The Sentry Autonomous Underwater Vehicle: Field Trial Results and Future Capabilities

The Sentry autonomous underwater vehicle combines an efficient long range survey capability with the ability to maneuver at low speeds. These attributes will permit Sentry to perform a variety of conventional and unconventional surveys including long range sonar surveys, hydrothermal plume surveys and near-bottom photo surveys. Sentry's streamlined body and fore and aft tilting planes, each possessing an independently controlled thruster, enable efficient operation in both near-bottom and cruising operations. Sentry is capable of being configured in two modes: hover mode, which commands Sentry's control surfaces to be aligned vertically, and forward flight mode, which allows Sentry's control surfaces to actuate between plus or minus 45 degrees. Sentry is equipped for full 6-Degrees of freedom position measurement. Vehicle heading, roll, and pitch are instrumented with a TCM2 PNI heading and attitude sensor. A Systron Donner yaw rate sensor instrumented heading rate. Depth is instrumented by a Paroscientific depth sensor. A 300kHz RD Instruments Doppler Sonar provides altitude and XYZ velocity measurements. In April 2006, we conducted our first deep water field trials of Sentry in Bermuda. These trials enabled us to examine a variety of issues, including the control software, vehicle safety systems, launch and recovery procedures, operation at depth, heading and depth controllers over a range of speeds, and power consumption. Sentry employ's a control system based upon the Jason 2 control system for low-level control, which has proven effective and reliable over several hundred deep-water dives. The Jason 2 control system, developed jointly at Johns Hopkins University and Woods Hole Oceanographic Institution, was augmented to manage Sentry-specific devices (sensors, actuators, and power storage) and to employ a high-level mission controller that supported autonomous mission scripting and error detection and response. This control suite will also support the Nereus Hybrid ROV, also in development at Woods Hole. Both systems performed well during these engineering trials. Sentry's heading and depth controller was tested in a series of trials at different speeds. The heading set point was maintained within approximately ½ degree and appeared to be limited by the precision of the heading sensor. The depth set point, during level flight, was maintained within about 2 centimeters. Based on these tests, we can project Sentry's range as a function of speed. Vehicle speed was measured by a 300 khz bottom-lock Doppler sonar while energy consumption was measured using a coulometer and voltage measurement. The vehicle flew complementary courses at the same levels of forward thrust, which allowed the effects of ambient currents to be eliminated. Assuming a sensor power level consistent with plume survey and bathymetric survey and a 10 kilowatt-hour battery, Sentry will be able to survey 100 km at 2.5 knots and over 150 km at 1.5 knots. Upgrades to Sentry that are presently funded and underway include the addition of an inertial navigation system, improved batteries, a camera system, and a multibeam sonar.