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Fiber Bragg grating (FBG) strain sensing can eliminate the interference of environmental temperature fluctuations on pure strain measurement results through specific packaging structure design or multi-channel compensation mechanisms.
I. Analysis of Physical Principles
The Bragg reflection wavelength \lambda_B of an FBG is simultaneously affected by axial strain \varepsilon and environmental temperature change \Delta T . The basic equation for wavelength drift is:
\Delta \lambda_B = K_\varepsilon \varepsilon + K_T \Delta T
where K_\varepsilon is the strain sensitivity coefficient of the grating and K_T is the temperature sensitivity coefficient (determined jointly by the thermo-optic effect and thermal expansion effect of the fiber material). Physically, a single bare FBG cannot distinguish whether the wavelength drift is caused by stress stretching or temperature change, so temperature sensitivity stripping must be performed through engineered packaging structures or system demodulation algorithms.
II. Self-Compensating Packaging Structures and Implementation Mechanisms
In sensor engineering design, packaging structures that can eliminate or offset temperature interference typically employ the following methods:
1. Built-in Dual-FBG Self-Compensating Structure
Two or two sets of FBGs are packaged inside the same sensor housing or tube:
- Strain FBG: Pre-stressed and fixed to the sensor substrate or housing, it senses both structural mechanical strain and environmental temperature changes. Its wavelength drift is \Delta \lambda_1 = K_{\varepsilon 1} \varepsilon + K_{T1} \Delta T .
- Temperature Compensation FBG: Placed in a free-hanging/stress-free cavity within the same packaging chamber, it only senses environmental temperature changes and does not bear structural mechanical strain. Its wavelength drift is \Delta \lambda_2 = K_{T2} \Delta T .
The demodulation system can accurately eliminate the temperature effect by taking the difference of wavelengths:
\Delta \lambda_{\text{pure strain}} = \Delta \lambda_1 - \frac{K_{T1}}{K_{T2}} \Delta \lambda_2 = K_{\varepsilon 1} \varepsilon
2. Mechanical Self-Compensation by Thermal Expansion
This method utilizes the combination of two materials with different coefficients of thermal expansion (CTE), such as different metals or alloys, to fabricate the sensor’s packaging base. When the temperature rises, the tensile displacement caused by the thermal expansion of the main material is offset by an equal and opposite displacement from the thermal expansion of the auxiliary material. This ensures that the mechanical strain of the fiber remains unchanged under pure temperature variations.
III. OFSCN® Official Technical Implementations and Recommendations
Within the OFSCN® (Beijing Dacheng Yongsheng Technology Co., Ltd.) product system, to address the cross-sensitivity issue between temperature and strain, two engineering paths are provided: customized built-in self-compensating structures and external independent compensation:
1. Built-in Self-Compensating Structure Customization
Major tubular and planar FBG strain sensors under OFSCN® support customization with built-in self-compensating structures that include temperature compensation:
- OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor
- OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor (1.5mm/2.3mm diameter)
- OFSCN® Fiber Bragg Grating Strain Gauge
2. Official Engineering Application Recommendations
Although various strain sensors can be manufactured with built-in self-compensating structures, in practical engineering applications, it is more recommended to use external independent Fiber Bragg Grating temperature sensors in combination with strain sensors for temperature compensation. The reason is that external independent temperature compensation sensors have closer contact with the measured component, eliminating issues of heat transfer delay or uneven thermal gradients within the tube. This provides higher calculation accuracy in environments with large spans or drastic temperature fluctuations.