I. Overview of power transmission of automatic transmission Automatic transmission consists of hydraulic components, transmission mechanism, control system and main transmission parts. Transmission mechanism can be divided into three types: fixed parallel shaft type, planetary gear type and metal belt type continuously variable transmission. In the vehicles in use in our country, the automatic transmission mostly adopts planetary gear transmission mechanism, which is also the focus of this paper. The planetary gear mechanism is generally composed of two or more planetary gear sets with different combinations, and its function is to generate forward gear, reverse gear and neutral gear with different speed ratios by driving or braking different components. The function of shift actuator is to restrain some components of planetary gear mechanism, including fixing and making its rotation speed zero, or connecting a component to make it rotate at a specified rotation speed. By properly selecting the basic elements and constraints of the planetary gear mechanism, different transmission ratios can be obtained and different gears can be formed. The shift actuator includes three different parts: clutch, brake and one-way clutch. The function of the clutch is to connect or drive the input shaft (active part) of the transmission mechanism with a certain part (passive part) of the planetary gear mechanism to realize power transmission. The function of the brake is to fix the basic elements in the planetary gear mechanism. When it works, it connects the braked element with the transmission housing so that it cannot rotate. One-way clutch has the characteristic of one-way locking. When the rotation trend of the connecting element makes its stress direction the same as the locking direction, the element is fixed (braked) or connected (driven). When the direction of the force is opposite to the locking direction, the element is released (disconnected). It can be seen that the function of one-way clutch is the same as that of clutch and brake in different States. As can be seen from the above introduction, mastering the motion law of different combination planetary gear mechanisms is the basis of automatic transmission fault diagnosis. ……