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Researchers Design Battery-Free Sensor For Underwater Exploration
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Researchers Design Battery-Free Sensor For Underwater Exploration

Underwater Sensors

To investigate the vastly unexplored oceans covering most of our planet, researchers at MIT’s Media Lab aim to build a submerged network of interconnected sensors that send data to the surface — an underwater “internet of things”. All using almost zero energy.

The proposed interconnected system of sensors is part of a communications system that avoids a key problem with underwater electronics, which is the pollution from batteries. Researcher at MIT developed a work around for this problem. Scrap the batteries all together. The system of sensors uses a battery-free underwater communication system that uses near-zero power to transit it sensor data.


How It Works

In order to achieve this feat, the system uses two natural phenomena. One, called the “piezoelectric effect,” occurs 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.

Piezoelectric materials, which have been around and used in microphones and other devices for about 150 years. 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. Using these properties researchers believe they can reliably transmit data without the need of batteries.

“That reversibility is what allows us to develop a very powerful underwater backscatter communication technology,” Adib says. 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

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. 

Its creators at MIT are calling the project the Piezo-Acoustic Backscatter System, and detailed their findings at last week’s SIGCOMM conference in Budapest, where they won “best paper”.

In the Piezo-Acoustic Backscatter 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 — 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.

“Once you have a way to transmit 1s and 0s, you can send any information,” says Adib. “Basically, we can communicate with underwater sensors based solely on the incoming sound signals whose energy we are harvesting.”

“When you’re tracking a marine animal, for instance, you want to track it over a long range and want to keep the sensor on them for a long period of time. You don’t want to worry about the battery running out,” Adib says. “Or, if you want to track temperature gradients in the ocean, you can get information from sensors covering a number of different places.”


Long-Term Applications

The system would enable better monitoring of marine life and ocean temperatures over longer periods of time with minimal pollution and human interference — factors integral to better understanding climate change and predicting the rise in sea levels.

Researchers say this system could even sample waters on distant planets in the most extreme conditions imaginable. 

You can read more about the project on MIT Media Labs website here.

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