A compact and powerful EMAT design for contactless detection of inhomogeneities inside the liquid volume of metallic tanks
A compact and powerful EMAT design for contactless detection of inhomogeneities inside the liquid volume of metallic tanks
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DOI：
10.1515/teme-2019-0124
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发表时间：
2019-12
期刊：
影响因子：
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通讯作者：
K. Rieger;D. Erni;D. Rueter
中科院分区：
文献类型：
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作者：
K. Rieger;D. Erni;D. Rueter
摘要：提出了一种简单、功能强大的电磁声换能器设计方案，无需笨重的永磁体。EMAT以脉冲回波方式工作，产生约1mhz的纵向超声。与横波不同，这些纵向超声脉冲可以在液体中传播。一般处理的应用场景是检查金属容器或罐内的液体体积，例如，检测液体内部的不均匀性。本文提出的EMAT几乎适用于所有金属容器，也就是说，它适用于由铝或铁磁性钢制成的容器壁，甚至适用于非铁磁性（不锈钢）。此外，与压电换能器不同，EMAT技术允许非接触式超声换能器：EMAT传感器线圈与油箱金属表面之间的气隙可延伸至2mm。即使有相对较大的气隙，偏置磁场在表面的磁通密度接近3.2 T，比传统的和体积较大的emat的永磁体所能达到的还要高。强场提高了主要是低效率的EMAT机制的耦合效率，这对超声波的传输和接收都很重要。为了获得优异的磁场强度，通过EMAT线圈的细绕组（0.5 mm）施加高达3.6 kA的单极电流脉冲。本文提出了一种适用于这种强电流和兆赫脉冲电压的新型固态EMAT电路。作为一项特别精细的任务，强大的电路还必须检测到μV范围内相当微弱的回波信号。在如此强烈的发射爆发后，需要很短的恢复时间。所讨论的电路由三个单极大电流模块组成，每个模块都可以独立启动。这使得接收到的回波信号可以独立计时，例如，可以明确定位液体罐深处的物体。总的来说，本研究主要集中在新型并联电路、一般脉冲回波功能和磁场方面。通过不同EMAT线圈几何形状的超声场的详细分析和成形将超出本贡献的范围，并将单独报告。
Abstract A simple and powerful design of an electromagnetic acoustic transducer (EMAT) without bulky permanent magnets is presented. The EMAT is operated in a pulse echo modality and generates longitudinal ultrasound at about 1 MHz. Unlike shear waves, these longitudinal ultrasound pulses can propagate in liquids. The generally addressed application scenario is the examination of a liquid volume inside a metallic container or tank, e. g., the detection of inhomogeneities within the liquid. The herein proposed EMAT operates for virtually all metallic containers, i. e., it succeeds for container walls made of aluminum or ferromagnetic steel, and even for non-ferromagnetic (stainless) steel. Moreover, unlike piezo transducers, EMAT techniques allow for a non-contacting ultrasound transduction: the air gap between the EMAT sensor coil and the tank’s metallic surface extends up to 2 mm. Even with this relatively large air gap, the biasing magnetic field approaches a flux density of 3.2 T at the surface, more than what is possible to achieve with the permanent magnets of conventional and bulkier EMATs. Strong fields improve the coupling efficiency of the principally low-efficiency EMAT mechanism, which is important for both ultrasound transmission and reception. For that superior field intensity, a unipolar current pulse of up to 3.6 kA is applied through the thin windings (0.5 mm) of the EMAT coil. This paper presents a novel solid-state EMAT circuitry for such strong currents and MHz pulsed voltages >1 kV. As a particularly delicate task, the powerful circuitry must also detect the rather weak echo signals in the μV range. A very short recovery time is required after such a strong emission burst. The discussed circuitry consists of three unipolar high-current modules, which can each be independently launched. This allows for received echo signals that can be timed independently, e. g., objects deep inside the liquid tank can be specifically addressed. In general, this work concentrates on the novel circuitry in parallel connection, the general pulse-echo functionality and the magnetic fields. A detailed analysis and shaping of the ultrasonic fields through different EMAT coil geometries would exceed the scope of this contribution and is to be reported separately.