A three-dimensional (3D) data acquisition strategy for automatic tolerance control in industrial parts is described in detail. Instead of using a classical mechanical coordinate measuring machine, which takes a lot of time to gain the data, and applies forces to the parts that in turn can distort the measurement, a laser range sensor is mounted on a coordinate measuring machine. The approach uses an existing computer-aided design (CAD) model of the part, and an error model, in order to optimize sensor placement with respect to the accuracy and time of the inspection task.

The error model for the positioning of the range sensor is based on a noise characterization with respect to certain parameters, like the distance from the sensor to the surface, and the incidence angle between the laser beam, when it reaches the surface, and the normal vector to the part. The movement of the range sensor, and its optimal position with respect to the surface, is determined by evaluating the CAD model (initial graphics exchange specification (IGES) format), and its nonuniform rational B-spline (NURBS)-based surfaces. The approach focuses on a scan-time efficient distribution of the sensor positions. Occlusion is checked by tracing a ray from the sensor toward the area to be digitized, using a voxel model of the part. In the case of occlusion, a new sensor position is determined by changing the angle between the surface normal and the laser beam, with respect to the occluded area. In addition, the voxel model is also used for collision-detection. Regions that cannot be digitized with sufficient accuracy are indicated by the system.

The authors’ main contribution is the planning of the sensor movement and positioning, using the available CAD model. A second contribution is the error model to estimate the accuracy of the measurement. The proposed approach handles arbitrary geometries, as well as curved surfaces, and improves the scanning accuracy up to 35 percent compared to the standard digitization process, where the part is swept by the laser from four fixed viewpoints.