A Tutorial on Robotics Part II

7/29/2019 A Tutorial on Robotics Part II

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Copyright Robotix Team, IIT Kharagpur

February 3rd-5th, 2006


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A Tutorial on Robotics

Part II : Motors & Motor Drivers


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Basic Parts Of Our Mobile Robot

Locomotion system

Power supply system

Actuators Sensory devices for feedback

Sensor Data processing unit

Control system


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End Actuators

They convert the electrical energy into

meaningful mechanical work

Mechanical output can be rotational or

linear (straight line)

Motors provide rotational motion

Electromagnets provide linear motion


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Motors are of various kinds

AC Motors : Not used much in robotics

Stepper Motors : For controlled rotation

DC Motors : Finds extensive general use Servo Motors : DC motor with in built

feedback & error compensation


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Stepper Motors

Used for measured

rotation

Can be held at a

particular positionof the shaft

Ideal for many

autonomous robots

requiring higher

precision


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Stepper Motor Working

0001001001001000

Stepping Sequences for Single Coil Excitation

Only one coil is active at a given instant of time


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Single Coil Excitation


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Stepper Motor Working Cont

1001001101101100

Stepping Sequences for Double Coil Excitation

Two coils active at any given instant of time


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Double Coil Excitation


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Differences

Single Coil

Consumes Less

Power

Less Torquegenerated

Settling time is more

Double Coil

Consumes Double

Power

More torquegenerated

Settling time is less


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Interleaved Stepping (Half Step)


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Connection Diagram


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Problems with Stepper Motors

Very low torque to weight ratio

Torque decreases with increase in the

stepping frequency

For sufficiently high stepping speed the

stepper motor may skip steps due to

overshoot


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DC Motors

As the name suggests,

a motor which uses a

DC (Direct Current)

power Can run in both

directions

Speed Controllable


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DC Motor Working

A

-

+

DC MOTOR1

2

A

-

+

DC MOTOR1

2

V DC

V DC

Direction of rotation controlled by polarity of

current / voltage

Speed of rotation controlled by average

energy (power) fed to the motor


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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


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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 shafts rotational velocity &

torque


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DC Motor Characteristics Cont...

DC Motors are highspeed, low-torque

devices

Using gears, the high speed of the motor

is traded off into torque


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DC Motor Characteristics Cont

Zero speed at

maximum load

(stall torque)

Highest speedwhile free running

(zero load)

Highest power at

half speed & half

load

50%

50%

20%

10%

40%

100%

80%

60%

90%

SPEED ------>

POWER

60%

90%

40%

70%

70%

30%

LOAD

20%

30%

100%

80%

10%


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DC Motor Drivers

These are current amplifying circuits

A low current control signal is converted

into a proportionally higher current signal

that can drive the motor

Power Transistors can switch high

currents. The ICmax is usually of the order

of Amps as compared to small signal

transistors which have ICmax in mA


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DC Motor Direction Control

H Bridge Circuit Diagram

S1

S3

S2

S4

M1 2

VCCPower

Transistor

Switches


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H Bridge Working

S1 S2 S3 S4 Current

Direction

Effect

1 0 0 1 1 to 2 Motor spins

forward0 1 1 0 2 to 1 Motor spins

backward

1 1 0 0 - Braking

Occurs

0 0 0 0 - Free running


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Electronic Direction Control

H Bridge Circuit Diagram

GND

VCC

NOT GATE

1

2Q1 Q3

A- +

DC MOTOR

12

LQ4Q2

R

NOT GATE

1

2


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H Bridge Working

GND

OFF

Q2

0

VCC

Q3

A- +

DC MOTOR

12

VCC

ON

Q1

NOT GATE

1

2

ON

1

Q1

L

ON

GND

NOT GATE

1

2

NOT GATE

1

2

1

Q3

OFF

Q1

OFF

NOT GATE

1

2

0

A- +

DC MOTOR

12

Q1

OFF

OFF

A- +

DC MOTOR

12

GND

Q3

OFF

ON

L

L

Q4

NOT GATE

1

2

OFF

Q2

Q2

ON

Q2R

OFF

R

R

1

VCC

ON

L

R

Q3

1

NOT GATE

1

2

Q4

Q4

ON

A- +

DC MOTOR

12

ON

VCC

0

GND

NOT GATE

1

2

NOT GATE

1

2

Q4

0


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DC Motor Speed Control Circuit

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

depending upon the control signal

A

-

+

DC MOTOR1

2

OPERATING

VOLTAGE

VOLTAGE

CONTROL

CIRCUIT

CONTROL SIGNAL


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DC Motor Speed Control Cont

Controlling the current by passing it

through a variable resistor is not

advisable as it is extremely energy

inefficient

A trick is done to achieve reduced

average power

Power is supplied to the motor in short

intermittent bursts, as explained further


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Duty Cycle Fundamentals

8s5s

2s

9s

100%

80%

4s

10s

100% Duty Cycle

40%

60%

7s8s4s

VOLTAGE

------>

40%

40% Duty Cycle

80%

5s9s

20%

1s

80% Duty Cycle

2 s 3 s3s

20%

6s

TIME------>

4s

20% Duty Cycle

VOLTAGE

------>

8s

2s

100%

9s

VOLTAGE

------>

20%

10s7s

80%

60%

1s

80%

7s

100%

9s6s

1s

5s 5s

TIME------>

40%

60%

TIME------>

VOLTAGE

------>

6s3s 10s7s

6s10s

2s 1s

60%

40%

3s

20%

4s

100%

TIME------>

8s


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Duty Cycle Cont

The time period () is the duration after

the voltage waveform repeats itself

Duty Cycle is the % time of, the voltage

is equal to the operating voltage

The average voltage is equal to the Duty

Cycle % of the Operating Voltage


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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 the

popular multipurpose IC 555, used

widely for hobby electronics


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Both Speed & Direction Control

NOT GATE

1

2

Q2

VCC

Q4

Q3

A- +

DC MOTOR

12

AND GATE

1

2

3

AND GATE

1

2

3

NOT GATE

1

2

R

PWM SIGNAL GENERATOR

SPEED CONTROL SIGNAL

AND GATE

1

2

3

GND

AND GATE

1

2

3

Q1

L


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L293D Quad Half H-Bridge

A EN Y

0 1 LOW

1 1 HIGH

X 0 FLOAT


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IC L293D Connection Diagram


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IC 555 Connection Diagram


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PWM Using IC 555


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Copyright Robotix Team IIT Kharagpur

Thank You

In Part III : Control System & Sensors,

we will discuss various analog sensors,

various aspects of feedback control

Proportional Control will be discussed in

depth

We will take up the example of a line

follower robot

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A Tutorial on Robotics Part II

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