# Measuring Torque and Temperature in a Rotating Shaft Using Commercial SAW Sensors
## 4 Discussion
Since individual strain sensors are 9.0 \(\times\) 5.4 mm\({}^{2}\), half-bridge configuration requires at least a \(\sim\)15 \(\times\) 15 mm\({}^{2}\) flat area. If both strain and temperature sensors are to be assembled, the required flat area is \(\sim\)50 \(\times\) 15 mm\({}^{2}\) (temperature sensor attached to radome antenna) or \(\sim\)26 \(\times\) 32 mm\({}^{2}\) (temperature sensor without antenna). After the sensors have been fixed on the surface to be monitored, the temperature sensor can be connected to the antenna by means of a 27 cm-long coaxial cable, while strain sensors can be connected with a 22 cm-long W.FL coaxial cable; this allows the antennas to be placed away from the sensors at a more convenient location, if necessary. However, if the shaft is rotating, the COTS system is not adequate for immediate use and a series of constraints were identified:  
1. Due to the large dimensions of the sensor antennas and to the difficulty to guarantee that sensors are in continuous line of sight with the interrogation unit, the antennas have to be replaced by a RF coupler. The mechanical stability of the RF coupler plays a crucial role in the stability and accuracy of the measurements with rotation speed. If, however, the variable changes slowly (like in the case of temperature or steady-state strain measurements), an array of several frequencies can be obtained; in this case, the value of the variable is calculated from the average frequency value and the accuracy of the readings improves considerably. 2. A second problem that arises is related with the sensors themselves. The temperature sensor comes embedded in an aluminum cage that cannot be removed and its weight (7 g) may unbalance the shaft at high rotation speeds. Both temperature and strain sensors have to be connected to the antenna by means of coaxial or W.FL coaxial cables. The mass of the cables cannot be neglected, nor can the centrifugal forces that appear when the shaft is rotating, which distort and pull the cables away from the sensors; this induces a strain on the piezoelectric material that compromises the accuracy of the readings. This effect is worse in the case of the strain sensors: since the W.FL connector is located in one of the extremities of the piezoelectric slab, the centrifugal force applied on the cables induces a cantilever effect that is an additional source of error. 3. The third problem is related with the acquisition rate: the user may define a confidence interval for the readings or even force the interrogation unit to average the results obtained during several interrogation windows. This is not problematic in the case of temperature readings, since temperature is a slow-changing magnitude. Strain, however, is expected to change rapidly and the accurate monitoring requires a large interrogation rate (a few tenths of even hundreds of Hz). If the configuration is made based on the confidence interval or on the window averaging, the interrogation rate may be seriously compromised. 4. The user does not have access to the full frequency response of the sensors. Like what was said above, the accuracy of the resonance frequency reading may be improved by defining a confidence interval of by averaging multiple readings. However, this is a limited procedure, since it relies in defining the parameters of the interrogation unit. It would be more useful if the user could have access to the full range of measured frequencies at each interrogation cycle; this way, it would be possible for the user to use different interpolation algorithms in order to improve the accuracy of the resonant frequency.