Patent Publication Number: US-2023160858-A1

Title: Electro-Magnetic Acoustic Transducer (EMAT) having Electromagnet Array for Generating Configurable Bias Magnetic Field Patterns

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/863,455 filed on Apr. 30, 2020, all of the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to electro-magnetic acoustic transducers (EMATs). 
     BACKGROUND 
     An electro-magnetic acoustic transducer (EMAT) is a transducer (i.e., sensor) for non-contact, acoustic (i.e., ultrasonic) wave generation and reception in metallic targets. EMATs are used for in-line inspection and non-destructive testing of metallic targets. 
     An EMAT requires a bias magnetic field to operate. Particularly, an EMAT requires a bias magnetic field having a particular pattern (i.e., shape, field lines, etc.) for the EMAT to transmit a corresponding type of ultrasonic wave. For instance, an EMAT requires a bias magnetic field having a certain pattern for the EMAT to transmit a Lamb wave, a bias magnetic field having a different pattern for the EMAT to transmit a shear-horizontal (SH) wave, a bias magnetic field having a different pattern for the EMAT to transmit a shear-bulk wave, etc. An EMAT typically includes permanent magnets fixed in a specific configuration (i.e., a fixed permanent magnet array) to generate a bias magnetic field having a given pattern for the EMAT to transmit the corresponding type of ultrasonic wave. 
     SUMMARY 
     Some embodiments of the invention can provide an electro-magnetic acoustic transducer (EMAT) system comprising an electromagnet array including an electromagnet with a magnetic core. The system can include a power supply connected to a wound coil wrapped around the magnetic core of the electromagnet. The system can also include a first bias magnetic field generated by the electromagnet array when the wound coil is energized with a first current from the power supply and a second bias magnetic field generated by the electromagnet array when the wound coil is energized with a second current from the power supply. In some embodiments, the power supply can include a direct current (DC) power supply and an alternating current (AC) power supply. In some embodiments, the system can also include a controller in communication with the power supply, wherein the controller is designed to control an output power operation including the first current and the second current used to energize the wound coil. In some embodiments, the magnetic core of the electromagnet can include a first pole and a second pole facing a common plane, with a square footprint, and/or with a rectangular footprint. In some embodiments, first current can be a positive electric current and the second current can be a negative electric current. In some embodiments, the magnetic core of the electromagnet can include a first pole and a second pole designed to conform to a curvature of a non-planar surface or form an arc segment conforming to a curvature of a non-planar surface. 
     Some embodiments of the invention can provide a method of controlling EMAT system. A power supply of the EMAT system can be used to energize a wound coil wrapped around a magnetic core of an electromagnet of an electromagnet array. The method can include providing a first current at a first time to the wound coil using the power supply and generating a first bias magnetic field based on the first current. The method can also include providing a second current at a second time to the wound coil using the power supply and generating a second bias magnetic field based on the second current. In some embodiments, the method can further include controlling the first current and the second current using a controller of the EMAT system. In some embodiments, a first and second ultrasonic wave can be generated based on the first and second bias magnetic field, respectively. In some embodiments, energizing the wound coil with the first current causes a first pole and a second pole of the magnetic core to have a checkerboard magnetic polarization pattern. In some embodiments, energizing the wound coil with the second current causes a first pole and a second pole of the magnetic core to have a non-checkerboard magnetic polarization pattern. In some embodiments, the first current is a negative current and the second current is a positive current. 
     Some embodiments provide an EMAT system including an electromagnetic array having one or more electromagnets arranged in rows, each including a magnetic core wrapped in a wound coil. Each magnetic core can also include a first pole and a second pole arranged adjacently in a row of the electromagnet. The system can also include a power supply designed to energize the wound coil and an electrical coil with a first leg and a second leg aligned relative to the first and second poles of the magnetic core or each of the one or more electromagnets. In some embodiments, the first leg of the electrical coil extends over the rows across the first poles of the electromagnets and the second leg of the electrical coil extends over the rows across the second poles of the electromagnets. In some embodiments, the first pole of the magnetic core is configured to have a north magnetic polarization when the power supply provides a first current and a south magnetic polarization when the power supply provides a second current, wherein the first current is opposite in polarity to the second current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a block diagram of an exemplary system having an electromagnetic acoustic transducer (EMAT) in accordance with embodiments for ultrasonic inspection of a pipe; 
         FIG.  2 A  illustrates a schematic diagram of an electromagnet of an electromagnet array of the EMAT, the electromagnet being a U-shaped electromagnet; 
         FIG.  2 B  illustrates a schematic diagram of the associated magnetic polarization of the electromagnet shown in  FIG.  2 A ; 
         FIG.  3 A  illustrates a perspective view of the electromagnet; 
         FIG.  3 B  illustrates a frontal view of the electromagnet; 
         FIG.  4    illustrates a perspective view of the EMAT with the electromagnet array, having a plurality of the electromagnets, arranged adjacent to one side of a printed circuit board (PCB) of the EMAT; 
         FIG.  5    illustrates a schematic diagram of the EMAT, the EMAT having electrical coils in addition to the electromagnet array and the PCB, the schematic diagram of  FIG.  5    illustrating the electrical coils on the PCB and the placement of the electrical coils relative to the electromagnet array; 
         FIG.  6 A  illustrates a schematic diagram of the EMAT in operation to transmit a shear-horizontal wave; 
         FIG.  6 B  illustrates a schematic diagram of an arrangement of the magnetic polarizations of the electromagnets for the electromagnet array to generate a bias magnetic field having a particular pattern required by the EMAT for the EMAT to use to transmit the shear-horizontal wave; 
         FIG.  6 C  illustrates an illustrative view of the magnetic polarizations of the electromagnets shown in  FIG.  6 B ; 
         FIG.  7 A  illustrates a schematic diagram of the EMAT in operation to transmit a Lamb wave; 
         FIG.  7 B  illustrates a schematic diagram of another arrangement of the magnetic polarizations of the electromagnets for the electromagnet array to generate a bias magnetic field having a different pattern required by the EMAT for the EMAT to use to transmit the Lamb wave; 
         FIG.  7 C  illustrates an illustrative view of the magnetic polarizations of the electromagnets shown in  FIG.  7 B ; 
         FIGS.  8 A,  8 B, and  8 C  illustrate schematic diagrams of the EMAT in operation to transmit shear-horizontal waves having different wavelengths; 
         FIGS.  9 A,  9 B, and  9 C  illustrate schematic diagrams of the EMAT in operation to transmit shear-bulk waves at different locations and/or with different beam widths; 
         FIGS.  10 A and  10 B  illustrate respective schematic diagrams of the EMAT in operation to transmit a shear-bulk wave and to receive any reflected-back portion of the shear-bulk wave; 
         FIG.  11 A  illustrates a perspective view of the electromagnet, the electromagnet being a single-cylinder core electromagnet in this variation; 
         FIG.  11 B  illustrates a perspective view of the electromagnet array having a plurality of the single-cylinder core electromagnets; and 
         FIG.  12    illustrates a schematic diagram of the EMAT, having the electromagnet array shown in  FIG.  11 B , in operation to transmit a Lamb wave. 
     
    
    
     DETAILED DESCRIPTION 
     Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     Referring now to  FIG.  1   , a block diagram of an exemplary system  10  having an electromagnetic acoustic transducer (EMAT)  16  in accordance with embodiments for ultrasonic inspection of a part to be inspected is shown. The part to be inspected is in the form of a pipe  12 . System  10  further includes a power supply  14  and a robot  18 . Power supply  14  is for powering EMAT  16 . Power supply  14  and EMAT  16  are mounted on robot  18 . Robot  18  can access (e.g., move throughout and physically against/near) pipe  12 . 
     EMAT  16  includes an electromagnet array having one or more electromagnets  17  (only one shown in  FIG.  1   ) and an electrical coil  19 . Each electromagnet has a magnetic core and a wound coil wrapped around the magnetic core (not shown in  FIG.  1   ). Power supply  14  is connected to the wound coils of electromagnets  17  and is connected to electrical coil  19 . More particularly, power supply  14  includes (i) a DC power supply for connected to the wound coils of electromagnets  17  for supplying DC power (i.e., 0 Hz frequency) to the wound coils of the electromagnets and (ii) an AC power supply connected to electrical coil  19  for supplying AC power (e.g., high-frequency) to the electrical coil. Power supply  14  selectively outputs electrical power to the wound coils to energize electromagnet array in a given manner to generate a bias magnetic field and outputs electrical power to electrical coil  19  which thereby cause EMAT  16  to transmit an ultrasound in pipe  12 . The ultrasound travels through pipe  12  and a portion of the ultrasound may reflect backward to EMAT  16 . 
     A controller  21  (e.g., an electronic processor such as a computer) in communication with EMAT  16  detects wall thickness, wall loss, and defects such as cracks of pipe  12  based on the transmitted and received ultrasound. Controller  21  is further in communication with power supply  14  to control its operation in outputting power to EMAT  16 . Controller  21  is further in communication with robot  18  to control its operation. 
     Referring now to  FIG.  2 A , a schematic diagram of one electromagnet  17  of the electromagnet array of EMAT  16  is shown. EMAT  16  is positioned near a part  12 , such as a metal plate, that is to be inspected by the EMAT. Electromagnet  17  is a U-shaped electromagnet having a U-shaped magnetic core  22 . Electromagnet  17  further includes a wound coil  24  wrapped around magnetic core  22 . Wound coil  24  is an electrically insulated wire that is wrapped around magnetic core one or more times. Magnetic core  22  includes first and second poles  26   a  and  26   b . In correspondence with the U-shape of magnetic core  22 , poles  26   a  and  26   b  face in the same direction toward to-be-inspected part  12 . 
     An electric current injected into wound coil  24  causes poles  26   a  and  26   b  to have opposite magnetic polarizations. Particularly, an electric current of one polarity injected into wound coil  24  causes poles  26   a  and  26   b  to respectively have north (“N”) and south (“S”) magnetic polarizations. In this case, first pole  26   a  has a N magnetic polarization and second pole  26   b  has a S magnetic polarization. Conversely, an electric current of an opposite polarity injected into wound coil  24  causes poles  26   a  and  26   b  to have S and N magnetic polarizations. In this case, first pole  26   a  has a S magnetic polarization and second pole  26   b  has a N magnetic polarization. 
     For instance, as shown in  FIG.  2 A , a negative DC electric current (i.e., an electric current of one polarity) injected into wound coil  24  causes poles  26   a  and  26   b  to respectively have N and S magnetic polarizations.  FIG.  2 B  illustrates a schematic diagram of the associated magnetic polarization of electromagnet  17  in this case. A positive DC electric current (i.e., an electric current of an opposite polarity) injected into wound coil  24  causes poles  26   a  and  26   b  to respectively have S and N magnetic polarizations. 
     As such, the orientation of the N-S magnetic polarizations of electromagnet  17  depends on the direction of the electric current flowing in wound coil  24  of the electromagnet. Thus, the magnetic polarizations of electromagnet can be swapped by changing the direction of the electric flowing in wound coil  24 . 
     Electromagnet  17  generates a magnetic field (i.e., a bias magnetic field) according to the polarizations of electromagnet  17 . To-be-inspected part  12  is magnetized by the magnetic field such that the top surface of the part has the simplified distribution shown in  FIG.  2 B . 
       FIGS.  3 A and  3 B  illustrate perspective and frontal views of electromagnet  17 . Of course, the two toroid windings illustrated in each of  FIGS.  3 A and  3 B  belong to the single wound coil  24  of electromagnet  17 . The toroid windings are always constructively enhancing the magnetic field along the U-shaped electromagnet. 
     Referring now to  FIG.  4   , with continual reference to  FIGS.  2 A,  2 B,  3 A, and  3 B , a perspective view of EMAT  16  is shown. The electromagnet array of EMAT is an electromagnet array  30  having a plurality of electromagnets  17 . EMAT  16  further includes a printed circuit board (PCB)  32 . Electromagnet array  30  is arranged adjacent to one side of PCB  32 . 
     Referring now to  FIG.  5   , with continual reference to  FIGS.  2 A,  2 B,  3 A,  3 B, and  4   , a schematic diagram of EMAT  16  is shown. Electromagnet array  30  is not in view in  FIG.  5    as the electromagnet array is underneath PCB  32  in this view. In like manner of  FIG.  2 B , grid pattern  34  designates the arrangement including the locations and magnetic polarizations of the poles of electromagnets  17  of electromagnet array  30 . 
     In addition to electromagnet array  30  and PCB  32 , EMAT  16  further includes first and second electrical coils  36  and  38 . Electrical coils  36  and  38  are on (i.e., a part of) PCB  32 . In embodiments, electrical coils  36  and/or  38  are looped coils and/or meander coils. The placement of electrical coils  36  and  38  relative to electromagnet array  30  (i.e., relative to grid pattern  34 ) is also shown in  FIG.  5   . 
     Electromagnet array  30  can create a configurable bias magnetic field arrangement for EMAT  16  using electromagnets  17 . The configuration of the bias magnetic field arrangement is composed by controlling the direction flowing in wound coils  24  of electromagnets  17 . Examples of magnetic field arrangements which allow a single, two electrical coil EMAT to generate specific ultrasonic waves will now be described with reference to the remaining Figures. 
     Referring now to  FIGS.  6 A,  6 B, and  6 C , with continual reference to  FIGS.  4  and  5   , the magnetic field arrangement for EMAT  16  to generate a shear-horizontal wave for transmission will be described.  FIG.  6 A  illustrates a schematic diagram of EMAT  16  in operation to transmit a shear-horizontal wave.  FIG.  6 B  illustrates a schematic diagram of an arrangement of the magnetic polarizations of electromagnets  17  for electromagnet array  30  to generate a bias magnetic field having a particular pattern required by EMAT  16  for the EMAT to use to transmit the shear-horizontal wave.  FIG.  6 C  illustrates an illustrative view of the magnetic polarizations of electromagnets  17  shown in  FIG.  6 B . 
     As noted above, grid pattern  34  designates the magnetic polarizations of poles  26   a  and  26   b  of electromagnets  17  of electromagnet array  30 . In this magnetic field arrangement for EMAT  16  to transmit a shear-horizontal wave, poles  26   a  and  26   b  of electromagnets have alternating magnetic polarizations (i.e., poles  26   a  have N magnetic polarization and poles  26   b  have S magnetic polarization) forming a checkerboard pattern. 
     Grid pattern  34  includes rows  40  and columns  42  consistent with the layout of the poles of electromagnets  17 . (It is to be understood that the terminology “row” and “column” are interchangeable; that is, row(s) and column(s) may actually be column(s) and row(s), respectively.) As shown in  FIG.  4   , electromagnet array  30  includes a 4×2 layout of electromagnets  17  and each electromagnet has two poles  26   a  and  26   b  (i.e., 4×4 layout of electromagnet poles). Thus, in this example, grid pattern  34  includes four rows  40  and four columns  42 . Each row/column pair (e.g., (x2, y1), (x1, y3), etc.) of grid pattern  34  defines a respective space of the grid. Poles  26   a  and  26   b  of each electromagnet  17  correspond to adjacent spaces (e.g., adjacent spaces in the same row) of grid pattern  34 . The magnetic polarizations of the poles of electromagnets  17  are in the out-of-page direction. 
     Further, the grids of grid pattern  34  have a square footprint which approximately corresponds to the perimeter of the poles of electromagnets  17 . The poles of electromagnets  17  may have square, rectangular, or circular footprints. This could mean, for instance, that the poles of some of electromagnets  17  have rectangular footprints and the poles of other ones of electromagnets  17  have square footprints. 
     Further, the poles of the magnetic cores of the electromagnets may be shaped to conform to a curvature of a non-planar surface or form an arc segment conforming to a curvature of a non- planar surface. As such, EMAT  16  may be shaped to conform to the curvature of an inner pipe wall or may be shaped to conform to an opposite curvature of an outer pipe wall. PCB  32  and electrical coils  36  and  38  are likewise shaped to conform to the curvature of the non-planar surface. EMAT  16  having its electromagnet array  30 , PCB  32 , and electrical coils  36  and  38  conformed to a curved surface provides minimal clearance between the EMAT and the curved surface and hence maximizes the strength of the electromagnetic field that interacts with the non-planar material. Ultimately, this results in the production of a stronger guided wave and higher signal-to-noise ratio. 
     Herein, as a convention, rows  40  of grid pattern  34  run along the x-direction and columns  42  run along the y-direction. Further, as described in greater detail herein, the y-direction is the wave propagation direction and the x-direction is the in-plane transverse direction. 
     In sum, electromagnet array  30  includes electromagnets  17  arranged in rows and columns with the poles of the electromagnets placed at corresponding row/column pairs. Rows of electromagnets are separated from neighboring rows of electromagnets along horizontal interfaces or boundaries. Similarly, columns of electromagnets are separated from neighboring columns of electromagnets along vertical interfaces or boundaries. 
     As shown in  FIG.  6 A , and consistent with  FIG.  5   , each electrical coil  36  and  38  has a pair of opposed long, straight legs  44   a  and  44   b  running in the x-direction across the entire electromagnet array  30 . The placement of first and second electrical coils  36  and  38  relative to electromagnet array  17  is such that the legs of the electrical coils are positioned along a center line of a respective column of poles of the electromagnets of the electromagnet array. Put another way, each legs of each electrical coil extends across the poles located in respective columns of grid pattern  34 . 
     As noted, in this magnetic field arrangement for EMAT  16  to transmit a shear-horizontal wave, poles  26   a  and  26   b  of electromagnets have alternating magnetic polarizations (i.e., poles  26   a  have N magnetic polarization and poles  26   b  have S magnetic polarization) forming a checkerboard pattern. With this magnetic polarization checkerboard pattern, electromagnet array  30  thereby generates the bias magnetic field having the requisite pattern enabling EMAT  16  to transmit the shear-horizontal wave. In turn, first and second electrical coils  36  and  38  are pulsed with alternating current of the same frequency, amplitude, and phase. The resultant ultrasonic wave transmitted by EMAT  16  is a shear-horizontal wave propagating in the y-direction. 
     Further, as indicated in  FIG.  6 A , the shear-horizontal wave has a wavelength which corresponds to the length in the y-direction of two rows of the poles of the electromagnets of electromagnet array  30 . 
     Referring now to  FIGS.  7 A,  7 B, and  7 C , with continual reference to  FIGS.  4 ,  5 ,  6 A,  6 B , and  6 C, the magnetic field arrangement for EMAT  16  to generate a Lamb wave for transmission will be described.  FIG.  7 A  illustrates a schematic diagram of EMAT  16  in operation to transmit a Lamb wave.  FIG.  7 B  illustrates a schematic diagram of the arrangement of the magnetic polarizations of electromagnets  17  for electromagnet array  30  to generate a bias magnetic field having a different pattern required by EMAT  16  for the EMAT to use to transmit the Lamb wave.  FIG.  7 C  illustrates an illustrative view of the magnetic polarizations of electromagnets  17  shown in  FIG.  7 B . 
     In the magnetic field arrangement for EMAT  16  to transmit a Lamb wave, each column of poles have the same magnetic polarization according to a N-S-S-N pattern, as indicated in  FIGS.  7 A,  7 B, and  7 C . As such, for first and second electromagnets  17  in each row, first pole  26   a  of the first electromagnet has a N magnetic polarization, second pole  26   b  of the first electromagnet has a S magnetic polarization, first pole  26   a  of the second electromagnet has a S magnetic polarization, and second pole  26   b  of the second electromagnet has a N magnetic polarization. With this magnetic polarization pattern, electromagnet array  30  thereby generates the bias magnetic field having the requisite pattern enabling EMAT  16  to transmit the Lamb wave. In turn, first and second electrical coils  36  and  38  are pulsed with alternating current of the same frequency, amplitude, and phase. The resultant ultrasonic wave transmitted by EMAT  16  is a Lamb wave propagating in the x-direction. 
     Further, as indicated in  FIG.  7 A , the Lamb wave has a wavelength which corresponds to the width in the x-direction of the columns of the poles of the electromagnets of electromagnet array  30 . 
     For both of the shear-horizontal wave and Lamb wave examples subject of  FIGS.  6 A,  6 B,  6 C,  7 A,  7 B, and  7 C , as well as for other ultrasonic wave transmissions, electromagnet array  30  may have more or less electromagnets  17  and/or EMAT may have just one electrical coil or more than two electrical coils. 
     Referring now to  FIGS.  8 A,  8 B, and  8 C , with continual reference to  FIG.  6 A , schematic diagrams of EMAT  16  in operation to transmit shear-horizontal waves having different wavelengths are shown. Compared with  FIG.  6 A , electromagnet array  30  in  FIGS.  8 A,  8 B, and  8 C  includes two additional rows of electromagnets (i.e., 6×2 array of electromagnets; or 6×4 array of electromagnet poles). 
     The magnetic polarization checkerboard pattern shown in  FIG.  8 A  is the same magnetic polarization checkerboard pattern shown in  FIG.  6 A  with the two additional rows of electromagnets. In  FIG.  8 B , the magnetic polarization checkerboard pattern is extended by an additional row compared with the magnetic polarization checkerboard pattern shown in  FIG.  8 A . In turn, the wavelength of the shear-horizontal wave transmitted by EMAT  16  according to the magnetic field arrangement of  FIG.  8 B  is twice as large as the wavelength of the shear-horizontal wave transmitted by EMAT  16  according to the magnetic field arrangement of  FIG.  8 A . In FIG.  8 C, the magnetic polarization checkerboard pattern is extended by an additional pair of rows compared with the magnetic polarization checkerboard pattern shown in  FIG.  8 B . In turn, the wavelength of the shear-horizontal wave transmitted by EMAT  16  according to the magnetic field arrangement of  FIG.  8 C  is three times as large as the wavelength of the shear-horizontal wave transmitted by EMAT  16  according to the magnetic field arrangement of  FIG.  8 A . 
       FIGS.  9 A,  9 B, and  9 C  illustrate schematic diagrams of EMAT  16  in operation to transmit shear-bulk waves at different locations and/or with different beam widths.  FIGS.  9 A,  9 B, and  9 C  depict a shear-bulk wave (single-element pulse-echo) application. In this condition, only part of electromagnet array  30  is active and only one electrical coil (e.g., only first electrical coil  36 ) is excited to provide a single point source. The resultant wave is a shear-bulk wave propagating in the z-direction. The combination of active electromagnets  17  and active electrical coil  36  can be used generate a wave at different locations, subject of  FIGS.  9 A and  9 B , and generate a wave with different beam widths, subject of  FIG.  9 C . 
     The magnetic field arrangements shown in  FIGS.  9 A,  9 B, and  9 C  can be modified to make a dual-element EMAT useable for thickness measurements. This is the subject of  FIGS.  10 A and  10 B , which illustrate respective schematic diagrams of EMAT  16  in operation to transmit a shear-bulk wave and to receive any reflected-back portion of the shear-bulk wave. In this condition, one electrical coil (e.g., first electrical coil  36 ) is used as the ultrasonic wave generator (Tx) and the other electrical coil (e.g., second electrical coil  38 ) is used for the receiver (Rx). The transmission and reception wave path in the to-be-inspected part  12  is shown in  FIG.  10 B . 
     In a variation, electromagnets  17  are single-cylinder core electromagnets. In this case, magnetic core  22  has a cylindrical shape as opposed to a U-shape.  FIG.  11 A  illustrates a perspective view of a single-cylinder core electromagnet.  FIG.  11 B  illustrates a perspective view of electromagnet array  30  having a plurality of single-cylinder core electromagnets  17 . As shown, each electromagnet  17  includes a wound coil  24  wrapped around cylindrical-shaped magnetic core  22 . Magnetic core  22  includes first and second poles at respective ends of the magnetic core. In correspondence with the cylindrical shape of magnetic core  22 , the first and second poles of electromagnets  17  face in opposite directions. In this case, only one of the poles (e.g., first pole  26   a ) of electromagnets  17  face in the same direction toward to-be-inspected part  12 . 
       FIG.  12    illustrates a schematic diagram of EMAT  16 , having electromagnet array  30  shown in  FIG.  11 B , in operation to transmit a Lamb wave. With the use of single-cylinder core electromagnets  17  in electromagnet array, the magnetic polarizations of first poles  26   a  of the electromagnets can all be the same (e.g., first poles  26   a  all have N magnetic polarization). As such, electromagnet array  17  has the magnetic polarization arrangement shown in  FIG.  12   . With this magnetic polarization arrangement, electromagnet array  30  thereby generates the bias magnetic field having the requisite pattern required by EMAT  16  for the EMAT to transmit a Lamb wave propagating in the +x and −x directions. In turn, first and second electrical coils  36  and  38  are pulsed with alternating current of the same frequency, amplitude, and phase. The resultant ultrasonic wave transmitted by EMAT  16  is a Lamb wave propagating in the +x and −x directions. 
     Further, as indicated in  FIG.  12   , the Lamb wave has a wavelength which is ½ the length of the wavelength of the Lamb wave transmitted by EMAT  16  according to the arrangement shown in  FIG.  7 A . 
     Of course, for EMATs in accordance with embodiments, the physical orientation of any of the individual electromagnets  17  in electromagnet array  30  can be rearranged for EMAT to generate different types of bias magnetic fields. One or more electromagnets  17  can be movable between first and second positions to change the physical orientation. In this regard, the EMAT assembly may further include an actuator(s) (not shown) to mechanically move electromagnets  17  between first and second positions. Similarly, the EMAT assembly may further include an actuator (not shown) to mechanically move PCB  32 , and thereby first and second electrical coils  36  and  38 , relative to electromagnet array  30 . 
     Further, as an exemplary application of the physical orientation of electromagnets  17  in electromagnet array  30 , electromagnet array  30  illustrated in  FIG.  11 B  comprising single-cylinder core electromagnets  17  can produce any magnetic polarization patterns shown in  FIGS.  6 A,  6 B,  6 C,  7 A,  7 B,  7 C,  9 A,  9 B, and  9 C  for the same ultrasonic waves to be generated. 
     As described, an EMAT in accordance with embodiments has an electromagnet array that is used for the EMAT to generate multiple types of ultrasonic waves. The electromagnet array provides a configurable pattern of magnetic fields for the EMAT. This allows a single EMAT to transmit different types of ultrasonic waves, including Lamb wave, shear-horizontal wave, and shear-bulk wave. 
     Further, EMATs in accordance with embodiments have been described herein in a transmit mode of operation. Of course, such EMATs may be used in a receive mode of operation. More descriptively, for an EMAT in accordance with embodiments, without changing the energizing pattern of electromagnet array  30  but using first and second electrical coils  36  and  38  as sensing (i.e., receiving) coils without pulsing alternating current, the EMAT functions as an ultrasonic wave receiver. As long as grid pattern  34  of the EMAT is the same for the transmitter and receiver, the transmitter and the receiver are corresponding to the same type of ultrasonic wave. This is due to the reciprocity of wave generation and reception. 
     As the EMAT has the ability to transmit different types of ultrasonic waves, the EMAT may be used in place of multiple EMATs each having the ability to transmit only one type of ultrasonic wave. As such, the function of multiple EMATs may be combined into just one EMAT in accordance with embodiments. This provides a reduction in size, weight, and cost. Benefits of an EMAT in accordance with embodiments may further include the ability to be used on smaller objects, an extended runtime in battery powered robotic deployment, and a lower economic threshold for potential applications. 
     Further, unlike a permanent magnet array, the electromagnet array of the EMAT in accordance with embodiments does not have the side effect of attraction and retention of ferromagnetic debris. Collected ferromagnetic debris can foul mechanisms and contaminate nearby electronics and the attraction and retention of ferromagnetic debris is particularly detrimental to robotic involvement. Any ferromagnetic debris collected by the electromagnet array can be released by turning off the electromagnets of the electromagnet array. 
     Potential users of the EMAT in accordance with embodiments include utility and gas companies with metallic containment vessels and metallic pipe distribution assets, government and commercial concerns with metallic ships and planes, and metal processing facilities. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the present invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the present invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the present invention.