MAGNETIC BRIDGE DEVICES

Abstract: Various devices are constructed with gap-modified magnetic bridge circuits. In a sample evaluation device, a magne­ tic bridge circuit (10, 20, 30, 40, 40′) has at least four arms (P1-P2; A12-B12, A13-B13, A14-B14, A15-B15), a driving leg (A11) and sensing leg (B11), one arm (A15-B15) being utilized for evaluating a sample (S), pole pieces (P1-P2) of a sample-­ evaluating arm having end faces defining between them a sample-evaluating gap (G15). An electrically conductive gap in­ sert (C) produces an enhanced fringing flux exteriorly of it. The sample (S) is placed in juxtaposition to the gap. A driving circuit (WA1) provides a time-varying flux within the arms of the bridge. A sensing device (WB1) senses changes in the sensing leg caused by changes in the fringing flux. A single-side modification provides an accelerometer (40′). Other em­ bodiments may be used for evaluating a tubular structure (T), for flows. In one form, the device (Fig. 16) is inserted within the tubular structure, and includes driving and sensing legs (54, 56) having axes which intersect the symmetrical axis of el­ ongation of the structure and are mutually perpendicular, being preferably spaced apart along the direction of elongation to introduce time-varying flux within the walls of the structure to form four effective arms of the magnetic bridge. A struc­ tural anomaly causes an imbalance sensed by a coil wound about the sensing leg.

Description: Background and Summary of the Invention This invention relates to magnetic bridges and more particularly to improved magnetic bridge devices providing electromagnetic evaluation of various samples and materials by measurement of the complex magnitude and phase components of an induced magnetic flux. Magnetic bridges driven by time-varying magnetic fields (US Patent 3379969 of O. H. Zinke) can be controlled through combinations of resistances and reactances coupled to the bridge arms (US Patent 3534253 of O. H. Zinke) and used to measure conducting and/or magnetic characteristics of samples either inserted directly in the gaps in the bridge arms or inserted in the fringing fields of the gaps. The present invention relates to a modification of such a bridge device. To achieve maximum sensitivity in the prior art bridge device described above, samples have to be placed directly in the gaps in order to intersect the maximum amount of time-varying flux. If, for reasons of convenience, samples are placed in the fringing flux, the sensitivity of the bridge to the characteristics of the sample changes in the approximate ratio of the fringing flux intersected by the sample to the total flux in the gap. Thus, in the magnetic bridges described in the above two patents, the most efficient use occurs when the sample is placed directly in the gap in a bridge arm. Under these circumstances, the size of the sample which can be inspected is restricted by the width of the gap which can be used and the length of the bridge arm. For example, if an aircraft wing were to be inspected for fatigue in the aluminium skin, the bridge arms would have to be about half the width of the aircraft wing, and the gap in the bridge would have to be larger than the thickness of the wing. Such a wide gap would greatly reduce the time-varying magnetic flux because of the tendency of flux to spread as it leaves a magnetic pole (the face of the gaps) . Further, and possibly more important, the eddy currents induced in the sample under such circumstances may not have paths which are appropriate to the detection of the characteristic sought. In the above example, the eddy-current paths could be of the order of the sample, the wing, while the characteristic being searched for would be small cracks appearing near the rivets in the wing. The sensitivity of the bridge would be further reduced by the ratio of the volume of the crack to the volume of the whole wing. US-A-3626344 discloses an eddy current transducer used for evaluating characteristics of metal samples. The transducer incorporates an inductance coil provided with a ferrite core in which there is a gap. When an electromagnetic field occurs within the core, a fringing flux is established around this gap. A non-magnetic insert of high conductivity is placed within the gap, and extends beyond the gap on one side only of the gap, so as to cause the fringing flux on the opposing side of the gap to be enhanced for more accurate evaluation of samples, such that changes of the time varying flux sensed by the sensing coils result from changes in the fringing flux adjacent the sample evaluating portion caused by changes in the sample characteristics or the sample position. This eddy current transducer also enables a single side detection. THE INVENTION The invention provides a magnetic bridge for the evaluation of samples, said bridge comprising: a magnetic circuit having at least four arms, a driving leg and driving means associated with the driving leg for providing a time varying flux within the four arms of the bridge, a sensing leg and sensing means associated with the sensing leg for sensing changes of the time varying flux within one or more of the four arms of the bridge, and at least one of the four arms being used for evaluating a sample, the or each sample evaluating arm including pole pieces forming end faces defining between them a gap having a fringing flux; characterised in including a conductive gap insert in the gap between the pole pieces of the gap, there being substantially no extension of the conductive gap insert beyond the perimeter of the gap at a selected sample evaluating portion of the perimeter, this sample evaluating portion representing a minor proportion of the perimeter, and there being extension of the gap insert out beyond the perimeter of the gap all around the remainder of the perimeter, for enhancing the fringing flux adjacent said sample evaluating portion, such that when a magnetically permeable sample is placed adjacent said sample evaluating portion a fringing flux is established within the sample, and changes of the time varying flux sensed by the sensing means result from changes in the fringing flux adjacent the sample evaluating portion caused by changes in the sample characteristics or the sample position. This modification of the magnetic bridge device of US-A-3534253, involving use of a conductive metal insert in a gap of the bridge to serve as means for enhancing a fringing flux exteriorly of the gap, is termed "single-side" detection or "single-side" technique because the fringing flux produced by the bridge can be used to inspect the sample from one side of the sample. Further, the modification reduces the volume of the sample being scanned to only a few cubic centimetres (fractions of a cubic inch). It is possible to detect cracks near rivets in aircraft wings with a bridge which is less than a very few cubic centimetres (a cubic inch) in volume through the use of this modification. The effect of this modification, further, is to move all the flux in a magnetic gap carrying a time-varying magnetic field completely out of the gap and into the fringe field. Thus, single side detection utilizing magnetic bridge devices according to the invention can be carried out for nondestructive evaluation of variously configured structures and materials of ferrous or non-ferrous metal. The evaluation will reveal cracks, pre-crack or residual stress, surface flaws and volume voids or anisotropies. Accordingly, magnetic bridge devices of various configurations may be provided for carrying out non-destructive evaluation of ferrous and non-ferrous samples and structures, such as to detect cracks, pre-crack stresses, surface flaws, volume voids, anisotropies and other anomalies therein, regardless of the shape or size of the sample or structure, such as for the purpose of coin testing, pipe testing, surface testing, aircraft structure evaluation, and many other evaluative purposes; and which can detect hysteresis loop "signatures" such as can predictably and accurately reveal the characteristics. Such magnetic bridge devices may be utilized for other evaluative and measurement schemes, including measurement of temperature changes of samples of ferrous or nonferrous metals regardless of size, shape or condition of motion of the samples; which can detect and/or measure thicknesses of laminants on ferrous or non-ferrous metals (such as coins and other clad or laminated metals, whether the laminants or surface coatings thereof are metallic or non-metallic; which can be utilized for measurement of the dimensions of samples of ferrous or nonferrous metals and of indentations or imprints in such samples, such as, for example, on coinage or on ruled objects, such as metal rulers. THE DRAWINGS FIG.1 is a schematic diagram of a magnetic bridge and associated circuitry in accordance with the prior art. FIG.2 is a simplified, fragmentary view of pole pieces of a magnetic bridge device as provided with a conductive insert between them in accordance with the invention for magnetic evaluation of an adjacent sample. FIGS. 3A, 3B and 3C together represent fragmentary views similar to FIG.2, depicting magnetic evaluation of a sample for flaws by magnetic bridge devices of the present invention. FIGS. 4A and 4B are fragmentary illustrations of conductive insert and associated pole configurations in accordance with the invention. FIGS. 5A and 5B are respectively front and side views of a magnetic bridge device in accordance with the invention providing single side detection in accordance with the present disclosure. FIG.6 is a fragmentary plan view of portions of the bridge device of FIGS.5A and 5B as extended through an apertured face plate. FIG.8D is a bridge device for evaluating a cylindrical surface. Corresponding reference characters indicate corresponding elements throughout views of the drawings. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a magnetic bridge circuit 10 of the type illustrated in the aforesaid Zinke U.S. Patent 3,534,253 serves to explain the present invention. Magnetic bridge circuit 10 includes integral portions A11, A12, A13, A14 and A15. The bridge also includes integrally connected portions B11, B12, B13, B14 and B15. Portion A11 is the input leg of the bridge and has an input winding WA1 mounted thereon. Winding WA1 is connected to a changing or varying current source which generates a varying flux in the core structure. In similar manner, leg portion B11 is the output leg portion and it has an output winding WB1 mounted on it. Output winding WB1 responds to changes in the condition of the flux in the bridge. Output winding WB1 is connected to a suitable output circuit, device or other output means for measurement of the magnitude and phase of the signal induced in output winding WB1 and for comparison of its phase relative to that of a time-varying input signal. Thus, it will be seen that the bridge includes an input leg, an output leg and effectively four arms. For example, a single arm may be regarded as formed of portions A12, B12, for example, and with each arm having a respective gap G12, G13, G14 and G15 in which are positioned gap insert elements C12, C13, C14 and C15 for gap-coupling between the adjacent ends of the bridge portions defining the gaps. However, the ends of the arm-defining bridge portions so illustrated may in some cases be in direct contact or formed integrally, as for the purposes stated in said Zinke Patent 3,534,253. Each of portions A12, A13, A14 and A15 has a respective winding or coil W12, W13, W14 and W15 mounted thereon. These windings are connected, respectively, to passive circuits such as circuits D12, D13, D14 and D15. Each of the passive circuits includes adjustable passive elements which are used for nulling the circuit in order to produce a minimum output on winding WB1 when the bridge is in balanced or null condition. A typical passive resistive-capacitive (R-C) circuit is shown in block D15 and includes a variable resistor 12 and a variable capacitor 14 connected in parallel. Other combinations of passive circuit elements can also be used in these blocks and it is usually desirable to include both resistive and reactive elements. By adjusting the elements in circuits D12-D15 a null condition is produced, as aforesaid, when the output on winding WB1 is a minimum. The null condition is produced by the effect the passive circuits have on the real and imaginary components of the flux in the respective arms of the bridge circuit. This simultaneously changes the phase and magnitude of the flux in the arms and hence also produces the desired minimum flux condition in the output leg B11, and hence in output coil WB1. By employing a time-varying or AC input source the reluctance becomes complex and the conditions and equations for balancing the bridge must take into account the fact that the reluctance is complex and that the reluctance has both real and imaginary components. The magnetic bridge device thus to be generally utilized for the present invention may be constructed of H-shaped pieces as described in said Zinke Patent 3,534,253, which pieces are of ferrite material having the desired flux concentrating and magnetic characteristics, although other flux concentrating materials may be utilized. Variously shaped members may be utilized for forming bridges in accordance with the invention, however. Throughout the drawings, material having a magnetic permeability greater than unity, such as ferrite or other ferrous materials, is illustrated by light cross hatching. Referring to FIG. 2, bridge 10 is modified in accordance with the present invention by configuring one of the bridge arms for sample evaluation by introducing within at least the gap-forming poles of the sample-evaluating leg, e.g., gap G15, a gap insert C of electrically-conducting metal such as copper or aluminum. Insert C has dimensions such that it extends well beyond the poles P1, P2 except that outer edge of insert C provides a flush face F along one side of the gap. This arrangement enhances fringing of the flux across the gap to such an extent that it provides a fringing field only on the side of the gap across face F, as the total flux is removed from the gap because of he circulating (or eddy) current produced within the conductive insert. The magnitude of the circulating current is dependent upon the conductivity of the insert which, in turn, is dependent upon the thickness D of the insert. From Lenz's Law it is known that the circulating current in the gap insert will produce a magnetic field which opposes the inducing field so that, by the theory of superposition, the field in the center of the gap will decrease as the two magnetic fields add. With appropriate gap width, being the same as the thickness D of the gap insert, most of the inducing field is inside the gap. That is, there is a very small fringe field. However, because the eddy current follows the outside dimension of the insert, it produces a field inside the gap and outside the gap. Outside the gap there is no inducing field to cancel the induced field so that as the conductivity of the insert increases (i.e. as D increases) the field inside the gap goes to zero and the field outside the gap increases. The net effect is accordingly to move the fringing field from within the gap to outside the gap and thus along face F. Accordingly, the bridge has been modified by modifying the insert within the gap in order to move the flux from inside the gap to outside the gap where it becomes a fringing flux. In some situations the insert may not extend right up to the perimeter of the gap at the sample evaluating portion (Figure 3C) although it extends beyond the gap for the remainder of the perimeter. For utilization of the bridge so modified to provide a single-side detector, a sample S is positioned adjacent face F by a distance W. If the sample is ferrous, the magnetic field will be attracted to the sample and alter the flux causing a change in the off-null signal of the bridge. If the sample is non ferrous, then magnetic field lines will be "squeezed" between the sample and the sample evaluating gap, again altering the off-null signal of the bridge. In either instance, the sample affects the field so that a signal results from the presence of the sample, and may be detected in the sensing leg B11 by sensing coil WB1. Moving the sample back and forth in a direction normal to the gap alters the off-null signal of the bridge, permitting the bridge to be used as a linear transducer measuring the distance W. Because of these principles of operation, the bridge can also be used in other ways taking advantage of the capability for measuring the distance W. If the sample is driven, for example, by changing air pressure, the magnetic bridge device may then be used as a manometer or a microphone. If sample S is a rotating shaft having eccentricity, and W represents the distance between the sample and the gap face F, the eccentricity of the shaft can be readily determined. Further, if the eccentricity is appropriately asymmetric, the bridge can be used as a tachometer. Further, if sample S is a vibrating device, and the bridge is placed at an average distance W from such vibrating device, the bridge will measure the rate of acceleration of the device. To further illustrate methods of usage and applications of the bridge, it may considered that sample S is a metal reed fastened to the same mechanical ground as the bridge. If the whole configuration is then vibrated, in time the reed will vibrate with respect to the gap at the same rate as the driving frequency and the bridge signal will now indicate the frequency. If the space represented by W is filled with material having different electrical characteristics from the sample, then the off-null signal from the bridge can be made to be representative of the thickness of that material whether the material is nonconducting, such as a carbide layer on a tool, or conducting, such as the cladding on the copper insert of coinage. By proper choices of frequency, the bridge can be made to "read" the fact that a coin has printing, one of the basic differences between genuine coins and most spurious coins (commonly called "slugs"). A great many, if not most, slugs have flat sides. The upset edge of a real coin can be registered on the gap so that W becomes the airspace generated by the printing on the coin. Slugs usually do not have such printing. Printing on some coins is frequently of a different average W as compared to other coins of essentially the same dimensions and electrical conducting. Choices of several frequencies can indicate both printing and cladding and provide a fool-proof test for coinage in automatic apparatus using very inexpensive circuitry. The single-side bridge can be used to detect flaws such as precrack and other residual stress, cracks, surface cracks, voids, metal fatigue, etc. in the manner shown in FIGS. 3A and 3B. First the bridge is nulled. Then the sample S is moved. If a flaw simulated by crack f has occurred, the flux lines in the sample are interrupted when within the fringing field, as shown in FIG. 3B or altered in some manner affecting phase and amplitude at the bridge output. While the basic physics of the flux paths differ between ferrous and nonferrous metals, the indication of the bridge is the same, that is, an off-null signal in the presence of a crack, void, pre-crack or residual stress, or other anomalous feature. The magnetic field of the embodiment shown in FIG. 3A will enter and leave the sample perpendicular to a plane. The net flux through such plane is zero because the flux both enters and leaves. By Faraday's Law, no circulating (eddy) current will be induced in that plane. A circulating current induced in that plane would spread throughout the sample dimensions as such a circulating current spreads with an unmodified bridge in accordance with Zinke Patent 3,534,253. The only circulating currents which can be induced in sample S outside the region near the insert must be perpendicular to the plane discussed above. These circulating currents will be limited in their depth of penetration from that plane by the skin effect. The skin effect can be controlled by the operator of the bridge through choice of the frequency of the time-varying signal driving the bridge, i.e., as provided across winding WA1. Thus, the volume of sample S which is being searched is limited to dimensions controlled by the size of the conductive metal insert and the frequency of the bridge which is contrary to the volume inspected by an unmodified bridge as in said Zinke Patent 3,534,253, which is essentially uncontrollable by the operator except as to depth from surface. A single side bridge in accordance with the present invention allows the operator to sharply limit the area of a sample being evaluated, such as for flaws, printing and so forth. For very high sensitivity measurements where very large magnetic fields are desired, i.e., where it is desired to concentrate the flux in a very small area, an insert C such as that in FIG. 4A having a narrow slot or gap g can be used where the base of the slot shown in the insert fits just at the point opposite from the sample when the poles and the insert are combined as shown in FIG. 4B. Referring to FIGS. 5A and 5B, an embodiment 20 of a magnetic bridge utilizing the single-side technique is illustrated, wherein the ferrite pieces of the bridge are of generally H-shaped configuration to provide a left bridge piece or half 21 and a right half or bridge piece 22 which are located on opposite sides of a conductive metal insert C20 of generally L-shaped configuration. The insert may, for example, be of copper and to which the bridge pieces are secured by suitable adhesive. For simplicity, the various windings upon the core have not been illustrated but will be understood to be utilized in accordance with the foregoing description and the teachings of Zinke Patent 3,534,253 which discloses H-shaped pieces, as of ferrite. Instead of copper, any conductor or superconductor may be used. for insert C20. In any event,the bridge pieces are elongated at an upper end to provide sample-evaluating pole portions 21, 21′, 22′ having outer extremities which lie in a plane 23 which is also coincident with the upper extremity of insert C20. In plan (as evident in FIG. 5B), insert C20 is of relatively greater width than corresponding portions of the bridge halves 21, 22 whereby to provide marginal portions C20 l and C20 r extending beyond corresponding surfaces of the-bridge portions 21, 22. The insert C20 is provided with an upper extension C20′ which extends between the upper extensions 21′, 22′ and similarly is of relatively greater width. Thus, between the bridge leg extensions 21′, 22′ there is provided a gap a between the upwardly extending bridge leg portions 21′, 22′. The gap is used for detection, whereas three other gaps b, c and d are similarly formed across corresponding portions of insert C20 to provide corresponding magnetic paths for balanced operation of the bridge. Generally, the sample-evaluating gap a is used for detection and the other gaps b, c and d are used for rough nulling of the bridge. Detection takes place above plane 23, across which a fringing field extends generally as shown in FIG. 5A. If desired, an aluminum face plate may be fitted around the upwardly extending portions 21′, 22′ and C20′ such as to provide a convenient surface across which samples may be moved and/or for shielding the bridge from unwanted samples. Such face plate may be closely conformed to the periphery of elements 21′, 22′ and C20′ will be insulated electrically from at least portions of the gap insert. Such assembly may be secured by appropriate adhesives, such as epoxies. Thus, there is formed a single-side bridge providing gap a for detection. Samples may be moved across gap a as shown in FIGS. 2, 3A and 3B. It will be evident that the surface across gap a is entirely flat and, if members 21′ and 22′ extend into a plate, as shown in FIG. 6, in accordance with the above-described modification, a polished surface may be provided. However, the surface of the gap can be shaped to conform to the particular aspect of the sample being examined. Of course, the thickness D of the insert may be varied in accordance with the usage to be made of the bridge and the material. In FIG. 6, a face plate 24 having an aperture 24a receives the bridge extension portions 21′, 22′ and the insert portion C20′ which separates said portions to form gap a. In this view, thicknesses are exaggerated to show a layer of adhesive such as epoxy, indicated at 25 which adheres the bridge portions 21′, 22′ and gap insert C20′ together and with the whole assembly then being secured in aperture 24a by a further layer 26 of adhesive, such as again of epoxy type. The epoxy layers also provide electrical isolation, serving as means for providing electrical isolation between the conductive gap insert and at least portions of the face plate. The face plate may be a wall or otherwise part of an electrically conductive enclosure completely enclosing the bridge device for providing electromagnetic shielding. In Fig 8D, an embodiment 37 of the bridge device has pole pieces 38a, 38b and a conductive insert, C38 having outer gap-defining surfaces lying all within a cylindric surface for evaluating a tubular structure 39 by movement along its surface as illustrated by arrow. The technique may be used to magnetically locate a weld 39′ which would not otherwise be visible. These embodiments are merely indicative of various device configurations having nonlinear pole and insert geometry for evaluating variously shaped samples with nonlinear surfaces. In such usages of a single side bridge, as herein described, data "signatures" are sensed in the output leg of the bridge which are sufficiently unambiguous to allow identification and characterization of defect morphology in metal samples and of lift-off of the bridge. A combination of scanning patterns is obtainable by utilization of bridges of the invention to provide such a "signature", as by measurement of bridge output variations in the presence of a flaw or lift-off of the sample, as measured, for example, by the electrical-parameters (e.g., resistance or capacitance) necessary to renull the bridge at particular frequencies, or by measurement of the amplitude and phase in the output leg. The variation of such parameter with translational distance of any one of these variables can be termed such a "scanning pattern". Among the several kinds of patterns presented by the bridge of this invention are the following: a. A central peak which seems to respond primarily to surface conditions of the sample. b. Satellite peaks which seem to respond primarily to subsurface conditions. c. Directional response to the angle between the scan and the flaw which can be made on the basis of width of the central peak. d. Individual scanning patterns for amplitudes and phases of the fundamental and harmonic frequencies which are impressed on the bridge through the input signal. e. Individual scanning patterns for amplitudes and phases of harmonics which are generated in the sample.

Claims: A magnetic bridge for the evaluation of samples, said bridge comprising: a magnetic circuit having at least four arms (A12-B12, A13-B13, A14-B14, A15-B15), a driving leg (A11) and driving means (WA1) associated with the driving leg for providing a time varying flux within the four arms of the bridge, a sensing leg (B11) and sensing means (WB1) associated with the sensing leg for sensing changes of the time varying flux within one or more of the four arms of the bridge, and at least one of the four arms (A15-B15) being used for evaluating a sample (S), the or each sample evaluating arm including pole pieces (P1,P2) forming end faces defining between them a gap (G15), having a fringing flux; characterised in including a conductive gap insert (C) in the gap between the pole pieces of the gap, there being substantially no extension of the conductive gap insert beyond the perimeter of the gap at a selected sample evaluating portion of the perimeter, this sample evaluating portion representing a minor proportion of the perimeter, and there being extension of the gap insert out beyond the perimeter of the gap all around the remainder of the perimeter, for enhancing the fringing flux adjacent said sample evaluating portion, such that when a ferrous sample is placed adjacent said sample evaluating portion the fringing flux is established within the sample and when a non ferrous sample is placed adjacent said sample evaluating portion, the magnetic field lines are squeezed between the sample and the gap, and changes of the time varying flux sensed by the sensing means (WB1) result from changes in the fringing flux adjacent the sample evaluating portion caused by changes in the sample characteristics or the sample position. A magnetic bridge according to claim 1 wherein more than one of the arms is used for evaluating a sample. A magnetic bridge device according to claim 1 where only one of the arms is used for evaluating a sample.

Inventors: ZINKE OTTO HENRY, ZINKE, OTTO, HENRY
Applicants: INT VALIDATORS INC, INTERNATIONAL VALIDATORS, INC.
Application year: 1988
Main classification: G01R33
IPC codes: G01N27, G01R33