A cascade control system is a multi-loop control system that can be implemented effectively for a controlled object that is capable of measuring any intermediate control variable directly affecting the primary control variable. In cascade control systems, the intermediate sensor and controller are used to effectively reject the disturbances before they affect the primary control variable.
In a cascade control system, the inner loop control system is usually designed as a fast response system, and the outer loop control system is designed as a little slower system than the inner loop system.
Due to the good disturbance rejection and fast convergence performance, many studies have been conducted to design a sliding mode control (SMC) for the inner loop controller of a cascade system. With the improvement of chattering effect, SMC controllers have been used for not only the inner loop system, but also the outer loop control system.
To the best of our knowledge, no efforts have been focused on the relationship between the outer loop sliding surface and the inner loop sliding surface of cascade control systems.
Kim Sok Min, a researcher at the Faculty of Automatics, designed the inner loop sliding surface as a hierarchical sliding surface containing the outer loop sliding surface, and proposed a hierarchical sliding mode controller so as to reduce the outer loop reaching phase and improve the entire convergence. Furthermore, in order to improve the steady-state performance, he designed an adaptive PID sliding surface to improve the reaching phase using the proportional-integral-differential (PID) sliding surface.
The numerical simulations verified the excellent performance of the proposed design method.
If more information is needed, you can refer to his paper “An improved fast convergent sliding mode control design of a cascade system based on the hierarchical structure and an adaptive PID sliding surface” in “Second International Conference on Electronics, Electrical, and Control System (EECS 2025)” (EI).