# Measurement of Structural Loads Using a Novel MEMS Extrinsic Fabry-Perot Strain Sensor
## 4 Experimental Setup and Discussion
### Measurement and Results  
The experimental setup included a demodulator (tunable laser source, detector, data acquisition, processing system, and coupler), loading device (fixed bracket, standard weight), and computer, as demonstrated in Figure 7. The wavelength range of the tunable laser source is 1510-1590 nm. By demodulating the reflected signal, the proposed sensor can output two channels of the wavelength signal, which can be taken as the measurement results to analyze. The wavelength resolution of the demodulator is 0.1 pm, with a wavelength measurement accuracy of up to \(\pm\)1 pm. Firstly, the sensor was fixed to the tested structure by welding; then, the two sensor signals were connected to the demodulator through the fiber jumper. Secondly, in order to avoid the temperature influence, an FBG sensor with a wavelength of 1539.97 nm was used to decouple the temperature. It was placed in a stainless steel tube with an internal diameter of 0.3 mm to ensure that it was only affected by the external environment (room temperature 25\({}^{\circ}\)C). Next, an axial force was applied to the structure by the loading device, and the constant loading state was maintained for about 2 min to ensure the accuracy of measurement data. Finally, five different loading cases were set to test the linearity, sensitivity, repeatability, stability, and hysteresis of the load measurement, as displayed in Table 2.  
The applied axial force changed from 0 N to 10 N, and the measurement was repeated three times to obtain the accuracy, linearity, and repeatability for the load measurement. It can be seen from Figure 8 that with an increase of the load, the interference signal of the cavity moves in the shortwave direction. The test data show that the wavelength shift of the sensor exhibits a linear relationship with the axial force. Figure 8a,b represent the test results and fitting curves obtained from three measurements of sensor signal 1 and sensor signal 2, respectively. It can be seen from the results that the strain sensing characteristics of the sensor have good linearity. The measured linearity (adjusted r-squared---Adj. R-Sq) for sensor signal 1 was 0.99367, 0.9952, and 0.99506; and for sensor signal 2, the measured linearity was 0.99684, 0.99767, and 0.99143. The sensitivity for sensor signal 1 was 33.0 pm/N, 31.0 pm/N, and 34.5 pm/N; and for sensor signal 2, it was 33.2 pm/N, 32.0 pm/N, and 35.6 pm/N. Considering the strain transfer efficiency, the measurement sensitivity of the EFF \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Case** & **Minimum (N)** & **Maximum (N)** & **Increment (N)** & **Sensor Location** \\ \hline
1 & 0 & 10 & 1 & Center (5.5 mm) \\
2 & 0 & 10 & 0.5 & Center (5.5 mm) \\
3 & 5 & 5 & - & Center (5.5 mm) \\
4 & 0 & 20 & 1 & Center (5.5 mm) \\
5 & 0 & 20 & 1 & Bottom (7.0 mm) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Description of the test (all cases). Figure 6: Structural strain distribution under external load. sensor for axial force is higher than that for the fiber Bragg grating sensor. The measurements results of the three different measurements are very similar, showing that the repeatability and accuracy of the sensor are high. According to these outputs, the 'loading' and 'unloading' curves provide different values of the sensor output values versus the same value of the applied force. The loading/unloading hysteresis for the sensor was also characterized in an experiment by measuring the wavelength corresponding to  
Figure 8: The static force (1.0 N increments) response and fitting curves of the Fabry–Perot (FP) sensor in three measurements, Case 1. (**a**) The curve of sensor signal 1. (**b**) The curve of sensor signal 2. Figure 7: The diagram of the experimental system. The proposed sensor was attached by welding a layer to the small structure. The load was applied by the load device. (**a**) Experimental device configuration. (**b**) Loading device. (**c**) Tested structure. (**d**) The experimental setup.