Creating an undersea network of interconnected sensors for monitoring and transmitting data faces one key challenge: powering the numerous sensors that remain underwater for long durations.
Researchers at the Massachusetts Institute of Technology have developed a battery-free underwater communication system that uses near-zero power to transmit sensor data. The scientists envision the system being used to monitor sea temperatures to study climate change and track marine life over extended periods.
The communications system makes use of the “piezoelectric effect,” which occurs when vibrations in certain materials generate an electrical charge. The system also relies on “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 operation, 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.
“Once you have a way to transmit 1s and 0s, you can send any information,” says co-author Fadel Adib, an assistant professor in the MIT Media Lab and the Department of Electrical Engineering and Computer Science and founding director of the Signal Kinetics Research Group. “Basically, we can communicate with underwater sensors based solely on the incoming sound signals whose energy we are harvesting.”
The researchers demonstrated their Piezo-Acoustic Backscatter System in an MIT pool, using it to collect water temperature and pressure measurements. The system was able to transmit 3 kilobytes per second of accurate data from two sensors simultaneously at a distance of 10 meters between sensor and receiver.
Inspiration for the system hit while Adib was watching “Blue Planet,” a nature documentary series exploring various aspects of sea life. Oceans cover about 72% of the Earth’s surface. “It occurred to me how little we know of the ocean and how marine animals evolve and procreate,” he said. Internet-of-things (IoT) devices could aid that research, “but underwater you can’t use Wi-Fi or Bluetooth signals … and you don’t want to put batteries all over the ocean, because that raises issues with pollution.”
Adib turned to piezoelectric materials, which have long been used in microphones and other devices. 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. They’re often used to detect sunken vessels, fish and other underwater objects.
“That reversibility is what allows us to develop a very powerful underwater backscatter communication technology,” Adib said.
Communication relies on preventing the piezoelectric resonator from naturally deforming in response to strain. The system incorporates 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 researchers hope to demonstrate that the system can work at farther distances and communicate with more sensors simultaneously. They would also like to see if the system can transmit sound and low-resolution images.