LINEAR PERIODIC CONTROL - A FREQUENCY-DOMAIN VIEWPOINT

Periodic control is a well studied design strategy in which the coefficients of the control law are varied periodically in time. The technique has been extensively studied in the timedomain and has been shown to lead to a range of perceived benefits including high gain and phase margins. However, in view of the periodic nature of the feedback, we argue that it is more natural to adopt an alternative viewpoint in the frequency domain. We show that, within this framework, the action of the periodic controller can be viewed as a form of amplitude modulation (as in communication systems). This gives new theoretical insights into the method, including a clearer understanding of the action of the controller and a complete characterisation of the frequency content ot the input and output signals.