| A pedelec measures its movement state through the torque sensor that detect the torque on the pedals and its speed,and adjusts the torque output of the motor accordingly.This ensures sound adaptability of the bicycle to various road conditions and as a result great riding experiences for users.The power motor in a pedelec and its control performance are of overriding importance since they are the key factors that determine the overall performance of the bicycle.Permanent magnet brushless DC motors,in particular,outperform other types of motors for its small size,light weight,and low prices and are widely used on electric bikes.Compared with traditional BLDC motor,sensorless BLDC motor when used can further reduce the motor volume,weight,and cost and enhance the reliability of the whole system to some extent.Therefore,study of the control strategies of high performance sensorless BLDC motor is of theoretical and practical significance for the application of electric bikes.Besides,the assist strategy of motors in pedelecs is another important factor that has impacts on riding experiences.Unlike the traditional assist strategy with an assist ratio of1:1,assist strategy featuring dynamic assist ratios that takes into account such factors as riding intention and environment can help pedelecs to provide a better riding experience.Thus,it can be said study of such assist strategy also has great value both theoretically and in application.It is those two issues,namely the algorithm of high performance sensorless BLDC motor and the new assist strategy featuring dynamic assist ratios,that are studied.First,the mathematical model,basic principles,and the traditional control methods of sensorless BLDC motor are given,followed by the introduction of a new control method that takes the advantages of line back EMF method and sliding mode observer method.By comparison,this proposed method does not need to construct the BLDC motor mid-point,rids of the delay circuit,and has good parameter robustness.It can also adapt to a wider speed range and effectively reduce the sliding mode vibration.The simulation and experiment results show that the proposed sensorless control method of BLDC motor can accurately obtain the line back EMF and its commutation point and has good control effect to meet the application requirements of pedelecs.Second,the principle of fuzzy control is introduced and based on which,a new assist strategy that takes into account riding intentions and riding environments is proposed.This new assist strategy includes two parts: static coefficient adjustment and dynamic coefficient adjustment.The static coefficient adjustment is achieved through the calculation of the physical parameters of the pedelecs,while the dynamic coefficient adjustment is based on the fuzzy control.These two parts together determine the assist ratio of the pedelecs.The simulation results show that,compared with the traditional assist strategy with a assist ratio of 1:1,this proposed method can dynamically adjust the assist ratios according to changing riding intentions and environments,so that better riding experiences can be guaranteed.Third,the overall control system and control method of pedelecs are introduced,the overall system of a moving pedelec is mathematically modeled,and the selection techniques for motors are provided.The simulation models of pedelecs on various road condition are also built and simulation analyses are conducted.The analysis shows that the proposed motor control strategy and the assist strategy with dynamic assist ratios can quickly respond to different riding intentions on a variety of road conditions to meet the need of pedelecs control and provide better riding experiences.Finally,a BLDC motor position sensorless control system is designed and tested.The results verify the effectiveness of the BLDC motor position sensorless current(torque)control strategy and a new assist strategy based on the principle of fuzzy control. |
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