Research On Vibration Suppression Technology Of Aerostatic Bearing Stage Driven By Linear Motor

Currently,the ultra-precision aerostatic bearing stage driven by linear motor(ABSLM)has the “zero transmission” advantage of linear motor and the “no friction” advantage of aerostatic bearing.The ABSLM is more and more applied to many fields,such as micro-nano processing,IC packaging,manufacturing and testing of large LCD panel,optical scanning detection.However,due to the low damping characteristics of the aerostatic bearing system,the ABSLM is easy to vibrate,and some small vibration will have a great impact on the quality of the machining.Therefore,improving the vibration suppression capability of the system is one of the key problems in the development of the ABSLM.In the design,manufacturing,assembly and testing process of the linear feed system,more attention has been paid to the positioning accuracy and dynamic characteristics of the stage.The study on the vibration mechanism and how to restrain the vibration of the ABSLM is not deep enough.Most of the existing researches focused on how to reduce the vibration of machine tools from the mechanical point of view,such as the optimization of aerostatic structure and the increase of damping.The direct drive system is an integration of the mechanical and electrical parts.The mechanical parts and the driving parts are strongly coupled.From the point of view of the drive system,there is not much research on the vibration suppression.This thesis focuses on ABSLM..The factors affecting the vibration of the ABSLM were analyzed from the electromechanical coupling point of view.An electromechanical coupling simulation method was proposed to obtain the vibration performance of the ABSLM rapidly effectively and accurately.Based on the vibration model analysis,two kinds of vibration suppression strategies for the ABSLM were proposed.One is to use linear motor damping to suppress the vibration,and the other is a vibration suppression strategy based on disturbance rejection and increasing electronic damping.These methods were tested on a direct drive experimental stage.It was found that the vibration of the ABSLM can be suppressed to some extent by using these methods.Finally the vibration suppression strategy was applied to a machine tool for chamfering of the KDP crystal.Results showed thatthe surface quality of KDP crystal was greatly improved.The main research works and results of this thesis are as follows:1)The effect of the various factors on the vibration characteristics of the ABSLM was comprehensively analyzed,including the aerostatic bearing,the flucutuation of the driving force of the linear motor,the feedback signals(the quality and disturbance of the grating signal),the control algorithm etc.An electromechanical coupling vibration model of the aerostatic bearing stage with a linear motor was established.The main factors affecting the vibration performance were found to be the low damping of the aerostatic bearing system,the electromagnetic force fluctuation of the linear motor,the closed loop control algorithm and the quality of the feedback signal.The key problem of vibration suppression was found.Through test analysis,it was found that the frequencty of the speed fluctuation is associated with the velocity and the cycle of the pitch signaland the maximum amplitude of the fluctuation occurs at the double frequency of the signal.2)In order to analyze the electromechanical coupling vibration performance of the ABSLM fast and accurately,a new modal simulation method with consideration of the mechanical and electrical coupling was proposed.Referring to the simulation method of the mechanical modal,an equivalent spring stiffness was used to simulate the dynamics characteristics of the servo drive.The servo stiffness of the drive system is equivalent to a spring element and acts on the ADSLM together with the spring-damper element of the aerostatic bearing.This method can quickly simulate the operating modal of the ABSLM.It is also possible to study the operating modal influenced by drive servo characteristics more directly.3)Aiming at the low damping characteristics of the ADSLM,a vibration suppression strategy based on the disturbance rejection and increasing electronic damping was proposed.The vibration suppression of ABSLM was mainly considered from the point of view of the drive.A feedforward+PD+LESO(FFPDL)controller was designed.The underlying idea is as follows: a LESO,which is essentially an observer,is designed to estimate of the position,the velocity and the disturbance of the ABSLM.The estimated disturbances are fed to the controller for real-time elimination of all disturbances.A PD controller is adopted to act as an electronic damper.Finally,the velocity and acceleration feedforward control is added to improve the dynamic response capability of the ABSLM.Experiments showed that the FFPDL controller can improve the positioning accuracy by 1 time and reduce the velocity fluctuation by half,compared with that of an optimally tuned PID controller.In addition,the FFPDL controller can restrain the fluctuation of speed caused by the grating cycle.Besides the vibration along the moving direction,the vertical(Y)and lateral(X)vibration of the stage were also reduced.4)A new damping method was proposed in this thesis.The linear motor is used as a mechanical damping to increase the external damping of the ABSLM,so as to control the vibration of the ABSLM.When the linear motor moves with the table,it is equivalent to a generator,resulting in an electromotive force from the motor.The electromotive force is applied to the load to produce an electric current,and the current cuts the magnetic lines to produce a damping force,which is related to the speed of the sliding table.Compared with the liquid and electronic dampers,linear motor damper has simpler structures and lower prices.It was proved by experiments that the damping of linear motor is advantageous to the improvement of speed fluctuation and the speed convergence of the load impact.Especially for aerostatic critical vibration,it can rapidly dissipate the energy and restrain the vibration.