MIT researchers have developed a battery-free, wireless underwater camera that is about 100,000 times more energy-efficient than other undersea cameras.
The device is capable of taking color photos, even in dark underwater environments, and transmits image data wireless through the water.
An estimated 95 per cent of the Earth’s oceans have never been observed, which means less of our planet’s oceans has been seen than the far side of the moon or the surface of Mars.
The high cost of powering an underwater camera for a long time, by tethering it to a research vessel or sending a ship to recharge its batteries, is a steep challenge preventing widespread undersea exploration.
The autonomous camera is powered by sound. It converts mechanical energy from sound waves traveling through water into electrical energy that powers its imaging and communications equipment. After capturing and encoding image data, the camera also uses sound waves to transmit data to a receiver that reconstructs the image.
Because it doesn’t need a power source, the camera could run for weeks on end before retrieval, enabling scientists to search remote parts of the ocean for new species. It could also be used to capture images of ocean pollution or monitor the health and growth of fish raised in aquaculture farms.
“One of the most exciting applications of this camera for me personally is in the context of climate monitoring. We are building climate models, but we are missing data from over 95 per cent of the ocean. This technology could help us build more accurate climate models and better understand how climate change impacts the underwater world,” said Fidel Adib, senior author on the research paper.
To build a camera that could operate autonomously for long periods, the researchers needed a device that could harvest energy underwater on its own while consuming very little power.
The camera acquires energy using transducers made from piezoelectric materials that are placed around its exterior. Piezoelectric materials produce an electrical signal when a mechanical force is applied to them. When a sound wave travelling through the water hits the transducers, they vibrate and convert that mechanical energy into electrical energy.
Those sound waves could come from any source, like a passing ship or marine life. The camera stores harvested energy until it has built up enough to power the electronics that take photos and communicate data.
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 gray scale images. And since most underwater environments lack a light source, they needed to develop a low-power flash, too.
“We were trying to minimize the hardware as much as possible, and that creates new constraints on how to build the system, send information, and perform image reconstruction. It took a fair amount of creativity to figure out how to do this,” Adib said.
They solved both problems simultaneously using red, green, and blue LED s. 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 LED s.
Even though the image looks black and white, the red, green, and blue colored light is reflected in the white part of each photo. When the image data are combined in post-processing, the colour image can be reconstructed.
Once image data are captured, they are encoded as bits (1 s and 0 s) and sent to a receiver one bit at a time using a process called underwater back scatter. 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 absorbed so that it does not reflect back.
A hydrophone next to the transmitter senses if a signal is reflected back from the camera. If it receives a signal, that is a bit-1, and if there is no signal, that is a bit-0. The system uses this binary information to reconstruct and post-process the image.
The researchers tested the camera in several underwater environments. In one, they captured color images of plastic bottles floating in a New Hampshire pond. They were also able to take such high-quality photos of an African starfish that tiny tubercles along its arms were clearly visible.
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