Method And Apparatus For Detecting At A Distance The Status And Identity Of Objects

Abstract

A non-contact system for distinguishing the presence, identity or status of an object and markers for use therein. The system comprises a marker including a ferromagnetic material to accompany each object to be detected, means for producing an alternating magnetic field within a zone through which the objects are to pass, means for monitoring magnetic flux changes within the zone and a circuit for detecting a flux change within the zone which corresponds to a signal characteristically produced by magnetization reversal of the marker ferromagnetic material. The marker comprises "n" sections of a ferromagnetic material and may include a remanently magnetizable control element to provide a sensitized and desensitized marker for demagnetized and magnetized states, respectively, of the control element. A plurality of magnetic field producing means are employed to produce in the zone magnetic fields of different orientations, virtually assuring production of a characteristic signal when a marker is passed into the zone.

Parent Case Text

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of copending patent application Ser. No. 840,973, filed July 11, 1969, now U.S. Pat. No. 3,665,449, by James T. Elder and Donald A. Wright.

Description

FIELD OF THE INVENTION

This invention relates in general to systems and materials for detection of an object or of the status of an object by use of alternating magnetic fields. More particularly, the invention relates to a system in which the object to be detected need not come in direct contact with the sensing apparatus. The object is provided with specifically chosen pieces of magnetic material, and an alternating magnetic field is provided in an area through which the object is to pass. Upon passage of the object through the area, the characteristic magnetic response of the specifically chosen pieces of magnetic material is sensed to distinguish the presence of the object and either or both the identity or status of the object.

BACKGROUND

Theft of books from libraries has become a serious problem both in the form of expense to the taxpayer for replacing stolen books and in terms of impairment of service rendered by the libraries. The annual loss of books from all libraries in the U. S. exceeds 20 million dollars and is increasing.

Systems for preventing such theft, in which instruments sensing evidence of theft actuate alarms, have been known since at least 1934. These systems generally comprise a "marker" element secured to each object to be detected and instruments for sensing signals produced by the markers. Obviously, in light of the foregoing statistics, such systems have been ineffectual.

A particularly serious problem of such theft detection systems is false alarms. After one or more false alarms, those using the system tend to ignore all alarms rather than risk being personally embarrassed or subjecting their establishment to a lawsuit. In addition to this kind of unreliability, such systems are readily compromised either by deliberately producing false alarm or "masking" signals or by shielding the marker to prevent it from producing a signal.

DESCRIPTION OF THE PRIOR ART

French patent No. 763,681, issued to Pierre Arthur Picard, discloses a non-contact detection system which employs dynamic magnetic phenomena to detect the presence of an object, e.g. a library book being carried through a doorway. The system of Picard is based upon his discovery that when a piece of metal is subjected to a sinusoidally varied magnetic field, an induced voltage which is characteristic of the metal composition is produced in a pair of balanced coils in the vicinity of the applied field. Analysis of this characteristic voltage thus permits classification of a metal present in the applied field. Hence, detection of a book to which a piece of metal of a special class has been attached is possible.

SUMMARY OF THE INVENTION

The present invention provides a non-contact system for detecting the presence, identity, or status of an object within an interrogation zone. The system comprises an interrogation zone, equipment for applying in the zone a periodically varying magnetic field which increases at a predetermined time rate of change, a marker secured to the object and equipment in the vicinity of the zone for detecting a magnetic pulse train produced in response to a marker being within the zone when the magnetic field is applied. The marker is capable of producing such a magnetic pulse train, and comprises "n" sections of ferromagnetic material, the sections having an aggregate saturation magnetization of at least 0.1 pole-centimeters, and each section differing from the other sections in A.C. coercivity such that when subjected to the periodically varying magnetic field, the magnetization of the sections reverses sequentially at equal intervals of time, each interval being less than or equal to T/4n where "T" is equal to the period of the varying magnetic field and wherein each alternation and set of sequential reversals produced in response thereto results in the magnetic pulse train.

In one embodiment, the ferromagnetic sections of the marker comprise "open-strips". The open-strips are selected such that their magnetization, when the strips are within and have a major dimension oriented parallel to a 60 Hz. sinusoidally varying magnetic test field of a predetermined peak magnitude of less than 20 oersteds, reverses for each alternation of the test field.

In the strictest sense, a complete magnetization reversal from one saturated condition to a saturated condition of opposite polarity is not required. By reversal we mean any cyclic magnetization change of at least 0.2 electromagnetic units per cubic centimeter in response to an applied field alternation. By an open-strip we mean one which when magnetized has separate poles, i.e. a strip which is not closed or wound upon itself. For a particular marker, the predetermined peak magnitude of the test field is the minimum field capable of reversing the marker's magnetization, i.e. the marker's "switching field". By oriented, we mean, for an applied field of a particular magnitude, the angular relationships between the applied field and marker major dimension are such that the applied field's vector component parallel to the marker major dimension is at least equal to the marker's switching field.

As used herein, "ferromagnetic" includes both conductive and non-conductive materials. Materials of the former include iron and its alloys with nickel; the latter class of materials includes ferrites. Conductive materials are generally preferred; they are capable of producing external magnetic fields on the order of 10 times greater than those produced by the same amount of non-conductive materials. Accordingly, a marker of conductive material may be smaller than an equivalent non-conductive marker. Such small size has dual advantages; material cost and, perhaps more importantly for anti-pilferage markers, concealability.

A further distinguishing characteristic of the signal produced by the "n" sections is that, for a particular composition, size and shape of material, the peak amplitude of the signal component produced by each section will occur for an applied alternating field of a particular waveform a predictable time after each applied field alternation. This signal characteristic can also be defined in terms of the absolute instantaneous value of the applied field at the instant when a particular section's net magnetization is zero. Hypothetically, it is at this instant that magnetization "reversal" occurs and we believe it corresponds to the peak point of the magnetization reversal signal produced by that section. We shall hereafter call this instantaneous applied field value the "AC coercivity" of the material, although it should be kept in mind that AC coercivity depends not only on material magnetic properties, but also on the waveform of the applied field. It is a convenient term for comparing responses of different markers subjected to the same applied field.

The open-strip embodiment of the present invention may take the form of a thin, flat ferromagnetic ribbon or wire having a magnetic moment of at least 0.1 electromagnetic unit. The ratio of the major dimension, i.e. the length, to the square root of the cross-sectional area of the ribbon or wire should be at least 150. At ratios below this, internal self-demagnetizing field effects in highly magnetic materials may increase the switching field beyond 20 oersteds. Also, for ratios below 150, the magnetization reversal signal amplitude decreases radically and becomes noticeably dependent upon orientation of the open-strip within the applied field. The open-strip may have one or more major dimensions satisfying this criterion.

The corresponding criterion for a thin, flat disc of a ferromagnetic material would be a ratio of its major dimension to thickness of at least 6,000. Conductive ribbon or disc markers should have a thickness of about 0.1 to 130 microns and conductive wire markers should have a diameter of 10 to 300 microns. For dimensions greater than these, the amplitude of the magnetization reversal signal decreases and the width of half-amplitude increases to become eventually indistinguishable from reversal signals of many common ferromagnetic metals likely to be carried by a person.

A thin, flat, narrow marker is particularly amenable for use with library books as it may easily be concealed either by insertion into the book binder or between two of the book pages. Commonly, a book includes two pairs of fly-leafs having a seam joining them along their entire length. Such seams are normally wider than a marker and thus a marker could easily be concealed in the seam. Or, by providing the marker with an adhesive coating on each face and a carrier web, a marker may be conveniently inserted near the binder between any two pages.

The preferred number and relative orientations of marker major dimensions depend upon the applied field characteristics in a manner which will be explained later. For reference purposes, we shall define an idealized marker having a single major dimension as a "one-dimensional" marker, a marker having two major dimensions perpendicular to each other, e.g. an "L", "T", or "plus " shaped marker, as a "two-dimensional" marker and a marker having three mutually perpendicular major dimensions as a "three-dimensional" marker.

The open-strip embodiment may be wholly inorganic or may comprise ferromagnetic laminae held together with an organic adhesive; or, it may be a dispersion of ferromagnetic particles in an organic binder such as vinyl chloride. It may even be closely spaced but physically separate ferromagnetic strips held in fixed geometric relation to each other on or within a nonmagnetic substrate (such as very fine wire filaments or ribbons within a piece of paper).

The key feature of the present invention is a marker having two or more sections of different AC coercivities employed as an integral unit. Such integrally joined sections, even if in physical contact with each other, do not magnetically influence each other enough to prevent each from providing its own characteristic pulse. The sections may be selected to sequentially reverse their magnetization following an applied field alternation at equal intervals of time, each interval being less than or equal to T/4n, where "T" is equal to the period of the applied field, and "n" is the number of sections. A sinusoidally varying field is only increasing during the first and third quadrants, thus such a series of pulses will occur in those quadrants, i.e., one-fourth of the total period T, at a point where the applied field has risen to a valve sufficient to exceed the AC coercivity of the sections and cause the magnetization to reverse. This series, or burst of pulses, actually becomes a short time signal having a characteristic frequency of occurrence whose period is the interval T/4n, and which can be unambiguously detected by unsophisticated apparatus.

Yet another suitable marker of the present invention would be the combination of one or more of the foregoing markers with at least one "control" element. The objective of a control element is to permit selective setting of a marker to either a sensitized or a desensitized state. By sensitized we mean a state in which the marker will produce a characteristic signal in response to an applied field. Conversely, by desensitized we mean a state in which the marker does not produce this characteristic signal in response to an applied field; instead, the marker will either produce a different distinguishable signal or fail entirely to produce a sensible signal.

I have found that a convenient way to prevent or alter magnetization reversal or a marker, i.e. to desensitize a marker, is to effectively bias the applied field at the marker by providing as the marker control element a remanently magnetizable material. When remanently magnetized, the remanent field of the control element alternately aids and opposes the applied field on successive half cycles. By providing a sufficiently large remanent field adjacent a marker, the net field to which the marker sections are subjected during each half cycle when the remanent and applied fields oppose each other is insufficient to reverse the marker section's magnetization in the characteristic manner. It is not necesary to completely prevent reversal of the marker section's magnetization for each applied field alternation. It is sufficient that reversal be so altered that the resulting signal is uncharacteristic of a marker. Accordingly, such a magnetic control element need not completely cover the marker surface. For example, a magnetized control element adjacent only a central portion of an open-strip section causes the segments on either side to behave approximately as two independent open-strips. Thus, the central portion should be large enough and positioned such that neither of the segments satisfies the aforementioned ratio criteria.

The remanent magnetization of the control element need not be uniform; in fact, non-uniformly magnetized control elements are generally desired because they are less costly to use.

An example of a non-uniformly magnetized control element would be one magnetized to have a series of bands of remanent magnetization, adjacent bands being oppositely polarized. Preferably, to minimize internal demagnetization effects and to provide the greatest external magnetic field, the respective directions of magnetization of the bands of such an alternately magnetized control element should be parallel to the control element length.

Conventional ways of magnetizing a control element are acceptable for desensitizing the marker. For example, to uniformly magnetize a control element, it could be exposed to the field of a large permanent magnet. Or, to provide a "band" type non-uniform magnetized control, the element could be exposed to a series of permanent magnets wherein adjacent magnets in the series were oppositely polarized. Some care is required in removing the magnetizing magnets. Movement of the magnetizing magnets along the direction of an axis parallel to the magnet polarizations would alter or skew said magnet polarizations from that intended. Such skew might reduce the magnetic influences of the control element to an amount less than that required to control or desensitize an open-strip. The use of a single pulsed magnetic field, whose geometrical field distribution resembles that produced by permanent magnets, for magnetizing markers would avoid such skew difficulties since it does not require controlled relative movement between the marker and source of magnetization while the field is applied. Means for providing such a field are well known. An example is "overdamped" discharge of a capacitor through a coil.

To sensitize markers, conventional demagnetizing apparatus based on the well-known principle of applying a relatively high frequency and diminishing amplitude magnetic field may be employed. For example, an apparatus for providing an "underdamped" discharge of a capacitor through a coil (i.e. a coil-capacitor combination similar to that which may be employed for magnetizing a control element but having a high "Q"). Alternatively, a demagnetizing apparatus comprising a series of permanent magnets in which adjacent magnets are oppositely polarized may be employed. When a control element and such a demagnetizing apparatus are moved relative to each other along a coordinate common to the series of magnets, the control element is effectively subjected to an alternating magnetic field. Such a demagnetizing apparatus can also be made to provide a field of diminishing amplitude through proper selection and arrangement of the magnets. By selecting the magnets to be of different strengths and by arranging them in an order ranging from highest to lowest (relative to the direction of travel) the magnetic field will appear to diminish in amplitude when passed over a control element. Magnets of the same field strength arranged like inverted ascending steps or like an inclined plane so that the amplitude of the field is progressively diminished would also produce the same result.

Because the external magnetic field of a control element may vary depending on the pattern of magnetization, it is not ordinarily necessary to demagnetize the control element in the strictest sense; rather, the magnetic influence of the control element need only be reduced to an extent permitting magnetization reversal of the marker sections by the applied field.

The control element should have a coercivity of at least 5 oersteds and be capable of producing when remanently magnetized a static external magnetic field of at least three-fourths oersted over at least a portion of an adjacent open-strip. For convenience of manufacture, the control element may be a thin, magnetic, uniform coating of gamma-ferric-oxide powder in a vinyl chloride binder on the surface of the open-strip.

Alternative marker "desensitization" techniques include deformation or rupture of the open-strip sections such that their resulting longest linear section is less than that required to satisfy the aforementioned length-to-square-root of cross-section ratio. Or, a marker may be desensitized by stressing the open-strip sections to change their magnetic response. For example, an open-strip may be employed in conjunction with, or as one element of, a thermosensitive bi-metallic strip.

Hereafter, in distinguishing between markers comprising "n" sections of ferromagnetic material and those further comprising one or more control elements, the markers with "n" sections alone will be referred to as "single-status" markers. Those markers also having one or more control elements will be referred to as "multi-status" markers.

The general requirement of the alternating applied field is that when a marker passes through the interrogation zone, the marker becomes oriented with the applied field at at least one, and preferably several, points in the zone to reverse the marker magnetization. Oriented was previously defined as the condition when the applied field vector component parallel to an open-strip major dimension was equal to or greater than the open-strip switching field. One combination of applied field and marker which would absolutely insure orientation would be the combination of a "one-dimensional" applied field and a "three-dimensional" marker wherein the strength of the applied field at every point in the zone was at least .sqroot.2 times the marker switching field. By a one-dimensional field, it is meant one in which all magnetic lines of force in the zone are parallel.

In a one-dimensional field, there exist two mutually perpendicular directions, which are also perpendicular to the field direction, along which there are virtually no components of the applied magnetic field. Similarly, a "two-dimensional" field is one in which there exists only one direction along which there are virtually no components of the applied magnetic field; and, a three-dimensional field is one in which there is no direction devoid of applied magnetic field components. It can thus be seen that the combination of a one-dimensional marker and a three-dimensional field, a two-dimensional marker and a two-dimensional field, and a three-dimensional marker and one-dimensional field, would absolutely guarantee "orientation" at each point in the zone.

With such combinations, the length of the path traversed by a marker passing through the zone could be very short, only slightly longer than the length of an open-strip. By increasing the path length, such an ideal combination which would assure orientation of the marker at every point in the zone is not required. To virtually assure at least one magnetization reversal whenever an open-strip passes through a zone having a relatively long path length, it is only necessary that magnetic field component vectors greater than the marker switching field along every direction of the unit sphere be present at many points in the zone.

This condition may be satisfied by producing sequentially, in time, at each point in the zone three one-dimensional fields, each of the fields being oriented along a different coordinate axis of the unit sphere. Alternatively, the condition may be satisfied by providing along the path through the zone a plurality of regions in which the applied field orientation does not vary, the fields of successive regions, however, being oriented differently.

In addition to the length of the zone through which the marker is to pass, other significant interdependent variables for designing a particular system which will insure at least one magnetization reversal of a marker passing through the zone include: the marker velocity, the number and orientation of the major dimensions of the marker sections, the applied field alternation rate, the peak magnitude of the applied field, and the applied field vector components at each point in the zone at each instant in time. Such an alternating field may also be in the form of a damped oscillating pulse or a modulated sinusoidally varying field. One such embodiment is to provide a second alternating magnetic field having a frequency at least 5 Hz. different than the first applied alternating field.

The alternating magnetic field may be produced by conventional methods, such as by application of an alternating current to an air core loop or to a coil of an electromagnet or by moving a permanent magnet such that the permanent magnet's field is made to effectively alternate throughout the interrogation zone.

General requirements of the flux monitoring system and detecting circuit are that it monitor magnetic flux changes within the interrogating zone and discriminate between magnetic flux changes produced by each marker section, and between all extraneous magnetic flux changes. Extraneous flux changes include the applied field, noise produced by electric motors, circuit breaker noise, etc., whose effect upon the sensor is dependent to some extent upon their strength and distance from the interrogating zone. Examples of magnetic flux monitoring means include types which indicate the rate of change of the field directly, as a coil, and types which indicate the instantaneous magnitude of the field from which the field rate of change can be derived. The latter includes magnetoresistive devices, Hall devices, and magnetodiode sensors. Both types may be used with flux gathering devices to improve their sensitivity. The coil type include at least one coil for inductively sensing magnetic flux. One large coil may be used with only a few turns, even as few as a single turn; or several small coils, each having relatively more turns, may be used. Small coils have the advantage of being less sensitive to magnetic noise such as that produced by electric motors and circuit breakers. On the other hand, one large coil responds more uniformly to a marker at different positions in the zone than do the individual small coils. Any number of coils may be arranged in opposition so that magnetic noise from a distant source can be minimized, while detecting a marker closer to one coil than the other. Further, when a plurality of coils are employed, they may be provided with individual detecting systems or may sequentially share a common detecting system.

The signal detecting circuit associated with the flux monitoring means, in its simplest form, will indicate the presence of a marker in the interrogating zone by sensing the magnetic pulses corresponding to a single magnetization reversal of a marker.

Implicit in an anti-pilferage system is initiation of some prescribed action upon detecting or sensing of an apparently stolen object. The particular action initiated is incidental to the operation of the present invention but may include production of electromagnetic or sonic waves such as light, ultrasonic transmissions or radio transmissions, either or both immediate to or remote from the interrogation zone. Action may consist of making a video or photographic record of the persons present in the interrogation zone at the time of sensing an apparently stolen object, or disablement of the automatic door opening mechanism of one or more exits. To complicate intentional compromise of the system, visible or audible indications may be slightly delayed from the instant of detection.

The same general sensing circuit may be employed for both a single-status and a multi-status marker. The signals produced by each marker section when an associated control element is demagnetized are substantially the same as those produced when the strips are employed as a single-status marker. However, when the control element is magnetized, the shape, amplitude and time occurrence of the signals are changed. The sensing circuit, in detecting such changes, can thus in essence sense the magnetization of the control element.

As applied to protection of the books in a library, it is readily apparent that by employing a multi-status marker, books may be desensitized during checkout to permit removal of the book from the library. Conversely, upon return of the book, it may be conveniently sensitized to prevent undetected removal of the book from the library, until it is again properly checked out.

With such sensitization and desensitization, the exact location of a marker need not be known. Accordingly, clever concealment of one or more markers on an object renders a system of the present invention virtually invulnerable to compromise as by shielding or removal of the marker.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment of a multi-status marker particularly amenable for use in protecting the stock of a library comprises "n" open-strip sections, one consisting of an annealed permalloy ribbon of composition 4% molybdenum, 79% nickel and 17% iron about 25 microns thick, 18 centimeters long and 0.6 centimeter wide, each section differing from the other sections in A.C. coercivity. A suitable control element is formed of a gamma-ferric-oxide strip of the same width and length as each open-strip. Such a control element may be produced by first dispersing 100 parts by weight of a recording-tape grade gamma-ferric-oxide pigment and 2 parts by weight of a wetting agent such as Ross and Rowe Yelkin TTS with a solvent such as toluene to produce a mixture of 25% solids. To this is added fifty parts by weight of a resin composition, e.g. 75% of a copolymer of 89 parts vinyl chloride and 11 parts vinyl acetate (VYHH) and 25% dioctyl phthalate. A small amount of a mixture of equal parts of methyl ethyl ketone and toluene may be also added as required to make a coatable solution. The solution is knife-coated on a silicone coated release sheet. After the solution has dried, the release sheet is peeled away and strips of the dried solution of a uniform 230 micron thickness are selected for use as control elements. For the previously described open-strip, thicknesses of 230 microns have been found to be sufficient to desensitize the open-strip when the control element is magnetized by a one-inch gap magnetron magnet. Each element is then laminated to an open-strip. For more efficient production of large quantities, the mixture may be coated on a wide sheet of the open-strip material which is then subsequently slit into 0.6 centimeter wide strips.

These and other characteristics and advantages of the invention will become more apparent when considered in light of the following description of preferred embodiments taken in light of the accompanying drawings wherein:

FIG. 1 is a combinational block diagram and schematic wiring diagram of an anti-pilferage system such as may conveniently be used at the exits of a building in which the objects to be protected are kept;

FIG. 2, View A, is a front elevational view of a preferred embodiment of the applied field producing means of the system of FIG. 1; and FIG. 2, View B, is a schematic illustration of the windings of a portion of the applied field producing means of View A;

FIGS. 3 and 4 show representations of magnetic fields produced by the applied field producing means of FIG. 2;

FIG. 5 in three views is a superimposition of two sets of magnetic field lines of the applied field producing means of FIG. 2;

FIG. 6 is a schematic diagram of an embodiment of the field sequencing circuit of FIG. 1;

FIG. 7 is a perspective view of a book containing a single-status marker comprising "n" strips of ferromagnetic material adjacent the back cover of the book;

FIG. 8 is a perspective view of a book showing a multistatus marker comprising "n" strips of ferromagnetic material together with control elements concealed inside the heel of the book;

In FIG. 1, three zone units, 10A, 10B and 10C, are shown positioned to form a pair of exit ways, the spaces between opposing units 10A and 10B and between 10B and 10C each thus forming an "interrogation zone". For the exemplary embodiment shown, zone units 10A and 10C comprise field producing and flux monitoring means whereas unit 10B includes only field producing means.

Unit 10A is partially cut away, revealing a pair of electromagnets, 12A and 12B, an air core loop 14, and four smaller coils, 16A, 16B, 16C and 16D. The electromagnets and air core loop form an applied field producing means and each have a terminal (for reference purposes, an "input" terminal) coupled to a "hot" lead 18 of a filtered alternating current source and another "output" terminal respectively coupled to a field sequencing circuit 20 by leads 22, 24 and 26. Units 10A and 10C are identical, but unit 10B differs in that it does not contain the four smaller coils. These smaller coils, each formed of 900 turns of enameled wire 0.01 centimeter in diameter wound in a bundle around a form 10 centimeters in diameter, form a magnetic flux monitoring means, designated as 29. They are connected in series with RG 58 A/U coaxial cable, placed in separated locations on one side of the interrogating zone, and very carefully oriented to balance out as much as possible of the magnetic noise produced by the field producing means and other sources. In the embodiment shown, the upper coils 16A and 16B are shown oriented vertically and the lower coils 16C and 16D are shown horizontally oriented. Each coil is wrapped with one layer of 12.7 micron-thick aluminum foil (not shown) to shield it from electrostatic noise while permitting magnetic signals to pass. These foil shields are connected to the shield of the inter-connecting coaxial cable.

Coupled to the magnetic flux monitoring means of unit 10A by coaxial cable 28 is a signal detector circuit 32A; an identical signal detector 32B is coupled by coaxial cable 30 to the magnetic flux monitoring means of unit 10C. When a book 31 carrying a sensitized marker 33 passes into an applied field interrogating zone, the marker magnetization reverses at each applied field alternation to produce a magnetic pulse train. The flux monitoring means of that zone responds to this change in magnetic flux within the zone and provides a signal corresponding to the pulse to its associated detector circuit 32. The detector circuit responds to the pulse train and provides a signal for activating its alarm and indicator circuit 34.

FIG. 2, View A, is a front elevational view of a preferred embodiment of a zone unit 10A. The electromagnets 12A and 12B of the applied field producing means are each formed by solenoids surrounding a 5.1-centimeter square bar. The bars are laminates formed of 0.0457-centimeter thick by 142-centimeter long by 5.1-centimeter wide sheets of transformer steel, type M-19. Each solenoid comprises a pair of 125-turn windings of 0.205-centimeter diameter enameled wire distributed uniformly along the length of the bar and an additional pair of 60-turn windings of enameled wire 0.259 centimeter in diameter at each end. Both additional 60-turn windings are uniformly wound in a bifilar aiding fashion with the end-most 60 turns of the corresponding 125-turn windings.

The 60-turn windings 36A and 36B are wound in series with each other and in series with the parallel combination of 125-turn windings 38A and 38B as shown in FIG. 2, View B. This combination of windings is referred to herein as the coil "winding", and its "input" terminal is shown as 35 and its "output" terminal is shown as 37. We have found that the poles of the polar magnets are located approximately 7.5 centimeters in from the bar ends.

A pair of applied field producing means separated by about one meter and of the foregoing dimensions is suitable for providing an interrogation zone of about one meter by two meters by two meters.

An air core loop 14 of the preferred embodIment is simply a multiturn closed loop. For the preferred embodiment shown, loop 14 is circular and has a diameter equal to the pole separation of electromagnets 12, and is made of 80 turns of enameled wire 0.205 centimeter in diameter. If the electromagnets were not of the same length, the loop geometry would remain curvilinear but would be adjusted so that the loop periphery still passed adjacent the poles of both polar magnets for reasons which will become apparent following a discussion of the relationships between the electromagnet and air core loop magnetic fields.

FIGS. 3 and 4 illustrate free space characteristic magnetic fields of an electromagnet and an air core loop respectively, such as those of FIG. 2. (The figures illustrate the magnetic field lines lying in the plane of the paper.) As shown, the field of the vertically oriented electromagnet includes components which are vertical (parallel to the "y" axis) and also includes significant horizontal "x" components in the regions near its poles. Similarly, the magnetic field components of the air core loop include components parallel to the "x" axis and components parallel to the "y" axis.

By passing the air core loop adjacent the electromagnet poles, the two forms of field producing means complement each other. This will become more apparent following a discussion of the views of FIG. 5 wherein a superimposition of two free space magnetic field lines from each of means 12A, 12B and 14 of one zone unit are shown. For the sake of clarity, the lines representing the field produced by means 12A are shown uniformly dashed; the lines representing the field produced by means 12B are shown in alternately long and short dashes; and the lines representing the field produced by means 14 are shown in solid lines. As shown, one set of lines intersects at point M; the other at point N. A vector representation of the angular relationship of the magnetic field strengths is shown at Views B and C of FIG. 5 for points M and N respectively. Components r, s, t of View B represent the magnetic field strength of the fields at point M respectively produced by means 12B, 14, and 12A; components r', s', t' of View C are corresponding magnetic field strength vectors of the fields at point N. As shown, the components of each set are almost mutually perpendicular even though the r, s, t component directions are different from the r' , s', t' directions. Inspection of FIG. 5 makes it readily apparent that the vector components of intersections of other sets of lines will be nearly perpendicular, too. Accordingly, by proper selection of individual magnetic field strengths, means 12A, 12B and 14 would produce a nearly ideal field.

By producing fields of not less than .sqroot.2 times the marker open-strip switching field, such fields, even in the interrogating zone extremities adjacent the ends of the polar magnets where the respective directions of the fields change rapidly, are nearly "ideal" or "three-dimensional". Of course, an absolutely uniform three-dimensional field is not required because it is virtually impossible that a marker could pass through an entire zone without the marker becoming oriented at least once with a component of the applied field which is greater than the marker switching field.

The fields of FIGS. 3, 4 and 5 are those which would be produced absent any external magnetic influence. Thus, for the embodiment of FIG. 1, wherein three zone units are shown, the middle zone unit cooperating with each of the other two to form a pair of interrogating zones, it would be necessary to consecutively energize each of the nine individual field producing means in order to produce fields as shown. Because of the relatively long path length, for the reasons previously set forth, sequential production of the nine fields is unnecessary. Indeed, it may be undesirable, for the peak amplitude of currents driven through the windings of the electromagnets and air core loops, and hence the associated circuitry costs, can be reduced by simultaneously producing one field of each zone unit.

FIG. 6 is a schematic diagram of an embodiment of the field sequencing circuit 20 of FIG. 1. The particular circuIt shown in FIG. 6 simultaneously turns on like field producing means of each of the three zone units, and sequentially turns on each of the three separate means. The basic sequencing cycle thus consists of three phases, each phase lasting eight complete cycles of the AC line voltage.

With reference to FIG. 6, the field sequencing circuit is shown to comprise, each shown generally, a power line filter 39, a phase selector 40, phase timer 42 and a stepdown transformer 44. Power line filter 39 is included in the field sequencing circuit to minimize noise on the power line and is shown to comprise an inductor 41 in series with the hot line of a 117 volt power source 43 and a capacitor bank 45 coupled between the hot lead 18 and common line 47. Inductor 41 is comprised of 130 turns of enameled wire 0.259 cm in diameter wound in a bundle about 4 centimeters wide around a form 18 cm in diameter. Preferably such a coil would be cast in a potting compound such as an epoxy resin after winding. Capacitor band 45 is comprised of a number of AC capacitors in parallel, each having a voltage rating greater than 117 VAC, and totaling about 450 microfarads capacitance. Although the primary function of these capacitors is to reduce the effects of harmonic noise on the power line, their exact value should be chosen so that they also operate to compensate for the poor power factor presented by the field producing coils. This is done by adding capacitors while observing the AC current drawn from the line, and stopping when this current reaches a minimum.

Phase selector 40 is shown to comprise a modulus 3 ring counter 46 and three identical line switches (one for each phase) shown generally as 48, 50 and 52, with switch 48 connected by lead 54 to the counter phase "1" output, switch 50 connected by lead 56 to the counter phase "2" output and switch 52 connected by lead 58 to the counter phase "3" output. The modulus 3 ring counter input is coupled to the output of phase timer 42. Phase timer 42 comprises a modulus 8 binary counter 62 (three cascaded integrated circuit toggle flip-flops). Counter 62 has its input driven by a Schmitt trigger 64 which in turn has its input coupled by lead 66 to the AC line voltage hot lead 18.

In operation, the Schmitt trigger circuit switches states at about zero volts for each negative-to-positive line voltage transition, switching just before the voltage crosses zero. At this instant, transistor 68 conducts and also switches on transistor 70. This causes the voltage at input pin 72 of integrated circuit flip-flop 74 to drop sharply, toggling the flip-flop and incrementing counter 62 by one. When the counter reaches a count of seven cycles, the next line voltage negative-to-positive transition will cause it to revert to zero. At that time, the output terminal of flip-flop 76 will drop sharply in voltage, sending a negative pulse through capacitor 78 to the base of transistor 80. Normally, transistor 80 is biased into saturation by resistor 82. However, this short negative-going pulse turns the transistor off momentarily, to increment by one the Modulus 3 ring counter 46 formed by transistors 80, 84, 86 and 88, in a manner well known in the art. Thus, such negative pulses are the counter 46 "input" pulses. Only one of transistors 84, 86 and 88 is normally conducting and upon receiving an input pulse that transistor stops conducting and the next transistor in the sequence conducts. Transistors 84, 86 and 88 respectively correspond to phases "1", "2" and "3" of phase selector circuit and thus it can be seen that said transistors provide a sequence of three phases, each phase lasting for the modulus of counter 62, which for the particular embodiment is 8 alternations of the line voltage.

The outputs of counter 46 are coupled to identical line switches 48, 50 and 52, only one of which, 48, is shown schematically; the counter phase "1" output is coupled to switch 48 by lead 54, the counter phase "2" output is coupled to switch 50 by lead 56, and the counter phase "3" output is coupled to switch 52 by lead 58. The line switch operation will be described with reference to switch 48, although it is to be understood that operation of switches 50 and 52 is similar. Switch 48 is shown to include normally nonconducting transistors 90, 92, 94, 96 and 98. The collectors of transistors 92, 94, 96 and 98 are respectively coupled to gate leads of triacs 100, 102, 104 and 106. "Load-switching" triacs 100, 102 and 104 are bi-directional conducting devices having their anodes respectively coupled to an output winding of like field producing means of zone units 10A, 10B and 10C and also having their anodes commonly resistively coupled to the anode of triac 106. Triacs 100, 102 and 104, when conducting, thus permit alternating current flow through the windings of each of horizontal electromagnet field producing means 12B of zone units 10A, 10B and 10C.

The function of triac 106 is very important to false-alarm-free operation of a field sequencing circuit such as that of FIG. 6, because of the noise spikes which the load switching triacs produce each time they pass from one quadrant of operation to another, i.e. to switch from a negative to a positive conduction state or vice versa. If such a switching instant occurs at nearly the same time as the corresponding load current alternation instant, the marker characteristic signal will be masked by the load switch noise spikes because it too occurs at about the load current alternation point. Triac 106 prevents such masking by causing the load switch to switch states sufficiently in advance of the load current alternation that the noncommitant noise spikes have decreased adequately to prevent masking of a marker signal. Because of the inductance of the field producing means windings, the current from said coils appearing at the anodes of triacs 100, 102 and 104 will lag the current appearing in the secondary winding of transformer 44. By connecting the cathode of triac 106 to said secondary winding such that the polarity of the current passed through resistors 108, 110 and 112 to the anodes of the load switches opposes the load current for the part of each half cycle where the load current is approaching zero, the switching instants of said load switches are caused to lead the corresponding load current alternation in the field producing means windings. Because the marker magnetization reversal of the present embodiment lags the corresponding field producing means current alternation, it is preferable that the transformer secondary winding be connected across triac 106 as shown; were the connection reversed, the load switch switching instant would lag instead of lead line current and hence the load switch noise spikes might mask a marker signal.

FIG. 7 is a perspective view of a book 250 provided with a marker 252 comprising "n" single status open-strip sections 254, 256, 258 and 260 of ferromagnetic material adjacent a rear cover of the book. At least three sections, 254, 258 and 260, labeled as "1", "n-1" and "n" are required to produce at least three separate magnetic pulses, so as to provide the necessary equally spaced intervals therebetween. Section 256 depicted in dashed lines, is used to represent a number of sections determined by the value of "n".

FIG. 8 is a perspective view of a book 262 provided with a concealed multi-status marker 264. A portion of an outer covering 266 over the heel of the book is shown broken away to reveal the marker 264, which comprises "n" sections 268, 270, 272 and 274. Sections 268, 272 and 274 are representative of section numbers "1", "n-1" and "n" respectively. Section 270, shown in dash lines, represents optional sections, the number of which depends upon the value of "n". Sections 268, 270 and 272 consist of open-strips 276 to which are laminated control elements 278 extending the full length of each open-strip. Section 274 consists of an open-strip 280 to which is laminated control elements 282 and 284, each of which extends over only a portion of the open-strip 280.

The placement and selection of either a single or multi-status marker is entirely arbitrary, the showing in FIGS. 7 and 8 being merely exemplary of possible placements and uses for the markers of the present invention.

The present invention has utilities other than protection against theft of library books or other articles of merchandise. For example, the system may be used for sortation. Objects belonging to one class may be each provided with a marker having "n" open-strip sections having AC coercivities selected to produce a pulse train having first characteristic frequency, while objects belonging to another class would be provided with a marker having open-strip sections having AC coercivities selected to produce a pulse train having second characteristic frequency, etc. Accordingly, objects may be sorted into groups by the frequency of their characteristic signals. Similarly, strips of different coercivities may be combined to form a binary code. Each bit of the code would correspond to strips generating a particular frequency. The presence or absence of marker strips generating a particular frequency would correspond to a "0" or "1" for the digit or "bit" position corresponding to those strips. A selectively alterable marker may be provided by including all strips in every marker and by providing each strip with a control element of the type previously described.

It is to be recognized that each library book could be provided with machine readable indicia of the book's identity, e.g. the book's Library of Congress number. By automatically desensitizing a book's marker in response to decoding and recording both these indicia and the pertinent data on a user's identity card, an automated checkout system is provided.

Claims

What is claimed is:

1. A marker capable of producing magnetic pulses which can be unambiguously detected by unsophisticated apparatus out of direct contact with said marker, said marker comprising "n" sections of ferromagnetic material, said sections having an aggregate saturation magnetization of at least 0.1 pole-centimeters and each section differing from the other sections in A.C. coercivity such that when subjected to a periodically varying magnetic field which increases at a predetermined time rate of change, the magnetization of the sections reverses sequentially at equal intervals of time, each interval being less than or equal to T/4n where "T" is equal to the period of said varying magnetic field and wherein each alternation and set of said sequential reversals produced in response thereto results in a magnetic pulse train.

2. A marker according to claim 1 wherein said ferromagnetic sections comprise "n" open-strips.

3. A multi-status marker capable of producing magnetic pulses which can be unambiguously detected by unsophisticated apparatus out of direct contact with said marker, said marker further capable of being deactivated to prevent production of said magnetic pulses upon interrogation, comprising:

a. "n" sections of ferromagnetic material, said sections having an aggregate magnetization of at least 0.1 pole-centimeters and each section differing from the other sections in A.C. coercivity such that when subjected to a periodically varying magnetic field which increases at a predetermined time rate of change, the magnetization of the sections reverses sequentially at equal intervals of time, each interval being less than or equal to T/4n where "T" is equal to the period of said varying magnetic field, and wherein each alternation and set of said sequential reversals produced in response thereto results in magnetic pulses; and

b. at least one control element of a ferromagnetic material having a coercivity of at least 5 oersteds producing when remanently magnetized a static external magnetic field of at least three-fourths oersted over at least a portion of each section to at least partially magnetize each said portion whereby the amplitude and time characteristics of a signal produced by said sections in response to said applied magnetic field differs distinguishably from the corresponding signal when the control element is demagnetized.

4. A marker according to claim 3 wherein said ferromagnetic sections comprise "n" open-strips.

5. A marker according to claim 4 having at least two control elements which when remanently magnetized produce a static external magnetic field over at least two portions of each section.

6. A marker according to claim 4 having one control element which produces a static external magnetic field of at least 3/4ths oersted over the entire length of each of said "n" sections of open-strips.

7. A system for detecting the presence of an object within an interrogation zone, which object has secured thereto an identifying marker, comprising:

a. means defining an interrogation zone;

b. means for applying in said interrogation zone a periodically varying magnetic field which increases at a predetermined time rate of change;

c. a marker secured to said object, said marker being capable of producing magnetic pulses, and comprising "n" sections of ferromagnetic material, said sections having an aggregate saturation magnetization of at least 0.1 pole-centimeters, and each section differing from the other sections in A.C. coercivity such that when subjected to said periodically varying magnetic field, the magnetization of the sections reverses sequentially at equal intervals of time, each interval being less than or equal to T/4n where "T" is equal to the period of said varying magnetic field and wherein each alternation and set of sequential reversals produced in response thereto results in said magnetic pulses; and

d. means in the vicinity of said interrogation zone for detecting said magnetic pulses.

8. A system according to claim 7 further comprising means for desensitizing said marker to render said marker incapable of producing said magnetic pulses when subjected to said varying magnetic field.

9. The system of claim 8 wherein said desensitizing means when in operation is out of contact with said marker.

10. The system of claim 8 further comprising means for altering said marker from a desensitized to a sensitized state to render said marker capable of producing said magnetic pulses when subjected to said varying magnetic field.

11. The system of claim 10 wherein each marker further comprises at least one control element of a ferromagnetic material having coercivity of at least five oersteds capable of producing when remanently magnetized a static external magnetic field of at least three-fourths oersted over at least a portion of said sections to at least partially magnetize said portion whereby the amplitude and time characteristics of a signal produced by said sections in response to said varying magnetic field differs distinguishably from the corresponding signal when the control element is demagnetized, and wherein said desensitizing means includes means for remanently magnetizing said control element; and said altering means includes means for demagnetizing said control element.

12. An object to which is secured a marker which can be detected without direct contact comprising "n" open-strips where "n" is an integral number greater than one, of ferromagnetic material as herein defined with an aggregate saturation magnetic moment of at least 0.1 pole-centimeter, the magnetization of each of said open-strips when subjected to a 60 Hz. sinusoidally varying magnetic test field of a predetermined peak magnitude of less than 20 oersted reversing at each field alternation to in turn produce a pulse of external polar magnetic field having appreciable flux components within the range of 1,000 to 16,000 Hz., the gross time rate of change of which flux components defines a signal having a width at half amplitude less than 0.1 millisecond, said strips selected to sequentially reverse their magnetization following a said applied field alternation at equal intervals of time, each interval being less than or equal to T/4n where "T" is equal to the period of said applied field, said sequential reversals producing a magnetic pulse train which can be unambiguously detected by unsophisticated apparatus.