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A Random-displacement Measurement by Combining a Magnetic Scale and Two Fiber Bragg Gratings

Published: September 30th, 2019



1School of Instrument Science and Opto-electronics Engineering, Beijing Information Science and Technology University, 2Beijing Engineering Research Center of Optoelectronic Information and Instruments, Beijing Key Laboratory for Optoelectronics Measurement Technology, 3School of Precision Instrument & Opto-electronics Engineering, Tianjin University of Science and Technology, 4School of Precision Instrument and Opto-electronics Engineering, State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, 5School of Instrument and Opto-electronics Engineering, Hefei University of Technology

A protocol to create a full-range linear displacement sensor, combining two packaged fiber Bragg grating detectors with a magnetic scale, is presented.

Long-distance displacement measurements using optical fibers have always been a challenge in both basic research and industrial production. We developed and characterized a temperature-independent fiber Bragg grating (FBG)-based random-displacement sensor that adopts a magnetic scale as a novel transferring mechanism. By detecting shifts of two FBG center wavelengths, a full-range measurement can be obtained with a magnetic scale. For identification of the clockwise and counterclockwise rotation direction of the motor (in fact, the direction of movement of the object to be tested), there is a sinusoidal relationship between the displacement and the center wavelength shift of the FBG; as the anticlockwise rotation alternates, the center wavelength shift of the second FBG detector shows a leading phase difference of around 90° (+90°). As the clockwise rotation alternates, the center wavelength shift of the second FBG displays a lagging phase difference of around 90° (-90°). At the same time, the two FBG-based sensors are temperature independent. If there is some need for a remote monitor without any electromagnetic interference, this striking approach makes them a useful tool for determining the random displacement. This methodology is appropriate for industrial production. As the structure of the whole system is relatively simple, this displacement sensor can be used in commercial production. In addition to it being a displacement sensor, it can be used to measure other parameters, such as velocity and acceleration.

Optical fiber-based sensors have great advantages, such as flexibility, wavelength division multiplexing, remote monitoring, corrosion resistance, and other characteristics. Thus, the optical fiber displacement sensor has broad applications.

To realize targeted linear displacement measurements in complex environments, various structures of the optical fiber (e.g., the Michelson interferometer1, the Fabry-Perot cavity interferometer2, the fiber Bragg grating3, the bending loss4) have been developed over recent years. The bending loss requires....

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1. Fabrication of the fiber Bragg grating

  1. To enhance the photosensitivity of fiber core, put a standard single-mode fiber into a hydrogen-loaded airtight canister for 1 week.
  2. Fabricate the fiber Bragg grating using the scanning phase-mask technique and a frequency-doubled, continuous wave argon-ion laser at a wavelength of 244 nm.
    1. Focus on the optical fiber with a cylindrical lens and an ultraviolet (UV) laser beam. Imprint the grating (periodic modulation of refractive index) in the pho.......

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The distance, ranging from 1 mm to 3 mm11, between the magnetic scale and the detector enabled the detection of the linear displacement with a sinusoidal function. A distance of 22.5 mm between two detectors enabled this approach to realize detection of the direction of an object's movement with a phase difference of 90°. The two detectors were separated from each other for (m ± 1/4)τ (m is a positive integer) and (m &#.......

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We have demonstrated a new method for random linear displacement measurements by combining a magnetic scale and two fiber Bragg gratings. The main advantage of these sensors is random displacement without limitation. The magnetic scale used here generated a periodicity of the magnetic field at 10 mm, far beyond the practical limits of conventional optical fiber displacement sensors, such as the displacement mentioned by Lin et al.7 and Li et al.8. The temperature-dependent .......

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The authors thank the Optics Laboratory for their equipment and are thankful for financial support through the Program for Changjiang Scholars and Innovative Research Team in University and the Ministry of Education of China.


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Name Company Catalog Number Comments
ASE OPtoElectronics Technology Co., Ltd. 1525nm-1610nm
computer Thinkpad win10
fiber cleaver/ CT-32 Fujikura the diameter of 125
fiber optic epoxy /DP420 henkel-loctite Ratio 2:1
interrogator BISTU sample rate:17kHz
motor driver Zolix PSMX25
optical circulator Thorlab three ports
optical couple Thorlab 50:50
optical spectrum analyzer/OSA Fujikura AQ6370D
permanent magnet Shanghai Sichi Magnetic Industry Co., Ltd. D5x4mm
plastic shaped pipe Topphotonics
power source RIGOL adjustable power
single mode fiber Corning 9/125um
Spring tengluowujin D3x15mm
stepper motor controller JF24D03M

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