Electricity: measuring and testing – Magnetic – With compensation for test variable
Reexamination Certificate
1999-02-02
2001-09-11
Williams, Hezron (Department: 2862)
Electricity: measuring and testing
Magnetic
With compensation for test variable
C324S230000, C324S234000, C324S207120, C324S207160
Reexamination Certificate
active
06288536
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an eddy current sensor with at least one measuring coil that can be supplied with alternating current, and with an evaluation circuit.
Eddy current sensors of the kind under discussion may be used, for example, for measuring a distance, or even for determining the conductivity of a measuring object. Depending on material and quality of the measuring object, an eddy current sensor may be used to determine even other properties of the surface coating of the measuring object, such as, for example, coating thickness. A further field of application for an eddy current sensor is the thickness determination of thin foils.
Eddy current sensors are frequently used in industrial surroundings, since they are also suitable for measurements under problematic environmental conditions, such as, for example, in the presence of strong magnetic fields, at high temperatures, and at a relatively high degree of pollution. However, strong temperature fluctuations also affect measurements with eddy current sensors. The present invention therefore concerns compensation of such temperature influences on measured data that are obtained with the aid of an eddy current sensor.
Temperature fluctuations affect the measurement results of an eddy current in two respects. First, the complex impedance of the measuring coil changes, in particular of the real parts of the complex impedance. Besides that, however, it is also possible to detect even a small temperature dependence of the imaginary part of the complex impedance. Second, the conductivity of the measuring object or the material thereof changes likewise along with the temperature. In certain temperature ranges, there exists an almost linear relationship between the temperature of the measuring object and its conductivity. Since conductivity of the measuring object material becomes effective on the eddy currents that are induced in the material via the measuring coil, the conductivity of the measuring object material influences again the measuring coil or its impedance via the feed back of the coupling. A complete compensation of the temperature influence on the measured data will therefore be possible only when both effects are compensated, i.e., both the temperature influence on the impedance of the measuring coil and the temperature influence on the conductivity of the measuring object material.
International Patent Application PCT/DE 93/00703, Publication WO 94/03778 and corresponding U.S. Pat. No. 5,629,619, disclose a method of compensating temperature influences on the output signal of a measuring coil, wherein a dc voltage is superposed on the ac voltage that supplies the measuring coil. The ac component of the output signal of the measuring coil is influenced both by the measuring object and by temperature influences. Contrary thereto, the dc voltage component of the output signal is subjected only to temperature influences, so that the temperature influences can be determined, isolated via the dc voltage component, and thus be considered and compensated in the evaluation of the dc voltage component of the output signal.
In the known method, the dc voltage component of the output signal of the measuring coil thus assists in only compensating the temperature influence on the real part of the measuring coil impedance. It is thus not possible to eliminate a possible temperature influence on the imaginary part of the measuring coil impedance. The temperature influence on the output signal of the measuring coil, which is due to a temperature dependence on the conductivity of the measuring object, is likewise compensated in this instance only when the measuring coil and the measuring object are subjected to the same temperature. In particular at high temperatures, this condition is often not met.
It is therefore the object of the invention to describe an eddy current sensor of the kind under discussion that facilitates with a simple construction and an evaluation circuit a reliable compensation of temperature influences both on the real part and on the imaginary part of the impedance of the measuring coil.
SUMMARY OF THE INVENTION
The above and other objects and advantages of the present invention are achieved by the provision of an eddy current sensor having at least one measuring coil and at least one compensating coil, both of which can be supplied with alternating current. The compensating coil is arranged in the direct vicinity of the measuring coil, i.e., in thermal contact therewith, so that the electromagnetic fields of the compensating coil and the measuring coil are orthogonal to each other.
In accordance with the invention, it has first been recognized that for evaluating the measuring signal of an eddy current sensor, it is normally necessary that the real part R
A
and the imaginary part X
A
of impedance Z
A
of the measuring coil be known per se, so that it is also possible to determine the phase angle &phgr; of complex impedance Z
A
via the quotient of imaginary part X
A
and real part R
A
, namely via the equation
tan &phgr;=X
A
/R
A
.
this end, it is necessary to eliminate not only the temperature influence on the real part of the complex coil impedance, but also the temperature influence on the imaginary part. In accordance with the invention, it has further been recognized that it is possible to determine the temperature influence on the imaginary part of the impedance of the measuring coil in the simplest manner with the aid of a compensating coil that is subjected to the same temperature influences as the measuring coil. In accordance with the invention, it is therefore proposed to associate to the measuring coil a compensating coil that can likewise be supplied with alternating current and is in thermal contact with the measuring coil, so that no temperature gradient can develop between the measuring coil and the compensating coil. It has still further been recognized by the present invention that it is possible to reliably detect with the aid of the compensating coil the temperature influence on the imaginary part of the measuring coil impedance only when the measuring object does not influence the impedance of the compensating coil. It is therefore proposed by the present invention to arrange the compensating coil such that the electromagnetic fields of the compensating coil and the measuring coil are orthogonal to each other. In this instance, the measuring object will practically have no influence on the impedance of the compensating coil, when the eddy current sensor of the present invention is positioned such that the presence of the measuring object is maximally effective on the impedance of the measuring coil. If one subtracts the complex impedances of the measuring coil and the compensating coil from each other in components by suitable circuit measures, one will obtain exclusively distance-dependent and material dependent impedance values. The sensor arrangement as proposed by the present invention facilitates in this manner a satisfactory compensation of the temperature influences on the impedance of the measuring coil.
Basically, there are different possibilities of a constructional realization of the coil arrangement of the eddy current sensor in accordance with the invention. In a particularly advantageous variant, the measuring coil has an annular form around which the compensating coil is wound in a fashion similar to the winding of a toroidal core transformer. The spatial vicinity of the measuring coil and compensating coil that results from this coil arrangement, ensures that the two coils are at the same temperature. Furthermore, it is here ensured that the electromagnetic field generated by the compensating coil is orthogonal to the electromagnetic field of the measuring coil.
As regards stability of the coil arrangement, it will be of advantage, when the measuring coil is wound around a coil form. The coil form may be of a dielectric material, for example, plastic. In this instance, the coil form would not affect the impedance of the measuring co
Mandl Roland
Mednikov Felix
Netschaewsky Mark
Wisspeintner Karl
Alston & Bird LLP
Andersen Henry
Microepsilon Messtechnik GmbH & Co. KG
Williams Hezron
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