A typical SAW wireless sensing system includes:
Surface Acoustic Wave-based sensors exploit Surface Acoustic Waves (SAW), and in the case of SENSeOR’s sensors, more specifically, the Rayleigh waves, named after Lord Rayleigh, who first demonstrated their existence in 1885.
Thanks to their piezoelectric properties, certain materials like Quartz create on their surface a mechanical displacement (surface acoustic wave) if an electrical field is applied. (Piezo means “Pressure” in Greek.)
Sensing with acoustic waves is based on measuring the variations of the acoustic propagation velocity of the wave, or wave attenuation. These variations imply changes in wave properties (frequency for resonators, delay for delay lines) which can be translated into the corresponding change of the physical parameter measured.
Remote Interrogation of wireless passive SAW sensors by radio waves
The Radio-Frequency (RF) transceiver sends an electromagnetic pulse.
The pulse is converted into a Surface Acoustic Wave (SAW) on the sensor (piezoelectric effect).
The properties of the Surface Acoustic Wave will be modified under the effect of the physical parameter which is sensed (e.g. temperature).
The SAW sensor response transmits these modifications back to the RF transceiver.
The transceiver computes the data and converts the frequency variation into the measurement monitored with patented algorithms.
It acts as an acquisition and storage unit.
When connected to a network or a computer, it sends the measurement in real-time to the customer gateway or HMI/GUI for display.
SAW sensors are powered by the energy of radio waves emitted by the associated transceiver unit when remotely interrogating the sensors in real-time.
Inverse piezoelectricity induces a mechanical strain (e.g. a surface acoustic wave) on the substrate due to an electrical field generated at the electrodes of the sensor, whereas the direct effect allows for an electrical detection of acoustic wave propagation under the electrodes. The velocity of a surface acoustic wave is very sensitive to the surface state. Under the effect of the physical parameter which is sensed, the velocity or the delay length is modified.
We can measure:
- the delay or corresponding phase
- the attenuation
- the frequency of the sensor’s electrical response
An adequate choice of crystal cut enables to significantly minimize unwanted sensitivities.
In order to compensate for unwanted effects due to other physical parameter variations, an independent reference sensor can be implemented on the same chip in order to benefit from a differential measurement.
Differential structure advantages
When an independent reference SAW sensor is added on a chip, the SAW sensor provides a differential measurement that compensates for unwanted effects due to other physical parameters variations.
SENSeOR’s SAW sensors are based on two resonators working at two different frequencies for optimal performance.
The use of this innovative differential structure offers improved accuracy in measurements and enables SENSeOR to provide the most time stable and robust SAW sensors in the industry.
SAW-based sensors are built on single-crystal piezoelectric materials like Quartz (SiO2), Lithium Niobate (LiNbO3), Lithium Tantalate (LiTaO3), Langasite (LGS) and Aluminum Nitride (AlN) or Zinc Oxide (ZnO)/Silicon compounds.
In the case of single crystals, different cut-angles produce largely different results. The design of the sensor needs to be adapted for each application by selecting the appropriate design alternatives:
- Wave type: Rayleigh waves, Love waves etc.
- Choice of structure: delay lines, resonators
- Choice of material: see examples above, according their properties (high coupling, high velocity, temperature compensation, high quality factor etc.)
- Choice of frequency: from 30 MHz up to 2.45 GHz and more
For temperature measurements and strain measurements in standard ranges, SENSeOR’s sensors are mostly based on Quartz resonators, using Rayleigh Surface Acoustic Waves, interrogated wirelessly.
The SAW technology can also be used in wired mode, with very high levels of sensitivity, very appropriate for chemical or biological sensing for instance.
Other types of waves, like shear acoustic waves are used in R&D projects, for instance in the Love Food project for biological sensing (Love waves).