4 – Feedforward Control

Publisher Summary This chapter focuses on the feedforward control of disturbances which are either deterministic or random. The deterministic disturbances are considered as being decomposed into their constituent frequency components. The effect of feedforward control on each of these discrete frequency components can then be considered independently, because the system under control is assumed linear and so superposition applies. Random disturbances represent a more severe problem, because of the requirement that the feedforward controller has a prescribed response at all frequencies in the range of interest, rather than just at a number of discrete frequencies, and so there is an additional constraint of causality for a realisable controller. The chapter discusses the single-channel feedforward systems for the control of harmonic, and then random disturbances, in which the performance limits of such controllers, are present. The filtered-x LMS algorithm is introduced for the automatic adjustment of an FIR digital filter used as the controller in a single-channel feedforward control system, and some of the properties of this algorithm are discussed. Adaptive controllers are described, in the frequency and time domains, for the feedforward control of systems with multiple sensors and actuators. Finally, an interesting interpretation of adaptive feedforward controllers as equivalent fixed feedback controllers is described.