Calibration and automatic focusing of zoom vision system for microassembly

In microassembly, zoom vision system was usually used to solve the contradiction between measuring scope and accuracy. However, new problems of dynamic calibration and real-time automatic focusing were also introduced as a side-effect. For this reason, the calibration and automatic focusing technology of zoom micro-vision system were described. For calibration, the principal point of image was determined by method of convertible magnification. Based on planar target, the linear calibration under fixed magnification was firstly performed by homography matrix decomposition of single view. Then the distortion model and the quantum-behaved particle swarm optimization (QPSO) were employed sequentially to do nonlinear optimization for the linear calibration result. After nonlinear optimization, the maximum re-projection error was 0.13 pixel and the average re-projection error was about 0.1 pixel. Furthermore, the calibration for magnification at arbitrary working condition was completed by Gaussian curve fitting. For real-time automatic focusing, method of maximum gradient threshold of eight-neighborhood and gradient threshold were used, for the traditional gray gradient function only considered the fixed gradient direction and was susceptible to noise. Compared with other several gray gradient focusing function, this method had good unimodality and noise immunity.