Study On A Displacement Measuring System Based On Phase Grating Interferometry

With the rapid development of precision manufacturing technology, the requirements of measurement is more and more strict. Micro and even nano-scale displacement measurement has become a main research direction in the field of measurement. The traditional approach is to use He- Ne laser interference measurement technique, but the expensive cost, complex structure and high requirements for measuring environment limits its widespread use. On the contrary, the displacement measurement based on diffraction grating interferometer have been developed rapidly in recent years since its low cost and insensitive to the environment. The large range and high resolution become the main research direction of grating interferometry technology.In this paper, we devote on the design of interference measurement system, digital subdivision, the error analysis and error compensation based on the phase grating interferometry principle with the ultimate goal of 10 nm resolution and 12 mm range displacement measurement according to the subsequent research for profilometer.The first step of grating interferometer measurements is to obtain the photoelectric signals which are closely related to the displacement. In this paper, the model of interferometer was built based on the requirements of experiment and the spatial arrangement after the analyses for the existing two principles of the optical interferometers. With the help of ZEMAX, optical model is built and the feasibility is verified. According to the requirement of the final displacement measurement, the analog signal conditioning circuit and digital circuit was designed. And we completed the experiment platform of interferometry and PCB of the signal conditioning circuit.Nanometer resolution measurements request the subdivision method must be high-expansion adopting in digital way. In this paper, we construct the tangent function segmentation method to realize high multiples of segmentation. According to the analyzing of the main error sources and error characteristics of the actual signals, we analyzed the segmentation error of optical signal with the aid of Matlab.By analyzing the practical characteristics of noise and nonlinear error, this paper preprocessed the original signal before subdivision in the form of digital signal processing. IIR band-stop filter and Kalman filter were designed to reduce the noise. Also, the nonlinear error compensation model was built, and we have completed the nonlinear error compensation based on the theory of least squares.Finally, we completed digital signal processing of original signals including data collecting, digital filtering, correction of nonlinearity and digital subdivision based on the software platform of Labview. And the multi-threaded operation of labview is used to calculate those parameters of nonlinearity and updates it in a short time in order to improve the accuracy of the interferometer system since those nonlinearity parameters changing slowly in continuous measurement. The design of the grating interferometry system was completed in preliminary. And we analyzed the error of subdivision and resolution of this system according to the contrast test. The experiments show that this system with a resolution of 10 nm. Also the standard deviation of subdivision can be controlled within 10 nm after the correction of nonlinearity.