According to a new article published in the journal Nature Communications, MIT engineers have built a battery-less wireless underwater camera that can harvest power on its own while consuming very little power. The system can take color photos of remote underwater objects, even in dark environments, and transmit 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 various methods of taking underwater images, but according to the authors, “most of the oceanic and marine organisms have not yet been observed”. This is partly due to the fact that most existing methods require connection to ships, underwater drones, or power plants for both power and communications. Methods that do not use tethering must incorporate battery power, which limits their 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 more bulky and expensive underwater camera.
Then the MIT team began developing a solution for a batteryless wireless imaging method. The design goal was to minimize the required hardware as much as possible. Since they wanted to minimize power consumption, for example, the MIT team used standard inexpensive imaging sensors. The trade-off is that such sensors only produce grayscale images. The team also had to develop a low-power flash, as most underwater environments don’t get a lot of natural light.
The solution to both challenges turned out to be incorporating 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 in black and white, according to the authors, but the three colors of the LED light are reflected in the white part of each image. In this way it is possible to reconstruct a color image during post-processing.
“When we were kids in art classes, we were taught that we could create all colors using three basic colors,” said co-author Fadel Adib. “The same rules follow for the color images we see on our computers. We just need red, green and blue, these three channels, to build color images.”
Instead of a battery, the sensor relies on piezo-acoustic backscatter for ultra-low power communication after the image data has been encoded as 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 data one bit at a time. This data is collected by a remote receiver capable of retrieving 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 run for weeks.
Naturally, the team built a proof-of-concept prototype and ran some tests to prove their method worked. For example, they filmed pollution (in the form of plastic bottles) at Keyser Pond in southeastern New Hampshire, as well as images of 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 five arms of the starfish.
The team was also able to use their underwater wireless camera to monitor the growth of an aquatic plant (Aponogeton ulvaceus) for several days and to detect and locate visual tags 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 and a half 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 harvest and communication capabilities, according to tests conducted in the Charles River in eastern Massachusetts. As expected, both critical capabilities decrease with distance, although the camera successfully transmitted data 40 meters (131 feet) away 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. Increasing their approach requires more sophisticated and efficient transducers, as well as higher powered underwater acoustic transmissions. It is possible that existing mesh networks of ocean surface buoys, or networks of underwater robots such as Argo floats, could also be used to remotely power the cameras for energy harvesting.
“One of the most interesting applications of this camera for me personally is in the context of climate monitoring,” said Adib. “We are building climate models, but we lack data from more than 95 percent of the ocean. This technology could help us build more accurate climate models and better understand how climate change affects the underwater world.”
DOI: Nature Communications, 2022. 10.1038 / s41467-022-33223-x (Information on DOIs).