The main indicators to measure the performance of the servo system are the frequency bandwidth and accuracy. The frequency bandwidth is referred to as the bandwidth, which is specified by the frequency response characteristics of the system and reflects the rapidity of the tracking of the servo system. The larger the bandwidth, the better the speed. The bandwidth of the servo system is mainly limited by the inertia of the control object and the actuator. The greater the inertia, the narrower the bandwidth. The bandwidth of the general servo system is less than 15 Hz, and the bandwidth of the servo system of large equipment is below 1 to 2 Hz. Since the 1970s, due to the development of torque motors and high-sensitivity tachometers, the servo system has realized direct drive, eliminated or reduced nonlinear factors such as backlash and elastic deformation, and enabled the bandwidth to reach 50 Hz, and has been successfully applied. In long-range missiles, artificial satellites, precision command instruments and other places. The accuracy of the servo system is mainly determined by the accuracy of the measuring elements used. Therefore, high-precision measuring elements must be used in the servo system, such as precision potentiometers, self-aligning machines, resolvers, photoelectric encoders, gratings, magnetic grids and ball grids. In addition, additional measures can also be taken to improve the accuracy of the system, such as connecting the measuring shaft of the measuring element (such as a self-aligning machine) to the rotating shaft through a reducer, so that the rotation angle of the rotating shaft is enlarged to improve the relative measurement accuracy. The servo system that adopts this kind of scheme is called the fine measuring and rough measuring system or the double-channel system. The angle measuring circuit that meshes with the rotating shaft through the reducer is called the fine reading channel, and the angle measuring circuit directly taken from the rotating shaft is called the coarse reading channel.