THE VERIFICATION OF A HIGH-PRECISION 2-DIMENSIONAL POSITION MEASUREMENT SYSTEM

In order to realize the performance limits of dimensional measuring ma chines, 'measurement redundancy' calibration techniques are widely emp loyed. The method involves the measurement of a reference object at se veral different positions within the area or volume to be calibrated a nd the measurement data are then mathematically analysed to identify a nd map the systematic errors in the machine. A small-area, two-dimensi onal position measurement system developed at the NPL was calibrated w ith a 'neutral axis' 80 mm x 80 mm optical scale using a redundancy te chnique. The scale consisted of a symmetrical array of location featur es and the method employed required the scale to be measured at eight different positions involving the rotation and inversion of the scale. Mathematical analysis of the measurements provided two-dimensional sy stematic error maps both for the system and for the scale with a dimen sional accuracy of nearly one quarter of the value for the repeatabili ty of the system's setting. This improvement was achieved by a combina tion of the metrology strategy employed and an innovative approach for the isolation of the machine errors. The design and construction of t he instrumentation will be described together with the results of the calibration with the optical scale.