MIT engineers have built a wireless, battery-less underwater camera that can harvest energy on its own while consuming very little power, according to a new paper published in the journal Nature Communications. The system can take color photos of distant submerged objects, even in dark environments, and transmit the data wirelessly for real-time monitoring of underwater environments, aiding in the discovery of new rare species, or monitoring ocean currents , pollution or commercial and military operations.
We already have a variety of methods for taking underwater images, but according to the authors, “most oceanic and marine organisms have yet to be observed.” This is partly because most existing methods require tethering to ships, underwater drones or power plants for power and communication. Methods that do not use a connection must incorporate battery power, which limits their lifespan. While it is in principle possible to harvest energy from ocean waves, underwater currents or even sunlight, adding the necessary equipment to do so would result in an underwater camera much bulkier and more expensive.
The MIT team therefore set out to develop a solution for a wireless, battery-less imaging method. The design goal was to minimize the hardware needed as much as possible. Since they wanted to keep power consumption to a minimum, for example, the MIT team used inexpensive 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 most underwater environments don’t get much natural light.
The solution to both challenges turned out to incorporate red, green and blue LEDs. The camera uses the red LED for in-situ illumination and captures that image with its sensors, then repeats the process with the green and blue LEDs. The image may appear black and white, according to the authors, but all three colors of light from the LEDs are reflected in the white portion of each image. Thus, a color image can be reconstructed during post-processing.
“When we were kids in art class, we were taught that we could create any color using three basic colors,” said co-author Fadel Adib. “The same rules apply to color images that we see on our computers. We just need red, green and blue, those three channels, to build color images.”
Instead of a battery, the sensor relies on piezoacoustic backscatter for ultra-low-power communication after image data has been encoded into bits. This method does not need to generate its own acoustic signal (as with sonar, for example), relying instead on modulating the reflections of incident underwater sounds to transmit the data one bit at a time. These data are picked up by a remote receiver capable of recovering the modulated patterns, and the binary information is then used to reconstruct the image. The authors estimate that their underwater camera is around 100,000 times more energy efficient than its counterparts and could operate for weeks.
Naturally, the team built a proof-of-concept prototype and ran tests to demonstrate that their method worked. For example, they imagined pollution (in the form of plastic bottles) in Keyser Pond in southeastern New Hampshire, as well as an African starfish (Protoreaster lincklii) in “a controlled environment with external lighting”. The resolution of this last image was good enough to capture the various tubercles along the starfish’s five arms.
The team was also able to use their underwater wireless camera to monitor the growth of an aquatic plant (Aponogeton ulvaceus) over several days, and to detect and locate visual beacons often used for underwater tracking and robotic manipulation. The camera achieved high detection rates and high location accuracy up to a distance of about 3.5 meters (about 11.5 feet); the authors suggest that longer detection ranges could be achieved with higher resolution sensors. Distance is also a factor in the camera’s energy harvesting and communication capabilities, according to tests conducted in the Charles River in eastern Massachusetts. As expected, both of these critical capabilities decrease with distance, although the camera managed to transmit data 40 meters (131 ft) from the receiver.
In short, “the wireless, inexpensive, and fully integrated nature of our method makes it a desirable approach for massive ocean deployments,” the authors wrote. Scaling up their approach requires more sophisticated and efficient transducers, as well as higher power underwater acoustic transmissions. It is possible that existing mesh networks of buoys on the ocean surface, or networks of underwater robots like Argo floats, could also be used to remotely power energy harvesting cameras.
“One of the most exciting applications of this camera for me personally is in the context of climate monitoring,” Adib said. “We are building climate models, but we lack data for more than 95% of the ocean. This technology could help us build more accurate climate models and better understand the impact of climate change on the underwater world. .”
DOI: Nature Communications, 2022. 10.1038/s41467-022-33223-x (About DOIs).
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