Disturbance rejection and reference tracking in control systems with saturating actuators.

This dissertation is devoted to the study of disturbance rejection and reference tracking problems in control systems with saturating actuators. Control systems are usually required to track reference signals while operating under the influence of disturbances. Often, this requires large control effort. However, due to power and material limitations, control effort is always limited, either in amplitude, or in rate, or both. Neglecting these limitations in the design process may severely degrade the closed-loop system performance and may even drive the system to instability. On the other hand, selecting controller gains so as to avoid saturation may lead to substantial performance loss. Consequently, actuator saturation must be explicitly taken into account in the design process to ensure acceptable performance. Based on the method of stochastic linearization, this dissertation develops analysis, synthesis and design methods for disturbance rejection and reference tracking problems in the face of actuator saturation. In the analysis part, a method for evaluating the quality of disturbance rejection and reference tracking is developed. In the synthesis part, the classical LQR/LQG theory is extended to systems subject to actuator saturation. In the design part, several practical design methods are proposed. The results obtained are illustrated by two practical control problems, namely a ship roll stabilization problem and a servo position control problem. One of the main conclusion of this research is that actuator saturation is not to be avoided by limiting the controller gain but rather allowed to improve the performance of the system. It is also concluded that the achievable level of disturbance rejection or reference tracking with saturating actuators is fundamentally limited and cannot be made arbitrarily good, even when the plant is minimum-phase.Ph.D.Applied SciencesElectrical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132574/2/9977161.pd