NON-CONTACT TORQUE SENSORS BASED ON SAW RESONATORS J. Beckley 1 , V. Kalinin 1 , M. Lee 1 , K. Voliansky 2 1 Transense Technologies plc., 66 Heyford Park, Upper Heyford, Bicester Oxon. OX25 5HD, UK 2 St.-Petersburg State University of Aerospace Instrumentation, 67 Bolshaia Morskaia str., St.-Petersburg, Russia Abstract - A detailed model of an interrogation system performing a continuous tracking of the resonant frequencies of the two SAW resonators is developed and the results of computer simulations are presented. A relationship is established between the amount of noise in the system, the Q- factor of the resonators, parameters of the frequency tracking system and the system bandwidth and the resolution. A variation of the SAW torque transducer sensitivity with temperature is discussed. A theoretical model describing this variation is presented and SAW sensors with a reduced sensitivity variation are suggested and experimentally tested. Keywords – SAW resonator, Sensor, Torque I. INTRODUCTION Torque sensors play an important role in automatic controllers for a great variety of complex mechanical systems, from electric drills to submarines. One of the major consumers of torque sensors is the automotive industry [1]. Sensors are needed to measure torque on driveshafts and crankshafts of engines in order to optimise transmission and engine operation and improve vehicle stability. Torque sensors are also required for electrical power assisted steering systems (EPAS) that will be installed even in small cars. Most conventional torque sensors employ a weak link (e.g. a torsion bar) to translate torque into a relatively large mechanical movement that can be measured by a potentiometer, capacitive, inductive, magnetic or optical angular position sensor. Alternatively a strain produced by the torque on the surface of the torsion bar can be measured using piezoresistive or piezoelectric strain gauges. In any case, because of the big difference between nominal measured torque (10 to 20 Nm for EPAS) and specified overload capability (10 to 30 times the nominal torque) a mechanical stop that complicates a mechanical design is usually required. Besides, many of the above mentioned sensors need a clock-spring wire connecting the shaft and the stationary interrogation unit. This also adds to complexity and cost of the sensor. A search for a wireless device sensitive enough to omit the weak link and thus not requiring the mechanical stop has led to two most promising cost-effective candidates for high-volume applications. The first one is a magneto-elastic sensor the long-term stability of which still needs to be proven. The second one is a SAW sensor that has already demonstrated its high potential in wireless measurements of temperature, pressure, torque, force, humidity etc. [2] including those in automotive industry [1,3]. The sensitivity of SAW devices to strain is sufficient to perform the measurements on a shaft that is not weakened [4]. It greatly simplifies mechanical design and reduces the cost of the whole system. Besides, SAW sensors can withstand heat, dirt and mechanical vibration which represent problems for other types of sensor, (e.g. optical ones). The fact that SAW sensors work at radio frequencies makes it easier to arrange a non-contact coupling between the rotating shaft and the stationary interrogation unit. A careful design of the latter allows reduction of the influence of electromagnetic interference to an acceptable level. Application of SAW devices to non-contact torque measurement was first suggested and patented in [5]. Since then the authors of the patent have fabricated and supplied torque sensors based on SAW resonators to a number of industrial customers [6,7]. Non-contact torque sensors based on SAW reflective delay lines were also introduced in [8,9]. Although the operation of both types of sensor was successfully demonstrated the abovementioned publications did not cover such important aspects as limitations on the accuracy of the sensors resulting from their non-contact interrogation and the temperature stabilisation of their sensitivity to torque. The aims of this paper are to: - compare advantages and disadvantages of SAW reflective delay lines and one-port resonators as passive strain sensing elements in torque transducers, - overview non-contact interrogation methods and build a model for a selected interrogation scheme that enables the establishment of a relationship between system parameters, the system bandwidth and resolution, - discuss the influence of the RF non-contact coupling devices on the performance of the sensor, - build a theoretical model describing temperature characteristics of the SAW sensors and suggest a configuration of the sensor that has reduced temperature variation of the sensitivity to torque. Theoretical results presented in the paper will be compared with the experimental results and the most important parameters of the sensor such as sensitivity, linearity and hysteresis will be discussed. II. RESONATORS VS.DELAY LINES Wirelessly interrogated passive torque sensors can employ one of the two basic types of SAW devices – either one-port resonators [1,5-7,9] or reflective delay lines [2,3,8]. Obviously, ordinary delay lines and two-port resonators can also be used as passive sensing elements although they are