PID control (fuzzy control) is one of the earliest and most widely used control laws in process control applications. It can obtain satisfactory control results for most industrial objects. People use PID control (or PI, PD control) in a variety of controllers in frequency control system. However, with the rapid development of modern large-scale industry, and the degree of automation is getting higher and higher, the scope of applications in frequency control technology has been continuously expanding, while it has wider and deeper requirements for the performance of speed control system. AC drive system has uncertain factors, such as the existing parameter variability, load disturbance, nonlinear and strong coupling in under controlled AC motors, they will seriously affect the performance of speed control system. The traditional PID control seems a little powerless. The fuzzy control is a nonlinear control in essence, it can significantly improve the robustness of the system comparing to linear PI controller, and it can more effectively overcome various nonlinear factors in transmission system. For a multi-loop speed control system, the outer loop is the fundamental factor in determining the system performance. Inner loop is mainly for changing object properties in order to facilitate the controlling actions of the outer loop. The sampling frequency of inner loop is lower than the one in outer loop, which is facilitating for the realization of intelligent control.
