ABSTRACT
This chapter shows how open-loop and closed-loop frequency response are related. It introduces the Nichols chart and its use and presents a frequency response stability criterion. The chapter describes integral action and its effects on static error and outlines the steps of a simple frequency response design procedure. It discusses some ways in which computer tools can aid control system design. In practice, though, the situation is usually far more complicated. In closed-loop position control system case, therefore, increasing controller gain has again had the effect of speeding up the response but only at the expense of increasingly oscillatory behaviour, which is likely to be undesirable in any real control system. In the terms of a second-order system, the damping ratio becomes zero and the closed-loop system is on the verge of instability since oscillations, once established, never die away.
