Electricity: measuring and testing – Magnetic – Hysteresis loop curve display or recording
Patent
1989-11-27
1991-07-02
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
Hysteresis loop curve display or recording
324235, G01N 2772, G01N 2780, G01R 3314
Patent
active
050288695
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process for the nondestructive measuring of magnetic properties of a test body which in set-up technique is magnetized in a magnetic field of an exciter coil fed with an alternating current, as the hysteresis curve of the test body is run derived several times with the frequency of the alternating current. The tangential field strength generated by the exciter coil on the test body surface is continuously detected with the aid of a magnetic field sensor during the traversing of the hysteresis curve, there being derived during the traversing of the hysteresis curve a signal whose variation with time is correlated with the variation with time of the tangential field strength i.e. the amplitude of the tangential component of the magnetic field which is parallel to the surface of the test body.
The invention relates likewise to an apparatus for the nondestructive measuring of magnetic properties of a test body with a magnetization yoke which may be placed on the test body to be investigated with its two magnetic poles. The exciter coil of the yoke is connected with a bipolar network apparatus for the traversing of the hysteresis curve of the test body and between the magnetic poles of which there is arranged a magnetic field strength sensor for the detection of the tangential field strength. The sensor is connected to a switching circuit which gives out the magnetic magnitudes determined by evaluating the variation with time of the tangential field strength.
Such an apparatus as well as a process of the type mentioned at the outset are known from DE-Al-30 37 932, in which in addition to the magnetic field strength sensor there is provided a second receiver for the detection of the Barkhausen noise or of the superheterodyne permeability of the test body during the traversing of the hysteresis curve. While the known apparatus equipped with an inductive receiver determines the coercive field strength of the investigated material by evaluating the noise maximum and determining the tangential field strength allocated to the noise maximum, the apparatus equipped with an eddy current coil determines the coercive field strength by means which measures the impedance of the eddy current coil dependent on the superheterodyne permeability of the investigated material. Further, the tangential field strength allocated to the maximum impedance is used for the determination of the coercive field strength.
Both the apparatus equipped with the inductive receiver for the determination of the Barkhausen noise and also the apparatus provided with the eddy current coil require, in addition of the magnetic field strength sensor, a senor for determining the magnitude of a second physical characteristic. Furthermore, by the use of the inductive receiver for the detection of the Barkhausen noise as well as of the eddy current coil for the detection of the superheterodyne permeability there are imposed restrictions in regard to the detectable measuring depth, which is especially disadvantageous when the hardening depth is to be determined, or for sorting of material wherein it is necessary to detect large volumes of material.
Proceeding from the above-mentioned state of the art, underlying the invention is the problem of creating a process and an apparatus which make it possible to detect magnetic properties, especially coercive field strengths, a well as the shape of the hysteresis curve at the coercive field strength point without using a sensor other than the magnetic field strength sensor also in the case of great measuring depths.
This problem is solved according to the invention in a process of the type mentioned at the outset by the means that the derived signal is determined from the variation with time of the tangential field strength signal, by means of a true-to-phase extremely narrow-band filtering, i.e. filtering which does not change the phase of the signal, from the periodic total signal of the tangential field strength. In this way the harmonic signal constituent is separated from the constitue
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Dobmann Gerd
Pitsch Holger
Fraunhofer-Gesellschaft Zur Forderung der Angewandten Forshung E
Strecker Gerard R.
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