DD143959A5 - METHOD AND DEVICE FOR CONTACTLESS MEASUREMENT OF THE VOLTAGE CONDITION IN FERROMAGNETIC MATERIALS - Google Patents

Abstract

The invention relates to a method and a device for contactless measurement of elastic Strains in ferromagnetic bodies, especially in sheets, due to the dependence between magnetic properties and elastic stresses of the material are based. To a high Measuring accuracy also during industrial production, e.g. when rolling, to reach, were to solve the following tasks: Correct and correct-sign display of anisotropy changes, Display also of small anisotropy changes, sinusoidal temporal dependence of Induction change and Unsobhänigkeit the measurement result of air gap changes. At a Measuring head and a detector whose excitation windings are largely perpendicular to each other arranged U-cores are arranged according to the invention, the measuring head is rotatably arranged, wherein during rotation, the output signal of the detector is detected phase sensitive. hereby are the direction of the larger of the two running parallel to the surface of the specimen Main tensions and the difference between them can be determined. It is preferred with worked different measuring frequency. The sinusoidal Induktionsverisuf in the test specimen is through secured a Rückkuppungsschleife. Fig.6 -

Description

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The invention relates to a method and a device for contactless measurement of elastic stresses in ferro-magnetic bodies - in particular in sheets.

Characteristics of the _n known. technical

The invention is based on the well-known phenomenon that the magnetic properties of ferromagnetic materials by elastic voltages are changed (Kagnetoelastizität), in such a way that the elastic stress causes a magnetic anisotropy or the existing magnetic anisotropy modified · Thus becomes a magnetic field exciter Coupled with a magnetic field detector via a ferro-magnetic specimen, the detector signal is influenced by the elastic stresses of the specimen. This creates

a measuring principle for the magnetic measurement of elastic stresses in ferromagnetic bodies. A realization of this principle is described, for _o example, in GB-PS No. 1 116th

The based on this principle, previously known magnetic devices for measuring the elastic voltages have the following disadvantages

a) Even for tapes made of the same material (for example, a single ingot), it is necessary to calibrate the electrical signal of the measuring device specifically to the elastic tension for each strip thickness and to choose a suitable exciting current. Without calibration and adjustment of a suitable excitation current, the known measuring devices can not only indicate the size but also the sign of the voltage in the case of metal sheets, if the magnetic operating point of the measuring head is substantially shifted during the yawl by the change of the magnetic anisotropy.

b) Since the device senses the combined effect of the already existing magnetic anisotropy and the elastic strain to be measured, it can only be calibrated for such large external elastic stresses which cause magnetic anisotropy, in addition to which the original magnetic anisotropy of the device under test can be neglected ,

c) In the known devices, the purely sinusoidal temporal change of the induction is not secured, so that the Fourier spectrum of the measuring signal several Fourie! 'Components. Enthalt, which significantly reduces the accuracy of measurement. · '. ,

d) the measures that have hitherto been taken to _o achieve the independence of the electrical output voltage of the air gap had not secured the stability of the magnetic Arbeitapunkttests why the measurement of vibrating or not perfectly flat belts only with severe accuracy was possible, not It can be ruled out that the measurement in extreme cases provides sensitive information.

The difficulty mentioned under point (a) requires a considerable calibration and adjustment work, especially in reversing machines. Each of the defects mentioned in points b) to d) can measure not only in reversing but also in single-pass rolling, even falsify the dressing.

With the invention, the deficiency of the prior art should be eliminated.

Presentation of the _n y / es _i en.8 _{ii ii} invention;

It has been recognized that the drawback a) can be avoided by allowing the measuring device to operate at a frequency where the skin depth is smaller than the thickness of the specimen. The skin depth is known to depend on the frequency of the exciter field, the magnetic permeability and determines the specific electrical conductivity of the specimen.

It also has other advantages when the frequency of the magnetic field is variable in a wide interval. By changing the frequency, the depth of the skin can be changed, and so the average mechanical tension under the surface can be measured down to different depths. "

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known measuring automatic set a suitable frequency program so you can, for. During the temper rolling, the distribution of the elastic tension across the thickness of the strip can also be measured continuously, whereby the rolling table can be adjusted to the optimum stress distribution or the homogeneity of this distribution can be continuously controlled by means of a suitable device.

It has further been recognized that the disadvantage b) can be avoided if the effect of the magnetic anisotropy already present in the test piece is canceled out by a suitable static magnetic compensator pair. In this way, regardless of the size of the magnetic anisotropy already present in the material, arbitrarily small external mechanical stresses can be measured with sufficient accuracy *

It has also been recognized that the Hachteil c) 'by a known per se, but never used in the magnetic measurement of elastic stresses negative magnetic feedback in the magnetic excitation circuit can be avoided. The excitation current then contains harmonics to the extent and in the manner that the induction flux assumes a pure sinusoidal shape, as a puncture of time, and thus one achieves that it does not excite disturbing eddy currents. At the same time, this also reduces the y / f current loss to a minimum.

Further, it was recognized that ^r can be prevented by the negative feedback magnetic potential in the magnetic excitation circuit also the disadvantage d). It is automatically ensured that a change in the air gap does not shift the specimen's original magnetic working point. If this measure according to the invention is supplemented with one of the arrangements already occasionally used in the previously known devices,

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which make the detector independent of the air gap, the voltage sensing is independent of the changes in the air gap.

It has also been recognized that when feedback is provided to a permanent operating point and a suitable choice and modification of the skin depth, it is possible to develop a method and a device which permit continuous testing of the ferromagnetic strips on the rolling mill stand. The test consists in that the spatial changes the parameter of the magnetic anisotropy can be determined in suitable directions or in all three dimensions.

Finally, the invention is based on the recognition that a non-contact magnetic strain measuring device in industrial production is suitable for recording the magnetic anisotropy of large ferromagnetic materials in direction and magnitude, regardless of whether the magnetic anisotropy is due to the crystallographic texture, for example. internal magnetic stresses, magnetic heat treatment or impurities. From the magnetic anisotropy on the physical and technological properties of the specimen (ζ · Β _β on the directions of the major axes of the internal stresses, the differences of the principal stresses, the texture, the inhomogeneity of the Hardness, flatness errors, etc.). In many practical cases, at least in the depths _s, which can still be detected with the usual magnetic measurements, two principal stresses parallel to the surface of the specimen run in all of the late r described inventive method is assumed that the candidate was in such a Spannungszuatand

The invention is aleo a method in which in ferro-

magnetic bodies - preferably in sheets - a contactless measurement of parallel to the surface extending two principal voltages is made possible, with a measuring head, the exciter, which is aligned in the direction 1, generates a variable magnetic field in the test piece, and its detector, the one is arranged in the direction 1 almost vertical direction 2, this magnetic field measures. The method is characterized in that the measuring head is to be rotated about an axis which is perpendicular to the surface of the specimen, and that during rotation, the output signal of the detector is detected phase-sensitive. Thereby, the direction of the larger of the two mentioned principal stresses and the difference between the two principal stresses can be determined.

Since the method of measuring the elastic stresses to be described makes no difference between the internal and external stresses, the local sum of the internal and external stresses is always measured with each measurement.

The invention further relates to a method for the contactless measurement of the change of the stress state in the direction perpendicular to the surface, in which a Meßkopferreger, which is aligned in a first direction, generates an alternating magnetic field in the sample, while the detector, which in a is arranged to this direction almost perpendicularly standing second direction, this magnetic field measures. This method is characterized in _t that continuously changes the frequency of the magnetic field during the measurement.

The device according to the invention is used for non-contact measuring dea Spannungszustandes in ferromagnetic bodies »The device consists of a pathogen, the

is aligned in the first direction and in the ferromagnetic test specimen generates an alternating magnetic field and from a detector which is aligned in a direction 2 almost perpendicular to the first direction 2 and measures this alternating magnetic field. The device is characterized in that it has a pair of compensators existing magnetic Kompensato rpaar whose two compensators can be energized simultaneously and independently with DC and which are both with the exciter and with the detector in a minimum magnetic coupling.

Examples t

The invention will be explained in more detail with reference to the drawings, which illustrate the appropriate embodiments or «applications.

Fig. 1; shows the schematic structure of a

known device for the magnetic measurement of elastic stresses.

Pig * 2% shows the output voltage of the Mn

direction as a function of the angular position of the measuring head consisting of the exciter and the detector «

Pig. J>% shows the gchem of the device that shifts and rotates the probe.

Pig »4s shows the distribution of internal stress over the strip thickness in a typical strapped steel strip as a function of the depths under the surface«

Pig. 5a and 5b: show the arrangement of the magnetic cores of the device according to the invention in axonometric and in the DUT viewed her view.

Pig. 6: shows the electrical circuit of the

DC powered magnetic compensators.

Pig. Fig. 7J shows the arrangement of the negative ones

magnetic voltage feedback.

Pig. {3: shows the arrangement of the on a

Kaltwalzanlage mounted erfindungsgeinäßen measuring heads, which serve as a sensor for a control device or a Eegelungssystern.

Pig. 1 shows the structure of the measuring head known per se for the magnetic measurement of the elastic stresses. The measuring head 1 consists of the exciter 26 and the detector 27. The exciter 26 is composed of the ferromagnetic U-core 2 and the coil 3 mounted thereon. The detector 27 is composed of the ferromagnetic U-core 4 and of the coil 5 mounted thereon. The U-core is nearly perpendicular to the U-core 2, and the cores are adjusted so that there is minimal magnetic coupling between the i-detector 27 and the exciter 26. The coil 3 is fed by the generator 6. The output signal of the coil 5 is fed to the phansenempfindlichen amplifier 7 "In Pig · 1, the measuring head 1 is located above the test piece. 8

Due to the alternating current in the coil 3, the test object 8 is periodically magnetized. * Is the material of the test object 8

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Magnetically isotropic, the magnetic lines of force run symmetrically -.um the plane of the exciter 26 and do not penetrate into the core 4 of the detector 27 a. If the material of the specimen 8 is magnetically anisotropic, the symmetry of the magnetic lines of force of Anisotror pie is distorted accordingly, a part of the lines of force closes over the core 4 of the detector 27, so that in the detector coil 5, an AC voltage is induced. The amplitude of the induced AC voltage depends on the ansiotropy of the investigated material. Their phase angle in relation to the exciting magnetic flux is approximately + 90 ° or -90 °, depending on which quarter of the magnetization of the device under test is.

The per se known measuring head 1 can be used according to the invention for determining the spatial distribution of the parameters of the magnetic anisotropy in the following manner. _E If the hosing head 1 is rotated about an axis perpendicular to the surface of the test piece and the output signals with the phase-sensitive amplifier are detected : 7, one obtains, when the output signals are represented by the angular position of the measuring head, a diagram as shown in Fig. 2. The mean of the curve G is the difference between the two principal stresses, which are chosen according to the Prerequisites are parallel to the surface of the specimen 8, proportionally "The zeros of the curve G ran the Eichtungen the said principal stresses." The phase relationships of the voltage induced in the detector 2 uniquely determine which zeroth corresponds to the direction of the larger of the two principal stresses.

On a test specimen whose original magnetic anisotropy can be neglected in relation to the effect of the external load, under "known" stress conditions, both the phase relationships of the sighting

For the purpose of this calibration, one must determine the angular dependence of the output signal (see Pig. 2). For this purpose, turn the measuring head 1 around one on the surface of the specimen perpendicular axis, while the specimen is loaded in a calibrated tensile testing machine. The voltage sensitivity is obtained from the maximum of the angle dependence curve. In the bullet of the curve, which shows the direction of the larger of the two mentioned principal stresses, the sign of the first derivative of the curve is independent of the properties of the test object. This sign can be determined during the calibration described above, because here the direction of the larger of the two principal stresses is just the direction of the known external load. As long as the sensor ratios are determined by the characteristics of the measuring device and can be determined with a single calibration forever, the voltage sensitivity of the measuring device depends essentially on the characteristics of the device under test and, consequently, the calibration of the voltage sensitivity should be re-performed for each device under test become.

With the aid of a magnetic tension measuring device calibrated for a particular ferromagnetic workpiece, it is possible, under the conditions of industrial production, to determine the above-mentioned two most important parameters of the internal stress state of the otherwise homogeneous, unloaded ferromagnetic workpieces. "

A new type of non-destructive material testing or -Sortierung of ferromagnetic bands during production is thus that the magnetic measuring head 1 so on a slidable and rotatable

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Mount frame so that it can be adjusted over any point of the belt in any angular position. One possible embodiment of this auxiliary device is shown in Pig. 3. The ferromagnetic test piece 8, a sheet in Fig. 3, is placed on a table with the edge mounted Rail 9 and 10 arranged on the rail] 9 and 10, the carriage 11 can be moved in the longitudinal direction. On the carriage 11 is the carriage 12, which can be moved in the transverse direction. On the carriage 12, the magnetic Hesskopf 1 is rotatably arranged »

The displacement of the carriage 11, 12 and the rotation of the measurement · head 1 is effected by a motor with appropriate translations and connectors ^ (the motors can, for example, servo motors to be _0, the position sensor for the longitudinal and transverse directions are in direct coupling with the balance 11, 12, the encoder of the angular position is connected to the rotatable measuring head 1. For example, helical potentiometers can be used as position encoders.

With the aid of the device shown in Fig. 3, the above-described two parameters of the internal Spannungszuatandes can be determined at any point of the ferromagnetic Blechprüflings. This provides a nondestructive testing method that can be automated in a manner known per se and provides information suitable for characterizing and sorting the sheets.

On the homogeneity of the above two parameters of the stress state, one can refer to the, otherwise, chemically and structurally homogeneous ferromagnetic !! Also provide an overview of the sheets in a continuous rapid process, provided that the measuring head has previously been calibrated to the sheet to be examined

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In this fast process, the measuring head 1 is switched to the in Pig. 3 shown auxiliary device placed. First, the measuring head is set via an arbitrarily selected calibration point of the sheet, whereupon the output signal is recorded as a function of the angular position of the measuring head (see Pig. 2). Thereafter, the measuring head is fixed in the 'angular position 1, which is assigned in the calibration point of the direction of the larger of the two parallel to the surface main stresses. The InhoHiogenitätskurve 1 is obtained by scanning with the measuring head iö the angular position 1, the sheet. If the measuring head is scanned in an angular position 2 at which the measuring head delivers a maximum output signal in the calibration point, the inhomogeneity curve 2 is obtained.

The inhomogeneity curve 1 represents the change of the direction of the larger of the two parallel to the surface main stresses, the inhomogeneity curve 2 represents the change of the difference of the two principal stresses. The first curve fluctuates around zero and the change of the second is small compared to the amplitude of the calibration curve On the other hand, when the two registered curves show a large change such that they are comparable to the amplitude of the calibration curve, the sheet is mechanical and / or magnetic strongly inhomogeneous. This means that the fast method is unsuitable for further testing. In this Pail Dian would have to examine the otherwise homogeneous sample from point to point in the manner described above, if this is necessary in terms of material testing.

The test methods described previously _s can be greatly expanded if the frequency, the AC

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generator 6 varies. Namely, at each frequency, information about the mechanical stress state of the devices under test can only be obtained up to a certain frequency-dependent magnetic skin depth. With the change in the frequency, the magnetic field can therefore also detect the changes in the stress state in the direction perpendicular to the surface. It is zv / eckmäßig to vary the angular frequency of the measurement in the following interval:

where ν is the obtained depth of the test, with / u the magnetic permeability of the body along the axis of strong magnetisation and with der the specific electrical resistance of the specimen.

When sorting on dressed belts, it is particularly important to know the distribution of the internal mechanical stresses over the belt thickness. FIG. 1 shows the depth dependence of the internal stress in a typical trained band. The horizontal coordinate indicates the distance from the surface of the belt, the vertical represents the component of the internal tension in the rolling direction * Using alternating magnetic fields of different skin depths one measures average values of the voltage for different depths, and the depth dependence of the average values the actual local dependence of the two principal stresses parallel to the surface can be determined.

This leads to a test method with which you can follow the tempering non-destructive. Aua de. Measurement results can even be drawn to the aging,

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An essential requirement for the usefulness of the methods described above is that the air gap, i. the distance of the surface of the specimen 8 from the end faces of the magnetic cores of the measuring head 1 during the calibration and the measurement is the same size. Unless in Pig. I apparatus shown for measuring the magnetic strains by a magnetic circuit that realizes the later-described negative magnetic voltage feedback and one of the devices that ensure the independence of the detector 27 of the air gap, there is the possibility, even body with not completely level surface, eg also to measure and check wavy and vibrating bands.

Fig. 5 shows the magnetic cores of the magnetic measuring head, which were supplemented according to the invention by the static magnetic compensators 28, 28A.

Fig. 5a is an axonometric view of the magnetic cores of the measuring head.

Fig. 5b shows the magnetic cores from the specimen 8-looked. The excitation core 2 and the detector core 4 are arranged in almost mutually perpendicular planes to each other, while the planes of the magnetic cores 13, 13 A of the static magnetic compensators 28; 28A approximately the bisectors of the planes of the cores 2; 4 are. The cross section of the magnetic cores which produce the required magnetic field of change is determined by the necessary maximum magnetic flux which, in turn, depends on the material to be examined. The cores may have not only a square cross section as shown in the figure but also Example also a circular one. T _he material of the exciting core 2 and the core 4 · detector is to have a large 'Anfangspermiabilität and a high electrical resistivity and be ferromagnetic linear, it is conveniently a manganese-zinc ferrite. The cores 13 »13A the

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static magnetic capacitors 28; 28A are made of a material having a large electrical resistivity and a high saturation magnetization, and whose axis of strong magnetization coincides with the longitudinal axis of the cores. It is two times a permalloy alloy.

The exact position of the excitation cores 2 and the detector core 4 is adjusted so that without the ferromagnetic test piece 8 - so in open magnetic circuits - the magnetic coupling between them is minimal.

From Pig. 5b shows the span of the U cores. The detector core 4 may be e.g. have the smallest span, so that the change of the magnetic anisotropy is detected in a homogeneous field as possible. The span of Kompensatorkerne 13; 13A is larger than that of the exciter core 2, so that the magnetic resistance of the DC-powered compensators 28; 28A as small as possible by the specimen 8 and by the exciter core 2 is possible large. With a not too large air gap one can avoid, with the conditions that can be seen from the figure, that a significant part of the compensation flow through the Err% erkern 2 and through the detector core 4 and a significant part of the excitation flux through the Kompensatorkerne 13; 13A runs. (These magnetic "short circuits" would falsify the measurements.)

In Fig. 6, the electrical circuit of the static magnetic compensators 28; 28A. On the opposite legs of the cores 13 (or 13A), the coils 14; 15 (or, 16; IT) connected in series so that their excitations add up. The series connected coils.14; 15 or »16; 17 are fed by the regulated double DC power source 18 »The ratio of the two currents is determined by the Richtun

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the magnetic anisotropy prevailing in the test piece, which is to be compensated determined. The magnitude of the currents should be selected according to the magnetic permeability of the specimen and the air gap. In order to ensure a minimum magnetic coupling with the smallest possible spans of the cores 13r 13A, it is expedient to the coils 15; 16; 17; IS capacitors 34; 35; 36; 37 in parallel, as is the case with Pig. 6 can be seen.

The basic prerequisite for a faultless measurement with the magnetic measuring head, by the static magnetic Koinpensatoren 28; 28A, is that the magnetic induction caused by the exciter 26 in the device under test 8, i. the magnetic operating point, during the calibration and the measurement, also in the Pail remains constant, if one can not provide for a constant size of the air gap.

The measuring accuracy of the magnetic measuring heads can also be reduced by another circumstance. Since the ferromagnetic materials often exhibit non-linear behavior, the magnetic flux induced by the exciter 26 contains many harmonics, although it is fed by a sinusoidal AC electrical voltage. The eddy current loss induced by the harmonics in the test specimen considerably reduces the measuring accuracy. "

The independence of the magnetic operating point from the air gap and the minimum content of the magnetic excitation fluence to harmonics is ensured by a known negative magnetic voltage feedback.

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The circuit that causes the negative magnetic voltage feedback is in Pig. 7 is shown. The magnetic exciter circuit is the DUT 8, the magnetic exciter core 2 and the air gaps 25; 25A formed. The purely sinusoidal AC voltage of the generator 6 is applied to the input of the power amplifier 20 and to the feedback coil 19, which is connected in parallel thereto, and which is located on the sputter core 2. If the voltage gain of the power amplifier 20 is large enough (at least 100 times), the excitation coil 3, which is located on the excitation core 2 and consists of two series-connected parts, flows. a - not necessarily purely sinusoidal - alternating current, under the influence of which the magnetic induction flux in the magnetic Erxgerkreis and consequently also in the specimen 8 as a function of time is purely sinusoidal. If the negative feedback is still present, the magnetic flux of induction remains purely sinusoidal in its dependence on time, and with an unchanged amplitude, if the magnetic resistance of the magnetic excitation circuit z ₍ B.) changes due to an enlargement or reduction of the air gap.

Consequently, with the help of the negative mangetic voltage feedback, the independence of the magnetic operating point from the air gap as well as the minimum content of the magnetic induction in the test piece at harmonics are ensured. Of course, the size of the air gap can not be changed indefinitely because the magnetic field of the magnetic excitation circuit can not be changed by several orders of magnitude.

The amplitude of the sinusoidal magnetic flux can be adjusted to the desired value by a suitable choice of the voltage amplitude of the generator 6 *

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However, to achieve sufficient accuracy of the magnetic measuring head, it must be ensured that the content of harmonics in the output signal of the detector 27 is less than 5 % .

The independence from the air gap is required not only in the magnetic Wechselinduktionsfluß, but also in the static magnetic circuits of the compensators. There is no possibility for direct feedback because it is a static price. If you use the one in Pig. 7, an electrical signal indicative of the size of the air gap may be taken directly from the poles of the coil 3. With this AC voltage, the double DC power source 18 shown in Fig. 6 can be controlled in a conventional manner so that the ratio of the two currents remains unchanged even with a change in the Luftspal'tes.

If the independence of the size of the air gap in the detector 27 of the magnetic measuring head, which was provided in its field circuit with a negative magnetic voltage feedback, guaranteed, the measuring head can also be used in cases where you can not ensure a constancy of the air gap , So you can, for example also measure vibrating, wavy bands »

The independence of the detector 27 on the size of the air gap can alternatively be achieved by various known solutions, which are enumerated below is not exhaustive?

a) The detector coil 5 is connected in parallel to a suitably dimensioned capacitor (Ju.D, Sheiesnow, A * G. Shurswski, AI Kondratovy Probory i sistemy upravlen _f jija,

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No 3J 25, 1972). This solution can not be used in measuring devices with variable frequencies.

b) The air gap dependent AC voltage of the excitation coil 3 can be used to control the amplification factor of the phase sensitive amplifier 7 so that the output signal is independent of the air gap (J.R.Dahm, Instrumentation Technology, May, 54, 1970).

c) The air gap-independent signal is produced from the voltage of the excitation coil 3 and from the output signal of the detector 27 in a conventional manner in a digital manner. , ·

It is expedient that the above measures for stabilizing the magnetic operating point and for achieving the air gap independence of the detector 27 also in the above-described measuring devices for industrial production, not with the static magnetic Kornpensatoren 23; 28A are to be used. The thus-added device ₅₁ can for example also be used for sorting sheets or wavelength of samples with not completely plane surface Z.

It is important and therefore important to note that in the case of the magnetic head of the magnetic head no major losses may be caused by the alternating magnetic field. With the elimination of such power losses, the measurement accuracy increases, and the smallest change in the stress that can still be resolved wi smaller. In addition, it is essential that the Leistungsbedai the exciter is reduced, and so the electrical input voltage can be lowered below 15 V, which is advantageous for safety reasons. The losses can be reduced with two measures? the cores of the magnetic. HesskoBfea are said to be made of a ferromagnetic material

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minimum specific electrical conductivity is produced (it is expedient to use magnetic cores made of sintered material or of sheet metal) and the mechanical elements for adjusting the sputtering head are to be manufactured from insulators. In order to reduce the electrical currents in the winding wire, the latter should have a small cross-section or be in the form of a strand and / or consist of a material with a high electrical resistance (z _o B _e manganin).

The above-described device with static compensators and circuits for independence of the air gap and a minimum harmonic content, which operates with multiple frequencies, can be used in the production of ferromagnetic bands as sensors for controlling technological processes or as a control device on a large scale.

Before performing with the magnetic voltmeter operation · ³ measurements and quality checks, it must be calibrated to the to be examined Ferromagnetic plate. It is convenient to carry out the calibration either with a tearing machine or with a special device built for this purpose allowing a constant tensile load. When calibrating, it must be ensured that the relative position v. .On measuring head and tape is exactly as with the measuring heads, which are arranged on the rolling stand.

Also, the unloaded band is generally anisotropic, and this anisotropy can be canceled by means of the static compensators. After the electrical output signal in the unloaded state became hull under the action of the compensators, one can obtain the proportionality factor between the mechanical stress and the output signal determine the voltage measuring device by attaching a known tensile load.

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The calibration to the external mechanical stress does not have to be carried out at each strip thickness if one chooses the operating frequency of the measuring head high enough so that the magnetic skin depth becomes smaller than the thickness of the finest sheet to be examined.

The calibration may be carried out on sheet metal samples taken after any essential technological process. The individual technological processes are in a certain sense also characterized by what type of magnetic anisotropy the tape produced possesses. Since the anisotropy of the tape is uniquely characterized by the suitably chosen current intensities of the static capacitors of the magnetic measuring head, a table-like detection of this Y is achieved / erte to a simple control. The thus prepared anisotropy table can be used as a test specification in material control. If one the tabulated DC values ^e; feweils just ended technological process are assigned on a Ivießkopf, the technological process is then error-free if the output signal of the measuring head is zero. Of course, with this control Ie no external mechanical stress may stress the band.

Pig. Fig. 8 shows the arrangement of the magnetic measuring heads 21 mounted on a rolling stand, which are provided with static compensators and serve to control the rolling process. The magnetic measuring heads 21, which are mounted on the stand-.24 are located below the plane of the ferromagnetic strip 8A, which runs out of the work rolls 22 and is supported by the roll 23.

With these measuring heads 2/1 it is possible, with the setting of the tabulated Gieichstroni values, which correspond to the technological process, to decide whether the production is necessary.

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her error-free course. If the output voltage of the measuring heads 21 Hull or remains within a given (depending on the make and the demands placed) tolerance limit, it can be started with the further rolling of the belt 8A. If the measuring heads 21 are used in pairs in differential circuit during rolling, then the differential voltage UuIl or the quotient signal one means that the band 8A is homogeneously deformed over the transverse direction and subsequently no flatness errors are to be expected. If one of the measuring head pairs indicates a deviation, the error can be compensated by changing the rolling pressure, by rolling deflection, etc. The tapes rolled with such a control have a uniform thickness and are not prone to subsequent flatness defect formation. ,

The tempering process can be directly controlled in the manner described above with a corresponding frequency program already on the rolling train and so the quality of the bands can be significantly improved.

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Claims (14)

213153 Invention sanction: 1. A method for the non-contact measurement dea elastic strain state in ferromagnetic bodies, in particular bands in which two principal voltages parallel to the surface of the specimen, with a measuring head whose exciter is aligned in a first direction and generates a magnetic alternating field in the specimen, and its detector for this purpose is oriented almost perpendicularly in a second direction and measures this magnetic field, characterized in that the measuring head is rotatable about an axis which is perpendicular to the surface of the specimen, and that during the rotation, the output signal of the detector is detected phaEenempfindlich and thereby determining the direction of the greater of the two named principal stresses and the difference between them 2. The method according to item 1, characterized in that the direction of the larger of the two said main voltages from the zero point of the output signal, which was detected in phase relative to the excitation voltage, is determined. 3 «method according to item 1 or 2, characterized in that the difference of said main voltages in relation to the excitation signal is detected and determined phase sensitive. 4. The method according to any one of items 1 to 3 *, characterized in that one repeats the method after the displacement of the measuring head along the surface of the ferromagnetic test specimen once or several times. 131 Method according to item 2, characterized in that the detector's output signal is measured along the surface of the ferromagnetic specimen during displacement of the sputtering head, maintaining the angular position of the measuring head in which the output signal was zero at the beginning of the measurement. 6, method according to item 1, characterized in that the output signal of the detector during the displacement of the measuring head along the surface of the ferromagnetic Prüflings is geinssen, v / ob ei the angular position of the measuring head, in which the output signal at the beginning of the measurement showed a maximum, is maintained. 7. The method according to any one of items 1 to 6, characterized in that in the ferromagnetic test piece, an induction flux whose amplitude is constant and whose course is sinusoidal as a puncture of the time is generated. 8 "method according to item 7, characterized in that the amplitude of the induction flux is chosen so that the content of harmonics in the output signal of the detector is less than 5 % . 9. A method for contactless measurement of the change in the voltage state in the direction perpendicular to the surface of the specimen in ferromagnetic bodies, preferably in sheets, with a measuring head whose exciter, which is aligned in a first direction and generates a magnetic alternating field in the test specimen , while its detector, in a direction almost to the first direction c
aligned perpendicularly extending second direction, this magnetic field measures, characterized in that during the measurement, the frequency of the alternating magnetic field is varied. .-as- 213153 10. The method according to item 9, characterized in that the frequency of the magnetic field in the test specimen, in the cycle frequency interval is varied continuously or stepwise, the circle frequency, ν, the depth of the test to be achieved, yes, the static magnetic permeability of the body along the axis of the strong Mangetisierung and ^ the af afische electrical resistance of the specimen is. Apparatus for carrying out the method according to item 1 to S _5, characterized in that, in addition to the exciter and the detector, it comprises a pair of static magnetic compensators whose two compensators (28; 28A) are energized with direct current from a direct current source (18) , wherein between them and your exciter (26) and the detector 27 is a minimal magnetic coupling. 12. Device according to item 11, characterized in that the compensators (28; 28A) are aligned in two directions, which are determined by the bisectors of the directions of the exciter (26) and the detector (27). Device according to points 11 and 12, characterized in that the two compensators (28; 28A) are U-shaped and consist of cores (13; 13A) with high electrical resistivity, preferential prices Permalloy, and coils wound around these cores (13; 14, 15 or 16, 17) » 14. Device according to one of the ^ points 11 to 13, characterized in that the two compensators (28j 28A) from ve - 26 213153 each independently variable DC sources (18) are fed. 15ο 'Device according to one of the points 11 to 14, characterized in that the exciter of a generator (6) is fed with a variable between 0.1 and 10 kHz frequency. 16. Device according to one of the items 11 to 15, characterized in that the exciter (26) is U-shaped and made of a core (2) with high initial permeability and high electrical resistivity, preferably of Mn-Zn ferrite and a Coil (3) wound around this core, which is made of a wire with high electrical resistivity, preferably of manganin or of a stranded wire. 17. Device according to one of the points 11 to 16, characterized in that the U-shaped core (4) of the detector is almost perpendicular to the U-shaped core (2) of the exciter (2β) and made of a material with high Anfangspermiablität and high electrical resistivity, preferably of Mn-Zn ferrite. 18. The device according to item 13, characterized in that by a change in the air gap between the cores (13; 13A) and the test piece (8), the DC power source (18). the compensator coils (14; 15) and (16j 17), respectively, are regulated in such a way that the set direction of the static magnetization in the test object remains constant despite the change in the air gap. 19. The device according to item 15, characterized in that the exciter (26) from the generator (6) via a Rückkopplungakreis (19; 20), the a purely sinusoidal temporal -27- 2 13153 running of the induction with constant amplitude secures, is fed » 20. Device according to item 13, characterized in that capacitors (34; 35; 36; 37) are connected to the coils (15; 16 and 17; 18) on the compensator cores (13; 13A). 21. The device according to item 11 to 20, characterized in that the detector is provided with one of the known devices which ensure an air gap independence of the detector. Sttn drawings - 28 -