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DC MOTOR CONTROL SYSTEMS FOR ROBOT APPLICATIONS
By: Rick Bickle
11/7/2003

Motor control questions
Why do we need speed control? How is DC motor speed controlled? How is motor direction controlled? What circuits can be used?

Reasons for accurate speed control
Motor speed should be independent of load. Differential drive platforms need to synchronize wheel speed to go in a straightline.

Speed control with PWM
Pulse Width Modulation

Simple PWM circuit
U1A 74HC14A

U1B

1

2

3

4

74HC14A

D1 DIODE

U1C

MG1
6

1

5

1

2

2
74HC14A

D2 DIODE

R2 POT
9

MOTOR DC
U1D

8

C1 CAP NP

3

74HC14A

U1E

11

10

74HC14A U1F

13

12

74HC14A

H-Bridge motor driver circuit
CIRCUIT INPUTS A 0 1 1 0 X B 0 1 0 1 X C 00 0 0 1 Output N Channel Brake P Channel Brake Forward Reverse Motor Off G D
M1
V+

R1 4.7K

R2 4.7K

S
Q1 IRF9510

S
Q2 IRF9510

G D

+5

DC Motor

+5

D
14

D
Q3 IRF510
Q4 IRF510

G
R3 4.7K

G S
Q6 2N2222
R4 4.7K
6

14

U1B 74HC08

S
Q5 2N2222

Input A

1

4

3
2
U1A 74HC08

Input B

5

7

7
V+

+5

R5

+5

D
14
9
U1C74HC08

Q7 IRF510

4.7K

U1C 74HC08

G S
R6 4.7K
Q8 2N2222

14
9

8
10

8
10

PWM Input

7

7

Optical encoder circuit
11

U27E 74HC14 10

13

U27F 74HC14 12

R11

C14

10K

0.001uF

R6 4.7K
L. Motor

1
Tach Input A

U27A 74HC14 2
U27B 74HC14 4

U6A
1
3

9
8

10

2

R7 4.7K
3
Tach Input B

74HC86
R10

U6C 74HC86
C15

D3 5V
R84.7K
R. Motor

D4 5V
U27C 74HC14 6

10K
0.001uF

U6B
4
6

13
11

5
Tach Input A

12

R9 4.7K
9
Tach Input B

U27D 74HC14 8

5

74HC86

U6D 74HC86

D5 5V

D6 5V

Motor control diagram
H-BRIDGE CIRCUIT DC MOTOR

PROCESSOR

ENCODER

Control systems
What is a control system? What are some examples? What are the types of control systems? How are controlsystems represented?

Open loop control systems
The output of the plant does not affect the input. (No feedback) Less common today than closed loop control systems. Examples include:
Stereo volume control Electric drill speed control

Open loop control system

INPUT

GAIN

PLANT

OUTPUT = INPUT X GAIN

Closed loop control systems
Use a measurement of output to control the input(Feedback) Examples include:
Air conditioning thermostat Automobile cruise control

Closed loop control system
INPUT

SUM

GAIN
FEEDBACK

PLANT

OUTPUT = (INPUT – OUTPUT) X GAIN

Motor control diagram
H-BRIDGE CIRCUIT DC MOTOR

PROCESSOR

ENCODER

PID Closed loop control system
PID controls the gain portion of the closed loop control system. PID algorithms adjust the gainto the plant based on several characteristics of the feedback, not just the current value.

PID control system diagram
P GAIN

INPUT

SUM

I GAIN

SUM

PLANT

D GAIN

FEEDBACK

OUTPUT = (INPUT – OUTPUT) X (P GAIN + I GAIN + D GAIN)

Sample PID output chart
Set point Rise time Overshoot Settling time Peak time Overdamped Underdamped

PID implementation
What is themathematics of PID? How is it programmed? What are some common problems? How is the PID behavior optimized?

PID variables
Error term P – Proportional gain I – Integral gain D – Derivative gain

Error term
The error term is derived by subtracting the feedback (motor speed) from the set point (set speed). This is the error in terms of a number of encoder counts per unit time.
P GAIN

INPUTSUM

I GAIN

SUM

PLANT

D GAIN

FEEDBACK

Proportional term
Simple proportional coefficient Kp is multiplied by the error term. Provides linear response to the error term.
P GAIN

INPUT

SUM

I GAIN

SUM

PLANT

D GAIN

FEEDBACK

Integral term
Integral coefficient Ki is multiplied by the error term and added to the sum of all previous integral terms. Provides...
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