2-D ABSOLUTE POSITIONING SYSTEM FOR REAL TIME CONTROL APPLICATIONS

INTRODUCTION The present research introduces a 2-D absolute position feedback method with application to real time position control for manufacturing equipment, such as machine tools. The desired planar displacement command of the tool is produced by an active element or active target on a liquid-crystal display (LCD). A fixed vision sensor is located so that camera plane is parallel to the LCD, which moves freely in the XY plane based upon control action. The sensor observes the motions of the active target and also provides a 2-D coordinate frame based on which in-plane position errors are determined. The machine axes move to reduce these errors causing the planar positioning stage to follow the motions of the target image on the display. Due to the direct-sensing nature of the position transducer no geometric error compensation is required. An image processing algorithm has been developed to retrieve high resolution position and orientation information of a crosshairs target, displayed on the LCD. This algorithm allows positioning resolution of less than 1/100th of the pixel size on the display. The time delay in the feedback signal to the control system associated with slow hardware frame acquisition rate and long image processing times is addressed through a Smith predictor control scheme. Experimental results are presented.