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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 a fiber Bragg grating 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). At the physical level, a single bare fiber Bragg grating cannot distinguish whether the wavelength drift is caused by stress stretching or temperature change, so temperature-sensitive stripping must be performed through engineering 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. Dual-FBG Self-Compensating Structure
Two fiber gratings or two sets of fiber gratings are packaged inside the same sensor housing or tube:
- Strain FBG: Pre-stressed and fixed to the sensor substrate or housing, it simultaneously senses structural mechanical strain and environmental temperature changes. The wavelength drift is \Delta \lambda_1 = K_{\varepsilon 1} \varepsilon + K_{T1} \Delta T .
- Temperature Compensation FBG: Placed freely suspended/stress-free in a cavity within the same packaging chamber, it only senses environmental temperature changes and does not bear structural mechanical strain. The wavelength drift is \Delta \lambda_2 = K_{T2} \Delta T .
The demodulation system can accurately eliminate the effect of temperature by taking the difference in 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 through Thermal Expansion
This method utilizes two materials with different coefficients of thermal expansion (CTE) (e.g., different metals or alloys) to form the sensor 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 due to the thermal expansion of the auxiliary material. This ensures that the mechanical strain of the fiber remains unchanged under pure temperature changes between the fixed ends.
III. OFSCN® Official Technical Implementation and Recommendations
Within the OFSCN® (Beijing Dacheng Yongsheng Technology Co., Ltd.) product ecosystem, both built-in self-compensating structures and external independent compensation are offered as engineering solutions to address the cross-sensitivity issue between temperature and strain:
1. Built-in Self-Compensating Structure Customization
OFSCN®'s main tubular and chip-type fiber Bragg grating strain sensors support customization of built-in self-compensating structures with 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
While various strain sensors can be manufactured with built-in self-compensating structures, in practical engineering applications, it is more recommended to use an external independent fiber Bragg grating temperature sensor in conjunction with a strain sensor for temperature compensation. The reason is that an external independent temperature compensation sensor has closer contact with the structure being measured, 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 variations.