Engineers build battery-free, wireless underwater camera that uses sound

A team of US engineers has developed a battery-free, wireless underwater camera, powered by sound, that is about 100,000 times more energy-efficient than other undersea cameras. The device takes colour photos, even in dark underwater environments, and transmits image data wirelessly through the water. The autonomous camera converts mechanical energy from sound waves traveling through water into electrical energy that powers its imaging and communications equipment. After capturing and encoding image data, the camera also uses sound waves to transmit data to a receiver that reconstructs the image.

Since it doesn't need a power source, the camera could run for weeks on end before retrieval, enabling scientists to search remote parts of the ocean for new species, said the team from Massachusetts Institute of Technology (MIT) in the US in a paper published in the journal Nature Communications. It could also be used to capture images of ocean pollution or monitor the health and growth of fish raised in aquaculture farms.

"We are building climate models, but we are missing data from over 95 per cent of the ocean. This technology could help us build more accurate climate models and better understand how climate change impacts the underwater world," said Fadel Adib, associate professor in the Department of Electrical Engineering and Computer Science and senior author of the paper. The camera acquires energy using "transducers made from piezoelectric materials" that are placed around its exterior.

"Piezoelectric" materials produce an electric signal when a mechanical force is applied to them. When a sound wave travelling through the water hits the transducers, they vibrate and convert that mechanical energy into electrical energy.

"Those sound waves could come from any source, like a passing ship or marine life. The camera stores harvested energy until it has built up enough to power the electronics that take photos and communicate data," said the study.

Once image data is captured, they are encoded as bits (1s and 0s) and sent to a receiver one bit at a time using a process called "underwater backscatter".

The receiver transmits sound waves through the water to the camera, which acts as a mirror to reflect those waves. The camera either reflects a wave back to the receiver or changes its mirror to an absorber so that it does not reflect back.

Even though the image looks black and white, the red, green and blue coloured light is reflected in the white part of each photo. When the image data is combined in post-processing, the colour image can be reconstructed.

The researchers tested the camera in several underwater environments. In one, they captured color images of plastic bottles floating in a New Hampshire pond. They now plan to enhance the device so it is practical for deployment in real-world settings.