Research On Disturbance Rejection Technology Of Ground-based Large Telescope Control System

Large aperture telescope is a high precision observation equipment for space exploration,which is widely used in astronomical observation,laser communication,target tracking and other fields.With the improvement of target observation accuracy in space exploration,telescopes are required to have stronger optical properties,so as to achieve accurate observation for small,faint and distant targets.In engineering,one of the most important methods to enhance the concentrating ability and resolution of the telescope is to improve the aperture of the telescope,but this will result in higher requirements for the servo control system of the telescope.With the expanding of telescope aperture,the rotational inertia and external load disturbance also increase,which requires the servo control system to provide more power and torque to maintain the normal operation of the motor.The lower power density of conventional brushless direct current(DC)motors indicates that larger volume is required to drive high-power loads,which affects the imaging optical path of the telescope.In addition,the mechanical spark inevitably exists in the commutation process,which not only affects the service life of the motor,but also leads to nonlinear torque to the motor control system and reduces the tracking performance of the servo system.The servo control system based on DC motor can no longer satisfies the requirements of large telescope tracking.Therefore,permanent magnet synchronous motors(PMSM)are widely used as servo driving mechanism of large telescopes in academia and industry.PMSM has high power density and can be equipped with corresponding cooling facility to eliminate the heat noise in high-power applications.Meanwhile,it also has the advantages of high torque ratio and excellent low-speed tracking performance,which can greatly improve the tracking accuracy of the telescope.For the research of PMSM servo control system,foreign countries started earlier,and the technology is more mature,although the domestic research in this field has made some achievements,generally there is still a gap.In order to improve the servo tracking accuracy of large telescope,this paper takes PMSM as the background,explores and analyzes the relevant factors affecting the tracking accuracy of large telescope,and develops corresponding control strategies according to the existing engineering problems of permanent magnet synchronous motor.The merits and demerits of servo control performance are mainly reflected in three aspects: the dynamic response of the system,the steady-state accuracy,and the robustness to external load interference.The main factors affecting the dynamic response of the control system are model uncertainty and time delay.The uncertainty of the model will lead to that the zero points in PI controller cannot cancel with the poles in the controlled model,so that the closed loop of the control system cannot reach the ideal first-order inertia link,thus producing overshoot and oscillation.In addition,the PMSM control system inevitably has time delays,which will lead the system to reach the steady state after a long adjustment time,thus affecting the dynamic response of the system.The main factors affecting the tracking accuracy of servo system are time variation of dead effect of inverter,current sampling error and torque pulsation generated by motor.These disturbances are usually equivalent to periodic nonlinear disturbances,which are equivalent to a second-order elements through Laplace transformation.Therefore,it is difficult to eliminate their influence on steady-state accuracy by using traditional linear PI controller.The external load disturbance is usually equivalent to the slow time-varying linear disturbance,and the steady-state error can be eliminated by using the traditional control method,but it usually needs a long transition process to reach the steady state.According to the above analysis,it is difficult to achieve precise control of the telescope servo system by using traditional control methods due to the comprehensive influence of various disturbances.Therefore,the disturbance rejection control technology of large telescope based on PMSM is studied.By designing an advanced control scheme,the dynamic response and tracking accuracy of the telescope servo system are improved,and the anti-interference capability of the control system is also enhanced.The research of this paper can be summarized as follows:(1)Based on the mathematical model of permanent magnet synchronous motor,the velocity loop dynamics equation and current loop voltage equation are established,and the disturbance from various source is analyzed.The principle of Space vector pulse width modulation technology(SVPWM),coordinate transformation,vector control(VC)algorithm is introduced,and the Matlab/Simulink simulation of telescope spindle based on PMSM is established.(2)The mathematical model of the two-mass spring of the servo spindle system is established,and the influence of the locked rotor frequency of the mechanical system on the bandwidth of the control system is analyzed,which provides guidelines for the controller design.On the basis of VC technology,the speed and current PI controllers are designed,which verifies the feasibility of the classical algorithm,analyzes the limitations of the traditional algorithm,but also provides a theoretical basis for the realization of PMSM digital control.(3)When the servo system of large aperture telescope operates at low speed,the torque ripple in the control loop will seriously affect the tracking accuracy of the control system.It is difficult to suppress torque ripple effectively by using a single control algorithm owing to the fact the torque ripple has periodic characteristics;In order to suppress the torque ripple at low-speed condition,a simple and effective method is to combine PI with resonant controller(PI-R),which utilizes the high amplitude gain of resonant controller at the central frequency to suppress the periodic disturbance in the frequency domain.However,the traditional PI controller has poor compensation effect for unmodeled and load disturbances.Therefore,the robustness of the controller to the lumped disturbance should be considered while compensating the torque ripple.In order to overcome the shortcomings of the traditional control algorithm,an improved adaptive robust sliding mode algorithm based on resonant control is proposed in this paper.The traditional adaptive sliding mode controller is extended to the case of periodic disturbance,and the torque ripple is suppressed by using a series-structure resonant controller.Firstly,a typical sliding mode controller is designed to ensure the speed tracking error converge to zero and obtain a certain anti-interference ability.Then,the improved adaptive law is used to estimate the load disturbance and torque ripple.The improved adaptive law can more accurately observe the periodic pulsation in the system because of the embedded resonant controller.Finally,the observed torque ripple and other disturbances are feedforward to the current loop and compensated online.(4)The reasons for disturbance of large aperture telescope servo system include uncertainty of controlled object,parameter time-varying and wind-borne disturbance.These factors will worsen the servo tracking performance of the telescope and even cause the instability of the control system.Based on disturbance analysis of PMSM,a robust two-degree-of-freedom controller(2DOF)controller based on extended sliding mode parameter observer is proposed in this paper.The robust 2DOF controller is simple in structure and can realize preset dynamic response,and can also suppress unmodeled interference by setting the bandwidth of the embedding filter.The sequential iterative learning module embedded in the forward channel of the controller can suppress the periodic torque ripple in the system and maintain a good dynamic response.In order to ensure robust stability of the system,an extended sliding mode observer is introduced,and the mechanical parameters of the motor can be obtained by acceleration and deceleration method,which is used in the design of robust internal model controller.In addition,the disturbance of the system can be further compensated by using the sliding mode parameter observer.(5)When the large aperture telescope servo system runs at low speed,the torque pulsation in the loop will seriously affect the tracking accuracy of the control system.Torque ripple is difficult to be suppressed by a single control algorithm because of its periodic characteristics.The periodic harmonics in the current loop(torque loop)will result in the velocity fluctuation of the same frequency,which will affect the tracking accuracy of the speed loop.The application of fractional calculus in the control system is systematically introduced,and a robust internal model control algorithm based on fractional order vector resonant controller is proposed.The improved resonant controller is embedded with inverse of the controlled plant and fractional calculus operators,which can reduce the phase delay and improve the gain and phase margin,so as to suppress harmonics more effectively.In addition,a robust internal model controller is designed to improve the robustness of the controller to slow time varying disturbance.The proposed control algorithm can not only suppress the unmodeled interference in the system,but also reduce the influence of certain sub-torque harmonics on the speed loop.(6)The arc splicing motor used in large telescope is a special case of the linear motor.Therefore,the relevant research of permanent magnet linear synchronous motor(PMLSM)is carried out.Based on the analysis of the mathematical model of linear motor,a thrust ripple observer is designed,and the stability of the observer is also analyzed by Lyapunov function.The simulation results show that the designed thrust ripple observer can accurately observe the thrust harmonics,and the thrust ripple can be effectively suppressed after the observation results are feedforward to the current loop.