Digital tracking control for PWM systems with unacceptable zeros

Perfect output tracking, which requires inversion of the input-output dynamics, is not always a practical control objective. Difficulties are encountered for systems with zeros which are unstable, or stable but lightly damped. When the zeros are unacceptable in the above sense, perfect output tracking would require the control to be either unbounded, or bounded but highly oscillatory. In this paper, an approximate output tracking control design method is introduced for pulsewidth modulated (PWM) systems with unacceptable zeros. The design method is applied to the nonlinear sampled-data model of the PWM system, and is based on output redefinition. Although the new approach leads to approximate rather than perfect output tracking, it guarantees bounded and nonoscillatory responses