Chapter 1 INTRODUCTION

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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.

Textbook: Automatic Control Systems

5

Author: B. C. Kuo