Automatic generation of high level inspection plans for coordinate measuring machines

Abstract

The topic of this thesis is the automatic generation of high-level inspection plans for mechanical parts, when inspection is performed by Coordinate Measuring Machines (CMMs). CMMs are very accurate Cartesian robots equipped with tactile probes. A high-level plan is a collection of setups of the part on the CMM, features to be inspected in each setup, and the probes and probe orientations to be used for each feature. The problem is formulated as search in a space of partially-completed plans. Intermediate states contain a collection of plans, some of which might be invalid or too expensive. Operators map plans into more refined plans, by imposing geometric and optimization constraints, until a feasible and nearly optimal plan is reached. The state is represented as a set of 4-tuples of the form (setup, probe geometry, probe direction, surface feature), and constraints of type same on the tuple variables. Operators are organized into problem spaces according to the goal they achieve. Some of these spaces group several geometric methods of varying power and execution cost for achieving the same goal. Accessibility is one of the most important geometric constraint for inspection planning. To analyze the accessibility of a surface feature probes are abstracted by half-lines, and all the half-line directions that can inspect the feature without colliding with the workpiece are computed. The set of these directions form the accessibility cone of the feature. The half-line abstraction is suitable for straight probes of relative large length with respect to the workpiece dimensions. Accessibility analysis however, can be extended to the case of finite length probes, and bent probes (a bent probe has several non-aligned components). A general method for computing accessibility cones is presented, based on Minkowski (sweep) operations on faces and solids, and on Boolean operations on direction cones. The method is implemented for polyhedral objects by an algorithm that uses Minkowski operations on 2-D sets. A prototype system implementation shows that the approach is feasible. Examples are presented of good-quality plans generated by the system for realistic parts. (Copies available exclusively from Micrographics Department, Doheny Library, USC, Los Angeles, CA 90089-0182.)