Jacinta Bowler
Exploring the depths of Earth’s oceans is no picnic. Both humans and electronics are not built for traversing the huge body of water surrounding our planet.
But a new study has come closer to a solution, creating an incredibly energy efficient, underwater camera powered by sound.
“It took a fair amount of creativity to figure out how to do this,” says MIT electrical engineer Fadel Adib.
“We were trying to minimise the hardware as much as possible, and that creates new constraints on how to build the system, send information, and perform image reconstruction.”
Depending on who you ask, scientists suggest that more than 80 percent of our ocean has never been mapped, observed, or explored. This would make it less understood than Mars or the far side of the Moon.
For their energy requirements, underwater cameras can be tethered to a research vessel, or a ship can be sent to recharge the camera’s batteries, but neither of these are ideal solutions when there’s so much ocean to cover.
The team, with new research published in Nature Communications, developed a battery-free, wireless underwater camera. It’s about 100,000 times more energy-efficient than other undersea cameras, and can take colour photos, even in dark underwater environments, transmitting that image data wirelessly through the water.
The camera acquires energy using ‘transducers’ placed around the outside. These transducers are piezoelectric, which produce an electric signal when a mechanical force is applied to them.
So, when a sound wave traveling through the water hits the transducers, they vibrate and convert that mechanical energy into electrical energy.
The camera stores harvested energy until it has built up enough to power take photos and communicate data.
Read more: Underwater cameras: Flash photography is fine, but don’t touch
The next problem the team had was how to take the actual photos. To keep power consumption as a low as possible, the researchers used off-the-shelf, ultra-low-power imaging sensors. But these sensors only capture grayscale images. And since most underwater environments lack a light source, they needed to develop a low-power flash, too.
They solved both problems simultaneously using coloured LEDs. When the camera captures an image, it shines a red LED and then uses image sensors to take the photo. It repeats the same process with green and blue LEDs.
So, while the three images are all black and white, the red, green, and blue coloured light is reflected in the white part of each photo. When the image data are combined in post-processing, colour image can be reconstructed.
Finally, the team created a new system to transmit the information back through the water in a process called ‘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 doesn’t reflect.
“This whole process, since it just requires a single switch to convert the device from a nonreflective state to a reflective state, consumes five orders of magnitude less power than typical underwater communications systems,” says ocean electrical engineer Sayed Saad Afzal.
Unfortunately, the new camera is not ready for deployment yet – it can only transmit 40 meters away and the amount of memory makes it limited for long term use. But it’s still an exciting step forward.
“We demonstrate wireless battery-free imaging of animals, plants, pollutants, and localization tags in enclosed and open-water environments,” the team write in their new paper.
“The method’s self-sustaining nature makes it desirable for massive, continuous, and long-term ocean deployments with many applications including marine life discovery, submarine surveillance, and underwater climate change monitoring.”
The research has been published in Nature Communications.