Intersample Ripple Resulting from Discrete-Time Feedforward Control

Adding feedforward controllers to feedback-only control systems can often improve the transient response performance. In the literature several architectures have been proposed, which combine feedback and feedforward controls together, such as Feedforward Closed-Loop Inverse (FFCLI) and Feedforward Plant Inverse (FFPI). In this paper, we investigate discrete-time feedforward control designs and show how some designs lead to large intersample ripple in the response of the continuous-time plant that is being controlled. If the feedforward controller cancels high frequency zeros with small damping factors in the discretized plant, large intersample ripple can result. Ignoring such zeros in the feedforward controller will suppress intersample ripple significantly, while slightly slowing the response. In this work, this phenomenon is demonstrated with a second-order plant and a fourth-order Torsional Disk System (TDS) plant.