Study On A Laser Interferometer For Measuring Straightness And Its Position Based On Heterodyne Interferometry

Straightness error is one of the fundamental geometric toleraces which can affect machines and devices'performances, precision and quality, Straightness measurement is widely used in fabricating and testing of pricision instruments, installation and location of large-scale apparatus, manufacture of war products, and so on. Current laser interferometers used for measuring straightness can measure straightness with high-accuracy while seldom give the relative position of the straightness error, which results in inconvenience for users, to solve this problem, a laser interferometer for measuring straightness and its position based on heterodyne interferometry is proposed which is to come to long-range and high precision measurement of straightness and its position.After the analysis and research work of ultrahigh-accuracy methods and laser interferometers for measuring straightness at home and abroad, an optical configuration based on heterodyne interferometry was designed to come to measuring straightness and its position simultaneously and the measurement principle was analyzed theoretically in detail; The methods of interferometric signal processing of Integral counting, fractional fringe counting and the integration of them were designed; The data acquisition and processing circuit board based on FPGA was designed to process the signals of the measurement system and to communicate with computers; The corresponding measurement software based on Visual Basic language was designed; Finally, a laser interferometer system for measuring straightness and its position based on heterodyne interferometry was designed and constructed.In order to verify the method proposed in this paper and to test the measurement system constructed, experiments were done separately:(1) Compared experiments with advanced interferometers and ultrahigh-accuracy guide rails, such as Agilent straightness interferometer, PI linear stage-M-521.DD, PI flexure-hinge stage-PI-517.3CL. Experiment results of millimeter level with the measurement range of 100mm and displacement step increment of lmm show that the straightness error is 0.16438μm, the average of the displacement errors is 0.006μm and the linear correlation coefficient is better than 0.999; Experiment results of micron level with measurement range of 5μm and displacement step increment of 0.1μm show that the straightness error is 0.18nm, the average of the displacement errors is 1.09nm and the linear correlation coefficient is better than 0.999; Experiment results of nanometer level with measurement range of 500nm and displacement step increment of 10nm show that the straightness error is 1.058nm, the average of the displacement errors is 1.43nm and the linear correlation coefficient is 0.998. (2) Experiments of phase measurement to test the signal processing system designed in this paper. Three experiments with the step increment of 100°,10°nd 5°were carried out and the experiment results show that the linear correlation coefficient is better than 0.999. (3) Experiments of measuring straightness and its positon to test the signal processing system designed in this paper. Two experiments with the step increment of displacements of 1mm and 1μm were carried out, the former experiment result show that the straightness error is 1μm, the average of the displacement errors is 1.3μm and linear correlation coefficient is better than 0.999; the latter experiment show that the straightness error is 0.05μm, the average of the displacement errors is 0.054μm and linear correlation coefficient is also better than 0.999. The experiment results above show that the measurement system designed in this paper based on heterodyne interferometry can come to long-range and ultrahigh-accuracy measurement of straightness and its positon.