Dual Fiber Bragg Grating Strain Sensor with Passive Temperature Compensation
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Solution Overview
Problem
Existing optical fiber strain sensors face challenges in accurately discriminating between strain and temperature effects, particularly in providing practical packaging methods that are simple, cost-effective, and maintain temperature independence without complex calibration or active systems.
Innovation Solution
The use of a method involving a pair of fiber Bragg gratings (FBGs) with different grating periods embedded in a composite laminate structure, where one FBG compresses and the other extends under strain, allowing for optical interrogation to determine peak reflection wavelength separation representative of induced strain, while the passive design compensates for temperature effects using a strain neutral layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a dual head sensor or multiple FBGs are used to discriminate strain and temperature, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The sensor fiber is divided into multiple sections, each containing an FBG with specific grating periods. The first FBG has a first grating period and the second FBG has a second grating period, creating segmented sensing regions that respond differently to strain and temperature, enabling discrimination through their combined wavelength responses
Solution Approach 2:
The sensor employs a composite packaging structure combining a rigid outer tube with a flexible inner tube made of elastomeric material. This composite structure provides mechanical protection while allowing controlled deformation, and the elastomeric material's Poisson's ratio contributes to temperature compensation effects
2Device complexity
If passive temperature compensation devices are used, then device complexity is reduced, but manufacturing precision requirements increase
Solution Approach 1:
The patent utilizes the Poisson's ratio parameter of the elastomeric material to achieve temperature compensation. By selecting materials with specific Poisson's ratios (typically 0.4-0.5 for rubber-like materials), the lateral contraction during axial stretching creates a wavelength shift that compensates for the direct temperature-induced wavelength shift of the FBG
Solution Approach 2:
The flexible inner tube made of elastomeric material exhibits thermal expansion characteristics that counteract the thermal contraction of the optical fiber. The elastomeric material's greater thermal expansion coefficient compared to the glass fiber creates a compensating mechanical strain that offsets temperature effects on the FBG wavelength
3Measurement precision
If active feedback systems are used for temperature monitoring, then measurement precision is improved, but use of energy increases
Solution Approach 1:
The sensor system performs self-temperature-compensation using the inherent thermal and mechanical properties of the elastomeric packaging material. The system requires no external power source, active feedback control, or additional temperature sensors - the physical properties of the materials automatically compensate for temperature effects throughout the sensing range
4Stability of the object's composition
If precision-made components are used for temperature compensation, then temperature independence is improved, but ease of manufacture deteriorates
Solution Approach 1:
The patent employs a flexible elastomeric inner tube that can be easily formed and assembled. The elastomeric material's flexibility allows it to be stretched and shaped during assembly, and it automatically returns to its original shape during operation, providing consistent temperature compensation without requiring precision machining or complex assembly procedures
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
This approach results in a robust, low-cost, high-sensitivity strain sensor with accurate pressure/load/displacement readings, independent of temperature perturbations, and minimizes component complexity, making it suitable for various applications including smart structures and industrial use.
Implementation Method 1
an optical fiber having at least a first fiber Bragg grating (FBG) and a second FBG formed therein; subjecting the optical fiber to a strain inducing force such that a grating period in the first FBG compresses and a grating period in the second FBG extends; and optically interrogating the first and second FBG to determine peak reflection wavelengths
Implementation Method 2
passive devices that utilize the thermal characteristics of materials/structures to modify the response of the FBG wavelength to temperature. Since the refractive index is hard to control, passive temperature compensation devices generally operate by controlling the elongation with temperature of the optical fiber containing the FBG
Implementation Method 3
G. W. Yoffe et al has proposed a passive temperature compensating package for an optical fiber grating, in which the grating is mounted under tension in a package comprising two materials (a silica tube and an aluminum tube) with different thermal-expansion coefficients (TEC). As the temperature rises, the strain is progressively released, compensating the temperature dependence of the Bragg wavelength
Data Source
AI summary
An optical fiber strain sensor, a method of fabricating the same, and a method of sensing strain (1200). The method of strain sensing comprises providing an optical fiber having at least a first fiber Bragg grating (FBG) and a second FBG formed therein (1202); subjecting the optical fiber to a strain inducing force such that a grating period in the first FBG compresses and a grating period in the second FBG extends (1204); and optically interrogating the first and second FBG to determine peak reflection wavelengths of the first and second FBGs respectively (1206), whereby a separation between the peak reflection wavelengths of the first and second FBGs is representative of the strain induced.