Study On Development Of Electric Power-Assisted Steering System

EPAS (Electric Power-Assisted Steering) adopts motor to provide assisttorque, which is controlled by ECU (Electric Controller Unit) according to thevelocity of the vehicle and the torque of the steering wheel. With thedevelopment of automobile industry and the popular using of automobile indaily life, people are not content with using a Car as a tool instead of walk,but to purse an driving experience with intelligent, safe and comfortable. Atthe moment, after electric technique being applied to engine, transmissionand brake system successful, EPAS is going to be perfect. Almost all thefamous automobile companies abroad have developed their own EPAS andhave the ability to volume-produce EPAS. Only in the recent years, theresearch on EPAS is carried out in china, for many reasons. But it springsup like mushrooms. EHPS (Electric Hydraulic Power Steering) is going to bereplaced by EPAS, which is one of future development direction of powersteering.This paper focuses on the hardware design and parameters calibrationof EPAS, with which the experiments carried out in the bench of EPAS androad test, the author study on the control strategy systematically, andpropose a reasonable and innovational method on target current estimate,CAN calibration protocol and numeric filter for EPAS.The ECU of EPAS is adopted an 8-bit microcontroller produced byFreescale semiconductor. MC68HC908GZ60 is suitable for automotiveapplication, and it's a highly-performance, low-cost device which comespacked with valuable features -16 channel, 10-bit analog to digital converter;8 channel, 16-bit pulse-width modulator and a MSCAN08 (controller areanetwork 2.0 ). Because of the low-cost and highly integrated performance, itwill reduce the quantity of device on the board, and make the productcost-effective. And then, the power source circuit, power drive circuit,predrive circuit and signal modulate circuit are most adopting the device withvery good EMC (electric magnetic compatible) performance suitable forautomobile and industry area, which is most useful for the process ofproduct application.The assist characteristic of EPAS is the rule that assist torque ischanging with the motion of automobile (such as steering handwheel torque,vehicle velocity). Assist characteristic in theory should meet the requirementof steering maneuverability and road feel synchronous. Straight-line type,broken-line type and curve-line type assist characteristic are discussedsystematically. Based on the request of the driver for the ideal relationshipbetween steering handwheel torque with vehicle velocity, by changingstraight-line type assist characteristic coefficient can make steeringhandwheel torque at different lateral acceleration agree with the request ofthe driver at a certain velocity, then the formula of assist characteristiccoefficient at different velocity is fitted. Using the parameters of Md, Mdmaxdefined by assist characteristic and the coefficient at different velocity, thevalue map of decision-making target current is derive from the interpolationanalysis in Matlab, which lays the foundation of study on control strategy.Using the embedded CCP protocol for ECU calibration in the interfaceto peripheral will make the process of calibrating for PID parameter simpler.And then, CCP module will transmit the variety sampling parameter to upperPC at 250kbps velocity by CAN bus, which is more faster and highlyperformance than the sampling process solved by serial communicationinterface.Proposing a novel fast filtering algorithm based on weighted median forthe signal of sensor. Because amounts of electromagnetic noise of engineand impact signal of road surface exist in the original signal simultaneous,so this paper applies a suitable numeric filter technology to preserve impactsignal on an 8-bit microcontroller with short filter periods. With the analysisof the sampling data from CCP module in Matlab, we can prove that thenovel filter algorithm is effective than the common numeric algorithm onmaintaining the impact signal of road, reducing the size of filter window andcosting few operation instructions.Analyzing the experiment result based on the bench for EPAS and pilottest is carried out. Through the experiment carried on the bench of EPAS toevaluate the assist performance and simulate the impact of road surface,the result show that the assist current follow the steering handwheel torquestably and the impact signal of road is preserved efficiently.