11 - Combined feedback and feedforward control

Abstract

This chapter discusses the concept and strategy of combined feedback and feedforward control. It describes the aspects, which have to be considered when using feedback and feedforward control. In a combination of feedforward, feedback and cascade control, the method of initialization is important. The value for the initialization of the controller output (OP) of the primary controller (TC) is calculated from the sum of the value of the feedforward signal from the lead/lag block and the SP value of the secondary controller (FC). As fluctuations occur to the inlet temperature, the inlet flow rate varies and depending on which direction they occur, the output of the lead/lag block varies in accordance with the functional algorithm and lead/lag time constants comprising the feedforward control. Because strong feedback control has a tendency toward instability, it should be avoided if possible. Therefore, if the feedforward control is already doing the major part of the required control, feedback control is just there to eliminate the drift of PV.