Modeling And Control Of Ultra-precision Positioning Stage Driven By Pneumatic Bellows

Ultra precision positioning is one of key technologies in precision machining, and was more and more widely used in the field of precision engineering, such as IT production, ultra precision CNC, large area liquid crystal panel, scanning in the micro world. Ultra precision positioning technology is an important symbol of ultra precision machining in a country.The ultra precision positioning systems are usually driven by the electric unit, such as piezoelectric or high precision motors. The positioning accuracy is enough but the stroke is limited in the system driven by piezoelectric, and the stroke is large but the positioning accuracy is low in the system driven by motors. In some fields such as IT manufacturing etc., electromagnetic interference is an important problem in electric driving system, which affects the products quality. Pneumatic actuators will produce a weak electromagnetic interference, but the strong nonlinearity in pneumatic actuators will affect the positioning accuracy. The double driving actuators can solve the problem between stroke and positioning accuracy, but the complicated mechanical structure is not conducive to the positioning accuracy. The large stroke is realized with electrically actuators in this system, and this still can not solve the problem of electromagnetic interference.In order to improve the positioning accuracy and stroke, and reduce electromagnetic interference in ultra precision positioning system, a new positioning stage driven by pneumatic mental bellows was designed. The author tried to do some research works on large stroke and ultra precision positioning in bellows driven positioning stage, with the optimization in structure design and control algorithm to improve the positioning accuracy, the main works and innovations are summarized as follows:1. The mechanical structure of the positioning stage was designed. Grating ruler was used to detect displacement, and aerostatic slideway was used to support the movement platform. The pressure in the bellows was controlled by electric proportional pressure valve to adjust the bellows expansion to control the displacement. The metal bellows itself has no sliding frictions, and the friction between the movement platform and the support platform was decreaseded with aerostatic slideway, which can help to improve positioning accuracy.2. The static characteristics of bellows driven stage was analyzed, the dynamic model is established, and the model was verified with experiments. The mass flow equation of bellows, pressure-flow equation of electric proportional pressure valve and kinetic equation of bellows were analyzed to establish the dynamic model of positioning stage. The hysteresis characteristics of positioning stage were analyzed with the experiments, PI model was established with initial loading curves formed by the experimental data, parameters were identified to establish the PI model of system, and the hysteresis model is verified with experiments,the accuracy of the different model was compared.3. Compounding control scheme was designed. PID controller was used to improve the positioning accuracy of system. A feed forward control scheme based on the inverse PI model was analyzed. The compounding control scheme was designed based on inverse PI model, and inverse PI(Prandtl-Ishlinskii) model was used as feedforward controller to compensate the output of PID controller, the positioning accuracy was improved. In order to reduce the computation, improve the model accuracy, the backlash operators were modified to establish the generalized PI model. The compounding control scheme based on the generalized inverse PI model was used on the positioning stage, and the positioning accuracy and real time performance of the system were improved.4. The elastic creep characteristics of bellow driven stage were analyzed, and elastic creep model of the positioning stage was established, the creep compensation scheme was designed. In this scheme, the inverse PI model was used to compensate the hysteresis characteristics, the output of PID controller and elastic creep model were superposition to realize creep compensation control. The positioning accuracy with slow rate input signal or in points positioning mode were improved.5. The upper computer combined with lower computer was employed as control system. In the upper computer, LabVIEW software was used to design the man-machine interface, while the STM32 Series MCU was used as control unit to communicate with upper computer to control displacement. The effectiveness of the model and control algorithm was verified based on the control system.