Design And Implementation Of The Simultaneous Measurement System Of Large Range Displacement And Angle Based On Laser Heterodyne Interferometry

With the rapid development of the advanced technology such as the microelectronictechnology, the ultra-precision machining technology and the biomedical engineering, somehigher requirements are put forward in the nanometer measurement technology and instruments.The multi-degree of freedom measuring system with large-range, high-precision has become theresearch focus in the nanometer measurement technology. Since the simultaneous measurementof displacement and angle is the key to realize the multi-degree of freedom nanometermeasurement, more and more attention has been payed to it. Based on the method that the opticalstructure can orthogonally return the linearly polarized light along the original path by Faradayeffect, the related theory and technology of the laser heterodyne measurement which can achievelarge range simultaneous measurement of displacement and angle was studied in detail in thisdissertation, the optical structure was designed, and the mathematical model was built. What’smore, nonlinear error analysis of the simultaneous measurement of displacement and angle wereimplemented, and the laser heterodyne interference signal processing method was considered.Finally, the feasibility and effectiveness of the proposed measurement method were verified bysome relative experiments. The main work and innovations of this dissertation are as follows:(1) The research status of the nanometer displacement and angle measurement at home andabroad were analyzed comprehensively, and the laser heterodyne interferometer principle andthe Faraday effect were described. Based on the optical structure which can orthogonally returnthe linearly polarized light along the original path by the Faraday effect, the system scheme forthe laser heterodyne interferometer which can realize simultaneous measurement of large rangenanometer displacement and angle was designed, and the method for the laser heterodyneinterferometer based on the Faraday effect was put forward. In addition, the optical andmechanical structures of the measurement system were designed.(2) According to the designed system scheme of the simultaneous measurement systemof displacement and angle, the overall layout and the position relationships ofthe optical components along the optical path were considered, and the equilibrium position ofthe light path structure was defined. Starting from various motion modes of the measured object,the two path optical path differences was analyzed while the measuring mirror translating,aiming at measurement object’s rotation with clockwise and counter clockwise direction respectively. The relations between the two optical path differences andthe rotation angle were derived, the general expression of between optical path differences andthe rotation angle was induced, and the mathematical model for simultaneous measurementof displacement and angle was established. Based on two cases of movement, beginning fromthe equilibrium position and an arbitrary position, the methods for simultaneous measurement ofdisplacement angle were discussed, the specific formula (were)derived, and the measurementrange and accuracy were presented. The simulation results show that when the separationdistance of two beams is150mm, the angle measurement range is about11.00°and thedisplacement measurement resolution is0.293nm, and the angle measurement resolution is0.0001°when the optical path difference is1nm.(3) The nonlinear errors of the laser heterodyne interferometer with the Faraday effect forlarge range and high accuracy displacement and angle measurement were studied. Starting fromthe optical path structure of the measuring system, the sources of nonlinear errors were discussed.The nonlinear errors of the laser source, the polarization beam splitter, the optical rotator, theghost reflection and the installation error of the analyzer were discussed, and an integratedmathematical model of the nonlinear errors was set up. The comprehensive influences of eacherror source were discussed, and some methods and measures to eliminate the nonlinear errorswere put forward. The theoretical analysis and simulation results show that the polarizationmixing and frequency mixing of the laser source, the polarization beam splitter and the opticalrotator are the main sources of nonlinear errors. The maximum nonlinear phase error offrequency mixing is up to0.60o, and the phase error of polarization mixing is up to0.45o.(4) The signal processing methods of the laser heterodyne interference were studied, and thepretreatment circuits of the interference signal were designed for frequency reduction, waveformtransformation and optimization, which provided the basis for signal processing in aprogrammable device. Based on the reference signal, four intervals with phase difference of90obetween each other were set up. The real-time movement direction identification and the integerfringe counting were realized by detecting the times that the rising-edge of the measurementsignal crossed the intervals. A laser heterodyne interferometer signal processing method offour intervals across the border was put forward, the discontinuous problem of phasemeasurement was solved when the initial phase difference was not equal to the zero phase difference, and the phase correction approach was proposed in detail. Take one of the outputsignals of the signal generator as the measurement signal and another as the reference signal,some simulation experiments of the decimal phased measurement and integral phasemeasurement were carried out. The experimental results show that the methods of the motiondirection identification and the phase compensation are correct, the measurement average error is-0.0401o, and the standard deviation is0.0683o in stability experiment. The displacementmeasurement accuracy of the system will be better than0.1758nm corresponding to the laserwavelength of632.8nm.(5) According to the designed optical structure of the laser heterodyne interferencemeasurement system based on the Faraday effect, the single channel measurement system wasbuilt, some measurements such as the stability measurement, the nanometer level displacementmeasurement experiment, also the micron and millimeter level displacement measurementexperiment were implemented to gauge the precision and stability of the displacementmeasurement. The measurement system based on the laser heterodyne interferometer which canmeasure the displacement and angle simultaneously was set up, and the angles measurementrange and accuracy were detected. Take the linear displacement stage as the measurement object,some simultaneous measuring experiments of displacements and angles were carried out. Theexperimental results show that the displacement measurement system stability is better than5nmwhen the platform was immobile. The displacement measurement average error is better than10nm, the standard deviation is better than8nm in the nanometer level displacement experiments;the measurement average error is better than1μm, the standard deviation is better than3μmin the micron and millimeter level displacement measurement experiments. The anglemeasurement range is about4.3o, and the measurement resolution is betterthan0.0001o. The average error and standard deviation are in the range of micron level in thedisplacement and angle measurement experiment, and the angle deviation of the lineardisplacement stage within10-5o, which are completely consistent with its parameters. All theresults of the experiments validate the correctness and feasibility of the simultaneousmeasurement system of displacement and angle.