Dynamics Of The Ultra-precision Positioning Stage With Gas-lubricated Bearings

The positioning stage with air bearings, which based on the air-supported bearing and direct electromagnetic-drive technology, is the new-typical machine of the precision motion which can overcome the shortages of the traditional positioning stage (The traditional positioning stage usually contains rotary motor, ball-screw and oil-lubricated bearings), such as more driving taches, big lag of response, and existed friction between the moving parts etc, therefore it has a prosperous application in the fields of semiconductor lithography equipment, precision measurement and biomedicine etc. For the existence of nonlinear effect caused by the air-supported bearing, the traditional modeling theory and analyse method couldn't solve the problems on the system dynamics and positioning control of this kinds of ultra-precision moving machine.The methods of the partial differential and the numerical analysis with limited elements are used to research the characteristic of the precision air bearing, and the changing regulation of the dynamic and static characteristic of the ultra-precision air bearing, as well as the models of the gas flow field, the pressure distribution, and characteristic parameters of the air bearing is obtained. Especially the effect rules of the recess shape and inner planar shape of the air lubricated bearings on the characteristic of the bearings are firstly opened out in the world, and the characteristic parameters of the air beanring are identified and the load capacity and the stiffness of the air bearing are obtained. Therefore, based on the researches mentioned above, the optimal structure of the air bearing is presented for the ultra-precision stage.Through the theoretical and experimental studies, the effects of the nonlinearity caused by the air supported-bearing on the frequency characteristic of the precision positioning stage are proved up, and the researches on the dynamic performance of the ultra-precion positioning stage varied with the characteristic parameters of the air-supported bearings are carried out. The mathematical model of the positioning stage with air supported bearing is established, and a novel control tactic which based on the mixture trending slide mode control (MTSMC) is presented and the control model based on the MTSMC position feedback controller is established accordingly. The parameter sensitive analysis of the positioning stage is carried out. The results show that the effects the nonlinearity and unmodel factors can be eliminated and the positioning accuracy of the stage with air supported bearings can reach nanometer class when the MTSMC controller is used.The dynamical experiment and positioning experiment of the stage are carried out too. The results show as follows: In the positioning stage without air supported bearings, the existence of the gap and the friction between the moving parts decrease the position accuracy of the stage seriously. Whereas, in the positioning stage with air supported bearings, though the gap between the moving parts exists yet, but there is pressured air in the gap, the stiffness of the air bearings play the role of the flexable connection between the moving parts, which makes the stage possess excellent repeatable positioning accuracy and smooth motion. Furthermore, the air-supported bearings eliminate friction between the moving parts, the positioning accuracy of stage is improved extremely, the maximum error of linearity(3σ) can reach 9.8256 nm in Y direction or Z direction, and the maximum error of pitch(3σ) can reach 3.8238×10-8 rad and the maximum error of positioning can reach not more than±10 nm , which means that the theoretical and simulating results above are correct.