What is an EMAT?

Full Matrix's EMAT designs are at the heart of our ultrasound sensing technology.

The word EMAT is an acronym for an electromagnetic acoustic transducer. EMATs are used for the transmission and reception of ultrasound within conducting materials. Full Matrix designs and manufactures EMATs for condition monitoring systems, which create ultrasonic guided waves for non-destructive evaluation of critical industrial components e.g. pipes, rails, cables. These resilient transducers require no surface preparation and offer particular advantages for measurements in challenging environments.

How do EMATs work?

A simple EMAT can be built from a coil of wire and a permanent magnet. Passing an alternating current through the coil generates an oscillating force within the surface of a nearby conducting material, which creates sound waves in the body.

Similarly, vibrations of a conductive body near to the EMAT will induce a current in the coil, which can be amplified and recorded. Hence, EMATs can be used both as transmitters of ultrasound -and also by the inverse mechanism- receivers of ultrasound. In practice, the attributes of a good transmitter are rarely conducive to a good receiver. Therefore, the design of EMAT arrays are often optimised for their role.

A simple EMAT design generates sound waves by using a magnet and a coil to induce eddy currents at the surface of a material.

All EMATs generate forces without contact, using a dynamic electromagnetic field produced by current flowing in the coil. The primary mechanism by which this occurs, however, differs from instance to instance. There are three dominant mechanisms of sound generation.

Mechanism 1: The Lorentz Forces

The EMAT configuration pictured above primarily generates a horizontal force.

Electromagnetic fields exert their influence on electric charges and currents through the Lorentz force. When an alternating current is passed through the EMAT coil, a small, dynamic electromagnetic field is produced in the space around the coil, inducing eddy currents at the surface of the nearby conducting material. In the presence of the larger static magentic field of the permanent magnet, the electrons moving in these eddy currents experience a Lorentz force, which is translated into movements of the metal. The direction of this force is perpandicular to both the direction of the eddy current, and the orientation of the permanent magnet, thus the EMAT can be configured to generate sound waves preferentially in a desired direction.

The Lorentz Force can be used to transmit and receive sound in all metals, both magnetic and non-magnetic, and is particularly well suited to creating shear waves.

Mechanism 2: The Magnetisation Forces

There is a much weaker, but still present, force contribution that comes from the magnisation force. This is the product of magnetic moments in the material responding to the changing electromagnetic field of the coil.  Magnetisation forces are greatest in ferromagnetic materials (iron, some steels, nickel, cobalt) which sustain a magnetic moment by themselves, but are nonetheles present in all metals, as the static magnetic field of the EMAT aligns the paramagnetic moment to a small degree. At operational currents, this contribution is small for most materials, hence EMATs are not generally designed to consider the magnetisation force as the primary force generation mechanism. The magnetisation force can, however, act in opposition to the Lorentz force, reducing the efficiency of sound generation.

Mechanism 3: Magnetostrictive Forces

The volume of ferromagnetic metals such as iron, nickel and some steels can be shifted ever so slightly by applying an external magnetic field. This effect is known as magnetostriction, and although slight, it can be exploited to transmit sound in magnetic materials. In such instances, the oscillating field generated by the coil translates directly into vibrations of the ferromagnetic material. Although directional dependence of magnetostriction in each material is quite complex, it is often dominant over the Lorentz force, and is the preferred mechanism for generating out-of-plane vibrations in ferromagnetic metals.

Can EMATs be used on non-conductive materials?

Yes they absolutely can! Vibrations can be coupled into a non-conductive material mechanically by attaching metallic material to the surface. Ultrasound which is then generated in the metal is then transferred to the insulating material through contact. This technique is applied when inspecting plastic water pipes using guded wave ultrasound testing, by coupling EMATs through a conducting metal jacket.

Why choose EMATs?

Non-contact transmission and reception:

The main strength of EMATs is the fact that they can transmit and receive ultrasound in a component without needing to be in physical contact with it. This is because the methods of transmission and reception for EMATs are based on the interaction of fields with the material. This is especially important because, for much critical infrastructure, either the conditions are too harsh for continual contact (very hot components e.g. metal processing), the environment is too messy for transducer-component coupling (muddy environment - buried water pipes), the operating conditions too hostile for good coupling over the measurement lifetime (remote railway and track inspections) or the component has a coating or is painted (outdoor pipework e.g. industrial processing).

The other main approach to transmission and reception of ultrasound in components involves the use of piezoelectric transducers instead of EMATs. The piezoelectric effect occurs in some crystals where a voltage across the crystal causes a strain and vice versa, this means that ultrasound can be produced by putting a high frequency voltage signal across a piezoelectric element. This element is coupled to the component so that the transducer physically vibrates the surface of the component. Using Piezoelectric transducers it is possible to transmit higher amplitude ultrasound but their use becomes either impossible or at least challenging in complex environments.

The non-contact nature of EMATs allows them to be a far more versatile and widely applicable technology than other ultrasound transducer technologies, having key uses in sectors from nuclear to water to railways.

Low damping of oscillations:

Whilst Piezoelectric transducers can produce signals an order of magnitude larger, the contact force between transducer and component adds significat damping which can change the results. This consideration is only relevant in the case where the pulse of ultrasound is travelling back and forth within a particular component, an example of this is included below.

EMAT Transducers Ring longer (After Hirao & Ogi 2003)

Geometry

EMATs can take a large number of different forms, shapes, orientations and sizes due to their fundamentally simple design; a coil and a permanent magnetic field. This means that EMAT design can be optimised for the transmission or reception of particular wavemodes or waveforms - examples of this include Permanent Periodic Magnet (PPM) EMATs and Meander Line Coil EMATs.

Full Matrix Case Studies:

Nuclear Reactor Cooling System Condition Monitoring:

Full Matrix have developed an EMAT condition monitoring system for performing guided wave ultrasound testing on the cooling system pipework of a nuclear energy reactor. EMATs are necessary for this application as the pipework cycles from about 600°C to about 50°C, this is a level of thermal cycling that would make coupling a piezoelectric transducer to the pipe impossible, any couplant would degrade quickly. By maintaining a constant EMAT-pipe distance measurements can be performed consistently and reliably over time without being affected by the extreme temeprature cycling.

<picture of EMAT ring on a pipe and the cassic graphic of a pulse going down a pipe with bends>

Remote Rail Monitoring:

Full Matrix have developed a low-cost [...]

<picture of EMATs on a rail>

Buried Water Main Inspection:

Full Matrix are developing a keyhole inspection technology to perform in-situ non-destructive testing on buried water mains. This involves cutting a narrow hole in the earth from ground to pipe surface through which a probe is inserted and ultrasound measurements performed at the surface of the main. These measurements will provide data essential to water companies to calculate the remaining life of such mains. EMATs are necessary for this application as the environment will be muddy and wet and the surface likely significantly corroded, all these things considered good physical coupling between transducer and pipe would not be possible, rendering the piezoelectric transducers not useful for this application.

<picture of a buried trunk main>