Research On BLDC System And Sensorless Control Strategy For Power Tools

At present,the brushless direct current motor(BLDCM)occupies a rising share of the power tools market due to its simple structure,high power density,high efficiency and stable operation.Appropriate controlling methods are of great significance to the improvement of BLDC's controlling performance.As one of BLDC's key technologies,sensorless controlling method is just an important issue in study of motor drive.Based on the engineering practice of portable power tool,this dissertation open up deeply study with BLDC's sensorless controlling method and controlling strategy of power tool.As a result,a new sensorless controlling method and several new controlling strategies of power tool are proposed.Experiments are carried out after fininshing the design of prototype.According to the shortcomings of conventional sensorless controlling method,a new method of rotor position detection is given at first.By means of theoretical analysis,a conclusion that power switches' body-diodes have great influence on accuracy of position detecting signals with low bus voltage is draw.What's more,it's confirmed that body-diodes make position detecting signals invalid with discontinuous phase currents.Taking existing problems into account,a new position detecting method based on the synchronous rectifier technology is proposed.With three-stages start-up method,the sensorless operation in full-speed-range is achieved.Secondly,considering the complex working conditions and demand of low cost,several control strategies are designed according to the practical application requirements in start-up,const-speed running and braking.In terms of the situation with frequent locked-rotor operation,inpluse current startup strategy is taken.By selecting the parameters of current pulse properly,impacting start-up is achieved with high torque output of motor.Considering high-inertia-load situation,the quasi-constant current controlling method for feedback braking is put forward.Compared to the converntional method,peak value of bus current can be controlled without setup of more current sensors,and braking performance is good as well.Finally,the construction of experimental platform is finished.With the experimental results,the feasibility of sensorless controlling method and system controlling strategies are validated.