Surface Acoustic Wave (SAW) technology

A wireless, passive, non-contact sensing system consisting of two main components: SAW sensing elements connected to antennas and an electronic interrogation unit called a reader, connected to its antenna. The reader launches the interrogation signal which is picked up by the sensor's antenna. The sensing element does not contain any DC power source, and works as a passive back-scatterer reflecting the interrogation signal back to the reader. The backscattered signal reflects the frequency of oscillation affected by a physical measurand. The reader analyses the received back-scattered signal and calculates the value of the physical measurand.

The read range for passive wireless sensors is limited by the interrogation power and depends on the required sensor resolution and the update rate. For an interrogation power of 10mW and requiring an update period of tens of milliseconds the typical sensor read range does not need to exceed a few meters. In most non-contact, rotating, torque measurements applications the distance between the sensor antenna and the reader antenna does not need to be more than 10mm, and the torque update period can be as small as 100 microseconds.

SAW sensing elements

The key part of the SAW sensing element is a small lightweight SAW device made on a quartz wafer substrate. The surface acoustic wave propagates on its free surface after being excited by a miniature Inter-Digital Transducer (IDT). The IDT is a thin-film aluminium structure formed by a standard photolithographic process which is the same process as used for manufacturing integrated circuits and MEMS devices. The IDT converts electric RF pulses, with a frequency of hundreds of MHz, into the SAW 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 aluminum 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 physical dimensions of the substrate and the SAW velocity also change. 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. Several resonators are needed to cancel the influence of a potentially variable reactance of the antenna on the measured value, and measure accurately and independently both strain and temperature. The latter is achieved by selecting a particular orientation of SAW resonators on anisotropic quartz substrates, and selecting a particular cut of the quartz for maximizing the strain sensitivity and optimisation of the temperature characteristics.

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. 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 developed by Transense can wirelessly measure up to five resonant frequencies of the SAW sensing elements. 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 are up to 30,000 rpm. Most of them are based on an inexpensive PCB material and can often be used as a standard solution for several different applications.