Smart Tether for Relative Localization of Moored and Towed Bodies

Sponsored by: Office of Naval Research
Team Member: University of California, San Diego

There exists a present need in the Navy to accurately detect the position of underwater mines and countermeasures. Currently, a demonstration effort within the Navy exists to explore the use of crawling robots for performing reacquisition-id-neutralization missions against mines. The current effort uses multi-channel radio floats that contain a global positioning system (GPS) for monitoring the location of the robot. There is a push to eliminate the float in order to remove any “surface expression” from the system through the use of a tether, but a means to determine the underwater location of the robot is required to replace the GPS functionality. Based on previous experience with shape measurement of towed sonar arrays using fiber Bragg grating (FBG) strain sensors, AITHER and the University of California San Diego (UCSD) are working to develop a smart optical fiber tether (SOFT). The SOFT will be able to accurately determine the location of the crawling robot with respect to the mooring vessel in real-time. The problem of determining dynamic shape of long, flexible bodies extends to other applications including cables, towed arrays, oil risers, aircraft wings, and the off-shore drilling industry.

AITHER has developed robust methods to embed FBG sensor arrays into the polyurethane hose structure of the towed sonar systems, composite structures, and tow cable assemblies. This experience is being used to determine a robust method for integrating the FBG sensor arrays into a tether structure. AITHER’s proprietary strain-to-shape algorithm developed for towed sonar arrays was modified and improved for this application. The algorithm takes the embedded strain readings and determines the shape of the tether along its entire length. The LabVIEW-based program, which implements the algorithm using real-time FBG wavelength data from Micron Optics, Inc.’s si425 to display the shape in real-time, developed under the towed array effort was also modified for the tether geometry and improvements to the shape algorithm. Test plans in the Phase I program include constructing a 20-foot smart tether with retrofit FBG sensors and completing other simple and complex shape tests on the tether using the strain-to-shape algorithm.