MEMS acoustic directional finder for small flying UAS

Abstract

When compared with the electromagnetic counterparts, acoustic sensors have many advantages that include non-line-of-sight, passive, low-cost, and low power, weight, and size. Acoustic sensors are the primary sensors employed in most unattended ground sensor systems because they can provide detection, direction finding, classification, tracking, and accurate cueing of other high-resolution sensors. The ability to detect and localize aerial UAS with acoustic sensors is not straightforward and require a sensor that can filter out intense background noise and spectral bands not related to the intrinsic acoustic signature of the source. In addition, highly accurate directionality is required. Our team is developing bio-inspired acoustic MEMS sensors to perform this specific task. In order to accomplish that it is necessary to understand the acoustic signature of the target. Typically, the acoustic spectrum of a quadcopter in hovering regime is dominated by high and sustained tone noise at the blade passing frequency and shaft rate and their harmonics up to the mid frequency range of hearing. Broadband noise in the spectra become dominant in the mid and high frequency ranges indicating that rotor self-noise could be significant part of the system noise. Variations in the acoustic spectra of different rotor-sets at the same thrust are significant around the mid and high frequencies at which the broadband component dominates. However between 600 and 1300 Hz some spectral features seem to be perennial (independent of flight regime, payload, etc.). In a realistic operational environment, topography, landmarks, obstacles, weather and different fight regimes influences must be added to the intrinsic signature. In this context, the 18th JIFX was instrumental to provide a variety of environmental and topographical conditions as well as different platforms to be analyzed. Acoustic signatures of 8 different UAS were recorded so far and preliminary observation of the data show that indeed very strong narrow spectral features were located between 600 and 1300 Hz, as shown in Figure 1, for a Yuneec Typhoon 600 Hexacopter. These spectral characteristics will be used as design requirements for the MEMS sensor, whose resonant response is ideal for increasing the sensitivity (10 to 1000 times better than convectional microphones) and naturally filter out the undesirable acoustic spectral bands for enhancing the detection capabilities.