# Measuring Torque and Temperature in a Rotating Shaft Using Commercial SAW Sensors
## 4 Discussion  
The possibility of using SAW sensors to measure temperature and torque in the shaft of a power gearbox was evaluated with a COTS solution fabricated by SENSEOR. This system is composed of the interrogation unit WR D005, the temperature sensor TSE F162 and strain gages SSE E015 and SSE E017 mounted in half bridge configuration. The communication between the interrogation unit and the sensors can be made with coaxial cables or with two omnidirectional antennas. The temperature sensor was calibrated in a thermal chamber, while strain gages were calibrated using a custom-designed and fabricated mechanical setup. A second mechanical setup that allowed the application of controlled torque while the load cell was rotating was also fabricated in order to emulate a simplified gear box shaft. The COTS solution (interrogation unit + sensors + antenna) can in principle be used without any need of adaptation to remotely measure temperature and strain/torque in any flat surface. The temperature sensor can be glued or fixed with screws; fixation by magnets in the metallic basis is also possible ([[http://www.senseor.com/senseor-monitoring-solutions/wireless-temperature-monitoring/tsa-h161-v](http://www.senseor.com/senseor-monitoring-solutions/wireless-temperature-monitoring/tsa-h161-v)]([http://www.senseor.com/senseor-monitoring-solutions/wireless-temperature-monitoring/tsa-h161-v](http://www.senseor.com/senseor-monitoring-solutions/wireless-temperature-monitoring/tsa-h161-v))). The accuracy of the temperature sensor (after proper calibration) is +/\(-\)1 \({}^{\circ}\)C. The strain sensors need to be glued to the surface of the part to be monitored with proper adhesive. The typical strain sensitivity at room temperature is 0.94 and \(-\)0.08 ppm/(\(\mu\)m/m) in the \(x\)- and \(y\)-direction, respectively), which leads to a gage factor higher than 3. Since the gage factor of a metallic  
Figure 13: Torque measured by the strain sensors as a function of the torque applied by the braking system. extensometer is typically 2, SAW strain sensors have potentially higher precision and accuracy. If, however, the surface to be monitored is not flat (like for instance the surface of a shaft) a flat area needs to be machined before gluing the sensors; contrary to a flexible extensometer, for instance, SAW sensors are fabricated on a rigid piezoelectric material and, as such, do not bend. In [12], for instance, the surface of the shaft was processed before assembling the SAW sensors. In some cases, however, this procedure may make the shaft itself more fragile and would need to be avoided. One way to overcome this issue would be to clamp the temperature sensor to the shaft, like in [20]. In this case, the sensor would not be measuring the temperature of the shaft directly and the pre-calibration of the set sensor + clamping ring + shaft would be required. Following a similar approach, the SAW strain gages could also be glued on a plate transducer or on an outer ring previously designed to fit the curvature of the shaft. However, this would mean adding another interface so the transfer of strain between the shaft and the plate or ring transducers should be previously determined. In addition to the issues related with the assemblage of the sensors, it should be also taken into consideration that the Young's modulus of the piezoelectric material is much higher than the one of adhesives that typically are used. Any asymmetries in the process of gluing different gases will have an impact in the transfer of strain from the part to be monitored to the sensor, especially at higher temperatures when the viscoelastic properties of the adhesive begin to change. The assemblage of the temperature sensor requires a circular 35.5 mm-diameter flat base (if attached to the antenna radome) or, alternatively, a -26 \(\times\) 16 mm\({}^{2}\) rectangular flat base (to accommodate the aluminum cage).