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

We addressed the challenge of long distance displacement measurements. Your then, optical fibers. The technical can be used in both basic research and industrial production.

With the right set up displacement can be measured. Other measured of the optical fiber. This methology is appropriate for use in industrial settings.

The user just needs to pull the magnetic scale on guard rail. This method could provide insight into the research area of optic fiber senses. It can be used to measure other parameters such as velocity and acceleration.

Create the Fiber Bragg gratings with the scanning phase-mask technique. For this, use single mode optical fibers that have been in a hydrogen loaded air tight container for one week. The phase-mask technique involves focusing a laser beam through a phase-mask onto an optical fiber to create periodic modulation of its refractive index.

Once two fibers are inscribed, place them in a 100 degree celsius oven for 48 hours to remove any residual hydrogen. The retrieved fiber gratings parameters will no longer change after the annealing step. Implement the design of the magnetic scale with the appropriate magnets.

The scale has slots to hold a set of cylindrical magnets along its length. The north and south poles of the permanent magnets alternate along the scale with a 10 millimeter pitch. The magnets in the study are 5 millimeters in diameter and have a magnetization of 750 kiloamperes per meter.

2 detectors set at a fixed distance apart will feel different forces as they move along the scale. Choose the separation so the forces have a phase difference of 90 degrees. In this case create a stainless steel clamp to hold the two detectors 22.5 millimeters apart.

To fabricate the sensor begin by preparing heat curable fiber optic epoxy. Once the epoxy is ready get one of the two Fiber Bragg gratings. Place a ruler alongside the fiber.

Starting at a point just beyond the grating, measure approximately 10 millimeters along the fiber and place a mark there. The fiber optic stripper, remove the coating from the marked position away from the grating. Clean the surface of any remaining polymer with alcohol and dust free paper.

When done, take the fiber to a high precision fiber cleaver to cleave the stripped region. Next, set up other elements of the sensor. Put a permanent magnet on a hot plate at 150 degrees celsius, then place a 15 millimeter spring on top of the magnet.

Inside the spring epoxy the grating end of the prepared fiber to the magnet. Allow the epoxy to cure at 150 degrees celsius for 30 minutes. To continue get the magnet spring grating assembly.

In addition, have a tapered and threaded tube that can go over the assembly. Put the assembly inside of the tapered tube. Push the magnet to compress the spring.

Use adhesive tape to fix the magnet in position. Next, insert a tapered tail pipe at the open end of the tube. Once it is in place, get optical fiber with epoxy at its end and insert it into the tail pipe to bond with the internal fiber.

Cure the applied adhesive on a hot plate, at 150 degrees celsius. Have the fiber oriented parallel to the surface of the hot plate. After 30 minutes retrieve the assembly from the hot plate.

Then, remove the tape to allow the spring to apply a force, to straiten the fiber. Fusion splice and APC style single mode connector to the end of the fiber coming from the tube. This is one of two detectors after splicing the connector.

It is ready for use in the system. When detectors have been made of both fibers fix them into the slot of the clamp using a screw. Take the clamp with the detectors to the testing system.

The systems main components are a micro displacement platform that is parallel to the magnetic scale. A high speed wave length interrogator with built in amplified spontaneous emission and is power source and an optical spectrum analyser with a minimum 200ths of a nanometer resolution. Mount the clamp with the detectors to the micro displacement platform.

Adjust the height of the detectors above the magnetic scale and fix the clamp. This schematic provides an overview of the testing system after the detectors have been connected. The output of the interrogator goes into the first port of a three port circulator.

From there, the light goes on to the detectors. Reflection spectra from the detectors pass through a cupular, and then in to the second port of the circulator. Output from the circulator is input to the optical spectrum analyser.

Use a position controller circuit to control the stepper motor of the micro displacement platform. Connect this controller and the interrogator to a computer. Position the detectors at different positions along the scale to vary the force on the fibers.

When the detectors are at a suitable height above the scale, there's a sinusoidal relationship between the displacement along the scale and the center wavelengths shifts due to strain in the fibers measured under static conditions. Fix the detectors at the height that produces a sinusoid and set parameter for dynamic measurements. Measure the wavelength shifts while using the stepper motor to move the detectors in one direction for a distance before bringing them to rest.

Then continue measurements while moving the detectors in the opposite direction. Next, perform temperature calibration of the sensors. Keep the sensors connected to the instruments but remove them from the clamp.

Then place the sensors on a hot plate. Measure their central wave length change at temperatures from 25 to 90 degrees celsius. Static calibration measurements of the presented detector system, revealed the relationship between the displacement and the two Fiber Bragg gratings wavelength shift.

The wavelength shifts are about half a nanometer. The residual errors are less then 10 picometers. This plot demonstrates the detectors ability to identify forward and reverse motion.

Initially with forward motion, the center wavelength of grating number 2 leads that of grating number 1 by a phase of 90 degrees. Then, motion stops and reverses. Now, the center wavelength of grating number 2 lags that of grating number 1 by 90 degrees.

These data represent multiple measurements made when detector grating number 1 is positioned so its polarity and that of the magnetic scale are the same and measurements made when the polarities are opposite. Over the course of ten measurements, those where the detector and scale have the same polarity are more stable. Here is the measured wavelength as a function of temperature for the two detectors.

When temperature interference is taken into account the temperature sensitivity of the detectors is the same. Which allows for temperature compensation. Were loading the force and the temperature was supply.

By compressing the spring with a magnet is essential for success with the technique. This technique can be a useful tool employ the magnetic strengths. Because it'll detect the periodical variation of the magnetic force directly.

Which is then transform into displacement.