说明：  离散系统的数字PID仿真:针对离散系统的阶跃信号、正弦信号和方波信号的位置响应，设计离散PID控制器。其中，S为信号选择变量，S=1时为阶跃跟踪，S=2时为方波跟踪，S=3时为正弦跟踪。
(PID digital discrete system simulation: discrete-time systems for the step signal, sinusoidal signal and the square wave signal in response to the location, design of discrete PID controller. One, S for signal selection variable, S = 1 when tracking step, S = 2 for the square-wave tracking, S = 3 for sinusoidal tracking.)