串级pid控制，系统结构上，串级控制系统有两个闭合回路：主回路和副回路，主、副调节器串联工作。副回路先调、粗调、快调，主回路细调、慢调。(This simulink model is a cascade pid control system, which uses two pid controller to control the transfer function. Cascade control system includes outer loop and inner loop.)