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The quality of an assembly largely depends on the predictive accuracy of the assembly deviation model in the selective assembly process. Presently, three-dimensional deviation analysis methods only offer tolerances that indicate the overall deviation of a part feature. This limitation fails to accurately represent the fit of the parts during the assembly process, resulting in initial inaccuracies in the selective assembly model. To address this issue, an improved Jacobian-Torsor model is proposed that incorporates measured data from part assembly features into the deviation analysis model. Initially, the actual contact state of the assembly is determined by measuring the contact surface of the plane fit and the shaft-hole fit. Next, the Jacobian matrix, representing the transmission path of assembly deviation, is constructed by analyzing the assembly connection diagram. Subsequently, by integrating the actual contact state characterized by small displacement torsor (SDT) into the unified Jacobian-Torsor model, the advantage of high credibility tolerance propagation is provided. Finally, the feasibility of the proposed improved Jacobian-Torsor model is verified using the double-joint manipulator.

Background:When performing the spatial large-scale measurements, the measurement accuracy of laser tracker would decreased with the increase of the measurement distances due to the refraction difference of most optical digital measurement devices. Therefore, this paper proposed a method based on photogrammetry system to improve the large-scale measurement accuracy of the laser tracker. Purpose:The purpose of this method is to improve the large-scale measurement accuracy of the laser tracker by considering the advantages of photogrammetry system such as high measurement accuracy and good portability. Methods:The measurement data from the photogrammetry system would be used as a reference to do correction on measurement results from laser tracker. The coordinate correction method based on Rodrigues’ rotation formula has been discussed. The measurement accuracy of the long-distance point of laser tracker can be significantly improved through the coordinate correction method. Conclusion:Based on the advantages of using photogrammetry system, the proposed method can achieve higher accuracy when measuring the common points at a closer distance than the laser tracker can do when measuring objects far away from instrument. The feasibility of the proposed method has been demonstrated by experiment.