New Direct Drive Method For Large Telescope Base On Arc PMLSM

Because the traditional rotary motor can’t meet the dimension of the the nextgeneration large telescope, at the beginning of21st century, a new driving system forlarge telescope based on Arc permanent magnetic linear synchronous motors (ArcPMLSM) was developed. Compared with other traditional drive systems, Arc PMLSMsare manufactured piecewise and assembled on site, which can reduce the cost ofmachining and transportation. And the Arc PMLSM is a kind of brushless motor, whichcan avoid the commutation spark and the serious electromagnetic interference.But thisking of motor has disadvantage of large torque ripple. It induces a disturbance for thepositioning precision at low speed, which will affect the tracking accuracy for telescope.This paper focus on how to weaken the torque ripple of this motor, and then proposesome methods from the point of motor design and control methods to reduce the torqueripple.In this paper, a test model of Arc PMSLM used on the next generation largetelescope of China was designed, which was based on the theory of rotary motor andlinear motor. The work mainly include the mechanical design and electromagneticdesign, such as the analysis and calculation of the work point of the magnet,the designof the winding of the stator, the analysis and calculation of the coefficient of magneticdispersion, the mode of connection of the uint stator wingding, and so on.In order to decrease the torque ripple, some methods were adopted to optimize thestructure of the motor. Firstly, the slot effect was analysed and verified by the finiteelement method (FEM) analysis, and the slot force was decreased by adoptingfractional-slot structure. Secondly, the end effect was analysed and verified by the FEManalysis, and the end force was decreased by optimizing the length of the stator core andthe angle between the two stators, which was firstly proposed. Thirdly, the problem ofunbalanced three-phase winding was analysed, and by connecting different phasewindings of different stators, the problem of unbalanced three-phase winding wassolved. Compared with the primary three-phase winding, there were less harmonics inthe current. At last, the shape of the magnet was optimized, and compared with the traditional magnet, there were less harmonics in the back-EMF.On this basis, an Arc PMLSM servo system including hardware platform andsoftware algorithms was established based on DSP+IPM. The key technology includethe vector control method and the space vector pulse width modulation(SVPWM) werestudied. Based on the simulation model, the current loop and the speed loop of thecontrol system were developed. At last the disvantages of the M and T speed measuremethods were analysed, and the M/T speed measure method was adopt in this paper.In addition, some control methods were studied in this paper to improve thetracking accuracy of the servo system. Firstly, the forward feed control(FFC) methodwas studied, this method can decrease the phase delay of the control system. And thesimulation and test result show that this method can improve the tracking accuracy ofthe servo system to a large extent. Secondly in order to decrease the end force, a currentcompensator was added in the control system to compensate the torque ripple. After thismethods was adopted, the torque ripple of the motor was decreased. At last, in order toimprove the robustness of the servo system, the the slide mode control(SMC) methodwas studied. Compared with other control methods, the simulation and test result showthat this method can improve the ability of anti-interference for the servo system.To validate the optimization results and control methods, the parameters of the testmodel of Arc PMSLM were test, such as the torque ripple, the bandwidth of system, thetracking accuary and so on. And the test result show that the Arc PMLSM can satisfythe requirement of tracking accuracy for large telescope.