The design of robust fractional order PI controller for liquid level control of a spherical tank

In this paper, robust stabilization of liquid level for a spherical tank is considered. This system is a First Order plus Time Delay (FOPTD) system, and the goal is to design a fractional order proportional integral (FOPI) controller. FOPI controller has three free parameters to achieve the desired performance. For this purpose, a stable region is obtained based on these free parameters which satisfy the desired phase and gain margins and flat phase constraints. And the proper parameters are selected based on this region. The main advantage of FOPI in comparison with traditional PID is its robustness. Computers simulations verify the robustness property and also show the effectiveness of the proposed FOPI controller.