. Winding resistance dominates load losses,
whereas hysteresis and eddy currents losses contribute to over 99% of the no-
load loss. The no-load loss can be significant, meaning that even an idle
transformer constitutes a drain on an electrical supply, which encourages
development of low-loss transformers (also see energy efficient transformer).
Transformer losses are divided into losses in the windings, termed copper loss,
and those in the magnetic circuit, termed iron loss.
Losses in the transformer arise from:
WINDING RESISTANCE
Current flowing through the windings causes resistive heating of the conductors.
At higher frequencies, skin effect and proximity effect create additional winding
resistance and losses.
HYSTERESIS LOSSES
Each time the magnetic field is reversed, a small amount of energy is lost due
to hysteresis within the core. For a given core material, the loss is proportional to
the frequency, and is a function of the peak flux density to which it is subjected.
103
EDDY CURRENTS
Ferromagnetic materials are also good conductors, and a solid core made from
such a material also constitutes a single short-circuited turn throughout its entire
length. Eddy currents therefore circulate within the core in a plane normal to the
flux, and are responsible for resistive heating of the core material. The eddy
current loss is a complex function of the square of supply frequency and inverse
square of the material thickness.
MAGNETOSTRICTION
Magnetic flux in a ferromagnetic material, such as the core, causes it to
physically expand and contract slightly with each cycle of the magnetic field, an
effect known as magnetostriction. This produces the buzzing sound commonly
associated with transformers, and in turn causes losses due to frictional heating
in susceptible cores.
MECHANICAL LOSSES
In addition to magnetostriction, the alternating magnetic field causes fluctuating
electromagnetic forces between the primary and secondary windings. These
incite vibrations within nearby metalwork, adding to the buzzing noise, and
consuming a small amount of power.
STRAY LOSSES
Leakage inductance is by itself largely lossless, since energy supplied to its
magnetic fields is returned to the supply with the next half-cycle. However, any
leakage flux that intercepts nearby conductive materials such as the
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transformer's support structure will give rise to eddy currents and be converted to
heat. There are also radiative losses due to the oscillating magnetic field, but
these are usually small.
6.7 EQUIVALENT CIRCUIT
The physical limitations of the practical transformer may be brought together as
an equivalent circuit model (shown below) built around an ideal lossless
transformer. Power loss in the windings is current-dependent and is represented
as in-series resistances RP and RS. Flux leakage results in a fraction of the
applied voltage dropped without contributing to the mutual coupling, and thus can
be modelled as reactances of each leakage inductance XP and XS in series with
the perfectly coupled region.
Iron losses are caused mostly by hysteresis and eddy current effects in the core,
and are proportional to the square of the core flux for operation at a given
frequency. Since the core flux is proportional to the applied voltage, the iron loss
can be represented by a resistance RC in parallel with the ideal transformer.
A core with finite permeability requires a magnetizing current IM to maintain the
mutual flux in the core. The magnetizing current is in phase with the flux;
saturation effects cause the relationship between the two to be non-linear, but for
simplicity this effect tends to be ignored in most circuit equivalents. With
a sinusoidal supply, the core flux lags the induced EMF by 90° and this effect can
be modelled as a magnetizing reactance (reactance of an effective
inductance) XM in parallel with the core loss component. RC and XM are
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sometimes together termed the magnetizing branch of the model. If the
secondary winding is made open-circuit, the current I0 taken by the magnetizing
branch represents the transformer's no-load current.
The secondary impedance RS and XS is frequently moved (or "referred") to the
primary side after multiplying the components by the impedance scaling factor
(NP/NS)2.
Fig 6.8 Transformer equivalent circuit, with secondary impedances referred to the primary side
The resulting model is sometimes termed the "exact equivalent circuit", though it
retains a number of approximations, such as an assumption of linearity. Analysis
may be simplified by moving the magnetizing branch to the left of the primary
impedance, an implicit assumption that the magnetizing current is low, and then
summing primary and referred secondary impedances, resulting in so-called
equivalent impedance.
The parameters of equivalent circuit of a transformer can be calculated from the
results of two transformer tests: open-circuit test and short-circuit test.
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6.8 TYPES OF TRANSFORMER
A wide variety of transformer designs are used for different applications, though
they share several common features. Important common transformer types
include:
6.8.1 AUTOTRANSFORMER
Fig 6.9 An autotransformer with a sliding brush contact
An autotransformer has only a single winding with two end terminals, plus a third
at an intermediate tap point. The primary voltage is applied across two of the
terminals, and the secondary voltage taken from one of these and the third
terminal. The primary and secondary circuits therefore have a number of
windings turns in common.[44] Since the volts-per-turn is the same in both
windings, each develops a voltage in proportion to its number of turns. An
adjustable autotransformer is made by exposing part of the winding coils and
making the secondary connection through a sliding brush, giving a variable turns
ratio. Such a device is often referred to as a variac.
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6.8.2 POLYPHASE TRANSFORMERS
Fig 6.10 Three-phase step-down transformer mounted between two utility poles
For three-phase supplies, a bank of three individual single-phase transformers
can be used, or all three phases can be incorporated as a single three-phase
transformer. In this case, the magnetic circuits are connected together, the core
thus containing a three-phase flow of flux. A number of winding configurations
are possible, giving rise to different attributes and phase shifts. One particular
polyphase configuration is the zigzag transformer, used for grounding and in the
suppression of harmonic currents.
6.8.3 LEAKAGE TRANSFORMERS
Fig 6.11 Leakage transformer
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A leakage transformer, also called a stray-field transformer, has a significantly
higher leakage inductance than other transformers, sometimes increased by a
magnetic bypass or shunt in its core between primary and secondary, which is
sometimes adjustable with a set screw. This provides a transformer with an
inherent current limitation due to the loose coupling between its primary and the
secondary windings. The output and input currents are low enough to prevent
thermal overload under all load conditions—even if the secondary is shorted.
Leakage transformers are used for arc welding and high voltage discharge lamps
(neon lamp sand cold cathode fluorescent lamps, which are series-connected up
to 7.5 kV AC). It acts then both as a voltage transformer and as a magnetic
ballast.
Other applications are short-circuit-proof extra-low voltage transformers for toys
or doorbell installations.
6.8.4 RESONANT TRANSFORMERS
A resonant transformer is a kind of leakage transformer. It uses the leakage
inductance of its secondary windings in combination with external capacitors, to
create one or more resonant circuits. Resonant transformers such as the Tesla
coil can generate very high voltages, and are able to provide much higher current
than electrostatic high-voltage generation machines such as the Van de Graff
generator. One of the applications of the resonant transformer is for the CCFL
inverter. Another application of the resonant transformer is to couple between
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stages of a superheterodyne receiver, where the selectivity of the receiver is
provided by tuned transformers in the intermediate-frequency amplifiers.
6.8.5 AUDIO TRANSFORMERS
Audio transformers are those specifically designed for use in audio circuits. They
can be used to block radio frequency interference or the DC component of an
audio signal, to split or combine audio signals, or to provide impedance matching
between high and low impedance circuits, such as between a high
impedance tube (valve) amplifier output and a low impedance loudspeaker, or
between a high impedance instrument output and the low impedance input of
a mixing console.
Such transformers were originally designed to connect different telephone
systems to one another while keeping their respective power supplies isolated,
and are still commonly used to interconnect professional audio systems or
system components.
Being magnetic devices, audio transformers are susceptible to external magnetic
fields such as those generated by AC current-carrying conductors. "Hum" is a
term commonly used to describe unwanted signals originating from the "mains"
power supply (typically 50 or 60 Hz). Audio transformers used for low-level
signals, such as those from microphones, often include shielding to protect
against extraneous magnetically coupled signals.
110
6.8.6 INSTRUMENT TRANSFORMERS
Instrument transformers are used for measuring voltage and current in electrical
power systems, and for power system protection and control. where a voltage or
current is too large to be conveniently used by an instrument, it can be scaled
down to a standardized, low value. Instrument transformers isolate
measurement, protection and control circuitry from the high currents or voltages
present on the circuits being measured or controlled.
Fig 6.12 Current transformers, designed for placing around conductors
A current transformer is a transformer designed to provide a current in its
secondary coil proportional to the current flowing in its primary coil.[51]
Voltage transformers (VTs), also referred to as "potential transformers" (PTs),
are designed to have an accurately known transformation ratio in both magnitude
and phase, over a range of measuring circuit impedances. A voltage transformer
is intended to present a negligible load to the supply being measured. The low
secondary voltage allows protective relay equipment and measuring instruments
to be operated at lower voltages.
Both current and voltage instrument transformers are designed to have
predictable characteristics on overloads. Proper operation of over-current
111
protection relays requires that current transformers provide a predictable
transformation ratio even during a short-circuit.
CLASSIFICATION
Transformers can be classified in different ways:
By power capacity: from a fraction of a volt-ampere (VA) to over a thousand
MVA;
By frequency range: power-, audio-, or radio frequency;
By voltage class: from a few volts to hundreds of kilovolts;
By cooling type: air cooled, oil filled, fan cooled, or water cooled;
By application: such as power supply, impedance matching, output voltage and
current stabilizer, or circuit isolation;
By end purpose: distribution, rectifier, arc furnace, amplifier output;
By winding turns ratio: step-up, step-down, isolating (equal or near-equal
ratio), variable.
6.9 CONSTRUCTION
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6.9.1 CORES
Fig 6.13 Laminated core transformer showing edge of laminations at top of photo
6.9.1.1 LAMINATED STEEL CORES
Transformers for use at power or audio frequencies typically have cores made of
high permeability silicon steel. The steel has a permeability many times that
of free space, and the core thus serves to greatly reduce the magnetizing
current, and confine the flux to a path which closely couples the windings. Early
transformer developers soon realized that cores constructed from solid iron
resulted in prohibitive eddy-current losses, and their designs mitigated this effect
with cores consisting of bundles of insulated iron wires. Later designs
constructed the core by stacking layers of thin steel laminations, a principle that
has remained in use. Each lamination is insulated from its neighbours by a thin
non-conducting layer of insulation. The universal transformer equation indicates
a minimum cross-sectional area for the core to avoid saturation.
The effect of laminations is to confine eddy currents to highly elliptical paths that
enclose little flux, and so reduce their magnitude. Thinner laminations reduce
losses, but are more laborious and expensive to construct. Thin laminations are
generally used on high frequency transformers, with some types of very thin steel
laminations able to operate up to 10 kHz.
113
Laminating the core greatly reduces eddy-current losses
One common design of laminated core is made from interleaved stacks of E-
shaped steel sheets capped with I-shaped pieces, leading to its name of "E-I
transformer". Such a design tends to exhibit more losses, but is very economical
to manufacture. The cut-core or C-core type is made by winding a steel strip
around a rectangular form and then bonding the layers together. It is then cut in
two, forming two C shapes, and the core assembled by binding the two C halves
together with a steel strap. They have the advantage that the flux is always
oriented parallel to the metal grains, reducing reluctance.
A steel core's remanence means that it retains a static magnetic field when
power is removed. When power is then reapplied, the residual field will cause a
high inrush current until the effect of the remaining magnetism is reduced, usually
after a few cycles of the applied alternating current. Overcurrent
protection devices such as fuses must be selected to allow this harmless inrush
to pass. On transformers connected to long, overhead power transmission lines,
induced currents due to geomagnetic disturbances during solar storms can
cause saturation of the core and operation of transformer protection devices.
114
Distribution transformers can achieve low no-load losses by using cores made
with low-loss high-permeability silicon steel or amorphous (non-crystalline) metal
alloy. The higher initial cost of the core material is offset over the life of the
transformer by its lower losses at light load.
6.9.1.2 SOLID CORES
Powdered iron cores are used in circuits (such as switch-mode power supplies)
that operate above main frequencies and up to a few tens of kilohertz. These
materials combine high magnetic permeability with high bulk electrical resistivity.
For frequencies extending beyond the VHF band, cores made from non-
conductive magnetic ceramic materials called ferrites are common.[55] Some
radio-frequency transformers also have movable cores (sometimes called 'slugs')
which allow adjustment of the coupling coefficient (and bandwidth) of tuned
radio-frequency circuits.
6.9.1.3 TOROIDAL CORES
Fig 6.14 Small toroidal core transformer
Toroidal transformers are built around a ring-shaped core, which, depending on
operating frequency, is made from a long strip of silicon steel alloy wound into a
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coil, powdered iron, or ferrite. A strip construction ensures that the grain
boundaries are optimally aligned, improving the transformer's efficiency by
reducing the core's reluctance. The closed ring shape eliminates air gaps
inherent in the construction of an E-I core. The cross-section of the ring is usually
square or rectangular, but more expensive cores with circular cross-sections are
also available. The primary and secondary coils are often wound concentrically to
cover the entire surface of the core. This minimizes the length of wire needed,
and also provides screening to minimize the core's magnetic field from
generating electromagnetic interference.
Toroidal transformers are more efficient than the cheaper laminated E-I types for
a similar power level. Other advantages compared to E-I types, include smaller
size (about half), lower weight (about half), less mechanical hum (making them
superior in audio amplifiers), lower exterior magnetic field (about one tenth), low
off-load losses (making them more efficient in standby circuits), single-bolt
mounting, and greater choice of shapes. The main disadvantages are higher cost
and limited power capacity (see "Classification" above). Because of the lack of a
residual gap in the magnetic path, toroidal transformers also tend to exhibit
higher inrush current, compared to laminated E-I types.
Ferrite toroidal cores are used at higher frequencies, typically between a few tens
of kilohertz to hundreds of megahertz, to reduce losses, physical size, and weight
of switch-mode power supplies. A drawback of toroidal transformer construction
is the higher labour cost of winding. This is because it is necessary to pass the
entire length of a coil winding through the core aperture each time a single turn is
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added to the coil. As a consequence, toroidal transformers are uncommon above
ratings of a few kVA. Small distribution transformers may achieve some of the
benefits of a toroidal core by splitting it and forcing it open, then inserting a
bobbin containing primary and secondary windings.
6.9.1.4 AIR CORES
A physical core is not an absolute requisite and a functioning transformer can be
produced simply by placing the windings in close proximity to each other, an
arrangement termed an "air-core" transformer. The air which comprises the
magnetic circuit is essentially lossless, and so an air-core transformer eliminates
loss due to hysteresis in the core material. The leakage inductance is inevitably
high, resulting in very poor regulation, and so such designs are unsuitable for use
in power distribution. They have however very high bandwidth, and are frequently
employed in radio-frequency applications, for which a satisfactory coupling
coefficient is maintained by carefully overlapping the primary and secondary
windings. They're also used for resonant transformers such as Tesla coils where
they can achieve reasonably low loss in spite of the high leakage inductance.
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6.9.2 WINDINGS
Fig 6.15 Windings are usually arranged concentrically to minimize flux leakage.
Fig 6.18 Cut view through transformer windings. White: insulator. Green spiral: Grain oriented silicon steel. Black: Primary winding made of oxygen-free copper. Red: Secondary winding. Top left: Toroidal transformer. Right: C-core, but E-core would be similar. The black windings are made of film. Top: Equally low capacitance between all ends of both windings. Since most cores are at least moderately conductive they also need insulation. Bottom: Lowest capacitance for one end of the secondary winding needed for low-power high-voltage transformers. Bottom left: Reduction ofleakage inductance would lead to increase of capacitance.
The conducting material used for the windings depends upon the application, but
in all cases the individual turns must be electrically insulated from each other to
ensure that the current travels throughout every turn. For small power and signal
transformers, in which currents are low and the potential difference between
adjacent turns is small, the coils are often wound from enamelled magnet wire,
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such as Formvar wire. Larger power transformers operating at high voltages may
be wound with copper rectangular strip conductors insulated by oil-impregnated
paper and blocks of pressboard.
High-frequency transformers operating in the tens to hundreds of kilohertz often
have windings made of braided Litz wire to minimize the skin-effect and proximity
effect losses. Large power transformers use multiple-stranded conductors as
well, since even at low power frequencies non-uniform distribution of current
would otherwise exist in high-current windings. Each strand is individually
insulated, and the strands are arranged so that at certain points in the winding, or
throughout the whole winding, each portion occupies different relative positions in
the complete conductor. The transposition equalizes the current flowing in each
strand of the conductor, and reduces eddy current losses in the winding itself.
The stranded conductor is also more flexible than a solid conductor of similar
size, aiding manufacture.
For signal transformers, the windings may be arranged in a way to minimize
leakage inductance and stray capacitance to improve high-frequency response.
This can be done by splitting up each coil into sections, and those sections
placed in layers between the sections of the other winding. This is known as a
stacked type or interleaved winding.
Both the primary and secondary windings on power transformers may have
external connections, called taps, to intermediate points on the winding to allow
selection of the voltage ratio. The taps may be connected to an automatic on-
load tap changer for voltage regulation of distribution circuits. Audio-frequency
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transformers, used for the distribution of audio to public address loudspeakers,
have taps to allow adjustment of impedance to each speaker. A center-tapped
transformer is often used in the output stage of an audio power amplifier in
apush-pull circuit. Modulation transformers in AM transmitters are very similar.
Certain transformers have the windings protected by epoxy resin.
By impregnating the transformer with epoxy under a vacuum, one can replace air
spaces within the windings with epoxy, thus sealing the windings and helping to
prevent the possible formation of corona and absorption of dirt or water. This
produces transformers more suited to damp or dirty environments, but at
increased manufacturing cost.
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Future prospect:
APPLICATIONS OF PC RADAR
PC Radar is a Windows-based application which emulates a generic ARPA radar
display on a PC monitor. It is a powerful training tool used to familiarize a student
with the use of radar for ship navigation. The radar image incorporates realistic
simulation of landmass, moving targets, precipitation, sea clutter and other video
effects seen on a real radar; while the radar operational features include radar
video controls, dual EBL and VRM, multiple presentation modes, graphics
capabilities and range scaling. For ARPA support, PC Radar provides target
acquisition and tracking, leading vectors and history trails, trial maneuvers and
navigation points. Target track data is also output via the serial port or through a
network connection using NMEA-0183 format. This allows for integration with an
ECS or ECDIS system.
The PCRadar user interface models the SPS-73 Radar Display, and was
designed specifically for use by the US Navy and US Coast Guard in shipboard
and shore based training applications. PC Radar was developed using BCG's
industry standard radar simulation engine, is compatible with BCG's enhanced
Graphical User Interface (GUI) and may be used in conjunction with our PCS-
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100/200 radar simulators. PC Radar is available in a software only package
ready for installation on a customer's PC, or BCG also offers "turn key" systems
packaged in multiple configurations including a shock mounted chassis for
shipboard use, or a desktop / laptop PC for classroom use. BCG has added an
AIS capability as an option to PC Radar. Another new feature is a Virtual
Ownship Steering Control allowing a student to navigate his/her Ownship with
controls located on the same screen as the radar presentation.
Triggers alarm in computer when walking is recognized. Motion detection through
most doors, walls and foliage. If it moves, RADAR PC can detect, display, trigger
alarms, and record its velocity data. Since all movement by living creatures is
behavior, that movement can be detected, displayed, characterized, recognized,
and trigger alarms both auditory and visual, and may be optionally recorded. It is
like extra sensory perception. It is used in military applications to detect
incoming fighter planes.It can also detect and display coordinates of lost planes.
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7.2 PRECAUTIONS
When making connections to either a PC parallel port, or I/O pins of a
microcontroller, be sure to isolate the motor well. High voltage spikes of several
hundred volts are possible as back EMF from stepper motor coils. Always use
clamping diodes to short these spikes back to the motor's power bus. The use of
optical isolation devices (optoisolators) will add yet another layer or protection
between the delicate control logic and the high-voltage potentials which may be
present in the power output stage. Whenever possible, use separate power
supplies for the motor and the translator / microcontroller. This further reduces
the chance of destructive voltages reaching the controller, and reduces or
eliminates power supply noise that may be introduced by the motor.
If you're using a computer that has a parallel port as part of its onboard I/O, you
may want to consider purchasing a parallel port card to use instead. I've seen
them for as little as $9.99 at Fry's Electronics and other computer stores. Not
only does this reduce the risk of permanently damaging or destroying your
motherboard (it happened to a friend of mine!), but it will also allow you to
experiment without the need for swapping cables or flipping a switchbox when
you want to use your parallel printer, since your experiments won't be sharing its
port. It is much cheaper to throw out a $10.00 parallel port card than it is to
replace your motherboard.
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