“What-if” scenarios towards virtual assembly-state mounting for non-rigid parts inspection using permissible loads

Abstract

Recent developments in the fixtureless inspection of non-rigid parts based on computer-aided inspection (CAI) methods significantly contribute to diminishing the time and cost of geometrical dimensioning and inspection. Generally, CAI methods aim to compare scan meshes, which are acquired using scanners as point clouds from non-rigid manufactured parts in a free-state, with associated nominal computer-aided design (CAD) models. However, non-rigid parts are deformed in a free-state due to their compliance behavior. Industrial inspection approaches apply costly and complex physical inspection fixtures to retrieve the functional shape of non-rigid parts in assembly-state. Therefore, fixtureless inspection methods are developed to eliminate the need for these complex fixtures and to replace them with simple inspection supports. Fixtureless inspection methods intend to virtually (numerically) compensate for flexible deformation of non-rigid parts in a free-state. Inspired by industrial inspection techniques wherein weights (e.g., sandbags) are applied as restraining loads on non-rigid parts, we present a new fixtureless inspection method in this article. Our proposed virtual mounting assembly-state inspection (VMASI) method aims at predicting the functional shape (in assembly-state) of a deviated non-rigid part (including defects such as plastic deformation). This method is capable of virtually mounting the scan mesh of a deviated non-rigid part (acquired in a free-state) into the designed assembly-state. This is fulfilled by applying permissible restraining forces (loads) that are introduced as pressures on surfaces of a deviated part. The functional shape is then predicted via a linear FE-based transformation where the value and position of required restraining pressures are assessed by our developed restraining pressures optimization (RPO) approach. In fact, RPO minimizes the orientation difference and distance between assembly mounting holes on the predicted shape of a non-rigid part with respect to nominal ones on the CAD model. Eventually, the inspection is accomplished by examining the mounting holes offset on the predicted shape of the scan model concerning the nominal CAD model. This ensures that the mounting holes on the predicted shape of a scan model in assembly-state remain in the dedicated tolerance range. This method is evaluated on two non-rigid parts to predict the required restraining pressures limited to the permissible forces during the inspection process and to predict the eventual functional shape of the scan model. We applied numerical validations for each part, for which different types of synthetic (numerically simulated) defects are included into scan meshes, to determine whether the functional shape of a geometrically deviated part can be virtually retrieved under assembly constrains.