A novel plate torque transducer based on UHF SAW resonant sensing elements has been developed allowing an easy aftermarket installation of passive wireless torque sensors on large diameter shafts by means of clamping. A design of a large diameter RF rotary coupler for wireless interrogation of the transducer was proposed that had a gap of 20 mm between the stator and the rotor and was insensitive to axial and radial misalignment as well as the rotation angle. The two plate transducers and the RF coupler were installed on a high-speed shaft of a wind turbine gearbox. The torque sensor accuracy achieved after calibration on a test shaft was better than 1% full range within the temperature range from +20°C to +80°C.

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Resonant SAW strain sensing elements used in noncontact torque, force and vibration sensors are investigated from the point of view of their stability and durability. Results of fatigue testing of the SAW sensing elements bonded to metal shafts with a stiff adhesive are presented. They demonstrate the sensor durability and stability of the strain sensitivity up to 13 million strain cycles. Stability of the frequency of the SAW differential resonant sensing elements is also investigated demonstrating an estimated sensor zero drift of 0.8-0.15 microstrain per year.