The University of Edinburgh -
Division of Informatics
Forrest Hill & 80 South Bridge

Research Paper #720

Title:Free-Form Surface Matching for Surface Inspection
Authors:Bispo,EM; Fisher,RB
Date:Dec 1994
Presented:Submitted to the 6th Conference on Mathematics of Surface, Surrey
Abstract:Despite their great accuracy, coordinate measuring machines (CMM) for the inspection of mechanical parts present some drawbacks related to their low speed, which make impossible measure many points on the surface of an object. Another difficulty is programming the coordinate measuring machines. We are investigating the use of dense range data acquired using laser stripers for the inspection of mechanical parts. Laser stripers do not produce as accurate measurements as CMMs but are much faster and consequently are able to measure many more points on the surface of the object. This makes possible a better evaluation of shape. We concentrate on the inspection of free-form surfaces, which are particularly interesting because few inspection processes can inspect the surface shape across the whole surface. The key idea involved in our work is the comparison of previously stored model surfaces with range images of the part being inspected. Clearly, before any comparison between data and model can take place, it is necessary to align the model with the range image of the part. This process, called registration, is relatively simple in the case of objects that have distinctive geometric features (e.g. planes), but is a very complicated problem in the case of free-form surfaces when no a priori information about the rigid transformation that aligns data and model is known. We use a modified version of Besl's ICP (iterative closest point)algorithm [10] that uses a priori knowledge of an approximation to the right registration to make the method more robust to outliers and avoid the problem of convergence to local minima. After the registration, we can explicitly compare the complete measured surface to the model and inspect the part. In this paper, we present some results concerning the registration of free-form surfaces and the final inspection of mechanical parts after registration.

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