Modern Control System Theory and Design, 2nd Edition

7.1.  INTRODUCTION

After the stability of a feedback control system has been analyzed, by using any of the tools presented in Chapter 6, it will often be found that system performances is not satisfactory and needs to be modified. It is necessary to ensure that the open-loop gain is adequate for accuracy, and that the transient response is desirable for the particular application. In order for the system to meet the requirements of stability, accuracy, and transient response, certain types of equipment must be added to the basic feedback control system. We use the term design to encompass the entire process of basic system modification in order to meet the specifications of stability, accuracy, and transient response. The term stabilization is usually used to indicate the process of achieving the requirements of stability alone; the term compensation is usually used to indicate the process of increasing accuracy and speeding up the response.

There are several commonly used configurations for compensating (or stabilizing) a control system. The compensating (or stabilizing) device may be inserted into the system either in cascade with the forward portion of the loop (cascade compensation) as shown in Figure 7.1, or as part of a minor feedback loop (feedback compensation) as shown in Figure 7.2 [1, 2]. The cascade-compensation technique is usually concerned with the addition of phase-lag, phase-lead, and phase-lag–lead passive networks. The feedback-compensation technique is primarily ...