关键词: 万向支架, 微小螺纹副, 自动装配, 机器视觉, 拧紧策略 

Abstract: An automatic assembly system was developed aiming at solving the problems of weak automation and poor assembly consistency of miniature thread pairs during gimbal assembly. The extraction method of the central positions for the threaded parts and the tightening strategy of multiple miniature thread pairs were studied. Firstly， an improved clarity evaluating function was proposed， and the auto focus of machine vision device was realized based on the evaluating function. Then， aiming at the asymmetry of the end surface features of the threaded shafts and the threaded holes of the inner frame， the variable focused and regional image acquisition method was proposed for extracting the center of the inner and outer threaded parts. Finally， combining the relationship between the input rotation angle and the axial displacement of the inner and outer frames in the loading and unloading processes， the tightening strategy called cross unloading-loading in group was proposed for multiple threaded joints. The system prototype for assembly experiments was built， and the experiments were carried out. The results show that the coaxiality between outer frame and base frame is less than 5 μm， the axial displacements of the inner frame and the outer frame are less than 3 μm， which may meet the coaxiality requirements of the gimbal. 

Key words: gimbal, miniature screw thread pair, automatic assembly, machine vision, tightening strategy