Research On The Growth Of Czochralski Crystal With Ultra Low Velocity Motion Control Technology

The Czochralski method is a fast and effective growth method for stimulate the growth of laser crystal,the laser crystal pulling device running in the ultra-low speed mode for a long time.In this condition,the servo system is always with irregular vibrations because of the nonlinear friction torque,speed encoder resolution,motor torque,and system coupling torque.Among them,non-linear friction interference is the main factor that causes the servo system running in the condition of low speed crawling and shaking,which seriously affects the quality of the laser crystal.Frictional disturbances are suppressed from both mechanical structure optimization and control algorithm design.In terms of mechanical structure optimization,ball screw is used as the transmission mechanism.Rolling linear guide completes the achievement of the guide movement,and minimizing the system's frictional interference from the mechanical hardware.In the design of control algorithm,vector analysis of the servo system and the space vector voltage algorithm are studied in the two phase rotor coordinates.The modules in the control structure of the servo system are modeled,and the voltage equation and the magnetic chain equation in the two-phase rotor coordinate system are further analyzed.The Stribeck friction model is used as the interference model of system nonlinear friction,and finally the mathematical model of the servo control system is established.The proportional-integer parameter adjustment is performed on the speed of a specific point,and the gain change law of the low-speed controller is summarized.According to this rule,again-changed list control method is proposed to compensate the nonlinear friction of the system.Low-pass filters and digital notch filters are added to the system to suppress system resonance and improve system tracking accuracy.In the simulation section,results shows that the gain-changed list control method can better suppress the slow crawling and jitter of the system.The actual measurement shows that the optimization of the mechanical structure of the pulling device and the addition of filters and traps to the variable gain list speed controller can improve the tracking accuracy of the system and suppress the creep and jitter of the servo motor throughout the low speed range.As a result,there is no creep in the low speed range,and the maximum jitter error is less than 5%.The equal diameter growth process has no dislocations,the maximum diameter error is 1.186%,and the maximum error is less than 1mm,which meets the requirements of actual production and can produce high-quality equal-aperture laser crystals.