DE2431040A1 - METHOD AND DEVICE FOR DETERMINING A MAGNETIC CHANGE IN STATE BY MEANS OF EDBY CURRENTS - Google Patents

Description

DR.-ING. EUGEN MAIER DR.-ING. ECKHARD WOLF

TELEPHONE: (O711) 3 * 27 61/3 TELEQRAMS: MENTOR

PATENT LAWYERS

7 STUTTGART 1, PISCHEKSTR. 19th

2A31040

DRESDNER BANK AQ STUTTGART NO. 1 9SO 934 POSTSCHECK STQT. 25200-703

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June 18, 1974
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INSTITUT DE RECHERCHES DE LA SIDERURGIE PRANQAISE

185, rue President Roosevelt Saint-Germain-en-Laye / France

Method and device for determining a magnetic change in state by means of eddy currents

The invention relates to a method and an apparatus for determining a change in the magnetic behavior of an electrically conductive material, wherein in the material Eddy currents are generated by means of an induction coil and the changes in the impedance of the induction coil are analyzed will. In particular, the invention relates to such methods and devices in which the impedance of the induction coil is significantly influenced by changes in the air gap.

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Under a change in magnetic behavior becomes
here above all the transition of a material from one
Understood first non-magnetic into a second magnetic state of a material or a transition in the opposite direction, this transition occurring, for example, as a function of the temperature.

If an alternating current flows through a coil near an electrically conductive material, e.g. in
placed in the vicinity of a metal or an alloy, eddy currents are induced in the interior of the conductive material due to the magnetic field of the coil. These induction currents in turn produce a magnetic field that is opposite to the magnetic field generated in the coil. Compared to the case without the induced magnetic field, this apparently changes the impedance of the coil. Any change in the intensity or distribution of the induction currents in the material results in a change in the induced magnetic field, which causes an additional variation in the impedance of the coil.

Attempts have already been made in the relevant technical field to establish a correlation between the change in impedance on the one hand and on the other hand the phenomena which cause a disturbance of the induction currents.

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Certain phenomena draw a predominant change either in the ohmic or in the inductive component of the impedance. In this case it is Establishing such a correlation is relatively simple by adding the amplitude and phase changes a signal corresponding to the impedance can be determined.

On the other hand, it proves difficult, while eliminating ambiguities, the type of a certain impedance change to determine the cause of the appearance, if several appearances at the same time the two impedance components influence. The procedures used so far to evaluate the tapped on the coil Original signals are usually complicated; therefore the information that can be obtained through different decompositions of the original signal is very difficult to get in explain clearly. *

A change in the magnetic behavior of a material can be demonstrated without ambiguity by the fact that a · fed by an alternating current and arranged in an air gap that is constant with respect to the material to be examined Coil is used.

The change in the magnetic behavior of a material causes a change in the phase and amplitude of the output

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output signal of the coil after itself. If at the same time another phenomenon produces a similar effect, it is obviously impossible to have an immediate relationship occurring between one or the other Appearance and change in the output signal of the coil. The variation of the air gap represents a Appearance of the aforementioned type. Therefore, no simple means is known so far, whereby a change in the magnetic behavior of a material can be determined in a unique way if the air gap is not very precisely remains constant, for example, as in the case of a passing product that carries out vibrations or has a wavy surface.

The invention is therefore based on the task of a method indicate, which also then changes the magnetic behavior of a material in a simple manner can be determined when the distance between the induction coil and the material is subject to variations which significantly affect the impedance of the coil.

To solve this problem, it is proposed according to the invention that the coil by means of an alternating current source is fed with an essentially constant effective current strength and the at the terminals of the coil when

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The essence of the material to be investigated Voltage signal, a voltage signal corresponding in terms of amplitude and phase is directed in the opposite direction for adjustment and that it results from the change in the impedance of the coil in the presence of the material Measured unbalance signal and a phase shift of the unbalance signal with respect to a reference signal corresponding , the recognition of the state transition signal is generated.

The conductive material can advantageously consist of a metal that depends on the temperature a transition from a first paramagnetic crystal state to a second ferromagnetic crystal state, or performs a transition in the opposite direction. When the first crystalline state and the second crystalline

State are of the same kind, the signal corresponding to the phase shift qualitatively indicates that the Temperature of the material corresponds to the Curie point of that material. If, on the other hand, the first crystalline state and the second crystalline state are different in nature, shows said phase shift signal qualitatively a crystalline transition of the material from the first to the second crystalline state.

An apparatus for carrying out the invention

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The method advantageously includes one between an alternating voltage generator and the induction coil arranged current regulator for feeding the coil with a substantially constant current so that the Voltage changes at the terminals of the coil are an image of the impedance changes in the coil, a balancing circuit with regulating organs acting on the amplitude and the phase of the generator voltage for generating a too the signal corresponding to the voltage signal appearing at the terminals of the coil, a comparator, its inputs are acted upon by the coil signal on the one hand and the signal of the balancing circuit on the other hand, and one Phase discriminator for generating one of the phase shift of the output signal from the comparator signal corresponding to a reference phase.

This becomes complicated with the method according to the invention Problem of analyzing one affected by two different phenomena at the same time Signal traced back to a simple problem of phase determination, for which various solutions are known. In this way one can clearly establish a direct relationship between the change in one to one of the two phenomena associated parameter, in the present case the permeability of the material, and that of the phase discriminator produce derived information in the

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Way that the problem, which at first appears complicated, is completely solved.

In the following the invention will be explained with reference to the drawings explained in more detail. Show it

Fig. 1 is a complex impedance diagram of a powered by an alternating current and in a variable Removing a coil disposed on an electrically conductive material;

Fig. 2 is a diagram of the various output signals the coil, the behavior of which is shown in the diagram of FIG. 1;

3 shows an impedance diagram in which the method according to the invention is illustrated;

Fig. 4 is a diagram in which the type of change of the signal is shown according to the method illustrated in the diagram of FIG. 3 is produced;

Fig. 5 is a block diagram of a device for implementation of the method according to the invention;

6 shows a sectional detail view of an inductive measuring sensor; ,

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7 shows some diagrams of the course of the signals at various points on the device shown in FIG.

The electrical behavior of a coil, the terminals of which are subjected to an alternating voltage V, is determined by

V = ZI

described, where Z is the impedance of the coil and I is the strength of the current flowing through the coil. If such a coil is placed in the vicinity of an electrically conductive material, it induces through the coil current generated magnetic field inside the material eddy currents; these eddy currents in turn have a Magnetic field result, which is opposite to the field generated in the coil. This results in a change in the Impedance of the coil.

Assuming that a coil with the inherent impedance Z is fed with a constant current I, the variations in the voltage V at the terminals of the coil reflect the apparent variations in the Impedance of the coil. This allows the location of the end point of a vector corresponding to the impedance of the coil in the complex plane is drawn in this locus

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is called the impedance diagram in the following.

1 shows an impedance diagram of a coil with an ohmic inherent resistance R and an inherent inductance L, which is fed with a constant sinusoidal alternating current of frequency co.

In a complex plane, the abscissa of which corresponds to the ohmic component R of the impedance and the ordinate of which corresponds to the inductive component wL of the impedance, the complex impedance Z of the coil can be given by a vector with the module / R + GO L and the argument I ^ = arc tan ^ r are represented. In the diagram of FIG. 1, the vector with the module Z and the argument represents the impedance of the coil in the absence of all additional influences, in particular the influence of an adjacent material to be examined. The end point of the vector is in the diagram by the point Koo marked.

If the coil is influenced by an external phenomenon, for example when it approaches the material to be examined, its impedance is changed. If an amplitude and phase measuring device is connected to the coil, it allows the amplitude a and the phase _{to be determined on the basis of the voltage signal Vj 3 appearing at the terminals of the coil}

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of the signal, the phase being expediently defined with respect to a purely ohmic reference phase, the end point of the vector corresponding to the impedance of the coil can be entered in a table of values, the coordinates of each location point M being represented by a corresponding value pair (a cos ψ ', a sin y) "is given.

When the coil is approached to a non-magnetic conductive material, the point M describes a certain curve C i, which obviously runs through the point K ₀₉ , which corresponds to the self-impedance of the coil. If, on the other hand, the coil is brought closer to a magnetically conductive material, the point M describes a certain curve C- * which also runs through the point K ₀₀ . When the gap between the coil and the material is increased, the point M approaches the point K _<xf on one of the curves C, or C ₂ , depending on whether the material is magnetic or not. In the example case, the location of the point M was shown, which corresponds to a material that changes from a first non-magnetic state into a second magnetic state. A non-magnetic state is generally understood to mean a state of weak magnetic permeability of the material under consideration, for example the paramagnetic or diamagnetic state. A magnetic state is understood to mean a state with high magnetic permeability of the material under consideration,

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for example the unsaturated ferromagnetic state. The impedance diagram shown in FIG. 1 relates to a steel that changes from a paramagnetic to a ferromagnetic state under the effect of a decrease in temperature, this change in magnetic behavior being caused, for example, by the appearance of ferrite in the grain structure of the steel . Immediately, the belonging to the respective intermediate space impedance point at the point M located in front of this transition on the curve C ,. After the start of the transition moves the point M on a given curve C ₃ in the direction to the point M _f on the curve C - belonging to the gap under consideration, the gap being _{assumed to be constant in the course of curve C 3.} In the event of a change in the gap, point M describes another curve starting from point M to another point on the curve C ₂ , which corresponds to the new value of the gap.

The displacements of the point M on one of the curves C 1, C 1 or C represent a change in the phase and amplitude of the signal corresponding to the impedance of the coil. The nature of this change is illustrated in the sketch in FIG. 2, in which the signal V ₁₃₀ belonging to the self-impedance of the coil with the amplitude a and the phase Ψ

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June 18, 1974

and the _{signal V fe,} which is phase-shifted by Δ ψ and changed in amplitude by ^ 4a, is shown, the latter signal V, corresponding to a point M on the curve sequence Cl, C2 and C3. At first it seems "that there is no clear relationship between the variation of the phase and the amplitude of the signal, on the one hand, and one of the parameters under consideration, on the other hand?" in fact, the variation in the air gap and the eventual change in the magnetic behavior of the material constitute two parameters which simultaneously modify the phase and amplitude of the signal appearing at the terminals of the coil.

Assuming the above-mentioned special experimental conditions, i.e. a coil is supplied with a constant effective current, it can be seen that the curves C ₁ and C 2, which correspond to the changes in the gap for a certain magnetic state of the material, have an im have a substantially straight course. In other words, this means that the joint development of the amplitude and the phase of the signal tapped at the terminals of the coil under the aforementioned conditions results in a practically rectilinear shift of the end point of the vector representing the impedance of the coil in the complex plane.

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This experimental result can be theoretically underpin and applies especially to relatively flat coils, as they are usually used for eddy current sensors can be used in which the coil axis is essentially perpendicular to the surface of the product to be examined and which, as is customary in this context, operated at frequencies of a few kHz up to a few tens of kHz will. This result has been observed for numerous steels, especially those that are ferromagnetic Condition, experimentally confirmed.

On the basis of this experimental finding, a signal can now be derived that is representative of a Change in the magnetic behavior of a material. . ■

For this purpose, a coordinate transformation is carried out in such a way that the origin of the complex plane is _{shifted to the point K 00} , and that the phase of a signal is then determined which is formed in the new complex plane by a vector whose origin the point K ₀₀ and its end point is the point M of the impedance diagram for the associated impedance value of the coil.

In the diagram of FIG. 3, the geometric location of the points M is formed by the curves C ₁ , C ₂ and C ₃ , which are marked with

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coincide with the curves in the diagram of FIG. 2, and the curves C ₁ and C _{2 are} straight lines which intersect at point K «d in accordance with the above explanations. An axis transformation in accordance with a first proposed method step shifts the origin of the complex plane to point K * ©, the new axes being formed, for example, by the - axes XY in FIG. 3. It should be noted that the geometric location of the point M is essentially independent of the axis system chosen for the complex plane.

If the point M is shifted on one of the straight lines C ₁ or C ₂ , ie in the case of a variation of the gap for one of the magnetic states of the material, the phase shift of the _{signal represented by the vector K 00} M in the plane XY represents with respect to any one

Reference signal, for example the phase of a vector in the X-axis, represents a practically constant value θ, namely either Θ ,, if the point M _{shifts on the straight line C 1} and the material is not magnetic, or Θ ₂ # if the Point M moves on straight line C ₂ and the material is magnetic. The changes in the space in one or the other case are therefore transferred exclusively to changes in the modulus of the vector K ₆₀ M. These changes correspond to the changes Am in the amplitude

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of the signal represented by said vector, as illustrated in the sketch of FIG. 4a.

In the event of a change in the magnetic properties of the material, point M shifts along curve C _{3 of} the impedance diagram. The phase shift of the signal represented by the vector KcoM with respect to a reference phase now has a variable value θ + Δ9 , where θ is a constant and Δθ is a variable. This change in the phase shift therefore provides unambiguous information for a change in the magnetic properties of the material, regardless of how "ever the gap changes at the same time. It is understood that this result cannot be obtained if the experimental conditions prescribed above are not observed, that is, on the one hand, if an essentially constant current is not maintained in the coil and, on the other hand, if not in the event that the external material influences on the coil are not present, an adjustment is made. These two steps are indispensable and form the essence of the invention The relevant phase change Λ θ is plotted in the sketch of FIG. 4b; the amplitude changes of the signal have no clear meaning in this case.

To carry out the axis transformation in which the origin of the complex plane is in that of the self-impedance

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point K «» corresponding to the coil is moved that at the terminals of the coil in the absence of the external given due to the proximity of the material to be examined Influence on the coil tapped signal tuned to zero, this result by generating a in terms of phase and amplitude with the signal appearing at the terminals of the coil and identical signal by opposing these two signals to cancel each other out.

Any change in the impedance of the coil results in a non-zero signal that is in the complex XY "plane is represented by a vector KjoM. A possible Change in the magnetic behavior of the material under investigation can be achieved by simply determining the phase of the can be determined by the signal represented by said vector. As the parts of the impedance diagram that are pure Changes in the space correspond, in certain cases cannot be exact straight lines, are only such phase changes which are above a certain threshold value must be taken into account.

In Fig. 5 is a block diagram for the purpose of illustration, an apparatus for carrying out the method is shown, the various steps of which are shown above have been developed. This device includes a

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Oscillator 1, the output of which is connected to a first branch A and a second branch B, each of which in turn are connected to an input of a comparator, which in the case of the exemplary embodiment by a differential amplifier 2 is formed. The first branch A contains a current regulator 3, which mxt with its one terminal the coil 4 connected to the ground is fed with an alternating current of practically constant strength. When using a Oscillator 1, which supplies a constant sinusoidal alternating voltage, can be controlled by a simple current regulator Voltage-current converter be formed, for example an operational amplifier, which at its output despite possible Changes in impedance in the downstream consumer circuit (coil 4) emits a practically constant current I = kV, where V is the oscillator voltage. In the second branch B there is a balancing circuit 5 for generating a voltage signal adjustable in terms of phase and amplitude. This circuit includes a differentiator 6, two amplifiers with adjustable gain 7a, 7b and an adder 8. The output of the oscillator 1 is on the one hand with the input of the differentiator 6 and on the other hand with connected to the input of the controllable amplifier 7a. Of the . The output of the differentiator 6 is via the amplifier 7b connected to a first input of the adder 8, while the output of the amplifier 7b to a second input of the adder 8 is connected. The differentiator 6 can

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be formed by an operational amplifier which is connected in such a way that the sinusoidal oscillator signal phase shifted by IT / 2. The outcome of the Differential amplifier 2 is connected to a first input of a phase discriminator 9, the second input of which is connected to a point of the branch B, which point in the case of the embodiment is between the amplifier 7b and the adder 8. At the inputs of the phase discriminator 9 there is a flip-flop circuit 10a and 10b,

one
the outputs of which are connected to the inputs of the AND gate 11, which in turn is connected to a low-pass filter 12. In the case of the exemplary embodiment, the output of the phase discriminator 9 is connected to a threshold value switch 13 which controls a relay 14, which is immediately followed by a control lamp 15. Another band-pass filter 16 is located between the differential amplifier 2 and the flip-flop 10a.

6 shows a detailed sketch of an exemplary embodiment of a measuring sensor 18 which has an induction coil 4 includes. The coil contains a flat winding, the axis of which is perpendicular to the surface of the product to be examined, for example a sheet 22 running past. The coil is arranged on a hollow plastic core 19, which in turn is arranged in a sealed graphite container 20 surrounded by a protective cover 21. The coil will

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by means of water that is fed axially over the core and over a line 23 is discharged from the graphite container, kept at a constant temperature. This precaution avoids a heating of the coil in the case of an investigation of materials with a relatively high and not constant Temperature.

In the various slide arms of FIG. 7 is for illustration the function of the individual components of the

Signal device according to FIG. 5, the course indicated at various points of this device. The various diagrams and the signals in FIG. 7 are designated with the reference letters a, b, ... 1, η, which correspond to the designations a, b, ... 1, η in FIG. It should also be noted that the signals shown correspond to the method that was described in connection with FIG. The signal (a) generated by the oscillator 1 has a sinusoidal shape and corresponds to a voltage with a phase shift of zero. The signal (b) at the output of the differentiator 6 is a sinusoidal curve which is phase-shifted by Ύ / 2 with respect to the signal (a). The signal (c) at the output of the amplifier 7b corresponds to the signal (b) except for the amplitude and the signal (d) at the output of the amplifier 7a corresponds to the signal (a) except for the amplitude. The signals (c) and (d) are added in the adder 8 and result in the signal (e)

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The signal (d) can be represented as a vector with an adjustable module that is in phase with the vector corresponding to the signal (a), while the signal (c) can be represented by a vector also with an adjustable module that is in phase with the signal (a ) corresponding vector is phase-shifted by Ir / 2. It follows from this that the sum of these two vectors with an adjustable module is a vector corresponding to the signal (s), the phase and module of which can be adjusted externally at will. In particular, the phase and amplitude of the signal (e) can be brought into agreement with the signal (f), which corresponds to the impedance of the coil 4. According to one method step, the signal (e) can therefore be brought into agreement with the signal (f), which corresponds to the intrinsic impedance of the coil, ie in the absence of an external influence on the coil, for example due to the inductive coupling with the material to be examined will. It is understood that the controllable parameters that could change the impedance of the coil, in particular the temperature, are kept constant. In other words, the signal (f) corresponds to the signal represented in FIG. 1 by the vector OK «o with the module Z and the argument ^ o = arc tan wL / R.

Since the differential amplifier 2 has a positive and a Has negative input, the signals (e) and (f) are directed opposite to each other in the amplifier, so that on

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the signal zero results at the amplifier output. Through this the adjustment condition is implemented according to a further method step according to the invention.

The diagrams relating to the signals behind the differential amplifier 2 relate to the case that the Gap and the magnetic behavior of the material can be changed simultaneously or separately after beforehand the matching condition by varying the gain of the amplifiers 7a and 7b in the manner described above has been set.

If the product to be examined is present, an unbalance signal results at the output of the differential amplifier 2, the band-pass filter 16 tuned to the frequency of the oscillator 1 eliminating the harmonics, which can be generated by the differentiator 6 and the coil 4. This unbalanced signal is generated by the Signal (g) is formed at the filter output and has essentially the sinusoidal curve for which the filter is selective. From the following it will be seen that the course of the Signal (g) is of little importance if only the frequency of the zero crossings of this signal is identical with the frequency of the zero crossings of the pure sinusoidal signal, which can very easily be obtained by the Influence of the harmonics by a sufficiently selective

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Filter is kept small enough. The signal corresponds to a value d, the space and a value po is the magnetic permeability of the material to be examined, for example p. - 1, corresponding to one paramagnetic state of steel at high temperature. For a value d_ des that is different from the value d Between the space and the same permeability u one obtains an asymmetry signal (g) ,, with the same phase as that Signal (g) j ·, / but different amplitude, what with corresponds to the above observations made in connection with the explanation of FIGS. 3 and 4. In the event of a modification of the magnetic state of the material becomes the unbalance signal (g) ,, due to the variation

α, u

the magnetic .Permeability and a possible variation of the gap in amplitude changed and also experiences a phase change by the amount Δ θ compared to the signals g ,. that belong to pure changes in the space. In the case of the exemplary embodiment, the change in the magnetic permeability is a consequence of the steel being examined being cooled to a temperature at which the steel becomes ferromagnetic due to the occurrence of ferromagnetic ferrites. The corresponding change in the impedance of the coil is illustrated in the impedance diagram of FIG. 3 by a curve, the type of curve part C-. For the conclusions to be drawn within the scope of the present invention, the course of the curve C ₃ , the development of the phase shift 4θ and the labeling are required

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the position of the common point M _{f of} the curves C ~ and C no further investigation, since the existence of a phase shift A θ and its finding is sufficient for this alone.

In order to realize the above-mentioned adjustment condition and thus the shift of the axis zero point of the impedance diagram to be able to effect the point Koo, is located behind the bandpass filter 16 is a tap to which a known amplitude measuring device can be connected can.

A phase shift can be determined in various ways. In the case of the exemplary embodiment, the signal (c) given to a negative input of the flip-flop 10b «to form the inverse signal (c.) and the To trigger flip-flop at every positive half cycle of the signal inverted in this way. That through the tilting stage 10b generated signal is the signal (h). The current branch B located in front of the differential amplifier 2 originating signal (c) is a signal of constant phase, and thus forms a reference signal, so that the fronts of the Square wave signal (h) are fixed. Any other signal with constant phase could also be used as a reference signal be used. The signal (g) with variable phase

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is applied to a positive input of the flip-flop 10a, so that a signal (i) is generated whose value corresponds to the value of signal (h) in the course of the triggering periods. The signal (j) output by gate 11 is a square wave whose width corresponds to the coincidence between signals (h) and (i) and is therefore directly proportional for the phase shift between the reference signal (c) and the unbalance signal (g). The low-pass filter acts as an integrator that emits a signal (k) that is proportional to the area of the rectangle (j). At constant The height of the level of the rectangle is the area directly proportional to the width of the rectangle and thus to the Phase shift between signals (c) and (g). The signal (k) is given to the threshold switch 13, where the threshold value (1) can be set by applying a voltage ν, if the value of the signal (k) exceeds the threshold value (1), the threshold value circuit a signal (s) generated and given to the relay, which is thereby excited and switching on the Control lamp 15 causes. The yes-no information formed by the signal (s) can also be sent to a computer be given if the control device described is part of an automated process control. the Adjustability of the trigger level of the threshold circuit 13 allows phase shift signals with smaller To eliminate amplitude resulting from a certain non-

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linearity of curves C, and C ₂ , ------

It goes without saying that the adjustment signal before the differential amplifier 2 can also be used in a different manner, as here described, can be generated. Thus, instead of the circuit 5 in the current branch B, one with the coil 4 identical coil can be arranged. This solution is, as far as the circuit is concerned, simpler. Still hers is Realization difficult because it is practically impossible to have "two coils of the same ohmic resistance and the same To get inductance, what the setting of the equalization condition at the output of the differential amplifier 2 very much makes difficult and thereby significantly reduces the selectivity of the device.

ι '

The method and the device according to the invention finds its application in particular in the investigation a change in the magnetic behavior in steels from a paramagnetic to a ferromagnetic state can pass, for example in the course of a thermomechanical or thermal treatment. In utility steels and a large number of alloy steels of the above Art occurs on cooling below the Curie point ferrite. This ferrite is instantaneously ferromagnetic. It follows that the method and the device according to the invention make it possible to start the crystalline

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The transition from y-iron to <A.-iron can be observed, whereby this information of great interest especially in the case of continuous thermomechanical or thermal Treatment of iron is. In certain cases, for example with extra-soft steels, ferrite is formed already above the Curie point. However, this ferrite is not yet ferromagnetic at these temperatures. The inventive method allows in this case Determination of the Curie temperature of the steel, which corresponds to the temperature at which the ferrotnagnetism of the Ferrite uses. This information can also be of importance in the thermal or thermomechanical treatment be. The main advantage of the invention is that the influence of changing the distance between the Coil and the product to be examined is suppressed, which changes result in an interfering signal that it was previously

made it impossible to selectively observe changes in magnetic behavior in a passing product.

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Claims (8)

Claims (l.y method for determining the transition of an electrically ¹ conductive material from a first non-magnetic state to a second magnetic state, wherein eddy currents are generated in the material by means of a coil and the changes in the impedance of the coil are determined, characterized in that, - That the coil is fed with an alternating current with an essentially constant effective current strength, - That the at the terminals of the coil in the absence of the voltage signal occurring to overshoot material (f) a voltage signal (e) corresponding with respect to amplitude and phase is directed in the opposite direction for the purpose of adjustment will, - That the asymmetry signal resulting from the change in the impedance of the coil in the presence of the material (g) is determined, and that a signal (n) which corresponds to the phase shift (AQ) of the asymmetry signal (g) with respect to a reference signal (c) and serves to detect the state transition is generated. - 28 - 409885/0907 2. The method according to claim 1, characterized that the electrically conductive material is a metal that depends on the Temperature from a first paramagnetic crystal state to a second ferromagnetic crystal state or vice versa. 3. The method according to claim 2, characterized in that the first crystal state and the second crystal state are similar, so that the signal at a temperature of the Material is triggered which corresponds to the Curie point of the material. 4. The method according to claim 2, characterized in that the first and the second K-r-iet-ailstatus are different, so that the triggered Signal the crystalline transition from one crystalline state to the other crystalline state indicates. ., ._, .. 5. Device for performing the method according to a > - " of * Claims 1 - 4, ge ke η η ζ e. i, ch η _e t _w ., _ ", ...,. by an alternating voltage generator - 29 409885/0907 (Oscillator 1) and the induction coil (4) arranged amperage controller (3) for feeding the coil with a substantially constant effective current, so that the voltage changes at the terminals of the coil are an image of the impedance changes in the coil, a matching circuit (5) with on the amplitude and the phase of the generator voltage acting regulating organs (amplifier 7a, 7b) for generating one to the at the Terminals of the coil occurring voltage signal corresponding signal, a comparator (2), whose Inputs with the coil signal (f) on the one hand and the signal (e) of the balancing circuit (5) on the other hand applied and a phaseri discriminator (9) for generating one of the phase shifts of the output signal (g) of the comparator with respect to a reference phase (c) corresponding signal ^ (h). 6. Apparatus according to claim 5, characterized d u rc h one connected to the discriminator circuit (9) Threshold circuit (13) which outputs a signal (n) when the phase shift signal (k) is a certain threshold value (1) is reached. 7. Apparatus according to claim 5, characterized in that that the current regulator (3) as a voltage-Strora converter fed with a constant voltage is trained. - 30 - 409885/0907 8. Apparatus according to claim 5, characterized g e. indicates that the balancing circuit (5) comprises the following components: - a circuit (6) for shifting the signal (a) coming from the generator (oscillator 1) by 90 °, - A first amplitude regulator (7a) acting on the signal (a) coming from the generator, - a second to that of the phase shifter circuit (6) incoming signal (b) acting amplitude regulator '(7b) and - An adder, the inputs of which are supplied with the output signals (c) d) of the amplitude regulator and the output of which is connected to one input of the comparator (2). 409885/0907