Laser trackers are state-of-the-art measuring machines that are capable of measuring the dimensions of large objects (up to approximately 120 meters in length) with uncertainties as small as approximately 60 µm. They work by using lasers to measure distances and angular encoders to measure the direction the laser is pointing. This forms a spherical measuring system. The beam is directed to a target which reflects the laser beam back to the laser tracker. These systems can measure distance using either absolute (time of flight) or incremental (linear interferometer) systems. Commercially, they are used in large-scale applications such as the precise measurement of size and shape of aircraft wings, an example where accuracy is extremely important due to the significant cost of these high-value added manufactured assemblies.
Performance testing of these measuring machines (which can cost as much as $180,000) is difficult because they have large work-volumes with relatively low measurement uncertainties. At National Measurement Institutes, including NIST, most tests of these systems are performed at approximately 20 °C, the temperature at which length is defined. These instruments are designed to be portable and used in the field at ambient temperatures. So test data for these systems in actual field environments is not typically available.
Automated Precision Incorporated (API), the inventor and a U.S. manufacturer of laser trackers, visited PML's Semiconductor and Dimensional Metrology Division in December to discuss a collaborative research effort to facilitate the testing of laser tracker absolute distance meters in the shop floor environment where they are most often used and where the temperatures can vary significantly. An agreement was reached to work on a set of laser tracker tests to be performed at NIST over the next few months, the results of which are intended to help industry understand and improve measurement accuracy in manufacturing facilities. In response to industrial needs, API and NIST will jointly research and develop new performance tests, which will be performed over a much larger range of conditions to help the automotive and aerospace industries understand and improve their industrial measurements. The data may also be used to help U.S. manufacturers of laser trackers improve their product, enhancing U.S. industrial competitiveness.