Research Of Perpendicular Axis Macro/Micro Composite 2D Motion Platform And Straightness Error Compensation Technology

With the rapid development of micro-nano technology, higher requirements have been put forward for ultra precision positioning platform. But it is inevitably that geometric errors, include manufacture errors, installation errors and other errors exist in the production process even ultra precision machinery is chosen. The limitation of the machining accuracy of conventional equipment is that there are geometric errors. Straightness error is one of the most important geometric error that can not be compensated in conventional guide, and it can only be improved by ultra precision machining and equipment composite errors compensating. This paper proposed a design of perpendicular axis macro/micro composite platform, and established its dynamic and simulation analysis models, then optimized its structure parameters. The output displacement functions of the platform were obtained by orthogonal experiment and regression analysis. The straightness errors compensation software is developed based on the output displacement functions. Performance of the macro/micro composite platform was evaluated based on experimental study. The straightness error compensation on the conventional linear guide has been carried out by perpendicular axis macro/micro composite platform.Firstly, an innovative structure of perpendicular axis macro/micro composite platform was proposed in this paper. The micro motion platform is fixed on macro motion platform by base and the motion direction of the micro motion platform is perpendicular to the motion direction of the macro platform. The micro motion platform with a pair of six flexible parallel connecting rods was designed in order to improve the loading capacity and reduce the coupling displacement of the platform. The macro/micro composite platform was simplified as a second order system consisted of mass-spring-damper. Then the dynamic model was established and the simulation analysis of the step response was done by the Matlab. The optimal configuration of the system is that the higher natural frequency and lower damper will reduce the overshoot and improve the dynamic traceability.Secondly, static, dynamic and optimal models were designed to the micro motion platform with a pair of six flexible parallel connecting rods. The static stiffness model of the platform was deduced and the angular stiffness and maximum stress checking model was established. The analytical formula of natural frequency of the platform was established based on Lagrange equation. The optimal parameters of the platform were obtained by Matlab and using normalized coordinates based on the principle of static and dynamic performance balance. The dynamic response was increased from 0.80s to 0.58s by the hollowing and ribbing of bottom structure. The theoretical stiffness is 7.92N/μm and natural frequency is 349.9Hz by calculating with the established modeling.The output displacement feedforward functions of the platform were studied. The no-load performance of the piezoelectric ceramic was tested and the results shows that the hysteresis nonlinearity of the micro motion platform will increase with the increasing of the voltage and the maximum value of the hysteresis phenomenon and nonlinearity phenomenon mainly appears in where the voltage is less than one-second full range voltage. Orthogonal experiment was designed to the output displacement of the micro motion platform. The displacement data tables of the forward and inverse voltage were obtained based on the level of factors (voltage and pretension), then regression analysis was used on the data analysis, and the bivariate multiple functions was obtained. The bivariate quadratic function with cross terms was identified as the feedforward function of the output displacement by analysis. The output displacement experiment of the forward and inverse voltage was done when the pretension equals to 60N and the theoretical and experimental comparison chart was deduced.Then the straightness error of the macro motion platform was tested and the compensation software was designed. The macro motion recognition system of straightness error was built based on PMAC motion control card and laser interferometer. Then the straightness errors in 101 points with the interval 2mm on a 200mm trip were obtained and that is 25.32μm, and the straightness error of the macro motion platform was calculated based on least square method. Then the error compensation data table was obtained according to the ideal straight line. Finally the redevelop was done to the piezoelectric ceramics control software and added with the function of reading files by Visual C++. The output displacement control algorithm was optimized based on the displacement control function obtained by regression analysis. The study of this chapter laid the foundation of straightness error compensation.Finally, the performance and straightness error compensation of the macro motion platform were studied by experiment. The displacement experiment of the micro motion platform shows that the platform has good tracking performance and the maximum output displacement can reach 56.59μm at low frequency. Three experimental methods (by piezoelectric ceramics and pressure sensor, steel bar and pressure sensor, innovative direct force device) were applied to test the stiffness linearity of the micro motion platform. The result shows that the stiffness is 7.43N/μm when the force increases and the stiffness is 7.42N/μm when the force decreases tested by innovative device. The minimum error is 6.6% compared with the theory value. Moreover the two stiffness curves when the force increases and decreases almost match together, which shows the micro motion platform has good stiffness linearity. The loading experiment shows that non-uniform load has big influence on the output displacement. The natural frequency on motion direction is 342.2Hz tested by POLYTECH laser vibrometer with the error 2.3% compared with the theory value. The straightness error compensating experiment was done by using the innovative perpendicular axis macro/micro composite platform, and the result shows the straightness error is 5.71μm after compensated, and is reduced by 19.61μm compared with the original straightness and the effective rate of the compensation is 83%. The result shows the feasibility of the perpendicular axis macro/micro composite platform and its compensation method studied by this paper.