The Study Of Surface Strain Measurement Technology Based On Optical Fiber Distributed Sensing

The manufacture,assembly,and application of large-scale precision mechanical structures drive the development of surface strain measurement to spatial resolution of millimeters.Optical fiber distributed sensing has been rapidly developed and applied with the capability of continuous sensing.In rayleigh strain sensing based on optical frequency domain reflectometry,wavelength resolution and sensing spatial resolution are mutually restricted,resulting in strain resolution of micro-strain yielding a spatial resolution of centimeters.At the same time,the method of spectral shift calculation based on cross-correlation limits the feasibility of large strain measurement and can not meet the high spatial resolution and large range of distributed measurement of surface strain.Aimed at these problems,a distributed sensor signal processing method based on continuous wavelet transform and a strain calculation method based on local Rayleigh spectral similarity are proposed.The spatial resolution is raised to millimeters and the strain measurable range is increased from 1000 ?? to 3000 ??.The main content of this article includes the following four aspects:1.Scheme design of distributed strain measurement system based on optical fiber distributed sensing: Theoretical analysis of the optical frequency domain reflection and rayleigh scattering is performed;then the effect of the nonlinear tuning effect of the light source on the spatial resolution of the system and the influence of the wavelength accuracy of the output light source on the accuracy of the strain measurement are analyzed;finally,the structural design of distributed strain measurement system is completed according to the theory of fiber distributed sensing and analysis of the measurable influence factors.2.Research on optical fiber distributed sensing signal processing method: Firstly,demodulation performance of traditional distributed sensing signal processing method is analyzed.Then distributed sensing signal processing method based on continuous wavelet transform is studied,relationship between the wavelet transform parameters and distributed sensing signal is established,demodulation performance is analyzed and algorithm is optimized.Then the limited reason for measurable range is analyzed.Spectral shift calculation method based on local spectral similarity is proposed.By optimizing the local Rayleigh spectrum length,the high similarity in local Rayleigh spectral matching is obtained to improve the strain range.3.Research on information recovery method in surface strain measurement: Firstly,the relationship between strain of measurable surface and axial strain of sensing fiber is established based on microelement pure bending model.Then fiber layout model of regular surface is designed and distributed measurement point of spatial coordinates is determined.Finally,effects of sensor fiber bonding model parameters on strain transfer rate during surface strain measurement is analyzed and the appropriate bonding model parameters are determined.4.Performance test of the distributed strain measurement system: Build a measurement system and test distributed strain measurement performance of system.Experimental results show that the linearity error is 20 ?? during measurable range of 3000 ??.Strain variation is 3.9?? with one minute and 7.3?? with one hour in stable measurement.With a spatial resolution of 3mm,the strain resolution reaches 5??,and when the spatial resolution of the sensor is 20 mm,the strain resolution is 1??.Strain standard deviation is 10.1?? after ten repeted measurements.Finally,surface strain of the cantilever structure is measured with 4mm sensing spatial resolution,3mm measurement point spacing and total 440 distributed measurement points.The experiment results show that surface strain trends measured is consistent with the simulation results.In this paper,through design of strain measurement system and study of sensing signal processing method,distributed strain measurement with mm magnitude spatial resolution and 3000?? measurable range is realized.It solves the problem of continuous measurement of surface strain in large-scale precision mechanical structures and provides a solution for surface shape recovery.