FBG-Based Loosening Angle Measurement Sensor for Bolt Looseness Monitoring

For looseness detection of bolted joins in large structures with a wide distribution span, a high-resolution bolt loosening angle detection sensor based on fiber Bragg grating (FBG) with an adjustable measuring range and sensitivity is proposed. A fitting profile cam structure is adopted to transfer the bolt loosening angle to the axial elongation of the fiber and then to the central wavelength shift of the FBG. By building the theoretical model of the sensor and through numerical analysis, the cam profile is optimized to ensure the linear relationship between the wavelength shift and the bolt loosening angle within a desired measurement range and sensitivity. An FBG bolt looseness detection sensor with easy installation and precise adjustment for distributed bolted-joints group is designed. The bolt loosening angle measurement experiments were performed for the sensor designed with the proposed method. The experimental results show that the FBG-based bolt sensor has good linearity, resolution, and precision. With the current sensor structural parameters, bolt loosening angle measurement can reach 0.38°. As a new method for bolt looseness detection, the proposed method has a good application prospect in the distributed bolts group monitoring in large structures.