Implementation of Electromagnetic Acoustic Transducers for the measurement

Shaw.Din     TECLAB（CHINA）LIMITED     shaw*teclab.cn

Abstract: The article gives a brief description of basic principles used in electromagnetic acoustic transducers (EMATs). Main configurations of EMAT transducers, wave types generated, transducer excitation, electrical Impedance matching are described, as well as most common applications of EMATs.

Keywords: electromagnetic acoustic transducers, EMAT, non-contact ultrasonic.

Introduction

Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more. A typical pulse/echo inspection configuration 1-1 as illustrated below will be used:

Picture: 1-1

However Classic transducers have a limitation, that in order to transfer the ultrasonic energy, they need a contact with material under inspection and the appropriate couplant,as 1-2. Couplant is a material (usually liquid) that facilitates the transmission of ultrasonic energy from the transducer into the test specimen. Couplant is generally necessary because the acoustic impedance mismatch between air and solids (i.e. such as the test specimen) is large.    Therefore, nearly all of the  energy is reflected and very  little is transmitted into the test material. The couplant displaces the air and makes it possible to get more sound energy into the test specimen so that a usable ultrasonic signal can be obtained. In contact ultrasonic testing a thin film of oil, glycerin or water is generally used between the transducer and the test surface.

Picture: 1-2 and 1-3

   When scanning over the part or making precise measurements, an immersion technique is often used, as 1-3. In immersion ultrasonic testing both the transducer and the part are immersed in the couplant, which is typically water. This method of coupling makes it easier to maintain consistent coupling while moving and manipulating the transducer and/or the part.

The EMAT offers many advantages based on its couplant-free operation. These advantages include the abilities to operate in remote environments at elevated speeds and temperatures, to excite polarizations not easily excited by fluid coupled piezoelectrics, and to produce highly consistent measurements. In general, the development of EMAT research is following three main reasons:

1. EMAT works without need of contact or liquid coupling. Typical distance between the transducer and material under test is tenths of mm up to number mm. A gap between the tested material and EMAT transducer prevents wearing of the transducer.

2. EMAT can work at high temperatures, where immersion evaporates and gaseous mixture of evaporated couplant causes deterioration or loss of coupling. The transducer can be cooled by cold air, with no influence on the signal transfer.

3. EMAT enables generation of following wave types:

a. SH shear horizontally polarized

b. LH longitudinal horizontally polarized

c. Rayleigh waves

d. Lamb waves

e. SH plate waves

   The principles of PZT and EMAT as illustrated below 1-4:

Picure: 1-4

Principles of Electromagnetic Acoustic Transducers (EMATs)

As discussed on the previous, one of the essential features of ultrasonic measurements is mechanical coupling between the transducer and the solid whose properties or structure are to be studied. This coupling is generally achieved in one of two ways. In immersion measurements, energy is coupled between the transducer and sample by placing both objects in a tank filled with a fluid, generally water. In contact measurements, the transducer is pressed directly against the sample, and coupling is achieved by the presence of a thin fluid layer inserted between the two. When shear waves are to be transmitted, the fluid is generally selected to have a significant viscosity.

Electromagnetic-acoustic transducers (EMAT) acts through totally different physical principles and do not need couplant. In simple terms, a magnet, a wire, an air gap and a metal surface can build an EMAT, as2-1. When a wire is placed near the surface of an electrically conducting object and is driven by a current I at the desired ultrasonic frequency. An alternating current I in the wire induces an equal and opposite eddy current I_e in the metal surface. When the eddy current occurs in a magnetic field, the magnetic field form the magent act on I_e and creates a Lorentz force F across the air gap.

Picture: 2-1

This force can generate elastic sound wave from the surface.

Conversely, a sound wave moves the metal surface in the magnetic field to generate an eddy current that induces a current in the wire.

For EMAT coil, only first term of the basic Maxwell equation is applied:

Practical EMAT configurations and wave type

    The most important application of EMATs has been in nondestructive evaluation (NDE) applications such as flaw detection or material property characterization. Couplant free transduction allows operation without contact at elevated temperatures and in remote locations. The coil and magnet structure can also be designed to excite complex wave patterns and polarizations that would be difficult to realize with fluid coupled piezoelectric probes. In the inference of material properties from precise velocity or attenuation measurements, using EMATs can eliminate errors associated with couplant variation, particularly in contact measurements.

    Although a great number of variations on these configurations have been conceived and used in practice, consideration of these practical EMAT configurations shown below should suffice to introduce the fundamentals. The biasing magnet structure, the coil, and the forces on the surface of the solid are shown in an exploded view. The first three configurations will excite beams propagating normal to the surface and produce beams with radial, longitudinal, and transverse polarizations, respectively. The final two use spatially varying stresses to excite beams propagating at oblique angles or along the surface of a component.  

    Cross-sectional view, Configuration of EMAT for radially polarized Shear Wave(3-1 ), Spiral coil(3-2 )Spiral coil of EMAT generates radially polarized shear wave, which propagates perpendicularly to the surface.

Picture: 3-1 and 3-2

 Cross-sectional view, Configuration of EMAT for plane polarized shear wave( 3-3) a normal field EMAT for exciting plane polarized shear waves propagating normal to the surface.

Picture: 3-3

Cross-sectional view , Configuration of EMAT for polarized longitudinal waves(3-4 ), Butterfly coil( 3-5) Magnetic field oriented parallel to the surface for generation of longitudinally polarized wave, which propagates perpendicularly to the surface.

Picture: 3-4 and 3-5

   Cross-sectional view, Configuration of EMAT (3-6 ) with Meander Coil(3-7 ) a meander coil EMAT for exciting obliquely propagating L or SV waves, Rayleigh waves, or guided modes (such as Lamb waves) in plates. 

Picture: 3-6 and 3-7

   Cross-sectional view, Configuration of EMAT (3-8 ) with Periodic Magnetic Field a periodic permanent magnet EMAT for exciting grazing or obliquely propagating horizontally polarized (SH) waves or guided SH modes in plates.

Picture: 3-8

   Practical EMAT designs are relatively narrowband and require strong magnetic fields and large currents to produce ultrasound that is often weaker than that produced by piezoelectric transducers. Rare-earth materials such as Samarium-Cobalt and Neodymium-Iron-Boron are often used to produce sufficiently strong magnetic fields, which may also be generated by pulsed electromagnets.

EMATs’ advantages are tempered by low efficiencies, and careful electronic design is essential to applications. An EMAT to meet the application requirements should be designed with appropriate coil type, correct and accurate Interval between the wires of coil, precise magnetic field.

Transducer excitation

    EMATs excite very weak signal in the metal sample. Therefore, for the present it is used only in specific applications and does not replace general classic ultrasonic testing. In order to get good signal to facilitate our analysis, we have two basic ways to improve the EAMTs signal. First of all, it is necessary to choose generator with high power to drive EAMTs. Sometimes, an extra amplifier connected the signal generator with EMATs. In the next place, it is helpful to add a low noise preamplifier between EMATs and receiver. Sometimes, the EMATs integrate low-noise preamplifier.

Tone burst generator is a good choice in EAMTs excitation. It takes low-voltage signals and converts them into high-power pulse trains for the most power-demanding applications. Their purpose is to transmit bursts of acoustic energy into a test piece, receive the resulting signals, and then manipulate and analyze the received signals in various ways. High power radio frequency (RF) burst capability allows researchers to work with difficult, highly attenuation materials or inefficient transducers such as EMATs. A computer interface makes it possible for systems to make high speed complex measurements, such as those involving multiple frequencies.

The High power tone burst pulser/receiver/AD card all in one (4-1 ),  and EMATs (4-2 )  as illustrated below:

Picture: 4-1 and 4-2

Electrical Impedance matching

  EMATs’ transfer efficiency is much lower than a PZT, so a high power signal generator to drive the EMATs is requisite. At the same time, In order to ensure high power signal be loaded into the EAMTs to the max, impedance matching should be made. There is usually large impedance mismatch between an EMAT and a pulse generator that drives it, causing inefficient transfer into acoustic waves. Gary Petersen describes a L- matching network (5-1 ) can achieve optimal design. The usual way of representing the impedances for this network is to use the series forms.

Picture: 5-1

 The output impedance of the pulse generator is, therefore, given by:

The EMAT impedance is given by:

The condition to maximize the power spent at the EMAT is:

The applicable equations for obtaining the matching impedances are:

Here, Q is defined by

    If the transducer impedance is given in parallel form, it can be converted to series form using relationship equations. Conversions from series to parallel are also valuable when examining the networks.

    Note that this formalism assumes that the shunt element of the L-Matching circuit is in parallel with the pulse generator. If one exchanges the values of Z_i for those of Z_E and vice versa, another solution is obtained in which the shunt element is in parallel with the EMAT.

   If the matching network is properly designed, the power output is given by:

 The power applied to the EMAT becomes:

Here, V_out and V_EMAT denote the RMS Voltage levels at output and at the transducer contacts. is the absolute magnitude of the complex EMAT impedance. The solution for the ratio of the EAMT and pulse generator voltages(step-up ratio) is given by:

Applications of EMATs

    EMAT techniques are developing intensively in the world now. Basic application branches are:

l On-line texture monitoring of steel sheets

l Acoustoelastic stress measurements

l Measurements on high-temperature steels

l Measurements of induction-hardening depth

l Detection of flaw and corrosion

l Average grain size of steels

l Remaining-life assessment of fatigued metals

l Creep damage defection of steels

l EMAR analysis

EMAT enables measuring at high temperatures. It can test hot bars of round, steel plate, tubes and pressure vessels in the production process at temperatures of 700-1200 °F. For this reason, many steelworks have designed and built their own destructive testing and evaluation EMATs system. An industrial EAMT test systems diagram (6-1 ) as illustrated below:

Picture: 6-1

Conclusion

     In order to achieve measurements using EMATs technology, and get good signal to facilitate   analysis, the EMATs should be designed carefully based on the actual test requirements. High power excitation to drive the EMATs ensure adequate signal, it constitute the key part for EMAT function. At the same time, it’s helpful to pay attention to the design of EMAT test system electrical Impedance matching.

Acknowledgements

   The authors wish to acknowledge Gary Petersen for his wonderful discussions and his insights into electronics and instrumentation, Brian F. Larson for his NDT Resource Center in Iowa State University.

References

Hirotsugu OGI, Masahiko HIRAO《EMATs for science and industry non-contacting ultrasonic measurements》

Stanislav Štarman “non-contacting ultrasonic testing using EMAT transdusers”

Brian F. Larson “Introduction to Ultrasonic Testing”