How it works
Remote Interrogation of wireless 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).
- Properties of the 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.
One of the outstanding values of acoustic wave-sensing is that it allows creating wireless sensors which require no local energy source!
Sensors are powered by the energy of radio waves emitted by the associated reader 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 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
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. Also an adequate choice of crystal cut enables one to significantly minimize unwanted sensitivities.
SENSeOR’s sensors are based on two resonators working at two different frequencies in the 434 MHz ISM band [433.05 MHz, 434.79 MHz]. Use of a differential structure offers improved accuracy in measurements and enables SENSeOR to provide the most time stable devices in the industry.