Continuous Fibre Bragg Grating Sensing for HTS Quench Detection
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Solution Overview
Problem
Conventional fibre Bragg grating (FBG) sensors for detecting quench in High Temperature Superconducting (HTS) machines face challenges such as high EMI sensitivity, limited spatial resolution, and high cost, especially in noisy and cryogenic environments, and require significant changes in sensing regions for accurate detection.
Innovation Solution
An optical fibre sensing system with a substantially continuous fibre Bragg grating that reflects light at a different wavelength and intensity when temperature or strain changes occur, allowing for quick detection of changes along the fibre length using a processor to analyze the reflected spectrum, even with non-separated peaks, and enabling higher reflectivity and longer fibre lengths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional point FBG sensors are used with high reflectivity, then sensing sensitivity is improved, but the sensing length is limited to about 10 mm and spatial resolution is reduced
Solution Approach 1:
The optical fibre is divided into multiple discrete sensing regions along its length, each with a different centre wavelength. This segmentation allows multiple sensing points to be distributed along the fibre while maintaining high reflectivity at each point, resolving the contradiction between sensing sensitivity and sensing length.
Solution Approach 2:
The patent transitions from a single-point sensing approach to a multi-dimensional distributed sensing approach by using wavelength division multiplexing. Each sensing region is assigned a unique wavelength, adding the wavelength dimension to distinguish between different spatial locations, thereby enabling long sensing lengths while maintaining high sensitivity.
2Quantity of substance
If multiple point FBG sensors are connected in series with different centre wavelengths, then the number of sensors per fibre increases, but the area/length that can be monitored is limited
Solution Approach 1:
The patent utilizes wavelength as a distinguishing parameter for each sensing region. By assigning different centre wavelengths to different sensing regions along the fibre, the system can monitor a much larger area/length compared to traditional methods that rely solely on spatial separation, thus resolving the contradiction between quantity of sensors and monitored area.
3Length of stationary object
If continuous FBG sensor with low reflectivity is used, then the fibre length can be extended, but the sensing sensitivity is reduced
Solution Approach 1:
Instead of using a continuous low-reflectivity grating, the patent segments the grating into discrete high-reflectivity regions spaced along the fibre. This allows the fibre length to be extended while maintaining high sensing sensitivity at each discrete sensing point, resolving the contradiction between fibre length and sensing sensitivity.
4Measurement precision
If high reflectivity FBG is used, then sensing sensitivity is improved, but light transmission through the fibre is attenuated
Solution Approach 1:
The grating is segmented into discrete regions with high reflectivity, separated by sections of normal fibre. This segmentation allows light to be reflected strongly at each sensing point while still transmitting through the intervening normal fibre sections to reach subsequent sensing regions, resolving the contradiction between sensing sensitivity and light transmission.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system effectively detects temperature and strain changes with high sensitivity and resolution over long distances, capable of identifying hotspots and quench events in HTS coils, even at cryogenic temperatures, with improved cost-effectiveness and faster response times compared to conventional methods.
Implementation Method 1
The grating 106 reflects light of a certain wavelength, and transmits other wavelengths. An incident light source 109 provides a spectrum of incident light 110 to an upstream end 112a of the sensor 100. Some of the incident light 110 is transmitted to a downstream end 112b of the fibre 102 to provide a transmitted spectrum 111. Some of the incident light 110 is reflected by the grating 106 to provide a reflected spectrum 114.
Implementation Method 2
FBG sensors are sensitive to temperature and strain. FIG. 1B shows the sensor 100 of FIG. 1A subject to a change in temperature and/or strain. Changes to temperature and strain change the spacing ∧ of the grating 106, either due to thermal expansion or elastic elongation. This change in spacing changes the effective refractive index of the grating 106, causing the peak in the reflected spectrum 114 to shift.
Implementation Method 3
Changes to temperature and strain change the spacing ∧ of the grating 106, either due to thermal expansion or elastic elongation.
Data Source
AI summary
An optical fibre sensing system (300) with an incident light source (309), a wavelength spectrum interrogator (313), and an optical fibre (302) with a substantially continuous fibre Bragg grating (306). An upstream portion (318) of the optical fibre has an attenuation length to light to the interrogator (313) at a first equilibrium wavelength. A downstream portion (322) of the optical fibre reflects light to the interrogator (313) when a change in temperature and/or strain at the downstream portion (322) causes a portion of the fibre Bragg grating to reflect light to the interrogator (313) at a second wavelength and at a second intensity. A processor (315) is configured to analyse the reflected spectrum (314) to determine when a portion of the fibre Bragg grating (306) is experiencing a change in temperature and/or strain based on deviation in the reflected spectrum (314) from an initial peak corresponding to the initial wavelength.