Method and apparatus for non-destructively detecting defects in metal materials by using rotating magnetic fields generated by multiphase ac current

Method and apparatus for non-destructive inspection of flaws in metal materials include exciting coils centered on a longitudinal axis of a cylindrical or columnar metallic material to be examined being arranged to surround the examinee material. A multiphase alternating current of such a relatively low frequency as to cause the exciting coils to generate a rotating magnetic field which rotates about the longitudinal axis of the examinee material is superimposed with a high frequency alternating current having an inspection frequency selected on the basis of the necessary sensitivity for detection of surface flaws of the examinee material. The composite multiphase current is applied to the exciting coils so that an electromagnetic effect induced in the surface of the examinee material by the high frequency alternating current is rotated for shifting circumferentially about the examinee material along with the rotating magnetic field. A detecting element group disposed in the proximity of the surface of the examined material detects the changing of the electromagnetic effect, dependent on the presence or absence of a surface flaw of the examinee material.

TECHNICAL FIELD 
This invention relates to a method and apparatus for detection of defects 
in steel materials by using a rotating magnetic field and more 
particularly to a method and apparatus for the non-destructve inspection 
of flaws in a to be examined cylindrical or columnar metallic material by 
applying a rotating magnetic field which rotates around the outer 
circumferential surface of the to be examined material or examinee 
material. 
BACKGROUND ART 
Known as a method for non-destructive inspection of surface flaws or crack 
flaws in a cylindrical or columnar metallic material such as, for example, 
a steel bar or a steel pipe is an eddy current inspection method wherein a 
high frequency magnetic field of several KHz to several of tens of KHz is 
applied to a material to be examined so that the state of an eddy current 
generated in the surface of the examinee material is changed dependent on 
the presence or absence of a flaw; followed by a change in magnetic flux 
generated by the eddy current, and the change is detected by a detecting 
element such as a detection coil disposed in the proximity of the examinee 
material. In the case of the material to be examined being non-magnetic, a 
magnetic inspection method is known wherein a detecting element such as a 
Hall element or a detection coil disposed in the proximity of the material 
to be examined detects that the state of leakage magnetic flux generated 
at the surface of the examinee material by a high frequency magnetic field 
mentioned as above is changed dependent on the presence or absence of a 
flaw. Principles of these detection methods are disclosed in detail in 
"Industrial Instrumentation Handbook" published by Asakura Shoten, March, 
1982, pp. 633-641. 
When inspecting the entire circumference of the material to be examined in 
accordance with any of the above inspection methods, it is necessary to 
rotate the material to be examined while an inspection apparatus, 
comprised of an exciter for application of the magnetic field and the 
detecting element, is kept stationary; or it is necessary to rotate the 
inspection apparatus around the material to be examined while the examinee 
material is kept stationary. In either case, a rotation mechanism is 
required, therefore, increasing the overall size of the apparatus. 
Furthermore, the inspection is time-consuming. To solve these problems, a 
method is proposed in the specification of British Pat. No. 1,436,186, 
according to which a cylindrical exciter of a multiphase alternating 
current winding structure such as a stator of an induction motor is 
provided. A multiphase alternating current is passed through the exciter 
to generate a rotating magnetic field; and surface flaws of a material to 
be examined, which is inserted for passage through the exciter, are 
detected by a toroidal coil disposed in the proximity of the surface of 
the examinee material and surrounding the same. In this method, however, 
the multiphase alternating current to be passed through the exciter is of 
a single frequency as described in the aforementioned British Patent 
Specification; and this method is suitable for use with a low frequency. 
But if use with a high frequency, the core of the exciter mut be removed; 
alternatively the core must be made of a special material such as ferrite. 
When the low frequency magnetic field is used, sensitivity is degraded, 
and especially sensitivity to surface flaws of the material to be examined 
is seriously degraded and the detection becomes almost impossible. 
Especially where the high frequency is used with the core made of such a 
material as ferrite, an attendant increase in the number of revolutions of 
the rotating magnetic field gives rise to an increase in noise, thus 
preventing the frequency from increasing to an extent that practically 
sufficient sensitivity can be obtained. 
Accordingly, an object of this invention is to provide a method and 
apparatus for non-destructive inspection of flaws in metal materials which 
does not require relative mechanical rotation between a material to be 
examined and the inspection apparatus and which can exhibit sufficient 
sensitivity of detection of surface flaws of the material to be examined. 
SUMMARY OF INVENTION 
To accomplish the above object, according to the method and apparatus of 
the invention, exciting coils centered on a longitudinal axis of a 
cylindrical or columnar metallic material to be examined are arranged to 
surround the examinee material. A multiphase alternating current of such a 
relatively low frequency as to cause the exciting coils to generate a 
rotating magentic field which rotates about the longitudinal axis of the 
examinee material is superimposed with a high frequency alternating 
current, of an inspection frequency selected on the basis of necessary 
sensitivity for detection of surface flaws of the examinee material, and 
applied to the exciting coils so that an electromagnetic effect created at 
the surface of the examinee material by the high frequency alternating 
current is rotated for shifting circumferentially the examinee material 
along with the rotating magnetic field. And a detecting element group 
disposed in the proximity of the surface of the examinee material detects 
the electromagnetic effect changed by the presence of a surface flaw of 
the examinee material. 
In a more preferable mode of the invention, respective phases of the low 
frequency multiphase alternating current for generation of the rotating 
field are independently balance-modulated with the high frequency 
alternating current and applied to the exciting coils.

BEST MODE FOR CARRYING OUT THE INVENTION 
A first embodiment of the invention will be described with reference to 
FIG. 1. In FIG. 1, 1 designates a cylindrical or columnar metallic 
material to be examined. An exciting unit 2 for generating a rotating 
magnetic field which rotates about a longitudinal axis 0 of the material 1 
to be examined is so disposed as to surround the examinee material 1, and 
a sensor group 3 is disposed between examinee material 1 and exciting unit 
2 along the inner circumferential surface of the exciting unit 2. The 
exciting unit 2 has a similar construction to a stator of a multiphase 
induction motor and comprises, as shown in FIGS. 2A and 2B, a core 22 
supported by a cylindrical outer support 21, a multiphase winding 23 
fitted in slots formed in the inner circumferential surface of the core 
22, and an inner insulating cylinder 24 disposed interiorly of the 
multiphase winding. For simplicity of explanation, when describing an 
instance using a three-phase alternating current the winding 23 comprises 
three coils which are spaced at intervals of an electrical angle of 
2.pi./3 and under excitation using a three-phase alternating current at a 
frequency fHz, a rotating magnetic field of a revolution number R=120f/P 
(rpm) is generated, where P is the number of poles. 
The sensor group 3 comprised of a number of sensors 30 is arranged in line 
in the circumferential direction along the inner wall of the inner 
insulating cylinder 24. As illustrated in detail in FIGS. 3A and 3B, each 
sensor 30 is connected to a connector 34 by a lead wire 33. A group of 
connectors 34 are arranged on a disc having a larger diameter than that of 
the insulating cylinder 24 to assure suffcient spacing between elements. 
For the sensor, a magnetic detecting element such as a Hall element, a 
magnetoresistive element or a detection coil may satisfactorily be used; 
and the direction of flux to be detected may be either radial or 
tangential with respect to the outer circumference of the examinee 
material. FIGS. 4A and 4B illustrate the posture of a coil when a 
tangential component of a magnetic field is detected by using a detection 
coil 30 as the sensor. When detecting a radial component, the coil as 
illustrated in FIG. 4A is rotated through 90.degree. so that the coil 
surface may be directed tangentially. The smaller the detection coil, the 
higher the sensitivity becomes, and a detection coil of 6 to 8 mm diameter 
is typically used. 
Returning to FIG. 1, the winding of sensor group 3 of the exciting unit 2 
is applied with a composite current, which is obtained by synthesizing, by 
means of a waveform synthesizer 7, a three-phase alternating current 
generated from a three-phase alternating current power source 4, having a 
low frequency elected in accordance with a desired revolution number of 
the rotating magnetic field, and a high frequency alternating current 
generated from a high frequency power source 5, having a frequency 
suitably used as an inspection frequency. This composition may be effected 
in a manner of either addition or multiplication; and preferably, a 
composite wave as shown in FIG. 10 is used. The composite wave is obtained 
by balance-modulating the low frequency alternating current with the high 
frequency alternating current, as will be described later. Preferably, the 
frequency of the three-phase alternating current for generating the 
rotating magnetic field may be 1 to 1000 Hz when utilizing eddy currents 
and 1 to 900 Hz when utilizing leakage flux, and the inspection frequency 
may be 10 to 1000 KHz when utilizing eddy currents and 1 to 100 KHz when 
utilizing leakage flux. 
The exciting unit 2 responds to the composite alternating current to 
generate a rotating magnetic field which rotates circumferentially about a 
cross section of the examinee material, i.e., about the longitudinal axis 
0 at a rotating speed determined by the frequency of the low frequency 
alternating current. The sensor group 3 detects that an electromagnetic 
effect such as eddy current or leakage flux generated at the surface of 
the examinee material by the superimposed high frequency alternating 
current is changed dependent on the presence or absence of a surface flaw 
of the examinee material. Thus, respective sensors 30a, 30b, 30c are 
connected to a detector 8 whose output is recorded on a recorder 6. Since, 
in the presence of a surface flaw, the output of a sensor in association 
with the position of the surface flaw is changed, the presence and 
position of that flaw can be recognized from an output waveform recorded 
on the recorder 6. This will be explained by referring to FIG. 5. 
Referring to FIG. 5, the ordinate represents the sensors 30a, 30b, 30c, . . 
. arranged circumferentially about a cross section of the examinee 
material, the abscissa represents the longitudinal direction of the 
examinee material, and a hatched portion represents a sensor which detects 
an electromagnetic effect (eddy current or leakage flux) induced in the 
surface of the examinee material by the rotating magnetic field. Since the 
magnetic field generated by the exciting unit rotates at a predetermined 
period circumferentially about the cross section of the examinee material 
as described previously, the electromagnetic effect induced in the surface 
of the examinee material by this magnetic field becomes as if it were 
rotating circumferentially about the cross section of the examinee 
material; and accordingly, the position of a sensor which detects this 
electromagnetic effect shifts circumferentially about the cross section in 
synchronism with the roating speed of the magnetic field, as indicated at 
hatched portions in the same figure. Assuming now that a crack flaw 50 
exists in the surface of the examinee material as shown in the same 
figure, a flaw signal is obtained as indicated at 51 each time a sensor 
directly above the crack flaw 50 is activated. 
A modification of the first embodiment will be described with reference to 
FIG. 6. In FIG. 6, components like those of FIG. 1 are designated by 
identical characters. A detecting portion (a) of FIG. 6 is the same as the 
corresponding portion of FIG. 1. A transmitting circuit (b) has amplifiers 
7a which amplify respective phases of a composite current produced from 
the waveform synthesizer 7. A receiving circuit (c) resembles that of FIG. 
1 by comprising the recorder 6 but has a sampling circuit 9, a 
differential amplifier 1 and a synchronization controller 11 which 
substitutes for the detector 8 of FIG. 1. The synchronization controller 
11 outputs a pulse signal synchronous with the rotating frequency of the 
rotating magnetic field obtained from the waveform synthesizer 7. The 
sampling circuit 9 has a switch group (not shown) arranged in 
correspondence to individual sensors 30a, 30b, . . . of the sensor group 3 
and is opreable to connect an output of each sensor to the differential 
amplifier 10 through a corresponding switch, the individual switches being 
sequentially closed by the pulse signal from the synchronization 
controller 11 to sequentially sample the individual sensor outputs. Since 
an electromagnetic effect, for example, leakage flux induced at the 
surface of the examinee material by the rotating magnetic field also 
shifts circumferentially about the surface of the examinee material in 
synchronism with the rotating magnetic field, the phase of the pulse 
signal from the synchronization controller 11 is adjusted to ensure that a 
switch connected to a sensor at a position corresponding to the shifting 
leakage flux can be closed. This phase adjustment is carried out by 
providing a phase adjuster (not shown) in the synchronization controller 
11 and by adjusting the phase of the pulse signal by means of the phase 
shifter in such a manner that when the apparatus is operated with a 
material to be examined, which has a small flaw corresponding to a single 
detection coil scratched at a previously known location, inserted in the 
detecting portion (a), the output of the differential amplifier 10 becomes 
maximum. The differential amplifier 10 in receipt of the sensor output 
amplifies an output of a level in excess of a predetermined reference 
level and applies an amplified output to the recorder 6. 
A second embodiment of the invention will now be described with reference 
to FIG. 7. A detecting portion 70 is the same as that of FIG. 1. A 
transmitting circuit 71 comprises a two-phase alternating current 
generator 72 for generating a two-phase alternating current of a low 
frequency determined in accordance with a revolution number of the 
rotating magnetic field, and phase shifter circuits 72a and 72b for 
respective 240.degree. and 120.degree. phase shiftings of the output of 
the generator 72. The outputs of the 240.degree. and 120.degree. phase 
shifter circuits 72a and 72b and of the two-phase alternating current 
generator 72 are respectively applied to balanced modulators 73a, 73b and 
73c so as to balance-modulate a high frequency alternating current of 
several KHz to several of tens of KHz generated from an inspection 
frequency generator 80, thereby producing a three-phase alternating 
current of A phase, B phase and C phase. This three-phase alternating 
current is amplified by amplifiers 74a, 74b and 74c and applied to the 
three-phase winding of the detecting portion so that a rotating magnetic 
field superimposed with the high frequency alternating current of the high 
inspection frequency can be obtained. 
A sensor group 30 is constructed similarly to that of the first embodiment, 
wherein each of the sensors 30a, 30b, . . . 30n has one end grounded and 
the other output end connected to a respective phase detector 76a, 76b, . 
. . or 76n through a corresponding amplifier 75a, 75b, . . . or 75n. Each 
phase detector phase-detects an inputted sensor output by using as a 
reference wave the output of inspection frequency generator 80 which is 
phase-shifted through a phase shifter circuit 79 and generates a 
phase-detected output. 
In the absence of a material to be examined, the output voltage of each 
sensor is zero because no magnetic flux interlinks the coil. When a 
material to be examined whose circumferential surface is devoid of flaw 
and uniform is inserted, each sensor detects a uniform eddy current 
reaction signal but this signal, can be cancelled out by suitably setting 
the reference wave phase of phase detection, and can be prevented from 
developing in the phase detector. The phase shifter circuit 79 is 
phase-adjusted such that the output of the phase detector becomes zero 
when the examinee material has no flaw. 
With an examinee material has a flaw, the eddy current is locally disturbed 
and a sensor near that flaw produces an induced voltage which is different 
from that produced from another sensor in phase and amplitude. The induced 
voltage changes as the magnetic field rotates. Consequently, the 
phase-detected output originating from the induced voltage of that coil 
becomes a sine wave having the same period as that of the rotating 
magnetic field. And the amplitude of the output from a sensor most 
proximate to the flaw becomes maximum and the amplitude of the sensor 
output becomes smaller in proportion to deviation from the flaw position. 
This condition is illustrated in FIG. 11. In this figure, assuming that a 
sensor nch is situated directly above a flaw, (n-1)ch represents a coil 
next to the coil nch to the left, (n+1)ch reprsents a coil next to the 
coil nch to the right, (n-2)ch represents a coil next to the (n-1)ch to 
the left, (n+2)ch represents a coil next to the (n+1)ch to the right and 
so on. These waveforms correspond to the phase detector outputs which are 
passed through low-pass filters 77a, 77b, . . . 77n so as to be removed of 
the inspection frequency component. 
Outputs from the low-pass filters are sent to a band-pass filter 84 through 
electronic switches 78a, 78b, . . . 78n. Each of the switches 78a, 78b, . 
. . is constituted by, for example, a field effect transistor and opened 
or closed by an output from a ring counter 83. One phase of the 
three-phase alternating current for generating the rotating magnetic field 
is inputted to a phase shifter circuit 81, which phase-shifts the current 
to a desired phase within a range of from 0.degree. to 360.degree.. The 
current is then inputted to a frequency multiplier 82 of phase-locked 
type. Assuming that the number of the sensors 30a, 30b, . . . is n, the 
frequency multiplier 82 performs n multiplication and, accordingly, given 
the frequency of the three-phase alternating current being f, it outputs 
nf which in turn is supplied to an n-step (n-nary) ring counter 83. The 
ring counter 83 has n switch control outputs and hence each of the 
switches 78a, 78b, . . . is rendered on once during one period of the 
three-phase alternating current. The phase of the output from the phase 
shifter circuit 81 is so adjusted as to ensure that the output of the ring 
counter 83 can close a switch connected to a sensor situated at a position 
opposing a shifting eddy current generated in the surface of the examinee 
material by the rotating magnetic field. Each switch remains on during an 
interval of time equal to 1/n of the period of the three-phase alternating 
current, n being 120, for example. In this manner, the outputs of the 
individual sensors are sequentially sampled. In the absence of the 
examinee material, the outputs of the low-pass filters 77a, 77b . . . are 
zero and the outputs sampled by the switches 78a, 78b, . . . are also 
zero. Where the material to be examined having no flaw is inserted for 
loading, owing to eccentricity in the insertion as well as rotationally 
asymmetric shape and unevenness in quality of the material, there occurs 
an output of a relatively small amplitude having a period which is equal 
to or about half the period of the rotating magnetic field. The outputs 
sampled by the switches 78a, 78b, . . . are sent to the band-pass filter 
84 where they are removed of signals other than the flaw information, that 
is, a gradually varying signal due to the uneven quality and a spite 
signal attendant on switching. 
The output of the ring counter 83 is fedback to the frequency multiplier 82 
so that the final stage output pulse of the counter may be locked to a 
reference phase of the sinusoidal output from the phase shifter circuit 
81, for example, a phase of zero-cross point. 
The relation of the position of the rotating magnetic field relative to the 
sampling timing for the sensor output can be changed by effecting the 
phase shift by means of the phase shifter circuit 81, and it is possible 
to sample a sensor output at a position where a maximum flux density 
portion of the rotating magnetic field is operating or to sample a 
detection coil output at a position where a zero flux density portion is 
operating. 
FIG. 8 shows addition to FIG. 7 of cancellation voltage generator 85a, 85b, 
. . . and differential amplifiers 86a, 86b, . . . . These components 
function to cancel out inherent outputs to the set (noises) due to 
irregularity in magnetic pole and winding of the rotating magnetic field 
generator or irregularity in assemblage of the sensor group. More 
particularly, in the absence of the material to be examined, outputs of 
the phase detectors 76a, 76b, . . . should ideally be zero. But actually, 
there occur distorted wave outputs due to the irregularity described 
above. Specifically, the sensors are not uniform and the sinusoidal waves 
of the same period as that of the rotating magnetic field outputted from 
the low-pass filters 77a, 77b . . . have different peak values. 
Accordingly, these sinusoidal waves are sampled to generate irregular 
signals leading to noises. These signals having different peak values for 
the individual coils are cancelled out by means of the differential 
amplifiers 86a, 86b, . . . and cancellation voltage generators 85a, 85b, . 
. . . The cancellation voltage generator 85a, 85b, . . . generate a sine 
wave which is of the same period as that of the rotating magnetic field 
and of variable amplitude and phase, the amplitude and phase of the sine 
wave being made coincident with those of the outputs delivered out of the 
filters 77a, 77b, . . . in the absence of the examinee material to enable 
the sine wave to be cancelled out by means of the differential amplifiers 
86a, 86b, . . . The amplitude and phase can be adjusted manually or 
automatically. 
FIG. 9 shows an improvement on FIG. 8 capable of cancelling noises due to 
vibrations of the examinee material. Electronic switches 87a, 87b, . . . 
connected to the outputs of the differential amplifiers 86a, 86b, . . . , 
respectively, and a differential amplifier 88 constitute a means for this 
purpose, the switches 87a, 87b, . . . being controlled by the outputs of 
the ring counter 83 which are displaced by one from those for the switches 
78a, 78b, . . . . More particularly, the same output turns on the switches 
87a and 78b, the same output turns on the switches 87b and 78c, . . . and 
the same output turns on the switches 87n and 78n. The difference between 
outputs of the switches 78a, 78b, . . . and outputs of the switches 87a, 
87b, . . . is determined by the amplifier 88 and applied to the band-pass 
filter 84. 
In comparison with FIG. 8, the output from differential amplifier 88 of 
FIG. 9 will be described with reference to FIGS. 12A and 12B. The sensor 
outputs are as shown at a bar graph in FIG. 12A and these outputs are 
applied to the recorder in the case of FIG. 8. Since the differential 
amplifier 88 sequentially determines the difference between the two 
adjacent outputs A and B, (A - B) outputs are obtained as shown in FIG. 
12B. Considering that changes in coil outputs due to a vibration of the 
examinee material develop in adjacent two coils (in this example, 120 
coils are arranged per circumference and the coil spacing is 3.degree.) in 
the same amount, the changes are cancelled out through the above 
difference operation and a zero-cross point in the (A - B) outputs is 
considered to be representative of a flaw position. 
For generation of the rotating magnetic field, use is not to be limited to 
the three-phase alternating current, as arbitrary m coils may be arranged 
with 2.pi./m displacement to generate an m-phase alternating current for 
excitation. The present invention may also use such a multiphase 
alternating current system. 
INDUSTRIAL CAPABILITY 
In the present invention, since a rotating magnetic field generated by a 
low frequency multiphase alternating current and a high frequency 
alternating current at an inspection frequency is superimposed on the 
rotating magnetic field so that a surface flaw of a material to be 
examined may be detected by using an electromagnetic effect induced in the 
examinee material by the high frequency alternating current, optimum 
values of the revolution number of the rotating magnetic field and the 
inspection frequency can be selected independently of each other in 
accordance with a material of the examinee material, position and size of 
a flaw to be detected, thereby providing method and apparatus for 
non-destructive inspection of metal matrials which can detect a surface 
flaw of a metal material with high efficiency and high sensitivity.