GB2069708A - An eddy-current test device for detecting surface faults on metal workpieces - Google Patents
An eddy-current test device for detecting surface faults on metal workpieces Download PDFInfo
- Publication number
- GB2069708A GB2069708A GB8104508A GB8104508A GB2069708A GB 2069708 A GB2069708 A GB 2069708A GB 8104508 A GB8104508 A GB 8104508A GB 8104508 A GB8104508 A GB 8104508A GB 2069708 A GB2069708 A GB 2069708A
- Authority
- GB
- United Kingdom
- Prior art keywords
- frequency
- eddy
- test
- test device
- current test
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/72—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables
- G01N27/82—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws
- G01N27/90—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws using eddy currents
- G01N27/9046—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws using eddy currents by analysing electrical signals
- G01N27/9053—Compensating for probe to workpiece spacing
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/72—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables
- G01N27/82—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws
- G01N27/90—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws using eddy currents
- G01N27/9046—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating magnetic variables for investigating the presence of flaws using eddy currents by analysing electrical signals
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- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Physics & Mathematics (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analyzing Materials By The Use Of Magnetic Means (AREA)
Abstract
The device comprises a high frequency oscillator (11) including a test coil (10) as part of the tuned circuit thereof, a frequency demodulator (14) connected to provide an output signal representative of variations in the test frequency of the oscillator, a band-pass filter (15) arranged to pass as a fault signal only those frequency components of the demodulator output signal indicative of faults in the workpiece being tested, and circuit means (16,17) regulating the test frequency of the oscillator automatically to reduce variations thereof caused by changes in the distance between the test coil and the workpiece. Amplifier 19 has its amplification factor varied in accordance with the test coil workpiece distance. <IMAGE>
Description
SPECIFICATION
An eddy-current test device for detecting surface faults on metal workpieces
The invention relates to an eddy-current test device for detecting surface faults on metal workpieces.
In one aspect the invention provides an eddycurrent test device for detecting surface faults on metal workpieces, the device comprising a high frequency oscillator including a test coil as part of the tuned circuit thereof, a frequency demodulator connected to provide an output signal representative of variations in the test frequency of the oscillator, a band-pass filter arranged to pass as a fault signal only those frequency components of the demodulator output signal indicative of faults in the workpiece being tested, and circuit means regulating the test frequency of the oscillator automatically to reduce variations thereof caused by changes in the distance between the test coil and the workpiece.
The circuit means may comprise a low-pass filter connected to receive an output signal from the demodulator and to pass as a control signal representative of said distance changes only those frequency components below the fault signal frequency components and means responsive to the control signal to regulate the test frequency. Said means responsive to regulate the test frequency may comprise at least one voltage variable capacitor connected as part of the tuned circuit of the oscillator.
An eddy current test device according to the invention may include a fault signal amplifier having a controllable amplification factor and means also responsive to said control signal to control said amplificaction factor to reduce variations in the amplitude of the fault signal resulting from said distance variations. Said means responsive to con trol said amplification factor may include a logarithmic amplifier.
In another aspect the invention provides an eddycurrent test device for detecting surface faults on metal workpieces, the device comprising a highfrequency oscillator supplying a test coil and followed by an amplifier, a frequency demodulator, a band-pass filter and a signal amplifier having an amplification factor which depends on the distance between the test coil and the work-piece, wherein a circuit regulating the test frequency in dependence on the distance between the test coil and the workpiece is disposed between the frequency demodulator and the high-frequency oscillator.
The control voltage is the output voltage of the frequency demodulator. To prevent regulation occurring during steep pulses, produced e.g. by cracks or similar faults in the material, according to another feature of the invention a tuned low-pass filter may be incorporated in the regulating circuit.
All slower frequency deviations are immediately compensated by the regulating circuit in cooperation with voltage-variable capacitors in the high-frequency oscillator circuit. As a result of this advantageous circuitry the test frequency remains stable and does not change even if
(1) The test coil is disposed at a variable distance from the workpiece,
(2) The test coil is adjusted to a fixed value but the distances from the test coil vary as a result of differences in tolerance between the workpieces,
(3) The distance from the test coil varies during each rotation as a result of constant faults in the shape of the workpieces (e.g. if they are oval, out of true or insufficiently coaxial),
(4) There may be variations in the properties of the material, or
(5) The surface structure of the material varies.
All the aforementioned kinds of faults may occur, particularly during automatic testing. As a result of the advantageous circuitry of the eddy-current test device according to the invention, automatic testing can continue for a long time without supervision and without maintenance, because the eddy-current test device automatically adapts to the continually changing conditions at the test site.
Since the test frequency is continuously readjusted and thus remains constant, the frequency demodulator can hvae a very steep characteristic, thus advantageously attaining high sensitivity and high resolution of faults. Since the oscillator frequency is constant, the operating position is always at the centre and thus, irrespective of the aforementioned influences, is always in the steepest region of the characteristic, which is steep in any case. As a result, the sensitivity is uniform besides being high as previously described.
The circuit for regulating the amplification factor in dependence on the distance between the test coil and the workpiece may contain a logarithmic amplifier.
The amplification of a signal amplifier is controlled in known manner, via a regulating circuit, so as to compensate for the drop in the sensitivity of the eddy-current test device when the distance between the test coil and the workpiece increases. As a result, the sensitivity is substantially uniform over a certain range of distance. Preferably the regulating circuit contains a logarithmic amplifier, since there is no linear relation between the frequency demodulator output signal and the distance from the test coil. The accuracy of the control system can be increased as a result.
The output voltage from the frequency demodulator in the single channel of the device may be the regulating voltage for both regulating circuits.
The frequency demodulator output voltage, used for adjusting the frequency of the high-frequency oscillator, can also be used for adjusting the sensitivity. This can save a number of components.
An embodiment of the invention will now be described by way of example, reference being made to the accompanying drawings, of which:
Figure lisa block circuit diagram of an eddycurrent test device for detecting surface faults, with frequency and sensitivity regulation, and
Figure 2 is a graph of the fault-signal level at varying distances from the test coil.
Figure 1 is a block circuit diagram of an eddycurrent test device comprising a test coil 10 connected as part of a high-frequency oscillator 11 and having a single winding. Coil 10 co-operates with a capacitor 12 to determine the oscillation frequency, which is also the test frequency. The high-frequency signal is fed through an amplifier 13 to a frequency demodulator 14 which converts the change in frequency caused by a fault in the workpiece into a low-frequency fault signal. A downstream band-pass filter 15 transmits the actual fault signal and attenuates interfering signals. An output voltage from the frequency demodulator 14 is fed to a low-pass filter 16, which transmits a control voltage corresponding to only those frequency changes in the demodulator output which are lower than the fault signal.The control voltage is used to control voltage-variable capacitors 17 which are connected in parallel with the tuned circuit of the high-frequency oscillator 11.
The control voltage is also fed via a second control circuit comprising a logarithmic amplifier 18, to regulate the amplification of a signal amplifier 19 connected downstream of the band-pass filter 15.
The remaining components ofthe eddy-current amplifier are an adjustable signal amplification system 20, a comparator 21, a fault signal store 22 and indicators if required.
Figure 2 is a diagram showing the control properties of the eddy-current test device. The continuous curve shows the variation in the signal level at the signal amplifier output in response to variations in the distance between the test coil and the workpiece. The measurements were made in a test under realistic conditions in which a cylindrical workpiece having a small surface crack rotated in a holder and the test coil was switched on atvarying distances.
The graph shows the high quality of the control system. The deviation is only + 1 dB when the distance of the test coil varies from near-contact to almost 0.5 mm. The test coil was very small, with a core about 0.5 mm in diameter. In the case of larger test coils, the control system is effective over a relatively wider range of distances.
The thick inclined or sloping broken line shows how, when the control system is switched off, the fault signal drops steeply with increasing distance from the test coil.
The control circuits shown in the present example for an eddy-current test device can also be used in slightly modified form for other test processes, e.g.
optical testing for adjusting the intensity of light or the ultra-sound method of regulating the test frequency.
Claims (10)
1. An eddy-current test device for detecting surface faults on metal workpieces, the device comprising a high frequency oscillator including a test coil as part of the tuned circuit thereof, a frequency demodulator connected to provide an output signal representative of variations in the test frequency of the oscillator, a band-pass filter arranged to pass as a fault signal only those frequency components of the demodulator output signal indicative of faults in the workpiece being tested, and circuit means regulating the test frequency of the oscillator automatically to reduce variations thereof caused by changes in the distance between the test coil and the workpiece.
2. An eddy-current test device as claimed in
Claim 1, wherein the circuit means comprises a low-pass filter connected to receive an output signal from the demodulator and to pass as a control signal representative of said distance changes only those frequency components below the fault signal frequency components and means responsive to the control signal to regulate the test frequency.
3. An eddy-current test device as claimed in
Claim 2, wherein said means responsive to regulate the test frequency comprises at least one voltage variable capacitor connected as part of the tuned circuit of the oscillator.
4. An eddy-current test device as claimed in
Claim 2 or3, and including a fault signal amplifier having a controllable amplification factor and meaiis also responsive to said control signal to control said amplification factor to reduce variations in the amplitude of the fault signal resulting from said distance variations.
5. An eddy-current test device as claimed in
Claim 4, wherein said means responsive to control said amplification factor includes a logarithmic amplifier.
6. An eddy current test device substantially as herein described with reference to and as shown in the accompanying drawings.
7. An eddy-current test device for detecting surface faults on metal workpieces, the device comprising a high-frequency oscillator supplying a test coil and followed by an amplifier, a frequency demodulator, a band-pass filter and a signal amplifier having an amplification factor which depends on the distance between the test coil and the workpiece, wherein a circuit regulating the test frequency in dependence on the distance between the test coil and the workpiece is disposed between the frequency demodulator and the high-frequency oscillator.
8. An eddy-current test device according to
Claim 7, wherein the circuit comprises a low-pass filter for regulating the test frequency.
9. An eddy-current test device according to
Claim 7 and 8, wherein a circuit for regulating the amplification factor in dependence on the distance between the test coil and the workpiece contains a logarithmic amplifier.
10. An eddy-current test device according to one of the Claims 7 to 9, wherein the output voltage from the frequency demodulator in the single channel of the device is the regulating voltage for both regulating circuits.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE3005915A DE3005915C2 (en) | 1980-02-16 | 1980-02-16 | Eddy current tester for surface defects |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2069708A true GB2069708A (en) | 1981-08-26 |
Family
ID=6094846
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8104508A Withdrawn GB2069708A (en) | 1980-02-16 | 1981-02-13 | An eddy-current test device for detecting surface faults on metal workpieces |
Country Status (4)
| Country | Link |
|---|---|
| DE (1) | DE3005915C2 (en) |
| FR (1) | FR2476318A1 (en) |
| GB (1) | GB2069708A (en) |
| IT (1) | IT1135435B (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2141234A (en) * | 1983-06-07 | 1984-12-12 | Mo Energeticheskij Institut | Eddy-current flaw detector |
| EP1906181A3 (en) * | 2006-09-29 | 2008-05-21 | Hitachi, Ltd. | Eddy current testing appartus and method |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3824948A1 (en) * | 1988-07-22 | 1990-01-25 | Fiz Energet I An Latvssr | Eddy-current device for non-destructive testing |
| DE4003330A1 (en) * | 1990-02-05 | 1991-08-08 | Foerster Inst Dr Friedrich | Eddy current tester |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB596182A (en) * | 1945-07-24 | 1947-12-30 | British Thomson Houston Co Ltd | Improvements relating to apparatus for detecting flaws in metal wire |
| GB870487A (en) * | 1958-01-24 | 1961-06-14 | Republic Steel Corp | Flaw detecting device |
| DE2049486A1 (en) * | 1969-10-10 | 1971-04-22 | Automation Ind Inc | Material testing device |
| AU3837372A (en) * | 1971-02-01 | 1973-08-02 | Automation Industries, Inc | Eddy current nondestructive testing system |
| GB1475517A (en) * | 1974-06-27 | 1977-06-01 | British Steel Corp | Detection of surface defects in elongate metallic members |
| US4095180A (en) * | 1975-12-29 | 1978-06-13 | K. J. Law Engineers, Inc. | Method and apparatus for testing conductivity using eddy currents |
| SE444079B (en) * | 1977-11-22 | 1986-03-17 | British Steel Corp | DEVICE FOR CHECKING THE SURFACE OF A LONG-TERM METAL FORM |
-
1980
- 1980-02-16 DE DE3005915A patent/DE3005915C2/en not_active Expired
-
1981
- 1981-02-04 FR FR8102162A patent/FR2476318A1/en active Granted
- 1981-02-12 IT IT19687/81A patent/IT1135435B/en active
- 1981-02-13 GB GB8104508A patent/GB2069708A/en not_active Withdrawn
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2141234A (en) * | 1983-06-07 | 1984-12-12 | Mo Energeticheskij Institut | Eddy-current flaw detector |
| EP1906181A3 (en) * | 2006-09-29 | 2008-05-21 | Hitachi, Ltd. | Eddy current testing appartus and method |
| US7772840B2 (en) | 2006-09-29 | 2010-08-10 | Hitachi, Ltd. | Eddy current testing method |
| US7872472B2 (en) | 2006-09-29 | 2011-01-18 | Hitachi, Ltd. | Eddy current testing apparatus and eddy current testing method |
Also Published As
| Publication number | Publication date |
|---|---|
| FR2476318A1 (en) | 1981-08-21 |
| DE3005915C2 (en) | 1985-01-10 |
| FR2476318B1 (en) | 1984-07-13 |
| DE3005915A1 (en) | 1981-08-27 |
| IT1135435B (en) | 1986-08-20 |
| IT8119687A0 (en) | 1981-02-12 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |