MIT researchers could use a battery-free underwater communication system that uses near-zero power to transmit sensor data. The system could be used to monitor sea temperatures to study climate change and track marine life over long periods and even sample waters on distant planets.
The system leverages two key phenomena. One, called the “piezoelectric effect,” taking place when vibrations in certain materials generate an electrical charge. The other is “backscatter,” a communication technique commonly used for RFID tags, that transmits data by reflecting modulated wireless signals off a tag and back to a reader.
In the researchers’ system, a transmitter sends acoustic waves through water toward a piezoelectric sensor that has stored data. When the wave hits the sensor, the material vibrates and stores the resulting electrical charge. Then the sensor uses the stored energy to reflect a wave back to a receiver. Alternating between reflection in that way corresponds to the bits in the transmitted data: For a reflected wave, the receiver decodes a 1; for no reflected wave, the receiver decodes a 0.
The researchers demonstrated their Piezo-Acoustic Backscatter System in an MIT pool, to collect water temperature and pressure measurements. The system transmitted 3 kilobytes per second of accurate data from two sensors at the same time at a distance of 10 meters between sensor and receiver.
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 included into the node by programming the microcontroller. An acoustic projector and underwater listening device, called a hydrophone (receiver), are placed some distance away.
The transmitter and receiver must have power but can be planted on ships or buoys, where batteries are can be replaced easier, or connected to outlets on land. One transmitter and one receiver can gather information from many sensors covering one area or many areas.
The researchers now aim to demonstrate that the system can work at farther distances and communicate with more sensors simultaneously. They are also hoping to test if the system can transmit sound and low-resolution images.
The work is partly sponsored by the US Office of Naval Research.