Engineers and crew of the research vessel Oceanus deploy and recover the AUV Sentry in April 2008. (Video by Chris German, WHOI)
Sentry produces high-resolution maps of the seafloor and takes digital photographs of the bottom to create a detailed picture of features like mid-ocean ridges, deep-sea vents, and cold seeps. (Animation by Jack Cook, WHOI)
This animation of a plume of chemicals flowing from the Deepwater Horizon oil spill shows Sentry making numerous criss-cross penetrations to map the plume 3,600 feet (1100m) beneath the surface of the Gulf of Mexico. (Animation by Jack Cook, WHOI)
Imagine you could design the perfect robot companion. Something to do the tasks you couldn’t do—or perhaps didn’t want to do. You would want it to be able to carry out your orders without constant supervision. You would also want to be able to trust that it would do what you told it to do, no matter how difficult or repetitive that might be.
Now imagine your new friend could dive 3 miles and swim continuously for nearly a day at a time and find underwater volcanoes and hydrothermal vents. You just described the autonomous underwater vehicle (AUV) Sentry.
Unlike human-occupied vehicles (HOVs) such as Alvin, it cannot transport scientists to the bottom to view in person the animals that live there or the processes that shape the planet. It also cannot take continuous commands and receive power from the surface like the remotely occupied vehicle (ROV) Jason does. Sentry can, however, survey large areas of the sea floor on dives lasting nearly a day without requiring that someone watch its every move. This is important when the job requires a long series of difficult or repetitive tasks thatwould be difficult and expensive to do with an ROV or HOV or that would very quickly become monotonous or overwhelming for a human to do manually.On a recent cruise to the Galapagos Rift, Sentry mapped about 100 square kilometers (40 square miles) of the seafloor to a resolution of 1 meter (3 feet), and it never complained once.
Following the Scent
Like many other AUVs, Sentry can be pre-programmed to follow a designated course or carry out a specific task. In the process, it can gather as many as 64 million data points per dive. It also has enough decision-making ability to avoid collisions with underwater obstacles or to change tasks and directions when it finds something interesting. Sentry is equipped with sonar to navigate and to map the seafloor and an array of sensors to measure things like water temperature and salinity. It can also be fitted with very advanced sensors that “sniff out” telltale chemicals that mark places or phenomena that scientists might want to investigate.
AUVs “work like bloodhounds,” said WHOI Senior Scientist Dana Yoerger, who teamed with WHOI engineers Al Bradley and Barrie Walden to design Sentry and its predecessor ABE. “When they find something we’re interested in, they can ‘bark’ and then return from the ‘hunt.’ ” Or, as Sentry did on a recent mission in the Gulf of Mexico, it can stay on the hunt and make detailed maps of an underwater chemical plume (see animation) connected to the Deepwater Horizon oil spill.
By pairing Sentry with Alvin, scientists can also help make the best use of the HOV’s limited time on the bottom. Sentry will generally do its work at night, before Alvin dives the following morning. The detailed maps that Sentry produces helps scientists and Alvin’s support crew fine-tune the day’s dive plan with much greater precision than before. Now, scientists can actually pinpoint volcanic features or coral reefs on the bottom before diving in the sub to get a closer look for themselves.
The Shape of Things to Come Sentry isn’t shaped like other AUVs, which usually resemble torpedoes. Instead, it has a streamlined body to reduce drag in the water, a body that one scientist said looks like a six-foot tall “flying bar of soap.” But that odd shape helps it submerge and surface quickly to maximize its time on the job and gives it stability when maneuvering over seafloor obstacles.
The vehicle also has four propeller-like thrusters built into its maneuverable foils, or wings, giving Sentry the ability to start, stop, change directions, and ‘fly’ in different ways underwater. It can even hover like an underwater helicopter to do close-up examinations of the seafloor. Traditional torpedo-shaped AUVs tend to keep moving in one direction.
Sentry is designed to dive very deep—as deep as 4,500 meters (3.1 miles)—putting nearly two-thirds of the global ocean within its reach. It is powered by more than 1,000 lithium-ion batteries similar to those in laptop computers, which allow it to dive up to 20 hours without a break. It might mean that scientists have an even larger mountain of data to sift through at the end of the day, but when you’re diving in the deep ocean, more information is often a good thing.
Click and drag on the Sentry vehicle for a 360 degree view: