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 v