Method for the operation and for the evaluation of signals from an eddy current probe and device for performing the method

• US 6,005,392 A
• Filed: 11/25/1997
• Issued: 12/21/1999
• Est. Priority Date: 11/26/1996
• Status: Expired due to Fees

• Current Assignee: Institut Dr Friedrich Forster Pruferatebau GmbH & Company KG
• Original Assignee: Institut Dr Friedrich Forster Pruferatebau GmbH & Company KG
• Inventors: Patzwaldt, Wolfgang
• Primary Examiner(s): STRECKER, GERARD R
• Application Number:
  US08/978,314
• Time in Patent Office: 756 Days
• Field of Search: 324/329, 324/335, 324/232, 324/233, 324/239
• US Class Current: 324/329
• CPC Class Codes: G01V 3/105 forming directly coupled primary and secondary coils or loops

Abstract

A method for the operation and for the evaluation of signals of an eddy cent probe wherein a transmitting coil is supplied with alternating current with preferably three frequency components. The receiving coil signal corresponding to an a.c. voltage is broken down into corresponding frequency components and by phase-control rectification projection signals are formed corresponding to a real part or an imaginary part of the coil signal. From at least two, optionally individually amplified projection signals of the same type (real or imaginary signal) differential signals are formed. Finally, by the combination of at least two differential signals a combination signal is formed and evaluated. In particular, quotients and amount or value sums of differential signals are formed. The method offers decisive advantages in seeking mines in connection with the suppression of disturbing ground or soil signals and in the identification and classification of mines on the basis of signals from an eddy current probe. The method also makes it possible to use an eddy current probe as the ground probe.

Claims

1. A method for operating an eddy current probe and for evaluating coil signals from the eddy current probe, the coil signals containing information about a region examined by the eddy current probe, the eddy current probe having a search coil system, which has at least one transmitting coil connected to an alternating current source and at least one receiving coil electrically connected to a signal evaluating device, the receiving coil being adapted to produce the coil signal U corresponding to an a.c. voltage, the method comprising the following steps:

supplying the transmitting coil with a primary alternating current I1, which contains a component of a discrete, first frequency f1 and a component of at least one discrete, second frequency f2,

arranging the transmitting coil with respect to the region to be examined such that a signal emitted by the transmitting coil and interacting with the ground is received by the receiving coil to produce the coil signal,

transmitting the coil signal to the signal evaluating device,

splitting up the coil signal into frequency components corresponding to the first frequency f1 and the at least one second frequency f2,

forming a projection signal associated with the first frequency f1 and a projection signal associated with the second frequency f2, a projection signal being a real signal UR1 or UR2 or an imaginary signal UI1 or UI2,

multiplying the projection signal with factors FR1, FR2, FI1, FI2 associated in each case with the projection signals for forming weighted projection signals P1=(FI1*UI1) or (FR1*UR1) or P2=(FI2*UI2) or (FR2*UR2),

forming at least two differential signals, wherein a differential signal comprises a difference between weighted projection signals of the same type (real or imaginary signal) and wherein a differential signal is either an imaginary differential signal or a real differential signal, the forming of differential signals being performed according to

space="preserve" listing-type="equation">DIm=(FI2*UI2)-(FI1*II2)

for an imaginary differential signal and

space="preserve" listing-type="equation">DR=(FR2*UR2)-(FR1*UR1)

for a real differential signal,

forming a combination signal differing from a linear combination by the combination of at least two differential signals of type DIm or DR,

evaluating the combination signal, the combination signal representing a processed coil signal.

2. The method according to claim 1, wherein an imaginary differential signal DIm and a real differential signal DR are formed and wherein the combination signal is a real-imaginary combination signal KRIS formed from the sum of the amounts of the differential signals according to

3. The method according to claim 1, wherein an imaginary differential signal DIm and a real differential signal DR are formed and wherein the combination signal is a real-imaginary combination signal KRIQ, formed from the quotient of the differential signals according to one of the group consisting of

4. The method according to claim 1, wherein the alternating current I1 contains a component of at least one discrete, third frequency f3, wherein at least one weighted projection signal p3=(FI3*UI3) or P3=(FR3*UR3) associated with the third frequency is formed, wherein from the difference between two weighted projection signals of type P1, P2, P3 is formed a first projection differential signal and at least one projection differential signal differing therefrom, the weighted projection signals P1, P2, P3 in each case all corresponding to a real signal or all corresponding to an imaginary signal and wherein the combination signal is a projection combination signal KP formed by the combination of at least two projection differential signals.

5. The method according to claim 4, wherein the projection combination signal KP is formed from the quotient of two projection differential signals.

6. The method according to claim 4, wherein the projection combination signal KP is formed by a summation of the amounts of at least two projection differential signals.

7. The method according to claim 1 comprising the following step:

forming a frequency-specific conductivity signal from one of the group consisting of a combination signal KRIQ corresponding to a quotient of differential signals DIm and DR and a combination signal KP corresponding to a quotient of at least two projection differential signals.

8. The method according to claim 7, wherein the conductivity signal is used for identifying the search object material.

9. The method according to claim 7, wherein from conductivity signals for at least two frequencies is formed a frequency dependence signal representing the frequency dependence of the conductivity of the search object material.

10. The method according to claim 9, wherein the frequency dependence signal is used for identifying the search object material.

11. The method according to claim 1, wherein one of the group of the primary alternating current I1 contains at least one component of a low frequency F0 and the transmitting coil in time-staggered manner to the primary current I1 at least for a time interval is supplied with an alternating current I0 with the low frequency f0, the low frequency f0 being between 10 and 1000 Hz.

12. The method according to claim 11, wherein at least two different low frequencies of type f0 are used and wherein at least one of the group of an imaginary differential signal and a real differential signal of two low frequencies is formed.

13. The method according to claim 11, wherein using a low frequency signal resulting from at least one low frequency a transformation signal is formed and wherein by means of the transformation signal a transformed coil signal is formed, which essentially contains no signal component resulting from an optionally present magnetizability of the search object.

14. The method according to claim 1, wherein the frequencies of the primary current differ from one another by identical integral factors.

15. The method according to claim 14, wherein the integral factor is equal to one of the group of 2 and 4 and 8.

16. The method according to claim 14, wherein the first frequency f1 is between 1.5 and 2.5 kHz, wherein a second frequency is between 12 and 20 kHz and wherein a third frequency is between 96 and 160 kHz.

17. The method according to claim 1, wherein the combination signal is a ground signal UB, the method for a frequency f comprising the following stages:

forming a projected metal signal M, which corresponds to one of a real component and an imaginary component of a metal signal component of the probe signal caused by a metal part,

multiplying the projected metal signal with a factor C specific for the frequency f for forming a weighted, projected metal signal,

subtracting the weighted, projected metal signal from the weighted projection signal associated with the frequency f, the weighted projection signal and the weighted projected metal signal being of the same type (real signal or imaginary signal).

18. The method according to claim 17, wherein the step of forming the projected metal signal comprises the following steps:

forming a conductivity signal (L2/R2) proportional to the electrical conductivity of the metal piece,

forming a coupling signal GI representing the magnetic coupling between the transmitting coil and the search object and between the search object and the receiving coil, which with respect to the imaginary axis and the frequency f1 is formed as follows;

space="preserve" listing-type="equation">GI=DIm/(G*(1/(S+1)-V21.sup.2)/(S+V21.sup.2))

with

G=FI1*f1*I11, in which I11 is the amplitude of the primary current of the component of the first frequency f1

space="preserve" listing-type="equation">S=(R2/(f1+L2)).sup.2 ;

space="preserve" listing-type="equation">V21=f2/f1,

forming the projected metal signal, which with respect to the imaginary axis and the frequency f1 is formed in the following way;

space="preserve" listing-type="equation">UI1=(GI/FI1)*G(1/(S+1)).

19.

19. A device for ground identification and foreign body detection in a search area with at least one eddy current probe having a search coil system, which has at least one transmitting coil connected to an alternating current source, to which can be supplied a primary alternating current with a component of a first frequency f1 and at least one second component f2, and at least one receiving coil, which is electrically connected to a signal evaluating device and which in particular on approaching an electrically conductive search object produces a coil signal U corresponding to an a.c. voltage, wherein the signal evaluating device has the following elements:

frequency splitting means, electrically connected to the at least one receiving coil for splitting the coil signal into frequency components corresponding to the first frequency f1 and the at least one second frequency f2,

projection signal forming means electrically connected to the frequency splitting means for forming two projection signals associated with the first frequency f1 and two projection signals associated with the at least one second frequency f2, wherein a projection signal is a real signal UR1 or UR2 or an imaginary signal UI1 or UI2 and represents a real or an imaginary component of a frequency component,

amplifying means electrically connected to the projection signal forming means for multiplying the projection signals with factors FR1, FR2, FI1 or FI2 associated with the particular projection signals for forming weighted projection signals,

at least two subtractors electrically connected to the amplifying means for forming at least two differential signals DIm or DR, a differential signal representing the difference between two weighted projection signals of the same type (real or imaginary signal) associated with the frequencies f1 or f2, and

combination signal forming means electrically connected to the subtractors for forming a combination signal by combining at least two differential signals of type DIm or DR in a manner differing from a simple linear combination, the combination signal representing a processed coil signal.

20. The device according to claim 19, wherein the combination signal forming means have at least one amount forming element electrically connected to an output of a subtractor.

21. The device according to claim 20, wherein each subtractor is followed by an amount forming element.

22. The device according to claim 20, wherein the outputs of at least two amount forming elements are electrically connected to inputs of a summator.

23. The device according to claim 19, wherein the combination signal forming means have at least one dividing element with at least two inputs, whereof one input is connected to the output of a subtractor constructed for forming an imaginary differential signal and one input is connected to the output of a subtractor constructed for forming a real differential signal.

24. The device according to claim 19, wherein the combination signal forming means have at least one computing element with at least one first and at least one second input, the first and second inputs being electrically connected to one of the group of the outputs of two different subtractors for forming an imaginary differential signal and the outputs of two different subtractors for forming a real differential signal.

25. The device according to claim 19, wherein the device is a mine search device.