November 14, 2012
Engineers from the Woods Hole Oceanographic Institution (WHOI) have developed a new wireless underwater communication system to control remotely operated vehicles (ROVs) in real time. This new method may eliminate the need for long tether cables, offering a new degree of freedom in underwater robotics.
The system, recently deployed on the WHOI battery-powered Nereus vehicle, demonstrated a capability to operate the vehicle to take samples and perform work with no tether.
“Usually, for real-time work with an ROV, tether cables are a requirement. You need to be literally hard wired,” says Andy Bowen, WHOI Principal Engineer and leader of the vehicle concept development. The WHOI system, by contrast, uses acoustic signals and pulses of light to send digital information through seawater. At peak speed, it can transmit data wirelessly at 10 megabits per second—a rate similar to a broadband Internet connection, adds WHOI Senior Engineer Norman Farr, who led the team that created the optical communications technology.
The WHOI engineers conducted a field demonstration of the system near Guam in September 2012, using as a test bed the unique hybrid robotic vehicle Nereus, which is capable of operating in both ROV and autonomous underwater vehicle (AUV) modes. The tests, say Farr and Bowen, showed immediate advantages over existing ROV control systems.
In order to handle the heavy tether cables used on conventional ROVs, support ships need specialized crane and winch systems that can increase operating costs. The tether cables themselves also limit the environments in which the vehicle can be used—if operating near manmade structures or sea ice, they canbe easily broken or damaged.
The battery-powered Nereus was originally designed for survey and sampling of the deep ocean trenches at depths of up to 11,000 meters or 36,000 ft.. To enable this extreme mission, a thin fiber optic cable for sending real-time control signals was developed. Such a tether reduces the need for large tether handling and management equipment and allows long-range excursions from its launch point of up to 12 miles. However, fiber optic tethers are fragile—almost as thin as a human hair—and can only be used once before being discarded.
The newly developed system eliminates these problems by marrying two different wireless communication systems: acoustic and optical. Acoustic methods, which transmit digital signals through seawater as sound waves (much like a dial-up modem), can operate over long distances. However, due to the speed of sound in water, they can send only a small amount of information, and can’t be used to control complex operations of an ROVs in real time. Optical signals, on the other hand, can send a high-bandwidth signal that allows real-time control, but only works within a 100-meter radius. Beyond that, the light is absorbed by seawater and the signal lost.
In order to work around these limitations, the WHOI team created a device that provides vehicles with what Bowen calls “adaptive autonomy”’—an ability to switch seamlessly between different communication bandwidths and exhibit varying degrees of automation as needed to perform a mission.
During the tests in Guam, the team sent Nereus 700 meters (2296 feet) below the surface under acoustic control and tracking using the Sonardyne Ranger 2 ultrashort baseline (USBL) tracking system. Then they lowered a small optical transmitter on a cable beneath the ship. When Nereus came within range of the transmitter, it automatically picked up the optical signal, and began wirelessly transmitting high-definition video back to the ship. This video link gave the vehicle’s operator visual feedback, allowing him to manipulate a robotic arm via acoustic control.
Ralph Rayner, Senior Director for Energy and Environment at BMT marine consulting group, thinks this represents a major advancement. “When you’re operating a vehicle from the surface remotely, you need to see what you’re doing,” he says. “You might be able to control it acoustically, but you certainly could not look at what it’s doing acoustically at the same time. That’s where the big potential breakthrough this new technology could provide comes in.”
High-speed wireless communication may be useful for more than just ROVs, however. Farr thinks it might simplify the process of collecting data from fixed underwater sensors. Without a wired connection to shore, he says, these devices normally store data locally until a research ship can upload the information. “If you try upload it acoustically, you might have to park a ship over the sensor for a day or two to transmit a few gigs of data,” says Farr. “With an optical device though, we’re talking 10 to 20 minutes.”
In addition to sending scientific data, the new communication system could also be used to transmit strategic data between Navy submarines or drones. At the moment, says Rayner, a sub has to rely either on a sophisticated low-frequency radio system—which has significant bandwidth limitations—or it has to release a buoy to the surface to use a conventional radio. “So, as you can imagine, the military would be very interested in a high-speed wireless system like this,” he says.
Bowen thinks the oil and gas industry, which uses ROVs extensively to keep tabs on deep ocean wells, could benefit from the technology as well. “There’s a large amount of equipment on ocean floor, and improving monitoring and interventions with such technology is a future requirement,” he says. “Imagine being able to do that wirelessly – perhaps even with resident systems.” Large jobs that require multiple vehicles might also be made safer and easier with the new communication system. “If you’ve got multiple vehicles in the water, you’ve got multiple umbilicals, and they can potentially get tangled or damaged. It’s very difficult situation,” Bowen adds. Wireless operation would eliminate that problem.
The next step, says Farr, will be perfecting a two-way optical system that can both send and receive data simultaneously. He’s tested such a system successfully in the lab, but has yet to use it in field trials.
In the meantime, Bowen and his team are working on a new class of robots that are specially designed to use the new communication system: the Nereid HT (for Hybrid Tether), and the Nereid UI (for Under Ice). The two vehicles will have the ability to switch seamlessly between a traditional tether cable, fiber optic line, and combined acoustic/optic wireless modem, offering a new degree of flexibility for undersea exploration.
The Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the ocean’s role in the changing global environment. For more information, please see www.whoi.edu.