New roles for feedforward in multivariable control by individual channel design

Abstract

Considerable though the benefits of feedback control are, they do not extend to ameliorating adverse multivariable plant dynamical characteristics that are of a structural nature. Feedforward control on the other hand, is shown in this paper to complement multivariable feedback control in a simple but powerful way by providing a strikingly direct resolution of such structural difficulties as non-minimum phase plant behaviour, plant excessive structure sensitivity (uncertainty in the number of channel RHPZs), plant excessive phase sensitivity, and decoupling without increasing system uncertainty. This use of feedforward is believed to be new and is completely different from the well-known application of feedforward control to anticipate and counteract the effect of some known plant disturbance before it affects the output, as widely used in the process industries. The paper ends with a complete statement of the philosophy and methodology of multivariable control, feedforward and feedback, within the context of the individual channel design of 2-input 2-output systems (O'Reilly and Leithead 1991). In particular, it is pointed out that it is immaterial which multivariable technique is used to determine the feedback controller for the plant amended by feedforward, although diagonal controller matrices are preferred (Leithead and O'Reilly 1992b) otherwise, if the feedback controller is non-diagonal, the extent to which the sensitivity to system uncertainty is increased should be checked (Leithead and O'Reilly 1992b, O'Reilly and Leithead 1993).