Analysis and design of a double two-degree-of-freedom control scheme

Robust analysis and design for a double two-degree-of-freedom (TDF) scheme are discussed in the paper. It is shown that the double TDF scheme is a TDF version of the internal model control (IMC) structure for integrating and unstable processes and retains the property of the IMC structure for stable processes. A robustness measure is proposed assuming that the controlled process has a general uncertainty description. The measure is convenient for comparing the robustness of different control systems and can be used as a guideline for robust tuning. Finally, a TDF-IMC method is proposed to design and tune the parameters of the double TDF scheme to achieve improved robust performance.