| Large optical telescopes play an important role in exploring the nature of dark energy,observing the first generation of celestial bodies,and observing the atmospheric composition of habitable planets outside the system.As the aperture of the telescope increases,the Stewart platform as the secondary mirror adjustment mechanism becomes a key component in the construction of large telescopes.Compared with other driving methods,the motor drive mode of Stewart platform has many advantages,such as low development and processing cost,fast response speed,strong programmability and easy maintenance in later period,and the permanent magnet synchronous motor(PMSM)has become the first choice for the drive motor of the servo control system of the Stewart platform,because of its high power density,good dynamic performance and high reliability.The larger the aperture of the large telescope,the greater the disturbance caused by the time-varying load of the Stewart platform.However,domestic research on the time-varying load of the servo control system of the Stewart platform was relatively incomplete.Therefore,this paper conducted in-depth research on the key technologies and related control strategies of the Stewart platform servo control system.(1)According to the motion mechanism of the Stewart platform,the joint space control scheme was determined for the servo control system of the platform.The mathematical model of permanent magnet synchronous motor was established,and the mathematical expressions of the transfer functions of current loop,speed loop and position loop were derived according to the mathematical model and vector control strategy.According to the classical frequency domain method,the controller parameters of each loop of the control system were determined and the simulation model of the servo control system was set up for analysis.The relationship between speed loop bandwidth and load was established,and the influence of load time variation on speed loop control performance was analyzed.(2)The sliding mode variable structure control was used to improve the speed loop controller,which was sensitive to load change and has poor robustness.In order to effectively suppress the chattering of sliding mode control,a new adaptive approach rate based on piecewise composite function was proposed,and a velocity integral sliding mode controller was designed according to the new approach law.In view of the time-varying load characteristics of Stewart platform,a load torque sliding mode observer was designed to reduce the influence of timevarying load on the servo control system by feed-forward compensation to the speed controller.Aiming at the defects of steady-state error and phase lag in position loop proportional controller facing more than one order position instructions,the strategy of feedforward compound control wass adopted to effectively improve the response speed and dynamic performance of position loop.(3)TMS320F38279S microcontroller was selected as the core to complete the servo control hardware circuit design,including power supply circuit,microcontroller basic circuit,inverter circuit,etc.,and on the basis of hardware platform to complete the development of the main program,control algorithm and other program code.(4)The control strategy of single-axis servo control of Stewart platform was verified by experiments.The test results show that the servo control system has short response speed and good dynamic performance.The position tracking test and repeated positioning accuracy tested in X,Y and Z directions were carried out on the Stewart platform prototype.The test results showed that the servo control system designed in this paper improved the overall control performance of the Stewart platform. |
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