## 3 Experimental Results and Discussion  
Because the tilt sensor is composed of two suspended cylindrical floats, each connected with FBG, we can do two-dimensional tilt sensing (see Figure 2). We have verified this proposed sensor's performance in four tilt directions: east-west, south-north, northeast-southwest, and northwest-southeast. In the sensing process, when the FBG is stretched or contracted, the reflection center wavelength of the FBG will change. In this research, we assume that the FBG's tension becomes larger and the tilt angle is positive. On the other hand, the tension of FBG becomes smaller and the tilt angle is negative. Figure 4 shows that the cylindrical float is immersed in a different height of liquid, and the buoyancy of the cylindrical float is also different, causing the pitch of the fiber grating to change. From the reflection center wavelength shift, we can calculate the inclination of the azimuth and the inclination angle of the test object. Figure 3: The pre-tension schematic of FBG: (**a**) the sustained weight was 0 g; (**b**) the sustained weight was 52 g.  
Figure 2: Schematic diagram of the proposed FBG tilt sensor. Figure 5 shows the measurement of the wavelength shift of the FBGs when the sensor was tilted in the east-west direction. The angle of inclination of the sensor to the east is positive, and the angle to the west is set to be negative. As the buoyancy of the cylindrical float changes to make the FBG bear the weight change, the FBG located in the west can cause expansion or contraction. When the sensor is tilted to the east, the buoyancy of the cylindrical float decreases as the water level decreases. Therefore, FBG1 is stretched and the center wavelength of FBG1 is shifted to the long wavelength band. At the same time, the center wavelength of FBG2 does not change because it is not subject to the impact of the tensile. When the sensor is tilted towards a negative angle (west), FBG1 gradually shrinks as the angle increases, and the center wavelength of FBG1 begins to move toward the short wavelength band. The center wavelength of FBG2 does not change because it is nt subject to the impact of the tensile. From the experimental results, the sensitivity of FBG1 is 0.132 nm/\({}^{\circ}\). In Figure 5, the coefficient of determination of FBG1, \(R^{2}\), is 0.998. When the sensor was tilted in the south-north direction, the principle was the same as for east-west. While the tilt angle was gradually tilted from 0 degree to a positive angle (south), the FBG2 was gradually stretched as the tilt angle increased, and the reflection center wavelength of the FBG2 began to shift to a long wavelength band. The measurement results are shown in Figure 6. Since the FBG1 is located in the center of the south-north direction, the center wavelength did not change because it was not affected by stretching. Similarly, if the sensor was gradually tilted from 0 degrees to a negative angle (north), FBG2 gradually shrank as the tilt angle increased, and the reflection center wavelength of FBG2 began to shift to a short wavelength band. The center wavelength of FBG1 also did not change because it was not subject to the impact of the tensile. From the experimental results, the sensitivity of FBG2 is 0.128 nm/\({}^{\circ}\) and the coefficient of determination of FBG2, \(R^{2}\), is also 0.998. Figure 4: The schematic of the tilt sensor: (**a**) was tilted toward a negative angle, (**b**) not tilted; and (**c**) tilted toward a positive angle. Figure 5: Measured wavelength shift of FBG1 and FBG2 in the east–west direction when the tilt angle is increased from \(-5^{\circ}\) to \(5^{\circ}\). Figure 7 shows the tilt measurement results of the center wavelength shift of FBG1 and FBG2 in the southeast-northwest direction when the tilt angle was increased from \(-5^{\circ}\) to \(5^{\circ}\). The angle of inclination to the southeast is set to positive, and it will reduce the buoyancy of the cylindrical float on the two FBGs, so the FBG's reflection center wavelength drifts to the long wavelength.