Post on 18-May-2015
BASIC MOBILE ROBOT WORKSHOP
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ROBOTICS:-
MOBILE ROBOT PLATFORMS Mobile robots have the capability to
move around in their environment and are not fixed to one physical location.
Land or home robots Aerial robots are usually referred to
as unmanned aerial vehicles(UAVs) . Underwater robots are usually called
autonomous underwater vehicles(AUVs).
Humanoids. Animal-like
movements.
MOBILE ROBOT NAVIGATION METHODS Manual remote or tele-op - Under control
of a driver with a joystick or other control device.
Guarded tele-op - Ability to sense and avoid obstacles.
Example:- bomb defusing robot, material handling robots.
Autonomously guided robot - Have different capabilities of sensing , navigating on their own. Also incorporate wireless technologies . Example:- Nurse robot, Patrol bot,etc.
WHAT IS A LINE FOLLOWER? Simple and early form of an automated robot. Earliest
Automated Guided Vehicles (AGVs) were line following mobile robots. Follow a visual line painted or embedded in the floor or ceiling or an electrical wire in the floor.
Basic System of Line follower robot
SENSOR SYSTEM
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SENSOR SYSTEM
Simple sensors like- Infrared transmitters and receivers pairs. (IR LED and Photo transistor). Led and Light dependent resistor.
High level like- Image processing technology. Using a
camera interfaced to Image processor.
LED & LDR PAIR:It is simple to interface and reliable sensors. Very compact in size. Low voltage and current requirements.
LED :-
When a light-emitting diode is forward biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor.
LDR:
A photo resistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.
MEASURING THE LDR RESISTANCE:-
Measure the resistance in dark.
Measure the resistance in bright light.
DATASHEET ANALYSIS AND LED LDR SELECTION
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LED:-
LDR:-
SENSOR PLACEMENT:-
TOP VIEW
FRONT VIEW
SENSOR WORKING:
Ideally LDR detects light reflected from the black and white surfaces only.
Practically ambient light has to be considered. This causes an change in résistance. Therefore some tolerance is always kept
SENSOR MOUNTING
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CONTROL LOGIC
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TRANSISTOR: Bipolar Junction Transistors (BJT):
NPN :- Forward active: base higher than emitter,
collector higher than base (in this mode the collector current is proportional to base current).
Saturation: base higher than emitter, but collector is not higher than base. This mode corresponds to a logical "on", or a closed switch.
Cut-Off: base lower than emitter, but collector is higher than base. It means the transistor is not letting conventional current to go through collector to emitter. This corresponds to a logical "off", or an open switch.
Reverse-action: base lower than emitter, collector lower than base: reverse conventional current goes through transistor.
PNP:-o Forward-active (or simply, active): The base–emitter
junction is forward biased and the base–collector junction is reverse biased.
o Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates high current conduction from the emitter to the collector (or the other direction in the case of NPN, with negatively charged carriers flowing from emitter to collector). This mode corresponds to a logical "on", or a closed switch.
o Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current, which corresponds to a logical "off", or an open switch.
COMPARISONS :-
Characteristics of cut-off region NPN:-
o IB = 0; IC = 0; VCE = very large (large depletion region).
The input and Base are grounded (0v)
Base-Emitter voltage VBE < 0.7V
Base-Emitter junction is reverse biased
Base-Collector junction is reverse biased
Transistor is "fully-OFF" (Cut-off region)
No Collector current flows (IC = 0)
Vout = VCE = VCC = "1" Transistor operates as
an "open switch"
The input and Base are connected to VCC
Base-Emitter voltage VBE > 0.7V
Base-Emitter junction is forward biased
Base-Collector junction is forward biased
Transistor is "fully-ON" (saturation region)
Max Collector current flows (IC = Vcc/RL)
VCE = 0 (ideal saturation)
Vout = VCE = "0" Transistor operates as a
"closed switch"
Characteristics of saturation region NPN :-
PNP Transistor Switch:-
Load is connected to ground (0v) and the PNP transistor switches power to it.
To turn the PNP transistor as a switch "ON" the Base terminal is connected to ground or zero volts (LOW) as shown.
DATASHEET ANALYSIS AND TRANSISTOR SELECTION
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BC 548:-
PN 2907:-
VOLTAGE DIVIDER :-
Vout = Vcc ×(R1 / R1 + R
ldr)
In dark: Vout = ?
In Bright light:
Vout = ?
PRACTICAL DESIGN AND CALCULATIONS:- Under test conditions:-
Calculations:- Calculations:- Case 1:-1. VB = R2 / (R1+R2) * Vcc. Here, R2=2.21 KΩ (POT) ; R1 = 27 KΩ(LDR) ; Vcc
=6V VB = 0.45V.
2. VE = VB - VBE VE = 0.45 - 0.7 = -0.3 VTherefore transistor is off and IE doesn't flow.
LDR Resistance
Ambient light
27 KΩ
White LED
500 Ω
Case 2:- 1. VB = R2 / (R1+R2) * Vcc. Here, R2=2.21 KΩ (pot) ; R1 = 500Ω
(LDR) Vcc =6V VB = 4.89V. 2. VE = VB - VBE
0.45 - 0.7 = 4.19 V.Therefore transistor is on.3. Now , IE = VE / RE. Since RE =0 ; IE = Large4. The transistor is on so IC ≈ IE. Therefore IC is also large.
This amplifies the current which is used to drive the motor.
Advantage of using voltage divider – It is immune to changes in βDC.
βDC = IC / IB dc current gain.
Hands on with NPN Transistor:-
Check the circuit. Observe the output of
transistor with LDR exposed to light and in dark condition.
Verify theoretical calculation with practical observation.
HANDS ON WITH PNP TRANSISTOR:-
Check the circuit. Observe the output of
transistor with LDR exposed to light and in dark condition.
Verify theoretical calculation with practical observation.
TRANSISTOR CONTROL LOGIC:-
BC 548 PN 2907A
PRACTICAL ADJUSTMENTS:-
Intensity of the ambient light changes with different environments.
Therefore to operate robot in different light conditions the potentiometer is to be adjusted every time so as to obtain precise control.
DRIVE SYSTEM
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DRIVE SYSTEM:-
Most popular drive technologies-
Electric – DC motor.(servo ,stepper,etc.)
Hydraulic – utilize pressurized fluid for generation, control, and transmission of power.
Pneumatic – compressed air or other inert gases.
DC MOTOR:-
Operation is based on electromagnetism. A current-carrying conductor generates a magnetic field; when this is then placed in an external magnetic field, it will experience a force proportional to the current in the conductor, and to the strength of the external magnetic field
DC MOTOR SELECTION:- Speed range:- Very wide dynamic speed range can be obtained with
armature voltage control Speed variation with torque:- Depends on Applications – 1. Constant speed at all torque shunt-wound dc motor 2.Speed must decrease as the load increases compound or
series-wound dc motors 3. Speed change with load must be minimized a dc motor
regulator (employing feedback from a tachometer). Reversing:- Affects power supply and motor's brush adjustment. If
motor cannot be stopped for switching before reverse operation then compound and stabilizing dc motor windings with armature-voltage control system.
Peak torque:- The dc motor's peak torque depends on the duration and frequency of occurrence of the overload. Dc motor peak torque is often limited by the maximum current that the power supply can deliver.
Heating:- Dc motor temperature is a function of ventilation and electrical/mechanical losses in the machine. Thermal capability curves available from the dc motor manufacturer or estimated by the power-loss method. This method requires a total losses versus load curve or an efficiency curve.
MOTOR SPECIFICATIONS:-
PARAMETERS VALUE
Voltage 6 V dc
Current :No-load current Load current
60 mA(Max), 300 mA(Max)
RPM 150
Torque 2 Kgcm
Weight 125 g
H BRIDGE It is an “interface circuitry ”.Used to drive
the dc motors as current requirements of dc motors is high.
Switches are opened and closed in a manner so as to put a voltage of one polarity across the motor for current to flow through it in one direction
Case 1:-S1 and S4 –closed ,S2 and S3 are open, current will flow from left to right in the motor i.e. positive voltage across the terminals.
Case2:- S2 and S3 – closed,S1 and S4 are open, current will flow from right to left, reversing the voltage polarity.
Case3:- Terminals of the motor are open, the motor will freewheel (vicious braking).
Case4:- Terminals are short circuited, the motor will brake(dynamic brake).
Flyback Diodes :-
Motor is an inductive load. If the switch opens suddenly, then the
current wants to go to zero quickly and dI/dt would be a large negative value.
Since, V=LdI/dt.
Therefore, Vb >> Va in fig(b), there are voltage spikes which blow up switching devices.
Therefore flyback diode in the reverse direction across the inductive load. So that voltage spike will forward bias the diode creating a return path for the current.
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WORKING OF ROBOT
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WORKING OF TRANSISTORIZED CONTROL CIRCUIT:
Two independent circuits that control the two motors by detecting the amount of light and controlling the output current..
Variable resistor is to be adjusted in such a way- Case 1:- Under black tape the sensor detects
low light. Therefore, for NPN Transistor: Vbe < Vcut-in.
Case 2:- Under white background the sensor detects normal light the Vjunction >Vbe (saturation).
Transistor operation:- Case 1:- Under black tape NPN transistor goes to cut-off
region. Hence the current through the motor is low. So the motor speed reduces.
Case 2:- Under white background
NPN transistor goes to saturation region. Hence the current through the motor is high.
Since one motor rotating and one motor stopped, there will be an effect of turning.
The purpose of PNP transistor is to supply sufficient amount of drive current to the motor.
PCB nomenclature
1. bc 548 = 2 (Q1) 2. pn 2907 = 2 (Q2) 3. 1n4007 = 2 (D1,D2) 4. 10 k pot = 2 5. ldr = 2 (S1,S2) 6. led = 2 (L1,L2) 7. 100 ohm = 2 (R2) 8. 22 ohm = 2 (R1) 9. 220 OHM = 2 (R3)
END