Issues in the testing of Portable Coordinate Measuring Systems (CMS)

Abstract

Historically, coordinate measuring machines (CMMs) are delivered to a room provided by the customer, with environmental controls (primarily temperature) that meet the CMM manufacturer's requirements. More sophisticated compensation methods and the use of advanced materials have led to the ability to place CMMs on the factory floor, but there are still environmental limits which must be satisfied in order for the CMM to perform as specified. Performance testing of CMMs follows (in general) the rubric that the error observed in a length measurement of a reference artifact must not exceed the CMM specification, provided the required environmental conditions are met. If we consider portable coordinate measuring systems (CMSs), such as articulating arm CMMs and laser trackers, the same general guidelines pertain to performance testing. There are, however, two differences in these instruments that introduce ambiguity with respect to the testing and calibration of these instruments. The first difference is that these instruments use an operator to perform measurements, where a CMM is computer controlled and largely independent of the operator. It is possible that an inexperienced operator may have difficulty in successfully completing a performance test of the instrument. If a technician representing the instrument manufacturer can successfully complete the test, is this adequate? Or must it be possible for any properly trained operator to achieve a successful result? The second difference in portable CMS is that . due to their portability . they are often sent to an offsite laboratory for performance testing and calibration. These offsite laboratories often have very good temperature control, performing the tests at 20 .C ™} 2.C, while the instruments are specified to perform at (for example) .5 .C to +40 .C. How then is the user able to be confident that the instrument will perform as designed in their own environment? What avenues are available to determine that the instrument continues to remain in conformance to the manufacturer specifications?