Power-assist Curve Design And Performance Evaluation Analysis Of A Passenger Car’s REPS System

Electronic Power Steering(EPS)system has replaced the traditional hydraulic power steering and mechanical power steering system due to its advantages in economy,environmental protection as well as adjustable power,and has become the first choice for passenger car.At present,Column-type Electronic Power Steering(CEPS)systems are widely used in passenger cars,but people pay increasing attention to the steering performance of vehicles with the improvement of people’s living standards,for which CEPS has already could not meet people’s requirements due to its shortcomings at steering stiffness and high noise.Rack-type Electronic Power Steering(REPS)system transfers the steering power directly to the rack through the reduction mechanism,reducing the overlapping of torque transmission path and the motion transmission path,improving the stiffness of the steering system.And the placement of the booster motor is far away from the cockpit,avoiding the shortcomings of loud motor noise during the steering process,which is more advantageous than CEPS system.The article first briefly introduced the structure,classification,advantages and disadvantages of different EPS systems,analyzed the current research status of EPS systems at domestic and abroad,and found that the current research results mainly focus on CEPS systems,and there was very little research on REPS systems,so the main content and research purpose of this paper were put forward.From the perspective of forward drive and reverse drive of the steering system,the equivalent stiffness of the steering wheel and the equivalent stiffness of the steering gear pinion were theoretically deduced to analyze the performance’s difference between the REPS system and the CEPS system.The results showed that the transmission relationship between the steering wheel angle and the wheel angle of the REPS system is more stable,and the equivalent stiffness of the steering wheel is less affected by the boost gain,which can effectively avoid steering wheel torque fluctuations.At the same time,the REPS and the CEPS systems’ relationship between rack stroke and the steering wheel angle were tested on the steering gear platform in order to analyze the steering performance when the steering wheel is near the middle position.The test results showed that the relationship between the rack stroke and the steering wheel angle of the REPS system is more linear and the slope is greater,which represent the initial response of the REPS system is better.Secondly,the advantages and disadvantages of the traditional boost power-assisted curve of the EPS system were analyzed.And the paper designed a new basic power-assisted curve combining straight line and curve based on the REPS system.At the same time,the high-frequency gain curve,blending frequency and return-to-correction power-assisted curve were also designed,which is carried out by real vehicle calibration.Finally,the steering performance of the test vehicle equipped with the REPS system and the benchmarking vehicles equipped with the CEPS system was evaluated objectively and subjectively.The objective evaluation mainly included steering portability,steering returnability,performance in the middle of the steering wheel,and steering accuracy.The results showed that the test vehicle’s steering portability,steering returnability,performance in the middle of the steering wheel and steering accuracy are significantly better than those of the benchmarking vehicles.Subjective evaluation mainly contains four first-level evaluation indexes,such as low-speed maneuverability,straight maneuverability,lane change maneuverability,and corner maneuverability,whose corresponding second-level and third-level evaluation indicators are set up.After processing the index scores at all levels,it is concluded that the low-speed maneuverability,straight-going maneuverability and corner maneuverability of the test vehicle are better than those of the two benchmarking vehicles,while the lane change maneuverability is slightly inferior to the benchmarking vehicle 2.The comprehensive steering performance of the test vehicle is even better.