Determination of Aspect Ratio Limitations, Accuracy and Repeatability of a Laser Line Scanning CMM Probe

Abstract

Coordinate measurement machine (CMM) probing techniques can involve direct mechanical contact (e.g., tactile probing) or diverse non-contact principles (e.g., laser line scan probing). For some applications, contact methods are not capable of measuring fast enough to ensure 100% quality controlled parts. A laser line scanning probe uses a laser triangulation-based method to acquire 3D measurement points on a workpiece relative to a sensor. Mounting the sensor in a 3D coordinate frame, e.g., in a CMM provides enough information to fully examine the workpiece. These techniques are most commonly exploited in medical industry and industries involving plate materials. A high data density and measurement speed are significant advantages when measuring free-form surfaces by laser line scanning, making the process much more time-efficient. However, high-precision geometrical features (such as cylinders, spheres, etc.) must be measured for locating and aligning the free-form shapes. The accuracy of the equipment therefore has to be assessed. Probe Maximum Permissible Error (MPE_P) values below 10 μm have been reported for cutting-edge laser line scanners. This paper compares the major influences on measurements on cylindrical features. First, the aspect-ratio limitations are considered by comparing two inherently different techniques. The stable inspection of reference features is important, while trying to maximize the spatial extent of the measured features. Second, the measurement method is analyzed in two ways: by using a limited sample of the features to increase stability and eliminate interference from neighboring features; by varying the number of scan tracks, which greatly affects the measurement time.