电磁声换能器的辐射声场研究

分析了一种圆环形螺线圈、垂直偏置磁场结构的电磁声探头在非铁磁性介质铝中的辐射声场特性。首先根据电磁感应原理对探头进行了物理和数学建模,将表面力源近似成水平和竖直分布的两种,且水平分量远大于竖直分量,推导出该种力源产生的声场的理论解析解。分别数值计算出了横波和纵波的切向和法向指向性,并和实验结果进行对比。实验结果和理论计算符合较好,为此种结构的电磁声换能器的实际工程应用提供了理论依据。      

Sound field of an electromagnetic acoustic transducer

The characteristic of radiated sound field excitated within a non-ferromagnetic aluminum by an electromagnetic acoustic transducer consisting of circle-spiral coil and vertical bias magnetic field is studied.This article presents an improved force source model according to the principle of electromagnetic induction.Then the analytic displacement solution of sound field produced by that force source model is deduced.The tangential and normal directivities of shear and longitudinal sounds are calculated numerically and compared with experimental results.Experimental results conform well to the theoretical prediction,which not only shows that the electromagnetic acoustic transducer of this conformation has a good directivity of vertical shear-wave radiation,but also confirms the given force source model is effective to describe the actual physical behavior of the probe.Theoretical and experimental study lays a foundation for optimization design and engineering application of the electromagnetic acoustic probe.     

基于电磁超声换能器的火车轮探伤研究

利用电磁超声探伤方法对检测火车轮表面及近表面缺陷进行实验探究,从而保证车轮质量,避免事故发生。文章介绍了火车车轮电磁超声探伤的原理和方法。根据电磁超声表面波辐射扩散角的分布情况,得出利用电磁超声表面波进行车轮踏面探伤的可行性。将制作的小巧换能器探头与便携式电磁超声探伤仪配合,能够实现对车轮的快速探伤检测。通过大量的车轮探伤实验,检出了典型车轮踏面缺陷。根据检测波形特点并结合生产工艺情况,分析得出产生缺陷的原因。研究表明:电磁超声无损检测方法能够快速、有效检出车轮踏面缺陷。     

Quantitative Schlieren measurements of a high energy electromagnetic transducer acoustic shock field

Schlieren photography has been used to analyse quantitatively the acoustic field of an electromagnetic acoustic transducer (EMAT). By measuring the angle through which the rays are refracted it is possible to compute the refractive index gradient and thus determine both the absolute and complex pressure related structures of the images. Using this method, planar and focused shock transients generated by the EMAT have been evaluated and compared with transducer derived pressure measurements. The peak pressure in the unfocused shock was found to be 3.2 MPa and 4.6 MPa for the Schlieren and piezoelectric transducer measurements respectively. Corresponding values for the focused shock-wave agreed to within experimental error at about 19 MPa.     

一种磁场优化结构电磁超声传感器设计*

为了进一步增强电磁超声检测技术在管道厚度测量领域的检测能力,该文对电磁超声传感器(EMAT)的结构进行了优化。提出了多磁铁对称分布型EMAT,能实现更小的磁铁体积,产生更强的表面剩磁强度。采用在硅钢表面开槽的方式限制涡流形成的区域,解决了涡流对测量的影响。建立厚度测量实验系统,对比出单磁铁型与多磁铁对称分布型EMAT在不同提离距离上检测信号的变化规律。结果表明,多磁铁对称分布型结构可通过增强EMAT的偏置磁场达到信噪比更优的效果。采用耐高温探头外壳和钐钴磁铁,提高了EMAT探头在高温环境下的检测性能。     

一种新型Cymbal换能器的研究

提出并研究了一种新型的Cymbal换能器。与传统的Cymbal换能器相比,该换能器中的压电陶瓷圆盘由一个金属圆环和压电陶瓷圆盘的径向组合体所代替。论文首先探讨了压电陶瓷圆盘和金属圆环组合体的径向振动,用解析方法得出了其径向振动的机电等效电路。在此基础上,得出了新型Cymbal换能器的机电等效电路,并得出了其共振及反共振频率方程。其次,利用数值计算方法,对这种新型的Cymbal换能器的共振频率和振动性能进行了研究。分析了Cymbal换能器的几何尺寸对其共振频率及有效机电耦合系数等参数的影响,探讨了其输入导纳的频率特性。所得结论为此类Cymbal换能器的优化设计及工程应用奠定一定的理论基础。本文提出的新型Cymbal换能器具有较大的几何尺寸、较高的机械强度以及功率容量等优点,可望在较大功率的水声及超声辐射器中获得较为广泛的应用。     

Torsional wave experiments with a new magnetostrictive transducer configuration

For the efficient long-range nondestructive structural health inspection of pipes, guided waves have become widely used. Among the various guided wave modes, the torsional wave is most preferred since its first branch is nondispersive. Our objective in this work is to develop a new magnetostrictive transducer configuration to transmit and receive torsional waves in cylindrical waveguides. The conventional magnetostrictive transducer for the generation and measurement of torsional waves consists of solenoid coils and a nickel strip bonded circumferentially to test pipes. The strip must be premagnetized by a permanent magnet before actual measurements. Because of the premagnetization, the transducer is not suitable for the long-term on-line monitoring of pipes buried underground. To avoid the cumbersome premagnetization and to improve the transduction efficiency, we propose a new transducer configuration using several pieces of nickel strips installed at 45 degrees with respect to the pipe axis. If a static bias magnetic field is also applied, the transducer output can be substantially increased. Several experiments were conducted to study the performance of the proposed transducer configuration. The proposed transducer configuration was also applied for damage detection in an aluminum pipe.     

Sound field separation with a double layer velocity transducer array (L)

In near-field acoustic holography sound field separation techniques make it possible to distinguish between sound coming from the two sides of the array. This is useful in cases where the sources are not confined to only one side of the array, e.g., in the presence of additional sources or reflections from the other side. This paper examines a separation technique based on measurement of the particle velocity in two closely spaced parallel planes. The purpose of the technique is to recover the particle velocity radiated by a source in the presence of disturbing sound from the opposite side of the array. The technique has been examined and compared with direct velocity based reconstruction, as well as with a technique based on the measurement of the sound pressure and particle velocity. The double layer velocity method circumvents some of the drawbacks of the pressure-velocity based reconstruction, and it can successfully recover the normal velocity radiated by the source, even in the presence of strong disturbing sound.     

Dual-acquisition phase-sensitive fat-water separation using balanced steady-state free precession

Balanced steady-state free precession (SSFP) sequences use fully re-focussed gradient waveforms to achieve a high signal and useful image contrast in short scan times. Despite these strengths, the clinical feasibility of balanced SSFP is still limited both by bright fat signal and by the signal voids that result from off-resonance effects such as field or susceptibility variations. A new method, dual-acquisition phase-sensitive SSFP, combines the signals from two standard balanced SSFP acquisitions to separate fat and water while simultaneously reducing the signal voids. The acquisitions are added in quadrature and then phase corrected using a simple algorithm before fat and water can be identified simply by the sign of the signal. This method is especially useful for applications at high field, where the RF power deposition, spatial resolution requirements and gradient strength limit the minimum repetition times. Finally, dual-acquisition phase-sensitive SSFP can be combined with other magnetization preparation schemes to produce specific image contrast in addition to separating fat and water signals.     

Conventional electromagnetic acoustic transducer development for optimum Lamb wave modes

Lamb waves are normally utilized for inspecting thin metal sheets. Wheel type probes using piezoelectric oscillators have generally been used as the sensors for Lamb waves. Recently, the electromagnetic acoustic transducer (EMAT) has been developed and is beginning to be used as a Lamb wave detector. We have developed a useful type of transducer for Lamb waves. The new EMAT consists of a meander coil with a narrow distance of 2.5 mm and has a symmetrical structure in the vertical direction for both surface sides. The new EMAT can generate Lamb waves with variable wavelengths corresponding to the frequency range from approximately 300 kHz to 2.5 MHz and multiple modes, and can also generate selected symmetrical and anti-symmetrical mode Lamb waves. It is demonstrated that the optimum Lamb wave mode could be produced by the appropriate positioning of the EMATs and controlling the phase (same or inversed) of the electrical signal driving the device. The described EMAT can be used to examine steel (or other material) sheets of different thickness. It is also shown that the S0 (0.3 MHz) mode Lamb wave is the most effective for the deepest (up to 6 mm) penetration.     

The design of a module of a new electromagnetic calorimeter for COMPASS II

A new-generation high-granularity Shashlyk EM calorimeter read out by micropixel avalanche photodiodes with thermostabilization based on the Peltier element is designed and constructed.     

Bolt axial stress measurement based on a mode-converted ultrasound method using an electromagnetic acoustic transducer

A method is proposed to measure the stress on a tightened bolt using an electromagnetic acoustic transducer (EMAT). A shear wave is generated by the EMAT, and a longitudinal wave is obtained from the reflection of the shear wave due to the mode conversion. The ray paths of the longitudinal and the shear wave are analyzed, and the relationship between the bolt axial stress and the ratio of time of flight between two mode waves is then formulated. Based on the above outcomes, an EMAT is developed to measure the bolt axial stress without loosening the bolt, which is required in the conventional EMAT test method. The experimental results from the measurement of the bolt tension show that the shear and the mode-converted longitudinal waves can be received successfully, and the ratio of the times of flight of the shear and the mode-converted longitudinal waves is linearly proportional to the bolt axial tension. The non-contact characteristic of EMAT eliminates the effect of the couplant and also makes the measurement more convenient than the measurement performed using the piezoelectric transducer. This method provides a promising way to measure the stress on tightened bolts.     

双驱动电动换能器电磁场和声-结构耦合场分析

电动换能器的设计涉及电磁场和声-结构耦合问题,本文利用COMSOL多物理场分析软件提供的该两种分析模块进行组合,以电磁理论为基础推导出了适合COMSOL数值计算的公式,把两种耦合场的分析结合起来,利用电磁场分析过程计算所需要变量,代入声-结构耦合场进行分析,得到声场分布情况。文章也大致介绍了COMSOL的有限元建模和该两种模块的使用方法。本文对电动换能器结构如何优化不做讨论。     

采用骨导语音自适应的语句分割方法*

为了解决含噪语句分割问题,也为了解决某些低信噪比环境下传统气导语句分割算法分割效果差、分割准确度低且算法自适应性弱等问题,提出一种基于骨导语音自适应的分段双门限语音分割方法。将骨导语音和气导语音同步采集,获取抗噪性能更好的骨导语音,然后在融合过零率与短时能量中引入随机动态阈值的自适应方法进行端点检测,最后利用分段双门限和语音聚类等手段实现语音分割,提高语音分割算法的鲁棒性。通过实验验证了所提算法的有效性和可行性,同时与其他语音分割算法进行了对比,证明该文所提分割算法精度更高,效果更好。     

On new electromagnetic waves in a multicomponent insulator

The dispersion equation for additional transverse electromagnetic waves in a multicomponent amorphous insulator is analyzed in the vicinity of a narrow absorption line. Such waves can be excited due to spatial dispersion associated with fluctuation of the polarizability of insulator molecules. The conditions under which the dispersion relation of an additional (new) wave is mainly determined by the mean square of the polarizability fluctuations are determined.     

骨导振子佩戴位置对重放声的影响

骨导振子佩戴位置会影响骨传导声重放的性能,但已有研究还不够全面和完善.针对11名双耳听力正常的受试者,采取不同的骨导振子佩戴位置,对骨传导声重放在听力阈值、主观音质评价和语言清晰度3个方面进行了研究.实验结果表明不同的佩戴位置会影响骨传导声重放性能,髁骨位置在3个实验都表现最好,髁骨的听力阈值比乳突位置平均低8.7 d...     

自聚焦换能器的声场研究

高强度聚焦超声（HIFU）治疗局部肿瘤是一项无创伤的新技术,有着很好的应用前景,因此对其声场的研究是有实际的价值的。对声场的研究不仅有利于换能器的设计,提高治疗的效果;而且有助声场测量方案的设计,提高对治疗超声剂量的安全保证。本文采用时域有限差分法研究了自聚焦换能器的声场,分别对简谐波,脉冲波进行了研究。比较清楚地看出在焦区的声场可以认为是平面波,这对光纤端面法声场测量提供了有力的支持。     

A new design on broadband flextensional transducer

The new flextensional transducer presented is driven by a tube stacked by longitudinally polarized piezoelectric ceramic rings. The rings are compressed between two annulus steel end plates which are coupled by a dual convex aluminum shell with slotted gaps. The transducer is a free-flooded design with the interior of the tube open to the surrounding water. Three main vibrating modes including the cavity, the longitudinal and the radial can be utilized by appropriately coupling design to broaden the working bandwidth. A prototype is fabricated and measured. The results confirm the three vibrating modes mentioned above and the broad band of transmitting voltage response is gained successfully with difference less than 10 dB from 2200 Hz to 9000 Hz.     

基于电磁谐振分离的宽带低雷达截面超材料吸波体

基于超材料的电磁谐振特性, 设计、制作了一种极化无关的宽带低雷达散射截面 (radar cross section, RCS)超材料吸波体. 通过场分布和反演法分析了其吸波机理, 利用波导法和空间波法测试了其吸波率和RCS特性. 理论分析表明: 在平面波的作用下, 该吸波体对某一吸波频率在不同的位置分别提供电谐振和磁谐振, 对不同的吸波频率, 利用不同的介质层提供主要的能量损耗, 从而有效减弱了电磁耦合, 保证了宽频带的强吸收特性. 实验结果表明: 设计的三层结构吸波体吸波率达90%以上的带宽是单层结构的4.25倍, RCS减缩10 dB以上的带宽为5.1%, 其单元尺寸为0.17λ, 厚度仅为0.015λ. 该吸波体的低RCS特性还具有极化无关、宽入射角的特点, 且通过改变吸波体的夹层结构可以实现工作带宽的灵活调节. 关键词： 超材料吸波体 雷达散射截面 宽带 电磁谐振     

Blind phase retrieval and source separation of electromagnetic fields

Blind source separation of two electromagnetic fields is investigated. The difficulty of this task lies in the fact that only the power, which is the square of the sum of the electromagnetic fields, can be directly measured; the cross term of the electromagnetic fields is inevitable, and a strong correlation occurs in blind deconvolution. However, the relative phase is physically different from the field intensities, and, hence, extracting the phase during separation seems inconceivable. Our results demonstrate that the intensities and the relative phase of two electromagnetic waves can be determined with eigenvalue problem formalism even when the mixing processes are completely unknown.