5.1 Introduction

The PTOS and MSC schemes discussed in the previous chapter have two controllers, i.e. a nonlinear controller and a linear controller, operating in two different time stages. The switching elements in these control schemes generally yield a system response not as smooth as what we would expect and thus limit the overall performance of the corresponding servo systems. Furthermore, both schemes do not pay any special attention to the transient performance of the overall design, which is in fact one of the important issues in tracking control problems. In general, quick response and small overshoot are desirable in most practical situations. However, it is well understood that quick response is often accompanied by a large overshoot. Thus, most of the design schemes have to make a tradeoff between these two transient performance indices. Inspired by a recent study of Lin et al. [130], which was introduced to improve the tracking performance under state feedback laws for a class of second-order systems subject to actuator saturation, we have developed in this chapter a nonlinear control technique, the so-called composite nonlinear feedback (CNF) control, for a more general class of systems with measurement feedback. It is applicable to systems with or without external disturbances.

The CNF control consists of a linear feedback law and a nonlinear feedback law without any switching element. Unlike the MSC and PTOS control laws, both the linear and nonlinear controllers in CNF are in operation all the time. The linear feedback part is designed...