Power Efficiency Autonomy for Long Duration AUV Operation

Abstract

The power required to move an Autonomous Underwater Vehicle (AUV) depends on the body drag coefficient and the cube of the water velocity. This relationship suggests that the most efficient use of power is obtained at the slowest speed required to maintain steerage and level flight. However, AUVs also have hotel loads, which are independent of vehicle propulsion, leading to an optimization problem for the speed that allows for maximum distance traveled per unit energy. This paper documents a power-efficienct behavior for AUVs that considers this distance optimization and the impact of apparent ocean currents. Our approach is applicable to all propeller driven AUVs and is explored in detail with the REMUS 100 vehicle (Huntington Ingalls, MA USA) with the goal of maximizing transit distances for a given battery capacity. The REMUS 100, a two-person portable vehicle, is a convenient surrogate for larger AUVs. It allows for maturing autonomy in a rapid, build-testbuild design spiral that can be applied to larger variants of the REMUS system. The framework for our optimization problem is an accurate power model that includes the hotel load and the hydrodynamics of the AUV. The power model is validated against a series of ocean tests that involve transits through a tidally-forced harbor, and extended to show the gains that might be possible for a power efficiency behavior in realistic head/tail currents. An in-situ behavior is also developed that dynamically adjusts its speed for optimized power-efficiency given on-board apparent ocean currents, further demonstrating energy saving potentials for long duration transits.