US3502968A - Eddy current inductive flatness measurement device - Google Patents

Description

March 24, 1970 H. G. TQBIN, JR, ET AL EDDY CURRENT INDUCTIVE FLATNESS MEASUREMENT DEVICE Filed May 4, 1966 2 Sheets-Sheet 1 I INVENTORS i'z'zadz? J ukzzzea/ll 7&12 co yaa' 2 I a I z Ks Z i ATTORNEYS March 24, 1970 H. a. TOBlN, JR, ET AL 3,502,968

EDDY CURRENT INDUCTIVE FLATNESS MEASUREMENT DEVICE 2 Sheets-Sheet 2 Filed May 4, 1966 INVENTOR5 y 6' FOX Z12, dz?

M Warzz 6003 00 2&0 ATTORNE Y6 United States Patent U.S. Cl. 32440 Claims ABSTRACT OF THE DISCLOSURE An eddy current inductive flatness measurement device which employs a driving coil having a high frequency signal therein for developing eddy currents on the surface of a moving sheet of material and pick-up coils for sensing changes in the eddy current level in the sheet due to the closeness of the sheet to the driving coils. Detector means are provided for removing a high frequency carrier from the pick-up signal, and filter means are provided to separate signals indicative of the overall undulation in the sheet from ripple in the sheet which may be of a higher frequency.

This invention relates to a device for measuring the displacement of a conducting material and in particular relates to an inductive flatness measurement device for detecting buckles and undulations in a moving metal sheet while being at all times maintained in spaced relationship with the sheet.

Sheet metal such as sheet steel or the like may be produced by either cold rolling or hot rolling an ingot through successive stages to gradually shape the desired sheet product. As is well understood, sizable pressures are required to be applied to the individual rolls for gradually compressing the ingot into an eighth inch sheet, for example.

Generally, the pressure necessary to compress the ingot at the face of the associated rolls is applied to the end bearings by hydraulic mechanisms or the like. However, the force applied to such bearings tends to deflect the roll at the longitudinal center as the ingot is being compressed. To compensate for this deflection, the rolls are generally provided with a convex profile or radial crown. Effectively, the radical crown adds an excess diameter to the center portions of the roll when the roll is undeflected, however, when the roll is under compression, the resulting deflection at the center is compensated by the radial crown such that the ingot is met with the desired pressure across the length of the roll. Theoretically, the result is that a radial crown of this nature will compensate for excessive pressure at the ends of the rolls and produce a sheet having a desired thickness.

In contrast with the theoretical application of sheet metal rolls having compensating radial crowns, actual practice indicates that the results are not entirely desirable. Due to variations in the material being rolled and due to variations in the size and shape of the ingot used as well as due to wear experienced at the crown surface, an appreciable degree of non-uniformity of pressure is encountered across the roll surface. The result of non-unifo m pressure applied to the ingot or sheet being rolled is that a buckling is generated in the sheet material.

It is apparent, therefore, that it would be highly desirable to provide a system for detecting buckles or undulations in a moving sheet, being either cold or hot rolled. and for utilizing the sensed information to adjust the pressure applied at the rolls to elimintate such buckling of the sheet.

This invention concerns a device for detecting the presence of buckles or undulations in a sheet and for developing an electrical signal response which is usable for controlling the pressure applied to the associated rolls.

The signal which is derived when buckles are present can be used not only for controlling the pressure applied to the rolls, but, may also be used to trigger an alarm. In addition, this signal may be used with a recorder to obtain a permanent record of the flatness of each of the sheets as rolled. These might be of importance in applications where provision for controlling the roll pressure is not provided. The instrument could be useful as an inspection device for quality control applications. A permanent chart recording of the output of the sensor would, in addition, be of value to the roller so as to allow him to determine the optimum rolling conditions for maximum flatness.

The problem of detecting buckles or undulations in a moving sheet may be viewed as the problem of detecting displacements of portions of the sheet in a direction perpendicular to the general plane of movement of the material. Therefore, a detection device may be placed above or below the plane of the moving sheet for detecting the apparent motion of the sheet relative to the detecting device due to the rise and fall of the moving undulated surface.

Various devices have been developed for measuring waviness of moving sheet material such as a device causing a detection arm to ride along the surface of the sheet and to be mechanically lifted and dropped by the undulations. Another device may use the detection arm as an armture in conjunction with a sensing coil wherein ripples in the sheet material will cause the contacting armature to have motion relative to the sensing coil. The movement of the armature then affects the inductance of the coil and, accordingly, renders a response which is indicative of sheet flatness.

Either the detection arm used alone or in combination with a sensing coil has the principal disadvantage of being "required to physically contact the surface of the moving sheet. Such a contacting requirement presents significant mechanical obstacles for hot rolled sheets in the order of 1700 F. and for sheets moving at high speeds such as 3500 to 5000 feet per minute.

In addition, simple inductive techniques for detecting flatness have the disadvantage of relying upon the magnetic properties of the sheet material. The magnetic properties of a given sheet may vary from location to location sufliciently to distort the output response of the sensing coil and, accordingly, to give incorrect information as to the flatness quality of the sheet being sensed. The magnetic properties may also be affected by temperature of the material being sensed which in the case of hot rolled steel or the like may not only fluctuate but may be well above the Curie temperature at which the material loses its magnetic properties entirely. In addition, altering the composition of the material being rolled would require continuous adjustment of the sensing apparatus.

Accordingly, it is a principal object of this invention to provide a non-contacting measurement device for detecting the displacement of a conductive material.

It is also an object of this invention to provide a flatness measurement device for detecting the flatness of a moving metallic sheet which is maintained in spaced relation from the device and wherein the detection is accomplished substantially independently of the magnetic properties of the sheet.

It is another object of this invention to provide a sensing device for detecting the flatness of a moving metal sheet wherein eddy currents are developed at the surface of the sheet having a magnitude which is a function of the instantaneous distance of the sheet from the device.

It is a further object of this invention to provide a sensing coil for detecting the flatness of a relatively moving electrically conductive sheet wherein a high frequency carrier signal is applied to the coils and wherein the high frequency carrier signal is modulated according to the instantaneous distance of the sheet from the coil.

It is also an object if this invention to provide an instrument for detecting the flatness of a moving conductive sheet wherein a high frequency carrier Signal is applied to a driver coil for developing eddy currents at the surface of the sheet and wherein a pickup coil is disposed adjacent to the sheet for detecting the instantaneous magnitude of the resulting eddy current flux generated in accordance with the displacement of the sheet relative to the driver coil.

It is an additional object of this invention to provide a flatness measurement detecting device for a moving conductive sheet which includes a driver coil for applying a high frequency flux to the surface of the sheet for generating eddy currents thereon in response to the distance of the sheet wherein the output signal of the pickup coil is for sensing the magnitude of eddy currents developed at the sheet wherein the output signal if the pickup coil is detected for distinguishing the rate, magnitude, and nature of a non-flat condition.

It is also an object of this invention to provide a flatness detection device for a moving conducting sheet having a driver coil for applying a high frequency carrier signal to the surface of the sheet and having first and second pickup coils which are disposed adjacent to spaced portion of the sheet wherein the pickup coils are wound in series opposition for detecting the relative movement of one portion of the sheet to another.

It is another object of this invention to provide a flatness measurement instrument having a driving coil for developing eddy currents at the surface of a moving sheet and having a pickup coil for sensing the modulated eddy current response and having a displacement coil connected through a feedback system for automatically adjusting the gain of a carrier signal applied to the driver coil in order to compensate for overall displacements of the sheet relative to the detection device.

These and other objects, features and advantages of the present invention will be understood in greater detail from the following description and the associated drawings wherein:

FIGURE 1 is a top view of a series of lower rolls showing the orientation of flatness detection devices according to this invention;

FIGURE 2 is an end view of the roll system shown in FIGURE 1 showing the positioning of upper rolls and a relatively moving sheet and indicating the orientation of a flatness detection device according to this invention;

FIGURE 3 is a diagrammatic illustration showing the operation of a set of sheet metal rolls under compression at the end bearings and producing a non-flat sheet product thereby;

FIGURE 4 is a block diagram showing the operation of a control circuit used in conjunction with the flatness measurement device of this invention;

FIGURE 5 is a diagrammatic view of an illustrative embodiment of the flatness detection coils as used within the block diagram of FIGURE 4; and

FIGURE 6 is a diagrammatic illustration of a modulated carrier signal as received at the pickup coils of this invention which contains the buckle amplitude and frequency information.

The flatness detection device of this invention consists generally of a driver coil which is placed adjacent to a moving sheet in such a manner as to induce eddy currents in the portion of the sheet instantaneously adjacent to the coil. The strength of the eddy currents so induced is indirectly proportional to the distance of the sheet from the driving coil. In this way, the magnitude of the eddy currents is a direct reflection on the instantaneous displacement of the sheet.

The magnitude of the eddy currents can be detected either by the original driving coil itself or more preferably by a secondary or pickup coil. The flux generated directly by the driving coil will induce eddy currents in the surface of the moving sheet which, in turn, have a generated flux which is opposed to the initiating flux of the driving coil. Therefore, the eddy currents produced in the moving sheet tend to reduce the effective inductance of either the driving coil or the pickup coil depending upon the method employed. This is in direct contrast to the magnetic system which utilizes a moving armature or the like for increasing the inductance of the coil due to the proximity of the moving sheet. This means that the detection device of this invention will have an inductance which is reduced progressively as the sheet moves closer to the driver coil, while devices using the magnetic properties of the sheet will tend to have an increased inductance as the sheet moves closer to the device.

Referring to the drawings in greater detail, FIGURES 1 and 2 show a series of rolls used to reduce the thickness of production type sheet material and to convey the sheet to a detector. The rolls 10, 11 and 12 provide the lower rolls in a series of three roller sets. The rolls 10 must be mounted to sustain the tremendous pressures applied to compress the sheet 13 and, accordingly, end bearings 14 are securely mounted on bracket support 17 which, in turn, are disposed on a machine frame 20 or the like. Also bearings 15 and 16 are mounted on the supports 18 and 19 to convey the sheet to a detector.

An upper roll 21 is disposed adjacent to the lower roll 10 for compressing the sheet 13 into a reduced thickness as at 24. The sheet 13 may be several feet in width and may be travelling at linear speeds up to 5000 feet per minute. Also, the sheet 13 is continuous, and for the purposes described herein may be considered to be infinite in length.

If, however, due to an undesirable pressure applied to the surface of the rolls, the sheet 13 is caused to have excess length in certain areas, the sheet will buckle in order to accommodate the excess length of material.

In FIGURE 3, for instance, upper and lower rolls 27 and 28, respectively, have roll surfaces 29 and 30 which are cooperable for squeezing or compressing a continuously moving sheet 31. The sheet 31 passes between the rolls 27 and 28 and is compressed such that the thickness of the sheet is significantly reduced as compared to its original thickness prior to entering the rolls 27 and 28. The roll 27 has end bearings 32 and 33, and the roll 28 has end bearings 34 and 35. Pressure is applied to the bearings 32 through 35 in the directions indicated by the arrows in FIGURE 3, and the compressing force is transmitted through the rolls 27 and 28 to the moving sheet 31.

Since the pressure applied to the rolls 27 and 28 is applied at the ends of the shafts, the rolls are vulnerable to deflection at the longitudinal centers thereof as at 36. The deflection of the rolls 27 and 28 in FIGURE 3 is substantially exaggerated for illustrative purposes, however, even slight deflections of the rolls 27 and 28 can be significant in considering the speed and thinness of the associated sheet 31.

When the rolls 27 and 28 deflect as in FIGURE 3, the effect is to increase the pressure at the edges of the sheet and to decrease the pressure at the center thereof. Increased pressure at edges 36 and 37, for instance, causes those edges to have less thickness and greater length than the center 38. The result is that the sheet 31 tends to buckle as at points 39 and 40 to accommodate the inconsistently rolled areas 36, 37 and 38. It is to be understood that the sheet may have uniform thickness and still buckle due to excess length as a consequence of irregularities in the initial ingot, for example.

To produce a uniformly flat sheet, therefore, a highly specialized force must be developed across the contacting faces of the rolls 27 and 28. However, due to the nature of the material being rolled, the wear at the surface of the respective rolls, and the temperature of the sheet material, the condition of the sheet must be continuously monitored in order to adjust the hydraulic force applied to compensate for a given set of variable rolling conditions. The adjusting force may either change the bearing pressure as at 32 and 33 or may otherwise alter the roll configuration.

The detecting coils of this invention used to sense variations in the flatness of the moving sheet may be placed adjacent to the sheet as shown in FIGURES 1 and 2. In particular, a supporting strip 41 may be disposed longitudinally between the rolls 11 and 12 for maintaining the individual coils in a proper spaced relationship with the sheet 13. Any number of coils 42 may be disposed within the supporting strip 41 for detecting variations in the flatness of the passing sheet. The number and combination of coils employed may depend upon the particular characteristics desired to be detected in the sheet material.

In operation, an RF signal generator 43 is used to pass a current through the coil 42 which, in turn, generates a magnetic field varying in time as a function of the current. When the conducting sheet 13 is brought in the vicinity of the coil 42, eddy currents are produced in the sheet which create a magnetic field of their own. The magnetic field produced by the eddy currents in the sheet 13 are opposed to the original field generated by the coil 42 with the result that the apparent inductance of the coil 42 is reduced. Furthermore, the strength of the eddy current field and, hence, the reduction in the inductance of the coil 42 is a direct function of the closeness of the sheet 13 to the coil; i.e., the closer the sheet, the greater the reduction in the coil inductance. Therefore, changes in the coil inductance are a direct indication of the displacement of the sheet relative to the stationary detection coil. The variations in inductance of the coil 42 can be detected by several methods. For instance, inductance variations of the coil 42 can be determined by a measurement of the current in the coil, if a constant voltage generator is used to apply the RF signal thereto. An alternate technique is to measure the coil inductance by means of a bridge circuit. Also, if a constant current generator were available, the coil voltage could be used to indicate the inductance.

The method of images may be used to describe the mechanism by which the inductance varies. If a perfectly conducting plate is brought into the vicinity of a coil, an image of that coil may be considered to be present at the same distance behind the sheet as the actual coil is in front of it. This image coil couples the actual coil by means of a mutual inductance. This mutual inductance is such as to decrease the voltage of the primary coil, assuming a constant current generator were used. As the sheet draws closer to the actual coil, the image coil also approaches closer, and the mutual inductance increases thereby causing a further decrease in the voltage of the actual coil. Thus, the displacement measurement technique is an indication of how the mutual inductance between a real and an image coil changes as a function of the separation between the real coil and the conducting sheet.

In contrast to inductive techniques which utilize the sheet material as an armature for increasing the inductance of the detection coil, the eddy current technique functions independently of the magnetic properties of the sheet. For instance, the eddy current technique is operable at temperatures above the Curie level where inductive techniques would fail. However, the eddy current technique yields more predictable results at a higher frequency.

In particular, it has been found that eddy currents are sensitive to certain properties of the moving material, such as the presence of flaws or local changes in permeability and conductivity. In addition, thickness variations of the moving sheet can eflect the magnitude of the eddy currents and hence change the inductance of the sensing coils.

It has been found, however, that variations in the magnitude of eddy currents developed due to variations in the property of the sheet material can be minimized by reducing the penetration of the eddy currents into the sheet. The nature of the penetration can be determined analytically from the skin depths equation,

5 P "f M where =skin depth in meters f frequency in cycles per second =permeability of the material in henrys per meter =resistivity of the material in ohm meters.

If p and ,u. are fixed parameters, it is apparent that the skin depth may be reduced by increasing the frequency. The exact frequency chosen, however, depends upon the coil selected and the power required to drive the coil. It has been found that a frequency of approximately 20,000 cycles per second is desirable and that frequencies in the range of 10,000 to 50,000 cycles per second may be used to minimize the variations in the moving sheet.

While the reduction of the inductance in the detection coil 42 may be used directly to sense the displacement of the sheet 13, it is generally more eflicient if a secondary or pickup coil 44 is employed. A preferable arrangement for these coils is that the two coils be coplanar and coaxial with the pickup coil being smaller in diameter than the driving coil. This orientation assures that all the flux from the driving coil links the pickup coil. When no conducting material is in the vicinity of these two coils, an output voltage will be noted at the pickup coil. As the combination is brought closer to a conducting material, the output voltage decreases due to a change in the mutual inductance. An image coil may again be considered to be located behind the sheet at the same distance that the driving coil is in front of the sheet. This image coil also links the pickup coil. However, its phase is such as to oppose that of the driving coil. Therefore, the output voltage of the pickup coil decreases as the conducting sheet is brought closer to the coil combination.

By placing a first set of coils toward the ends of the rolls 10, 11 and 12 as shown in FIGURE 1 and a second set of coils at the center of the rolls 10., 11 and 12, output voltage readings can be achieved for sensing buckles and variations in the moving sheet. In particular, the buckle occurring at the edge of the sheet, as shown in FIGURE 3, will produce a fluctuating voltage at the coil combination adjacent the ends of the rolls 10, 11 and 12, while a constant voltage will be sensed by the coil combination at the center of the rolls. Therefore, by comparing the voltage at the center coil combination with the voltage occurring at the end coil combination, both the magnitude and rate of the sheet buckles can be determined.

The method of using a single pickup coil is effective for producing a direct reading of the displacement of the moving sheet, however, an output signal is always present at a pickup coil, and it is necessary to measure changes about this initial value. Since it: is usually easier to measure changes in voltage about a zero level, two pickup coils can be used to accomplish this result. In this connection, several dilferent configurations are possible. For instance, in FIGURE 1, the coil combination at the end of the rolls 10, 11 and 12 may be linked to the coil combination at the center of the rolls or at some other position along the length of the rolls. Inthis case, two sets of driving coils are used which are substantially identical and which are connected in series to assure that the same current will exist in both coils. Also, two identical pickup coils are used which are physically arranged with respect to their respective driving coils so as to have the same voltage induced under identical environmental conditions. The pickup coils, however, are connected in series opposition so as to have their sum equal to zero under identical environmental conditions. If, however, a buckle should occur at the edge of the sheet as shown in FIGURE 3, the output voltage of the coil combination would vary about a Zero level. The magnitude and the frequency of the variations then could be used as an indication of the magnitude and frequency of the buckles occurring at the surface of the sheet.

A second technique for measuring the flatness quality of the sheet is shown in FIGURE and involves using a single driving coil 45 and two pickup coils 46 and 47. In this arrangement, the two pickup coils are identically wound on fiat pancake type coil forms, and the driving coil is then wound about the outside of the pickup coils. This orientation assures that each of the pickup coils is similarly linked by the flux from the driving coil.

The output of the coil arrangement shown in FIGURE 5 is zero if both coils are equidistant from the moving sheet, however, if a ripple appears, it will cause the metal to be closer to one of the coils than to the other. This will lead to an output signal which can then be related to the amplitude of the buckle.

The circuit arrangement for utilizing the output of the pickup coils of this invention is shown in FIGURE 4. In particular, the signal generator 3 may be a standard source of RF energy and may have an output in the order of 50,000 cycles per second. The output of the signal generator is applied directly to the driving coil 42 which is mutually coupled to the sensing coil 44. The driving coil 42 generates eddy current at the surface of the moving sheet 13 which for explanation purposes may be considered to develop an image coil at the opposite side of the sheet 13. The image coil then links both the driving coil 42 and the sensing coil 44 and modulates the carrier signal applied to the driver coil in accordance with the displacement of the sheet 13.

For example, a carrier signal 49 may be applied to the driver coil 42 from the signal generator 43. Eddy currents then generated at the surface of the moving sheet 13 will produce a flux associated with the image coil which subtracts from the fiux of the driver coil for reducing the output voltage sensed by the sensing or pickup coils 44. As the sheet 13 moves toward and from the coil combination, the high frequency output voltage of the sensing coil will be modulated in accordance with the ripples or buckles present in the sheet material. This modulation takes the form of an envelope 50 associated with the carrier signal 49. It can be seen from FIGURE 6 that the envelope 50 contains the necessary information as to both the amplitude of the buckle occurring at the ends of the sheet and to the frequency of the buckles. This information then can be converted through the circuit of FIGURE 4 and utilized for varying the pressure applied to the ends of the rolls 27 and 28 as shown in FIGURE 3 with the end objective of achieving a uniform pressure across the surface of the sheet 31.

As the information which is required to adjust the pressure applied to the rolls 27 and 28 is contained in the envelope of the carrier wave 49, a detector 51, as is well understood, is used to separate the high frequency carrier from the modulated information.

In addition to the buckles which occur at the edge of the sheet due to a non-uniform pressure on the rolls 27 and 28, a general waviness or undulation may be present in the entire sheet due to aerodynamic causes or less frequently due to improper adjustments of the speeds of various sets of roll combinations. Accordingly, it is necessary to distinguish between overall undulations of the sheet and buckles and ripples which require adjustment of pressure applied to the rolls 27 and 28.

It has been found that overall undulations in the sheet have a larger wavelength which at operating speed results in frequencies of less than 10 cycles per second, for example, whereas buckles formed at the edges of the sheet have a substantially shorter wavelength which at operating speeds result in frequencies of 10 to 500 cycles per second. When the detector 51 separates the high frequency carrier, which is in the order of above 20,000 cycles per second, for example, from the information signal, the information signal retains both the frequency of the overall undulations of the wave and also the frequency of the buckle occurring at the ends of the sheet 31. In order to separate these two forms of information, high pass and low pass filters may be used in the circuit of FIGURE 4.

For instance, a high pass filter 52 may be connected to the output of the detector 51 for passing only the frequencies associated with the buckles at the edge of the sheet 31. A filter passing frequency signals above 10 cycles per second, for example, would serve this purpose. Finally, the output of the high pass filter may be detected by a voltmeter 53. It is understood, of course, that the output information may be fed directly to an electrical mechanical transducer for varying the hydraulic pressure applied to the rolls 27 and 28.

In order to automatically adjust for overall displacements in the sheet which may be a result of adjusting the positioning of the roll combinations or of adjusting the positioning of the entire bracket 41 Which holds the coils therein, a displacement coil 53a is mutually coupled to the driving coil 42. The displacement coil is a single coil which has a constant voltage output as a direct function of the closeness of the sheet to the displacement coil. The output of the displacement coil is then fed back through an automatic gain control amplifier 54 to the signal generator 43. In this way automatic compensation can be achieved for various positions of the coil relative to the sheet. For example, if the sheet is brought closer to the coil mechanism due to an adjustment of the rolls, a corresponding voltage will be detected by the displacement coil 53a and the signal generator can be automatically compensated through the feedback network for adjusting the signal applied to the driver coil 42.

A low pass filter 55 is connected between the displacement coil and the automatic gain control amplifier 54 in order to detect the overall undulations in the sheet which were separated from the output signal of the detector 51 by the high pass filter 52 Also, the low pass filter 55 prevents high frequency displacement due to the buckles at the ends of the coil from affecting the amplifier 5 4.

It will be understood that various modifications and combinations of the features disclosed herein may be accomplished by those versed in the art, but we desire to claim all such modifications and combinations as come within the scope and spirit of our invention.

We claim as our invention:

1. A device for measuring the displacement of an electrically conducting material comprising:

a driver coil for being disposed in spaced relation from the surface of the material being sensed,

signal generator means for applying a relatively high frequency carrier signal to said driver coil for generating eddy currents in the surface of the material being sensed,

a pickup coil mutually coupled with said driver coil for being likewise disposed in spaced relation from the surface of the sensed material,

said pickup coil detecting an amplitude modulated high frequency eddy current carrier signal in the material being sensed wherein the amplitude modulation is the displacement response of the sensed material,

low pass filter means for removing the carrier signal, means coupling the output of said pickup coil to said low pass filter,

high pass filter means for removing signals indicative of overall undulations in the sheet and passing signals indicative of sheet ripple, and

means for coupling the output of said low pass filter to the output of said high pass filter.

2. A displacement measurement device in accordance with claim 1 wherein a second pickup coil is connected in series opposition with the said pickup coil of claim 1 and wherein both pickup coils are substantially equally mutually coupled to a driver coil such that equal displacement of the sensed material relative to each of said pickup coils causes Zero output signal thereacross and such that non-equal displacement of the sensed material relative to each of said coils develops an AM high frequency output signal across said series wound pickup coils,

whereby overall displacement of the material being sensed is discriminated by said pickup coils from displacement of portions of the sensed material due to buckling or the like.

3. An instrument for sheet metal flatness measurement comprising:

a driver coil for being disposed adjacent to the surface of a moving metal sheet,

a signal generator for applying a high frequency carrier signal to said driver coil,

said signal generator maintaining a relatively high output frequency for developing low penetration eddy currents in the adjacent surface of the moving metal sheet and for reducing the effects of the magnetic properties of the sheet on the flux level within said coil wherein the strength of the eddy currents within the moving sheet is a direct function of the closeness of the sheet to said driver coil,

a pickup coil for sensing the said eddy current level in said sheet caused by said driver coil signal,

low pass filter means for removing the carrier signal, means coupling the output of said pickup coil to said low pass filter,

high pass filter means for removing signals indicative of overall undulations in the sheet and passing signals indicative of sheet ripple, and

means for coupling the output of said low pass filter to the output of said high pass filter.

4. An instrument for sheet metal flatness measurement in accordance with claim 3 wherein said driver coil and said pickup coil are constructed in the form of a flat pancake type coil whereby the ability to detect the change in eddy current strength at the moving metal sheet is maximized due to a given displacement of the sheet relative to the driver coil.

5. An instrument for sheet metal flatness measurement in accordance with claim 3 wherein said pickup coil is disposed adjacent to and spaced from the sheet,

said pickup coil being mutually coupled to said driver coil,

a high frequency signal is induced across said pickup coil which is amplitude modulated in accordance with the relative movement of the sheet toward and away from the coil,

the amplitude of the signal induced across said pickup coil being indicative of the amplitude of movement of the sheet and the frequency of the demodulated signal across the coil being indicative of the rate of change of the amplitude of sheet movement.

6. An instrument for sheet metal flatness measurement in accordance with claim 5 wherein a second pickup coil is Wound in opposition to the said pickup coil of claim 5, and wherein the summation of the outputs of both coils is applied to said low pass filter means, each of said pickup coils being disposed adjacent to different portions of the moving sheet, whereby overall undulations in the sheet will develop substantially equal and opposing voltages at said pickup coils for yielding a substantially zero summation voltage at the output thereof and whereby buckles in the sheet will develop unequal and opposing voltages for yielding a non-zero summation voltage at the output thereof.

7. An instrument for sheet metal flatness measurement in accordance with claim 6 wherein one of said pickup coils is disposed adjacent one of the longitudinal edges of the moving sheet and the other of the pickup coils is disposed adjacent the longitudinal center of the sheet, whereby buckles due to unequal crown pressure of sheet rolls is sensed by said spaced pickup coils.

8. An instrument for sheet metal flatness measurement in accordance with claim 5 wherein said high :pass filter separates the signal from said low pass filter into frequencies indicative of the buckle rate of the sheet and frequencies indicative of the overall undulation rate of the sheet.

9. An instrument for sheet metal flatness measurement in accordance with claim 6 wherein a displacement coil is disposed adjacent to and spaced from the moving metal sheet, said displacement coil being mutually coupled to said driver coil, whereby the output of said displacement coil is indicative of the absolute displacement of the sheet, and feedback means for controlling the amplitude of the signal generator is accordance with the output of the displacement coil.

10. An instrument for sheet metal flatness measurement in accordance with claim 9 wherein an additional low pass filter is connected to the output of said displacement coil for recording overall undulations in the sheet movement.

References Cited UNITED STATES PATENTS 2,629,004 2/ 1953 Greenough 32434 3,252,084 5/1966 Krobath 324-40 3,256,610 6/1966 Brys 32434 3,281,667 10/1966 Dobbins et al 324-40 3,371,272 2/1968 Stanton 324-34 RUDOLPH V. ROLINEC, Primary Examiner R. J. CORCORAN, Assistant Examiner US. Cl. X.R. 33147

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