Development of an FBG Sensor for Measuring Large Range and Multi-Directional Settlement

In this paper, one advanced settlement sensor with the advantages of wide measurement range and multi-directional displacement measurement has been designed and fabricated based on multidisciplinary technologies, including fiber Bragg grating (FBG) sensing technology, 3D printing technology, and others. First, one novel structure of settlement sensor was designed by introducing the concept of a hinge joint, which mainly consists of four components, including internal rigid beam, external rigid beam, elastic FBG sensing beam, and hinge joint. The FBG sensing element inside the sensor can rotate unconstrained without disturbing stress field underground, overcoming the strengthening problem commonly existed in conventional settlement sensors. It is also worth noting that all sensor components were designed and fabricated with 3D printing technology, while the two most popular and effective 3D printing technologies, including fused deposition modeling (FDM) and stereolithography (SLA) technology were separately used for fabricating elastic sensing beam and rigid beam segments, respectively. Furthermore, the FBG sensors were successfully embedded into the printed polylactic acid (PLA) without sacrificing its sensing performance, and the superiority of FBG sensing technology, such as high sensitivity, high accuracy, strong resistance, light weight, and serial connection have been achieved. After completing the fabrication of developed sensor, the measurement performance parameters of elastic FBG sensing beam (includes two paralleled FBG sensors) including measuring linearity, repeatability, and measurement range have been calibrated and verified. Finally, working performance of the assembled settlement sensor from calibration was examined, characterized by a maximum measurement range -160 ~ +160, and a measurement sensitivity of 0.0066 pm/° (in terms of inclination degree) or 0.012 mm/pm (in terms of settlement).