Contact probe radius compensation using computer aided design models

Probe radius compensation is necessary in metrology applications that employ contact probes, but it can be a signi® cant source of systematic measurement errors when dealing with freeform part geometry. The paper presents implementation and performance analysis of a proposed new compensation technique based on the nominal computer aided design (CAD) model, which is assumed to be de® ned using non-uniform rational B-splines (NURBS). Errors associated with the conventional compensation approach are assessed on the basis of experiments using a modern coordinate measuring machine (CMM), providing clear motivation for this work. The proposed method consists of a number of steps, including measurement, generation of oVset nominal surfaces, registration, surface ® tting, data smoothing and calculation of compensating oVsets. Critical steps include registration and NURBS surface ® tting and their implementation is presented. Simulation studies are used to analyse the registration accuracy and the accuracy of the overall method in comparison with the conventional one. The proposed method is shown to produce superior results in situations involving non-uniform measurement distribution, measurement noise, free-form geometry with no clear datums, deformation relative to the nominal shape and component misalignment.