说明：  用MATLAB程序软件完成临界比例带法对PID控制器的参数整定，并求出PID控制器的三个参数Kp，Ti，Td。最后绘制出参数整定前后的单位阶跃响应图进行比较。
(The parameters of PID controller are adjusted by MATLAB software and the three parameters Kp, Ti and Td of PID controller are obtained. Finally, the unit step response diagram before and after parameter tuning is drawn.)