DC motor control for robot

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
straight line.
Speed control with PWM
 Pulse Width Modulation
Simple PWM circuit
U1A
74HC14A
1
2
U1B
74HC14A
3
4
U1C
74HC14A

5
6
U1D
74HC14A
9
8
U1E
74HC14A
11
10
U1F
74HC14A
13
12
C1
CAP NP
D1
DIODE
D2
DIODE
R2
POT
1
3
2
MG1
MOTOR DC
1
2
H-Bridge motor driver circuit

V+
V+
+5
+5
+5
+5
M1
DC Motor
Q1
IRF9510
Q5
2N2222
Q8
2N2222
R6
4.7K
R2
4.7K
R1
4.7K
R3
4.7K
R5
4.7K
U1C
74HC08
9
10
8
14

7
U1B
74HC08
4
5
6
14
7
Q4
IRF510
U1C
74HC08
9
10
8
14
7
R4
4.7K
Q6
2N2222
Q2
IRF9510
Q3
IRF510
U1A
74HC08
1
2
3

14
7
Q7
IRF510
Input A
Input B
PWM Input
CIRCUIT INPUTS
SS
SS
D
DD
D
G
GG
G
A B C Output
0 0 0 N Channel Brake
1 1 0 P Channel Brake
1 0 0 Forward
0 1 0 Reverse
X X 1 Motor Off
D
G
S
Optical encoder circuit
U6B
74HC86
4
5

6
R6 4.7K
R7 4.7K
D4
5V
D3
5V
R9 4.7K
D6
5V
D5
5V
R8 4.7K
U6A
74HC86
1
2
3
U6D
74HC86
12
13
11
U6C
74HC86
9
10
8
R10
10K

C15
0.001uF
C14
0.001uF
R11
10K
U27A
74HC14
1
2
U27B
74HC14
3
4
U27D
74HC14
9
8
U27C
74HC14
5
6
U27F
74HC14
13
12
U27E
74HC14
11
10

Tach Input A
Tach Input B
Tach Input A
Tach Input B
L. MotorR. Motor
Motor control diagram
PROCESSOR
DC
MOTOR
ENCODER
H-BRIDGE
CIRCUIT
Control systems
 What is a control system?
 What are some examples?
 What are the types of control systems?
 How are control systems 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
GAIN
INPUT
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
GAIN
INPUT
PLANTSUM
FEEDBACK
OUTPUT = (INPUT – OUTPUT) X GAIN
Motor control diagram
PROCESSOR
DC
MOTOR
ENCODER
H-BRIDGE
CIRCUIT
PID Closed loop control system
 PID controls the gain portion of the
closed loop control system.
 PID algorithms adjust the gain to the
plant based on several characteristics of
the feedback, not just the current
value.
PID control system diagram
I GAIN
INPUT
PLANTSUM

FEEDBACK
P GAIN
D GAIN
SUM
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 the mathematics 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.
I GAIN
INPUT

PLANTSUM
FEEDBACK
P GAIN
D GAIN
SUM
Proportional term
 Simple proportional coefficient Kp is multiplied by
the error term.
 Provides linear response to the error term.
I GAIN
INPUT
PLANTSUM
FEEDBACK
P GAIN
D GAIN
SUM
Integral term
 Integral coefficient Ki is multiplied by the error term
and added to the sum of all previous integral terms.
 Provides response to accumulated error.
I GAIN
INPUT
PLANTSUM
FEEDBACK
P GAIN
D GAIN
SUM
Derivative term
 Derivative coefficient Kd is multiplied by the difference
between the previous error and the current error.

 Responds to change in error from one PID cycle to the next.
I GAIN
INPUT
PLANTSUM
FEEDBACK
P GAIN
D GAIN
SUM
PID calculation example
 Error_term = Set_Speed – Encoder_Count;
 P_Term = P_Gain * Error_Term;
 D_Term = D_Gain * (Error_Term – D_State);
 D_State = Error_Term;
 I_State = I_State + Error_Term;
 I_Term = I_Gain * I_State;
 PWM_Set = PWM_Set + P_Term + I_Term + D_Term;
Factors to consider
 PID cycle time (0.1 sec)
 Motor speed (30 rpm)
 Encoder resolution (500 counts/rev)
 PWM frequency (1kHz)
 Interrupt driven PID trigger
 Eliminates code tuning
 Maintains accurate PID timing