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BRIDGE RECTIFIER

This circuit is used to convert AC to DC. It uses 4 diodes and results in a full wave rectified, regulated, DC output.


TWO DIODE CIRCUIT FOR RECTIFIER • Using a centre-tap transformer, it is possible to create a full-wave rectifier using only two diodes. • This circuit is simpler to implement than the bridge rectifier circuit • But the four-diode bridge rectifier is necessary in cases where the transformer is not a centre-tap one.


VOLTAGE REGULATION

The diode is to prevent any reverse bias when the input supply is switched off while the output remains active for a short time due to high inductive load in the motor. The capacitors across the input and output aid in the stability of the regulator and may be anywhere between 100 and 330 nF. The third capacitor is around 100 microF and helps smooth out the supply.


ELECTROLYTIC CAPACITOR •

Constructed from two conducting aluminium foils, one of which is coated by an insulating oxide layer, and a paper space soaked in electrolyte.

Rolled up and placed in an aluminium casing.

Have a polarity and it is very important to maintain that.

The longer of the two legs represents the positive terminal, but look at the arrow (negative terminal)

Upper left picture is of a burnt capacitor


NON-ELECTROLYTIC CAPACITOR • Either ceramic (the upper figure) or tantalum (the lower one). • Absence of polarity between the terminals. • Non-electrolytic capacitors provide a much lower capacitance (in the range of pF) while electrolytic capacitors give that in the range of 1 – 100 microfarad. • They are also not suitable for high frequency applications due to long internal leads, which have high resistance, and losses in the dielectric.


Holonomic Drive •

A holonomic drive allows you to control all its degrees of freedom of motion. • This is in contrast to say a differential drive, which is constrained. • The drive shown here is called an Omni Drive / Kiwi Drive

Differential Drive

Omni Drive Can move in any direction (Image courtesy KRSSG)


Introduction to Autonomous Robotics • Autonomous robots are robots that can perform desired

tasks in unstructured environments without continuous human guidance.


Basic Parts of a Autonomous Robot • Power Supply • Locomotion System • Sensory devices (Feedback from Environment) • Data Processing • Control system • End actuators/motors


Actuators • They convert electrical energy into meaningful

mechanical work. • Mechanical output can be rotational or linear (straight line). • Motors provide rotational motion. • Electromagnets provide linear motion.


Motors are of various kinds • Motors can be either AC or DC. We use DC motors so

batteries can be used on robots. • For special purposes of controlled rotation, we use stepper or servo motors.


DC Motors •

As the name suggests, a motor which uses a DC (Direct Current) power • Can run in both directions • Speed Controllable

Sumandeep Banerjee, IIT Kharagpur


+

+

A

A

2

-

V DC

DC MOTOR

-

2

V DC

DC MOTOR

1

1

DC Motor Working

• Direction of rotation controlled by polarity of current /

voltage • Speed of rotation controlled by average energy (power) fed to the motor


DC Motor Specifications •

Operating Voltage : Recommended voltage for powering the motor • Operating Current : Current drawn at a certain load on the shaft • Stall Current : Maximum current drawn, when motor not allowed to rotate • Stall Torque : Rotation force needed to hold the motor in stall condition


DC Motor Characteristics • • • • •

Free running torque & current are ideally zero. Increased load implies, increased torque, current drawn & power consumption. Power supplied by a motor is the product of output shaft’s rotational velocity & torque. DC Motors are inherently high–speed, low-torque devices Using gears, the high speed of the motor is traded off into torque


DC Motor Characteristics 100% 90% 80% POWER

70% 60% 50% LOAD

40% 30% 20%

90%

100%

SPEED ------>

80%

70%

60%

50%

40%

30%

10%

20%

Zero speed at maximum load (stall torque). • Highest speed while free running (zero load). • Highest power at half speed & half load.

10%

Sumandeep Banerjee, IIT Kharagpur


Electronic Direction Control H – Bridge Circuit Diagram

1

+

A

L

-

2 Q2

Q3

NOT GATE

1 Q4

GND

2

DC MOTOR

Q1

1

NOT GATE

2

VCC

R


H – Bridge Working


MOTOR BRAKING • Electromagnetic Energy – The very fast throwing of the

switches causes sparking (due to a high momentary potential drop across the inductor coils in a motor) which causes a loss of the electromagnetic energy stored in such coils while the motor is rotating. • Mechanical Energy – When the motor is stalled, its rotation

due to inertia makes it become a dynamo, generating a backwards current. This flows through the circuit with finite resistance, leading to dissipation of heat energy which is the avenue for loss of the mechanical energy of the rotating motor.


MOTOR TORQUE VS GEAR TORQUE •

The relation between the input power, the torque and the angular velocity (P = T.w) is true for both motors and gear trains, but the concept that a reduction in torque immediately leads to an increase in the angular velocity, is relevant only to gear trains. In gear trains, a number of gear wheels are connected to each other through their teeth, and so a constant power is transmitted through the system. For motors, a reduction in torque is brought about by reducing the input power, which in turn reduces the angular speed.


DC Motor Speed Control • • • •

Operating Voltage is kept constant. Reducing Operating Voltage will reduce speed of rotation but also the output torque & power. A trick is done to achieve speed control without sacrificing the output torque. Instantaneous Operating Voltage is kept constant, but it’s average value is reduced.


DC Motor Speed Control Circuit

VOLTAGE CONTROL CIRCUIT

1

CONTROL SIGNAL

DC MOTOR

+

OPERATING VOLTAGE

A 2

-

• Input is the operating voltage & control signal • Output is a part of the operating voltage

depending upon the control signal


Duty Cycle Fundamentals 100% Duty Cycle VOLTAGE ------>

100% 80% 60% 40% 20%

VOLTAGE ------>

100% 80% 60% 40% 20%

20% Duty Cycle

1s 2s 3s 4s 5s 6s 7s 8s 9s 10s

1s 2s 3s 4s 5s 6s 7s 8s 9s 10s

TIME ------>

TIME ------>

80% Duty Cycle VOLTAGE ------>

100% 80% 60% 40% 20%

VOLTAGE ------>

100% 80% 60% 40% 20%

40% Duty Cycle

1s 2s 3s 4s 5s 6s 7s 8s 9s 10s

TIME ------>

1s 2s 3s 4s 5s 6s 7s 8s 9s 10s

TIME ------>


Duty Cycle •

The time period (τ) is the duration after the voltage waveform repeats itself. • Duty Cycle is the % time of τ, the voltage is equal to the operating voltage. • The average voltage is equal to the ‘Duty Cycle’ % of the Operating Voltage.


Pulse Width Modulation • • •

PWM is a technique using which we can modify the duty cycle of a waveform depending upon an input control voltage. This forms the backbone of our speed control circuit It can be easily implemented using various ICs which we will study in further classes.


Sensors • Analogous to human sensory organs. • Eyes, ears, nose, tongue, skin

• Sensors help the robot knowing its surroundings better. • Improves its actions and decision making ability. • Provides feedback control.


LDR - Light Dependent Resistor • Made of cadmium sulphide. • Resistance between two terminals vary depending on the

intensity of light. • Can be used to differentiate contrast colours.


Thermistor • Manufactured from the oxides of the transition metals -

manganese, cobalt, copper and nickel. • Resistor depends on temperature.


IR Photo Diode • • •

Detects presence of Infra Red radiations. Used for obstacle proximity sensing. IR Data Communication.


• •

LM339, LM393 and LM311. They consist of more than one comparator. LM311 consists of single comparator.


LM339


LM339




Light Sensing Module using LED-LDR combination


K.R.A.I.G. 103