Surface Acoustic Wave (SAW) technology

A wireless, passive, sensing system consisting of three main elements: SAW sensing elements connected to antennas known as RF Couplers mounted on the part to be measured and an electronic interrogation unit called a reader, connected to its stationary RF coupler. The reader generates and transmits an interrogation signal across the RF Coupler to the sensing element. The sensing element is excited by this RF signal creating a Surface Acoustic Wave (SAW) on the surface of a Piezoelectric substrate between the fingers of an interdigital transducer (IDT). The IDT reflects the SAW back to the reader. The frequency of this reflected signal is affected by the physical measurement of either strain or temperature in the sensing element. The reader analyses the received signal and calculates the value of the physical measurement.

In most non-contact, rotating, torque measurement applications the distance between the sensor antenna and the reader antenna should be less than 5mm, and the torque update period can be as small as 100 microseconds or a frequency of 10kHz. For other applications where slower update frequencies are possible larger separation distances are possible up to several metres.

SAW sensing elements

The key part of the SAW sensing system is a small lightweight SAW sensing element made on a piezoelectric substrate. The surface acoustic wave propagates on its free surface between the 2 sides of an Inter-Digital Transducer (IDT). The IDT is a thin-film conductive structure fabricated by a standard photolithographic process which is the same process as used for manufacturing integrated circuits and MEMS devices. The IDT converts electric RF signals, with a frequency of hundreds of MHz, into the mechanical SAW on the surface and due to the piezoelectric effect returns them into a RF signal. If the IDT is surrounded by the Bragg reflecting gratings, in the form of periodic arrays of thin conductive strips, then the energy of the SAW is trapped inside the cavity between the gratings and the SAW device works as a resonator.

The Q-factor of SAW resonators is very high, in the order of 10,000-20,000, which is something that is difficult to achieve in other technologies at UHF. This is what makes the SAW resonator suitable for wireless and passive sensing. The long ringing resonance in response to the interrogation pulse allows a very accurate remote measurement of its resonant frequency.
To turn the resonator into a wireless device, a simple antenna is connected to the electrical terminals of the IDT. The size of the efficient antenna at UHF is relatively small, which is another advantage.
Since the SAW velocity is 105 times smaller than the velocity of light, the size of the resonator is just 1-3mm in length and less than 0.4mm in width at UHF. Further advantages of SAW devices are that they are very small, rugged, and stable over many years.

When the strain is applied or the temperature of the substrate changes, the SAW velocity changes, the changing SAW velocity creates a frequency shift in the reflected RF signal. This makes the resonant frequency sensitive to strain and temperature, and allows the SAW resonator to be used as a strain and temperature sensing element.

Transense has developed SAW sensing elements having two to three SAW resonators each. For Strain measurement 2 resonators are used at 90 degrees to each other which allows the principal resultant strain between them to be measured. If required the third resonator allows temperature to be measured at the point of measurement enabling calibration offsets to be applied across the full working temperature range of the system.

To turn the SAW device into a practical sensing element its is packaged to protect it from the environment and efficiently coupled to the physical component to be measured. Because it has a lid formed from Quartz, Transense calls its proprietary design of packaged SAW device an "All Quartz Package" or AQP. Transense has developed AQP's for pressure and temperature (up to 150 psi), as well as torque/strain and temperature measurement (up to 500-800 microstrain) within the temperature range of -40˚C to +150˚C. AQP's are especially well-suited for high-volume and low-cost applications.

SAW Interrogation Units

The electronics reader, or Sensor Interrogation Unit developed by Transense can wirelessly measure up to five resonant frequencies of the SAW sensing elements in a typical torque measurement system or up to 12 sensing elements in a temperature only measuring system.

A key component of the reader is a Radio Frequency Application-Specific Integrated Circuit (RF ASIC) developed by Transense, which allows a very fast and accurate measurement of the SAW resonant frequencies and reduces the size and cost of the electronics to a level acceptable for high-volume applications. Apart from the RF ASIC, there is another key component, a Digital Signal Processor (DSP) to control the operation of the transceiver, perform spectral analysis of the SAW response, and calculate strain/torque/pressure and temperature.

The standard Transense SAW electronics reader has both analogue and digital CAN and SPI outputs transmitting data up to 6.67 kHz. The accuracy of the frequency measurement is better than 1 kHz at 430 MHz.

Transense has also developed specialised readers with various form factors, multichannel readers with up to eight RF channels and a high-speed reader with a strain/torque update rate of 16 kHz; the fastest available on the market.

RF Rotary Couplers for SAW Torque Sensors

A pair of near-field close-coupled antenna, the stator and rotor, referred as RF rotary couplers, are used to wirelessly transmit the RF signal from the ASIC to the rotating SAW sensors. Transense has developed RF rotary couplers with a planar and coaxial cylindrical geometry for shaft diameters from 10 to 420mm.

The gap between the stator and rotor couples are from 1 to 20mm, and rotational speeds up to 30,000 rpm have been produced, higher shaft speeds are also possible. Most of the RF coupler designs are based on an inexpensive PCB material and can often be used as a standard solution for several different applications.