Optimization Of Control Strategy On Electric Power Steering System

Recently, with the development of Electric Power Steering System (EPS), the demands on its performances are increasing. As the core technology of EPS, the control strategy affects its safety and comfort directly. The more excellent the control strategy is, the better the performances are.Firstly, a series of improvement and perfection was made in this thesis on the existing control strategy of our research group because of its several problems and unfinished parts. The parameters on estimating motor rotational speed were modified and inertia compensation was added in order to enhance the rapidity of EPS under acceleration and deceleration condition. By the means of adjusting PI parameters and group control according to vehicle speed, the vibration of current-circle fell off. For improving the stability of high speed, damp control was plugged and non-disturbance switch between damp mode and assistance mode was realized. Fault diagnosis of EPS and nonlinear compensation of the restricted bipolar mode were improved. And the problems that the motor actual current was nonzero in the torque dead zone and controller didn’t work in a single direction intermittently were also resolved. The safety and comfort of EPS were increased through the mentioned strategies.The effects of EPS parameters on its stability, such as assistant gain, time delay constant of current circle and damping coefficient, were analyzed from time and frequency domains respectively based on the mechanic model of EPS. It can found that the reduction of assistant gain and time delay constant of current circle, the increase of damping coefficient contributed to the stability of EPS. Two types of phase compensation method, which is speed feedback and torque differentiation, as well as the effects of actual integration on the rapidity of EPS were discussed. The former ideal differentiation was replaced by actual differentiation as the phase compensation method of EPS, and the algorithm was designed. The bench test results showed that the phase compensation based on actual differentiation could balance the contradiction between phase advance and enlargement of undesired signal; moreover, the hand feeling got better while steering. Finally, return control was discussed and the algorithm was designed. The constitution and influence factors of align torque and return resistant torque was prescribed and the target of return control was proposed according to work-energy theory. Regarding to EPS with steering angle sensor, the return control strategy based on PID and work-energy theory was discussed theoretically. By analyzing the steering wheel torque and rotational speed of free return process, that the steering wheel rotational speed and torque change rate being less than respective thresholds and rotational speed opposite to the steering torque direction was used to judge the return status, and the current variation combining return control with active damping control was designed. Furthermore, through designing return control algorithm, the test results proved that the above method could avoid misjudgment of return status, and the returnability of EPS was strengthened.