Figure 2. Typically, the quartz die is packaged in a stainless pellet circa 11 diameter x 2 mm thick with mass <2 gm.

As shown in Figure 2, the quartz die, carrying SAW resonators, is typically packaged within a stainless steel pellet circa 11 mm diameter by 2 mm thick, mass <2 gm.

For tire pressure monitoring, the die rests on two ledges and is deformed by the upper surface diaphragm pressing against it via a central pip. Interrogation and backscatter signals are broadcast from a simple whip antenna.

Figure 3. For torque sensing, the die is bonded to the base of the pellet.

For torque sensing (Figure 3), the die is bonded to the base of the pellet, which is in turn attached to the component under torque. The interrogation and return signal are transmitted via non-contacting planar couplers.

In TPMS and torque sensing dies, three SAW resonators generate two frequency differences, one proportional to pressure or torque, the other to temperature. This provides temperature-compensated pressure and torque and independent temperature monitoring.

For TPMS, SAW sensors can be mounted in various ways including on the tire using a bonded rubber patch, on the valve (both rubber and metallic) or on a runflat band clamped in the wheel well. Sensitivities can be adjusted to suit trucks at 10 bar down to racecars at 2 bar. Typical accuracy is 1% of full scale.

Figure 4. While for TPMS, SAW sensors can be mounted in a variety of ways on the tire, the torque sensors are attached to shafts or disks with resonators in ±45° orientation to sense the tension/compression components of shear strain.

For torque measurement (Figure 4), sensors can be attached to shafts or disks with resonators in classic ±45° orientation to sense the tension/compression components of shear strain.

Measurement accuracy in the 1% class can be obtained with full-scale strains between 50 and 500 microstrain. Applications to date include steering shafts for EPAS, engine flexplates, auto-transmission output shafts and drive shafts.

SAW-sensing technology provides new opportunities for sensing pressure, torque and temperature, especially on rotating components, within the automotive environment. In many cases, there is the opportunity to switch from indirect parameter measurement, in order to estimate a wanted variable, to real-time output sensing for both monitoring and closed loop control.

Gary O'Brien is an Engineering Fellow for the Sensing and Control Division of Honeywell International in Freeport, IL. Ray Lohr is technical director for Transense Technologies plc in Oxon, UK.