简单伺服系统设计。定义了伺服系统数据结构，对外提供目标值和跟随值的两个输入接口，和伺服系统对外输出值。用户也可以根据自己需要改变伺服系统中的宏，以满足实际需要(Simple servo system design. The servo system data structure is defined. Two input interfaces of target value and following value are provided externally, and the output value of the servo system is output externally. Users can also change the macros in the servo system according to their needs to meet the actual needs.)