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JP2003319493

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DESCRIPTION JP2003319493
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
electromagnetic ultrasonic transducer, and more particularly to an electromagnetic ultrasonic
transducer using an array of a plurality of strip magnets.
[0002]
2. Description of the Related Art An electromagnetic ultrasonic transducer (abbreviated as EMAT:
Electromagnetic Acoustic Transducer) is a device for transmitting and receiving ultrasonic waves
in a noncontact manner in metal by electromagnetic action. Roughly speaking, EMAT consists of
a magnet and a coil, and when it is brought close to metal and a high frequency current is
applied to the coil, an eddy current is excited on the metal surface, and this eddy current and the
static magnetic field by the magnet Mechanical vibration occurs in the vicinity of the metal
surface by Lorentz force induced in the metal due to the interaction between the two metals to
generate ultrasonic waves. Also, in this reverse process, mechanical vibrations propagating in the
metal can be converted into electrical signals and detected. EMAT is used, for example, for
ultrasonic flaw detection.
[0003]
One type of EMAT is PPM (Period Permanent Magnet) type EMAT. In the PPM type EMAT, as
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shown in FIG. 18, as a static magnetic field source, strip-like permanent magnets 110 whose
upper and lower surfaces are N and S pole surfaces are laterally reversed while alternately
reversing the polarity. Use a superimposed array 100. The two arrays 100 are arranged in
parallel on the respective straight portions of the spiral coil 120 respectively. This PPM type
EMAT is used as a means for transmitting and receiving a surface SH (Shear Horizontal) wave to
a plate material, as also shown in the column of "Prior Art" in JP-A-11-125622.
[0004]
The generation mechanism of the surface SH wave in the PPM type EMAT illustrated in FIG. 18
will be described with reference to FIG. FIG. 19 is a view of the PPM EMAT shown in FIG. 18 in
the lateral direction (direction shown by arrow A in FIG. 18), where the horizontal direction in the
figure is the x direction, the vertical direction is the y direction, A three-dimensional coordinate
system is set with a positive direction as the z direction.
[0005]
As shown in FIG. 19, the PPM EMAT is installed so that the spiral coil 120 abuts on the surface of
the object 130 which is a conductive plate material. Each permanent magnet 110 of the array
100 forms a magnetic field B which is perpendicular to the surface of the object 130 and works
alternately in opposite directions corresponding to the orientation of the magnetic poles of each
permanent magnet 110. On the other hand, when a high frequency current I (ω) (ω is an
angular frequency) flows through the spiral coil 120, an eddy current J (ω) parallel to the
direction of I (ω) is induced on the surface of the object 130. The interaction between the
magnetic field B and the eddy current J (ω) generates a Lorentz force F which is parallel to the
surface of the object 130 and perpendicular to both the magnetic field B and the eddy current J
(ω). In addition, to explain the mark indicating the Lorentz force F, the mark with a black dot at
the center of the circle indicates a force directed from the back to the front perpendicular to the
paper of the figure, and the mark with an x in the circle It shows the force from the near side to
the far side perpendicular to the paper of the figure. Also, the size of the circle indicates the
magnitude of the force.
[0006]
The direction of the Lorentz force F is reversed at the pitch of the width of the permanent
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magnet 130 in the x direction. Although FIG. 19 shows the magnetic field B, the eddy current J,
and the Lorentz force F in separate stages in order to avoid complexity of the drawing, these are
actually for the same position in the object 130.
[0007]
According to such a mechanism, by inputting a burst wave of a high frequency current whose
wavelength of the surface SH wave and the period of the Lorentz force F coincide with each other
to the spiral coil 120, it is possible to transmit and receive the surface SH wave with high
strength.
[0008]
SUMMARY OF THE INVENTION In the conventional PPM type EMAT described above, the striplike permanent magnet 110 is thin (that is, the width in the x direction is small) in order to
transmit and receive surface SH waves of high frequency due to its structure. There is a need to.
However, it is difficult to process a permanent magnet thinly, and if it is too thin, the magnetic
force weakens, so there is a limit to the thinness that can be put to practical use. Therefore,
conventionally, it has been extremely difficult to realize a PPM type EMAT capable of
transmitting and receiving surface SH waves having a frequency of 1.5 MHz or more.
[0009]
The present invention has been made in view of such problems, and it is an object of the present
invention to provide an electromagnetic ultrasonic transducer capable of transmitting and
receiving surface SH waves having a frequency higher than that of the prior art.
[0010]
SUMMARY OF THE INVENTION The present invention is an electromagnetic ultrasonic
transducer for performing at least one of transmission and reception of an ultrasonic wave to an
object by electromagnetic action in a state in which a predetermined action surface is in
proximity to the object. A magnet array in which a plurality of strip-shaped magnets are arranged
along the arrangement direction parallel to the working surface, and adjacent strip-shaped
magnets are arranged such that magnetic pole faces of different polarities face each other An
electromagnetic ultrasonic transducer including a magnet array and a flat coil disposed between
the magnet array and the working surface in parallel with the working surface.
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[0011]
In a preferred aspect of the present invention, the coil is a spiral coil, and the magnet array is
positioned with respect to the spiral coil such that the arrangement direction coincides with the
direction in which the spiral coil wire extends. It features.
[0012]
Further, in another preferred aspect of the present invention, the coil is a spiral coil, and the
magnet array is mounted on the spiral coil so that the arrangement direction coincides with the
direction orthogonal to the extending direction of the spiral coil. It is characterized in that it is
positioned relative to it.
[0013]
Further, in another preferred aspect of the present invention, the magnet array is configured so
that a component perpendicular to the surface of the magnetic field formed in the vicinity of the
surface in the object is inverted at predetermined intervals along the arrangement direction. The
spacing between the adjacent strip-like magnets is set, and the coil is a meander line coil
meandering with a period of twice the predetermined spacing, and the arrangement direction of
the magnet array is the meander line It is disposed to be perpendicular to the linear direction of
the coil.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present
invention (hereinafter referred to as embodiments) will be described based on the drawings.
[0015]
FIG. 1 is a view schematically showing a main part configuration of an electromagnetic ultrasonic
transducer (EMAT) according to the present invention.
[0016]
As shown in this figure, the EMAT of this embodiment is the conventional one shown in FIG. 18
and FIG. 19 in that a strip-like (thin rectangular parallelepiped) permanent magnet is
superimposed and arranged as means for forming a magnetic field. It is the same as PPM type
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EMAT.
The present embodiment differs from the above-described conventional configuration in that
permanent magnets 13 whose pole faces are on the both sides, that is, faces on the side of the
array direction of the magnet array 11 (ie, the x direction) face each other. It is a point arranged
to do.
That is, the array 11 is formed in an array such that the surface of the N pole and the surface of
the S pole of the adjacent permanent magnets 13 face each other.
A nonmagnetic spacer 15 is provided between the permanent magnets 13 adjacent to each other
to set the distance between the opposing magnetic pole surfaces.
[0017]
In the EMAT of this embodiment, as shown in FIG. 2, such a magnet array 11 is aligned on the
linear portion 17 a of the spiral coil 17 with the arrangement direction of the permanent
magnets 13 aligned with the direction of the conducting wire of the linear portion 17 a To be
arranged.
In FIG. 2, illustration of the spacer 15 is omitted to avoid complexity.
Further, in FIG. 2, the magnet array 11 is disposed only in one of the two linear portions 17 a in
the spiral coil 17, but the magnet array 11 is disposed in both as in the conventional
configuration shown in FIG. 18. Of course, it is possible to set it up.
In this case, the orientations of the two magnet arrays 11 are reversed with respect to the
arrangement direction.
[0018]
Although only the main part of the EMAT is shown in FIGS. 1 and 2, in an actual probe, this main
part is accommodated in the case together with the necessary circuit configuration.
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[0019]
When transmitting and receiving ultrasonic waves to and from the object using this EMAT, the
lower surface of the spiral coil 17 (that is, the surface opposite to the surface on which the
magnet array 11 is disposed) is brought close to the object.
That is, the lower surface side becomes an action surface (probe surface) of the transmission and
/ or reception action of the ultrasonic wave by EMAT.
[0020]
FIG. 3 is a diagram showing a configuration example of an inspection system using this EMAT.
In this system, the EMAT probe 10 is a probe incorporating the EMAT configured as shown in
FIGS. 1 and 2. The transmission circuit 22 generates a burst signal of a high frequency pulse
based on a command from the controller 20 that performs transmission control, and supplies the
burst signal to the spiral coil 17 in the EMAT probe 10. Due to the input of the high frequency
pulse, an ultrasonic wave is transmitted from the EMAT to the object to be inspected. When this
transmission ultrasonic wave is reflected in the object and returns to the EMAT, a current is
induced in the spiral coil 17 in the reverse process of the transmission time. This current is
amplified by the preamplifier 24, and after being subjected to amplification, filtering and the like
by the receiving circuit 26, is input to the signal processing device 28. The signal processing
device 28 executes predetermined signal processing and arithmetic processing for inspection
based on the reception signal input from the reception circuit 26.
[0021]
In this EMAT structure, the surface SH waves can be transmitted and received by appropriately
setting the distance between the permanent magnets 13 (that is, the width d of the spacer 15).
The mechanism of transmission and reception of this surface SH wave will be described with
reference to FIGS. 4 and 5. FIG. 4 is a view schematically showing magnetic lines of force in a
structure in which magnetic pole faces of different polarities are arranged opposite to each other
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by lines with arrows, and FIG. 5 shows the configuration of the EMAT shown in FIG. FIG. 2 is a
view showing a state viewed from the z direction). In FIG. 5, the magnetic field B by the magnet
array 11, the eddy current J (ω) induced by the high frequency current I (ω) in the coil 17, and
the Lorentz force F are shown separately in different stages. As with FIG. 19 of this, this is for the
convenience of illustration, in fact they are for the same position in the object 30.
[0022]
As shown in FIG. 4, most of the magnetic lines of force are almost straight toward the N pole face
of a permanent magnet 13 and the S pole face of the permanent magnet 13 opposite thereto, but
from near the periphery of the N pole face of the permanent magnet 13 The exiting magnetic
field lines enter the opposite S-pole surface in a path that expands outward of the magnet array
11. There are also magnetic lines of force that wrap around from near the N pole of the same
permanent magnet 13 to near the S pole.
[0023]
From this, by adjusting the distance between the adjacent permanent magnets 13 (that is, the
width of the spacer 15), as shown in FIG. 5, in the portion near the surface of the object 30 of
ultrasonic transmission / reception, It can be seen that the magnetic field B can be formed such
that the direction of the y-direction component of the magnetic field is alternately reversed, at
substantially equal pitches along the arrangement direction (x-direction). The spacer width d
suitable for forming the magnetic field B whose direction is reversed at equal intervals in this
way varies depending on the magnetic flux density and dimensions of the permanent magnet 13,
the material of the spacer 15, etc. Can. Generally speaking, the preferred spacer width d is equal
to or less than the width D of the permanent magnet 13 in the x direction. Therefore, the pitch of
reversal of the y-direction component of the magnetic field B is equal to or less than the width D
of the permanent magnet 13. From this, the change in the y direction component of the magnetic
field B illustrated in FIG. 5 is similar to the change in the magnetic field near the object surface
when using the conventional EMAT shown in FIG. 19 as a pattern, If the thinness (width D) of the
permanent magnet 13 is the same, it is understood that the pitch of the change is about half or
less of that of the conventional EMAT.
[0024]
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Thus, when the high frequency current I (ω) is caused to flow through the coil 17 by the EMAT
using the magnet array 11 configured by arranging the permanent magnets 13 in this manner,
the eddy current J (ω) is induced in the object 30. Ru. Then, due to the interaction between the
eddy current J (ω) and the magnetic field B by the magnet array 11, the mechanism similar to
that of the conventional EMAT is parallel to the surface of the object 130 against both the
magnetic field B and the eddy current I. A vertically oriented Lorentz force F is generated. The
pitch of the change of the Lorentz force F in the x direction is equal to the pitch of the change of
the magnetic field B, and the Lorentz force F is alternately reversed in direction at this pitch. Due
to the Lorentz force F, the vicinity of the surface of the object 30 vibrates to generate a surface
SH wave. This is the transmission mechanism of surface SH waves. Reception can be performed
in the reverse process.
[0025]
In this configuration, when the width D in the x direction of the permanent magnet 13 is the
same as in the conventional PPM EMAT shown in FIG. 19, the pitch of the change of the
magnetic field B near the surface of the object 30 (x direction Inversion period) can be about half.
As a result, in the PPM type EMAT of the conventional configuration, an effect close to that in
which the width D of the permanent magnet is about half can be expected. That is, when
permanent magnets 13 having the same width D are used, it is possible to transmit and receive
surface SH waves having a frequency twice as high as that of the conventional configuration.
Therefore, according to the configuration of the present embodiment, it is possible to transmit
and receive surface SH waves having a frequency higher than that of the conventional
configuration.
[0026]
Next, an experimental example of EMAT adopting the structure shown above will be described. In
this experimental example, as the permanent magnet 13, the width D (the length in the x
direction) is 0.9 mm and the height is made of a neodymium-based trade name NEOMAX
(manufactured by Sumitomo Special Metals Co., Ltd .: magnetic flux density 1.143T). A length
(length in the y direction) of 15 mm and a depth (length in the z direction) of 10 mm were used.
The width d of the spacer 15 is 0.6 mm (see FIG. 1). The dimension of width D = 0.9 mm is a
value close to the limit of thinness that can be processed with this material at present. Further, in
this experimental example, a magnet array 11 using five permanent magnets 13 is used.
Moreover, the spiral coil 17 was created using the enameled wire of diameter 0.1 mm. In
addition, a plate of an aluminum alloy (Al2017-T3) with a thickness of 8 mm was used as the
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target object 30 to be subjected to ultrasonic transmission / reception.
[0027]
FIG. 6 shows a graph of the measurement result of the magnetic field in the vicinity of the
surface of the object 30 when the working surface of the EMAT of this configuration is brought
close to the object 30 of the conductive material. The solid line graph shows the change of the
value of the x direction component Hx of the magnetic field in the x direction, and the broken
line graph shows the change of the value of the y direction component Hy of the magnetic field
in the x direction. As can be seen from this figure, the y-direction component Hy of the magnetic
field contributing to the generation of the surface SH wave is alternately reversed in direction at
a pitch close to the width of the permanent magnet in the x direction. Therefore, it is understood
that the surface SH wave can be transmitted and received by the above-described mechanism. In
this experimental example, by inputting a signal having a frequency of 2 MHz to the spiral coil
17, it was possible to excite a surface SH wave of about 2 MHz.
[0028]
Also, the peak value of the magnetic field strength shown in this graph is about twice the value of
the magnet array of the conventional structure (see FIG. 19) using permanent magnets of the
same magnetic flux density and the same size. There is. As described above, according to the
magnet array structure of the present embodiment, since a stronger magnetic field can be
formed than in the conventional case, the transmission output can be higher than in the
conventional aspect in ultrasonic transmission, and in the conventional aspect in ultrasonic
reception. Reception sensitivity can be increased. In the experiment, it was possible to output an
ultrasonic wave with a sound pressure of about 4 to 5 times that of the conventional structure.
[0029]
FIG. 7 shows that a burst signal consisting of five sine waves of 2 MHz in frequency is input to
the EMAT of this example to generate surface SH waves on the object 30, and it is received by
the EMAT of the same structure at a distance of 70 mm. FIG. 6 is a diagram showing received
waveforms. Here, a RAM 10000 manufactured by RITEC Corporation was used as a transmission
circuit of EMAT. Also, in the reception system, RITEC PAT-0.1-20 was used as a preamplifier, and
RITEC BR640 was used as a reception circuit, and the waveform of the output signal of this
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reception circuit was obtained with a digital oscilloscope IWATSU-LeCoy LT342. In this
experimental example, the received signal was sampled at a sampling frequency of 500 MHz, and
a waveform obtained by averaging the waveforms of the sampling results for 250 times was
determined as a received wave system. In this figure, the horizontal axis represents time, and the
vertical axis represents the amplitude of the received signal.
[0030]
In this graph, a relatively large waveform of the surface SH wave appears between 20 and 30
μs. Such a large waveform can not be detected when using a PPM type EMAT of a conventional
configuration using permanent magnets of the same magnetic flux density and the same size.
[0031]
As described above, according to this embodiment, it is possible to transmit and / or receive
surface SH waves having a frequency higher than that of the conventional PPM type EMAT. In
addition, higher transmission power and higher reception sensitivity than in the prior art can be
realized.
[0032]
As described above, in the EMAT of the present embodiment, it has been shown that transmission
and reception of the surface SH wave can be performed by appropriately setting the width and
the like of the spacer 15 of the magnet array 11.
[0033]
Next, it will be described with reference to FIG. 8 that in the present embodiment, by adjusting
the width of the spacer 15, an EMAT capable of transmitting and receiving the transverse wave
of the volume wave to the object 30 can be configured.
The configuration of FIG. 8 is the same as the configuration illustrated in FIG. 5 except for the
spacer distance d.
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[0034]
In this example, the distance between the adjacent permanent magnets 13 (that is, the spacer
width d) is made smaller than in the configuration of transmitting and receiving the surface SH
wave shown in FIG. Thereby, the distribution of the magnetic field formed near the surface of the
object 30 changes. Roughly speaking, in the example of FIG. 5 described above, the magnetic
field of the same strength is matched in a change pattern in which the magnetic field is
alternately reversed at approximately equal intervals, while the change period (interval) is
reduced by reducing the spacer width d. Will be larger than that.
[0035]
In the example of FIG. 8, the period of change of the y-direction component of the magnetic field
in the vicinity of the surface of the object 30 is about twice the array length of the magnet array
11 (in this example, the array 11 is composed of five permanent magnets). The spacer width d is
set to be as follows. In this case, the magnitude of the y-direction component of the magnetic
field in the vicinity of the surface of the object 30 is maximum at both ends of the magnet array
11 in the x-direction (the arrangement direction of the magnets). The interaction between such a
magnetic field B and the eddy current J (ω) induced by the high frequency current applied to the
coil 17 causes the Lorentz force F to fluctuate at a period of about twice the array length along
the x direction. It is excited. Such a pattern of Lorentz force F induces a transverse wave as a
volume wave in the object 30.
[0036]
An experimental example of the configuration of FIG. 8 will be described with reference to FIG. 9
and FIG. In this experiment, except that the spacer width d of EMAT is 0.1 mm, EMAT having the
same configuration as the above-described experiment of surface SH wave is used, and the
configuration of the experiment system is basically the same as the above-described experiment.
It is.
[0037]
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FIG. 9 shows a graph of the measurement result of the magnetic field in the vicinity of the
surface of the object 30 when the working surface of the EMAT of this configuration is brought
close to the object 30 of the conductive material. It can be seen from this figure that the ydirection component Hy of the magnetic field has the pattern shown in FIG.
[0038]
FIG. 10 shows a received waveform when an ultrasonic wave (transverse wave) is transmitted to
the object 30 by the EMAT of this configuration, and multiple reflections of the ultrasonic wave
in the object 30 are received by the same EMAT. From this graph, it can be seen that multiple
reflections are detected at intervals of about 5 μs. The velocity of sound of this shear wave was
about 3140 m / s.
[0039]
Thus, in the EMAT configuration of the present embodiment, the transverse wave of the volume
wave can be transmitted and / or received by reducing the spacer width d.
[0040]
When the spacer width d is set to a size intermediate between the example of FIG. 5 and the
example of FIG. 8, the period of change of the y direction component of the magnetic field along
the x direction is the same as that of the example of FIG. It has an intermediate length.
In this case, the ultrasonic wave that can be induced into the object 30 by EMAT is an
intermediate wave that includes both the surface SH wave component and the shear component
of the volume wave. As the overall tendency, the smaller the spacer width d, the stronger the
transverse wave component of the volume wave, and conversely, the larger the spacer width d,
the relatively stronger the surface SH wave component. However, if the spacer width d is made
larger than the width D of the permanent magnet 13, the magnetic field between the adjacent
permanent magnets 13 becomes weak, and a sufficiently large y-direction magnetic field can not
be formed in the object 30. The width d is preferably equal to or less than the width D of the
permanent magnet 13.
[0041]
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Next, a modification of the present embodiment will be described. FIG. 11 is a view schematically
showing the configuration of the EMAT of this modification. The EMAT of this modification is a
modification of the EMAT configuration shown in FIGS. 1 and 2, in which the arrangement of the
magnet array 11 is rotated by 90 degrees in the plane of the spiral coil 17. In the EMAT of this
modification, longitudinal waves of SV (Shear Vertical) waves and volume waves can be
transmitted and received by adjusting the width d of the spacer 15 appropriately.
[0042]
FIG. 12 is a diagram for explaining a transmission mechanism of SV waves by the EMAT of this
modification. This figure shows a cross-sectional view of a portion of the straight portion 17 a of
the spiral coil 17 in a plane perpendicular to the conducting wire. By setting the width d of the
spacer 15 to an appropriate value, as shown in FIG. 12, the component of the magnetic field in
the x direction (arrangement direction of the permanent magnets 13) is the arrangement pitch of
the permanent magnets 13 near the surface of the object 30. An alternating magnetic field B can
be formed with an approximately half pitch of. For example, in the magnet array 11 (spacer
width d = 0.6 mm) mentioned in the experimental example of the surface SH wave described
above, the x-direction component Hx of the magnetic field has a pattern shown by a solid line in
FIG. It is close to the pattern shown in.
[0043]
In the EMAT having such a configuration, when a high frequency current I (ω) is supplied to the
spiral coil 17, an eddy current J (ω) in the z direction is induced in the vicinity of the surface of
the object 30, as shown. Due to the interaction between the eddy current J (ω) and the magnetic
field B, a Lorentz force F is generated which is reversed at the same pitch as the reversal pitch of
the magnetic field B in the direction perpendicular to the surface of the object 30 as shown. An
SV wave is induced in the object 30 by this Lorentz force. Also, SV waves can be received in the
reverse process.
[0044]
The transmission and reception mechanism of the SV wave has been described above. On the
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other hand, it is also possible to transmit and receive longitudinal waves of volume waves by
reducing the spacer width d. The mechanism of this longitudinal wave transmission and
reception will be described with reference to FIG. As shown in this figure, by setting the spacer
width d to an appropriate value smaller than the example of FIG. 12, the x-direction component
of the magnetic field is in the same direction and near the surface of the object 30 as shown in
FIG. A magnetic field B whose intensity changes periodically can be formed. For example, in the
magnet array 11 (spacer width d = 0.1 mm) mentioned in the experimental example of the abovementioned volume wave transverse wave, the x direction component Hx of the magnetic field has
a pattern shown by a solid line in FIG. It is close to the pattern of the magnetic field B shown in
FIG.
[0045]
In the EMAT having such a configuration, when a high frequency current I (ω) is supplied to the
spiral coil 17, an eddy current J (ω) in the z direction is induced in the vicinity of the surface of
the object 30, as shown. Due to the interaction between the eddy current J (ω) and the magnetic
field B, a Lorentz force F whose direction is perpendicular to the surface of the object 30 and
whose strength changes periodically in the x direction is generated as illustrated. The Lorentz
force F generates a periodic density pattern in the x direction in the vicinity of the surface of the
object 30, thereby generating a longitudinal wave. Also, longitudinal waves can be received in the
reverse process. FIG. 14 shows a reception waveform when transmitting the ultrasonic wave of
the longitudinal wave to the object 30 by the EMAT and receiving multiple reflections in the
object 30 by the same EMAT. From this graph, it can be seen that multiple reflections are
detected at intervals of about 3 μs. The velocity of sound of this shear wave was about 6360 m /
s.
[0046]
Further, according to the EMAT configured as shown in FIG. 11, it is possible to transmit and
receive Rayleigh waves and Lamb waves.
[0047]
First, the transmission / reception mechanism of the Rayleigh wave will be described with
reference to FIG.
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FIG. 15 is a partially enlarged view of FIG. However, while FIG. 12 shows only the x-direction
component of the magnetic field B, FIG. 15 shows the magnetic field B as a vector in the xy plane
in which the y-direction component is also taken into consideration.
[0048]
It can be understood from the example of the distribution of Hx and Hy shown in FIG. 6 that the
magnetic field B can be made to have such a pattern in the EMAT illustrated in FIG. The Lorentz
force F excited by such a magnetic field B and the eddy current J (ω) exhibits a change pattern
as shown. The change pattern of this Lorentz force F is similar to the amplitude distribution of
the Rayleigh wave. Therefore, in the EMAT configured as shown in FIG. 11, by appropriately
setting the spacer width d and the frequency of the input signal to the spiral coil 17, the
component of the Rayleigh wave among the vibrations excited in the object 30 is strengthened. It
can be adjusted. The input signal in this case may be a burst wave in which the wavelength of the
Rayleigh wave is set to be the same as the period of the change pattern of the Lorentz force F.
The appropriate spacer width d and the frequency of the input signal depend on various
parameters such as the magnetic flux density and dimensions of the permanent magnet 13, the
material of the spacer 15, and the material of the object 30, but it is possible to specify by
experiments etc. . The reception of the Rayleigh wave is possible in the reverse process.
[0049]
Further, in the case where the EMAT having the configuration shown in FIG. 11 is used, when the
object 30 is a very thin plate (for example, the plate thickness is 1 wavelength or less of the
sound wave to be excited), It can generate waves. Reception is possible in the reverse process. In
this case, in order to be able to transmit the Lamb wave efficiently, it is necessary to set the
spacer width d and the frequency of the input signal to the spiral coil 17 appropriately, but this
can be determined by experiment or the like.
[0050]
Moreover, as another modification using the magnet array 11 shown in FIG. 1 etc., the structure
combined with a meander line coil (serpentine coil) 18 as shown in FIG. 16 is also possible. In
this configuration, the positional relationship between the two is set so that the direction of the
linear portion 18 a of the conducting wire that constitutes the meander line coil 18 is
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perpendicular to the arrangement direction of the permanent magnets 13 of the magnet array
11. Here, if the spacer width d is set appropriately, a magnetic field B in which magnetic field
components in the x direction as shown in FIG. 17 are alternately reversed at a constant cycle
can be formed near the surface of the object 30. For example, in the magnet array mentioned in
the experimental example of the surface SH wave, as shown in FIG. 6 and FIG. 12, a relatively
strong magnetic field B satisfying the conditions can be formed. Then, as shown in FIG. 17, by
making the meandering pattern of the meander line coil 18 coincident with the reversal pattern
of the magnetic field B, the direction and the magnitude of the induced Lorentz force F of the
magnet array 11 are obtained. It can be aligned almost uniformly over almost the entire array
direction. Then, the magnitude of the Lorentz force F generated in each portion in the
arrangement direction changes in unison at almost the same time according to the change of the
high frequency current input to the coil 18, so that a strong longitudinal wave can be generated
as a whole. Reception can be realized in the reverse process.
[0051]
As described above, in the EMAT of this embodiment, transmission and reception of various
types of ultrasonic waves are possible.
[0052]
In the above example, the case where the ultrasonic wave is excited in the object 30 by the
Lorentz force F has been described.
In the case where the object 30 is a nonmagnetic material such as aluminum exemplified as the
experimental example, only the Lorentz force F is generated, so that description may be made.
On the other hand, when the object 30 is a ferromagnetic substance such as iron, in addition to
the Lorentz force F, a magnetostrictive force is also generated in the vicinity of the surface of the
object portion 30. However, since the magnetostrictive force exhibits the same change as the
Lorentz force F in the arrangement direction of the permanent magnets 13, it is a generation
source of ultrasonic waves. Therefore, when the object 30 is a ferromagnetic body, the sum of
the Lorentz force F and the magnetostrictive force is the ultrasonic source. In particular, when
the direction of the Lorentz force F coincides with the direction of the magnetostrictive force,
strong ultrasonic waves can be transmitted. In the experiment, an ultrasonic wave of 4 to 5 times
as strong as that of the conventional PPM type EMAT can be transmitted to the ferromagnetic
material. However, in the configuration for transmitting and receiving the longitudinal waves of
the volume waves shown in FIG. 13, the directions of the Lorentz force F and the
magnetostrictive force are opposite to each other, so the efficiency of the ultrasonic wave
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transmission is not good.
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