Washout filters in feedback control: benefits, limitations and extensions

Advantages and limitations of washout filters in feedback control of both continuous-time and discrete-time systems are discussed, and generalizations that alleviate the limitations are presented. The generalized washout filters presented involve feedback through auxiliary state variables that are intercoupled. This is in contrast to the more traditional washout filter-aided control, in which these variables couple to the system state but not to each other. The generalized washout filter results obtained in the paper include a systematic feedback control design procedure, which is not available for traditional washout filter-aided control. Some previously unpublished results in the Ph.D. dissertation of one of the authors (Lee, 1991) are presented in the context of their relation to the generalized results and to recent publications on delayed feedback control. We observe that delayed feedback control for discrete-time systems, used in a number of controls of chaos studies, is a special case of washout filter-aided feedback. Moreover, the limitations of delayed feedback control can be overcome by the use of washout filter-aided feedback, which gives rise to the possibility of stabilizing a much larger class of systems.