An acoustic remote sensing method for high-precision speed estimation of micro-UUVs

Abstract

Understanding the dominant sources of acoustic noise in unmanned underwater vehicles (UUVs) is important for passively tracking UUVs and for designing quieter propulsion systems. This work describes how the speed of a vehicle can be passively measured by the unique high frequency acoustic signature of a brushless DC motor propulsion system. First, the causes of high frequency tones were determined through direct measurements of two micro-UUVs and an isolated thruster. From this analysis, the vehicle noise was mapped to speed and the common and dominant features of noise were established: strong tones at the motor's pulse-width modulated frequency and its second harmonic, each of which is a carrier with sidebands at frequency intervals equivalent to the propeller rotation frequency multiplied by the poles of the motor. Field experiments were performed where the speed of two micro-UUVs was predicted by measuring the sidebands of the two dominant tones. When the mapping of rotational to translational speed of the UUV was known, this method produced speed predictions with 0.004 m/s accuracy. These findings are applicable to many off-the-shelf vehicles available today which rely on brushless DC motors and can be easily integrated into passive acoustic security systems for target motion analysis. [Work supported by the Department of Defense NDSEG program, ONR, and Draper.]