Feedforward control under the presence of uncertainty

In this paper, we study the effect of model errors on the performance of feedforward controllers. In accordance with the sensitivity function for feedback control, we define the feedforward sensitivities, S-ff (feedforward from disturbance) and S-ff,S-r (feedforward from setpoint), as measures for the reduction in the output error obtained by the feedforward control. For "ideal" feedforward controllers based on the inverted nominal model, the feedforward sensitivities equal the relative model errors, which must thus remain less than 1 for feedforward control to have a positive (dampening) effect. For some common model error classes we provide rules for when the feedforward controller is effective, and we also design mu-optimal feedforward controllers.