| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 95 === DVD pick-up head applies the astigmatic principle to let the data track on the disc be focused and read out. Furthermore, based on the same principle we can also design and develop a high-precision optical displacement measurement system. However, the original DVD pick-up head is limited to a narrow measuring range by the short focal length of the objective. In order to expand the measuring range, this thesis proposes varying focal length of the objective to modify the beam convergence, which makes the imaging laser spot completely project onto the photodiode-IC. Through the optical analysis software, the optical system of the DVD pick-up head is thoroughly analyzed and studied to find out the relationship between the variation of astigmatic spot and the defocus signal. Besides, a high precision laser interferometer is adopted as displacement reference to experimentally acquire the calibration curve between the defocus signal and the displacement. Firstly, the measuring stability and the inclination interference are investigated; and further, a systematic study on four objectives with different focal length is conducted in terms of measuring range, linearity, resolution and repeatability. For the verification testing of the measuring performance, a precision shear force type piezoelectric stepping actuator is used as the testing subject to comprehend the repeatability and the error driving the measurement of the long-distance motion. For the error analysis, the infrared thermo graph and the thermocouple are utilized to measure the temperature variation inside the operating DVD pick-up head; moreover, the thermal influence on the imaging laser spot is also analytically researched. And from the variation of the imaging laser spot the corresponding defocus signal can be derived. Through the comparative evaluation between the analytical and the experimental data the analytical model can be improved to achieve higher reliability.
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