Research On High-power Ultra-low Frequency Electromagnetic Transducer

With the rapid development of seabed resources exploration,marine environmental monitoring and long-range underwater target detection technology,high-power ultra-low frequency sound sources have become the key equipment of ocean engineering.Compared with piezoelectric transducers and magnetostrictive transducers,the output force of electromagnetic transducers is larger,and its output displacement can reach millimeter level.Electromagnetic transducers can achieve high-power acoustic radiation output with manageable size and weight,which has great advantages in the ultra-low frequency band(10-400 Hz).Since 1994,the United States,Russia and Japan have used electromagnetic transducers to observe the marine climate,and have carried out the Acoustic Thermography of Ocean Climate(ATOC)and Global Ocean Observing System(GOOS),establishing low frequency acoustic emission and receiving networks.However,the research on electromagnetic transducers in China is still situated at initial stage.The design of electromagnetic transducers involves electro-magneto-mechanoacoustical coupling and must consider a few key problems,such as magnetic flux leakage,magnetic saturation,output nonlinearity and instability,which considerably increases the complexity of modeling.The existing design methods lack in systematic and comprehensive modeling measures.The influence of magnetic flux leakage and saturation in the magnetic circuit is often ignored in the existing research of electromagnetic transducers,which brings great errors to the design and analysis.In addition,due to the nonlinear influence of driving mechanism,mechanical springs and magnetic saturation,the output harmonic and instability problems must be considered in real application,which have a great impact on the performance of electromagnetic transducers.The main contents of this paper are as follows:1.Aiming at the inaccuracy of electromagnetic force estimation caused by magnetic flux leakage in air gaps of high-power ultra-low frequency electromagnetic transducers,based on the equivalent magnetic circuit model,two calculation methods of electromagnetic force considering magnetic flux leakage are proposed,which greatly improves the accuracy of electromagnetic force calculation.In the first method,the leakage flux in the air gap is divided into several flux tubes with simple geometries to calculate the leakage reluctance,and then the leakage reluctance is brought into the magnetic equivalent circuit to calculate the parameters of magnetic flux and electromagnetic force.The second method uses the finite element simulation to calculate the leakage coefficient of the air gaps when the moving iron moves to different positions,and gets more accurate leakage reluctance by curve fitting,so as to reduce the influence of magnetic flux leakage on the accuracy of model.Then,according to Maxwell's force formula and magnetic circuit model of electromagnetic transducer,the electromagnetic force model within the stroke is obtained.2.Aiming at the situation that the nonlinearities of electromagnetic transducers in underwater acoustic field are normally neglected,this paper uses an analytical method of electrodynamics to investigate the output harmonics and stability of electromagnetic transducers.Firstly,the output harmonic components of the electromagnetic transducers are calculated by the proposed analytical method.On this basis,a simple and fast method to solve the harmonic components is provided.Finally,the stability of the electromagnetic transducers is analyzed,and its stability boundary of vibration displacement is determined.3.Considering the shortage of accurate and systemic modeling methods for highpower ultra-low frequency electromagnetic transducers,this paper established an analytical model and a numerical model(finite element simulation model)of electromagnetic transducers,and proposed a set of design process.Firstly,based on the magnetic equivalent circuit and motion equation,the electrodynamics equations of the electromagnetic transducers are constructed,and the MATLAB Simulink module is built for preliminary design.Then,the mechanical and acoustical characteristics of the electromagnetic transducers are introduced into the electromagnetic analysis as loads,and the dynamic finite element simulation model of multi-field coupling is established to further optimize the design parameters.Through the mutual confirmation of analytical model and numerical model,the comprehensive modeling method of electromagnetic transducers is established.4.This paper briefed the manufacturing process of the high-power ultra-low frequency electromagnetic transducer prototype,and carried out indoor vibration characteristic test and acoustic characteristic test in the lake.The parameters such as resonance frequency,vibration displacement,sound source level and output harmonic content are measured.What's more,the reason of current waveform distortion in the lake test is further analyzed.The influence of inductance parameter caused by hydrostatic pressure is mainly analyzed.Based on the proposed design method,a prototype of high-power ultra-low frequency electromagnetic transducer is fabricated successfully and its underwater performance is tested.The test results show that the analytical value of electromagnetic force considering magnetic leakage is in good agreement with the experimental value,the total harmonic content of the output displacement is less than 1% and the stability of the electromagnetic transducer is good,which verifies the correctness of the proposed method.The sound source level of the prototype is 185.5 d B at 74 Hz under the condition of 10 m and 5.6 A.