MIT engineers have built a wireless underwater camera without a battery, capable of accumulating power on its own while consuming very little power, according to new paper Published in Nature Communications. The system can remotely capture color images of submerged objects – even in dark places – and transmit data wirelessly for real-time monitoring of underwater environments, aiding in the discovery of new rare species, monitoring of ocean currents, pollution, or commercial and military operations.
We already have different ways of taking pictures underwater, but according to the authors, “most marine and ocean creatures have not yet been observed.” This is partly because most current methods require them to be connected to ships, underwater drones, or power plants for both power and communications. Those methods that do not use tethering must include battery power, which limits its life. While it is in principle possible to harvest energy from ocean waves, underwater currents, or even sunlight, adding the equipment needed to do so would result in a much larger and more expensive underwater camera.
So the MIT team set out to develop a solution for a battery-free wireless imaging method. The design goal was to reduce the hardware required as much as possible. Because they wanted to keep power consumption to a minimum, for example, the MIT team used cheap, off-the-shelf imaging sensors. The trade-off is that these sensors only produce grayscale images. The team also needed to develop a low-power flash as well, since most underwater environments don’t get much natural light.
It turns out that the solution to both challenges involves red, green, and blue LEDs. The camera uses a red LED to illuminate the location and captures that image with its sensors, then repeats the process with the green and blue LEDs. The image may look black and white, the authors say, but the three colors of light from the LEDs are reflected in the white portion of each image. So a full-color image can be reconstructed during post-processing.
“When we were kids in art class, we were taught that we can make all colors using three primary colors,” Co-author Fadel Adeeb said:. “It follows the same rules for the color images we see on our computers. We only need red, green, and blue – these three channels – to create color images.”
Instead of a battery, the sensor relies on piezo acoustic backscattering for very low-power communications after the image data is encoded as bits. This method does not need to generate its own audio signal (as with sonar, for example), and instead relies on modulating the reflections of underwater sounds to transmit data one bit at a time. This data is captured by a remote receiver capable of retrieving the modified patterns, and then the binary information is used to reconstruct the image. The authors estimate that their underwater camera is about 100,000 times more energy efficient than its counterparts, and can run for weeks on end.
Naturally, the team built a proof-of-concept prototype and ran some tests to prove their method worked. For example, they photographed pollution (in the form of plastic bottles) at Keyser Pond in southeastern New Hampshire, as well as photographed the African starfish (Protorster Linkley) in a “controlled environment with outdoor lighting”. The resolution of the last image was good enough to capture the various tubercles along the starfish’s five arms.
The team was also able to use the wireless underwater camera to monitor the growth of an aquatic plant (Aponogeton ulvaceus) over several days, detecting and locating visual tags often used for underwater tracking and automated processing. The camera achieved high detection rates and high localization accuracy up to a distance of about 3.5 meters (about 11 and a half feet); The authors suggest that longer detection ranges can be achieved with higher-resolution sensors. Distance is also a factor in the camera’s energy harvesting and communication capabilities, according to tests conducted at the Charles River in eastern Massachusetts. As expected, these two vital capabilities diminish with distance, even though the camera did manage to transmit data up to 40 meters (131 feet) from the receiver.
In summary, the authors write: “The unrestricted, inexpensive, and fully integrated nature of our method makes it a desirable approach to massive ocean dispersals.” Scaling up their approach requires more advanced and efficient transducers, as well as higher power underwater acoustic transmissions. It is also possible that one can take advantage of existing mesh networks of ocean surface buoys, or networks of underwater robots such as Argo buoys, to operate energy-collecting cameras remotely.
“One of the most exciting applications of this camera for me personally is in the context of climate monitoring,” Adeeb said. “We are building climate models, but we are missing data from more than 95 percent of the ocean. This technology can help us build more accurate climate models and better understand how climate change is affecting the underwater world.”
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