Precision Positioning of a 3-PRR Planar Parallel Manipulator Driven by Linear Ultrasonic Motors Based on Machine Vision

Abstract

A 3-PRR planar parallel manipulator actuated by liner ultrasonic motor is set up and investigated. The structure of the planar parallel manipulator(PPM) is presented, and then the kinematic model of the manipulator is developed based on vector chain method. To realize the global closed-loop feedback, charge coupled device camera is adopted to measure the output of the manipulator, namely the 3 degrees of freedom of the manipulator. By using the technologies of image processing, the center of the artificial features fixed on the moving platform is located to realize the pose measurement of the moving platform. A perturbation observer-based joint control is adopted to overcome the disadvantage of the traditional PID control in real time control. Experimental results show that the accuracy of the semi-closed loop control of the PPM is around 500 μm while the target position is(20 mm,20 mm), but after the procedure of the visual based closed loop control, the positioning accuracy is further improved to ±5 μm. Compared with the semi-closed loop control, the visual based positioning control provides a substantial increasing in positioning accuracy of the PPM, which could satisfy the positioning requirements of the PPM.