EuroWire March 2020

Technical Article

Fibre Optic Strain Gauge Using Pseudorandom Code Interrogation By William E Hodges, Abhishek P Jawale, Damon E Pate, Ryan C McEntire, Simon G Jackowski and Elliot J Panicco, University of North Carolina at Charlotte, Engineering Senior Design, Charlotte, North Carolina, USA

Abstract In this paper, the purpose and function of a fibre optic strain gauge using pseudorandom code will be explained and discussed. The research and completion of this project was done for the purpose of creating a laboratory instrument that will be used by students in the mechanical engineering programme at the University of North Carolina at Charlotte. Laboratory instruments that measure strain are already present for mechanical engineering students to use and learn about, but a more accurate industry- related strain gauge was desired for students to use and learn with. 1 Introduction Fibre optic cables have become a very versatile medium for things like data transfer and sensors, though these sensors required special order fibre optic cables called FBG (fibre Bragg grating). These fibre optic cables require precise inscribing of the cable to create the needed reflective index, which results in a more expensive end product. The purpose of this study is to create a strain gauge without the use of FBG cabling. Instead, the strain applied to the fibre optic cable will be calculated by measuring the cable’s elongation. This study began with the team doing an immense amount of research of applications of fibre optics and fibre optic laser hardware, as well as research for specific software needed to process the data from the fibre optics. The utilisation of consultation with faculty advisors for this project played a huge part in the advancements made. The team came to a conclusion that the hardware required for this project will need to be a single-mode fibre optic cable that will be connected to a ten gigabit laser transceiver via LC connectors. To reach the required data transfer speed, it was decided that the

transceiver will need to be an SFP+ form factor that will have an Ethernet data converter to send/receive and process data. Pseudorandom code will be gene- rated and sent through the fibre optic cable. The cable will have a base source code that will continuously compare the code received. As strain is applied to the cabling an offset will be displayed due to the increased travel distance of the code. In conclusion, highly accurate strain sensors can be made with standard single mode fibre optic cables, instead of using expensive fibre Bragg grated cables. The pseudorandom code sent through the cable will be used to measure strain forces applied to the cable. These long strings of code will allow the program to sense a measurable time delay from the data that was initially received from when there was no strain. The University of North Carolina at Charlotte desired to create and implement an additional lab to the instrumentation course that utilised a fibre optic strain gauge. The course currently has a lab on wire strain gauges and another on fibre optics. Another lab was desired in order to show the wide range of applications for fibre optic cables, including those in strain applications. The proposed solution was to design and create a fibre optic strain gauge that utilises pseudorandom generated code to measure strain. This solution was desirable because it is the cheapest and simplest design for a fibre optic strain gauge and has a higher level of precision than a traditional wire strain gauge. At the end of the project, a fully functional fibre optic strain gauge utilising pseudorandom code is to be delivered along with a test apparatus that can be used in the instrumentation lab. 2 Main body 2.1 Project overview

All documents describing the functionality of the test apparatus and the logic of the code need to be delivered in case of trouble shooting in the future. 2.2 Project specification 2.2.1 Performance specification PS1: The fibre optic strain gauge should be as accurate as the fibre Bragg grating method, which measures to 2x10 -3 micro-strain. PS2: LabVIEW will gather data at 10 Hz and display results in real time while in operation. PS3: Hardware cost should remain under $1,000. PS4: Fibre optic cable used in testing will be under one metre in length. Pre-calibrated wire strain gauges will be used to initially test the results to verify. Strain will be constantly applied and released in quick succession to verify PS2. Cost-effective parts will be chosen to keep cost below $1,000 for the final product to verify PS3. The fibre optic cable used in testing will be measured to ensure it is less than one metre in length to verify PS4. 2.3 Design narrative In the beginning it was not clear that this project would end up being a lab project for the university. Therefore, the first intention of the project was to demonstrate that the fibre optic strain gauge worked properly and the results could be presented. We started to design for an apparatus that could be placed in an environmental chamber that would expand and contract with changes in temperature. That meant it was important that all of our components would function in a wide range of temperatures. Displaying strain in real time was a consideration but not the main focus. V2: V3: V4: 2.2.2 Performance verifications V1:

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March 2020