Electromagnetic acoustic transducer and method therefor

Abstract

An electromagnetic acoustic transducer (EMAT) assembly (200, 300) is described that comprises: an EMAT probe (212) comprising: a coil (262) arranged to produce eddy currents in an object under inspection; and a permanent magnet (270) connected to the coil; and an EMAT probe position mechanism comprising: at least one actuator (232) operably coupled to the EMAT probe and arranged to locate the EMAT probe onto or adjacent the object under inspection. At least one movable device (620) is operably coupled to the actuator and arranged to move the EMAT probe around the object under inspection; wherein the actuator and the movable device in combination, are arranged to control: a magnetic field strength between the permanent magnet and the object under inspection; and a variable frictional force that is exerted on to the EMAT probe as the EMAT probe is guided over a surface of the object under inspection.

Description

Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_FinalELECTROMAGNETIC ACOUSTIC TRANSDUCER AND METHOD THEREFORTechnical Field

[0001] , The technical field relates generally to a mechanical assembly that includes an electromagnetic acoustic transducer (EMAT) that enables scanning over large areas of an object to be inspected or tested. In particular, in some examples, the technical field relates to scanning an object of ferromagnetic material with minimal friction between the EMAT and the surface of the ferromagnetic material.Background

[0002] , Piezo electric transducers and ultrasonic EMATs are extensively used in non-destructive testing (NDT) applications and non-destructive evaluation (NDE) to inspect the properties of components, such as an object’s thickness or to detect defects in the object, e.g., cracks in a ferromagnetic material such as steel. It is known that piezo electric transducers and ultrasonic EMATs may also be used to test other material properties that are measurable, such as a speed of sound through the object being tested, which is related to e.g., modulus of elasticity and density of the object.

[0003] , Here, ultrasonic waves are generated within a material through electromagnetic interaction, without requiring direct mechanical coupling or a liquid couplant to transmit the electromagnetic wave from the moving transducer element into the component under test. The EMATs directly induce the electromagnetic wave in the component surface via electromagnetic induction and they can even be lifted off the surface by a few mm and still perform well. This characteristic, together with the fact that EMATs do not require the presence of a liquid couplant, makes them particularly well-suited for environments where contact-based ultrasonic methods are impractical or undesirable, such as for (mobile) robotic inspection or for locations that are difficult to access or where positioning aids are unable to use very strong actuators to control the EMAT position.

[0004] , Despite the advantages of non-contact operation, EMATs typically still require close proximity to the surface of the test material to ensure efficient transmission and reception ofAttorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final ultrasonic signals. This proximity often necessitates the use of mechanical assemblies to maintain the EMAT at a consistent stand-off (sometimes referred to as Tift-off) distance from the material surface during scanning operations. For large-scale inspections, such as those involving pipelines, structural steel components, or ship hulls, existing EMAT scanning systems often struggle to maintain a uniform gap to navigate surface irregularities, or minimize frictional drag, which can impact scanning speed, signal quality, and operator fatigue.

[0005] , Referring now to FIG. 1, a conventional EMAT scanning assembly 100 is illustrated. In a first illustration 110, a thickness measurement is performed by an ultrasonic EMAT probe 112 located on an uneven 3D surface 116 that requires movement of the scan / measurement in all directions 114 of the uneven 3D surface 116.

[0006] , In a second illustration 140 of FIG. 1, the EMAT probe 112 is illustrated in more detail, with a magnet (having a negative polarity 144 and a positive polarity 146 connected to a coil 148) that is in contact with a component being inspected 150. The coil 148 produces eddy currents in the object under test. Typically, EMATs are required to create strong magnetic fields and therefore most EMATs for field testing contain permanent magnets. Here, as illustrated, the strong magnetic attractive force 142 of the EMAT probe 112 exerts a very strong ‘pull force’ onto the surface of the component being inspected 150 (in a form of a ferromagnetic material), which attaches the EMAT probe 112 onto the surface and produces frictional forces that make it difficult to manipulate the EMAT probe 112 around the surface due to friction 152 between the surface and the EMAT probe 112. It is known that the EMAT probe 112 may be located in a housing that is easy to hold by hand or by an actuator, whereby the housing may also contain cable connectors to the coil 148 (not shown). It is known that, in general, light-weight mobile robots are only able to exert the force of their own weight (or power that their locomotion device can produce) onto a structure, otherwise the actuator lifts off the light-weight mobile robot rather than exerting more force onto the structure.

[0007] , In a third illustration of FIG. 1, the EMAT probe 112 shows a further problem due to the strong permanent magnets (having a negative polarity 144 and a positive polarity 146 connected to a coil 148) attracting small ferromagnetic particles, e.g., ferromagnetic particles 172Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final that can form on surfaces being tested. The build-up of these particles then distorts the magnetic field and produces friction and wear, and potentially interference with the measurement in addition to making it difficult to operate the EMAT properly.

[0008] , Thus, conventional EMAT scanning / inspection assemblies often rely on sliding contact surfaces, wheels, or fixed mounts that can generate significant friction when moved across large or rough surfaces, particularly ferromagnetic surfaces. This friction can lead to wear on the EMAT housing, inconsistent scanning motion, or even damage to the test surface. Additionally, the magnetic attraction between the EMAT and the ferromagnetic material can exacerbate these issues, thereby increasing the difficulty of scanning and reducing the efficiency of inspection processes.

[0009] , Therefore, the inventors have identified a need for an improved mechanical assembly that facilitates the movement of an EMAT across large areas of, say, ferromagnetic material with reduced friction and consistent scanning quality. Such an assembly would ideally maintain the necessary proximity for effective signal transmission, whilst also accommodating surface irregularities and minimizing mechanical resistance during operation.Summary

[0010] , In a first aspect, an electromagnetic acoustic transducer, EMAT, assembly is described that comprises: an EMAT probe comprising: a coil arranged to produce eddy currents in an object under inspection; a permanent magnet connected to the coil; and an EMAT probe position mechanism. The EMAT probe position mechanism comprises at least one actuator operably coupled to the EMAT probe and arranged to locate the EMAT probe onto or adjacent the object under inspection; at least one movable device operably coupled to the at least one actuator and arranged to move the EMAT probe around the object under inspection. The at least one actuator and the at least one movable device, in combination, are arranged to control a magnetic field strength between the permanent magnet and the object under inspection and a variable frictional force that is exerted on to the EMAT probe as the EMAT probe is guided over a surface of the object under inspection.Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final

[0011] , In this manner, an improved mechanical EMAT assembly is provided that facilitates the movement of the EMAT assembly across large areas of ferromagnetic material with reduced friction and consistent scanning quality. In addition, the improved EMAT assembly may ideally maintain the necessary proximity for effective signal transmission, whilst also accommodating surface irregularities and minimizing mechanical resistance during operation.

[0012] , In a second aspect, a method for moving an electromagnetic acoustic transducer, EMAT, probe across an object under inspection is described, wherein the EMAT probe comprises a permanent magnet connected to a coil arranged to produce eddy currents in an object under inspection. The method comprises: locating the EMAT probe onto or adjacent the object under inspection by at least one actuator of an EMAT probe position mechanism; moving the EMAT probe around the object under inspection using at least one movable device operably coupled to the at least one actuator; and controlling by the at least one actuator and the at least one movable device in combination, a magnetic field strength between the permanent magnet and the object under inspection and a variable frictional force that is exerted on to the EMAT probe as the EMAT probe is guided over a surface of the object under inspection.Brief Description of the Drawings

[0013] , Further details, aspects and embodiments will be described, by way of example only, with reference to the drawings. In the drawings, similar reference numbers are used to identify like or functionally similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.

[0014] , FIG. 1 illustrates a conventional EMAT scanning assembly.

[0015] , FIG. 2 illustrates a first example EMAT assembly to control the magnetic field strength at the location of the EMAT coil, adapted in accordance with some examples.

[0016] , FIG. 3 illustrates a second example EMAT assembly to control the magnetic field strength at the location of the EMAT coil, adapted in accordance with some examples.

[0017] , FIG. 4 illustrates an example diagram of a use of an actuator to control the magnets creating the flux in the EMAT assembly, in accordance with some examples.Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final

[0018] , FIG. 5 illustrates an example diagram of an actuator operation to control the flux created by the EMAT assembly, adapted in accordance with some examples.

[0019] , FIG. 6 illustrates a side view and a front view of an example of an EMAT assembly, adapted in accordance with some examples.

[0020] , FIG. 7 illustrates a graphical example of a variation of rolling resistance with wheel radius for a normal force N=10N acting on a wheel surface combination with parameter b=0.5mm, adapted in accordance with some examples.

[0021] , FIG. 8 illustrates a graphical example of a variation of EMAT signal amplitude as a function of Tift off, adapted in accordance with some examples.

[0022] , FIG. 9 illustrates an example flowchart for moving an electromagnetic acoustic transducer, EMAT, probe across an object under inspection, in accordance with some examples.

[0023] , Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and / or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various examples. Also, common but well- understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various examples. It will be further appreciated that certain actions and / or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.Detailed Description

[0024] , Examples herein described propose, inter alia, an EMAT assembly consisting of a coil, a permanent magnet and housing; one or more actuators arranged to integrate the EMAT components onto (which in some examples may be linear actuators, although other actuator typesAttorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final are envisaged as being equally applicable to the concepts described herein), a friction lowering manipulation mechanism consisting of a movable object (which has one or more wheels); and a compliant attachment point that controls a force to be exerted on the EMAT housing or wheel assembly to guide it over the surface.

[0025] , In this manner, the concepts described herein overcome the aforementioned problems by producing an integrated EMAT assembly with a mechanism that enables low friction manipulation of the EMAT over the surface that is to be inspected, despite the strong pull force that the magnet imposes between the EMAT and the surface of the structure that is to be inspected. Furthermore, the integrated EMAT assembly is arranged to enable or support an adjustment of a lift-off of the EMAT coil over the relevant surface of the structure that is to be inspected and independently enable adjustment of the magnet above the test aperture. In examples described herein, it is envisaged that the test aperture encompasses an area in which the interaction of the magnetic field from the magnet and the eddy current induced by the coil in the specimen under test excite an ultrasonic wave. This makes it possible to operate on surfaces that are non-uniform and exhibit some curvature without the EMAT coil making contact of the surface of the structure that is to be inspected. At the same time, it also makes it possible to reduce the magnetic flux / field strength and hence lowers the attraction force between the EMAT probe and the surface making it easy to remove the probe from the surface that is being inspected whilst also making it possible to remove ferromagnetic dust that becomes attracted to the front face of the transducer.

[0026] , The EMAT assembly enables ultrasonic inspections at multiple locations of a surface, for example an uneven surface, where the EMAT assembly is arranged to follow the direction instigated by an external system / controller, say, a main robot / manipulator. The EMAT assembly imparts very low resistive forces to the operator / robot as a result of the low friction between the EMAT and the test structure.

[0027] , In examples described herein, it is envisaged that the magnet and coil may be arranged in any number of different ways, as would be appreciated by a skilled person, for example placed in a housing that may be controlled by an actuator in order to locate the coil contained in the housing. In some examples, the housing may be arranged to additionally contain cable connectors to the coil.Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final

[0028] , FIG. 2 illustrates a first example EMAT assembly 200 arranged to control, and in some instances substantially reduce, the magnetic field strength at the location of the EMAT coil, in accordance with some examples. In one example application, the component or object 250 to be inspected or tested or scanned or investigated may be a ferromagnetic material, with the testing being an ultrasonic thickness measurement performed by an EMAT probe 210. Hereinafter, the component or object 250 to be inspected or tested or scanned or investigated, which may be a ferromagnetic material, is referred to as an ‘object under inspection’, which is intended to encompass all applications where the EMAT assembly and EMAT probe may be employed.

[0029] , The EMAT probe 210 comprises a magnet 212 (having a negative polarity 244 and a positive polarity 246) connected to a coil 262 that is to be placed in contact with an object 250 under inspection. In this example, the EMAT probe 210 may be located in an EMAT housing 214, which may also contain cable connectors to the coil 262 (not shown). In this example, the coil 262 may comprise or may be connected to a coil front face / plate 260, with the coil arranged to produce eddy currents in the object 250 under inspection. The coil front face / plate 260 may be attached to a movable object that enables low friction rolling of the EMAT probe 210 with the coil 262 across a planar surface of the object 250 under inspection. Examples are herein described in terms of the movable object comprising two wheels, whereas it is envisaged that the concepts described herein are equally applicable to any suitable movable object, such as one with a single (movable / rotatable) wheel, single or multiple rollers or castors.

[0030] , In some examples, it is envisaged that the EMAT assembly 200 may be made ‘wearproof, for example with changeable covering / coating on the surfaces of the EMAT assembly 200 particularly those surfaces that are in contact with the object 250 under inspection. Here, in some examples, the cover may be constructed of a harder material than the material that is tested, thereby avoiding scraping the material off quickly and wearing away the conductors in the coil, which could lead to a short and render the EMAT non-functional. In this manner, it is possible to ensure that the EMAT assembly 200 does not degrade during scanning with components that are easily cleanable / replaceable. In some examples, it is envisaged that the coil front face / plate 260 may be arranged to collect ferro-magnetic metal (sometimes referred to as ‘swarf), and when a wearproof front face / plate is used any intermittent contact with the surface may prevent damageAttorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final to the coil 262. In some examples, it is envisaged that the coil front face / plate 260 may be made out of a hard wearing material, such as a ceramic material that is transparent to magnetic fields (i.e., non-conductive and non-magnetic) yet hard and durable. This will prevent damage to the coil, should the EMAT assembly 200 accidentally come into contact with the surface due to, say, a disturbance.

[0031] , In some examples it is envisaged that the EMAT assembly 200 may include a compliant attachment point 202, where the EMAT assembly 200 may be connected to an external system / controller 290, such that a form of decoupling exists and the EMAT assembly 200 can be considered as rigid. The compliant attachment point for the external system / controller 290 (which may be robot or an ‘external actuator’ or another contact point / interface to the outside world), e.g., compliant attachment point 202 in FIG. 2, is arranged to guide the mechanism is then attached onto the EMAT housing via a rotary joint or a ball and socket joint or a pin or a universal joint 230 or indeed any other joint useable by a skilled person that is offset from the central axis of the EMAT (the measurement aperture). In some examples it is envisaged that the measurement aperture may encompass an area where the eddy currents that are induced by the coil and the magnetic field from the magnets interact, in order to produce an ultrasonic wave in the material that is to be tested. In some examples, the aperture may typically be located at the centre of the EMAT and may have typical dimensions of, say, 5x5mm or 10x10mm; although it is envisaged in other examples that the dimensions may be as large as 100x100mm and as small as 1x1 mm. Thus, in this manner, the compliant attachment point 202 enables motion of the EMAT assembly 200 in any direction on the surface of the object 250 under inspection and ensures that the EMAT assembly 200 follows the motion applied by (or instructed to) the compliant attachment point 202. In some examples, it is envisaged that the compliant attachment point 202 may also help with adapting to measurement tolerances. In some examples, the compliant attachment point 202 may exhibit a compliance C in the range of 0.01-1000 N / mm in any direction and (0.01-1000) * [1 / 0.0175] Nm / rad with respect to any rotation has been found to be adequate in most applications. Here, compliance is intended to encompass a displacement or rotation (mm / rad) of that part of the structure relative to the bulk of the structure represented by, e.g., the centre of gravity (COG) in response to an externally applied force (N) or moment (Nm).Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final

[0032] , In this example, the EMAT probe 210 is moved towards 220 the object 250 under inspection to enable the coil 262 to create a strong magnetic field. However, in some examples, it is envisaged that the magnetic field is controlled to ensure that any created friction between the coil front face / plate 260 and the surface is manageable, thereby ensuring that the frictional forces do not make it difficult to manipulate the EMAT probe 212. In order to move the EMAT probe 210 towards the object 250 under inspection, a pin or universal joint 230 (used in some examples to enable some tolerance in the parts so that the EMAT mechanism doesn’t become stuck) is connected to an actuator 232 that is attached to the magnet 212. In this example, it is envisaged that the magnet 212 may be fixedly connected 234 to the coil 262 and front face / plate 260, as illustrated. In a non-fixed arrangement 236, it is envisaged that the coil 262 and front face / plate 260 in combination may move away from the magnet 212, or the magnet 212 may be fixedly connected 234 to the coil 262 with the front face / plate 260 moveable towards or away from the coil 262, with the distances moved 264 being variable and controllable by a further actuator.

[0033] , FIG. 3 illustrates 3 optional arrangements. A first option where the magnet, coil and front face are fixedly connected 234 ensures that the whole magnet-coil-front face assembly moves together. Advantageously, this reduces the force required to remove the assembly, but may have a disadvantage in that particles may remain stuck to front face 260 when the actuator 232 retracts. A second option includes the front face 260 and the coil 262 connected together where there is no movement between them. Here, the force that holds the assembly onto the structure is removed. However, and advantageously, since the front face 260 stays out, the force on debris / wear particles that are stuck to the front face 260 also reduces, which makes removal of these particles easier. Furthermore, there is no movement of coil in this second option and hence no movement of connectors and cables. A third option (far right on FIG. 2) includes the front face coupled to the device being tested and the coil 262 is attached to magnet 212 stack. This achieves the functionality as the second option, but now the coil 262 moves and notably cable connections (not shown) also move, which might lead to fatigue in the connectors.

[0034] , Referring now to FIG. 3, a second example EMAT assembly 300 is illustrated that controls, and in some instances substantially reduces, the magnetic field strength at the location of the EMAT coil, in accordance with some examples. FIG. 3 substantially replicates FIG. 2,Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final other than with a second actuator to provide a second additional control of the distance from the magnet 212 to the object 250 under inspection. Thus, components in the second example EMAT assembly 300 that are similar to the first example EMAT assembly 200 of FIG. 2 will not be described again for clarity purposes only.

[0035] , In this second example, the EMAT probe 210 is moved towards 220 the object 250 under inspection to enable the coil 362 to create a strong magnetic field. However, in some examples, it is envisaged that the magnetic field is controlled to ensure that any created friction between the front face / plate 360 (of the coil 362) is manageable, thereby ensuring that the frictional forces do not make it difficult to manipulate the EMAT probe 212. In order to move the EMAT probe 210 towards the object 250 under inspection, a pin or universal joint 230 is connected to an actuator 232 that is attached to the magnet 212. In this second example, it is envisaged that the magnet 212 may be fixedly connected 234 to the coil 362 and front face 360 or in a non-fixed arrangement 236 to the coil 362 or front face / plate 360, as illustrated. In a nonfixed arrangement 236, it is envisaged that a second actuator 364 may move the coil 262 and front face / plate 360 away from the magnet 212, or the second actuator (not shown) may be arranged to move the front face / plate 360 away from the coil 362, with the distances moved 364 being variable and controlled by the second actuator 364. In this example, the additional actuator 364 of the EMAT assembly 300 may be controlled by a remote controller and used to change the normal component of the magnetic flux that passes through the front face by adjusting the relative, variable distance 364 between the front face 360 / coil 362 and the magnet 212 of the EMAT assembly 300.

[0036] , The inventors have recognised and appreciated that when testing / inspecting a ferromagnetic material, the EMAT assembly will be pulled strongly onto the structure that is to be inspected due to electromagnetic forces. With a weak EMAT assembly manipulator, such as a drone or crawler robot, the inventors have also recognised and appreciated that it might be difficult to overcome the pull force to take the whole assembly off the surface. In order to overcome this problem, an additional mechanism to reduce the magnetic field and pull force between the EMAT assembly and the structure that is to be inspected is described herein, with respect to FIG. 4 or FIG. 5. The concepts described enable the reduction in the magnetic pullAttorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final force so that the assembly can be easily removed from the surface. The reduction in magnetic field from the coil surface also advantageously aids the removal of ferromagnetic dust or debris that might collect on the coil or front face / plate surface due to the magnetic attraction force. Thus, in accordance with some examples, it is envisaged that mechanisms may be introduced that control, and therefore may substantially reduce / negate, the magnetic field strength at the location of the EMAT coil, when desired, as illustrated in FIG. 4 and FIG. 5.

[0037] , FIG. 4 illustrates an example diagram 400 of a use of an actuator 232 with an extendable arm 430 to control flux created by one magnet that has a north pole and a south pole 212 operably connected (not necessarily physically connected) to a coil 262 in the EMAT assembly, in accordance with some examples. In this example, the magnet 212 and actuator 232 are located within a housing 214 connected to the coil 262. FIG. 4 illustrates two example flux diagrams 410, 420 created by the magnet 212 as adapted / controlled by the actuator 232. As illustrated, a change in flux is achieved by changing the relative position of the magnet with respect to the coil 262. In this manner, the actuator 232 is able to move the magnet 212 towards the coil 262 and create a high flux condition with actuator arm 430 extended as illustrated in 410, or the actuator 232 is able to move the magnet 212 away from the coil 262 and create a low flux condition with actuator arm 430 retracted, as illustrated in 420. FIG. 4 also shows the aperture 415, which is where the magnetic flux lines cross the coil. As shown, a high flux state 415 exists with strong coupling and hence excitation of strong ultrasonic signals with the actuator arm 430 extended and a low flux state 420 exists with weak coupling. As a result, hardly any or very weak ultrasonic signal excitation exists at the aperture 415 with the actuator arm 430 retracted.

[0038] , In this manner, the actuator 232 is able to control the magnetic field strength that acts at the location of the coil 262 so that the EMAT assembly can be effectively (temporarily) switched ‘off’ and then very easily moved normally with respect to the surface of the object under inspection. Once the EMAT assembly is re-located, the actuator 232 is able to control the magnetic field strength that acts at the location of the coil 262 so that the EMAT assembly can be effectively switched ‘on’, in which case the magnetic force will pull the assembly onto the surface so that it is restricted to move tangentially to the surface. The aperture 415 is the location on the object under test / inspection, where the highest cross product of the eddy current that is induced by the coil 262 in the test object and the magnetic flux from the magnet 212 is achieved.Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_FinalSince eddy currents are only induced in very close proximity to the exciting coil, the aperture 415 here is described by the location where the maximum flux density from the magnet 212 is achieved, e.g., as illustrated in the high flux state 410 in FIG. 4. It is envisaged that in other examples several preferrable coil geometries may be used, such as a butterfly and a pancake coil. In both of these coil geometries the desirable aperture 415 has approximate dimension of 5x5mm (5mm diameter for the pancake), 10x10mm (10mm diameter for the pancake). In some alternative examples it is envisaged that some coil geometries may be in a range from 1-100mm characteristic dimensions in order to achieve practically functional devices.

[0039] , Referring now to FIG. 5 illustrates a further example diagram 500 of a use of an actuator 232 to control the flux created by the magnets 212 operably connected to a coil 262 in the EMAT assembly, in accordance with some examples. In this example, the magnets 212 are located within a flux guide or housing 214 of high magnetically permeability connected to a coil 262. In this example, as illustrated, a change in flux may be achieved by placing two magnets (having a north pole 244 and a south pole 246) in an aligned polarity to create high magnetically permeability, as shown in 510 and using the actuator 232 to change the relative polarity of one magnet with respect to the other magnet (i.e., place opposite polarities substantially adjacent to one another) to create low magnetically permeability, as shown in 520. In this manner, the actuator 232 is able to rotate one of the magnets 212 and create either a high flux condition as illustrated in 510 or create a low flux condition as illustrated in 520.

[0040] , In this manner, the actuator 232 is able to control the magnetic field strength that acts at the location of the coil 262 and thus the aperture so that the EMAT assembly can be effectively (temporarily) switched ‘off in 520 and then very easily moved normally with respect to the surface of the object under inspection. Once the EMAT assembly is re-located, the actuator 232 is able to control the magnetic field strength that acts at the location of the coil 262 so that the EMAT assembly can be effectively switched ‘on’ in 510, in which case the magnetic force will pull the EMAT assembly onto the surface so that it is restricted to move tangentially to the surface.Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final

[0041] , Thus, as illustrated in FIG. 4 and FIG. 5, a number of mechanisms are described to control the flux, where the operation can be implemented in several forms. For example, an additional actuator may be used to adjust the distance between the coil and the magnet surface as shown in FIG. 4, or within a magnetic assembly the position of one magnet is flipped so that the resultant external field cancels out, as shown in FIG. 5. In both approaches, a loss of magnetic field strength at the front face / plate of the transducer can be controlled.

[0042] , In some examples, it is envisaged that the EMAT assembly may be arranged such that the distance is adjusted so that the normal magnetic flux that passes through the front face is <100mT. When the magnetic field strength drops below lOOmT or preferably even lower to below 1 OmT the magnetic attraction force is weaker than the gravitational force that acts on small ferro magnetic particles / debris and thus the small ferro magnetic particles / debris and dust falls off the front face / plate of the transducer (where the coil is located). This facilitates the periodic cleaning of the transducer and guarantees that it can function properly.

[0043] , FIG. 6 shows a description of one optional implementation of an EMAT assembly 600, with a front view 610 and a side view 650, in accordance with some examples. In this example, the EMAT assembly 600 is arranged to control, and in some instances substantially reduce, the magnetic field strength at the location of the EMAT coil and move the EMAT probe 212 around an object 250 under inspection using at least one wheel-based movable device 620. It is envisaged that in other examples a roller-based or castor-based movable device may be employed, as would be appreciated by a skilled person.

[0044] , In this example application, the object 250 under inspection may be a ferromagnetic material, with the testing being an ultrasonic thickness measurement performed by an EMAT probe 210. The EMAT probe 210 comprises a magnet (not shown, having a negative polarity and a positive polarity) connected to a coil 262 that is to be placed in contact with the object 250 under inspection. In this example, the EMAT probe 210 may be located in an EMAT housing, which may also contain cable connectors (not shown) to the coil 262. In this example, the coil 262 may comprise or may be connected to a coil’s front face / plate (not shown), with the coil arranged to produce eddy currents in the object 250 under inspection.Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final

[0045] , In this example, the EMAT probe 210 is moved towards 220 the object 250 under inspection through use of the actuator 232 to enable the coil 262 to create a strong magnetic field. However, in some examples, it is envisaged that the magnetic field is controlled to ensure that any created friction between the front face / plate 260 (of the coil 262) is manageable, thereby ensuring that the frictional forces do not make it difficult to manipulate the EMAT probe 212. Thus, it is envisaged that the EMAT probe may be configured, in some examples, in accordance with the optional arrangements in any of FIG’s 2-5.

[0046] , In this example, a compliant attachment point 202 is attached to the wheel -based movable device 620 via, say, a cylindrical structure 618 as shown, and is arranged to respond to control instructions and manipulate the wheel-based movable device 620, and thus the EMAT probe 210 with the coil 262, in at least one and preferably two degrees of freedom (DOF) across a planar surface of the object 250 under inspection in a low-friction rolling manner. In this example, the compliant attachment point 202 may be able to rotate the wheel(s) of the at least one wheel -based movable device 620 across the surface of the object 250 under inspection in a first DOF 656, e.g., about the axis of the cylindrical structure 618 as illustrated, the cylindrical structure 618 being substantially more rigid and preventing motion in all other degrees of freedom, i.e., translations and rotations about the other axes. In this example, the wheel-based movable device 620 moves around a different DOF 658, which includes a rotation about the axis of the wheel(s) 620. Thus, the at least one wheel 620 comprises a first axis arranged to be substantially parallel to a primary plane of the coil 262 and comprises one joint that induces a rotary degree of freedom with a second axis that is perpendicular to, or substantially perpendicular (within a suitable tolerance as expected by a skilled person to achieve the same or similar result) to, the primary plane of the coil 262.

[0047] , In this example, the EMAT assembly 600 is suspended between a set of two wheels 620 of large radius ‘W’, 654, which can both independently rotate about their attachment point. In other examples it is envisaged that the EMAT housing may contain one or as shown two (or more) large radius wheels 620. In some examples, the inventors have determined that for an EMAT assembly as illustrated in FIG. 6, a wheel radius ‘W’ 654 of between 1 -300mm is advantageous, which enables for the assembly to be small and compact yet not very heavy so that it can be manipulated by a small to medium size robot. In some examples, having largeAttorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final diameter wheels ensures that there is low rolling friction on the surface, as the rolling resistance is inversely related to the wheel diameter via the commonly employed equation:F= N *b / W [1]Where: F is the rolling resistance force that is experience,N is the normal force that the wheel exerts onto the surface, and b is a parameter that is dependent on the materials that the wheel and the surface are made off.

[0048] , The actuator 232 is used to control and fine-tune a lift-off distance ‘L’ 616 between the coil 262 on the front surface of the EMAT probe and the surface of the object 250 under inspection, which is particularly important for curved and non-flat surfaces. In some examples, the actuator 232 may be controlled via a remote controller to adjust ‘L’ 616, dependent upon a feedback signal from a sensor or components of the system itself. In some examples, this lift-off control is achieved by measuring the signal strength on the EMAT probe and reducing the lift off distance ‘L’ 616 if the signal drops and increasing it when the signal strength increases, for example relative to a predefined reference value.

[0049] , In this illustrated example, a lift off sensor 614 may be employed to measure a distance ‘L’ 616 between the coil (or front face / plate 260) and the surface of the object 250 under inspection. Alternatively, in some examples, it is envisaged that a different sensor may be used, for example to measure an absolute position of an actuator. In some examples, it is envisaged that the different sensor may be an external proximity sensor, such as an ultrasonic sensor, an infrared sensor or to incorporate a sensor such as a spring-loaded Linear Variable Differential Transformer (LVDT) sensor arranged to measure displacement between the coil and the surface the surface of the object 250 under inspection. It is envisaged that in other examples, another feedback signal may be the impedance of the EMAT coil 262 itself, which advantageously avoids use of an additional sensor. The impedance of the coil is strongly affected by the lift-off distance ‘L’ 616 from the surface. Therefore, sensing of the coil impedance as a reference value and adjusting ‘L’ 616, such that the coil impedance remains unchanged is an effective strategy to control ‘L’ 616. It is envisaged that in other examples, another feedback signal may be to use the EMAT ultrasonic signal amplitude measurement itself to control ‘L’ 616. Here, in this example,Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final the amplitude of the ultrasonic signal, that is applied to the object 250 under inspection, strongly depends on the lift-off ‘L’ 616. Therefore, adjusting the distance ‘L’, such that the ultrasonic signal amplitude remains constant, is an effective control strategy. Since, the coil is used for the ultrasonic measurement this is also an option that does not require any additional sensors. Thus, it is envisaged that different sensors, settings and sensor inputs may be used by, say, a remote controller to set the lift off distance ‘Z ’ 616.

[0050] , The inventors have determined that, in some examples, a lift-off distance ‘L’ 616 in the range 0.01 and 20mm may be used, with a range of 0.02- 10mm representing an optimum tradeoff between delivering strong ultrasonic signals from the object 250 under inspection and providing enough clearance to navigate over the non-flat surface of the object 250 under inspection without the EMAT probe catching on any surface features. The inventors have also determined that, in some examples, a preferred ratio between the lift-off distance ‘L’ 616 and the radius of the wheel ‘W’ 654 functions best when L:W is in a range of 1:50 to 1: 10,000; noting that ‘L’ 616 « ‘W’ 654.

[0051] , In this example, the compliant attachment point 202 is able to provide a second, additional rotary degree of freedom, DOF2 656. An offset ‘O’ 652 measured between a centre of the compliant attachment point 202 and the centre of the coil 262, is able to influence the movement in the DOF2 656. In some examples, the offset ‘O’ 652 is required to induce a moment that the external system / controller (such as external system / controller 290 in FIG. 2) is able to exert onto the EMAT probe (which may be located in a housing) and wheels 620 that will turn the EMAT probe, if necessary on the spot, and orient it in any direction on the substantially planar surface. In some examples, the offset ‘O’ 652 may be selected / designed to be not too small and not too large, to ensure that there is sufficient moment exerted onto the wheels 620 to turn them without causing excessive deformations and deflections that could make any other part of the assembly other than the wheels 620 make contact with the surface, i.e., lead to jamming and unsmooth motion. The inventors have identified that, in some examples, the normal offset between the centre of the coil and the axis of rotation of the joint inducing said second, additional rotary degree of freedom DOF2 656 (identified as distance ‘O’) may take a value between 1 and 500mm, where in this range the moment exerted is large enough to easily turn the wheels on the spot whilst at the same time resulting in a small, compact and light weightAttorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final assembly that is not too heavy to be manipulated by a small or mid-sized robot. The inventors have identified that, in some examples, the ratio of the value of the distance ‘O’ 652 to the wheel radius ‘W’ 654 may be of the order of ‘O’ 652: ‘W’ 654 of 0.01-100, where this range of values will result in strong enough moment and force transmissions while also keeping the dimensions of the EMAT assembly 650 to a practical size, thereby avoiding any part of the EMAT assembly catching on the surface the surface of the object 250 under inspection and thus increasing friction.

[0052] , In some examples it is envisaged that the material is non-magnetic, such that the wheels do not affect the not affect the electromagnetic field and measurements. In other examples it is envisaged that the material may be magnetic for example if it is at a distance larger than, say, 5- 10mm from the magnets. In some examples it is envisaged that a further rotational degree of freedom may be supported, such that the direction of the wheels can be arbitrarily changed following the instruction / guidance / control of an external system / controller (such as external system / controller 290 in FIG. 2). In this manner, the external system / controller may control the movement of the EMAT assembly 600 in any direction on a substantially planar surface, where it is envisaged that the ‘R’ -radius of curvature (not shown) of the object 250 under inspection > ‘W’-radius of the wheels. In this example, it is envisaged that such an ‘external system / controller’ may be a drone or crawler robot moving the EMAT probe 210 over the surface of the object 250 under inspection. However, it is envisaged that in other examples the external system / controller may be a technician or engineer or such.

[0053] , Although the example EMAT assembly 600 is shown with two wheels 620, it is envisaged that the concepts described herein could work equally well with one or more wheels. For example, with a single wheel being arranged to apply another moment, for instance by use of an external actuator / controller or use of a single magnetic wheel. In another example, for some applications, it is envisaged that a single wheel may be employed with a symmetrical dummy EMAT added on the other side of the single wheel, so that the moment is balanced.Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final

[0054] , Referring now to FIG. 7, a graphical example 700 illustrates a variation of rolling resistance 710 versus wheel radius (in mm) 720 for a normal force N=10N acting on a wheel surface combination with parameter b=0.5mm, adapted in accordance with some examples. Here, as illustrated in the graphical example 700, the magnetic pull force between the EMAT probe and the object / component to be inspected / tested then assures that the EMAT is pulled towards the surface and normally oriented with respect to it. FIG.7 shows that it is desirable to have wheels of radius W exceeding 10mm, such that the rolling resistance force is much below IN.

[0055] , Referring now to FIG. 8, a graphical example 800 illustrates a variation of EMAT signal peak amplitude (in self-consistent yet arbitrary units ‘arb.’) 810 as a function of Tift off (in mm) 820, adapted in accordance with some examples. In graphical example 800, a first set of data 830 indicates measured signal amplitude vs lift off distance, whereas a second set of data 840 indicates an exponential curve fit) of the first set of data 830, in accordance with some examples. It is clearly visible that the signal amplitude is dependent on L and therefore is a measure of L and can be used for lift-off control. As shown, the signal peak amplitude 810 exponentially reduces as a function of lift-off distance 820 as described by the equation:S = Ae~cL[2]Where: S is the signal amplitude; and‘A’ and ‘c’ are arbitrary constants that depend on the specific coil and magnet configurations, as well as the amplitude of the driving current.

[0056] , The lift-off control enables both movement without contact with the surface and optimisation of the signal amplitude on the EMAT as the signal drops off exponentially with increasing lift off distance, where it is important to be as close as possible without touching the surface. As will be appreciated, a delicate trade off exists between these two competing requirements.Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final

[0057] , Referring now to FIG. 9, an example flowchart 900 is illustrated for moving an electromagnetic acoustic transducer, EMAT, probe across an object under inspection, in accordance with some examples. The EMAT probe comprises a permanent magnet connected to a coil arranged to produce eddy currents in the object under inspection. At 910, the flowchart 900 comprises locating the EMAT probe onto or adjacent the object under inspection by at least one actuator of an EMAT probe position mechanism. At 920, the flowchart 900 comprises moving the EMAT probe around the object under inspection using at least one movable device operably coupled to the at least one actuator. At 930, the flowchart 900 comprises controlling by the at least one actuator and the at least one movable device in combination, a magnetic field strength between the permanent magnet and the object under inspection and a variable frictional force that is exerted on to the EMAT probe as the EMAT probe is guided over a surface of the object under inspection.

[0058] , In particular, it is envisaged that the aforementioned inventive concept can be applied by a magnet manufacturer to any EMAT assembly configured to perform any of the aforementioned operations. It is further envisaged that, for example, a magnet manufacturer may employ the inventive concept in a design of a stand-alone device. It will be appreciated that, for clarity purposes, the above description has described example embodiments with reference to different functional units. However, it will be apparent that any suitable distribution of functionality between different functional units, for example with respect to the magnet and coil and the mechanisms that move the EMAT probe around a component / object to be tested / inspected may be used without detracting from the concepts described herein. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality, rather than indicative of a strict logical or physical structure, size or organization. Thus, it is envisaged that the elements and components of an example may be physically, functionally and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. Although the concepts have been described in connection with some examples, it is not intended to be limited to the specific form set forth herein. Rather, the scope is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that variousAttorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final features of the described examples may be combined in other examples. In the claims, the term ‘comprising’ does not exclude the presence of other elements or steps.

[0059] , Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by, for example, dependent on the EMAT configuration.Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and / or advantageous. Also, the inclusion of a feature in one category of claims does not imply a limitation to this category, but rather indicates that the feature is equally applicable to other claim categories, as appropriate.

[0060] , In accordance with examples herein described, an EMAT assembly and a method of testing / inspecting a surface of the ferromagnetic material are provided, wherein the aforementioned disadvantages with prior art arrangements have been substantially alleviated.

Claims

Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_FinalClaims:

1. An electromagnetic acoustic transducer, EMAT, assembly (200, 300) comprising: an EMAT probe (212) comprising: a coil (262) arranged to produce eddy currents in an object under inspection; a permanent magnet (270) connected to the coil; and an EMAT probe position mechanism comprising: at least one actuator (232) operably coupled to the EMAT probe (212) and arranged to locate the EMAT probe onto or adjacent the object under inspection; at least one movable device (620) operably coupled to the at least one actuator and arranged to move the EMAT probe around the object under inspection; and wherein the at least one actuator (232) and the at least one movable device (620) in combination, are arranged to control: a magnetic field strength between the permanent magnet and the object under inspection; and a variable frictional force that is exerted on to the EMAT probe (212) as the EMAT probe (212) is guided over a surface of the object under inspection.

2. The EMAT assembly (200, 300) of Claim 1, wherein the at least one movable device (620) is arranged to operate in a first degree of freedom, DOF, with an axis that is parallel to surface of the object under inspection.

3. The EMAT assembly (200, 300) of Claim 2, wherein the at least one movable device (620) is operably coupled to an attachment point (202) that is arranged to additionally apply a second, rotary DOF to the at least one movable device (620).

4. The EMAT assembly (200, 300) of Claim 3, wherein the second, rotary DOF to the at least one movable device (620) comprises an axis that is perpendicular to the surface of the object under inspection.Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final5. The EMAT assembly (200, 300) of Claim 3 or Claim 4, wherein a distance, O, identifies a distance between a centre of the coil and the attachment point (202) that induces the second, rotary DOF to the at least one movable device (620), wherein the distance, O, is a value less than 500mm.

6. The EMAT assembly (200, 300) of any preceding Claim further comprising a sensor (614) operably coupled to the at least one actuator (232), wherein the sensor (614) is arranged to determine a lift-off distance, L, between the surface of the object under inspection and the coil (262) or a front plate (260, 360) attached to the coil (262) and provide the determination to a controller that controls the force that is exerted on to the EMAT probe (212) via the at least one actuator in response to the determination to adjust the lift-off distance, L.

7. The EMAT assembly (200, 300) of Claim 6, wherein the sensor is one of: a spring-loaded Linear Variable Differential Transformer, LVDT, sensor, an ultrasonic sensor, an infrared sensor.

8. The EMAT assembly (200, 300) of any of preceding Claims 1 to 6 wherein a lift-off distance, L, between the surface of the object under inspection and the coil (262) or a front plate (260, 360) attached to the coil (262) is determined based on a measurement of an EMAT signal amplitude and the determination is provided to a controller that controls the force that is exerted on to the EMAT probe (212) via the at least one actuator in response to the determination to adjust the lift-off distance, L.

9. The EMAT assembly (200, 300) of any of preceding Claims 1 to 6 wherein a lift-off distance, L, between the surface of the object under inspection and the coil (262) or a front plate (260, 360) attached to the coil (262) is determined based on a measurement of at least one impedance property of the coil and the determination is provided to a controller that controls the force that is exerted on to the EMAT probe (212) via the at least one actuator in response to the determination to adjust the lift-off distance, L.Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final10. The EMAT assembly (200, 300) of any of Claims 6 to 9, wherein the lift-off distance, L, of the coil (262, 362) is arranged to be between 0.01mm and 20mm.

11. The EMAT assembly (200, 300) of any of preceding Claims 6 to 10, wherein the coil comprises, or is operably coupled to, a front face and the lift-off distance, L is arranged to be a distance from the object under inspection that enables the front face to be cleaned of debris or dust.

12. The EMAT assembly (200, 300) of any of preceding Claims 6 to 11, wherein the lift-off distance, L, is adjusted so that a normal magnetic flux that passes through the coil or front face is <100mT.

13. The EMAT assembly (200, 300) of any preceding Claim, wherein the at least one actuator comprises a second actuator arranged to change a normal component of magnetic flux that passes through the coil by adjusting a relative distance between the coil and the permanent magnet of the EMAT assembly (200, 300).

14. The EMAT assembly (200, 300) of any preceding Claim, wherein the permanent magnet is located in a housing whereby the force is exerted on the housing to guide the permanent magnet over the surface of the object under inspection.

15. The EMAT assembly (200, 300) of Claim 14 when dependent upon Claim 13, wherein the second actuator and permanent magnet are enclosed inside the housing.

16. The EMAT assembly (200, 300) of any of preceding Claims 13 to 15, wherein the change in flux is achieved by placing two magnets into a flux guide of high magnetically permeability wherein the second actuator is arranged to change a relative position of a polarity of a first permanent magnet with respect to a polarity of a second permanent magnet.Attorney Docket No. SON 2024-001WO 28-Nov-2025 Specification_Final17. The EMAT assembly (200, 300) of any preceding Claim, wherein the attachment point exhibits a compliance C in the range of 0.01-1000 N / mm in any direction and (0.01-1000) * [1 / 0.0175] Nm / rad with respect to any rotation.

18. The EMAT assembly (200, 300) of any preceding Claim, wherein the EMAT assembly (200, 300) is configured with a wearproof, changeable cover ing / coating on surfaces that are in contact with the object under inspection.

19. The EMAT assembly (200, 300) of any preceding Claim, wherein the at least one movable device (620) is at least one wheel having a radius, W, between 1mm - 300mm.

20. The EMAT assembly (200, 300) of any preceding Claim, wherein the object under inspection comprises ferromagnetic material.

21. A method for moving an electromagnetic acoustic transducer, EMAT, probe across an object under inspection, wherein the EMAT probe comprises a permanent magnet (270) connected to a coil (262) arranged to produce eddy currents in an object under inspection, the method comprising: locating (910) the EMAT probe onto or adjacent the object under inspection by at least one actuator of an EMAT probe position mechanism; moving (920) the EMAT probe around the object under inspection using at least one movable device (620) operably coupled to the at least one actuator; and controlling (930), by the at least one actuator (232) and the at least one movable device (620) in combination, a magnetic field strength between the permanent magnet and the object under inspection and a variable frictional force that is exerted on to the EMAT probe (212) as the EMAT probe (212) is guided over a surface of the object under inspection.

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