Download - A Tutorial on Robotics Part II
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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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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