Research On Low-speed Servo System Of Large Telescope Based On Permanent Magnet Synchronous Motor

Telescope a remote observation system with high precision,in which a variety of techniques are integrated,such as optical,mechanical,electrical control and image processing,et al.Telescope plays a very important role in the areas of celestial object observation,laser communication,monitoring and recognition of space artificial targets.With the development of the techniques on astronomy and space target observation,the faint and remote targets need to be tracked and monitored accurately in real-time.The telescope is required to have much stronger space observation capability.In order to guarantee a greater ability in gathering light and higher resolution,the diameter of telescope is required to increase.At the same time,the telescope servo system,which performs the tracking task,is also required to have a tracking accuracy within arcsecond.The increasing telescope diameter leads to a larger load on the drive motor in the servo system.As a result,the motor is required to provide a bigger torque to drive the telescope system.Generally,the motor torque needs to reach 10~5N.m.But the brushed motor,which has the corresponding drive ability,is large in volume and has poor ability in heat dissipation.Meanwhile,the nonlinear perturbations caused by mechanical commutation,lead to a bad low speed performance.Therefore,it is incompetent to drive the large telescope tracking system with high precision.The AC permanent magnet synchronous motor(PMSM)is the best candidate for the replacement of DC motor.It has the characteristics of high power density,high torque inertia ratio,high reliability and excellent low-speed performance.The PMSM has been employed in many large telescopes abroad,working as the actuator.But for the telescopes at home,the usage and application techniques of PMSM are relatively immature.Thus,making research on the telescope servo system,which is driven by PMSM in low speed,has great significance in engineering.In order to improve the whole performance of the telescope,the main factors that affect the control precision should be analyzed,and the corresponding control strategy should be designed,simultaneously.Tracking and pointing accuracy are the main indexes for performance evaluation of the telescope servo system,.In the traditional telescope tracking control methods,the nonlinear factors such as the time-varying parameters and disturbance torque,are not considered in the controller design.A relatively simple control structure can be obtained by the neglection of disturbance.However,it reduces the control accuracy level,which the telescope servo can achieve.In the process of actual target observation,the telescope is influenced by many kinds of disturbance factors inevitably.The model uncertainty perturbation caused by the parameter perturbation,the torque ripple of drive motor,the friction torque disturbance,and the external wind disturbance,are the dominate disturbance terms.They lead to an increasing tracking error in the telescope servo system.This article mainly makes research on the low-speed servo system of PMSM,which is used as drive motor in a certain type ground-based optical telescope,in Changchun Institute of Optics,Fine Mechanics and Physics.Advanced control strategy is proposed,for the purpose of tracking error reduction as well as improvement of the anti-disturbance ability of the telescope servo system.The work is divided into four aspects:(1)The structure and characteristics of PMSM are first introduced.Mathematical models are established,including voltage equation,torque equation,magnetic chain equation and motion equation.The principle and realization of the basic control method of PMSM are described,including space vector pulse width modulation,coordinate transformation and vector control.Finally,simulation analysis is made on the main axis servo system of telescope in Matlab/Simulink.The step speed and equivalent sine signal are set as the reference signals.Based on the results of speed and current response in simulation,the following conclusion can be made:the system has the characteristics of small fluctuations,fast response and good stability and reliability.The feasibility of the basic control method is verified,and the simulation results provide a foundation for the PMSM hardware system design.(2)The control model of the main axis servo system,is presented as a equivalent two-quality spring model.The causes of mechanical resonance are analyzed based on the control model.Meanwhile,the factors which have effect on the distribution of mechanical resonant frequency are also studied.For the controller design of advanced control strategy,a simplified dynamic model of the main axis servo system is proposed,in which the mechanical flexible coupling is not considered.In order to obtain the frequency characteristic of system,open loop test is conducted and the sinusoidal sweep frequency signal is employed as excitation of the main axis servo system.The test data are processed by spectral analysis method,and the transfer function is calculated based on the power spectral density function.The distribution of locked rotor resonant frequency can be identified from the frequency characteristic curve of the system.The control model and locked rotor resonant frequency provide theoretical basis for the following system controller design.(3)There are many kinds of comprehensive disturbances factors in the actual telescope servo control system,and the tracking precision is affected by them.The disturbance factors include model uncertainty disturbance,friction torque disturbance,drive motor torque ripple,and external wind disturbance,etc.Considering the ability of restraining the comprehensive disturbances,a control strategy with systemic anti-disturbance ability should be designed,to improve the tracking precision of the servo system.Therefore,the influences of above disturbances are analyzed first.For the sake of improving the robustness of the main axis servo system of telescope,sliding mode control and disturbance observation and compensation technology are combined in the proposed strategy.For the purpose of solving the contradiction between the reaching time and sliding mode chattering,a novel reaching law is proposed.The dynamic response speed of sliding mode controller is also improved by the novel reaching law.To make an improvement in the control performance of the whole system,the current and speed loop controller are both designed as sliding mode controllers with novel reaching law.Based on the characteristics of sliding mode control,a sliding mode observer is introduced to estimate the system disturbances online.The estimated value is utilized to compensate the current and speed controllers in order to eliminate the effects of the disturbance,and it can suppress the sliding mode chattering phenomenon simultaneously.In the presented composite strategy,disturbances of the system are all considered as the external disturbance torque.Then,a strategy with strong robustness is proposed to make an improvement in the control precision.The experimental results demonstrate that,with the employment of composite strategy,the RMS value of tracking errors of ramp signals with different slopes are 0.0083"and 0.021",respectively,and the RMS value of tracking errors of sine signal is 0.13".Compared to the PI control strategy,the position tracking accuracy is increased by 44.67%,57.1%and 43.5%,respectively.(4)In the low speed servo system,which is driven by PMSM,the periodic torque ripples of the motor is ubiquity and may have a great influence on the tracking precision.The disturbances characteristic of periodic are not considered in the above composite stragy,and the inhibition ability to the periodic disturbance is limited.The periodic torque ripples problem is detailed studied in this part.An anti-disturbance stragy based on robust iterative learning control is designed,in order to improve the control performance of the system which is greatly affected by periodic torque ripples.Robust iterative learning control improves the low speed tracking precision of the system mainly from the aspect of reducing the harmonic torque.The main causes of torque ripples in PMSM control system include flux harmonics,cogging effect,dead zone effect of inverter and current measurement errors,and are analyzed in detail.The corresponding torque ripples produced by the aforementioned factors are also described.The analyses indicate that periodic torque ripple of the motor will further lead to the pulsation of turntable speed at the same frequency.The solution methods of torque ripple are introduced systematically and the characteristics of different methods are studied.A robust iterative learning control(robust ILC)strategy achieved by sliding mode control is proposed,the advantages of sliding mode control and iterative learning control are both inherited.The system disturbances are divided into the periodic and non-periodic disturbance terms.The ILC is employed to inhibit the periodic disturbance,i.e.,periodic torque ripples.Meanwhile,the sliding mode control acts as the robust part.Sliding mode control deals with the parameter variations and external disturbances,to ensure the global asymptotic stability and speed up the system response.The robust iterative learning control can gurantee the robustness of the system,while effectively suppress the periodic speed fluctuation in the telescope system.The experimental results demonstrate that,with the employment of robust ILC,the system speed tracking accuracy is increased by 36.67%,the position tracking accuracy of ramp and sine signal are increased by 55.56%and 50%,respectively,compared to the PI-ILC method.