Research On Compensation Control Strategy Of PMSM EPS

Electric Power Steering System (EPS) has the advantages of energy saving，environment friendly and assistant torque varying with vehicle speed over traditionalHydraulic Power Steering System (HPS), EPS has the tendency to replace ECHPS withthe shortage of energy becoming serious increasingly. Compared with brush motor,Permanent Magnet Synchronous Motor (PMSM) has the advantages of bigtorque-inertia ratio, high efficiency and torque smoothness; therefore, PMSM EPS hasthe tendency to replace brush motor EPS in the future.The basic assistant control strategy of EPS is designed to solve the power steeringproblem at the steady state, while the dynamic performance and the return to centerability of the steering system should be improved by the compensation control strategy.The dynamic performance of the steering system is deteriorated by the assistant motor,which augmented the system inertia. This paper derived the rotor angular velocity signaland angular acceleration signal from the rotor angle signal through Kalman filteringmethod, and the inertia compensation was done with the derived angular accelerationsignal and the necessity of inertia compensation in PMSM EPS was also analyzed.Further more, the torque ripple during the steering process was suppressed with theangular velocity signal, thus improving the dynamic performance of the steering system.The design of the EPS control strategy has close relations with the vehicle, in orderto shorten the development cycle, improve the development efficiency and reducedevelopment cost, an accurate vehicle model is necessary, so as to do simulationresearch. Taking Chery A3as the research object, a column type EPS (C-EPS) vehiclesimulation model was built through Carsim vehicle dynamics simulation software andSimulink software, and the joint simulation model was verified through experiment. Theparking torque is important for the design of EPS assistant curves, but the theoreticalvalue is usually small than the experimental value. The torque equilibrium equation andthe tire-surface sliding friction coefficient were analyzed, thus solving the problem.Returnable control is an important content of EPS compensation control, generallythe steering wheel can’t return to center under low vehicle speed because of the frictiontorque, while there exist overshoot under high vehicle speed. This paper attempts to improve the vehicle returnabilty by adjusting the system equivalent damping. Firstly thealigning torque was analyzed and the steering wheel movement equation after the driverloosening the steering wheel was deduced, secondly the desired damping of the wholesteering system was calculated according to energy conservation principle, then thecompensation coefficient in the returnable control can be determined according thedesired damping and the real damping. Finally the design returnable control strategywas verified through simulation and experiment.