UBC Theses and Dissertations

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UBC Theses and Dissertations

On-axis self-calibration of angle measurement errors in precision rotary encoders Graetz, Richard J


Incremental angle encoders are widely used in industry as a standard sensor to measure rotational axis angular position and velocity. The best commercially available angle encoders have achieved measurement accuracy on the order of 1 arc-sec with proper installation. Higher-accuracy encoders are needed to measure rotational axis angular position and velocity in ultra-precision rotary table based manufacturing and measurement applications. One specific application is to enable the development of maskless lithography technology used for mass manufacture of next generation semiconductors. In order to achieve accuracy well below 1 arc-sec, repeatable errors in the encoder measurement need to be removed through a calibration process. Numerous high-accuracy encoder calibration techniques have been developed, but the fundamental problem of calibrating angle encoders remains unsolved; their calibration results cannot be directly applied to the manufacturing machine. Existing calibration methods involve calibrating the encoder on a specially designed angle comparator, but the calibrated error is useless after transferring the encoder back to its application axis, due to the sensitivity of error on the installation. Other calibration methods capable of calibrating the encoder on its application axis cannot determine all the encoder error harmonics. There still does not exist a calibration method to quickly calibrate an angle encoder on its application axis, providing all encoder error harmonics. In this thesis the development of a Time-measurement Dynamic Reversal (TDR) encoder calibration technique is presented and its accuracy is validated, through simulation and experiment, and shown to improve encoder accuracy to the thousandth of an arc-sec level. The accuracy of this method is analyzed in detail, and an accuracy limitation based purely on measurement repeatability identified. Through experiments performed on a custom-built precision rotary table, experimental accuracy of several thousandths of an arc-sec is validated through uncertainty analysis and spindle radial error motion comparison. A comparison with the industry standard, the Equal Division Averaged (EDA) calibration method, shows agreement within 0.01 arc-sec for error harmonics not multiple of four. Due to the missing multiple of four harmonics, the EDA method is found to be 0.3 arc-sec less accurate than the TDR method in calibrating this experimental setup.

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