A simple optical system for small rotation angle measurement is constructed in this paper. A laser diode is fixed on the rotation part of the mechanism and the laser beam is received by a screen. The position of the spot could be registered from the image captured by CCD camera using the gray centroid algorithm. Then rotation could be obtained from the spot movement according to geometrical relationship. In order to verify the system and evaluate its accuracy, a deflection test of a low carbon steel cantilever beam is performed. It shows that the maximum relative error of experiment results and theoretical value is less than 1.5%, the measuring accuracy and stability is satisfied. The system in this paper has many advantages, such as high resolution, low cost, great convenience and could be used to measure small rotation angles of very complex mechanisms under non-contact request.
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