To investigate oceans, researchers aim to build a submerged network of interconnected sensors that sends data to the surface. Supplying constant power to scores of sensors designed to stay for long durations underwater has been a major problem.

A battery-free underwater piezoelectric sensor transmits data by absorbing or reflecting sound waves back to a receiver, where a reflected wave decodes a 1 bit and an absorbed wave decodes a 0 bit and simultaneously stores energy. (Image courtesy of the researchers)

A battery-free underwater communication system was developed 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 makes use of two key phenomena: the piezoelectric effect that occurs when vibrations in certain materials generate an electrical charge; and back-scatter, a communication technique commonly used for RFID tags that transmits data by reflecting modulated wireless signals off a tag and back to a reader.

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 — or it doesn’t reflect one at all. 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. Establishing a way to transmit 1s and 0s allows any information to be sent.

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.

The transmitter and receiver must have power but can be planted on ships or buoys where batteries are easier to replace, or connected to outlets on land. One transmitter and one receiver can gather information from many sensors covering one area or many areas.

Applications include marine biology, oceanography, meteorology, or other areas requiring long-term, low-human-effort underwater sensing. The system also could be used to collect data in the recently discovered subsurface ocean on Saturn’s largest moon, Titan. In June, NASA announced the Dragonfly mission to send a rover in 2026 to explore the Moon, sampling water reservoirs and other sites.

For more information, contact Abby Abazorius at This email address is being protected from spambots. You need JavaScript enabled to view it.; 617-253-2709.


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This article first appeared in the November, 2019 issue of Tech Briefs Magazine.

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