Torque Optimization Research Of In-wheel Motor

With the energy and environment issues becoming more prominent, the electric vehiclebecomes the main development direction of automobile industry in the future. Its drivingmotor, which is divided into centralized driving motor and in-wheel motor, is the key poweroutput part, just as the engine in the traditional vehicle. Comparison with traditionalcentralized driving motor, the transmission system can be simplified and the mechanicalefficiency losses are deduced with the in-wheel motor. Meanwhile, each wheel can be drivenindependently and thus the dynamic control is easy to achieve. But the output torque islimited by the space in the wheel and is hard to meet the torque demands of vehicle. So inthis paper, the motor control strategy is researched in the aspect of torque optimization. Theinfluences on output torque are analyzed and the strategy of torque optimization is obtained.With the optimization, the proper torque of motor is generated as more as possible on thecurrent technology condition.For now, the motor control theories are focusing on the influence reducing on thecontrol system stability by the varying characteristics of motor parameters. But the motortorque decline problem caused by the varying motor parameters is less concerned. The motorcan’t generate the whole torque because of the inaccuracies field orientation which could becaused by varying motor parameters. In addition, the motor output torque is also affected bythe Pulse Width Modulation (PWM) signal frequency in the motor inverter. So from theperspective of optimizing the torque output, the variable PWM frequency control strategyand motor timing control strategy is researched in the paper.At first, the present research and application situation of in-wheel motor are introduced,and the types of in-wheel motor are analyzed comparatively. Finally the Permanent MagnetSynchronous Motor (PMSM) is chosen in the paper. Because the PMSM has higher powerdensity and higher torque at low speed which are the obvious advantages in the in-wheelmotor application. Then the motor theory development situation is elaborated, and based on the summarization of various research results, the torque optimization methods for in-wheelmotor are proposed.The two basic control algorithms of PMSM, the space vector modulation control andthe direct torque control, are analyzed comparatively. As the space vector modulation controlhas small torque ripple and could get full load output, which is more suitable for thein-wheel motors. The motor torque optimization algorithm is researched based on spacevector modulation control in the paper. Firstly, the torque optimization strategy is studiedthrough variable PWM frequency control. Based on analyzing the influence of PWMfrequency on motor performance, the conclusion that lower PWM frequency decreases thelosses on the motor inverter and getting higher output torque at low speed by utilizingharmonic torque is obtained. Then the corresponding variable frequency control strategy isdesigned to improve motor performance at low speed. Secondly, the torque optimizationstrategy is studied through timing control. The influences on field orientation accuracy areanalyzed, which include the influence of position sensor accuracy, varying motor parametersand commutation delay angle. For the complex and nonlinear disturbance, the controlstrategies based on direct-axis current feedback and neural network feedforward areproposed. By comparing the strategies, the neural network control is more suitable for timingcontrol, and the motor torque output is improved at high speed with the design purpose ofzero direct-axis current.A motor simulation model is built with Simscape, a commercial toolbox in Simulink.And a PWM signal generator with dead band module is built based on actual operatingprinciple of Motor control chip to simulate actual motor control process accurately. Thetorque optimization strategy, PWM frequency conversion control strategy and angle timingcontrol strategy, is proposed in this paper. The two strategies are verified in the simulationand the result shows that the low speed output torque of motor is increased with PWMfrequency conversion control by utilizing harmonic torque, and the high speed output torqueof motor is increased with timing control by decreasing commutation delay angle throughneural network Control algorithm. Finally, the torque optimization control combined with the two control strategy is simulated, and the result shows that the optimization strategyincreases the motor output torque in the whole speed range and the performance of the motoris improved.In order to verify physical model quickly, a rapid prototyping development platform formotor controller based on Simulink is built. Through the study of the mechanism of rapidprototyping technology in Matlab, a rapid prototyping method is obtained that can be usedwidely in all kinds of micro controller chip. This method, based on writing the system targetfile, can make various micro controller chip supported by the automatic code-generationfunction of Matlab, no longer will it be restricted by the type of chip. The method is anextension to the original Matlab rapid prototyping technology, to make the Matlab rapidprototyping technology no longer confined to the common chip platform. For any kind ofcontroller chip, this method can be used to make the chip supported by the Matlab rapidprototyping technology, which enables researchers to focus more energy on the research ofcontrol algorithm, instead of writing embedded codes. Based on the motor controllersolutions of Texas Instruments (TI) Company’s C2000series, the rapid prototyping design ofin-wheel motor controller is accomplished with the method, and rapid prototyping platformis set up aiming at the solution. The AD2S1210chip is used to decode the position sensorand the AD7606chip is used to measure current sensor output. Using the model of vectorcontrol algorithm which has been validated by computer simulation, the correspondingembedded C code is generated, and the rapid prototyping of in-wheel motor controller isachieved.Based on the motor controller designed, a motor test bench is built. The characteristic ofmotor is tested in this bench and the corresponding torque optimization strategy is proposedin this paper. Tests are also carried out to validate the optimization strategy. The Resultshows that the designed torque optimization strategy promotes the accuracy of fieldorientation of vector control and so the performance of motor.The factors that can affect the output torque of motor are analyzed, and thecorresponding torque optimization strategy is designed aiming at the factors. The Results of simulation and test show that the optimization strategies can promote the output torque andlay foundations for the applying of in-wheel motor in electric vehicles.