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Study on the Key Technology of High-resolution Diffraction Grating Interferometric Transducer of Linear Displacements

Author: ShangPing
Tutor: FeiYeTai
School: Hefei University of Technology
Course: Precision instruments and machinery
Keywords: Grating Interferometric Application characteristics of the Measurement Grating Diffraction Efficiency Signal Error Analysis
CLC: TN253
Type: PhD thesis
Year: 2012
Downloads: 234
Quote: 0
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Abstract


Due to the rapidly development of high-precision manufacturing technology, the devicedimensions can reach to micron and even nano scale. So the precision positioning measurementtechniques must be continuously improved. For the requirements of long range and high resolution,the He-Ne laser interferometer is often adopted as the top priority for its advantages of longcoherence length to mater scale and wave length subdivision to nanometer. Its drawbacks aresensitive to environmental conditions and bulky size.In the plant, there are some special advantages based grating interferometer measurementsystems. So there are some manufacturers and research institutions product and study the diffractiongrating interferometer measurement system in the international. They all have their own matureproduct. The technology is still in its infancy in our country. The measurement system of factoriesand workshops still bought from abroad in order to meet the demand. It is very expensive, and needlong maintenance cycle. Or we only study post-processing circuit of the system. In view of thissituation, it will have far-reaching significance for in-depth study of the subject and applied to actualproduction. The measurement system is also applicable to high-precision measuring equipment, suchas nano coordinate measuring machines and so on.Therefore, the key technologies of high-resolution diffraction grating interferometricmeasurement system were researched in the theory and experiment. The main research contentsand results obtained are as follows:1、This thesis presents a high-precision grating interferometer system based on laser diode. Thesystem uses a laser diode as a light source, which reducing the costs while increasing service life. Itcan reduce the environmental impact on the measurement results for using the grating pitch ratherthan the wavelengths of light as a measurement benchmark.2、The characteristics of the grating measurement element were analyzed including the number ofdiffraction of the diffracted beam and the diffraction order. This thesis was focus on calculating thediffraction efficiency and the impact of the grating morphology on the diffraction efficiency, such asthe shape of grating and surface roughness. Scalar and vector diffraction theory were used, thediffraction efficiency of the low lines and the high lines diffraction grating respectively. Thediffraction efficiency of rectangular and trapezoidal was calculated including consideration of theactual situation in the grating such as certain surface roughness.The impacts of the parameters ofgrating groove and the groove depth to the diffraction efficiency were discussed through simulation.This can provide a basis for the grating engraving.3、The error sources of system designed in this thesis were analyzed. Error sources are composed of self interference and external disturbances. Self-interference is mainly composed of the opticalstructure errors and circuit errors. Optical structure errors are introduced by the grating and lightsource, circuit errors are mainly photoelectric conversion error, count and Subdivision errors.External disturbances are composed of the environment errors and the grating guide straightnesserrors. In view of the structure of this system and circuit approach, the sensitivity to self interferenceand external interferences were reduced. This thesis was focus on analyzing and calculating thestraightness error of the guide. Correction method was used to reduce grating pitch error which wasone of the greatest impact factors. After then the accuracy of the system were greatly improved, themaximum positioning error of the system is reduced from±80nm to±30nm.4、Study on high precision signal approach of grating interferometer measurement system, includingcircuit design, high-precision sine wave count and subdivision, We using square-wave signal zerotrigger on the rising edge to count and direction distinguishing, the arctangent method to Subdividethe high-precision sinusoidal signal. This way can reduce the identical requirements of the amplitudeof the two signals. About the circuit part, grating optical signal test equipment was designed used totest the circuit reliability and practicality.5、Finally a series of verification tests were made, includes the displacement measuring experiment,the small step and long-travel experiment, the stability experiments and resolution of experiment.This system was verified meeting the measurement requirements.This thesis includes the innovative points as follow:1、A high-precision diffraction grating interferometic displacement sensor was designed, it mainlyapplies to high-end CNC machine tools, the positioning accuracy of it can reach to±20nm.2、The characteristics of the grating measurement element were analyzed. This thesis was focus oncalculating the diffraction efficiency and the impact of the grating morphology on the diffractionefficiency. Scalar and vector diffraction theory were used, the diffraction efficiency of the low linesand the high lines diffraction grating respectively. This can provide a basis for the grating engraving.3、The error sources of system designed in this thesis were analyzed. Correction method was used toreduce grating pitch error which was one of the greatest impact factors. After then the accuracy ofthe system were greatly improved, the maximum positioning error of the system is reduced from±80nm to±30nm.4、Study on high precision signal approach of grating interferometer measurement system, includingcircuit design. This system can achieved high-precision subdivision, count and directiondistinguishing.

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