Designed for underwater monitoring and exploration, the battery-free sensor system uses the vibration of piezoelectric materials to generate power and send and receive data. The system, say the researchers, 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 is based on two key phenomena: The "piezoelectric effect," which occurs when vibrations in certain materials generate an electrical charge, and "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.
With the system, a transmitter sends acoustic waves through water toward a piezoelectric sensor that has stored data. When the acoustic wave hits the sensor, the sensor material vibrates and stores the resulting electrical charge. The sensor then 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, say the researchers, 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.
"Once you have a way to transmit 1s and 0s, you can send any information,” says co-author of a paper on the research, 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 kbit per second of accurate data from two sensors simultaneously at a distance of 10 meters between sensor and receiver.
Applications for such a system, say the