A robust two degree-of-freedom controller for systems with delay

This paper presents a method to design a robust two DOF (Degree of Freedom) controller for LTI systems with delay. The delay places bandwidth limitations on the feedback control loop which limits the system's response to rapidly changing exogenous signals i.e. reference and/or disturbances. A non-causal feedforward controller augments feedback to extend the achievable bandwidth, thereby obtaining fast and accurate control performance. System model uncertainty and signal measurement uncertainty compromise the feedforward control performance. Therefore, the two DOF controller is designed to optimize the performance for given levels of uncertainty. Due to the delay in the system, the optimization procedure is irrational. By transforming the irrational optimal control problem into an equivalent rational form, the problem is solved using standard robust controller synthesis approaches. A case study on header height control for a combine harvester is shown as an example of the analysis applied to a particular real-world system.