Research On Modeling And Control Of Long Range Nanometer Positioning System Based On Ballscrew

Ultra-precision machining and nanotechnology are at the forefront of modern manufacturing industry. When the accuracy of machined part approaches nanoscale, machine tools have to be improved gradually. Positioning system is the key technology for realizing high accuracy. Precision positioning system with nanometer resolution and long stroke are becoming more important in industrial applications. Usually nanometer positioning systems based on ballscrews are of short stroke because of friction. Even positioning systems with dual-model control strategy or aerostatic mechanism can realize nanometer positioning with long strok, it is every expensive because of suppression or elimination of mechanical friction.In this thesis, a positioning system based on ballscrew and linear ball guides is designed and constructed. The mechanism, friction phenomenon and control strategy of the system are investigated. Experimental and simulated results indicate that the sigle-step nanometer positioning with long stroke can be realized on such a cheap frictional system with computer control closed-loop system, friction identification and compensation or dual-model control strategy are avoided. The fulfillment of this study proposes a simple and inexpensive method for realizing nanometer positioning. It will contribute to the improvement of theoretic and experimental method of nanotechnology for its further development.The precision positioning system based on ballscrew consists of ballscrew, linear ball guides, DC-motor and ball bearings, et al. Combining solid elements and spring elements, accurate model of the mechanism is built and Finite Element Analysis is carried out for calculating the system dynamics. The modal analysis and frequency response analysis are performed. The first four modals and natural frequencies are obtained. The frequency analysis of the positioning system under motor output is also obtained. The computation results provide theoretical basis for mechanism design and improvement.For ballscrew mechanism, models that do not account for friction can only be used to describe the macrodynamic behavior. However, because of friction, the behavior of the system prior to continuous slipping at the friction interface is completely different from that of its macrodynamics. Friction becomes the main obstacle for realizing nanometer positioning. The torque/displacement experiments and frequency response analysis experiments are executed for studying friction phenomenon and identifying the microdynamic characteristics of the positioning system. The theoretic model is founded including microdynamics of the mechanism. This provides the basis of controller design and theoretical analysis for nanometer positioning.Control strategy is the most important for nanometer positioning system design and it determines performance of the system. Based on the macrodynamics of the ballscrew mechanism alone, a high-gain PID control structure with proportional and derivative items placed in the feedback path is designed. Friction is considered as the outer disturbance of the closed-loop system. The controller is implemented on a personal computer. Experiment and simulation of point-to-point (PTP) positioning for step height from10nm to 10mm are examined and analyzed. Robustnes of the positioning system also be researched.Because of saturation of the amplifier in the system, even the anti-windup technique can guarantee stability of long stroke positioning, overshoot appeared in long stroke PTP positioning is unavoidable with the high-gain PID controller. Input trajectory design method and Bang-Bang control strategy are combined with the PID closed-loop system respectively. Time optimum nanometer positioning is realized without overshoot.