| The 3D profile measurement of large-scale components is of great significance in many fields,such as aeronautics and Astronautics,national defense and civil affairs.For up to 20 m long airplane wings,the interchangeable rivet hole patterns require 40?m accuracies on hole positions;Large spacecraft assembly and machining also have similar needs.At present,the instrument used for 3D profile measurement of large components usually adopts the method of high-precision absolute distance measurement combined with high-precision two-dimensional turntable.Among them,high-precision absolute distance measurement is the core technology.In the existing absolute distance measurement methods,the frequency scanning interferometry method has many advantages,such as large measurement range,no blind area,and non-cooperative target measurement.At present,the frequency scanning interferometry commonly adopts semiconductor laser as the swept laser.But as the semiconductor laser has a small frequency swept range?about 100 GHz?,which limits the measurement precision of frequency scanning interferometry.If the external cavity laser?frequency swept range 10THz?and frequency sampling method are used,a better precision theoretically could be obtained.But this method still has three problems.The first is the problem of environmental temperature changes lead to a drift of measuring datum;The second is the problem of when the environment vibrates,due to the influence of Doppler effect,the measurement error will be hundreds of times to the actual vibration,so the application environment is very limited;The third is that,due to the influence of dispersion,even though the frequency scanning range is increased,the precision does not increase.In order to solve the above problems,this paper proposes a new method for measuring the distance of large-scale high-precision targets.The principle of this method is analyzed,and a large distance measurement experiment has verified this method.The main research contents are as follows:The basic principle of frequency scanning interferometry is studied,and the problems in existing frequency scanning interferometry distance measurement systems are analyzed theoretically.In this paper,a wideband frequency scanning interferometry absolute distance measurement method is proposed.In this method,an external cavity laser?frequency swept range 10THz?is used.In order to overcome the influence of frequency modulation nonlinearity,the frequency sampling method is used,which is confirmed in experiment that compared to the phase comparison method frequency sampling method can obtain higher signal-to-noise ratio,more suitable for non-cooperative target measurement.In order to overcome the influence of temperature drift,this system introduces a gas absorption cell,and the mathematical model of gas absorption cell based on-line calibration technology is established.In order to overcome the influence of vibration of target,dual acoustooptic modulators module is introduced.In order to overcome the influence of dispersion mismatch,the chirp decomposition dispersion mismatch compensation algorithm is adopted.The analysis and experiment show that the method can measure the non-cooperative target with high precision,and has a high measurement stability.Arming to the problem of Doppler effect introduced by target vibration,the Doppler mathematical model is established.The results of frequency scanning interferometry under different amplitudes and frequencies are simulated and analyzed.A new Doppler effect correction method is proposed,and at the same time,a phaselocked loop module is introduced to ensure the signal-to-noise ratio of the vibration measurement signal when a non-cooperative target is measured.In the measurement experiment of a cooperative target located at 16 m,the standard deviation is 645?m when the Doppler effect is not corrected,and after correcting the Doppler effect,the standard deviation is 3.15?m.In the measurement experiment of a non-cooperative target located at 15 m,if the Doppler effect correction method without phase-locked loop is used,the standard deviation is 53.52?m,and when the Doppler effect correction method with phase-locked loop is adopted,even if the retroreflective power is only a few nanowatts,the standard deviation can reach 3.43?m.Theoretical analysis the influence of dispersion mismatch in frequency scanning interferometry,establish the theoretical model of dispersion mismatch.The influence of dispersion mismatch under different distances and FM bandwidths are simulated and analyzed.The problem that traditional dispersion mismatch correction algorithm will introduce an additional random error is pointed out,and a new chirp decomposition based dispersion mismatch compensation algorithm is proposed.This method doesn't need to determine other parameters before each measurement,and therefore doesn't introduce additional random error.At the same time for the huge computation problem of chirp decomposition,a combining sub-Nyquist sampling and chirp decomposition method is proposed.In the experiment,the premise without considering the influence of vibration,for a non-cooperative target located at about 3.9m,using chirp decomposition,the measurement standard deviation is 0.72?m,and using the traditional dispersion correction algorithm,the measurement standard deviation is only 2.11?m;when the sampling points number is 3200000,using common chirp decomposition will cost 269.8s,and using fast chirp decomposition will just cost 1.9s.An absolute distance measurement system based on the frequency scanning interferometry introduced above is designed and established.When a non-cooperative target located at 15 m is measured,the standard deviation can reach 3.43?m,which proved that this system can achieve higher measurement precision for long-range noncooperative targets.Different targets measurement experiments show that,when the light contains only a distance information,even the SNR is lower,this system also can obtain a good measurement precision.By measuring a non-cooperative target at 2.2m for two days,the standard deviation of the measurement is 3.70?m,which verifies the excellent stability of the system.Finally,a cooperative target distance measurement uncertainty of the system is better than 8.3?m in the range of 24 m,and the relative uncertainty can reach 3.5×10-7 at 24 m. |
PDF Full Text Request