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For a car, automatic transmission is the core component of the shifting system, and its performance will have a direct impact on vehicle safety and stability, so in its development progress, it is necessary to test its performance. If we do the testing on the vehicle, the results are relatively well, but the cost is high and there is a problem on security, so people design professional testing equipments to test its performance. And the testing devices have been more mature abroad, but in our country, it is still in developing stage, so we had to do some research and design.In this paper, we design a passive variable damping load simulator, The mechanical system includes a hydraulic cylinder and a variable throttle. When the external devices force hydraulic cylinder piston rod to move, the servo motor controls the area of the throttle, thereby controls the damp of the system, and the system will performance a corresponding force,which impedes the movement of the rod, and that is the load force to be simulated. This force has something to do with the moving speed of the piston rod and the area of the throttle. The movement of the rod has been given, so as long as you change the area of the throttle,the system will performance the corresponding force.After detailed analysis, to meet the requirements of the load simulation, I design the cylinder as analogous hydrostatic bearing hydraulic cylinder. Firstly I design the hydraulic cylinder and throttle, account the parameters of them, and make the mathematical description of cutting expenditure. Then I get the mathematical modeling of the whole system and do the simulation. As the design specifications require quick response and high accuracy, and the simulation analysis determines the parameters of the model, which is the guider of later specific design. After debug the PID controller control parameters, it is sure that classic PID algorithm can meet the requirements of the control of system.
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