Research On Development And Static-dynamic Characteristics Of Drag-free Aerostatic Positioning Stage

Ultra-precision positioning stage technology,one of the critical technologies in contemporary manufacturing,is often used as an essential part of machining and inspection to provide linear motion.Ultra-precision air-bearing positioning stages require higher accuracy,faster acceleration,more excellent range of motion,lower losses,and adaptation to more complex environments.There is a balance to be managed among these extreme indicators.Due to the characteristics of the stage,the air-bearing damping is too small to absorb vibration energy effectively.Therefore,the external load disturbance,the vibration of the gas in the bearing,and the vibration brought by the motor act directly on the transmission part of the stage,which affects the overall dynamic performance and positioning accuracy,and even affects the machining surface quality.In order to solve the above problems,this paper proposes a drag-free ultra-precise aerostatic positioning stage driven by a linear motor.In addition,this paper also analyzes and suppresses the effect of air-bearing and linear motor thrust ripple on the overall dynamic performance of the system.Firstly,this paper identifies the cable force in the classical ultra-precision positioning stage,determines the design idea of the drag-free ultra-precision aerostatic positioning stage,and completes the development of the drag-free positioning stage.In this paper,a dynamic magnet type permanent magnet linear synchronous motor is used to drive the ultra-precision aerostatic stage.The motor cables and air inlet tubes are arranged on the fixture and do not follow the slider to eliminate the cable force as a load interference.The author designed and optimized the mechanical structure of the stage.The disturbance caused by the drag-free structure is suppressed by adding a throttle-controlled air pressure parallel module.The correctness of the model is verified by finite element simulation analysis,and the ultra-precise drag-free aerostatic positioning stage is machined as designed.Next,the author studies the static characteristics of the air bearing that provides support and lubrication for the moving slider.The drag-free aerostatic positioning stage establishes the effective working air film structure model,and the working principle of the aerostatic guideway is analyzed.Based on the velocity slip and the thin effect,the Reynolds equation of the gas film is modified,and the changing boundary conditions of gas flow are established.There are two limit states during the motion of the mover,one is the balanced state where all the throttle holes on the air film are covered with a uniform distribution,and the other is the unbalanced state where the throttle holes are biased on the air film with one throttle hole half covered.The static performances of the above two states are analyzed.When the air supply pressure is different,we analyzed the distribution of the pressure and gas flow vector of the air film,identified the crucial parameters such as the load-bearing capacity and stiffness of the guideway,and solved the variation regulation between the bearing capacity and the thickness of air film.The analysis results are validated by comparing the difference between bearing carrying capacity and stiffness under two limit states in the experiment.Further,the linear motor is used as a drive unit to provide power,and the magnitude of the thrust ripple is directly reflected in the linear positioning motion.Output force conditions of the drive motor are optimized.An analytical model of the air-gap magnetic field is established to resolve the end effect of the long primary moving magnet type motor,and the thrust ripple and normal force ripple are derived.Based on two ways to optimize the shape of the permanent magnet and the length of the moving end core,the fluctuation of the thrust force and normal force can be thus improved.It has been proved that the combination of the two optimization methods is effective in suppressing the fluctuation of thrust and normal force through experiments.Finally,based on the coupled flow-solid-electric multi-physics field model,this paper analyzes the influence of fluid vibration and motor thrust ripple on the overall system vibration output displacement.The result of the overall vibration is not a simple superposition of several influencing factors.The model calculation results are verified by experiments.For the vibration problem caused by the structure characteristics,it can be reduced by the parallel module.It is proved by the experiment that the adjustment method can reduce part of the vibration and improve the stability of the movement speed.The positioning accuracy of the platform has been tested to verify the feasibility of the drag-free aerostatic positioning stage.