U.S. patent number 3,665,449 [Application Number 04/840,973] was granted by the patent office on 1972-05-23 for method and apparatus for detecting at a distance the status and identity of objects.
This patent grant is currently assigned to Minnesota Mining and Manufacturing Company. Invention is credited to James T. Elder, Donald A. Wright.
United States Patent |
3,665,449 |
Elder , et al. |
May 23, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
METHOD AND APPARATUS FOR DETECTING AT A DISTANCE THE STATUS AND
IDENTITY OF OBJECTS
Abstract
A non-contact method of and system for distinguishing the
presence, identity or status of an object. The system comprises a
marker including a ferro-magnetic 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 ferro-magnetic material. The marker includes a relatively
long, relatively thin, open-strip of a ferro-magnetic 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.
Inventors: |
Elder; James T. (Shoreview,
MN), Wright; Donald A. (Woodbury, MN) |
Assignee: |
Minnesota Mining and Manufacturing
Company (St. Paul, MN)
|
Family
ID: |
25283703 |
Appl.
No.: |
04/840,973 |
Filed: |
July 11, 1969 |
Current U.S.
Class: |
340/572.1;
340/572.3; 324/243; 335/284 |
Current CPC
Class: |
G08B
13/2474 (20130101); G08B 13/2477 (20130101); G08B
13/2442 (20130101); G01N 27/72 (20130101); G08B
13/2437 (20130101); G08B 13/2408 (20130101) |
Current International
Class: |
G08B
13/24 (20060101); G01N 27/72 (20060101); G01r
033/02 (); G08b 013/24 () |
Field of
Search: |
;340/280,258,258C,149A
;324/41 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Luck, David G. C., Young, Charles J., "The Prison Gun Detector
(Electromagnetic Devices Discloses Metal Generally)", Radio World,
Dec. 1936, pp. 50-56,.
|
Primary Examiner: Caldwell; John W.
Assistant Examiner: Partridge; Scott F.
Claims
What is claimed is:
1. A method for detecting the presence of an object within an
interrogation zone, comprising
a. securing a marker means to said object, which marker means
comprises an open-strip of a ferro-magnetic material, which is
sufficiently thin relative to its major dimension to produce a
characteristic pulse of an external magnetic field when its
magnetization is reversed by a sinusoidal alternating applied
magnetic field having a maximum time rate of change in excess of
300 oersteds per second, wherein said characteristic pulse
comprises a composite signal containing a band of frequencies
having a lower bound exceeding 1,000 Hz. and a width at least 10
times the frequency of said applied field;
b. passing said object into said interrogation zone;
c. applying in said interrogation zone an alternating applied
magnetic field having a maximum time rate of change in excess of
300 oersteds per second; and
d. detecting in the vicinity of said interrogation zone the
presence of a signal comprising a band of frequencies whose lower
bound exceeds 1,000 Hz. and whose width is at least 10 times the
frequency of said applied field.
2. A method according to claim 1 wherein step (c) further comprises
sequentially applying in said interrogation zone a plurality of
alternating applied magnetic fields such that while said object is
in said interrogation zone there is present in every direction at
least one magnetic field component vector greater than a field
sufficient to reverse the magnetization of said marker to thereby
assure at least one magnetization reversal whenever said object is
in said interrogation zone.
3. A method according to claim 1 wherein step (c) comprises
applying a pulsed alternating applied magnetic field.
4. The method of claim 1 wherein said
ferro-magnetic material has an aggregate saturation magnetic moment
of at least 0.1 pole-centimeter, the magnetization of each strip
when said strip is subjected over its full length of a 60 Hz.
sinusoidally varying magnetic test field of a predetermined peak
magnitude of less than 20 oersteds reversing for each alternation
of said test field, wherein step (c) further comprises
applying in said interrogation zone at least said applied
alternating magnetic field having at least 3/4 oersted peak
amplitude; and wherein the method further comprises
e. sensing the amplitude and time characteristics of at least one
of said detected signals to verify the presence of a said
object.
5. The method of claim 4 wherein step (e) further comprises
producing a timing signal and comparing the time occurrences of
said at least one said detected signal and said timing signal to
verify the presence of a said object.
6. The method of claim 1 wherein said applied magnetic field is a
modulated sinusoidally varying magnetic field.
7. The method of claim 4 wherein step (c) further comprises
applying in said interrogation zone at least a second alternating
magnetic field of a frequency at least 5 Hz. different from said
first applied alternating magnetic field, wherein
step (d) further comprises detecting the presence of at least one
further signal comprising a said band of frequencies whose lower
bound exceeds 1,000 Hz. and whose width is at least 10 times the
frequency of said second alternating magnetic field; and
wherein
step (e) further comprises sensing the amplitude and time
characteristics of at least one of said detected further signals to
verify the presence of a said object.
8. The method of claim 1 including the step of sensing for a
potential carrier of said object within said interrogating zone and
initiating step (c) only when a said carrier is sensed.
9. A method for detecting the presence of an object within an
interrogation zone, comprising
a. securing a marker means to said object, which marker means
comprises an open-strip of a ferro-magnetic material, which is
sufficiently thin relative to its major dimension to produce a
characteristic pulse of an external magnetic field when its
magnetization is reversed by a sinusoidal alternating applied
magnetic field having a maximum time rate of change in excess of
300 oersteds per second, wherein said characteristic pulse
comprises a composite signal containing a band of frequencies
having a lower bound exceeding 1,000 Hz. and a width at least 10
times the frequency of said applied field, and depending upon the
particular composition, size and shape of said ferro-magnetic
material, the peak amplitude of said composite signal will occur
for an applied alternating field of a particular waveform a
predictable time after each applied field alternation;
b. passing said object into said interrogation zone;
c. applying in said interrogation zone an alternating applied
magnetic field having a maximum time rate of change in excess of
300 oersteds per second;
d. detecting in the vicinity of said interrogation zone the
presence of a signal comprising a band of frequencies whose lower
bound exceeds 1,000 Hz. and whose width is at least 10 times the
frequency of said applied field; and
e. detecting in the vicinity of said interrogation zone whether the
peak amplitude of said composite signal occurs a predictable time
after each applied field alternation.
10. A system for detecting the presence of an object within an
interrogation zone, comprising
a. marker means secured to said object, which marker means
comprises an open-strip of a ferro-magnetic material, which is
sufficiently thin relative to its major dimension to produce a
characteristic pulse of an external magnetic field when its
magnetization is reversed by a sinusoidal alternating applied
magnetic field having a maximum time rate of change in excess of
300 oersteds per second, wherein said characteristic pulse
comprises a composite signal containing a band of frequencies
having a lower bound exceeding 1,000 Hz. and a width at least 10
times the frequency of said applied field, and depending upon the
particular composition, size and shape of said ferro-magnetic
material, the peak amplitude of said composite signal will occur
for an applied alternating field of a particular waveform a
predictable time after each applied field alternation;
b. means defining an interrogation zone;
c. means for applying in said interrogation zone an alternating
applied magnetic field having a maximum time rate of change in
excess of 300 oersteds per second;
d. means for detecting in the vicinity of said interrogation zone
the presence of a signal comprising a band of frequencies whose
lower bound exceeds 1,000 Hz. and whose width is at least 10 times
the frequency of said applied field; and
e. means for detecting in the vicinity of said interrogation zone
whether the peak amplitude of said composite signal occurs a
predictable time after each applied field alternation.
11. A system for detecting the presence of an object within an
interrogation zone, comprising
a. marker means secured to said object, which marker means
comprises an open-strip of a ferro-magnetic material, which is
sufficiently thin relative to its major dimension to produce a
characteristic pulse of an external magnetic field when its
magnetization is reversed by a sinusoidal alternating applied
magnetic field having a maximum time rate of change in excess of
300 oersteds per second, wherein said characteristic pulse
comprises a composite signal containing a band of frequencies
having a lower bound exceeding 1,000 Hz. and a width at least 10
times the frequency of said applied field;
b. means defining an interrogation zone;
c. means for applying in said interrogation zone an alternating
applied magnetic field having a maximum time rate of change in
excess of 300 oersteds per second; and
d. means for detecting in the vicinity of said interrogation zone
the presence of a signal comprising a band of frequencies whose
lower bound exceeds 1,000 Hz. and whose width is at least 10 times
the frequency of said applied field.
12. A system according to claim 11 wherein the means of paragraph
(c) further comprises means for sequentially applying in said
interrogation zone a plurality of alternating applied magnetic
fields such that while said object is in said interrogation zone
there is present in every direction at least one magnetic field
component vector greater than a field sufficient to reverse the
magnetization of said marker to thereby assure at least one
magnetization reversal whenever said object is in said
interrogation zone.
13. A system according to claim 11, wherein the means of paragraph
(c) comprises means for applying a pulsed alternating applied
magnetic field.
14. A system according to claim 11 wherein the system further
comprises
A. a plurality of markers, at least one of which markers is to be
secured to each object, each marker comprising at least one
open-strip of a ferro-magnetic material with an aggregate
saturation magnetic moment of at least 0.1 pole-centimeter, the
magnetization of a said marker when the marker is subjected over
its entire length to a test field of a 60 Hz. sinusoidal magnetic
field of a predetermined peak magnitude of less than 20 oersteds
reversing at each field alternation to in turn produce a said
characteristic pulse of external polar magnetic field having
appreciable flux components within the range of 1,000 Hz. to 16,000
Hz., the gross time rate of change of which flux components defines
a signal having a width at half amplitude of less than 0.1
millisecond; and
B. means for applying throughout said zone at least one alternating
magnetic field of a predetermined frequency having a peak magnitude
of at least 3/4 oersted.
15. A system according to claim 11 wherein said alternating
magnetic field applying means comprises a set of magnetic field
producing means including
a first substantially linear electromagnet;
a second substantially linear electromagnet orthogonal to said
first linear electromagnet; and
an air core loop for producing a magnetic field, said linear
electromagnets and said air core loop disposed in mutually parallel
planes and the perimeter of the air core loop passing adjacent the
magnetic poles of said first and said second linear
electromagnets.
16. The system of claim 11 further comprising means for
desensitizing said marker to render said marker incapable of
producing said characteristic pulse when subjected to said applied
field.
17. The system of claim 16 wherein said desensitizing means when in
operation is out of contact with said marker.
18. The system of claim 16 further comprising means for altering
said marker from a desensitized to a sensitized state to render
said marker capable of producing said characteristic pulse when
subjected to said applied field.
19. The system of claim 18 wherein each marker further comprises a
control element of a ferro-magnetic material having a coercivity of
at least 5 oersteds and capable of producing, when remanently
magnetized, a static external magnetic field of at least 3/4
oersted over at least a portion of said open-strip to at least
partially magnetize said portion wherein the disposition of said
control element with respect to said open-strip is such that an
electrical signal generated while the control element is
demagnetized differs distinguishably from an electrical signal
generated while the control element is magnetized and wherein
said desensitizing means include means for remanently magnetizing
said control element; and
said altering means includes means for demagnetizing said control
element.
20. An automated checkout system employing the apparatus of claim
16 and further comprising
machine readable indicia associated with each object, which indicia
identifies said object;
means for encoding said indicia and for providing a signal for
recording said indicia; and
means, responsive to sensing a machine readable user's identity
card and a said indicia, for actuating said desensitizing means and
for making a record of the contents of said identity card and said
indicia.
21. The system of claim 11 for detecting the presence, identity, or
status of an object within an interrogation zone, wherein
each marker comprises at least two open-strips, each open-strip
having a different AC coercivity; wherein
the magnetization of each of said open-strips reverses during a
different time interval following each said field alternation to
produce flux changes defining a characteristic signal for each said
open-strip, each said characteristic signal occurring a different
time interval after a said field reversal;
and wherein
said detecting means produces a status indication in response to
detecting at least one of each of said characteristic signals
during a time interval between successive field alternations.
22. The system according to claim 21 wherein the system comprises n
groups of markers, each open-strip of all markers in a particular
group having the same AC coercivity but the AC coercivity of each
group being different; wherein
the magnetization of the strips of each group reverses during
substantially the same time interval after each said field
alternation but the magnetization of strips of different groups
reverse during different time intervals to produce flux changes
defining signals respectively characteristic of each said group;
and wherein
said detecting means produces an indication in response to
detecting a said characteristic signal and wherein the indication
produced is different for each characteristic signal to provide an
indication of which group the characteristic signal corresponds
to.
23. The system according to claim 21 wherein the system comprises n
groups of markers, where n is an integral number greater than 1,
each said marker in said group comprising m open-strips, wherein m
is an integral number, each of the m open-strips of a said marker
having different AC coercivities, all of said markers of a said
group having the same combination of said open-strips but different
groups having different combinations of open-strips, each marker of
a said group producing a set of magnetic field pulses
characteristic of said group when said each marker is passed into
said applied field,
and wherein
said detecting means produces an indication representative of a
said group in response to detecting a set of said pulses
characteristic of said group.
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 a specifically chosen piece 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
piece 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 Pat. No. 763,681, issued to Pierre Arthur Picard, discloses
a remote 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.
Picard gives as one class of metals those which require a high
applied field in order to magnetically saturate. He shows that the
induced voltage from each such metal contains in addition to a
fundamental component a certain number of harmonic components. One
such metal, iron, is described as producing a voltage containing a
third harmonic and a little bit of the fifth harmonic.
Picard also teaches that high permeability metals, i.e. those which
will saturate in a weak applied field, produce an induced voltage
including higher order harmonics than the harmonics of metals such
as iron. He gives permalloy as one high permeability material and
points out that the permalloy characteristic voltage contains ninth
and eleventh harmonic components, unlike such common metals as
copper, iron, or aluminum, which produce practically no harmonics
of such a high order.
According to Picard, only the composition of a metal determines the
order of the harmonics present in its characteristic voltage. He
teaches that marker size and geometry affect the component
amplitudes proportionately. Accordingly, the ratio of two
individual harmonic components for a particular material would be
the same regardless of the material's size or geometry. Further,
Picard teaches that the ratio between at least certain selected
components is characteristically different for different materials.
Picard does, however, emphasize that size of the metal piece to be
used as a marker is important; not to control the order of the
harmonics present, but rather to provide a signal large enough to
be detected.
Picard thus teaches that high permeability metals, and specifically
permalloy, can be distinguished from other classes of magnetizable
metal by both the presence of harmonics on the order of the ninth
and eleventh and by the ratios of some of its characteristic
harmonic components.
Picard also felt that permalloy was a good material for use as the
special marker to be placed in library books because normally one
would rarely, if ever, carry such a material.
SUMMARY OF THE INVENTION
The present invention provides a non-contact method of detecting
the presence, identity, or status of an object within an
interrogating zone such as an exit from a library. The method
comprises securing to each object to be detected a marker
comprising at least one open-strip of a ferro-magnetic material
with an aggregate saturation magnetic moment of at least 0.1
electromagnetic units or "pole-centimeters". The open-strip is
selected such that its magnetization, when the strip is within and
has 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.
An alternating magnetic field is applied within the interrogating
zone. This applied field is at least equal to said predetermined
peak magnitude and is substantially free of frequency components
exceeding a predetermined frequency of at least 1,000 Hz. When an
object having an open-strip is passed into the applied magnetic
field, and a major dimension of the open-strip and a vector
component of the magnetic field become oriented with each other,
the magnetization of the open-strip reverses at each alternation of
the applied field. Each magnetization reversal produces a pulse of
external polar magnetic field. Means are provided for monitoring in
the vicinity of the interrogating zone an entire band of frequency
components of magnetic flux. The lower cutoff frequency of this
band is greater than the predetermined frequency and the band has a
width of at least 600 Hz. Means are provided for detecting a signal
corresponding to a said polar magnetic field pulse to verify the
presence, identity, or status of the object by sensing the
amplitude and time characteristics of at least one of the
signals.
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, "ferro-magnetic" 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.
We have found that an open-strip of a ferro-magnetic material, if
made sufficiently thin relative to its major dimension, will
produce a characteristic pulse of an external polar magnetic field
when its magnetization is reversed by an alternating applied field
having a maximum time rate of change in excess of 300 oersteds per
second. Unlike the marker described by Picard, the open-strip
marker we have described is not reliably distinguished by a single
frequency or a ratio of two frequencies. We have found that the
amplitude of a particular harmonic depends upon the amplitude and
frequency of the applied field and upon the orientation of the
marker with respect to the applied field. The composite of an
appropriately selected band of frequencies from our marker,
however, provides a signal which has distinguishing features which
are essentially independent of the orientation of the marker
relative to the field and of both the amplitude and frequency of
the applied field. Indeed, our marker is frequency independent to
the extent that it produces its characteristic signal in response
to other than a sinusoidal applied field, e.g. a pulsed field.
When an open-strip of the present invention is subjected to a
sinusoidal applied field, it produces an external polar pulse
distinguishable by its gross flux components in a band whose lower
bound (the predetermined frequency) exceeds 1,000 Hz. and whose
width is at least 10 times the applied field frequency to insure a
statistically representative group of frequencies. A signal
corresponding to the gross time rate of change of the flux
components within the band (a "magnetization reversal signal"),
such as the voltage induced in a coil linked by the flux, is very
narrow, less than 0.1 milliseconds at half amplitude.
A further distinguishing characteristic of the open-strip signal is
that, for a particular composition, size and shape of material, the
peak amplitude of the signal 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 the marker'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. 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 of the present invention may take the form of a
thin, flat ferro-magnetic 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
ferro-magnetic 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 at half-amplitude increases
to become eventually indistinguishable from reversal signals of
many common ferro-magnetic 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 flyleafs having a seam joining them
along their entire length. Such seams are normally wider than an
open-strip and thus a strip 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 may be wholly inorganic or may comprise
ferro-magnetic laminae held together with an organic adhesive; or,
it may be a dispersion of ferro-magnetic particles in an organic
binder such as vinyl chloride. It may even be closely spaced but
physically separate ferro-magnetic 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).
Still another version of a marker is two or more open-strips of
different AC coercivities employed as an integral unit. Such
integrally joined open-strips, even if in physical contact with
each other, do not magnetically influence each other enough to
prevent each from providing its own characteristic pulse. Open
strips may be selected to sequentially reverse their magnetization
following an applied field and "n" is the number of open-strips
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. 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 value
sufficient to exceed the AC coercivity of the open-strip material
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 open-strips 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.
We have found that a convenient way to prevent or alter
magnetization reversal of a marker, i.e. to desensitize a marker,
is to effectively bias the applied field at the open-strip 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 an open-strip, the net field to which the open-strip
is subjected during each half cycle when the remanent and applied
fields oppose each other is insufficient to reverse the open-strip
magnetization in the characteristic manner. It is not necessary to
completely prevent reversal of the open-strip 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 an open-strip surface. For example, a magnetized
control element adjacent only a central portion of an open-strip
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 influence 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 open-strip 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 such that its 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 to change its magnetic
response. For example, an open-strip may be employed in conjunction
with, or as one element of, a thermosensitive bi-metallic
strip.
Hereafter, a marker including one or more open-strips alone shall
be referred to as a "single-status" marker and a marker including
the combination of one or more control elements with one or more
open-strips shall be referred to as a "multi-status" marker.
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 open-strip
is to pass, other significant interdependent variables for
designing a particular system which will insure at least one
magnetization reversal of an open-strip passing through the zone
include: the open-strip velocity, the number and orientation of the
open-strip major dimensions, 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 from 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 a marker and 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 magneto-diode 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 time and amplitude
characteristics of a signal corresponding to a single magnetization
reversal of a marker open-strip. As previously stated, an
open-strip produces an uncommonly narrow signal and a signal which
occurs, for a particular applied field and a marker of a particular
AC coercivity, at a predictable time following an alternation of
the applied field. By sensing signals which occur within a
prescribed interval and are of some minimum amplitude and which
both begin and end during a minimal time interval, the detecting
circuit distinguishes between noise, marker signals and extraneous
signals from other objects which respond to an alternating magnetic
field.
The predictability of the time occurrence of marker characteristic
signals also permits building redundancy into a sensing circuit to
provide a safety factor against false alarms. The reference for
measuring successive signals may be a preceding signal or some
phase or time of the applied field, conveniently the alternation
points of the applied field. Also, a marker may be permanently
secured within the zone to provide a reference signal. Accordingly,
both the time intervals between signals as well as individual
signal characteristics provide a reliable indication of the
detection of a marker. In combination, the two provide a virtually
false-alarm-free system.
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 sensing means may take the form of a circuit which responds to
at least one signal which is both of at least a minimum amplitude
and less than a maximum width. Additionally, the circuit may check
the time occurrence of the detected signal against the time
characteristic of either the applied magnetic field signal or a
reference signal correlated with the applied magnetic field signal.
The same general sensing circuit may be employed for both a
single-status and a multi-status marker. The amplitude and width of
the signals produced by each open-strip 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 an
open-strip consisting of an annealed permalloy ribbon of
composition 4 percent molybdenum, 79 percent nickel and 17 percent
iron about 25 microns thick, 18 centimeters long and 0.6 centimeter
wide. A suitable control element is formed of a gamma-ferric-oxide
strip of the same width and length as the 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 percent
solids. To this is added 50 parts by weight of a resin composition,
e.g. 75 percent of a copolymer of 89 parts vinyl chloride and 11
parts vinyl acetate (VYHH) and 25 percent 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 more than 230 microns have been found to
be sufficient to desensitize the open-strip when the control
element is magnetized by a 1-inch gap magnetron magnet. These
elements are 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 on two sheets labeled 7A and 7B is a combinational block
diagram and schematic wiring diagram of preferred embodiment of the
magnetic flux monitoring means and signal detecting circuit of the
system of FIG. 1.
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 carrying a
sensitized marker passes into an applied field interrogating zone,
the marker magnetization reverses at each applied field alternation
to produce a pulse of external polar magnetic field. 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 unique amplitude and time characteristics of the
marker signal 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 electro-magnets 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 electro-magnet 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 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 8 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 centimeter in diameter wound in a bundle about 4
centimeters wide around to form 18 centimeters in diameter.
Preferably such a coil would be cast in a potting compound such as
an epoxy resin after winding. Capacitor bank 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 Schmidt
trigger 64 which in turn has its input coupled by lead 66 to the AC
line voltage hot lead 18.
In operation, the Schmidt 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" or 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 electro-magnetic 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 concommitant 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. 7A and B, shown on two sheets of drawings, is a combinational
block diagram and schematic wiring diagram of one of the two
identical combinations of magnetic flux monitoring means 29 and
signal detecting circuits 32 of the system of FIG. 1. As shown, the
coils 16A, 16B, 16C and 16D of magnetic flux monitoring means 29
are coupled to the input of detector 32, by coaxial cable 28.
Detector 32 comprises a signal shaping and amplifier section (SSAS)
128, a signal discriminator section (SDS) 130 and an output section
132, the output of the latter of which is provided by lead 134 to
alarm and indicator 34 of the system of FIG. 1.
SSAS 128 comprises, in series, an input filter 136, a first stage
preamp 138, second stage preamp 140, third stage preamp 142
coaxially coupled to a further filter 144, a further amplifier 146,
and a limiter stage 148, each of which is of a well known
construction. The pair of twin-T filters of filter 136 have a very
high transfer impedance at 60 Hz., the frequency of the field
producing means in this embodiment; they are made from low-cost,
10-percent tolerance components, require no tuning, almost
eliminate the very strong 60 Hz. interference and reduce other
interfering signals up to about 1,000 Hz. as well. In input filter
136, capacitors 201,202, 204 and 206 were selected to be 0.01
microfarads, capacitors 208 and 210 were selected to be 0.022
microfarads, resistors 212, 214, 216 and 218 were selected to be
270 .times. 10.sup.3 ohms and resistors 220 and 222 were selected
to be 120 .times. 10.sup.3 ohms. Preamplifier stage 138 employs a
transfer designed for very low noise, and is intended to contribute
as little self-generated noise as possible to the amplified signal
at its output.
Second and third preamplifier stages 140 and 142 each provide
signal gain with good linearity. Their interstage coupling
capacitors attenuate frequencies below about 1,000 Hz. A variable
resistor 150 is coupled between preamplifier 140 and 142 to permit
optimum adjustment of the system against false alarms in the form
of random signals present in, and perhaps peculiar to, the
particular system's environment. Optimum adjustment is made by
increasing the AC collector voltage of transistor 152 until
detector 32 detects a signal even though it is known that a marker
characteristic signal has not been produced in the interrogation
zone. The value of this AC collector voltage is observed and
variable resistor 150 then set to provide a transistor 152 AC
collector voltage of approximately one-half the observed value.
Filter 144 is essentially a high-pass filter with an additional
band-reject filter centered upon the applied field fundamental
frequency, the latter filter comprising a twin-T filter network
between high-pass filter capacitors. The twin-T filter network
provides a high transfer impedance at 60 Hz., the fundamental
frequency of the applied field, and the high-pass capacitors
substantially attenuate signals within the range of 60 Hz. to 1,000
Hz., the predetermined frequency of the instant preferred
embodiment. In filter 144, capacitors 224 and 226 were selected to
be 0.01 microfarads, resistors 228 and 230 were selected to be 270
.times. 10.sup.3 ohms, resistor 232 was selected to be 120 .times.
10.sup.3 ohms, and capacitor 234 was selected to be 0.022 ohms.
Further, amplifier 146 provides additional amplification and
limiter stage 148 provides still further gain and also provides
both a high input impedance for amplifier 146 and unsymmetrically
clipped signals to signal discriminator 130.
Signal discriminator 130 is designed to provide an output signal
for enabling alarm and indicator circuit 34 each time three signals
of an amplitude at least twice as great as the noise and less than
about 0.3 milliseconds in duration occur at successive intervals of
about 16 2/3 milliseconds. Each such signal corresponds to the
characteristic signal of a marker of the foregoing described
preferred embodiment. Further, the 16 2/3 millisecond interval
corresponds to the period of a 60 cycle signal and thus represents
alternate magnetization reversals of a sensitized marker. In
general, signal discriminator 130 is synchronized with the applied
field to sense for a characteristic signal only for about a 1.5
millisecond interval or time window beginning immediately following
each applied field positive-to-negative alternation. If such a
signal is detected, a signal is provided on lead 154 to the output
section 132. Upon receipt of three signals on lead 154 at
successive 16 2/3 millisecond second intervals, output section 132
provides a signal to alarm and indicator 34. After receiving a
first signal, output section 132 blocks incrementation of the
counter during the following 13 2/3 milliseconds and if another
signal is not received with 6 milliseconds after that, the counter
is cleared.
Signal discriminator section 130 is shown to comprise a disabling
transistor 156 which, together with trigger circuit 158, delay
timer 160 and coercivity gate 162 prevent passage of an output
signal from limiter stage 148 to the combination of a signal shaper
164 and signal width discriminator 166 for most of each cycle. In
operation, trigger 158 changes state at approximately the time the
AC line voltage goes through zero. When this transistion is
positive-to-negative, the trigger output makes a similar
transistion and couples a pulse through capacitor 168 to trigger
the delay timer, a monostable multivibrator, to its unstable state.
The delay timer stays in this state for a period of time selected
to be about 4 milliseconds, then switches off at approximately the
time that the applied field crosses through zero (the applied field
lags the line voltage by slightly less than 90.degree., or roughly
4 milliseconds). When the delay timer goes off, it sends a
negative-going pulse through capacitor 170 to trigger the
coercivity gate, also a monostable multivibrator, into its unstable
state, which lasts about 1.5 milliseconds before reverting to the
stable state. While in its unstable state, it supplies a low
voltage for resistor 172 so that no current flows into the base of
disabling transistor 156. Therefore, transistor 156 does not
conduct during the unstable state of gate 162, thereby enabling the
shaper circuit during this time. A high voltage level is
simultaneously placed on resistor 174 which conducts current into
the base of transistor 176 to in turn permit it to conduct current
to similarly enable the signal width discriminator circuit. When
the coercivity gate multivibrator returns to its stable state, the
situation is reversed. A high voltage level appears on resistor 172
so that transistor 156 conducts current through resistor 177,
causing transistor 179 to conduct current continuously, disabling
the shaper circuit. At the same time, a low voltage level is
impressed upon resistor 174, which then supplies no current to
transistor 176, and the signal width discriminator circuits are
disabled. Thus, it is only during the short unstable state of
coercivity gate 162 of each 60 Hz. period that a signal will be
accepted for processing. The characteristic signal pulses emanating
from a marker of the type used in this invention will fall within
this short interval.
When such a characteristic signal is timely received from limiter
stage 148, it switches the shaper circuit state if its peak
amplitude is greater than about 8 volts, transistors 179 and 181
switching from normal conducting and non-conducting states
respectively to their new opposite states. This sends a pulse
through capacitor 178 and diode 180 to trigger the signal width
discriminator multivibrator 182 into its unstable state.
Transistors 179 and 181 remain in their new states for so long as
the received signal exceeds about 6 volts, which for characteristic
marker signals is less than about 0.3 milliseconds. In their new
states, absence of current flow through the collector of transistor
179 cuts off transistor 184, which transistor will also be cut off
whenever either or both transistors 176 and 186 are non-conducting.
The non-conductive state of the former has been previously
described in connection with the description of coercivity gate
162. Transistor 186 is non-conductive during the stable state of
signal width multivibrator 182. Transistor 184 will thus conduct
only during the unstable states of the coercivity gate and signal
width multivibrator and during a normal state of shaper 164 when
the collector current of transistor 179 is providing forward bias
current to its base. Accordingly, the unstable state of
multivibrator 182 must be greater than the width of a
characteristic signal.
When a characteristic signal does switch transistor 184 into the
conducting state, a negative-going signal is provided on lead 154,
the output section 132 input lead.
The output section 132 is shown to comprise a counter 188, a period
time 190, a consecutive-pulse gate 192 and a counter-clear gate
194.
Period timer 190 is simply a monostable multivibrator coupled to
input lead 154 and adapted to switch into its unstable state in
response to a negative going signal thereon. Each transition of the
period timer from its stable to unstable state increments counter
188 by switching transistor 196. The period timer unstable state is
about 13 2/3 milliseconds; accordingly, counter incrementing is
limited to once per each 13 2/3 milliseconds or about once per a 60
Hz. cycle. Clearing of the counter is also inhibited during this 13
2/3 millisecond interval by the forward bias provided at node 198
because transistor 200 is cut off. Consecutive-pulse gate 192,
another monostable multivibrator, is switched into its unstable
state when the period timer reverts to its stable state. In its
unstable state, lasting about 6 milliseconds, gate 192 prevents
clearing of the counter by providing forward bias to the base of
counter clear gate 194. If the period timer has not been switched
into its unstable state when consecutive-pulse gate 192 reverts to
its stable state, the counter is cleared.
Relay driver circuit 133 responds to an output produced when the
counter is counted up to three to provide an output signal on lead
134 for activating alarm and indicator circuit 34.
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 an open-strip
of one AC coercivity, objects belonging to another class would be
provided with a marker having a second AC coercivity, etc.
Accordingly, objects may be sorted into groups by the time
occurrence of their magnetization reversal signals. Similarly,
strips of different coercivities may be combined to form a binary
code. Each bit of the code would correspond to a strip of a
particular coercivity. The presence or absence of a strip of a
particular AC coercivity in a marker would correspond to a "0" or
"1" for the digit or "bit" position corresponding to that strip. 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.
A further use of the marker of the present invention would be as a
tamper-proof seal, whose integrity could be determined by
instruments without direct contact as on a railway car moving past
an interrogation point. The seal would be secured such that it
could not be removed intact and subsequently be replaced. Moreover,
the seal would be secured such that any attempt to remove it would
result in a division of the seal such that the dimensional ratios
of each of the divided pieces would be less than the aforementioned
minimum value of 150.
The marker of a tamper-proof seal may comprise a high-strength
adhesive on a flimsy length of an open-strip of ferro-magnetic
material. The adhesive may be one such as that described in U.S.
Pat. RE No. 24,906 and the open-strip may be a strip of permalloy
0.6 centimeter wide, 25 microns thick and 18 centimeters long.
Applications of such a seal would include sealing railway car doors
or containers. The seal would be applied such that opening the door
would rupture the seal. Because of the high adhesive strength and
the low open-strip tensile strength, it would be impossible to peel
off and subsequently reapply a marker. The original length of the
seal and the positioning of the seal on the door and door frame
must be such that a piece of a ruptured marker would be
sufficiently short that it would no longer produce its original
characteristic magnetization reversal signal. Conveniently, the
means for producing an applied field and sensing means may be used
in conjunction with automated optical scanning equipment presently
used by the railroads to record car identities.
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.
* * * * *