Electromagnetic transducer

A magnet is used to create a static magnetic field adjacent to the surface of a conductive material. At least one conductor runs through the magnetic field. When a pulse of current flows through the conductor, an elastic wave is generated in the material. In a preferred embodiment, a row of individual permanent magnets is used to create a periodic magnetic field. One side of a coil is placed in the periodic magnetic field so that a sheet of current moves transverse to the magnetic field when a pulse of current flows through the coil. When the coil is oriented parallel to the row of magnets, a horizontal shear wave is generated in the material. When the coil is oriented transverse to the row of magnets, a Lamb wave is generated in the material.

BACKGROUND OF THE INVENTION 
A. Field of the Invention 
This invention relates to the field of nondestructive inspection and 
particularly to the field of transducers for use in ultrasonic inspection. 
B. Description of the Prior Art 
Elastic or ultrasonic waves are commonly used to inspect parts for cracks 
and other defects. Generally, a narrow beam of a longitudinal or a 
transverse type wave is injected into the part by a transducer which 
directly contacts the part or indirectly contacts the part through a 
transmitting medium such as water which contacts both the transducer and 
the part being tested. A piezoelectric crystal in the transducer is used 
to generate the elastic wave. 
More recently, non-contact type transducers have been developed which can 
generate elastic Lamb-type waves in an object of conductive material 
without any physical contact with the object. Non-contact transducers are 
particularly useful in applications where the transducer moves relative to 
the test object because it eliminates friction and wear of the transducer. 
One such non-contact transducer is the electromagnetic transducer 
described in U.S. Pat. No. 3,850,028. Such prior art non-contact 
transducer utilizes a meander coil placed within a static magnetic field 
to generate Lamb waves by means of a Lorentz force mechanism. 
The prior art meander coil electromagnetic transducer can generate only a 
Lamb type wave which fills the entire cross-section of the part. While 
Lamb waves can be advantageously used for inspecting parts such as large 
diameter pipelines (see for example, patent application No. 731,199), they 
have low radiation impedance and consequently require strong magnetic 
fields and high currents to obtain strong signals. For applications 
involving a small confined space, such as the inspection of tubes from the 
inside, it is difficult to provide the strong magnetic fields and high 
currents required to obtain strong signals using Lamb waves generated by 
meander coil transducers. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved noncontact 
transducer. 
It is an object of the invention to provide non-contact transducers which 
can generate horizontal shear waves or Lamb waves. 
It is an object of the invention to provide an electromagnetic transducer 
for generating torsional waves in tubes and cylinders. 
It is an object of the invention to provide an electromagnetic transducer 
having good efficiency at low frequencies. 
It is an object of the invention to provide a non-contact transducer whose 
efficiency can be increased without restricting its acoustic bandwidth. 
It is an object of the invention to provide a non-contact transducer which 
can be placed inside a tube to inspect the tube by transmitting and 
receiving elastic waves in the wall of the tube. 
According to the invention, a magnet is used to create a static magnetic 
field adjacent to the surface of a conductive material. At least one 
conductor runs through the magnetic field. When a pulse of current flows 
through the conductor, an elastic wave is generated in the material. In a 
preferred embodiment, a row of individual permanent magnets is used to 
create a periodic magnetic field. One side of a coil is placed in the 
periodic magnetic field so that a sheet of current moves transverse to the 
magnetic field when a pulse of current flows through the coil. When the 
coil is oriented parallel to the row of magnets, a horizontal shear wave 
is generated in the material. When the coil is oriented transverse to the 
row of magnets, a Lamb wave is generated in the material. 
Unlike prior art electromagnetic transducers, periodicity in the magnetic 
field is obtained by the design of the static magnetic field rather than 
by the use of a meander type coil to create periodicity in the alternating 
magnetic field created by the current flow. Further, at any given time, 
current in the coil exposed to the static magnetic field flows in the same 
direction rather than in opposing directions as in conventional coils. 
The transducer operates in a reciprocal manner as a receiver, generating 
electrical signals as elastic waves pass underneath it. 
These and other objects and features of the present invention will be 
apparent from the following detailed description, taken with reference to 
the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a transducer 2 for generating (or receiving) horizontal shear 
waves 4 in an object of conductive material such as metal test plate 6. 
Transducer 2 comprises a plurality of permanent magnets 7 abutting each 
other to form a row. Adjacent magnets in the row have opposite polarity, 
thus creating a periodic, static magnetic field adjacent to and in plate 
6. The period 8 of the static magnetic field is equal to the thickness of 
two abutting magnets. 
A coil 10 is wrapped around the complete row of magnets in the longitudinal 
direction so that a pulse of current can flow in the same direction in all 
the wires between magnets 7 and plate 6. Thus, according to the known 
Lorentz force mechanism, when a current flows in the wires of coil 10 
between magnets 7 and plate 6, an eddy current is induced in test plate 6 
and this eddy current interacts with the static magnetic field to produce 
horizontal shear forces 12 that launch ultrasonic waves 4 down plate 6. 
FIG. 2 shows a second embodiment of the invention used to generate (or 
receive) Lamb waves 13 (rather than the previously mentioned horizontal 
shear waves 4) in a test object 6. A plurality of permanent magnets 14 are 
stacked to form a row of magnets having alternating polarity and period 15 
in the same manner as described for horizontal shear wave transducers 2. 
However, in Lamb wave transducer 16, coil 18 is wound so that the wires 
run transverse to the length of the periodic, static magnetic field 
created by the row of magnets. 
Thus, when a signal is supplied to coil 18, a sheet of current flows in one 
direction in the periodic, static magnetic field adjacent metal plate 6. 
Such current induces an eddy current in plate 6 and this eddy current 
interacts with the static magnetic field to produce forces 20 which launch 
Lamb waves 13 down plate 6. 
FIG. 3 schematically shows a third embodiment of the invention used to 
generate torsional wave 22 in metal tube 24. Torsional wave 22 is 
basically a shear wave created in a cylindrical object and therefore 
torsional wave transducer 26 is wound similarly to shear transducer 2. 
Coil 28 extends along the length of the row of permanent magnets 30 and 
when current flows in coil 28 horizontal shear forces are created as shown 
by the arrows forming torsional wave 22. This wave will have a wavelength 
related to period 32. As shown in FIG. 3, the surface of magnets 30 that 
are used to establish the periodic magnetic field in tube 24 are curved to 
match the inside surface of the tube. 
A transmitting transducer which was constructed to inspect 7/8 inch 
diameter tubes having a wall thickness of 0.050 inch utilizes twelve 
samarium cobalt permanent magnets 30 capable of producing a periodic 2.4 
KG normal field. The magnets have a seven-sixteenths inch radius and about 
0.32 inch of arc length. Their thickness is 0.125 inch so that the period 
32 of the field is 0.25 inch. Coil 28 consists of 24 turns of #36 bifilar 
wire. A spring clip attached to the transducer allows it to pass through 
dented regions in the tube. 
A receiving transducer also used to inspect the seven-eigths inch diameter 
tubes is similar to the transmitting transducer except that only six 
magnets are used and the coil has 48 turns. The transmitter series 
resonant input impedance is approximately 8.OMEGA. with a Q of 3.3 at 0.5 
MHz. The receiver also has a Q of 3.3 with a parallel resistance of 
approximately 230.OMEGA. at 0.5 MHz. 
FIG. 4 shows a variation of a transducer for producing torsional waves in a 
tube. The magnets 34 in this transducer are curved on opposite sides to be 
concentric with diametrically opposite surfaces on the inside of the tube 
being inspected. The coil is wound so that the direction of the current is 
the same in the conductors on the upper surface 36 as it is on the lower 
surface 38. This transducer generates (or receives) a torsional wave 
having a period 40 from both the upper and lower surfaces. Consequently, 
the circumferential torsional wave generated by the transducer of FIG. 4 
is twice as strong as the circumferential torsional wave generated by the 
one-sided transducer of FIG. 3. 
The double-sided transducer of FIG. 4 can be wrapped with a coil in the 
manner shown in FIG. 1 so that current flows in opposite directions on 
diametrically opposed surfaces 36, 38. Such a transducer can excite the 
higher order circumferential torsional waves, but will not excite the 
lowest order circumferential wave. The transducers shown in FIGS. 3 and 4 
can excite both the higher order circumferential waves and the lowest 
order wave. 
A single conductor passing under the field created by either a single 
magnet or a periodic magnet can create an elastic wave in an adjacent 
conductive material according to the principles of the invention. However, 
the efficiency of a single conductor transducer is low because the 
efficiency of the transducer depends directly upon the number of 
conductors (turns in the coil) passing through the field. 
Likewise, a single magnet can be used in conjunction with a conductor to 
create an elastic wave in a conductive material. However, the wave 
launched will not have a particular frequency and can't be separated from 
other elastic waves. Therefore, it is preferred to use a static, periodic 
magnet field, as described above, and utilize an RF signal to drive the 
coil. For example, for thick metal plates the frequency of the RF signal 
used is matched to the periodicity of the static field according to the 
formula: 
EQU D = (V.sub.s /f sin .theta.), (1) 
wherein: D = the thickness of two adjacent magnets (8, 20, 32, and 40 of 
FIGS. 1-4), 
v.sub.s = the velocity of the shear wave in the material being tested, 
f = frequency of the RF signal, and 
.theta. = the angle between the surface normal and the shear wave beam 
radiated into the material. 
For thin plates, e.g. horizontally polarized plate modes, the periodicity 
of the static field is given by: 
EQU D = 1/.sqroot.(f/v.sub.2).sup.2 - (n/2t).sup.2 (2) 
where: n = mode index 0, 1, 2, etc., t = plate thickness 
Similarly, for a transversely wound coil, FIG. 2, used with a thick plate, 
a shear wave having a vertical component of polarization (sv wave) as well 
as a longitudinal wave can be excited. In such case, the periodicity D is 
given by equation (1) for the sv wave and by 
EQU D = (v.sub.1 /f sin .theta.) (3) 
for the longitudinal wave. In such case the relation for the Lamb wave in 
thin plates must be generated numerically according to known principles, 
see for example Acoustic Fields and Waves in Solids by B. A. Auld, (Wiley, 
1973). 
Increasing the quantity of adjacent magnets in the transducer increases the 
intensity or magnitude of the wave being generated and narrows its 
bandwidth. 
Means of generating, amplifying, and measuring RF waves are well known in 
the art of ultrasonic inspection. FIG. 5 shows a schematic arrangement of 
such known equipment for evaluating and using transducers 42, 44 according 
to the present invention. A transmitter 42 is placed on plate 6 and a 
receiver 44 is placed on the same plate 6 at a distance from the 
transmitter 42. A Matec RF pulse generator 46 is suitably power matched to 
the coil of transmitter 42. The frequency of the RF pulse generated by 
generator 46 is selected to match the period of the transmitter 42 and 
receiver 44 according to the previously mentioned formulas. 
A low noise amplifier 48 is suitably matched to the coil of receiver 44 and 
an oscilloscope 50 is driven by amplifier 48. When a pulse of RF power is 
sent by generator 46 to the coils of transmitter 42, an ultrasonic wave 52 
is created in plate 6, as previously described. This wave propagates 
through the material at a speed of v.sub.s according to known principles 
until it reaches receiver 44. At receiver 44, the wave creates electric 
signals in the receiver coil in a manner which is reciprocal to its 
generation. These signals are displayed in scope 50. 
Of course, a single transducer can be used as both a transmitter and as a 
receiver to locate defects in a material by reflection of the transmitted 
wave from the defect. Similarly, the transducers of the present invention 
can be used in other inspection systems that are analogous to conventional 
ultrasonic inspection systems using prior art piezoelectric and 
electromagnetic transducers. 
The static magnetic field can be created by means other than by the use of 
adjacent permanent magnets. For example, if every other magnet in the row 
is simply replaced by a non-magnetic material, a useable periodic magnetic 
field will be obtained from the spaced apart magnets of similarly oriented 
polarity. Or, a single horseshoe type magnet with a pole piece having a 
row of periodic projections between the poles of the magnet will create a 
periodic magnet field. In some applications, it may be advantageous to 
utilize electromagnets to provide the periodic magnetic field. 
In certain applications, the part being tested can itself be used (rather 
than a separate coil) as the conductor for passing the RF signal through 
the magnetic field. Additionally, curved transducers which mate with the 
outside of a pipe or cylinder can be used. Further, the surface of the 
magnets can be metallized to minimize inductance between the coil and the 
magnets. 
Numerous variations and modifications may be made without departing from 
the present invention. Accordingly, it should be clearly understood that 
the form of the present invention described above and shown in the 
accompanying drawings is illustrative only and is not intended to limit 
the scope of the present invention.