“No power source required”: four great words engineers like to hear when designing sensor devices. Kerem provides background on the Surface Acoustic Wave (SAW) technology that makes that statement possible.
Emerging market needs require very small sensors that can support data and monitoring functions that are passively and wirelessly interrogated with no power source required. In addition, industrial environments demand that sensors be inherently rugged, extremely sensitive, and intrinsically reliable and impervious to magnetic fields.
Acoustic wave sensor technology
Acoustic wave sensors generate an acoustic wave on piezoelectric material when a bias is applied. As the acoustic wave propagates through or on the surface of the material, changes to the propagation path produce a change in the wave’s velocity and/or amplitude. Changes in velocity can be monitored by measuring the sensor’s frequency or phase characteristics and can then be correlated to the corresponding physical quantity being measured.
The Rayleigh SAW delay line (see Figure 1) confines the propagating wave to the top surface of the substrate. As a result, the SAW is a highly sensitive probe for measuring mechanical properties such as stress or strain coupled in the SAW substrate, either through the packaging or on a diaphragm on which the SAW transducer is fabricated. Rayleigh SAW devices can be tailored with special cuts of piezoelectric substrate to create a linear SAW frequency-versus-temperature dependence. The result is a high-resolution temperature sensor.
Enabling wireless applications
The aforementioned characteristics are complemented by SAW sensors’ ability to operate without wires or batteries. Because these sensors have low input signal levels and high electrical efficiency, they are connected only by an RF link to a transceiver or reader unit.
A representation of a wireless sensor/identification system is shown in Figure 2. A high-frequency electromagnetic wave is emitted from an RF transceiver and received by the SAW sensor’s antenna. Interdigital Transducers (IDTs), the comb-like patterns of metal on the device that convert the electric field energy to mechanical wave energy and then back to an electric field, are connected to the antenna to convert the received signal into an acoustic wave, which propagates along the sensor and enables operation without a power source.
Depending on the construction of the device (using metal patterns such as reflectors), the IDTs can retransmit to the receiver. The received signal is amplified, converted into a baseband frequency in the RF module, and analyzed by a signal processor. Because the operating frequency is high, SAW sensors are protected from the electromagnetic interference that often occurs near industrial equipment such as motors and high-voltage lines.
A commercial SAW temperature sensor is a 433.78 MHz one-port SAW resonator structure specifically designed to have a linear frequency-versus-temperature characteristic (see Figure 3). With a temperature coefficient frequency of 16.2 ppm/°C (~7,028 Hz/°C), it is operable from 0 to +120 °C. The sensor has an unloaded Q of 8,000, is low loss (2.5 dB max), and is designed for a 50 ohm system. When combined with an antenna and interrogation unit, this SAW sensor chip can be used in numerous wireless temperature sensing applications.
Wireless sensors work within the 433.92 MHz Industrial, Scientific, and Medical (ISM) band for the defined operating temperature range. The sensors are designed to provide instantaneous wireless temperature measurements for embedded real-time in-line environments requiring high resolution and accuracy. SAW temperature sensors have exceptional stability characteristics, passing DIN IEC 68 T2-27 specifications for shock rating and screening according to DIN IEC 68 T2-6 standards for vibration rating. Temperature stability characteristics are assured by DIN IEC 68 Part 2-14 Test N standards.
These types of wireless temperature sensors are already installed in commercial products such as cooking ovens and are being evaluated for use in industrial applications to determine contact temperature in high-voltage breaker boxes and monitor temperature in rotating equipment.
Sensing the pressure
In the field of pressure sensing, the wireless SAW sensor presents a viable approach to installing portable, batteryless sensors that do not require wiring. In this type of sensor, various packaging features translate the external pressure to a mechanical force on the die where a pressure resonator is present. This pressure causes the die to flex, resulting in a SAW output frequency change proportional to the applied external pressure. A temperature resonator can be located on the same die to provide temperature compensation for the pressure data.
Wireless pressure sensors have already been tested for the tire pressure monitoring market, and engineers are developing approaches for other industrial markets. For example, efforts are under way to install pressure sensors in remote or mobile high-value fluid tanks for reporting tank levels. With their unique ability to operate without a power source, SAW sensors can achieve the sensitivity, accuracy, range, cost, stability, configuration logistics, and other performance characteristics needed in the semiconductor, medical, sanitary, and process control sectors and ultimately satisfy end users.
Kerem Durdag is director of sales and marketing at SenGenuity, a Hudson, New Hampshire-based division of Vectron International. Kerem has more than 14 years of experience in business development, partner and channel development, and corporate strategy. Prior to SenGenuity, he held key management positions at BiODE, STEAG HamaTech, and Conceptronic. He holds a B.Sc in Applied Physics from Saint John University and an M.Sc in Mechanical Engineering from the University of New Hampshire. In addition to holding several patents, Kerem has authored white papers for peer-reviewed journals and articles published in trade magazines.
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