APPENDIX B

CHARACTERISTIC RESPONSES OF SECOND-ORDER CONTROL SYSTEMS

The purpose of this appendix is to describe the transient response of a typical feedback control system which is referred to many times in this book. We consider a very common configuration in which a two-phase ac servomotor is enclosed by a simple unity feedback loop. Figure 4.1 illustrates the block diagram of this second-order system. For purposes of simplicity, the gain of the amplifier driving the motor is assumed to be unity.

The closed-loop transfer function of this system is given by

images

By defining the undamped natural frequency ωn and the dimensionless damping ratio ζ as

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Figure B.1 Second-order feedback system containing a two-phase ac servomotor.

Eq. (B.1) can be rewritten as

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The parameters ωn and ζ are very important for characterizing a system's response. Note from Eq. (B.3) that ωn turns out to be the radian frequency of oscillation when ζ = 0. As ζ increases from 0, the oscillation decays exponentially and becomes more damped. When ζ 1, an oscillation does not occur.

We assume that the initial conditions ...

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