Controlled process. _. Transducer. Controller. Example 1.1.3 Closed-loop idle-
speed control system. Textbook: Automatic Control Systems. 2. Author: B. C. Kuo
...
Chapter 1 INTRODUCTION 1.1
Control Systems
(1) What is a control systems? (2) Why control systems are important? (3) What are the basic components of a control system?
Example 1.1.1 Idle-speed control system Engine speed w
Load torque TL Engine Throttle angle a
Example 1.1.2 Printwheel control system
1
1.1
Class Notes of Control Systems
qr
MICROPROCESSOR
q r (reference input)
DC MOTOR
POWER AMPLIFIER
CONTROLLER
Position of printwheel
KEYBOARD
Instructor: Dr. Chih-Chiang Cheng
q v ( t )(output)
t1
0
Printing
Positioning
t2
Time
(4) Why feedback is incorporated into most control systems? I. Open-Loop Control System (Nonfeedback systems) Reference Input r
Actuating signal u Controller
Controlled variable c Contolled process
Major drawback: there isn’t a good way to control output. Advantage: simplicity, economy, used in many noncritical application. II. Closed-Loop Control Systems (Feedback Control systems) General block diagram: error detector +
output -
Controller
Controlled process
_
Transducer
Example 1.1.3 Closed-loop idle-speed control system Textbook: Automatic Control Systems
2
Author: B. C. Kuo
1.2
Class Notes of Control Systems
Instructor: Dr. Chih-Chiang Cheng
TL
wr
Error detector
w
we ENGINE
CONTROLLER
SPEED TRANSDUCER
Application of T L
Open-loop
Desired idle speed wr
Time
Application of T L
Closed-loop
Desired idle speed
Time
wr
Example 1.1.4 Closed-loop printwheel control system KEYBOARD
qr
MICROPROCESSOR CONTROLLER
POSITION ENCODER
1.2
DC MOTOR
POWER AMPLIFIER
Feedback
The Effects of Feedback on Control System
The reduction of system error is merely one of the many important effects that feedback may have upon a system. There are other important effects, such as stability, bandwidth, overall gain, disturbance, and sensitivity.
r +
y
e
G
_ H If H D 0 ) open loop system. The input-output relation is M D
Textbook: Automatic Control Systems
y G D r 1 C GH 3
Author: B. C. Kuo
1.2
Class Notes of Control Systems
Instructor: Dr. Chih-Chiang Cheng
1. Effect of Feedback on Overall Gain open-loop gain=G G 1 C GH The general effect of feedback is that it may increase or decrease the gain G, and the gain of the system could increase in one frequency range but decrease in another. closed-loop gain=
2. Effect of Feedback on Stability If GH D
1, the system becomes unstable.
Feedback can improve stability or be harmful to stability if it is not properly applied. 3. Effect of Feedback on Sensitivity Definition 1.2.1 The sensitivity of the gain of the overall system M to the variation in G is defined as @M percentage change in M SGM D M D @G percentage change in G G I. if M D
G (closed-loop), then 1 C GH SGM D
.1 C GH / GH @M G D @G M .1 C GH /2
G G 1CGH
D
1 1 C GH
) Sensitivity can be made arbitrarily small by increasing GH . Note: GH is a function of frequency. II. if M D G (open loop), then SGM D 1. 4. Effect of Feedback on External Disturbance or Noise A good control system should be insensitive to noise and disturbances and sensitive to input commands. Consider the following system: n + r
++
e
G1
e1 +
e2
G2
y
_
H
I. Open loop system: H D0
Textbook: Automatic Control Systems
) y D G1 G2 e C G2 n; 4
eDr
Author: B. C. Kuo
1.3
Class Notes of Control Systems
Instructor: Dr. Chih-Chiang Cheng
II. Closed-loop system: yD
1.3
G1 G2 G2 rC n 1 C G1 G2 H 1 C G1 G2 H
Types of Feedback Control System
Feedback control systems may be classified in a number of ways, depending upon the purpose of the classification. 1. linear versus nonlinear control systems
Linear: r
+ _
G1
G2
y
Nonlinear:
b
Saturation
Ideal relay
Dead band
2. time-invariant versus time-varying systems time-invariant system: Parameters of a control system are stationary with respect to time during the operation of the system. Otherwise: time-varying system. 3. Continuous-data versus discrete-data systems Continuous-data systems: the signals at various parts of the system are all functions of the continuous time variable t. Discrete-data systems: the signals at one or more points of the system are in the form of either a pulse train or a digital code.