# Measuring Torque and Temperature in a Rotating Shaft Using Commercial SAW Sensors
## 1 Introduction
Propagating velocities in common materials (such as quartz or lithium niobate) are of the order of \(10^{-5}\) of the speed of light. If an external magnitude (such as temperature or deformation) influences the propagation characteristics of the piezoelectric materials, the phase velocity of the SAW waves changes and the frequency \(f_{c}\) of the sensor changes accordingly. By probing the resonance frequency of the SAW sensor, the user is able to get an indirect measurement of the parameter under surveillance. The operating frequency of SAW sensors depends on the phase velocity of the acoustic waves and on the dimensions of the IDT fingers, which, in turn, are limited by the photolithographic process. Typical operating frequencies range between 30 MHz and approximately 3 GHz. Commercial devices usually operate in two different bands, 433.92 MHz and 2.45 GHz (ISM bands). SAW sensors have a few characteristics that make them the ideal choice for measurement of strain on moving parts or in harsh environments. The IDT can be physically connected to an RF antenna and the interrogation of the device can be made remotely, that is, without the need of a wired connection. In addition, the sensors respond with a fraction of the energy sent by the interrogation unit, meaning that they are passive devices that do not require external power or batteries. SAW sensors have other advantages as well. They are low-cost small devices (typically a few square millimeters), exhibit high immunity to EMI and are able to operate under harsh environments. Depending on the operating frequency, the antenna and the RF environment for each particular case, reading distance can be as high as 3 m.  
SAW devices have been used in telecommunications for several decades as resonators and filters. More recently they have started to be used as sensors and identification (RFID) tags. Due to their high resilience, SAW sensors have been used to monitor temperature under extreme conditions, such as the inside lining of metallurgical vessels [2], jet engine turbine blades [3] or within the metal-oxidecolumn of surge arresters [4]. Commercial solutions allow real-time and continuous monitoring of temperature in critical electric power transmission and distribution assets [5]. SAW sensors have also been used to measure the cutting forces in computer numerically controlled (CNC) turning [6; 7] and strain on rotating shafts [8]. The strain sensitivity of SAW sensors was studied on Chip-level by [PERSON] and his co-workers [9]. In the automotive industry, these sensors have been used for torque measurement [10; 11], in kinetic energy recovery systems [12] and to monitor in real time the tire pressure of mining and commercial [13] and high performance racing vehicles [14]. High acquisition rates of commercial interrogation units enable the use of these components to measure vibration of mechanical structures [15]. Besides sensing capabilities, SAW devices can also behave as RFID tags. Commercial solutions are already available for the railway [16] and metallurgical [17] sectors. This work intends to evaluate the possibility of using the commercial off-the-shelf (COTS) system fabricated by SENSeOR (Besancon, France) to measure torque and temperature of a rotating shaft. The system is composed of temperature and strain sensors and the corresponding interrogation unit. Since the COTS RF antenna is not appropriate to be used under rotation, a simple RF coupler was designed and fabricated. An aluminum load cell was fabricated and instrumented with the sensors. The temperature sensor was calibrated using a thermal chamber. The strain sensors were calibrated using a mechanical setup designed and fabricated specifically for that purpose. A second mechanical setup was designed to perform the measurements under rotation. The influence of the rotation in the accuracy of the measurements was evaluated at different temperatures and rotation speeds in the absence of external torque.