On-line calibration and uncertainties evaluation of spherical joint positions on large aircraft component for zero-clearance posture alignment

With the improvement of performance requirements for modern aircraft, such as speed and leak tightness, the posture alignment of large components without clearance is now extremely required in the aircraft industry. Nevertheless, the inherent errors of both the components themselves and the posture alignment system lead to high risk of collision during the inserting stage, among which the positional errors of spherical joints that connecting the large component to the numerical controlled locators are identified as the primary error source by existing studies. In this paper, a novel method for on-line correction and uncertainties evaluation of the positions of spherical joint centers (SJCs) is presented. Firstly, the rough positions of SJCs are identified based on the nominal model, and the finite element analysis (FEA) method is applied to preliminarily compensate the self-weight deformation of the component. Secondly, the on-line correction model of SJCs is further established, majorly on the basis of the displacements of locators and the relative postures between the initial posture and the new posture after each motion. Thirdly, to improve the correction accuracy, a new relative posture evaluation model considering the anisotropic measurement uncertainties of key points is suggested and solved by particle swarm optimization, and the correction uncertainties are then analyzed using Monte Carlo simulation. According to the numerical experiments, the proposed relative posture evaluation method has demonstrated more robustness evaluation results than the conventional approaches, and also leads to lower correction uncertainties of SJCs. Moreover, since the relative posture evaluation is a common problem in robot calibration, it also provides a promising alternative or supplement for the conventional optimal posture selection method to improve calibration accuracy. The practical application for a wing to fuselage assembly has verified the effectiveness of the correction method, in which the largest positional error of SJCs has decreased from about 14.2 mm to less than 0.4 mm after correction, and the displacement calculation error has been accordingly reduced from 0.1 mm to smaller than 0.01 mm. Therefore, the security of posture adjustment in confined clearance has been largely enhanced.