In this paper, we propose a scheme for measuring the focal length of a collimating optical instrument. First, a mathematical model for measuring the focal length of a collimator with double gratings is derived based on the moiré fringe formula and the principles of geometric optics. Mathematical simulation shows that a slight difference in the focal length of two collimators has an important influence on the imaging law of moiré fringes. Our solution has a good resolution ability for focal length differences within 5‰, especially in the small angle range below 4°. Thus, the focal length of collimators can be measured by the amplification of the slight difference. Further, owing to the relative reference measurement, the measurement resolution at the symmetrical position of focal length is poor. Then, in the designed experiment, a corresponding moiré image at different angles is acquired using collimators with known focal length. The experimental results indicate that the root mean square error (RMSE) of the collimator corresponding to grating angles of 2°–4° is better than 4.7‰, indicating an ideal measurement accuracy of the proposed scheme. This work demonstrates that our proposed scheme can achieve an ideal accuracy in the measurement of a symmetrical optical path.
Aiming at the measurement of roll deformation between two devices, we propose a novel scheme and elaborate the structure design in this paper. The scheme can realize the relative measurement of roll angle even if the system does not provide a reference datum, which effectively solves the problems of limited measurement range of moiré fringes and accuracy guarantee only in an extremely small angle range. First, the principle of measuring the roll angle by moiré fringes is introduced, and the effects of various error sources in the system are comprehensively considered. Then, the combined measurement scheme and the entire system composition structure are analyzed in detail. In addition, an experimental setup is designed to verify the accuracy of roll deformation measurement. The measurement results show that the root mean square (RMS) error can reach 6.9" within 3°, and is less than 7.8" in the range of 0°-90°. Hence, our proposed scheme has a good measurement stability in a large roll range. INDEX TERMS Roll angle, measurement sensor, system structure design, error analysis, moiré fringe.
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