According to a new article published in the journal Nature Communications, MIT engineers have built a wireless, battery-free underwater camera capable of self-harvesting while using very little power. The system can capture color photos of remote submerged objects – even in dark environments – and wirelessly transmit the data to monitor the underwater environment in real time, aid in the discovery of new rare species, or monitor ocean currents, pollution, or commercial and military operations.
We already have various methods for capturing underwater images, but according to the authors, “Most of the ocean and marine organisms have not yet been observed.” That’s partly because most existing methods for both power and communications need to be connected to ships, underwater drones, or power plants. The methods that do not use tethering must involve 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 equipment required to do so would result in a much bulkier and more expensive underwater camera.
So the MIT team set out to develop a battery-free, wireless imaging solution. The design goal was to minimize the required hardware as much as possible. For example, because they wanted to minimize power consumption, the MIT team used cheap, off-the-shelf image sensors. The downside is that such sensors only produce grayscale images. The team also had to develop a low-power flash because most underwater environments don’t get much natural light.
The solution to both challenges turned out to be to integrate red, green and blue LEDs. The camera uses the red LED for in situ illumination and captures this image with its sensors and then repeats the process with the green and blue LEDs. The image may appear black and white, according to the authors, but the three colors of light from the LEDs are reflected in the white portion of each image. In this way, a full-color image can be reconstructed during post-processing.
“When we were kids in art class, we were taught that we could create all colors using three primary colors,” said co-author Fadel Adib. “The same rules apply to color images that we see on our computers. We only need red, green and blue – those three channels – to construct color images.”
Instead of a battery, the sensor uses piezo-acoustic backscatter for ultra-low-power communication after image data is encoded as bits. This method does not have to generate its own acoustic signal (like sonar, for example), but instead relies on modulating reflections of incoming underwater noise to transmit data bit by bit. This data is picked up by a remote receiver able to recover the modulated patterns and the binary information is then used to reconstruct the image. The authors estimate their underwater camera is about 100,000 times more energy efficient than its counterparts and could run for weeks.
Of course, the team built a proof-of-concept prototype and ran some tests to show their method worked. For example, they photographed pollution (in the form of plastic bottles) in Keyser Pond in southeastern New Hampshire and an African starfish (Protoreaster lincklii) in “a controlled environment with external lighting”. The resolution of the latter image was good enough to capture the various tubercles along the starfish’s five arms.
The team was also able to monitor the growth of an aquatic plant using their wireless underwater camera (Aponogeton ulvaceus) over several days and to detect and locate visual tags commonly used for underwater tracking and robotic manipulation. Up to a distance of around 3.5 meters, the camera achieved high detection rates and high localization accuracy; The authors suggest that larger detection areas could be achieved with higher resolution sensors. Distance is also a factor in the camera’s power harvesting and communications capabilities, according to tests conducted in eastern Massachusetts’ Charles River. As expected, these two critical capabilities decrease with distance, although the camera successfully transmitted data at a distance of 40 meters (131 feet) from the receiver.
In short, “The cordless, low-cost, and fully integrated nature of our method makes it a desirable approach for massive marine operations,” the authors wrote. Scaling up their approach requires more sophisticated and efficient transducers, as well as more powerful underwater acoustic transmissions. It is possible that one could also use existing mesh networks of sea surface buoys or networks of underwater robots such as Argo-Floats to remotely power the energy harvesting cameras.
“One of the most exciting uses of this camera for me personally is climate monitoring,” said Adib. “We build climate models, but we lack data from over 95 percent of the ocean. This technology could help us create more accurate climate models and better understand how climate change is affecting the underwater world.”
DOI: Nature Communications, 2022. 10.1038/s41467-022-33223-x (About DOIs).
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