# Thermal-Mechanical Behaviour of Road-Embedded Wireless Charging Pads for EVs
## 4 Instrumentation  
The ferrites in both the partial and complete pads were instrumented with a combination of resistive strain gauges and fibre Bragg grating (FBG) sensors. FBG sensors are more ideal for IPT applications as they are unaffected by magnetic fields but are typically more expensive than resistive strain gauges [34]. The usage of both types of sensors in this study provides additional information regarding measurement methodologies for engineers who want to conduct similar thermal-mechanical investigations in the future. The TML resistive gauges were used, with the National Instruments NI-9219 module interfaced with LabVIEW for data acquisition. The FBG sensors used in this study were draw tower gratings (DTG\({}^{\otimes}\)s) from FBGS, with a diameter of 195 \(\upmu\)m and a grating length of 3 mm. A LUNA Si155 interrogator was used to measure the reflected wavelength of each FBG, and the ENLIGHT sensing analysis software (v 1.18.8) was used to visualize the reflected spectrum and record the peak wavelengths.  
The change in Bragg wavelength with strain (\(\Delta\lambda_{B\text{-}\varepsilon}\)) was found to be linear up until the failure strain of ferrite and is given by Equation (5) [35]. The sensitivity factor, \(K_{\varepsilon}\), is 8.2 \(\times\) 10\({}^{-7}\), and \(\Delta\varepsilon\) is the strain in microstrains. The change in Bragg wavelength with temperature, \(\Delta\lambda_{B\text{-}T}\), of an unadhered and unconstrained FBG had minor non-linearity, and thus was calibrated experimentally.  
\[\Delta\varepsilon=\frac{\Delta\lambda_{B\text{-}\varepsilon}}{\lambda_{B} \times K_{\varepsilon}} \tag{5}\]  
For thermally induced strains, it is impossible to isolate the wavelength shift caused by temperature and mechanical strain using a single FBG sensor, if the temperature is not known prior to the experiment. One method of solving this problem is to install a stress-free FBG close to the stressed FBG for temperature measurement. Once temperature is known, the shift in wavelength from temperature (\(\Delta\lambda_{B\text{-}T}\)) can be subtracted from the total wavelength shift (\(\Delta\lambda_{B}\)) to obtain the shift from thermally induced mechanical strain, as shown in Equation (6). Note that, with this method, the shift in wavelength associated with temperature (\(\Delta\lambda_{B\text{-}T}\)) for ferrite-adhered FBGs must still be known prior to measurement.  
\[\Delta\lambda_{B\text{-}\varepsilon}=\Delta\lambda_{B}-\Delta\lambda_{B\text{ -}T} \tag{6}\]  
The instrumentation scheme for the ferrite cores of the partial and complete pads is shown in Figure 7. FBGs were glued onto the ferrite using Loctite HY 4090, while Loctite 401 was used for resistance strain gauges. The lead wires of the strain gauges were twisted together to minimize drift in measurement over the long duration of the thermal experiment. The ferrites were numbered one to four, while the sensor locations on each ferrite were labelled using capital letters 'A', 'B' and 'C', with 'A' located closest to the outer edge. Sensors will be referred to by the \"Ferrite #\" \"Location\". For example, '2A' refers to the sensor on Ferrite 2 at Location 'A'. For the compensation of \(\Delta\lambda_{B\text{-}T}\) for thermally induced strains, temperature-sensing FBGs were located beside the strain-sensing FBGs on the complete pad. An additional temperature-sensing FBG was placed in the middle of the coil. To isolate the temperature FBGs, they were inserted into a Teflon tube (inner diameter 0.6 mm, outer diameter 1 mm). The temperature on the top surface of the complete pad was also measured using an FLIR T540 thermal camera.