Fig. 1. (a) Sensor structure design. (b) Initial cantilever design structure. (c) Sensor assembly

Fig. 2. Simulation results of (a) equivalent strain distribution by FEA and (b) the surfaces experiencing maximum strain

Fig. 3. Designed cantilever structure

Fig. 4. (a) The relation between the position distance from the neutral axis, applied force, and strain. (b) Amplified diagram showing the maximum strain of 1.0 N

Fig. 5. (a) Optimized cantilever structure design and (b) the corresponding mass block design

Fig. 6. Fabricated prototype of physical sensor. (a) Cantilever with prestressed FBGs. (b) Sensor structure design. (c) Assembled sensor

Fig. 7. Tilt sensor calibration system

Fig. 8. (a)Wavelength shifts of FBG1 and FBG2 for the four tilt tests. (b) Average wavelength shifts responses of FBG1 and FBG2

Fig. 9. (a) Wavelength shifts difference of FBG1 and FBG2. (b) Linear fit for the average values of the wavelength shift difference of the optimized and initial design

Fig. 10. Creep resistance test results

Fig. 11. Temperature compensation test setup

Fig. 12. Temperature compensation experimental results

Fig. 13. Tilt sensor installation. (a) Underground pipeline bay. (b) Tilt sensor 1. (c) Tilt sensor 2. (d) FBG interrogator and PC

Fig. 14. Tilt angle curve during the six-month monitoring period with the extraction of central value from the vibration signal for (a) tilt sensor 1 and (b) tilt sensor 2

Fig. 15. Extracted central value for (a) tilt sensor 1 and (b) tilt sensor 2