Electricity: measuring and testing – Magnetic – With compensation for test variable
Reexamination Certificate
2001-06-18
2002-11-12
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Magnetic
With compensation for test variable
C324S229000, C324S207120
Reexamination Certificate
active
06479990
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for operating an eddy current sensor with a measuring coil and an evaluation circuit for determining material or geometric parameters of a test object, wherein the measuring object is arranged at a distance from the measuring coil, wherein the impedance of the measuring coil is evaluated, while the measuring coil is being supplied via an alternating voltage of a predetermined frequency, and wherein the evaluation circuit determines the material and geometric parameters of the test object on the basis of the impedance of the measuring coil.
The invention further relates to an eddy current sensor, in particular for use in the present method, with a measuring coil, a compensation coil, and an evaluation circuit, used for determining the material and/or geometric parameters of an electrically conductive test object. The measuring coil and compensation coil can be supplied with alternating current, and the compensation coil is arranged in the vicinity of the measuring coil, so that it is exposed to the thermal environmental conditions of the measuring coil.
Eddy current sensors are frequently used in the industry for measuring the distance and for measuring the thickness on a flat test object, for example, a web, a layer, or a tape. In this process, they are used in most cases under difficult environmental conditions, for example, at high temperatures. Just in connection with high, strongly fluctuating temperatures, considerable falsifications of the measuring and evaluation results occur in known processes for determining material and geometric parameters. While in a measuring coil of the eddy current sensor, the temperature influence leads primarily to a change of the real part, it is also possible to detect an influence on the imaginary part of the complex impedance. However, on the other hand, the temperature has also an influence on the conductivity of the test object. Since the conductivity of the test object in turn exerts an influence on the induced eddy currents and, likewise, via the feedback of the magnetic coupling, on the impedance of the measuring coil, the conductivity of the test object is likewise a source for falsifying the measuring results.
U.S Pat. No. 6,288,536 discloses an eddy current sensor with a measuring and a compensation coil, which is arranged in the direct vicinity of, i.e., in thermal contact with the measuring coil. The electromagnetic fields of the compensation coil and the measuring coil are oriented in orthogonal relationship with each other. As a result of the orthogonal arrangement of the compensation coil, the test object has little influence on the impedance thereof, in case the eddy current sensor is positioned with its measuring coil such that the presence of the test object becomes maximally active on the impedance of the measuring coil.
A disadvantage of the known eddy current sensor lies in that the temperature influences on the conductivity of the test object are transferred, via the eddy current effect, into the measuring coil, so that the measuring results, for example, of the thickness of a layer, are influenced by the change of conductivity.
Furthermore, eddy current sensors are known, which are used in a method for a noncontacting measurement of the thickness of a foil, wherein the eddy current sensor is arranged on one side of the test object. This method has the disadvantage that as a function of the material properties, the eddy current sensor must be kept at a certain readjustable basic distance from the test object to perform exact measurements.
It is therefore an object of the invention to describe a method and an miniaturizable eddy current sensor for a noncontacting measurement of material and geometric parameters of electrically conducting materials, which provides a reliable compensation for disturbance variables on the measuring results. A disturbance variable means the impedance fluctuation of the measuring coil by changing the basic distance or the temperature, whereby the measuring results are influenced in a falsifying manner.
SUMMARY OF THE INVENTION
The above and other objects and advantages of the invention are achieved by the provision of a method of operating an eddy current sensor wherein the impedance of the measuring coil is measured at an alternating voltage of a first frequency, the impedance of the measuring coil is measured at an alternating voltage of a second frequency, and the evaluation circuit computes the material and geometric parameters of the test object based on the impedances of the measuring coil at the first and the second frequencies.
In accordance with the invention, it has been recognized that the temperature influences on the measurement of the material and geometric parameters of the test object can be excellently compensated, when the measurement is performed with an eddy current sensor at two different frequencies. The temperature influences are differently effective as a function of the frequency of the alternating voltage, which is supplied to the measuring coil. On the other hand, based on the impedances of the measuring coil at the different frequencies, with the knowledge of the mathematical relationships between temperature-influenced variables, for example, the conductivity of the test object, the evaluation circuit is capable of computing the material and geometric parameters of the test object. A complete compensation of the temperature influences affecting the measured values is possible, only when both the temperature influence on the impedance of the measuring coil and the temperature influence on the conductivity of the test object are compensated.
It is preferred to use the method of the present invention in connection with unilaterally measuring the thickness of a foil, with the eddy current sensor being arranged on one side of the test object. The method includes determining the thickness or strength of a flat test object, in particular a web, a tape, or a layer of an electrically conducting material, with the aid of at least one measuring coil through which an alternating current passes, the measuring coil being arranged at a basic distance from the test object. In so doing, the change of inductance and damping are evaluated via the impedance. As regards the basic distance, it is possible to perform first a measurement with the eddy current sensor without test object for purposes of calibrating the eddy current sensor. Thereafter, it is possible to perform measurements with the test object, wherein it is intended, in accordance with the invention, to compensate the impedance fluctuations resulting from the variation of the basic distance. The method of the present invention permits a calibration, even when measurements can be performed only with the test object. In this instance, a microcontroller may compute the compensated measured value from the measurement results at the different frequencies.
In accordance with the invention, the method may provide for the following measuring steps: determining the impedance, or the inductance value, and/or the damping value of the measuring coil at a first frequency. In this instance, either the test object is absent in the region of the eddy current sensor, or the spacing between the test object and measuring coil is greater than twice the radius R of the measuring coil, so that the influence of the measuring object is small. Subsequently, the inductance value and the damping value are determined in the presence of the test object, with the spacing between the measuring coil and the test object being smaller than the radius R of the measuring coil. Preferably, the measuring results are converted into a dimensionless value. Finally, a computer determines the thickness of the test object from the measured value of the measuring coil. To this end, one may use in the computation, for example, the conductivity of the test object. However, when conducting the measurement exactly in accordance with the invention at two different frequencies, it will not be absolutely neces
Mandl Roland
Mednikov Felix
Netschaevsky Mark
Alston & Bird LLP
Lefkowitz Edward
Micro-Epsilon Messtechnik GmbH & Co. KG
Zaveri Subhash
LandOfFree
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