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

Summary

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

Abstract

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.

Introduction

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....

Protocol

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 photosensitive core by using a phase mask (parallel with the fiber axis) placed in front of the fiber. The light output by the laser is shaped and perpen....

Results

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 ± 1/4)τ ≤ the total l.......

Discussion

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 .......

Materials

List of materials used in this article

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