Abstract
A simple noncontact force sensor based on an optical fiber Bragg grating attached to a small magnet has been proposed and built. The sensor measures the force between the magnet and any ferromagnetic material placed within a few millimeters of the sensor. Maintaining the sensor at a constant standoff distance, material loss due to corrosion increases the distance between the magnet and the corroded surface, which decreases the magnetic force. This will decrease the strain in the optical fiber shifting the reflected Bragg wavelength. The measured shift for the optical fiber used was 1.36 nm per Newton. Models were developed to optimize the magnet geometry for a specific sensor standoff distance and for particular corrosion pit depths. The sensor was able to detect corrosion pits on a fuel storage tank bottom with depths in the sub-millimeter range.
Highlights
Significant progress has been made towards the development of fiber Bragg grating (FBG) sensors, which have the advantages of being immune to electromagnetic interference, easy to multiplex, remotely accessible, stable for long-term measurements, and easy to cable
We used a different approach that does not rely on the bonding of the Bragg grating to a specific substrate to develop a contactless force sensor; the sensor employs one FBG and a small permanent magnet attached to the tip of the optical fiber
Theoretical analysis of the magnetic field caused by the sensor magnet was performed, and sensors were designed to maximize the magnetic force for standoff distances in the millimeter and sub-millimeter range
Summary
Significant progress has been made towards the development of fiber Bragg grating (FBG) sensors, which have the advantages of being immune to electromagnetic interference, easy to multiplex, remotely accessible, stable for long-term measurements, and easy to cable. FBG sensors have been used in the oil and gas industry to monitor corrosion in pipelines, offshore platforms, and fuel storage tanks [19]. We used a different approach that does not rely on the bonding of the Bragg grating to a specific substrate to develop a contactless force sensor; the sensor employs one FBG and a small permanent magnet attached to the tip of the optical fiber. The developed sensor can detect corrosion with sub-millimeter depths and has unique properties: it is passive with no need of local power sources, and can be used to monitor corrosion in remote sites and in places that are difficult to access
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