Abstract
In this research paper, a precision position-measurement system based on the image grating technique is presented. The system offers a better robustness and flexibility for 1D position measurement compared to a conventional optical encoder. It is equipped with an image grating attached to a linear stage as the target feature and a line scan camera as the stationary displacement reader. By measuring the position of the specific feature in the image and applying a subpixel image registration method, the position of the linear stage can be obtained. In order to improve the computational efficiency, the calculations for pattern correlation and subpixel registration are performed in the frequency domain. An error compensation method based on a lens distortion model is investigated and implemented to improve the measurement accuracy of the proposed system. Experimental data confirms the capability of the developed image grating system as ±0.3 µm measurement accuracy within a 50 mm range and ±0.2 µm measurement accuracy within a 25 mm range. By applying different optics, the standoff distance, measurement range, and resolution can be customized to conform to different precision measurement applications.
Highlights
Precision positioning of a workpiece is a fundamental function required in many areas such as dimensional metrology and surface finish inspection [1,2], where its applications require appropriate measurement technology adoption [3]
The measurement results have demonstrated that the developed image grating
The measurement results have demonstrated that the developed image grating system has a measurement error of ±0.2 μm within a 25 mm measurement range
Summary
Precision positioning of a workpiece is a fundamental function required in many areas such as dimensional metrology and surface finish inspection [1,2], where its applications require appropriate measurement technology adoption [3]. Laser interferometers based on the heterodyne phase-detection principle have been widely used in different industries for long displacement measurement with sub-nanometer resolution [4]. The integration of a laser interferometer for in situ measurement is not very straightforward due to its low robustness and high sensitivity to environmental conditions [5]. It is mainly used as a reference instrument instead of a feedback sensor for motorized stage calibration. Optical encoders are widely used feedback sensors, which are typically paired with precision positioning systems for close-loop control. Time grating encoders are a newly developed alternative approach to optical grating encoders for displacement measurement. By accurately measuring the time and the phase of a constant-speed electric field, the displacement can be calculated [6,7]
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