Jul 09 2015
The manner in which fish’s fins allow it to swim is the inspiration for the design of a unique underwater propulsion system being developed by the U.S. Naval Research Laboratory (NRL) in Arlington, Va.
WANDA, the Wrasse-inspired Agile Near-shore Deformable-Fin Automation, is a man-portable autonomous underwater vehicle (AUV) that uses two pairs of fish-like fins that allow it to conduct missions in littoral zones where low speed and high maneuverability are prized capabilities.
Jason Geder, aerospace engineer at the NRL’s Laboratories for Computational Physics and Fluid Dynamics, says WANDA was inspired by the pectoral fins of the reef fish, bird wrasse, and allowed NRL researchers to develop a vehicle with an actively controlled curvature robotic fin that provides scaled down AUVs with a low speed propulsion system.
“The actively controlled curvature fins allow the vehicle to maneuver more precisely than a propeller driven vehicle,” Geder points out. “Propeller systems have issues with dead zones and overshoot that WANDA does not.”
WANDA has two pairs of pectoral fins, two forward and two aft on the vehicle. Geder says WANDA’s uniqueness lies in the fact it actively controls the curvature of its fins by actuating individual rib bars within the fins, which allows it to swim through the water by an oscillating motion.
The current version of WANDA is 3-1/2 feet long with a nearly elliptical cross section that’s approximately 14 inches in width at its widest point and less than 10 inches at its highest point.
“The dry weight, depending on its payload, is approximately 25 pounds and can be picked up by a single person,” Geder says.
WANDA carries sensors on board for navigation, operation and maneuvering, relying on an inertial navigation system, angular rate gyros, accelerometers for measuring heading and the angle of the vehicle, a pressure sensor for measuring depth, and a GPS for fixing its location if it comes to the surface, Geder says.
Geder says NRL researchers have developed near-field pressure sensors that allow WANDA to detect current flows and static objects in the near field. “It’s like the lateral line sensors fish use to detect objects on their side and front,” he says. “The fish use a system of hair-like pressure sensing elements that allow them to sense changes in the flow field around their body. We are testing those kinds of sensors with WANDA for their navigational capabilities.”
WANDA’s modular construction allows it to be integrated for a number of different mission-specific payloads, Geder points out. “Besides the near-field sensors, we also are developing chemical sensors for detecting underwater chemical signatures in the environment, where they would detect a particular element and then use that information to navigate upstream to where it’s coming from,” he says. “If WANDA is close to the surface, it can communicate that information in real time by wireless data transfer and also store the data on board for later downloading.”
Geder says NRL has programmed a number of different maneuvers and missions into WANDA.
“These are closed loop maneuvers, where, for example, the vehicle would dive to a certain depth, do a certain pattern like a box or a lawnmower pattern, and then return to the surface to transfer the data,” Geder says. “It also has the capability for manual operation for testing purposes, where a user controls the vehicle.”
Geder says the next step in the development of WANDA is improving the vehicle’s robustness for field trials that will be conducted this summer in the Washington, D.C. area with a number of NRL collaborators.
About the author: Alan M. Petrillo is a Tucson, Ariz., journalist who writes for national and regional magazines and newspapers. He’s the author of several books on historical military firearms and two historical mysteries, Full Moon, and his latest novel, Asylum Lane, all available at www.amazon.com.