Variable-structure PID controller for level process

A variable-structure (VS) PID controller for the level process is proposed. A methodology of analysis of its stability and performance is given. It is proposed that stability of the VS system can be approximately analyzed via the describing function method. The describing function of the VS PID controller is derived. It is shown that the system with the VS PID controller is quasi-linear. Tuning rules for the VS PI controller for the level process are given. It is shown via the theory and simulations presented that, if properly tuned, the VS PI controller has higher performance than the conventional PI controller for the process considered.