The verification of a high precision two dimensional position measurement system

In order to realise the performance limits of dimensional measuring machines 'measurement redundancy' calibration techniques are widely employed The method involves the measurement of a reference object in several different positions within the area or volume to be calibrated and the measurement data is then mathematically analysed to identify and map the systematic errors in the machine. A small area, two dimensional position measurement system developed at the NPL was calibrated with a 'neutral axis' 80 mm x 80 mm optical scale using a redundancy technique. The scale consisted of a symmetrical array of location features and the method employed required the scale to be measured in eight different positions involving the rotation and inversion of the scale. Mathematical analysis of the measurements provided two dimensional systematic error maps for both the system and the scale with a dimensional accuracy of nearly one quarter of the value for the setting repeatability of the system. This improvement was achieved by a combination of the metrology strategy employed and an innovative approach for the isolation of the machine errors. The design and construction of the instrumentation will be described together with the results of the calibration with the optical scale.