High Anti-noise Displacement Sensor Base on Fiber Bragg Grating Peak Counting
The lateral creep of hydrate-bearing layers associated with natural gas hydrate decomposition can easily cause geo-engineering security risks.The triaxial shear instrument is an experimental apparatus for simulating lateral creep of the lateral creep of hydrate-bearing layers.To obtain the displacement of this creep accurately and in real time,a kind of displacement sensing scheme based on fiber Brag grating peak counting of strain shift curve with high noise resistance is proposed.The sensor is mainly composed of fiber Bragg grating,cantilever,transmission system,gear,rack,base and metal-tube.Inside of the transmission system is composed of gears with different teeth and installed on the base with bolts.Using the rack as the probe to engage with the input gear of the transmission system,another side of output shaft is fixed with the gear securely.One end of the cantilever beam press is fixed to the side plate of the base,while the other end presses against the gear.One end of the fiber Bragg grating is encapsulated in a metal tube using epoxy resin,and the tube is welded vertically to the free end of the cantilever beam press plate using a laser,while the other end is fixed in a similar manner on the side plate of the base after a pre-tension is applied.The interval between two metal tubes is 41 mm.The rack and transmission system are used to convert the lateral displacement of the hydrate-bearing layers into the rotation of the output gear.During the rotation of gear,the tooth of gear pulls on the free end of the cantilever to produce periodic stretching and resetting of the fiber Bragg grating.The average and minimum wavelength shift of the reflection center is 1405 pm and 1263 pm,respectively.By calculating the product of the number of peak jumps in the wavelength drift curve of the reflection center of the fiber Bragg grating and the standard step length of the rack passing between adjacent peak jump,we can determine the lateral displacement of the geological layer.Furthermore,we can build a linear relationship between the frequency of peak jumps and speed.The coefficient of sensor's transmission system is 2 mm/r and tooth number of gear is 20,respectively.In the measuring range of 40 mm,displacement sensor has a resolution up to 0.1 mm,maximum error of only 16 μm,and the range is adjustable through the rack length.The sensor displays excellent repeatability and multi-parameter detection capabilities,greatly enhancing the sensor's noise immunity.When the rack is pushed at three different speeds for 6 mm,it is found that the wavelength drift for the same number of gear teeth is only 28 pm,which is significantly smaller than the minimum wavelength drift of the reflection center 1 263 pm.In situations where the displacement change speed far exceeds temperature variations,temperature only offsets the entire wavelength drift curve of the reflection center.This offset is directly related to the temperature sensitivity of the fiber Bragg grating and range,temperature does not impact the wavelength drift of the reflection center resulting from displacement or the displacement of the rack between adjacent peaks.In this case,the fiber Bragg grating is only affected by noise sources like vibration and demodulation instrument error.The minimum optical signal-to-noise ratio achieved is 43.966 dB,meeting the requirements for hydrate-bearing layer creep monitoring and enabling simultaneous measurement of displacement and velocity.It's worth noting that when the displacement change speed matches the temperature change specifically,when the temperature changes rapidly form 0℃to 20℃,the optical signal-to-noise ratio decreases from 43.966 dB to 14.497 dB,greatly impacting displacement measurement accuracy.Therefore,it's essential to incorporate a free-state fiber Bragg grating for temperature compensation,ensuring the sensor maintains a high noise resistance.At this point,the sensor utilizes two fiber Bragg gratings to achieve three-parameter measurement of displacement,speed,and temperature,while maintaining a high cost-efficiency.Finally,four kinds of displacement sensors'performance are compared,with displacement sensor base on fiber Brag grating peak counting of strain shift curve standing out for its superior anti-noise and multi-parameter detection capabilities.
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维普
