Basics on SAW
Acoustic wave technology relies on piezoelectricity, discovered by Pierre and Jacques Curie in 1880 during their analysis of materials like Quartz.
Surface acoustic wave propagation mode and properties were first described by Lord Rayleigh in 1885.
Industrial and commercial applications of SAW technology were first made around the 1950’s.
The most common use of SAW technology today is for mobile phone filters.
SAW-based sensing devices have been in development for years and now perform optimally for a wide range of difficult measurement applications (industrial, chemical, medical).
Acoustic wave-based sensors exploit Surface Acoustic Waves (SAW – propagate on the piezoelectric material) or Bulk Acoustic Waves (BAW – propagate through the piezoelectric material) depending on application requirements.
Based on their piezoelectric properties, certain materials like Quartz create a mechanical displacement (acoustic waves) if an electrical field is applied. (Piezo means “Pressure” in Greek.)
Sensing with acoustic waves is based on measuring variations of acoustic propagation velocity of 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.
SAW-based sensors are built on single-crystal piezoelectric materials like Quartz (SiO2), Lithium Niobate (LiNbO3), Lithium Tantalate (LiTaO3), Langasite (LGS) and Aluminium 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 alternative:
- Wave type: Rayleigh waves, Love waves, Transverse waves
- Choice of structure: delay lines, resonators
- Choice of material (high coupling, high velocity, temperature compensation, high quality factor etc.)
- Choice of frequency: from 30MHz up to 2.45GHz and more
Typical SAW wireless sensing systems include:
- A packaged SAW sensor connected to an antenna (SENSeOR’s SAW sensors are wireless and can be mounted in various packages and housings like thermowells and PCB)
- A transceiver connected to one or multiple antennas