Programmable Logic Programmable Logic Controllers (PLCControllers (PLC’’s)s)

According to National Electrical Manufacturers’ Association According to National Electrical Manufacturers’ Association a PLC is:a PLC is:A digital electronic device using a programmable memory A digital electronic device using a programmable memory to store instructions and to implement specific functions to store instructions and to implement specific functions such as logic, sequence, counting, timing and arithmetic to such as logic, sequence, counting, timing and arithmetic to control machines and processes through digital or analog control machines and processes through digital or analog input/output modules.input/output modules.

DefinitionDefinition

What is a PLC?What is a PLC?

A special A special computercomputer for logic controls for logic controls

PLC History

In the late 1960's PLCs were first introduced

Bedford Associates (Bedford, MA) proposed something called a Modular Digital Controller (MODICON) to a major US car manufacturer

In the mid70's the dominant PLC technologies were sequencer state-machines and the bit-slice based CPU. The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Communications abilities began to appear in approximately 1973. The first such system was Modicon's Modbus. The PLC could now talk to other PLCs and they could be far away from the actual machine they were controlling

The 80's saw an attempt to standardize communications with General Motor's manufacturing automation protocol(MAP

The 90's have seen a gradual reduction in the introduction of new protocols

Basic PLC Operation

Pushbuttons (sensors), in this simple example, connected toPLC inputs, can be used to start and stop a motor connected toa PLC through a motor starter (actuator).

PLCs consist of input modules or points, a Central ProcessingUnit (CPU), and output modules or points.

PLC PLC ARCHITECTUREARCHITECTUREProgrammable controllers replace most of the relay panel wiring Programmable controllers replace most of the relay panel wiring

by software programming.by software programming.

It is consists of the following modules:

Power supply module

CPU module

Digital input module

Digital output module

Analog input module

Analog output

PLC PLC COMPONENTSCOMPONENTS

1. Processor1. Processor Microprocessor based, may allow Microprocessor based, may allow arithmetic operations, logic operators, arithmetic operations, logic operators, block block memory, computer interface, local memory, computer interface, local area area network, functions, etc.network, functions, etc.

2. Memory2. Memory Measured in words.Measured in words.

ROM (Read Only Memory), ROM (Read Only Memory),

RAM (Random Access Memory), RAM (Random Access Memory),

PROM (Programmable Read Only PROM (Programmable Read Only Memory), Memory),

EEPROM (Electric Erasable EEPROM (Electric Erasable Programmable Programmable

ROM), ROM),

EPROM (Erasable Programmable Read EPROM (Erasable Programmable Read Only Only

Memory),Memory),

PLC COMPONENTSPLC COMPONENTS3. I/O3. I/O Modular plug-in peripheryModular plug-in periphery

AC voltage input and output, AC voltage input and output,

DC voltage input and output, DC voltage input and output,

Low level analog input, Low level analog input,

High level analog input and output, High level analog input and output,

Special purpose modules, e.g., high speed Special purpose modules, e.g., high speed

timers, timers,

Stepping motor controllers, etc. PID, MotionStepping motor controllers, etc. PID, Motion

4. Power supply AC power4. Power supply AC power

5. Peripheral 5. Peripheral hand-held programmer (HHP)hand-held programmer (HHP)

printerprinter

simulatorsimulator

EPROM loaderEPROM loader

graphics processorgraphics processor

network communication interfacenetwork communication interface

modular PCmodular PC

Discrete I/O : Discrete I/O : -AC Voltage I/O or -AC Voltage I/O or -DC Voltage I/O-DC Voltage I/O

Discrete Input:Discrete Input:Selector switches, Selector switches, Pushbuttons, Pushbuttons, Photoelectric Eyes, Photoelectric Eyes, Limit switches, Limit switches, Circuit breakers, Circuit breakers, proximity switches, proximity switches, Level switch, Level switch, Pressure switch, Pressure switch, Temperature switches,Temperature switches, Motor starter contacts, Motor starter contacts, Relays contacts and Relays contacts and Thumbwheel switchThumbwheel switch

SPDT

Multiple through

DPST

Multiple pole

Normally open

Normally closed

lockingNon-locking

RELAYSRELAYS

A switch whose operation is activated by an electromagnet is called a "relay" . Coil circuit controls many contact points

contact

coil

input

Relay coil

R1

R1Output contact normally open

R1Output contact normally closed

Relays and Schematic

Symbols

no rm a llyo p e n

no rm a llyc lo se d

inp ut c o il

O R

O R

Logical Control with Relays

115VACwall plug

relay logic

input A(normally closed)

input B(normally open)

output C(normally open)

ladder logic

A B C

Relay Logic In a PLC

ladder

powersupply

+24V

com.

inputs

outputs

push buttons

logic

PLC

AC power

115Vac

neut.

A B C

light

Discrete OutputDiscrete OutputAlarms, Alarms, Control relays, Control relays, Fans, Lights, Horns, Fans, Lights, Horns, Valves, Valves, motor starters, and motor starters, and SolenoidsSolenoids

Analog I/OAnalog I/O

Analog inputs:Analog inputs:-Flow sensor, -Flow sensor, Humidity sensors, Humidity sensors,

Pressure sensors, Pressure sensors, Temperature sensors, Temperature sensors, Potentiometers, Potentiometers, load cell transducers load cell transducers

Analog outputs:Analog outputs:Analog metersAnalog meters Analog valvesAnalog valves ActuatorsActuators Chart recorders Chart recorders DC and AC motor drivesDC and AC motor drives

Some Special Some Special I/OI/OThermocouple input Thermocouple input

Low level analog signal, filtered, amplified, and digitized Low level analog signal, filtered, amplified, and digitized before sending to the processor through I/O bus.before sending to the processor through I/O bus.

Fast input Fast input

50 to 100 microsecond pulse signal detection.50 to 100 microsecond pulse signal detection.

ASCII I/OASCII I/O

Communicates with ASCII devices.Communicates with ASCII devices.

Stepper motor outputStepper motor output

Provide directly control of a stepper motor.Provide directly control of a stepper motor.

Servo interfaceServo interface

Control DC servo motor for point-to-point control and Control DC servo motor for point-to-point control and axis positioning. axis positioning.

PID control PID control

The Proportional Integral Derivative is used for closed The Proportional Integral Derivative is used for closed loop process control. loop process control.

Network moduleNetwork module

LADDER DIAGRAMLADDER DIAGRAMA ladder diagram (also called contact symbology) is a means A ladder diagram (also called contact symbology) is a means

of graphically representing the logic required in a relay of graphically representing the logic required in a relay logic system. logic system.

A

R1PB1 PB2

R1

R1

start emergency stop

Rail

Rung

LADDER DIAGRAMLADDER DIAGRAM

The left vertical line of a ladder logic diagram represents the power or energized conductor. The output element or instruction represents the neutral or return path of the circuit. The right vertical line, which represents the return path on a hard-wired control line diagram, is omitted. Ladder logic diagrams are read from left-to-right, top-to-bottom. Rungs are sometimes referred to as networks. A network may have several control elements, but only one output coil.

Reading Ladder Logic

HOT NEUTRAL

INPUTS OUTPUTS

A B X

C D

E F

G

H

Y

Note: Power needs to flow through some combination of the inputs (A,B,C,D,E,F,G,H) to turn on outputs (X,Y).

A Ladder Logic Example

Note: When A is pushed, the output B will turn on, and the input B will also turn on and keep B on permanently - until power is removed.

A

B

B

Note: The line on the right is being left off intentionally and is implied in these diagrams.

Statement listStatement list

A statement list (STL) provides another view of a set of instructions. The operation, what is to be done, is shown on the left. The operand, the item to be operated on by the operation, is shown on the right. The set of instructions in this statement list perform the same task as the ladder diagram.

0 0 0 0 00 0 0 0 10 0 0 0 20 0 0 0 30 0 0 0 40 0 0 0 50 0 0 0 6

L D NL DA N DL DL DA N DO R

AB

CD

A B

C D

X

E N D

the m ne m o nic c o d e is e q uiva le nt tothe la d d e r lo gic b e lo w

S T0 0 0 0 7 XE N D0 0 0 0 8

N o te : T he no ta tio n sho w n a b o ve isno t s ta nd a rd A lle n- B ra d le yno ta tio n. T he p ro gra m to theright w o uld b e the A - B e q uivale nt.

S O RB S TX IC AX IO BN X BX IO CX IO DB N DO T E XE O RE N D

Function Block Diagrams

Function Block Diagrams (FBD) provide another view of a set of instructions. Each function has a name to designate its specific task. Functions are indicated by a rectangle. Inputs are shown on the left-hand side of the rectangle and outputs are shown on the right-hand side.

Standard PLC scan cycle Standard PLC scan cycle

The PLC program is executed as part of a repetitive process referred to as a scan. A PLC scan starts with the CPU reading the status of inputs. The application program is executed using the status of the inputs. Once the program is completed, the CPU performs internal diagnostics and communication tasks.The scan cycle ends by updating the outputs, then starts over. The cycle time depends on the size of the program, the number of I/Os, and the amount of communication required.

PLC Scan

Software

Software is any information in a form that a computer or PLC can use. Software includes the instructions or programs that direct hardware.

Hardware

Hardware is the actual equipment. The PLC, the programming device, and the connecting cable are examples of hardware.

Basic Requirements

In order to create or change a program, the following items areneeded:• PLC• Programming Device• Programming Software• Connector Cable

Programming Devices

The program is created in a programming device (PG) and thentransferred to the PLC. The program for the S7-200 can becreated using a dedicated Siemens SIMATIC S7 programmingdevice, such as a PG 720 (not shown) or PG 740, if STEP 7Micro/WIN software is installed.

A personal computer (PC), with STEP 7 Micro/WIN installed,can also be used as a programming device with the S7-200.

Software

A software program is required in order to tell the PLC what instructions it must follow. Programming software is typically PLC specific. A software package for one PLC, or one family of PLCs, such as the S7 family, would not be useful on other PLCs. The S7-200 uses a Windows based software program called STEP 7-Micro/WIN32. The PG 720 and PG 740 have STEP7 software pre-installed. Micro/WIN32 is installed on a personal computer in a similar manner to any other computer software.

Connector Cables PPI

(Point-to-Point Interface)

Connector cables are required to transfer data from the (Point-to-Point Interface) programming device to the PLC. Communication can only take place when the two devices speak the same language or protocol. Communication between a Siemens programming device and the S7-200 is referred to as PPI protocol (pointto- point interface). An appropriate cable is required for a programming device such as a PG 720 or PG 740. The S7-200uses a 9-pin, D-connector. This is a straight-through serial device that is compatible with Siemens programming devices (MPI port) and is a standard connector for other serial interfaces.

A special cable is needed when a personal computer is used as a programming device. Two versions of this cable are available.One version, called an RS-232/PPI Multi-Master Cable, connects a personal computer’s RS-232 interface to the PLC’s RS-485 connector. The other version, called a USB/PPI Multi-Master Cable, connects a personal computer’s USB interface to the PLC’s RS-485 connector.

Connector Cables PPI

S7-200 Micro PLCs

The S7-200 Micro PLC is the smallest member of the SIMATIC S7 family of programmable controllers. The central processing unit (CPU) is internal to the PLC. Inputs and outputs (I/O) are the system control points. Inputs monitor field devices, such as switches and sensors. Outputs control other devices, such as motors and pumps. The programming port is the connection to the programming device.

S7-200 Models

There are five S7-200 CPU types: CPU 221, CPU 222, CPU 224,CPU 224XP, and CPU 226 and two power supply configurationsfor each type.

S7-200 Features

The S7-200 family includes a wide variety of CPUs and features. This variety provides a range of features to aid in designing a cost-effective automation solution. The following table provides a summary of the major features, many of which will be covered in this course.

Mode Switch and

Analog Adjustment

When the mode switch is in the RUN position the CPU is in the run mode and executing the program. When the mode switch is in the STOP position the CPU is stopped. When the mode switch is in the TERM position the programming device can select the operating mode.

The analog adjustment is used to increase or decrease values stored in special memory. These values can be used to update the value of a timer or counter, or can be used to set limits.

Optional Cartridge

The S7-200 supports an optional memory cartridge that provides a portable EEPROM storage for your program. The cartridge can be used to copy a program from one S7-200 PLC to a like S7-200 PLC.

Expansion Modules

The S7-200 PLCs are expandable. Expansion modules contain additional inputs and outputs. These are connected to the base unit using a ribbon connector.

The ribbon connector is protected by a cover on the base unit. Side-by-side mounting completely encloses and protects the ribbon connector.

The ribbon connector is protected by a cover on the base unit. Side-by-side mounting completely encloses and protects the ribbon connector.

External Power Supply

An S7-200 AC/DC/Relay would be connected to a 120 or 230 VAC power supply.

I/O Numbering

S7-200 inputs and outputs are labeled at the wiring terminations and next to the status indicators. These alphanumeric symbols identify the I/O address to which a device is connected. This address is used by the CPU to determine which input is present and which output needs to be turned on or off. I designates a discrete input and Q designates a discrete output. The first number identifies the byte, the second number identifies the bit. Input I0.0, for example, is byte 0, bit 0.

Inputs Outputs

Freeport Mode

The programming port has a mode called freeport mode.Freeport mode allows connectivity to various intelligent sensing devices such as a bar code reader.

Printer

Freeport mode can also be used to connect to a non-SIMATIC printer.

Interconnection

It is possible to use one programming device to address multiple S7-200 devices on the same communication cable. A total of 31 units can be interconnected without a repeater.

Symbols

Contacts

Coils

Boxes

Coils represent relays that are energized when power flows to them. When a coil is energized, it causes a corresponding output to turn on by changing the state of the status bit controlling that output to 1. That same output status bit may be used to control normally open and normally closed contacts elsewhere in the program.

Boxes represent various instructions or functions that are executed when power flows to the box. Typical box functions are timers, counters, and math operations.

Entering Elements

An AND Operation

PLC HARD WIRING PLC HARD WIRING DIAGRAMDIAGRAM

Externalswitches

Stored program

01 02 20

20

20 11

01

02

03

11

12

C

PLCInput OutputA

B

External switches with stored program ladder diagram and output devices can be presented using The shown PLC hard wired diagrams

• PLC• Programming Device• Programming Software• Connector Cable

Input Card Example

2 4 V A CP o w e rS up p ly

no rm a lly o p e n p ush- b utto n

no rm a lly o p e nte m p e ra ture sw itc h

P L C Inp ut C a rd2 4 V A C

it is in ra c k "b o b "s lo t 3

0 0

0 1

0 2

0 3

0 4

0 5

0 6

0 7

P ushb utto n (b o b :3 :I.D a ta .1 )

Te m p se nso r (b o b :3 :I.D a ta .3 )

C O M

N o te : inp uts a re no rm a lly high im p e d a nc e . T his m e a ns tha t the y w illuse ve ry little c urre nt.

H o t

N e ut.

Output Card Example (Transistors)2 4 V D CO utp ut C a rd

ra c k "sue "s lo t 2

C O M

0 0

0 1

0 2

0 3

0 4

0 5

0 6

0 7

2 4 V L a m p

R e la y

+ 2 4 V D CP o w e r

1 2 0 V A C

P o w e r

M o to r

S up p ly

S up p ly

M o to r (sue :2 .O .D a ta .3 )

L a m p (sue :2 .O .D a ta .3 )

N e ut.

C O M

Output Card Example (Relays)

1 2 0 V A C /D CO utp ut C a rd

in ra c k 0 1I/O gro up 2

0 0

0 1

0 2

0 3

0 4

0 5

0 6

0 7 2 4 V la m p

R e la y

2 4 V D CP o w e r

1 2 0 V A CP o w e r

M o to r S up p ly

S up p ly

Some Design Issues• DC voltages are usually lower, and therefore safer (e.g., 24V).• DC inputs are very fast, AC inputs require a longer on-time.• DC voltages can be connected to more electrical systems.• AC signals are more immune to noise than DC.• AC power is easier and less expensive to supply to equipment.• AC signals are very common.

RelaysContactor - Special relays for switching large current loads.Motor Starter - Basically a contactor in series with an overload relay to cut off when too much current is drawn.Arc Suppression - when any relay is opened or closed an arc will jump. This becomes a major problem with large relays. On relays switching AC this problem can be overcome by opening the relay when the voltage goes to zero (while crossing between negative and positive). When switching DC loads this problem can be minimized by blowing pressurized gas across during opening to suppress the arc formation.AC coils - If a normal relay coil is driven by AC power the contacts will vibrate open and close at the frequency of the AC power. This problem is overcome by adding a shading pole to the relay.

Problem: You are planning a project that will be controlled by a PLC. Before ordering parts you decide to plan the basic wiring and select appropriate input and output cards. The devices that we will use for inputs are 2 contact switches, a push button and a thermal switch. The output will be for a 24Vdc solenoid valve, a 110Vac light bulb, and a 220Vac 50HP motor. Sketch the basic wiring below including PLC cards.

L 1 N

P L CL 1 N

I:0 /0

I:0 /1

I:0 /2

I:0 /3

a c c o m

O :0 /0

O :0 /1

O :0 /2

O :0 /3

s to p s ta rt

C R 1

C R 1

C R 1

C R 1

M C R

P B 1

L S 1

P B 2

C R 2

C R 2

L 1 ND rill S t a tio n

0 1 0

0 2 0

0 3 0

0 4 0

0 5 0

0 6 0

0 7 0

0 8 0

0 9 0

1 0 0

11 0

1 2 0

1 3 0

R

G

L 1

L 2

S 1

0 3 5

0 5 0

0 6 0

0 7 0

0 9 0

1 0 0

11 0

1 2 0

9 0 -1

1 0 0 -1

1 1 0 -1

1 2 0 -1

9 0 -1

1 0 0 -1

1 1 0 -1

1 2 0 -1

Wiring Symbols

disconnect circuit interrupter

breaker (3 phase AC)normally openlimit switch

normally closedlimit switch

normally openpush-button

normally closedpush-button double pole

push-buttonmushroom headpush-button

(3 phase AC) (3 phase AC)

Input Input ConnectionsConnections

AC DC TTL

TTL input interfaces allow the controller to accept signals from TTL compatible devices including solid state controls and sensing instrumentation. TTL inputs are used for interfacing with some 5 VDC level control devices and several types of photoelectric sensors

The Dc input module allows interfacing of input devices which provide a DC output voltage. The range of input voltage for DC input module varies between 5 VDC and 30 VDC

AC input module allows interfacing of input devices which provide AC output. Converter is used to covert incoming AC voltage to a DC logic signal to be used by the processor

Output ConnectionsOutput Connections

AC DC TTL

DC output module interface is used to control DC loads by switching On and Off.

AC output module allows interfacing of output devices which provide AC input.

The TTL output interface allows the controller to drive output devices that are TTL compatible such as seven segment LED display

The difference between the two types is whether the load (in our case, the plc is the load) is switched to ground or positive voltage. An NPN type sensor has the load switched to ground whereas a PNP device has the load switched to positive voltage.

DC Inputs

Inside the sensor, the transistor is just acting as a switch. The sensors internal circuit tells the output transistor to turn on when a target is present. The transistor then closes the circuit between the 2 connections shown above. (V+ and plc input).

DC Inputs

AC Inputs

AC input modules are less common these days than dc input modules

sensors typically have transistor outputs

Relay Outputs

A relay is non-polarized and typically it can switch either AC or DC. Here the common is connected to one end of our power supply and the other end of the supply is connected to the load. The other half of our load gets connected to the actual plc output you have designated within your ladder program.

Transistor Outputs

Shown above is how we typically connect our output device to the transistor output. Please note that this is an NPN type transistor. If it were a PNP type, the common terminal would most likely be connected to V+ and V- would connect to one end of our load. Note that since this is a DC type output we must always observe proper polarity for the output. One end of the load is connected directly to V+ as shown above.

Typical I/O connections with PLC:Typical I/O connections with PLC:

The shown diagram is a typical PLC with input and output module to control pneumatic actuator

Advantages of PLCs:Advantages of PLCs:

Increase flexibility,Increase flexibility, Faster implementation of changes and Faster implementation of changes and

correction,correction, Lower cost,Lower cost, Increased visual observations,Increased visual observations, Increased operation speed,Increased operation speed, Increased reliability and maintainability,Increased reliability and maintainability, Increased security,Increased security, Reprogramming capability,Reprogramming capability, Elimination of wiring.Elimination of wiring.

SummarySummary

This sequence covered the following This sequence covered the following topicstopics

1-PLC’S definition1-PLC’S definition

2-PLC’s Components2-PLC’s Components

3-PLC’S advantages3-PLC’S advantages

5-PLC’S I/O connections5-PLC’S I/O connections