GB2386499A

GB2386499A - Electromagnetic acoustic transducer with delay capacitance between coils

Info

Publication number: GB2386499A
Application number: GB0305546A
Authority: GB
Inventors: David Paige; Robert Andrew Mercel
Original assignee: PII Ltd
Current assignee: PII Ltd
Priority date: 2002-03-15
Filing date: 2003-03-11
Publication date: 2003-09-17
Also published as: AU2003212520A1; WO2003078998A1; GB0305546D0; GB0206174D0

Abstract

An electromagnetic acoustic transducer for exciting ultrasound in a material under test (2) comprises at least one magnet (4) positioned so as to apply a magnetic field to the material (2), and a plurality of electrical windings (8) spaced along a direction parallel to the surface of the material (2). Adjacent windings (8) are interconnected by an associated capacitance (10), and a source of alternating current supplies one of the windings (8) with a pulsed signal, such that the pulsed signal is fed from the one winding (8) to the next adjacent winding (8), and subsequently sequentially to each next adjacent winding (8), with a time delay determined by the capacitance (10) between adjacent windings (8), and whereby each winding (8), creates an ultrasound pulse within the material (2). The spacing of the windings (8) and the electrical characteristics thereof are such that the directional characteristics and time duration of the ultrasound wavefront within the material (2) is enhanced.

Description

ELECTROMAGNETIC ACOUSTIC TRANSDUCER

This invention relates to an electromagnetic acoustic transducer for inspecting the integrity of metallic 5 components using ultrasonics.

There are numerous technologies and physics principles which have been used for detecting and measuring the size of defects in metal structures. For ferromagnetic metals, magnetic flux leakage techniques and ultrasonic testing 10 have evolved as the most useful procedures in practice.

Although there is considerable overlap in their areas of application, magnetic flux leakage is of most use in measuring significant metal loss, for example due to corrosion or gouging, and ultrasonics has its main 15 application in measuring cracks.

For conventional ultrasound transducers, a liquid is needed to conduct the ultrasound into the component under test. In many situations, however, the use of a liquid is inconvenient, and alternative transducer designs are 20 required, of which some of the most important types are electromagnetic acoustic transducers (hereinafter referred to as EMATs) which directly excite ultrasound in the component or structure under test and do not require an intermediate medium to convey the sound from the 25 transducer.

The main advantages of EMATs are in providing ultrasonic inspection without liquid or physical coupling, and the ability to excite several specific wave modes that are beneficial for inspection which cannot be excited by 5 conventional piezoelectric transducers.

Disadvantages of EMATs include high electrical power requirements and physically large transducers, because of the low energy conversion efficiency compared to piezoelectric variants of similar inspection performance.

10 A further disadvantage is that practical transducers designed to transmit ultrasound in a direction parallel to the surface of a component under test produce broad divergent beams of ultrasound, or compensate for this by using long tone pulses. In both cases the system resolving 15 power is reduced, and hence the ability to measure and discriminate defects is reduced.

One known method for improving the quality of ultrasonic inspection is to use a phased array of transducers. This is a system in which a group of 20 transducers are fired with a specific time delay relative to each other and relative to their spatial positions, so as to transmit the ultrasound within a narrower angular range or within pulses of shorter duration in the component or structure under test. A similar principle applies to 25 receive transducers, where the signals from a number of

transducers are combined in a time delay fashion so as to enhance the information content.

However, conventional systems to produce the benefits of a phased array suffer from the disadvantage of excess 5 complexity. Each transducer in the array is separately powered by a drive circuit each producing a similar high frequency pulse train but displaced in time by an amount which matches the physical displacement of the transducers and the velocity of sound in the component or structure 10 under test. In situations where there is a strict limit on space and/or power, this can restrict their application.

US-A-4295214, US-A-4408493 and US-A-4434663 each discloses an EMAT together with an external driver circuit, the EMAT incorporating a pair of coils fed by the external 15 driver circuit to produce signals having a pre-set phase shift of typically 90 .

It would be desirable to be able to provide an electromagnetic acoustic transducer incorporating the advantages of a phased array but without the inherent 20 complexities and bulk of the known arrangements.

According to the present invention there is provided an electromagnetic acoustic transducer for exciting ultrasound in a material under test, the transducer comprising at least one magnet positioned so as to apply a 25 magnetic field to the material under test, and a plurality

of electrical windings spaced along a direction parallel to the surface of the material, adjacent windings being interconnected by an associated capacitance, and a source of alternating current supplying one of the windings with a S pulsed signal, the arrangement being such that the pulsed signal is fed from the one winding to the next adjacent winding, and subsequently sequentially to each next adjacent winding, with a time delay determined by the capacitance between adjacent windings, and whereby each JO winding creates an ultrasound pulse within the material under test, the spacing of the windings and the electrical characteristics thereof being such that the directional characteristics and time duration of the ultrasound wavefront within the material under test is enhanced.

15 Thus it will be appreciated that the arrangement of windings and capacitances effectively constitutes a ladder network such that, when a single high frequency pulse is applied at one end of the network, the pulse is transmitted from winding to winding with a time delay, determined by 20 the inductance/capacitance characteristics, which matches the propagation speed of the ultrasound in the material under test, whereby the benefits of a phased array are achieved using a single drive, and without the necessity for active phase delay circuits.

The windings may comprise, for example, Ccoils, pancake coils or meander coils, while the transducer may be arranged to transmit an ultrasound pulse at any angle to the surface of the material under test including parallel 5 to the surface.

By way of example only, embodiments of the invention will now be described in greater detail with reference to the accompanying drawings of which: Fig. l is a schematic illustration of part of a 10 horizontally polarized shear wave transducer according to the invention, and Fig. 2 shows part of an alternative transducer according to the invention.

Referring to Fig. l of the drawings, a component under 15 test, which may be the ferromagnetic defining wall of a gas pipeline, is shown at 2. The transducer of the invention includes a permanent magnet 4 the longitudinal axis of which extends along the length of the component 2, opposed longitudinal faces of the magnet 4 being of opposite 20 polarity. In the illustrated example, the south pole face of the magnet 4 abuts or lies closely adjacent the component 2.

Arranged along the length of the magnet 4 are a plurality of equi-spaced C-cores 6 on each of which is 25 wound an associated coil 8, the coil system being such as

to induce high frequency magnetic fields and eddy currents

in the component 2 which interact with the magnetic pattern created by the magnet 4 to produce magnetostrictive or Lorentz or other forces.

5 More particularly, a coil 8 is connected to the next adjacent coil by means of a capacitor 10 to create a ladder network or delay line whereby each pair of coil 8 and capacitor 10 conveys to the next pair a delayed high frequency signal. The system is fed by a source providing 10 a high frequency (typically lMHz) pulse, the physical displacement of the cores/coils 6/8 being such that they form a natural phased array. Thus, when the high frequency pulse is applied to one end of the ladder network, the first core 6/coil 8 starts the ultrasound in the component 15 2. The first core 6/coil8 also passes the pulse to the next core 6/coil 8 with a time delay determined by the inductance and capacitance of the core 6/coil 8/capacitor 10 combination and which matches the propagation speed of the ultrasound in the component 2. This process continues 20 down the ladder network enhancing and shaping the ultrasonic pulse in the component 2 whereby the benefits of a phased array are realized with a single pulsed drive.

Thus the transducer of the invention combines the functions of generating the ultrasound, shaping the pulse 25 and controlling the phase time relationship.

The illustrated arrangement exemplifies a simple linear array of coils displaced from one another by an amount related to the acoustic signal wavelength, and with an inductance/capacitance delay related to the acoustic 5 frequency, whereby an enhanced horizontally polarized shear wave ultrasound pulse is transmitted at an angle to the surface of the component under test or parallel to the surface of the component if required. Such an arrangement thus transforms the typical slow build up tone burst of 10 conventional horizontally polarised shear wave EMATs into a sharp pulse with better inspection characteristics, and achieves this in a much more compact and economic manner than heretofore.

The precise construction of the transducer can vary 15 from that described and illustrated providing the electrical characteristics together with the physical displacements of the cores, coils and capacitances continue to form a natural phased array. In particular, the C-core coils may be replaced by pancake coils or meander coils, 20 while there may be, for example, a plurality of longitudinally aligned magnets, adjacent magnets having opposite poles abutting one another.

Fig. 2 shows such an alternative arrangement in which a plate material 20 under test has a magnetic field applied

25 thereto in the direction of arrow 'F' by a standard yoke

(not shown), the transducer incorporating a plurality of parallel, thin support plates 22 each carrying a plurality of interconnected pancake coils 24 and associated capacitors 26, the components on adjacent plates 22 being 5 interconnected as shown to produce a ladder network or delay line equivalent to that detailed with reference to Fig. 1.

Each plate 22 carries a ground plane conductor 28, adjacent conductors 28 being interconnected as shown to 10 comprise a continuous length, while a terminating resistor is shown at 30.

Other variations and modifications will be apparent to those skilled in the art. The described transducers may comprise either transmit or receive devices, in the latter 15 case the vibration of the component under test causing the magnetic pattern to induce high frequency signals in the coils, while the material under test may be any electrically conductive material.

Claims

1. An electromagnetic acoustic transducer for exciting ultrasound in a material under test, the transducer 5 comprising at least one magnet positioned so as to apply a magnetic field to the material under test, and a plurality

of electrical windings spaced along a direction parallel to the surface of the material, adjacent windings being interconnected by an associated capacitance, and a source 10 of alternating current supplying one of the windings with a pulsed signal, the arrangement being such that the pulsed signal is fed from the one winding to the next adjacent winding, and subsequently sequentially to each next adjacent winding, with a time delay determined by the 15 capacitance between adjacent windings, and whereby each winding, in use, creates an ultrasound pulse within the material under test, the spacing of the windings and the electrical characteristics thereof being such that the directional characteristics and time duration of the 20 ultrasound wavefront within the material under test is enhanced.

2. A transducer as claimed in claim 1 in which the windings comprise C-coils, pancake coils or meander coils.

3. A transducer as claimed in claim 1 or claim 2 and arranged to transmit an ultrasound pulse at any angle to the surface of the material under test including parallel to the surface.

4. A transducer as claimed in any one of claims 1 to 3 and including a plurality of longitudinally aligned magnets, adjacent magnets having opposite poles abutting one another.

5. An electromagnetic acoustic transducer substantially as described with reference to and as illustrated by the accompanying drawings.