Underwater communication system uses battery-free sensors: Page 2 of 3

August 23, 2019 //By Rich Pell
Underwater communication system uses battery-free sensors
Researchers at MIT have developed a battery-free underwater communication system that uses near-zero power to transmit sensor data.
researchers, extend beyond Earth's oceans, and could, for example, be used to collect data in the recently discovered subsurface ocean on Saturn's largest moon, Titan, where NASA is planning to send a rover in 2026 for exploration with its Dragonfly mission.

"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

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