Study On Visual Measurement And System Control In Precise Assembly

With the development of precision manufacturing technology, micro electromechanical products are developing towards miniaturization and integration. Assembly parts with size from a few millimeters to hundreds micrometers are commonly manufactured by using different process on different materials. At present in home and abroad, the assembly process of most micro electromechanical products is still finished by experienced workers under microscopes, which requires highly skilled workers and labor-intensive. As a result, it is hard to meet the consistency and stability of products.For industrial application, this paper designed a precision assembly system of miniature parts based on machinery vision to solve the problems in manual assembly of a small industry device. The assembly system is designed by modules, which consists of the loading module, the precise measure module, the assembly operation module and the human-computer interaction module.The operating structure of the assembly system is researched from both horizontal and vertical direction. In the horizontal direction,"look and move" vision control strategy was introduced to acquire edge image of parts. After image processing, edge point pickup and coordinates conversion, the pose information of the parts in the universal coordinate was acquired to guide the planar pose adjustment and alignment. In order to realize non-contact assembly control in the vertical direction, laser displacement sensor was used to detect contact states of parts.After studying image processing and method of edge acquisition, the way of image mosaic was applied to solve the difficulty of acquiring all the edge information in one single viewing field. On the basis of the iterative optimization in the least square method, the linear edges and circular arc edges of parts were fit, thereby the position of the part was fixed. In the meantime, manual adjusting ports that can be used to adjust light intensity of illumination were introduced in the system to increase the measuring accuracy and assembly success rate.The model of camera and assembly system was built to unify coordinates in the system and to convert them. As the assembly accuracy requirement of miniature parts is high and the relative position of modules, like precise measurement module and assembly operation module in the developed precise assembly system might change subtly after a period of use, the system needs to be calibrated regularly. A calibration board with diagrams of lines and circles is designed and fixed on one of the modules. The height of the calibration board takes the advantage of adjustable installation to make the calibration diagram plane parallel to the camera image plane. The influence of the initial angle of the lines on the calibration board in the image coordinate is eliminated by using the compensation algorithm, so there is no need to precisely set the angle of the fixed calibration board. An automatic calibration program is developed to calibrate the space distinguishability of the camera and angles between modules in the assembly system.The experimental results show that the adopted measuring method and the control strategy can satisfy the assembly precision requirement and improved the consistency of the product.