Eddy current sensor for analyzing a test object and method of operating same

• US 6,479,990 B2
• Filed: 06/18/2001
• Issued: 11/12/2002
• Est. Priority Date: 12/18/1998
• Status: Expired due to Term

• Current Assignee: Micro-Epsilon Messtechnik GmbH & Co., KG
• Original Assignee: Micro-Epsilon Messtechnik GmbH & Co., KG
• Inventors: Netschaevsky, Mark, Mednikov, Felix, Mandl, Roland
• Primary Examiner(s): Lefkowitz, Edward
• Assistant Examiner(s): Zaveri, Subhash
• Application Number:
  US09/883,773
• Publication Number:
  US 20010054896A1
• Time in Patent Office: 512 Days
• Field of Search: 324/225, 324/229, 324/233, 324/239, 324/232, 324/230, 324/207.16, 324/207.12
• US Class Current: 324/225
• CPC Class Codes: G01B 7/10 using magnetic means, e.g. by measuring change of reluctance
• CPC Class Codes: G01N 27/72 by investigating magnetic variables
• CPC Class Codes: G01N 27/9046 by analysing electrical signals

Abstract

A method for operating an eddy current sensor (10) with a measuring coil (2) and an evaluation circuit (4) for determining material or geometric parameters of a test object (5), in which the test object (5) is arranged at a distance (d) from the measuring coil (2). The impedance of the measuring coil (2) is evaluated, while the measuring coil (2) is being supplied with an alternating voltage of a predetermined frequency, and the evaluation circuit (4) determines the material and geometric parameters of the test object (5) based on the impedance of the measuring coil (2). The impedance of the measuring coil (2) is determined at an alternating voltage of a first frequency, and the impedance of the measuring coil (2) is determined at an alternating voltage of a second frequency, and the evaluation circuit (4) computes the material and geometric parameters of the test object (5) on the basis of the impedances of the measuring coil (2) at the first and the second frequencies. An eddy current sensor (1) is also disclosed, which can be used with advantage in conjunction with the method.

Claims

1. A method for operating an eddy current sensor having a measuring coil, a compensation coil, and an evaluation circuit for determining material or geometric parameters of a test object, comprising the steps of

positioning the test object at a predetermined distance from the measuring coil and the compensating coil,

measuring the impedance of the measuring coil while the measuring coil is being supplied with an alternating voltage of a first predetermined frequency, then

measuring the impedance of the measuring coil while the measuring coil is being supplied with an alternating voltage of a second predetermined frequency which is different from said predetermined frequency, and

computing in the evaluation circuit the material and geometric parameters of the test object based on the impedances of the measuring coil at the first and second frequencies, and including compensating for temperature influences on the measured impedances of the measuring coil with a measured impedance of the compensation coil.

2. The method as defined in claim 1 wherein the compensation coil is positioned so that the test object has substantially no influence on its impedance.

3. The method as defined in claim 2 wherein the compensation coil is spatially smaller than the measuring coil and is arranged inside the measuring coil.

4. The method as defined in claim 2 comprising the further steps of computing the conductivity of the test object in the evaluation circuit based upon the measured value of the impedance at at least one of the predetermined frequencies, and computing the thickness of the test object based upon the computed conductivity.

5. The method as defined in claim 1 wherein the first frequency is sufficiently high so that its impedance is substantially independent of the material and geometric parameters of the test object.

6. The method as defined in claim 1 wherein the computing step includes utilizing a nonlinear system which is dependent on the first frequency and the second frequency of the alternating voltage.

7. The method as defined in claim 1 wherein the computing step includes computing the thickness (D) of the test object from the following equation:

$D\approx <br><br>\mathrm{konst}.*\tan <br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\varphi <br><br>}}_c=-2<br><br>{\mathrm{\beta <br><br>}}_k<br><br>\frac{2<br><br>\sqrt{2}<br><br>{\mathrm{\beta <br><br>}}_k<br><br>\left(\mathrm{ch}<br><br>\left({\mathrm{\delta <br><br>}}_k\right)-\cos <br><br>\left({\mathrm{\delta <br><br>}}_k\right)\right)+3<br><br>\left(\mathrm{sh}<br><br>\left({\mathrm{\delta <br><br>}}_k\right)-\sin <br><br>\left({\mathrm{\delta <br><br>}}_k\right)\right)}{3<br><br>\sqrt{2}<br><br>\left(3<br><br>\mathrm{ch}<br><br>\left({\mathrm{\delta <br><br>}}_k\right)-2<br><br>\cos <br><br>\left({\mathrm{\delta <br><br>}}_k\right)\right)+2<br><br>{\mathrm{\beta <br><br>}}_k<br><br>\left(\mathrm{sh}<br><br>\left({\mathrm{\delta <br><br>}}_k\right)+\sin <br><br>\left({\mathrm{\delta <br><br>}}_k\right)\right)}.$

8. The method as defined in claim 1 wherein the computing step is limited to a range as a function of the conductivity of the test object and the first and second frequencies.

9. The method as defined in claim 1 wherein the range, to which the computation of the evaluation circuit is limited, is predetermined by the following equation:

$r<br><br>\sqrt{{\mathrm{\omega <br><br>}}_k<br><br>{\mathrm{\mu <br><br>}}_o<br><br>{\mathrm{\sigma <br><br>}}_k}<br><br>\text{\hspace{1em}<br><br>}\geqq <br><br>10.$

10. The method as defined in claim 1 wherein the temperature compensating step includes subtracting the complex impedance of the measuring coil from the complex impedance of the compensation coil.

11. A method for operating an eddy current sensor having a measuring coil, and an evaluation circuit for determining material or geometric parameters of a test object, comprising the steps of

positioning the test object at a first predetermined distance from the measuring coil which is greater than twice the radius of the measuring coil and measuring the impedance or the inductance value and/or the damping value of the measuring coil when supplied with an alternating voltage at a given frequency,

positioning the test object at a second predetermined distance from the measuring coil which is less than twice the radius of the measuring coil and measuring the impedance or the inductance value and/or the damping value of the measuring coil when supplied with an alternating current at the given frequency, and

computing in the evaluation circuit the material and geometric parameters of the test object based on the values measured at the first and second distances.

12. The method as defined in claim 11 comprising the further steps of determining the conductivity of the test object, and computing in the evaluation circuit the thickness of the test object based upon the measured values at the first and second distances and the conductivity.

31. The method as defined in claim 11, wherein the eddy current sensor further includes a compensation coil and wherein the computing step includes compensating for temperature influences on the impedance of the measuring coil utilizing the measured impedance of the compensation coil.

32. The method as defined in claim 31 wherein the compensation coil is spatially smaller than the measuring coil.

13. An eddy current sensor comprising

a measuring coil and a compensation coil which are each adapted for receiving an alternating current, and with the compensation coil arranged in the vicinity of the measuring coil so that it is exposed to the thermal environmental conditions of the measuring coil,

an evaluation circuit connected to the measuring coil and the compensation coil for determining material and/or geometric parameters of an electrically conductive test object from the impedances of the measuring coil and the compensation coil, and

the compensation coil being designed and arranged such that the test object has substantially no influence on its impedance.

14. The eddy current sensor as defined in claim 13 wherein the compensation coil is spatially smaller than the measuring coil.

15. The eddy current sensor as defined in claim 13 wherein the radius of the compensation coil is smaller than the radius of the measuring coil, and smaller than the spacing between the compensation coil and the test object.

16. The eddy current sensor as defined in claim 13 wherein the compensation coil is a flat coil and arranged in one plane, and wherein the compensation coil is arranged inside the measuring coil.

17. The eddy current sensor as defined in claim 13 wherein the measuring coil and the compensation coil are arranged relative to each other in a common plane.

18. The eddy current sensor as defined in claim 13 wherein the measuring coil and the compensation coil are arranged in concentric relationship with each other.

19. The eddy current sensor as defined in claim 13 wherein the measuring coil is arranged on one side of the test object, and the compensation coil is arranged on the opposite side thereof.

20. The eddy current sensor as defined in claim 13 wherein the measuring coil and the compensation coil are designed and constructed as helical coils which lie in a common plane.

21. The eddy current sensor as defined in claim 13 wherein the measuring coil and/or the compensation coil are constructed in multiple layers which are separated by an insulating layer.

22. The eddy current sensor as defined in claim 13 wherein the coil geometry of the measuring coil and that of the compensation coil are substantially the same.

23. The eddy current sensor as defined in claim 13 wherein the measuring coil and the compensation coil are each interposed in the feedback loop of an operational amplifier which is part of the evaluation circuit.

24. The eddy current sensor as defined in claim 13 wherein the evaluation circuit includes means for generating two complementary alternating voltages.

25. The eddy current sensor as defined in claim 24 wherein the two complementary alternating voltages are each applied to an input of an operational amplifier.

26. The eddy current sensor as defined in claim 13 wherein the evaluation circuit includes means for measuring the impedance of the measuring coil, the impedance of the compensation coil, and/or the coupling impedance.

27. The eddy current sensor as defined in claim 13 wherein the evaluation circuit includes an electronic device for determining the change of the phase angle of the measuring coil impedance, compensation coil impedance, and/or coupling impedance.

28. The eddy current sensor as defined in claim 13, wherein the measuring coil and the compensation coil are arranged with substantially parallel axes relative to each other.

29. The eddy current sensor as defined in claim 13, wherein the measuring coil and compensation coil have axes which form an acute angle with each other, or are arranged in orthogonal relationship with each other.

30. The eddy current sensor as defined in claim 13 wherein the compensation coil has a smaller number of winds, or a smaller diameter of the coil wire, or a different material of the wire or coil core, so that the influence of the test object is less than in the case of the measuring coil.