# Measurement of Structural Loads Using a Novel MEMS Extrinsic Fabry-Perot Strain Sensor
## 4 Experimental Setup and Discussion
### Measurement and Results
cyclic force change, and the result is shown in Figure 9. The blue line measured from low to high force represents the increasing process, and the red line measured in the opposite direction represents the decreasing process. The major contribution of hysteresis may come from the welding layer used to bond the sensor and structure. The maximum hysteresis error of sensor single 1 and sensor single 2 was 4.00% and 3.67%, respectively, which was observed under 2 N and 6 N. It should be noted that since the tested structure has a microdynamic response for mechanical swing owing to the small size, the real hysteresis error is relatively lower. Thus, these results indicate favorable reliability of the sensor [34], and that the hysteresis of the sensor is lower. When loading to 10 N by the stepwise loading method with a 0.5 N increment, the measurement sensitivity under this load configuration for sensor signal 1 and sensor signal 2 was measured as 33.0 pm/N and 32.0 pm/N, and the linearity of each sensor was 0.99006 and 0.99742, respectively. The linearity and accuracy of the sensor are also excellent when the tested structure is subjected to a small increment load, as shown in Figure 10.  
The constant load of 5 N was loaded onto the structural member to test the stability. The central wavelength data of the sensor was recorded per 1 s, and the total data collection time was 15 min. The test results are shown in Figure 11. For sensor signal 1, the maximum wavelength drift was  
Figure 10: The static force (0.5 N increments) response and fitting curve of the sensor, Case 2. (**a**) The measuring curve of sensor signal 1. (**b**) The measuring curve of sensor signal 2. Figure 9: Sensor hysteresis curve of a structure under loading (blue line) and unloading (red line), Case 1. (**a**) The curve of sensor signal 1. (**b**) The curve of sensor signal 2. 12.21 pm, while for signal 2, it was 11.79 pm. The measurement drift is believed to be due to the following reasons. One reason is due to the sensor itself, which generates a systematic shift. The other reason is that as the tested structure has light weight, the variation caused by swing needs to be considered. These results indicate that the stability of the sensor is good. The sensors with different placement were tested to compare the axial force measurement linearity of these two situations. The sensors were fixed on the bottom (7.0 mm) and center (5.5 mm) position of the tested structure. The loading range was set to 0-20 N (maximum instantaneous load), and the signal of the sensor was collected once at each 1 N increment to test the linearity of the sensor with different positions. It can be seen from Figures 12 and 13 that the linearity and precision of the sensors placed at different positions are different. For comparison, the test curve displayed in Figure 11 shows the measurement results when the sensor was placed in the center position, while Figure 12 depicts the results when the sensor was placed at the bottom of the structure. The fitting linearity of the different sensor placement obtained was 0.899 and 0.911 for the center position, and 0.981 and 0.989 for the bottom position. Therefore, when the measured structure is subjected to the same external load, the sensor at the center point has better linearity, because it is located within the uniform strain field of the structure. When the sensor is used to test the structural load, it is necessary to consider the placement of the sensor. Figure 11: Static stability test, Case 3. (**a**) The measuring curve of sensor signal 1. (**b**) The measuring curve of sensor signal 2. Figure 6:0 Measurement and fitting curve with center location, Case 4. (**a**) The curve of sensor signal 1. (**b**) The curve of sensor signal 2.