Research On Airborne Sonobuoy Employed Acous-Tic Vector Sensor And Suspension Technology

Airborne sonobuoys have been working as the prevalent acoustic detection equipments on airborne anti-submarine platform,and their performances largely determine the efficiency and accuracy of airborne detection on submarines.Acoustic vector sensor(AVS)is compact and lightweight,and exhibits excellent low frequency performance,which is capable of effec-tively improving the operational efficiency of airborne detection on submarines.However,the airborne sonobuoy platform is different from the general working platform of AVS,and the special characteristics have put forward new requirements for AVS,such as geometrical shape,construction form,suspension structure,etc.In this paper hence,AVS and its suspension structure applied to sonobuoy platform are investigated according to the characteristics of airborne sonobuoy platform and the inertial AVS respectively.Acoustically induced vibration of free and rigid spheroids in the low frequency plane acoustic field are theoretically derived in this paper,since spheroidally shaped AVSs lack of corresponding theoretical evidence for acoustic particle velocity reception.Firstly,mechanical relationship between fluid and structure is derived according to structure vibration and sound radiation in a fluid medium,and accordingly velocity amplitude of acoustically induced vi-bration of an arbitrarily shaped rigid body is obtained.Then,acoustically induced added masses to spheroids are computed in an ideal low-frequency sound field,and as a result,long waves driven motion to spheroids are derived including the translational and rotational movement.Moreover,the motion of a prolate spheroid in the presence of sound diffraction is calculated by finite element method,and the velocity magnitude is presented to decrease with the rising sound frequency,and is related to the eccentricity and acoustic incident direction.The rotational motion caused by asymmetrical pressure enhances with the increase of acoustic frequency and eccentricity,when acoustic oblique incidence occurs.To verify the feasibility of sonobuoy platform employing an inertial AVS,a two-dimensional composite inertial AVS prototype is designed and fabricated.In this proto-type,there are a hydrophone,a two-dimensional piezoelectric accelerometer,a magnetic compass as well as the signal conditioning electronics.AVS's house is composed of a metal cylindrical and two half oblate spheroidal sealing shells,with overall size of 80 mm in diame-ter,100 mm in height,average density 1.34 g/cm3,and operating bandwidth 5 Hz?2500 Hz.The hydrophone has an underwater resonant frequency of 8 kHz,and pressure sensitivity is-157 dB(0 dB re 1 V/?Pa,with preamplifier gain 41.6 dB).The resonance frequency of ac-celerometer is 7.5 kHz in the air,and the sensitivities according to acoustic pressure are-166 dB and-167 dB(0 dB re 1 V/?Pa,100 Hz,with preamplifier gain 40 dB)respectively.Mag-netic compass utilizes a magnetic resistance sensor HMC1022 to achieve magnetic direction finding,and the tested angular precision can achieve 1 degree.Signal conditioning circuit is mainly to provide amplification and filtering for sensors' signals.Besides,experimental stud-ies on the self-noise level and hydrodynamic noise level are carried out as well.Acoustic reception principle of inertial AVS demands the sensor have to be flexibly sus-pended when working underwater,and the suspension structure of AVS needs to be specially designed according to the characteristic of sonobuoy platform.In this paper,the influence on response of an ideal suspended acceleration-type AVS is analyzed by making use of lumped parameter equivalent and electromechanical analogy.Influences on the dynamic response of a neutrally buoyant sphere using different number of flexible componentsis are theoretivally analyzed.The single point suspension model for AVS is studied through the movement theory of pendulum and complex pendulum,considering vertical suspension as well as oblique sus-pension impact,which theoretically verified the feasibility of an AVS deployed in sonobuoy platform with the single point suspension.In addition,to mitigate the buoy fluctuations of sea surface,the cable dithering caused by the ocean current and other mechanical movement coupling to AVS,sonobuoy platform must be installed with the vibration-isolation structure.In this paper,multi-layer flexible suspen-sion structure is proposed to constitute the suspension system of AVS to achieve better vibra-tion-isolation effect.Vibration isolation effect of double-layer flexible suspension structure are analyzed theoretically as well as its influence on the acoustic particle velocity reception.Experiments on magnetic vibration exciter show that the vibration isolation effect of dou-ble-layer flexible suspension structure is 12 dB/OCT better than that of single layer suspen-sion structure.In addition,a three-layer flexible suspension structure for AVS based on the single point suspension is designed,and vibration-isolation effect was experimentally verified in the tank.For vertical oscillation interference,double-layer suspension is capable of provid-ing 8.9 dB isolation effect more than single-layer suspension,when the vibration-isolation ef-fect of three-layer suspension are able to improve 16.5 dB than that of single-layer suspen-sion.