Abstract: This paper describes a general two-degree-of-freedom control system with variable system structure. Based on the application of servo systems, the design and adjustment method of the two-degree-of-freedom controller is derived, and experimental results are given to demonstrate its tracking performance and disturbance rejection. Keywords: Two-degree-of-freedom PID controller; servo system; variable system structure 1 Typical Structure of Two-Degree-of-Freedom Control System Two-degree-of-freedom control systems have wide applications in industry and are simple to operate. Common structures of two-degree-of-freedom control systems include composite feedforward type, closed-loop compensation type, feedback compensation type, and given filter type, as shown in Figure 1. [Click for details] Figure 1 Common Structures of Two-Degree-of-Freedom Control Systems