Talbot Micro-displacement Measurement System Based On Optical Path Modulation

In recent years, due to the rapid development of information technique, superfinish technique, and aerospace technique, numbers of new material, new technology and new product is emerged, in which micro-displacement technology is often necessary. With the advantage of enabling the measurement accuracy to be at wavelength level, optical grating measuring system becomes the most typical one in micro-displacement technology. However, the high-resolution, high-groove line grating is very expensive, and difficult to make, so it is an effective method to seek new method using low-groove line grating and new Moiréfringe subdivision technology to improve the accuracy of the measurement.In this paper, a novel micro-displacement measurement method based on optical path modulation is proposed. In order to improve the measurement accuracy, a new Moiréfringe subdivision method with embedded differential type based on FPGA is proposed. This paper lays emphasis on researches about the principle of displacement measuring method based on optical path modulation and SOPC designing of embedded differential Moiréfringe subdivision method. The main contents are as follows:The common characteristic of most double grating interferometers is to use Moiréfringe detection to realize micro-displacement measurement. One method is to produce periodic Moiréfringe by using two gratings with large difference period and making relative movement between them. In this paper, Moiréfringe detection method based on optic path modulation is proposed, instead of moving grating, optic path is modulated by use of optical wedges. During measurement, the two gratings are fixed, no movement, the shift of Moiréfringe is induced by modulating optic path through optical wedges.From the interference theory of double-interferometer, the intensity distribution formulas in each image region of optic path modulation, based on Talbot interferometer are derived, and Moiréfringe pattern in each image region is simulated by Matlab. Variation regularity of Moiréfringe pattern, caused by modulating the optic path of first positive order beam, in the interference region formed by first positive order beam(or first negative order beam) and zero order beam, is given.The method of Talbot micro-displacement measurement based on optic modulation is proposed, in which two sine gratings with identical period to form Talbot interferometer, two optical wedges are placed in the interference region of first positive order beam and zero order beam, one wedge is fixed, the other can move along its incline plane, and the object to be measured to connect the moving wedge through a mechanism. Moiréfringe is shifted by changing optic path induced by the variation of wedge thickness. Micro-displacement is measured through detecting the moving numbers of Moiréfringes. Simulation and experiment are carried out with 50l/mm groove line grating, the measurement resolution is improved ten times compared to that of moving grating method.A new Moiréfringe subdivision method based on FPGA (Field Programmable Logic Gate Array) is proposed and the principle of Moiréfringe subdivision method using Mealy state machine is given. Adopting Altera's Cyclone II series FPGA chip EP2C8Q208 programmable logic chip, the function modules of subdivision-direction recognizing function, couting function, and optical parameter conversion function are integrated in one chip, realizing integration of Moiréfringe detection.Modular process design is adopted in the PSOC (Programmable System on Chip) of Moiréfringe subdivision method. The designed method of embedded differential Moiréfringe subdivision method using Mealy state machine is proposed. In the design process, subdivision and direction recognizing are combined in one functional module to avoid the traditional subdivision method constraints, and when optical parameters change, the only thing need to do is to modify the program code making program universial.