The Research Of Cilium MEMS Vector Hydrophone

With the advance in noise and vibration control technology, conventional sonar detection technology is under unprecedented challenges. All countries with noticeable navy force has drawn unprecedented attention to explore new type of transducer for underwater acoustic target detection basing on new principle, new technology and new process.Currently, underwater target detection is mainly realized by the use of the scalar sound pressure hydrophone and its array, which can only get the sound pressure information without vector information of sound field like water particle velocity and acceleration. Besides, for the micro-sized underwater weapon platforms such as mines, since the restriction in dimension, it is difficult to obtain effective array gain and beam width in low frequency band, which in turn affect the detection range and location accuracy. However, the vector hydrophone,which can realize the low frequency, long distance detection and positioning through a single hydrophone or small array, has motivated a worldwide interest. In respect of the manner of vibration picking, vector hydrophones can be classified as mobile circle type, pressure-gradient type,and co-vibrating type etc. Although the working principles are different, they all have a common feature of ―8‖-shaped cosine directivity. Cilium MEMS vector hydrophone is one kind of mini-sized hydrophone developed in recent years. Comparing with other hydrophones, the Cilium MEMS vector hydrophone has the advantages of small size, low cost and rigid mounting, which makes it feasible for applying in underwater small-sized weapon platform. However, there are also a series of weaknesses such as low sensitivity, narrow response frequency band, high level of self-noise, poor anti-flow-noise performance etc. In this paper, a series of researches have been carried out aiming at the existed weaknesses of MEMS vector hydrophones.Mechanical model is established based on the micro-stucture of foremost Cilium MEMS vector hydrophone and mathematical derivation of the relations between stress, resonant frequency and the geometry of micro-structure is made. After qualitive analysis, we can conclude that the sensitivity and resonant frequency is a pair of inherent contradictions for that micro-structure. In order to solve the problem, micro-structure of multiple stress concentration is proposed, aiming to focus the advantages of two or more different structures on one micro-structure so as to improve both the sensitivity and frequency response bandwidth. Besides, at the top of acoustic cylinder, a low density ball is integrated so as to increase the receiving area of sound and thus greatly improve the sensitivity of CiliumMEMS vector hydrophone without reducing too much the resonant frequency of the micro-structure. For the two kinds of above design, finite element fluid-structure interaction(FSI) models are established respectively and the feasibility are verified through the finite element simulation analysis.Since the optimized microstructure is non-standard MEMS design, a set of complete, dedicated MEMS processing process has been developed, which solve the stress groove etching, distribution of piezoresistors, ohmic contact and body silicon etching technology, etc. Mapped the lithography mask by L-Edit software, then delivered it to processing company and the MEMS chips have been successfully manufactured. Afterwards, the secondary integration between the acoustic cylinder and the MEMS chip, had been realized with the help of special micro-system integration platform.For solving the problems of insulation, pressure-tolerant and sound-transparent packaging of Cilium MEMS vector hydrophone chips, the Cilium MEMS vector hydrophone is sealed by sound-transparent cap, filled with insulated, sound-transparent oil. And one kind of ―orange-segment‖support structure is in the cap. Based on the combination of finite element simulation and experimental validation, the effect of this cap on performance of Cilium MEMS vector hydrophone had been deeply studied, which shows that the thinner the thickness and lower the elastic modulus of the sound-transparent cap, the less the sensitivity loss will be. Although, the first-order inherent frequency of the sound-transparent cap will be lower, causing the frequency response curve of Cilium MEMS vector hydrophone become more complex, the ―orange-segment‖ type package can effectively improve the first-order inherent frequency as well as prevent collision.A micro guided flow dome is designed to satisfy the anti-flow-noise ability of Cilium MEMS vector hydrophone during sea trials. Tests show that, although the micro guided flow dome slightly loweres the sensitivity of Cilium MEMS vector hydrophone(the maximum loss of 3 d B below 1 KHz), it can greatly restrain the flow-noise and improve the signal-to-noise ratio of the Cilium MEMS vector hydrophone. In order to reduce the noise caused by the follow-up circuit as possible, a weak signal extraction circuit is specially designed and test results show that the noise spectrum level of the circuit is less than-140 d B(for frequency higher than 80Hz).Finally, the sensitivity, directivity, dynamic range, equivalent background noise sound pressure level, resistance to vibration and other main performances of Cilium MEMS vector hydrophone are tested both indoor and in Xin’an river as well as Qingdao sea trial. The results show that, the sensitivity of newly designed Cilium MEMS vector hydrophone is 6~15d B higher than early hydrophone; the frequency response band changes from 20Hz~500Hz to 20Hz~1k Hz; the equivalent background noise level decreases by 10 ~ 20 d B. For the guided flow dome, typical environment test and sea trial results show that the flow-induced-noise could be significantly restrained and the signal-to-noise ratio could be sharply improved, which lay a technical foundation for the engineering application of Cilium MEMS vector hydrophone.