Induction coil as a non-contacting ultrasound transmitter and detector: Modeling of magnetic fields for improving the performance

Abstract

A simple copper coil without a voluminous stationary magnet can be utilized as a non-contacting transmitter and as a detector for ultrasonic vibrations in metals. Advantages of such compact EMATs without (electro-)magnet might be: applications in critical environments (hot, narrow, presence of iron filings…), potentially superior fields (then improved ultrasound transmission and more sensitive ultrasound detection). The induction field of an EMAT strongly influences ultrasound transduction in the nearby metal. Herein, a simplified analytical method for field description at high liftoff is presented. Within certain limitations this method reasonably describes magnetic fields (and resulting eddy currents, inductances, Lorentz forces, acoustic pressures) of even complex coil arrangements. The methods can be adapted to conventional EMATS with a separate stationary magnet. Increased distances (liftoff) are challenging and technically relevant, and this practical question is addressed: with limited electrical power and given free space between transducer and target metal, what would be the most efficient geometry of a circular coil? Furthermore, more complex coil geometries ("butterfly coil") with a concentrated field and relatively higher reach are briefly investigated.

Experimental Demonstration and Circuitry for a Very Compact Coil-Only Pulse Echo EMAT

“…Similar rise times and amplitudes have been observed in practice and simulated for the real transducer system, and—the purpose here—transient and strong magnetic fields and Lorentz forces occur close to the windings of the small coil. Although the field topology, the achieved flux density, and the forces between the coil and the target can be computed with a quasi-analytical method [11], a much more rough and simple calculation [10] suffices for this practical work: about 50% of the available electrical energy in the capacitor (here: 0.125 J) can momentarily convert into magnetic energy within the gap between the coil and the target metal. The volume of a 0.5 mm gap with an 8 mm diameter (the diameter of the coil) is close to 25 mm 3 , and then the momentary magnetic energy density E m in the gap becomes about 5 MJ/m 3 .…”

“…This approach has not been explicitly shown before, although it was predicted in a previous work [11]. The echo operation (contact-less ultrasound emission into a test object and echo detection) of a conventional EMAT with a permanent magnet is well-known and has been commercialized for many years.…”

“…Contactless ultrasound excitation in metallic test objects is also possible with a simple compact induction coil alone (Figure 1b), without a permanent magnet [5,6,7,8,9,10,11]. As the transduction effect is proportional to the RF eddy current and the exciting RF induction field itself (“self-biasing”), the RF Lorentz forces follow a quadratic characteristic.…”

“…This kind of transduction preferably excites normally oriented body forces, such as hydrostatic pressure, into the target, more suitable for generating longitudinal ultrasound waves in the volume and the near surface Rayleigh waves [8] in the lateral direction. A more detailed discussion about these pressure fields is given in [11] and is not extensively recapitulated in this practical demonstration. However, the scheme is less effective for exciting lateral body forces for shear wave emission into the bulk material.…”

“…This was briefly described in Ref. [11] but without a demonstration of the echo operation for a single coil. With a sufficiently intense magnetization current, an even stronger bias field can be provided than possible with a permanent magnet in a conventional EMAT.…”