Design of robust controllers with simple structures for stable, integrating and unstable processes

A method is presented to design a robust controller for a wide range of stable, integrating and unstable process dynamics. The controller with a simple structure can be a Pl, PID or higher-order type, whose parameters are determined simply by specifying the desired closed-loop response. However, the design of high-order controllers may result in good performance but very poor robustness, which is not detectable by the conventional indices of gain and phase margins. To avoid this, a robustness index is proposed as a replacement for the gain margin. Optimal controller design is then achievable by minimizing a designated error integral criterion for a load disturbance subject to inequality constraints on phase margin and the new robustness index. By a clever adjustment in control algorithms, the optimal design for a load disturbance can also give a fast response with a desired overshoot for a step set point change.