Mini-unmanned aerial vehicle (Mini-UAV) is widely employed in scientific research and entertainment due to its small size, low cost, easy operation, and high flexibility. However, the“abuse”of mini-UAVs has caused great hidden dangers to public security and personal privacy. Therefore, radio, radar, image recognition, and other detection methods have been proposed to meet the urgent need for mini-UAV detection and surveillance. However, since mini-UAVs have low altitudes, low speeds, and a small reflective cross-sectional area, it is difficult for radars to detect them quickly under the interference of the complex background. Additionally, radio detection is prone to false alarms due to severe electromagnetic interference at low altitudes. Although CNN-based image recognition has a high detection accuracy, the ability to accurately distinguish between birds and mini-UAVs is affected by image resolution, which needs to be improved. Meanwhile, the above methods have complex equipment, high detection costs, and poor real-time performance. In contrast, the mini-UAV can be quickly detected in noisy low-altitude environments by acoustic detection, which features sound real-time performance, simple equipment, and low cost. However, the current acoustic sensors adopted for acoustic detection have low sensitivity and do not recognize the sound source direction. Therefore, we fabricate a fiber-optic acoustic sensor with a resonant MEMS wheel-shaped diaphragm to detect acoustic signals with high sensitivity and high signal-to-noise ratio (SNR) near the resonance peak. The sensor has an“8”shaped directional response, which allows for the identification of the sound source direction. Finally, a new method is provided for mini-UAV detection.
To improve the sensitivity of optical fiber acoustic sensors and reduce the damping effect caused by the enclosed back cavity of the circular diaphragm, we adopt a wheel-shaped diaphragm with an open acoustic back cavity as the acoustic sensing diaphragm. The wheel-shaped diaphragm consists of a central diaphragm connected to four symmetrically distributed connecting arms on an outer base ring. Firstly, the geometric structure of the wheel-shaped diaphragm is modeled by acoustic vibration theory. According to the characteristics of the mini-UAV's radiated noise spectrum, the diaphragm eigenfrequency is set near the mini-UAV noise fingerprint frequency, and the geometric parameters of the wheel-shaped diaphragm at this frequency are calculated. The acoustic characteristics are simulated and verified via finite element analysis software. Then, the wheel-shaped diaphragm is fabricated using MEMS processing technology. Meanwhile, to optimize the sensor performance, we sputter a metal on the diaphragm surface to improve the optical reflectivity of the diaphragm. Finally, the fiber optic acoustic sensor of the silicon-based MEMS wheel diaphragm is assembled by mechanical micro-assembly. In addition, the cavity length of its static Fabry-Pérot (FP) interference cavity is adjusted to make the sensor work at the quadrature point, which ensures high sensitivity without signal distortion.
A fiber optic acoustic sensor is fabricated using the designed silicon-based MEMS wheel-shaped diaphragm (Fig. 5). The FP static cavity length is measured using interferometric spectroscopy. The experiment shows that when the laser wavelength is 1550 nm, the FP static cavity length is 144.457 μm, which meets the quadrature point (Fig. 6). An acoustic testing system is built to characterize the performance of the wheel-shaped diaphragm fiber-optic acoustic sensor (Fig. 7). The sensor has a resonance peak at 7.279 kHz and a relatively flat response in the frequency range of 2-6 kHz below the resonant frequency (Fig. 8). At normal incidence of 7 kHz sound, the sound pressure sensitivity is 1.8 V/Pa, the SNR is 71 dB, and the minimum detectable sound pressure is 99 μPa/Hz0.5 (Fig. 9). In outdoor mini-UAV detection experiments, mini-UAV noise can be accurately detected within a range of 65 m, with a detection capability about three times that of commercial ECM (Fig. 13).
To detect the radiation noise of mini-UAVs, we design and fabricate a fiber-optic acoustic sensor with a silicon-based MEMS wheel-shaped diaphragm. The wheel-shaped diaphragm consists of a central vibrating membrane and four symmetrically distributed joint arms, and it has high sensitivity near the resonance frequency and the ability to detect mini-UAV at a distance. The sensor has a resonance peak at 7.279 kHz. At the normal incidence of 7 kHz sound, the sound pressure sensitivity is 1.8 V/Pa, the SNR is 71 dB, and the minimum detectable sound pressure is 99 μPa/Hz0.5. Additionally, it has an“8”shaped directional pattern, which indicates its ability to identify the sound source direction. It can accurately identify the noise of mini-UAVs within a range of 65 m, and the detection ability is about three times that of commercial ECM. This indicates its advantages and potential in applications such as mini-UAV detection in some special situations.
