# Measuring Torque and Temperature in a Rotating Shaft Using Commercial SAW Sensors
## 1 Introduction  
Nondestructive testing techniques are widely used for the early detection of potential failure of a material, component or system. These techniques do not alter nor damage the article under inspection and, as such, provide a valuable tool to increase the safety while minimizing the associated costs. However, these inspections are usually prescheduled and do not give information about the real behavior of the components during operation. Real-time structural health monitoring techniques, on the other hand, provide real data obtained during the operation of the system, allowing for the early detection of potential structural failures and the corresponding adoption of mitigating actions before an accident occurs. The predictive maintenance of an industrial system relies, to a large extent, on the possibility of monitoring continuously and in real-time sensors that have been strategically placed in selected components. An example is the measurement of vibrations and forces in machines; in case a pre-established value is reached, the order of repairing is triggered before being reached the state of stop by failure. Furthermore, the new trends and the so-called Industry 4.0 is based on SCADA (supervisory control and data acquisition) systems, which are an integral part of industrial processcontrol and equipment. Sensing elements for parameters such as pressure, flow, temperature or strain, among others, are one of the key elements of such systems. Strain is usually measured with strain gage transducers. These widespread components are composed of a thin metallic sheet or wire whose electric resistance changes as it is stretched or compressed; this change is then converted into the strain value. However, the use of strain gages to measure strain in a rotating shaft requires the use of slip rings to connect the gages with the required interrogation electronic module; this increases the complexity of the system and may become a noise source, with a negative impact on the accuracy of the required measurements. Strain can also be measured with fiber-optic sensors using Fiber Bragg gratings. These sensors can be embedded in the structures to be monitored and, when the structure is deformed, the displacement felt by the optic fiber influences the propagation of the light and causes changes in the intensity, phase, polarization or wavelength of the radiation. In the case of Bragg gratings, for instance, the change in the internal reflections at the gratings caused by the fiber displacement can be monitored by light interferometry techniques. These sensors are the preferable solution for remote monitoring or when low electromagnetic interference (EMI) sensitivity is required. However, in the case of monitoring, the deformation of a rotating shaft, the optical signals need to be routed out and optical slip rings are tailored for digital data transfer only, so these sensors are also not suitable for the monitoring of a rotating shaft. Surface acoustic wave (SAW) technology provides a third technique to monitor the strain that is felt by a mechanical component. An SAW device is composed of a piezoelectric slab with one interdigitated (IDT) contact and some reflectors. When an alternating electric field is applied to the IDT of a SAW resonator, a mechanical deformation of the piezoelectric substrate takes place. If the distance between the IDT fingers is such that the frequency of the electric signal, \(f_{c}\), is related with the phase velocity \(V_{p}\) and the wavelength of the SAWs \(\lambda\) by \(f_{c}=\frac{V_{p}}{\lambda}\), a resonance condition happens and a SAW wave propagates across the surface of the piezoelectric material. SAW waves were described by [PERSON] in the 19 th century [1] and are similar to longitudinal seismic waves, in the sense that they only penetrate solids up to a depth of the order of magnitude of the wavelength, they undergo little attenuation as they progress on the surface, and the movement of particles in the solid is elliptical.