Calibration of non-contact incremental linear encoders using a macro-micro dual-drive high-precision comparator

The accuracy of a linear encoder is determined by encoder-specific errors, which consist of both long-range and cyclic errors. Generally, it is difficult to measure the two errors of a non-contact incremental linear encoder with a large measuring range and small signal period in one measurement because of the contradiction between long travel range and high resolution. To resolve this issue, a prototype high-precision interferometric comparator with a macro-micro dual-drive system is presented. The measurement and motion resolution of the comparator are 1 nm and 3 nm, respectively. A measuring range of 320 mm is realized and the theoretical maximum range of the comparator is 2 m. The comparator mainly includes a high-accuracy aerostatic linear-motion stage, a constant displacement ratio piezoelectric-driven stage, two laser interferometers, a 6-DOF grating pair position adjustment devices and a PC-based data processor. The measurable linear movement is afforded, respectively, by the long-stroke stage and the piezoelectric-driven stage for the long-range error and cyclic error measurement. The movement can be measured by the encoder and then be calibrated by the corresponding laser interferometer. In the experiment, the accuracy of a non-contact incremental linear encoder with a 20 mu m-long signal period and 320 mm measuring range proposed by our team was calibrated after proper mounting. The long-range error is measured to be 3.123 mu m, and the cyclic error is within +/- 0.159 mu m, which matches well with the theoretical estimation given by +/- 0.145 mu m. The measurement uncertainties are estimated and the results confirm the effectiveness and feasibility of the proposed scheme and instruments.