Filter design for UDE-based controllers

The control algorithm based on the Uncertainty and Disturbance Estimator (UDE) is a robust control strategy and has received wide attention in recent years. In this paper, the two-degree-of-freedom nature of UDE-based controllers is revealed. The set-point tracking response is determined by the reference model while the disturbance response and robustness are determined by the error feedback gain and the filter introduced to estimate the uncertainty and disturbances. It is further revealed that the error feedback gain and the filter introduced to estimate the uncertainty and disturbances can be designed independently in the frequency domain. Moreover, after introducing the UDE-based control, the Laplace transform can be applied to some time-varying systems for analysis and design because all the time-varying parts are lumped into a signal. It has been shown that, in addition to the known advantages over the time-delay control, the UDE-based control also brings better performance than time-delay control under the same conditions. An example is given to demonstrate the findings.