| Electric Power Steering system(EPS) is a new type of power steering system. It can solve the contradiction between handiness and sensitivity in the steering system and help to improve the steering performance and active safety, in addition, The EPS has the advantages about energy saving, environmental friendly, simple structure and easy installation, so once launched, the system has rapid developed. The assistant motor is the actuator of EPS, its torque output capacity and quality has a great influence on the performance of EPS. The early EPS products mainly adopt dc motor as assist motor, but the dc motor has some shortcomings such as large noise, low power density and poor reliability and so on, which limit the further improvement of the EPS performance. With the development of power electronics and motor control technology, the ac motor due to its better performance are applied more and more widely in EPS. In ac motor, the permanent magnet synchronous motor(PMSM) is gradually replace the DC motor and asynchronous motor and become the development direction of EPS power motor with the advantage of small volume, high efficiency, large power density and small inertia. The foreign famous EPS suppliers have launched their own EPS based on PMSM. However, for the disadvantaged of automobile electronic technology development, the domestic EPS products still is powered by DC motor and largely assembled in domestic minicar. Therefore, the development of EPS based on PMSM with independent intellectual property has positive significance for expanding the application range of domestic EPS products and breaking the market monopoly of foreign manufacturers.As a vehicle torque servo system, the EPS has higher requirements for the motor torque response: On the one hand, the assist torque can be quickly and accurately track the target torque, without sluggish feeling, on the other hand, the torque should be smoothing and ensure driving comfort. To meet the above requirements, the paper take research from the following aspects for the torque control of PMSM used in EPS: the torque control strategy, the predict current control under the vector control, field weakening control and inhibition of steering wheel torque fluctuation.The research contents of this dissertation are as follows:(1) The mathematical model and torque control strategies of PMSMFirstly, this article analysis the mathematical model of PMSM at different coordinates, and discusses the principle and realization method of vector control and direct torque control of PMSM. Finally, this paper compare the advantages and disadvantages of two kinds of torque control strategy with the requirements of the EPS to the assistant motor and choose the higher precision vector torque control strategy for the EPS control with the PMSM. At last, the paper analysis the influence of inverter nonlinear on the performance of torque control under the vector control strategy, and propose a on-line inverter nonlinear compensation method.(2) The predictive current control algorithm for permanent magnet synchronous motorUnder the vector control, the torque control is changed to two axis current controls in the synchronous coordinate system. The steady and fast tracking current performance is the basis of EPS torque control. Compared to the other control strategy, the predictive current control(PCC) strategy is more and more used in the control of PMSM for the perfect dynamic response and the lower current ripple. The performance of traditional PCC is affected by the parameter variations and with poor robustness. So the paper firstly study the parameter sensitivity for the traditional PCC and find that the stability mainly is influenced by the stator inductance and the stator resistance, inductance, and permanent magnet flux also affect the steady state performance of the PCC. Therefore, the paper established the system mathematical model of PMSM with the influence of the disturbance. And based on the model, the paper proposed a kind of adaptive robust predictive current control(ARPCC) algorithm, the algorithm use the luenberger observer to predict the motor current and improve the stability margin through the adjustment of observer gain. And design the disturbance observer for real-time estimation system disturbance and correction motor model, improve the prediction precision of the current and control precision. Experimental results show that the proposed algorithm can improve the robust performance of the system without a obvious dynamic performance degradation, which sets up the basis on later research.(3) The field-weakening control for PMSM used in EPSThe paper using field-weakening control algorithm to solve the issue of feels heavy in fast steering condition. The paper analysis the optimal current trajectory of PMSM considering the stator resistance voltage drop, and introduce the principle of feed forward and feedback field-weakening control method, and analyze the reasons of poor dynamic performance of feedback method, then propose a comprehensive field-weakening method based on feed forward and fuzzy PI. The controller is consisted with the improved feed forward element, fuzzy PI feedback element and the current trajectory planning element. The feed-forward link output d-axis current directly through online look-up table according to the motor running state to improve the dynamic performance of the field-weakening control. The feed forward element based on fuzzy is used to eliminate the feed forward errors quickly in the whole work ranges through online self-tuning PI based on fuzzy inference. The current trajectory planning module realizes the smooth transition from controlling d-axis current to the q-axis current through a simple q-axis reference current adjustment method to broaden the field-weakening depth of feedback control. Motor experiments show that the proposed method can make the motor output larger torque at high speed, and the two axis currents can follow its commands under sudden changes in motor speed, there is no obvious fluctuation in dynamic process, and show a good dynamic response performance.(4) The fluctuation suppression method for EPS steering wheel torqueThe major two factors for the fluctuation of steering wheel torque are the deficiency EPS system stability margin and the motor torque ripple. This paper firstly establishe the EPS system model and analyzed the factors which are affecting the stability of the system. The paper explained the principle and characteristic of speed feedback control and the common PD control in detail.The research using the speed feedback control to improve the phase margin of the middle frequency according to the characteristics of the permanent magnet synchronous motor. But the speed feedback control improve system stability margin at the cost of reducing the dynamic response performance of the system. So the PD+ speed feedback control method is used to improve the stability of the system without influence the dynamic response performance. Secondly, the paper analyzes the factors which lead to the motor torque ripple: cogging torque, back EMF distortion and the current detection errors, The harmonic injection method is used to suppress the 6 times harmonic torque caused by cogging torque and back EMF distortion idea of harmonic injection are adopted in the paper. The ripple torque caused by the current detection errors is eliminated with the current detection errors correction through a method based on integral in phase current about electrical angle. The experimental results indicate that the proposed method can reduce the ripple torque in steering wheel and improve the steering feeling.(5) The development of experimental environment and functional verification for EPSThe EPS controller is designed and the controller hardware and software architecture are optimized. The permanent magnet synchronous motor test bench is established to verify the motor control algorithms. Based on the real vehicle front suspension system, the EPS pivot steering test rig is established, and the measurement and control system is developed. Finally, the current control algorithm and the field-weakening control algorithm are verified with EPS experiments.The conclusions of the research are as follows:(1) The stability of traditional PCC is mainly affected by the stator inductance, and the change of the stator resistance, inductance, and permanent magnet flux will also affect the steady state performance of the PCC method. The proposed ARPCC can improve the system robust performance and eliminate steady errors without reducing the dynamic response performance of the PCC, that make the PCC more suitable for EPS power motor current control.(2) The voltage loop of feedback field-weakening control is a nonlinear system. The PI controller with fixed parameter cannot assure voltage loop has good dynamic performance at full speed ranges. In addition, the inherent hysteresis characteristic in the PI regulator is another important reason for reducing dynamic performance of the feedback method. Otherwise, the proposed method in this paper can improve the dynamic performance of the feedback field-weakening control without current ripple which happen under traditional method, and the motor can provide smoothing and sufficient assistance torque for the driver under rapid steering.(3) The stability margin deficiency and the power motor torque fluctuation can cause steering wheel torque ripples. The PD control and speed feedback control can improve system stability margin but all have shortcomings, and the PD + speed feedback control method proposed in the paper can enhancement the system stability margin without reducing the system dynamic response. The cogging torque and back EMF distortion result in the 6 times harmonic torque and current detection error lead to the 1 to 2 time torque ripples, and the proposed method based on the phase current integral and the harmonic injection method can effectively suppress the pulsating torque, and reduce the ripple torque in steering wheel. |
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