Absolute Distance Measurement Using Frequency Sweeping Interferometry With Large Swept Range And Target Drift Compensation

The distance measurement method with laser interferometry is the one of the highest accuracy at present.Compared with incremental laser interferometry,the absolute distance measurement（ADM）method can realize instantaneous measurement of distance without continuous movement of the target,and high-speed and high-accuracy measurement can be achieved without laying guide rail.Furthermore,the influence of cumulative error can be avoided.Therefore,ADM has broader application prospects in the fields of high-end equipment manufacturing,aircraft assembly and big scientific devices.Analyzing the current research status of ADM methods,a method of absolute distance measurement using frequency sweeping interferometry with large swept range and target drift compensation is proposed in this dissertation for long-distance,high-precision and fast distance measurement.Frequency sweeping interferometry（FSI）is a widely used absolute distance measurement method with the advantages of large measurement range,simple structure,and high flexibility.Three key issues in the realization of this method include calibration of the frequency sweeping range,compensation of target drift during the sweeping process,and accurate counting of interference fringes.In order to further improve the range and accuracy of ADM,in this dissertation,periodic continuous sweep of the laser frequency with large range up to 2.5 THz is performed by controlling the DC motor of an external-cavity diode lasers（ECDL）.A reference interferometer with known length is introduced,and the distance to be measured is obtained by detecting the phase changes of the measurement and reference interferometer caused by laser frequency sweeping.Thus,the use of external frequency calibration devices can be avoided and the complexity and cost of the system can be reduced.The target drifts of the measurement and reference interferometer during frequency sweeping process are monitored in real time by adding a monitoring interferometer which employs a frequency stabilized He-Ne laser and has a completely common optical path with the measurement and reference interferometer.Then,the applicability of the proposed ADM system in complex environments is improved and the stability requirements for the reference interferometer can be reduced.In order to realize synchronous demodulation of the four interferometric signals with high-precision,two electro-optical phase modulators（EOMs）are used to modulate the phase of ECDL and He-Ne laser at different frequencies,and the phases of the four interferometric signals are demodulated simultaneously by the phase generation carrier（PGC）demodulation method based on frequency division multiplexing technique.In order to improve the accuracy of absolute distance measurement and reduce the influences of random noise,the phases acquired during one frequency sweeping process is linearly fitted based on the least squares method to obtain the ratio of the phase changes between the measurement interferometer and the reference interferometer,and this ratio is multiplied with the calibrated length of the reference interferometer to obtain the absolute distance to be measured.Thus,the update rate of data is half of the frequency sweeping period.To verify the feasibility of the proposed absolute distance measurement method,an experimental setup of absolute distance measurement using laser frequency sweeping interferometry based on large swept range and target drift compensation was constructed.Firstly,the experiment of stability test was carried out for 60 min.The results show that,the drifts without compensation were larger than 20 nm,and they reduced to smaller than 3 nm after compensation,which shows the necessity of drift compensation.Next,the accuracy of the phase demodulation algorithm was verified by nanometer displacement measurement experiments,and the experimental results showed that the maximum step errors of 1 nm,5 nm and 10 nm step displacement measurements were 0.47 nm,0.72 nm and 2.5 nm,respectively,and the nonlinear error caused by phase demodulation was less than 0.1 nm,indicating that the phase demodulation accuracy was better than 1°.The calibration of the reference length was realized by inverting the measurement and reference interferometer,and the length of the measurement interferometer was provided by the corresponding monitor interferometer.A relative uncertainty of 2.97×10^-6 was obtained at a refenernce length of about 200 mm.In the stability test experiments of absolute distance measurement,20 times of frequency sweeps were conducted at a distance of 4.4 m with a frequency range of 2.5 THz,and the relative stability was improved from 5.27×10^-6to 4.71×10^-7by after drift compensation.In the accuracy experiment of ADM,repeated measurement experiments of 200 mm were conducted at a distance of 4.5 m compared with the relative displacement measurements.The experimental results show that at a swept frequency range of 1 THz,the residual fluctuation and standard deviation of the measurement results after drift compensation were 7.51μm and 2.46μm,respectively.When the swept frequency range is expanded to 2.5 THz,the residual fluctuation and standard deviation decrease to 3.37μm and 1.02μm,respectively.All experimental results have demonstrated the effectiveness of expanding the frequency sweeping range and the compensation of target drift in improving the stability and accuracy in FSI.At the same time,the system of absolute distance measurement was designed,including data visualization and storage module,laser control module and guide rail control module,and automatic absolute distance measurement was achieved.