Kinematics and Control of the 4-Axes Visual Alignment System

A central issue in a display mass production line is how to reduce the overall tact time by improving the alignment process between mask and panel. Usually, the visual alignment system employs a parallel stage with high stiffness which can determine 3-DOF planar motions and other vision peripherals. In this paper, we investigate the kinematics and control of an alignment stage with four independent axes where the one extra degree of freedom can be exploited to increase the overall stiffness and load capacity comparing with the 3-axes stages. First, the inverse kinematics of parallel alignment stage is investigated, where the solution represents the moving distance of active joints to compensate the misalignments. Then, the visual servoing system incorporating two CCD cameras and PC-based controller is presented. As well, the real-time estimation of the misalignment states and the vision algorithm for pattern recognition of alignment marks are discussed