Design and Control of an Active Stage for Suppressing Motion Induced Vibration in Optical Inspection Systems

Fast positioning and vibration suppression play important roles in virtually all tasks requiring high speed maneuvers with sufficient motion accuracy. One typical example is on the operation of optical inspection systems, which requires to perform product defect inspections by fast moving to the desired inspection locations and take images. Due to the inherent structural compliance and the inertial force generated during movement, residual vibrations would be inevitable and this implies that additional settling time would be required for avoiding blurred images. How to improve inspection quality and yield is therefore a traditional but non-trivial concern. In this work, a test stage built with elastomeric bearings and compliant structure mounted with a camera, is designed for developing control schemes. Through dynamic testing, the plant characteristics are identified and the mathematical model of the active stage is obtained. Both loop transmission shaping and input shaping methods are used as the first step to evaluate the effectiveness on suppression of motion-induced vibration. The positioning control results indicate that a 0.5% overshoot and a 0.09 second setting time or a 22 Hz bandwidth could be achieved. Finally, the stage is mounted on a linear servomotor to simulate the behavior during fast maneuver of typical AOI systems due to structure compliance. A webcam mounted on the stage is responsible to examine the effectiveness of vibration control in both of the acquired imaging quality and inspection yield. It is found that the time delay required for taking inspection clear image after arriving the destination is reduced from 9s to 0.5s. Thus the inspection yield could be significantly improved. In summary, this work successfully confirmed that the active stage control system can effectively reduce residual vibration during optical inspection. In the future, the developed stage and control scheme should be able to be applied in real design for improving the associate inspection efficiency.