Achievement Of Low-cost EPS Control System And Optimization Of Control Strategy

Electric power steering (EPS) is a state-of-the-art technology in auto industries. Compared to convertional hydraulic power steering (HPS), EPS has the superiority of lower fuel consumption, simpler structure, easier assembly process, and better controllability. EPS are thus being replacing HPS extensively. Therefore, research and development of EPS system with lower cost and better performance is a hot point of the auto and motor industries.In this thesis, development of the EPS system, particularly the EPS motor and control system, is reviewed, including the design of EPS motor, design of EPS motor controller, power electronic drive, and the strategy of EPS control. Then, the charateristies of an EPS motor used in the laboratory test system are introduced. Design of control system, including control and drive unit, position sensor unit and current sensor unit, as well as some particular design issues, are also discussed and detailed.A simulation modle with dual-closed-loop control is established on Matlab/Simulink platform in order to validate the field oriented control (FOC) of the EPS motor drive. The control system is then implemented on the laboratory test system with the DSP program which is designed based on the simulation modle. Meanwhile, in order to optimize the EPS motor performance, three pulse width modulation (PWM) techniques, viz, sine-wave PWM (SPWM), space vector PWM (SVPWM) and third harmonic injected PWM (THI-PWM), are comparatively studied on the control system. Moreover, two control strategies, viz, adaptive PI control and constant-parameters PI control, are also investigated. It is seen that the control system employing the adaptive PI control and SVPWM can exhibit an outstanding operation performance. In addition, a new FOC technique, in which the phase current sensing and the abc-dq0 frame transformation are not required, is presented in the thesis. It uses the q-axis current (i_q), which is directly obtained from the DC-link current for FOC. Therefore, the cost of the system and the calculation load of the DSP are low. Furthermore, the problems caused by the uneven measurement gain for different phases and the low measurement accuracy caused by the current pulsation do not occur any more. Thus, the control system employing this new FOC can perform well.