Research On The Control System Of Electrical Power Steering Based On Rapid Control Prototype

At present, the study on vehicle electric power steering system(EPS) is advancing front and hotspot in all the studies on vehicle steering system with the more and more demands for three goals of aotomobile safety, energy saving, environmental protection and fierce competition increasingly in aotomobile industry. The EPS is a very advanced aotomobile steering technology with low energy consumption, no pollution and easy assemble, furthermore, it controls assist power flexibly in all kinds of vehicle speed and road condition to make sure steering easiness at low speed and stability steering with road feel at high speed. As a result, it will become the vehicle standard equipment replacing the traditional hydraulic power steering system increasingly.The control system is not only the critical part of EPS, but also the important factor which can affect the EPS performance. EPS is a control target which is nonlinear and complex system with multi-input and multi-output(MIMO), at the same time, many demands of control performance indexs must be meeted, so the control effect is not good only by classical control algorithms. It is the main content in the paper to design the EPS control system for some vehicle which can meet the demands of all kinds of performance indexs, especallly meet the demand of assisting curve.Based on analyzing the dynamics of EPS, the mathematical models of important parts, such as linear two degrees of freedom vehicle including steering system, tyre and assist motor are established. As a result, multi-input and multi-output state space equation of EPS is established.Steering easiness and road feel are analyzed. The characteristics, type and applied range of assisting curve which can meet vehicle steering easiness and road feel are introduced. And characteristic parameters of assisting curve for some vehicle are calculated. Based on the above works, specific assisting curve for some vehicle is measured by test.In order to analyze the stability of EPS, transfer function from steering-wheel angle to assist torque is established. In order to analyze the effect on the system stability from road impact, transfer function from damped torque to assist torque is established, and differential link is applied to eliminate the effect on system stability from road impact.The control algorithm of target current output control and colsed loop feedback control based on signal filter is designed.As for signal filter, multi-function filter of magnitude filter and frequency filter is applied to filter noise and suppress interference. The noise of road roughness impact is filtered by magnitude filter, and electromagnet noise with high frequency is filtered by frequency filter.The different control algorithms are applied to target current output control according to different steering-wheel working state. There are three different working state for vehicle steering-wheel: assisting process, return process, damping process. During the assisting process, the motor target current is determined by vehicle speed and steering-wheel torque according to assisting curve, the fuzzy controller is designed to control the target current output for the nonlinear of assisting curve. During the return process and damping process, the proportion linear model controller is applied to control the target current output. The system model varies with variable assisting coefficient at different vehicle speed, so single PID control of closed loop feedback can't assure the system robust . The control parameters of PID are modified by fuzzy controller by using fuzzy-PID control of closed loop feedback to improve rapid response of system and decrease overshoot.After simulating the designed control algorithms with Matlab/Simulink in computer, in order to verify the reality control effect of the control algorithms, the EPS test table and vehicle are applied in the Hardware-In-Loop(HIL) simulation with rapid control prototype(RCP) method on the LabVIEW platform, simulation results show the control effect is good.After the control theory study and simulation, the concrete form of control system, namely ECU is designed. The ECU of EPS is made up of the circuit of motor drive and current direction change, the control circuit of motor drive signal, the drive circuit of alarm lamp and electric relay, the circuit of vehicle speed processing, etc. Embedded control system of EPS is programmed, this embedded system includes module of signal acquisition and processing, module of control algorithm, module of PWM voltage regulation output, module of assisting direction control, etc.Road tests are carried out with the vehicle equiped with electronic control unit which has been made. The results of road tests show that the control algorithms can improve the steering easiness, steering return performance and have good road feel for driver.