"How can you put a sensor under the water on Titan that lasts for long periods of time in a place that's difficult to get energy?" says Adib. "Sensors that communicate without a battery open up possibilities for sensing in extreme environments."
Piezoelectric materials, say the researchers, offer a solution. They produce a small voltage in response to vibrations, but that effect is also reversible: Applying voltage causes the material to deform. If placed underwater, that effect produces a pressure wave that travels through the water.
Communicating relies on preventing the piezoelectric resonator from naturally deforming in response to strain. At the heart of the system is a submerged node - a circuit board that houses a piezoelectric resonator, an energy-harvesting unit, and a microcontroller. Any type of sensor can be integrated into the node by programming the microcontroller. An acoustic projector (transmitter) and underwater listening device, called a hydrophone (receiver), are placed some distance away.
If the sensor wants to send a 0 bit, when the transmitter sends its acoustic wave at the node the piezoelectric resonator absorbs the wave and naturally deforms, and the energy harvester stores a charge from the resulting vibrations. The receiver then sees no reflected signal and decodes a 0.
When the sensor wants to send a 1 bit, when the transmitter sends a wave the microcontroller uses the stored charge to send a voltage to the piezoelectric resonator. That voltage reorients the material’s structure in a way that stops it from deforming, and instead reflects the wave. Sensing a reflected wave, the receiver decodes a 1.
The transmitter and receiver must be powered, say the researchers, but can be planted on ships