JPS63132151A - Surface defect detector - Google Patents

Abstract

PURPOSE:To improve the uniformity of a current distribution in a measurement area and to monitor the progressive state of a surface defect with high accuracy by forming plural couples of feed terminals so that they can be displaced, and providing a terminal position control part which displaces and positions the terminal couples according to the current distribution inside the terminal couples. CONSTITUTION:The plural feed terminal couples 3 are displaceable and displaced by the terminal position control part 6. When a current is supplied to the terminal couples 3 under the command of a computer 7 through a constant current source 9 and a polarity converting device 8, the current distribution is generated inside. The current distribution is measured through a potential difference measurement terminal 4, a scanner 10, and a voltmeter 11 and inputted to and processed by the computer 7. The terminal couples 3 are moved with a control signal from the control part 6 according to the result to uniform the current distribution in the measurement area. In such a state, signals from terminals 4 are processed by the computer 7 to detect whether or not there is a surface defect and its shape.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a non-destructive inspection technique for detecting cracks generated in metal structural members, and particularly to a surface defect detection device for detecting the shape of a surface defect.

[Prior art and its problems]

Conventionally, a defect detection device on the surface of a member using the potential method uses the so-called four-terminal method (Japanese Patent Publication No. 50-25
This four-terminal method uses a flaw detector in which a pair of power supply terminals and a pair of potential difference measuring terminals are arranged in a line as a whole inside the power supply terminal, and is used to detect metal structural members. This method scans the surface of the sensor and measures changes in potential distribution, thereby detecting surface defects such as cracks. That is, the potential difference in a defect-free area is set as a reference potential, and it is determined that a defect exists where the potential is higher than this.

In large metal structures, the lifespan does not end immediately when a surface defect occurs; in many cases, the structure can be used for a considerable period of time even after the occurrence of a surface defect. Sometimes it is longer than the period until the defect occurs. Therefore, monitoring the progress of surface defects is important to enable long-term effective use of structures and to ensure safety.

However, since conventional devices have one pair of power supply terminals, the uniformity of current distribution is poor due to changes in the shape of members in the measurement area, making it difficult to determine the reference potential during measurement.
Furthermore, there is a problem in that the shape of the surface defect is inaccurately determined, and as a result, the progress of the surface defect cannot be accurately monitored. Furthermore, there is a problem in that the current spread makes it susceptible to shape changes in the vicinity of the measurement area and to nearby defects.

SUMMARY OF THE INVENTION An object of the present invention is to provide a surface defect detection device that can improve the uniformity of current distribution in a measurement region and thereby monitor the progress of surface defects with high accuracy.

[Means/effects for solving the problem] The present invention provides a plurality of pairs of power supply terminals that are displaceable and displaces the pair of power supply terminals based on the current distribution inside the pair of power supply terminals. By providing a terminal position control unit for positioning, the terminal position control unit receives signals of nonuniform current distribution caused by changes in the shape of the measurement area, etc., and adjusts the positions of the power supply terminal pairs individually to cancel out the nonuniformity. This outputs a control signal to change the current distribution, thereby achieving uniformity of the current distribution.

In addition, by forming a plurality of power supply terminal pairs and providing a current control unit that changes the current supplied to each power supply terminal pair based on the current distribution inside the power supply terminal pair, uneven signals of current distribution can be suppressed. The current control unit outputs a control signal that is received by the current control unit and changes the current value supplied to each power supply terminal pair so as to cancel out the non-uniformity, thereby achieving uniformity of the current distribution.

Further, a plurality of pairs of power supply terminals are formed, and a pair of potential difference measurement terminals is formed to be variable, and only a region where the current distribution inside the pair of power supply terminals is substantially uniform is defined as a measurement scanning region of the pair of potential difference measurement terminals. By providing a control section, defects can be automatically detected only in areas where the potential distribution is uniform.

〔Example〕.

FIG. 1 shows an overall configuration diagram of a surface defect detection device including the invention described in claim 1. In the figure, 1 is a sensor head, and the sensor head 1 detects surface defects of a structural member 2. It is something to detect. This sensor head 1 includes a plurality of pairs of power supply terminals 3 that supply a constant current to a structural member 2, a pair of potential difference measurement terminals 4 that measure the potential distribution inside the power supply terminal pairs 3 that changes depending on the presence or absence or shape of a surface defect, and each of these terminal pairs. It consists of a sensor jig 5 that holds a pair of terminals.

During measurement, the sensor head 1 is pressed against the structural member 2 .

The power supply terminal pair 3 held by the sensor jig 5 is formed to be displaceable, and this displacement is performed by a control signal from the terminal position control section 6. This terminal position control section 6 is connected to a computer 7 via an interface. The power supply terminal pair 3 is a polarity converter 8 for removing thermoelectromotive force.
It is connected to constant current g9 via. Constant current source 9 is connected to computer 7. Further, the potential difference measuring terminal pair 4 is connected to a high-resolution micro voltmeter 11 via a scanner 10 so as to be able to measure the potential at multiple points, and this voltmeter 11 is connected to a computer 7 via an interface to process the measurement data. It is connected to the.

When an arbitrary current is supplied from the constant current source 9 to the power supply terminal pair 4 according to a command from the computer 7, a corresponding current distribution is generated inside the terminal pair 4 depending on the mutual arrangement of the terminal pairs 4. This current distribution is measured as a potential distribution by the potential difference measuring terminal pair 3, and the measured potential difference signal is input to the computer 7 and processed. A control signal is output to move the pair of power supply terminals 3 so that the measurement area is approximately uniform. Either the standard material is the same as the structural member with no surface defects, or the data is analyzed using computer analysis based on the shape of the actually measured structural member and the arrangement of the power supply terminals.The latter method is extremely effective in that no standard material is required. It has the effect of being easy to perform.

After completing the preparation stage to make the current distribution approximately uniform,
The measured potential difference signal of the actually measured structural member is input to the computer 7, but unlike the preparation stage, this signal is processed by a defect determination section (not shown) built in the computer 7 to determine whether there is a surface defect or not. The shape can be detected.

Furthermore, the detection results are displayed on an output device (not shown).

The sensor head 1 is attached to a drive device 12, making it possible to scan the entire structure.This embodiment shows a drive device for measuring cracks on the inner surface of a pipe.The structural member 2 of the sensor head 1
The pressing is performed by a pressing means 13 such as an air cylinder. The sensor head 1 is pivotally supported by a rotating shaft 14 so as to be able to scan in the circumferential direction of the inner circumference of the pipe. This rotating shaft 14 is connected to a motor 15. belt 1
It is rotated by a 6° pulley 17. Further, movement in the axial direction within the pipe is carried out by a motor 17. Zero drive device made up of rollers 18 and connecting belts 19! 12 is controlled by a drive control section 20, and this drive control section 20 is connected to the computer 7.

A 16-bit microcomputer system is used as the computer 7, and a printer and a 5-inch floppy disk are used as the output device and recording device for the detection results. A peripheral interface adapter (PIA) is used for the drive control section 20 and the terminal position control section 6, and a GP-IB interface is used for the voltmeter 11 and constant current source 9, and input/output with the computer 7 is performed. Ta.

FIG. 2 is an enlarged plan view of the sensor jig shown in FIG. 1. The movable structure of the power supply terminal pair 3 is that among the three terminal pairs, the center is fixed and the both ends are
and a movable power supply terminal pair 3b that is movable in the Y-axis direction. That is, the movable power supply terminal pair 3b consists of "levers 21 and 22 connected in a rectangular shape, and the levers 21.2
2, threaded shafts 23, 24 that are screwed together, and motors 25, 26 that rotate the shafts 23, 24. The motors 25 and 26 are connected to and controlled by the terminal position control unit 6, and are operated so that the power supply terminals are moved to the optimal arrangement analyzed by the computer 7, so that a uniform current field can be obtained. In addition, power supply terminal pair 3
Of course, all of them may be used as movable terminal pairs.

FIG. 3 is a configuration diagram showing another embodiment of the above invention, in which a large number of terminals 27 are arranged in a matrix on the sensor jig 5, and the terminals 27 selected from among all the terminals 27 as the power supply terminal pair are changed. It is made to be able to be displaced. Even if each terminal 27 is fixed, if a different combination of terminals for power feeding is selected, the terminals 27 will be displaced as a result. Some of the remaining terminals in the central group become a pair of potential difference measurement terminals. That is, each terminal 27 is connected to the matrix scanner 28, and the arrangement of the power supply terminal pair and the potential difference measurement terminal pair is the optimum arrangement analyzed by the computer 7. According to this embodiment, the arrangement of each terminal pair is can be freely selected, the range of application is wide and the structure is simple.

FIG. 4 is a configuration diagram showing an embodiment of the second invention as set forth in claim 3. In this embodiment, the power supply terminal pair 3 has seven
It consists of a pair, and unlike the first invention, its position is not variable but fixed.

The power supply terminal pairs 3 are each independently connected to a constant current source 9, and the current value of each terminal pair 3 can be individually varied by a control signal from a current control section (not shown) built into the computer 7. It has become. The W flow control section detects the current distribution generated inside the power supply terminal pair 3 due to the arbitrarily supplied current using the potential difference measuring terminal pair 4 as in the first invention, and determines a substantially uniform current based on the measurement signal. The current control section outputs a control signal so as to have a distribution. As a result, a substantially uniform current field is formed in the measurement area, and the preparation stage is completed, allowing the actual measurement stage to proceed.

FIG. 5 shows a flowchart for detecting surface defects in a structural member using the apparatus according to the present invention. This flowchart shows a case where both the first and second inventions are applied and one can be selected and used. .

Furthermore, instead of using standard materials to form a uniform current field in the preparation stage, preparation is performed by computer analysis by inputting data such as the shape of the structural member to be measured and data regarding the arrangement of the li current value power supply terminal pair. The case where a uniform current field is formed in stages is shown.

Optimization of placement of power supply terminal pairs (first invention)> Step 29
Input data on the shape of the measurement area of the structural member to be actually measured. This shape has the greatest effect on current distribution.

Input other items other than defects that affect the current distribution as appropriate. In step 30, optimization of the arrangement of power supply terminal pairs is selected.

Normally, this selection is more desirable; in step 31, the current value to be supplied to each power supply terminal pair 3 is input.

This current value is the same for all three terminal pairs. In step 32, data on the initial setting arrangement of the power supply terminal pair 3 is input. This results in a certain current distribution in the measurement area. This current distribution is analyzed by computer in step 33. That is, the current distribution is measured as a potential distribution by the potential difference measuring terminal pair 4, and the measured signal is input to the computer 7, and FEM analysis or simple analysis using a theoretical formula is executed.

In step 34, it is determined from the analysis result whether the uniformity of the current distribution is within or outside the allowable range. If it is outside the range, change the arrangement data of the power supply terminal pair 3 in step 35, and then proceed to step 33.
Return to , analyze the current distribution again, and repeat the same procedure until a uniform current distribution within the allowable range is finally obtained. In step 36, the supply f on the sensor jig 5 is
The positioning of the t-terminal pair 3 is completed.

A uniform current distribution can be obtained in the above preparation stages, and step 3
7 and subsequent steps, actual surface defects are detected.

First, in step 37, the same current as the current value input in step 31 is supplied to the positioned power supply terminal pair 3. In step 38, the potential difference measuring terminal pair 4 is scanned to measure the potential distribution in the measurement area. In step 39, the defect determining section determines the presence or absence and shape of defects on the surface of the member based on the measured potential difference signal.

Here, if the potential distribution is uniform, it means that there is no surface defect in that region, and if there is a non-uniform portion such as a peak, it means that there is a surface defect at the corresponding position of the structural member. In addition, the shape of the defect is determined by comparing it with a master curve etc. determined in advance. After completion of the first determination, in step 40, the drive unit
12, the sensor head 1 is moved to another measurement area.

If the data such as the shape of the measurement area of the structural member does not change, the same procedure is repeated from step 37. If there is a change in shape, etc., the process returns from step 41 to step 29, and the same procedure is repeated from the beginning.If the sensor head 1 is located outside the measurement range, the process ends.

Optimization of Supply Current (Second Invention)> Step 29 remains unchanged, and optimization of supply current is selected in Step 30. In step 42, the arrangement data of the power supply terminal pair 3 is input. This arrangement is fixed. In step 43, the initial setting supply current value is input. This results in
A certain current distribution occurs in the measurement area. This current distribution is analyzed by computer in step 44. The analysis method is the same as that described in detail in the first invention. When it is determined in step 45 that the uniformity of the current distribution is outside the allowable range, the supplied current value is changed in step 46, the process returns to step 44, and the same procedure is repeated until a uniform current distribution is finally obtained. Repeatedly, in step 47, a uniform current distribution is obtained in one or more preparatory stages in which the power supply is set in accordance with the final current value supplied to the power supply terminal pair 3, and in step 37 and onwards, the same current distribution as above is obtained. The procedure is used to detect actual surface defects.

Next, the present invention will be explained in detail.

The finite element method can be used to analyze current fields, but since the flow of current in a uniform continuum corresponds to electric charges and lines of electric force, it can be determined analytically. That is, the current density is replaced by the surface charge at the desired coordinates. Therefore, it is determined by the following basic formula.

Engineering → q ... (1) I:
Current, q:! load, x1: coordinate, ixl: xi acid component current density, ffKt? Xi component ^ Prayer density By incorporating a program using this basic formula into the computer 7, a simple current field analysis can be performed. In addition, in the case of a particularly complicated shape, the finite element method is used to find it.

The analysis results are explained below.

FIG. 6(a) shows a layout diagram of the terminal pair 3 in the conventional four-terminal method when there is one pair of power supply terminals 3, and FIG. 6(b) shows a current distribution diagram inside the power supply terminal pair 3. be. If the magnitude of the current at the center is 1, it decreases to 35% at the negative position of the distance between the terminal pairs from the center.

Figure 7 (a) is a layout diagram when four pairs of power supply terminal pairs 3 are provided adjacently at equal intervals, and Figure 7 (b) is an illustration of the inner current when the same current is supplied to each terminal pair 3. This is the figure on the right. The improvement is 78% at the negative position of the distance between the terminals from the center.

Figure 8 is a summary of the results in Figures 6 and 7.
FIG. 3 is a relationship diagram between the number n of power supply terminal pairs 3 and current distribution. n=
1, that is, it can be seen that the current distribution is flattened by providing a plurality of pairs of power supply terminals 3 compared to the case where there is only one pair of conventional power supply terminals 3. When n=4 or more, interference between terminals is also improved.

In Fig. 9(a), the distance 2Q between the power supply terminal pair 3 is 1.5 (
This is a layout diagram for the case where n=0.75 from the center, and (b) of the same figure is a corresponding current distribution diagram.

In Figure 10(a), the distance between the terminal pair is 2Ω, which is 1.0 (1=0
．． 6), and FIG. 6(b) is a corresponding current distribution diagram. FIG. 11 is a summary of the results shown in FIGS. 9 and 10.

It can be seen that the smaller the distance between the power supply terminals, the better the uniformity of the current distribution. However, if the irises become too small, mutual interference between adjacent terminals will decrease, and uniformity will worsen. This is determined by the relationship with the adjacent distance m between the pair of power feeding terminals 3, and as a result of various experiments, it was confirmed that uniformity is poor when m>a.

FIG. 12(a) is a layout diagram when the distance n' between the pair of power supply terminals 3 at both ends is made smaller than the distance n between the pair of terminals 3 at the center, and FIG. 12(b) is a corresponding current distribution diagram. here 0
=1, and Q' =0.8. Figure 13(a) is Ω
This is a layout diagram when ' = 0.7, and the same figure (b)
is the corresponding current distribution diagram. FIG. 14 summarizes the results of FIGS. 12 and 13. Q' =0.7.1
When 2=1.0゜m=0.66, the current distribution is -1≦
It can be seen that there is only a 5% variation in the range of X≦1, and it is almost uniform (Figure 12 (b)).If L, rudder, and 4' are made too small, the influence of the power supply terminal pair at both ends becomes large. Therefore, the uniformity is poor (Figs. 13 and 14). When there are 4 pairs of power supply terminals and 3m = 2.2 = 3-, Ω' =
It can be seen that the uniformity is best when the value is 0.8 to 0.9.

Figure 15(a) shows the current (0,5I) supplied to the inner terminal pair than the current CI supplied to both ends of power supply terminal pair 3.
This is a layout diagram in the case where is made small, and the diagram (b) is a corresponding current distribution diagram. In Figure 16(a), the current supplied to the inner terminal pair is further reduced (0,3I).
This is a layout diagram for the case L, and FIG. 3(b) is a corresponding current distribution diagram. If the difference in the supplied current values is 50% (Figure 15), the current distribution will fluctuate by 7%, but if the difference is increased to 70% (Figure 16), the current density at the center will decrease, resulting in a change in the current distribution. It can be seen that the uniformity deteriorates. FIG. 17 shows a summary of these results.

As the principle has been explained above, it can be seen that the current distribution can be made uniform by forming multiple pairs of power supply terminal pairs 3 and changing the arrangement of each terminal pair, or by changing the supplied current value to each terminal pair individually. . Based on this, even if the shapes of the structural members etc. differ, it is possible to cancel out the non-uniformity of the current distribution due to the difference and make it uniform by changing the arrangement of the power supply terminal pairs as described above.

FIG. 18 is a configuration diagram showing an embodiment of the third invention as set forth in claim 4. The power supply terminal pair 3 consists of three pairs,
Its arrangement is fixed, and the supplied current value is also fixed. A current distribution based on this arrangement and current value is generated inside the power supply terminal pair 3. In the present invention, only a uniform region of the current distribution generated in this manner is determined as the measurement scanning region of the pair of potential difference measurement terminals 4. That is, the potential difference measuring terminal pair 4 is formed to be movable. Here, one is formed as a fixed terminal 4a and the other is formed as a movable terminal 4b, thereby simplifying the structure. The movable terminal 4b is adapted to scan a certain area in response to a control signal from the measurement area control section 48. This measurement area control unit 48 determines a uniform area of the current distribution that is occurring from the arrangement of the power supply terminal pair 3, the supply current value, the shape of the structural member, etc. by analysis using the computer 7 similar to that described above. It outputs a control signal.

The movable terminal 4b has motors 49, 50, levers 51, 52,
It is movable in the X-axis and Y-axis directions by threaded shafts 53 and 54. The motors 49 and 50 are connected to and controlled by a measurement area control section 48, which is connected to the computer 7 via an interface.
The present invention allows the potential difference measuring terminal pair 4 to assess only areas with a uniform current distribution, and does not cover non-uniform areas.

FIG. 19 shows another embodiment of the third invention, in which the potential difference measuring terminal pairs 4 are arranged in a matrix. According to this embodiment, the potential difference measuring terminal pairs 4 are not directly moved, and the terminals to be used are This can be handled by changing the selection as appropriate, making measurement easier.

FIG. 20 shows another embodiment of the third invention.

The power supply terminal pair 3 is arranged in a double concentric circle, and positive and negative polarity currents are supplied to each of the inner and outer terminals. Power supply terminal pair 3 is a switching device! ! In this embodiment, current is not supplied to all the power supply terminal pairs 3, but only to some of the internal and external terminals. The switching device generates a uniform current distribution, and allows you to select the terminal that supplies the current and replace it with another! 2E56 is provided. After a uniform current field is obtained, a rotating current field is formed by switching over all the terminals while maintaining the relative positions and current values of the terminals to which current is supplied. The potential difference measuring terminal pairs 4 are formed in a matrix shape and can follow the rotating current field in synchronization. Incidentally, in this embodiment, since reverse polarity current is supplied to the inside and outside of the power supply terminal pair 3, there is no leakage of current outside the measurement area, and there is an effect that it is less affected by adjacent defects or changes in the shape of structural members.

Next, the yX expression of this embodiment will be explained. Figure 21(a)
1 is a layout diagram showing supplied current values and terminal positions, and FIG. 5B is a corresponding current distribution diagram.

FIG. 22(a) is a layout diagram in which the number of inner peripheral terminals is increased to three, and FIG. 22(b) is a corresponding current distribution diagram. It can be seen that a uniform current distribution can be obtained by using the arrangement as shown in FIG. 22(a). Switching in this condition! i! A uniform rotating current field can be obtained by switching and moving the terminals to be used using 55. A diagonal crack in a structural member can be easily detected without moving the sensor head 1 using the rotating current field.

[Effects of the Invention] According to the first invention described in claim 1, the terminal position control section displaces the non-uniformity of the current distribution due to the shape of the measurement area of the structural member in a direction to cancel the non-uniformity. Since the current distribution can be made uniform by moving the possible power supply terminal pairs without being affected by the shape, etc., the presence or absence of surface defects and the defect shape can be detected with high precision, and the defect progress state can be determined with high precision. It becomes possible to observe.

According to the second invention described in claim 3, the current value supplied to each of the plurality of pairs of power supply terminals can be individually controlled by the current control section to make the current distribution uniform, so that the current distribution can be made uniform. A similar effect can be obtained.

According to the third invention described in claim 4, the measurement scanning area of the potential difference measurement terminal pair is determined based on the current distribution,
Since the scanning area is a uniformly distributed area, surface defects can be detected with high accuracy.

[Brief explanation of the drawing]

FIG. 1 is an overall configuration diagram of the device according to the present invention, and FIG.
FIG. 3 is an enlarged plan view showing another embodiment of the above invention; FIG. 4 is an enlarged plan view showing an embodiment of the second invention; FIG. 5 is an enlarged plan view showing surface defects. The detection flowchart, Figure 6 (a) and (b) shows the arrangement of the power supply terminal pair! Figure 7 (a) and (b) are current distribution diagrams corresponding to the same layout diagram, Figure 8 is the number n of power supply terminal pairs.
Figures 9(a) and (b) are diagrams showing the relationship between current distribution and current distribution, and Figures 9(a) and (b) are layout diagrams of power supply terminal pairs and corresponding current distribution diagrams, and Figures 10(a) and (b).
is the current distribution diagram corresponding to the same arrangement diagram with different terminal distance trays, and Figure 11 is the distance between the terminals of the power supply terminal pair! 1g and current distribution, Figure 12 (a) and (b) are layout diagrams and corresponding current distribution diagrams when the distance between terminals is different, Figure 1
Figures 3 (a) and (b) are current distribution diagrams corresponding to different layout diagrams, Figure 14 is a diagram showing the relationship between the degree of difference in distance between terminals and current distribution, and Figure 15 is a diagram showing the layout when the supply current is different. Corresponding current distribution diagrams, Figures 16(a) and (b) are layout diagrams of different supply currents and corresponding current distribution diagrams, Figure 17 is a diagram of the relationship between the degree of difference in supply current and current distribution, and Figure 18 is a diagram of the relationship between the degree of difference in supply current and current distribution. 3. FIG. 19 is an enlarged plan view showing another embodiment. FIG. 20 is an enlarged plan view showing another embodiment. FIGS. 21(a) and 21(b) are terminals. Current distribution diagrams corresponding to layout diagrams, FIGS. 22(a) and 22(b) are current distribution diagrams corresponding to different terminal layout diagrams. 3...Power supply terminal pair, 4...Potential difference measurement terminal pair, 9...
... Current source, 20... Terminal position control section, 48... Measurement area control section.

Claims (1)

[Claims] 1. A current source, a pair of power supply terminals that supply direct current from the current source to the surface of a member, a pair of potential difference measurement terminals that measure the potential difference inside the pair of power supply terminals, and a signal of the measured potential difference. In a surface defect detection device comprising a defect determining section that determines a surface defect based on A surface defect detection device 2 characterized in that it is provided with a terminal position control section for displacing and positioning a terminal, in claim 1, a large number of terminals are arranged in a matrix, and a pair of power feeding terminals is selected. A surface defect detection device that is movable by changing the terminal, and in which some of the other terminals are used as a pair of potential difference measurement terminals. 3. A current source, a pair of power supply terminals that supply direct current from this current source to the surface of the member, a pair of potential difference measurement terminals that measure the potential difference inside this power supply terminal pair, and a surface defect detection method based on this measured potential difference signal. In a surface defect detection device comprising a defect determination unit that performs a determination, a current control unit that forms a plurality of pairs of power supply terminals and changes the current supplied to each pair of power supply terminals based on the current distribution inside the pair of power supply terminals. A surface defect detection device characterized by being provided with. 4. A current source, a pair of power supply terminals that supply direct current from this current source to the surface of the member, a pair of potential difference measurement terminals that measure the potential difference inside this power supply terminal pair, and determine surface defects based on this measured potential difference signal. In the surface defect detection device, a plurality of pairs of power supply terminals are formed, a pair of potential difference measurement terminals is formed to be movable, and only a region where the current distribution inside the pair of power supply terminals is substantially uniform is detected. A surface defect detection device comprising a measurement area control section that defines a measurement scanning area of a pair of potential difference measurement terminals. 5. A surface defect detection device according to claim 4, in which one of the pair of potential difference measuring terminals is a fixed terminal and the other is a movable terminal. 6. A surface defect detection device according to claim 4, in which pairs of potential difference measuring terminals are arranged in a matrix. 7. In claim 4, 5, or 6, the pair of power supply terminals is arranged in a double concentric circle, and reverse polarity current is supplied to each of the inner and outer terminals, and a current of opposite polarity is supplied to each of the inner and outer terminals. A substantially uniform current distribution is formed by a combination of current supplying terminals and current values, and a rotating current field is formed by moving over all terminals while maintaining the relative arrangement and current value of the terminals to which current is supplied. Surface defect detection device.