U.S. patent number 3,697,996 [Application Number 04/885,874] was granted by the patent office on 1972-10-10 for electromagnetic field producing apparatus and method for sequentially producing a plurality of fields.
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 |
3,697,996 |
Elder , et al. |
October 10, 1972 |
ELECTROMAGNETIC FIELD PRODUCING APPARATUS AND METHOD FOR
SEQUENTIALLY PRODUCING A PLURALITY OF FIELDS
Abstract
An apparatus and method for sequentially producing in a zone a
plurality of electromagnetic fields. The lines of each field are
produced to be generally curved and to have a direction at
substantially every point in the zone substantially different from
the direction of at least one other electromagnetic field line at
that point. The apparatus comprises a plurality of field producing
means each of which is responsive to applied electrical energy to
produce an electromagnetic field. Means are provided for
sequentially applying energy to the field producing means.
Inventors: |
Elder; James T. (Shoreview
Village, MN), Wright; Donald A. (Woodbury Village, MN) |
Assignee: |
Minnesota Mining and Manufacturing
Company (St. Paul, MN)
|
Family
ID: |
27126228 |
Appl.
No.: |
04/885,874 |
Filed: |
December 17, 1969 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
840973 |
Jul 11, 1969 |
3665449 |
|
|
|
Current U.S.
Class: |
324/260;
324/263 |
Current CPC
Class: |
G08B
13/2442 (20130101); G08B 13/2474 (20130101); G01N
27/72 (20130101); G08B 13/2437 (20130101); G08B
13/2477 (20130101) |
Current International
Class: |
G08B
13/24 (20060101); G01N 27/72 (20060101); G01s
001/02 () |
Field of
Search: |
;324/34,41 ;340/38L,258
;343/101 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Quarforth; Carl D.
Assistant Examiner: Potenza; J. M.
Parent Case Text
CROSS REFERENCES
This application is a continuation-in-part of our pending
application, Method And Apparatus For Detecting At A Distance The
Status And Identity Of Objects, U.S. Ser. No. 840,973, filed July
11, 1969, now U.S. Pat. No. 3,665,449.
Claims
What is claimed is:
1. An apparatus for sequentially producing a plurality of
electromagnetic fields within an interrogation zone comprising:
a plurality of field producing means, each being positioned for
producing in response to electrical energy supplied thereto an
electromagnetic field the lines of which in said interrogation zone
are generally curved and have a direction at substantially every
point in the zone substantially different from the direction of at
least one electromagnetic field line produced by another of said
field producing means at that point; and
means for sequentially applying electrical energy to each of said
plurality of field producing means.
2. An apparatus according to claim 1 wherein said plurality of
field producing means comprises:
a first means for producing a first electromagnetic field;
a second means for producing a second electromagnetic field each
line of which has at substantially every point in the zone a
direction substantially perpendicular to the direction of the line
of the first field at that point.
3. An apparatus according to claim 2 wherein
the first field producing means comprises a first long, straight
conductor, and wherein
the second field producing means comprises at least one other long,
straight conductor parallel to and co-extensive with the first
conductor.
4. An apparatus according to claim 3 wherein
the first field producing means comprises at least two long,
straight parallel conductors.
5. An apparatus according to claim 4 wherein
the second field producing means comprises at least two long,
straight parallel conductors and the conductors of at least one of
the first and second field producing means comprise parallel
segments of a generally rectangular planar loop.
6. An apparatus according to claim 5 wherein said loop comprises a
generally rectangular figure-8 coil and said parallel segments
comprise the end and center segments of the figure-8 coil.
7. An apparatus according to claim 6 wherein the conductors of the
other of said first and second field producing means also comprise
parallel segments of a generally rectangular planar loop, the loops
of the first and second field producing means being closely spaced
and lying in substantially parallel planes.
8. An apparatus according to claim 2 wherein
the first field producing means comprises a first long, straight
conductor and wherein
the second field producing means comprises a second long, straight
conductor orthogonal to and crossing the first conductor, the first
and second conductors crossing far from their respective ends.
9. An apparatus according to claim 8 wherein
at least one of the first and second field producing means further
comprises at least one additional long, straight conductor
positioned parallel to the conductor comprising the same field
producing means and lying in substantially the same plane as said
first and second conductors.
10. An apparatus according to claim 9 wherein alternate ones of the
conductors of said at least one field producing means are connected
to said energizing means as a first group of conductors and wherein
the other conductors of said at least one field producing means are
connected to said energizing means as a second group of conductors,
and wherein said energizing means energizes each of said groups in
sequence.
11. An apparatus according to claim 8 wherein
at least one of the first and the second field producing means
comprises a center segment of a figure-8 coil.
12. An apparatus according to claim 2 wherein
the first field producing means comprises a first substantially
linear electromagnet; and wherein
the second field producing means comprises a second substantially
linear electromagnet disposed close to and substantially orthogonal
to the first electromagnet, the first and second electromagnets
crossing near their mid-points.
13. An apparatus according to claim 2 wherein
the first field producing means comprises a substantially linear
electromagnet and wherein
the second field producing means comprises a substantially planar
air core loop having a diameter approximately equal to the
separation of the magnetic poles of said first substantially linear
electromagnet and wherein the electromagnet and air core loop are
close-spaced with the periphery of the air core loop passing
proximate the magnetic poles of the electromagnet.
14. An apparatus according to claim 2 further comprising:
third means for producing a third electromagnetic field the lines
of which have a direction substantially mutually perpendicular to
the directions of the first and second field lines at substantially
every point in the interrogation zone.
15. An apparatus according to claim 14 wherein said first and third
field producing means each comprise a generally rectangular
figure-8 coil, each coil comprising a generally rectangular
figure-8 winding which forms two generally rectangular portions of
nearly equal size, each portion having substantially parallel top
and bottom lengths and a top and a bottom length of the two
portions being substantially coincident to form a center segment of
the figure-8 coil and the other of said top and bottom lengths of
said portions forming end segments of the figure-8 coil, the
figure-8 coils being oriented so that their center segments are
orthogonal and cross near their mid-points; wherein said second
field producing means comprises parallel segments of a generally
rectangular o coil; wherein said figure-8 and o coils are closely
spaced and lie in substantially parallel planes and wherein each of
the parallel segments of the o coil are parallel to and lie between
the center segment and one end segment of one figure-8 coil.
16. An apparatus according to claim 15 wherein the means for
sequentially applying energy comprises
a source of electrical energy;
an energy transfer section coupled between each of said field
producing means and the source of electrical energy, each energy
transfer section including a switch responsive to a control signal
to enable the transfer section to pass a pulse of energy from the
energy source to the field producing means; and
a control signal generator for providing a control signal to each
of the switches in a predetermined sequence.
17. An apparatus according to claim 16 wherein each field producing
means is a coil having appreciable electrical inductance;
wherein said source of energy is a DC voltage;
and wherein each energy transfer section includes a capacitor and
wherein the energy transfer section and field producing means form
an inductive-capacitive circuit, the capacitor of the transfer
section acting to store said pulse of energy upon actuation of said
switch and to pass the energy to the field producing means as a
damped pulse of energy.
18. An apparatus according to claim 17 wherein said
inductive-capacitive circuit is an overdamped electrical circuit to
produce an overdamped electromagnetic field pulse.
19. An apparatus according to claim 17 wherein said
inductive-capacitive circuit is an underdamped circuit to produce
an underdamped electromagnetic field pulse.
20. A method of sequentially producing a plurality of
electromagnetic fields within an interrogation zone comprising the
steps of
providing a plurality of field producing means, each of said means
being positioned for producing in response to electrical energy
supplied thereto an electromagnetic field the lines of which in an
interrogation zone are generally curved and have a direction at
substantially every point in the zone substantially different from
the direction of at least one electromagnetic field line produced
by another of said field producing means at that point; and
sequentially applying electrical energy to each of said plurality
of field producing means;
thereby producing as a pulse sequence in the interrogation zone a
plurality of pulses of electromagnetic field, the lines of each
field being generally curved and having respective directions at
substantially every point in the zone substantially different from
the direction of at least one other electromagnetic field line
produced by another of said field producing means at that
point.
21. A method according to claim 20 wherein said step of producing
comprises producing the pulse sequence during a sequence period
comprising: producing at least one continuous period of at least
1.0 seconds during which no field is produced; and, producing said
pulse sequence during an interval of not longer than 0.4 seconds.
Description
BACKGROUND
This invention relates in general to methods and apparatus for
producing electromagnetic fields. More particularly, the invention
relates to methods and apparatus for producing electromagnetic
fields within a zone, such as an interrogation zone, into or
through which a responder may be introduced or passed. When
sufficient energy from the field is received by the responder
element, the responder produces or alters its characteristic
response. Typical responses include generation of a new signal and
modulation and disturbance of the quiescence of the electromagnetic
field.
Examples of systems which employ such apparatus and method are
anti-pilferage systems and sortation systems. In a typical
anti-pilferage system, each article to be protected against
pilferage is provided with a responder and an electromagnetic field
is produced in a zone through which the protected article would
necessarily pass when being pilfered. An example of such a system
is described in the above-identified pending application. In
accordance with the system disclosed therein, when a responder such
as the open strip, also discussed in the copending application, is
passed into an 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 which is monitored in the vicinity of the interrogation zone,
the presence of particular frequency components in the pulse being
indicative of the presence of the marker. One possible sortation
system application is sortation of passenger luggage at airport
terminals. In such an application, each piece of luggage would be
provided with a responder. Passage of the luggage and its responder
through a field could conveniently be accomplished in many
instances by producing the field proximate conveyor belts such as
those now in use in many airport terminals.
Another possible application of the method and apparatus of the
present invention would be a metal sensing system such as an eddy
current detector system. In such systems, a metallic object such as
a firearm acts as a responder. Upon introduction of such a
responder into the electromagnetic field of the system, the
quiescence of the field is disturbed.
A difficult problem of the foregoing type responder systems is
insuring that each responder which enters an interrogation zone
receives sufficient energy. Although energy is a function of both
time and intensity, to simplify discussion, when the term energy is
used hereafter, it is only the intensity of the energy that is
being referred to. The energy received by a responder at a point in
a zone depends upon both the field intensity at that point and the
orientation of the responder relative to the direction of the field
at that point. The orientation of a responder is conveniently
defined in terms of its geometry, usually the plane, axis, or
length of the responder. Known responders include a planar
receiving coil or sheet; an axial electric dipole; or a piece,
usually an elongated strip, of isotropic magnetizable material
having at least one long dimension. The orientation of such
geometric characteristics of the responder relative to the
direction of the field usually determines the proportion of the
energy of the field at that point that is received by the
responder. When the expression field direction or intensity is used
herein, it is meant the direction or intensity of the field at a
point; and, usually, this point is the point at which a responder
is positioned. For a responder having just one such geometric
characteristic, there is one orientation for which the responder
receives the most energy. For each other orientation, a lesser
amount of energy is received. The orientation for which the most
energy is received shall hereafter be referred to as the
"orientation of maximum sensitivity." Usually, the orientation of
maximum sensitivity of a receiving coil or sheet is that
orientation in which the field direction is perpendicular to the
plane of the coil or sheet; for an axial electric dipole it is that
orientation in which the field direction is parallel to the dipole
axis; and, for a magnetizable piece it is that orientation of the
piece for which the field direction is parallel to the longest
straight dimension of the piece. Hereafter, when reference is made
to the orientation or direction of a responder, it is meant the
direction of the geometric characteristic of the responder used to
define the responder's orientation of maximum sensitivity.
It will be appreciated that in some systems the direction of a
responder in a zone may be random and may change as it is moved
through the zone. This is true of anti-pilferage applications. In
most such applications a responder can conceivably assume every
possible direction and its direction can continuously change. In
other applications such as those in which a responder is on an
article carried by a conveyor belt, a responder is likely to, or
may actually be constrained to assume fewer than every possible
direction, and is not as likely to change its direction, as it
passes through the zone. These considerations and the reliability
desired of the system are important factors for determining the
field requirements; that is, for determining whether a field
component must be provided along every direction at every point in
the zone, along every direction at some points in the zone, or
along only certain directions at one or more points in the zone. By
field component, it is of course meant a vector component the
intensity of which provides "sufficient" energy to the
responder.
The field requirements also depend on the responder geometry. They
are of course less stringent for a "multi-dimension" than for a
"single-dimension" responder. A multi-dimension responder is one
which has two or more geometric characteristics each of which has a
different direction. Examples of such multi-dimensional responders
are L- or T-shaped magnetic strips such as those described in the
above identified pending application. The previously mentioned flat
coil, sheet, and electric dipole are single-dimension responders.
Reasons of cost and susceptibility to damage make single-dimension
responders generally preferable to multi-dimension responders. In
some applications, other considerations make single-dimension
responders highly desirable if not essential. Concealability, which
is a desirable feature of anti-pilferage applications, is one
example of such a consideration.
DISCUSSION OF PRIOR ART
Several apparatus for producing an electromagnetic field are
disclosed in French Pat. No. 763,681. One of those apparatus
comprises a single figure-8 coil. The field of such a coil at a
point has only one direction and has a total number of directions
equal to only a portion of all possible directions. Another of the
apparatus disclosed therein comprises a pair of coils energized
simultaneously but out of phase to produce what has come to be
referred to as a "rotating" field. Such a field provides at a
point, a field having a different direction at different times, but
these different directions still are only a portion of all possible
directions. The size of such a rotating field is also small
compared to the coil size. Of course by producing a field having a
large, as opposed to a "minimum," intensity the unreliability
resulting from there being no field in some directions is reduced.
Such a large intensity field would have more vector components of
sufficient intensity than would a minimum intensity field. (A
minimum intensity field is one which provides sufficient energy to
a responder when the responder has an orientation of maximum
sensitivity.) This effectively increases the directions provided by
the field. Increasing the field intensity, however, usually also
increases both the cost of the field producing means and the
probability that the device will interfere with the use of other
apparatus.
SUMMARY OF INVENTION
We have discovered field producing apparatus and a method of
energizing the apparatus which permits limitation of the applied
field intensity to an amount not much greater than the "minimum"
intensity. In one embodiment, the method and apparatus of our
invention sequentially produce a plurality of individual fields,
which together provide a field component along nearly every
direction at virtually every point in a zone. In another
embodiment, our invention sequentially produces only two
electromagnetic fields. The field lines of these two fields are
nearly everywhere in the zone orthogonal to each other. For a field
intensity a factor of .sqroot.2 times greater than the minimum
intensity, a field component of at least a minimum intensity exists
along many directions at virtually every point in a zone. At any
point defined by the crossing of two nearly orthogonal lines of
fields of such an intensity, the fields would effectively provide a
component along every direction in the plane containing the two
fields. The apparatus of our invention also produces a relatively
large zone.
For a particular application, the apparatus of our invention is
designed to produce fields and the sequence period (the time
required to energize each field producing means of an apparatus
once) is selected such that, for an assumed responder velocity, the
lines of each field are substantially unchanged in direction for a
distance equal to the distance a responder would travel during one
sequence period. In effect then, this distance becomes a "point" in
the zone. We are able to make the sequence period very short and
thus are able to employ fields the lines of which are generally
curved.
Briefly, the apparatus for sequentially producing a plurality of
electromagnetic fields within an interrogation comprises a
plurality of electromagnetic field producing means. The fields
will, when a responder having characteristics capable of being
produced by the fields is present in said zone, produce the
characteristic response of the responder. Each field producing
means is positioned for producing in response to electrical energy
supplied thereto an electromagnetic field the lines of which within
a zone are generally curved. The direction of each field line at
substantially every point in the zone is substantially different
from the direction of at least one electromagnetic field line
produced by another of the field producing means at that point.
Means are provided for sequentially providing energy to each field
producing means. The difference in the direction of the fields
depends upon both the total directions required to be provided and
the number of fields to be produced. When only two fields are to be
produced and it is desired to provide every direction within a
plane, to permit use of minimum intensity fields, the directions
should differ by 90.degree.. If more fields are produced, it may be
possible to correspondingly reduce the difference between the
directions of any two fields.
Each field producing means may comprise at least one long, straight
conductor. If a means comprises more than one conductor, they are
parallel to each other. The conductors of different field producing
means are preferably either parallel or orthogonal to each other.
When the conductors are orthogonal to each other, they preferably
cross each other far from their respective ends.
We have found that a convenient way of obtaining parallel
conductors is to use parallel segments of a generally rectangular
loop comprising a coil which has been wound in either a generally o
or a generally "figure-8" shape. Hereafter, such coils shall simply
be referred to as o or "figure-8" coils. Parallel segments of the
coil are then employed as the long, straight parallel conductors.
One embodiment constructed with such rectangular coils comprises at
least one o coil and at least one "figure-8" coil. The respective
parallel segments of the coils are parallel, lie in essentially the
same plane, and are spaced apart in that plane. When the coils are
sequentially energized, they have been found to provide fields
having directions which nearly everywhere within a relatively large
zone are substantially orthogonal to each other.
For critical applications, those requiring a high degree of
assurance of producing or altering the characteristic response of a
responder, a preferred embodiment of the apparatus of our invention
comprises three sequentially energized electromagnetic field
producing means. Each produces within a zone an electromagnetic
field the lines of which within the zone are generally curved. At
virtually every point within the zone, the directions of the lines
of the three fields are substantially mutually perpendicular. Such
an apparatus shall hereafter be referred to as a
"three-dimensional" field producing apparatus; and, an apparatus
which produces two fields the lines of which are substantially
orthogonal to each other at substantially every point in the zone
shall be referred to as a "two-dimensional" field producing
apparatus.
In a preferred embodiment, a three-dimensional field producing
apparatus comprises a pair of "figure-8" coils and an o coil. The
parallel end and center segments of a first "figure-8" coil act as
long conductors to produce a first field. Only the field produced
by the center segment of the second "figure-8" coil is required as
the second field. Each of the coils is conveniently generally
planar and the coils lie in closely spaced substantially parallel
planes. The two "figure-8" coils are orthogonal to each other and
their center segments intersect each other near their mid-points.
The o coil parallel segments which provide the third field are
parallel to the end and center segments of the first "figure-8"
coil. Each of the o coil parallel segments lies between the center
and one end segment of the first "figure-8" coil.
Another embodiment of a three-dimensional field producing apparatus
comprises a pair of ferromagnetic linear electromagnets orthogonal
to each other and crossing each other at approximately their
mid-points in combination with an air core loop the periphery of
which passes proximate the magnetic poles of the linear
electromagnets.
We have sequentially energized the field producing means, i.e., we
have produced pulsed fields, by switching a sinusoidally varying
energy source from one means to another. The field produced by each
means during one continuous energization is hereinafter referred to
as a "pulse of field" or a "field pulse." Also for those apparatus
in which each field producing means includes a coil, we have
produced each field pulse by discharging an underdamped inductive
capacitive (LC) circuit. The coil of each field producing means is
the significant inductive component of the circuit. A capacitor is
provided in series with the coil to provide the capacitive
component. Means are provided for sequentially supplying electrical
energy to each LC circuit. The discharge of this energy by the LC
circuit produces an underdamped sine-wave field pulse. In each of
the foregoing cases, a "pulse sequence" consists of one field pulse
from each field producing means and each field pulse comprises a
sinusoidal-like field the direction of which reverses or alternates
several times. It is to be appreciated, however, that by forming an
overdamped LC circuit in a similar manner, the direction of a pulse
would not alternate but would be a single uni-directional pulsation
for each discharge of the overdamped circuit.
It should be noted that a sequence period need not be equal to the
sum of each of the pulse field durations. Indeed, we have found in
some cases that it is desirable to include in each sequence period
a continuous interval at least one second long during which no
field is produced. By providing such a one second interval we find
that the operation of a heart beat timing control device, commonly
referred to as a heart pacemaker, is virtually unaffected by the
fields of our apparatus.
BRIEF DESCRIPTION OF THE DRAWING
The principle of the invention and the relative size and
arrangement of the different field producing means of various
exemplary embodiments of the invention will be better understood
from the following description taken in connection with the
accompanying drawings wherein:
FIG. 1 is an end view of two long, straight parallel conductors and
flux lines produced in response to a current flowing in the
conductors;
FIG. 2 is an end view of four long, straight parallel conductors
and the flux lines produced in response to a current flowing in the
conductors;
FIG. 3, View A, is a schematic plan view of a pair of generally
rectangular o shaped coils; View B is a perspective of View A
illustrating a zone within which the field lines of the coils are
nearly orthogonal;
FIG. 4, View A, is a composite plan view of a grid formed by
superimposition of three coils consisting of a pair of generally
rectangular figure-8 coils and one generally rectangular o coil;
Views B, C and D are individual schematic plan views of the coils
of View A;
FIG. 5, View A, is a side schematic plan view of two of the coils
of FIG. 4 and a cross-section of a zone within which vector
representations of the fields produced by the "horizontal" segments
of the coils of FIG. 4 are shown; View B is a top schematic plan
view of the coil of FIG. 4 not shown in View A, and a cross-section
of a zone within which vector representations of the field produced
by the "vertical" center segment of a "figure-8" coil of FIG. 4 are
shown;
FIG. 6 is a front schematic plan view of a "three-dimensional"
field producing apparatus comprising a pair of ferro-magnetic
linear electromagnets and an air core loop;
FIG. 7 is a circuit schematic illustration of the windings of an
electromagnet of FIG. 6;
FIGS. 8 and 9 show representations of magnetic fields produced by
the applied field producing apparatus of FIG. 6;
FIG. 10, View A, illustrates two sets of magnetic field lines of
the applied field producing apparatus of FIG. 6; Views B and C are
vector representations of the direction of each of the three fields
produced by the apparatus of FIG. 6 at an intersection of each of
the two sets of lines of View A;
FIG. 11 is a schematic of a circuit for sequentially energizing a
plurality of electromagnetic field producing means;
FIG. 12 is a schematic of another circuit for sequentially
energizing a plurality of electromagnetic field producing
means.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is an end view of two long, straight, parallel conductors 12
and 14 showing the flux lines of the magnetic fields produced by a
current flowing in each conductor at different intervals of time.
Circular line 13 is one member of a set of concentric circular flux
lines produced by a current flow in conductor 12; line 15 is a
similar flux line of the set produced by a current flow in
conductor 14. Other lines defining the electric field, which also
characterize an electromagnetic field produced when a current flows
in a conductor, are related to magnetic flux lines such as the set
including lines 13 and 15 in a well known manner. Thus, for
simplicity of illustration only the magnetic flux lines have been
shown in FIG. 1. For similar reasons, only the magnetic properties
of the electromagnetic fields will hereafter be discussed and
illustrated.
As shown, the lines of the fields of each of conductors 12 and 14
are curved yet substantially orthogonal to each other at their
points of crossing throughout the area 10 enclosed within dashed
line 17. Area 10 represents a cross-section of an interrogation
zone and thus is illustrated as rectangular because this is often
the shape of the cross-section of a passageway in which an
interrogation zone of an electromagnetic field is to be produced.
Doorways, hallways or other similar passageways are typical of such
areas.
Assume that dimension 19 of area 10 is the width of a doorway and
that conductors 12 and 14 are vertically oriented. Dimensions 19
and 21 thus correspond to the width and length respectively of the
interrogation zone. The considerations for determining the actual
length of a zone were previously discussed. The intensity of a
field is known to vary directly as the amplitude of the current
flowing in the conductor and inversely with respect to distance
from the conductor. For a specified application having a width 19,
having the conductors 12 and 14 spaced apart distance 23 and spaced
from the edge of the passageway distance 25, and for a responder
requiring a particular "sufficient energy," it can be determined by
vector analysis of each of the fields at their points of crossing
at the farthest point in the area (near dimension 21) what
amplitude of current flow in conductors 12 and 14 is required.
FIG. 2 is an end view of four long, straight parallel conductors
16, 18, 20 and 22 showing two sets of lines of magnetic fields
which could be produced by current flowing in the conductors. One
set of flux lines is shown in solid lines, the other in short
dashed lines. The solid lines are those which would be produced if
conductors 16 and 18 were simultaneously carrying like amounts of
current in opposite directions with no current flowing in
conductors 20 and 22; the short dashed lines are the field for a
converse current flow, i.e., no flow in conductors 16 and 18 and a
flow of like amounts but in opposite directions in conductors 20
and 22. It should be noted that for equal dimensions 23 of FIGS. 1
and 2 an area 10 of much longer length 21 can be produced in the
same amount of time (in both cases, only two "field pulses" need be
produced). The analysis for determining the current requirements
for the conductors of FIG. 2 is generally the same as that
described with reference to FIG. 1. It will also be appreciated
that by providing another "set" of such an arrangement of
conductors and orienting these two sets in closely spaced parallel
planes with the conductors of the respective sets orthogonal to
each other, a three-dimensional apparatus will be provided. With
such an apparatus there will be at substantially every point within
the area 10 at least once during each pulse sequence of four pulse
fields each of three nearly mutually perpendicular magnetic fields.
The "end" conductor of such another set is indicated in long-dashed
lines in FIG. 2 and identified as 27.
In FIG. 3 there is shown a two-dimensional field producing
apparatus. Referring now to FIG. 3, View A, the apparatus is shown
to comprise a pair of o coils 24 and 26. For purposes of analysis,
the vertical segments of the coils can be considered as long,
straight parallel conductors. The coils 24 and 26 lie in
substantially the same plane but are displaced such that adjacent
vertical segments are of different coils and such that the
horizontal separation between the inside vertical segments,
segments 28 and 30, is much less than the horizontal separation of
vertical segments of the same coil. In this way, the lines of the
fields between the inside vertical segments are approximately those
shown in FIG. 1. View B illustrates a zone of such unequal spaced
coils having a length 21, width 19 and height 34. If the coils were
equal spaced, i.e., if the separation between the inside segments
was equal to one-half the separation of segments of the same coil,
the fields produced would have lines like those of FIG. 2. We have
found that a combination of such equal-spaced coils having 31/2
foot long vertical segments, 4 foot long horizontal segments and
comprising ten turns of stranded No. 10 A.W.G. wire (such as Belden
No. 30610) will produce a substantially two-dimensional field in an
interrogation zone the volume of which has dimensions of about
three feet high by 3 feet wide by 5 feet long when pulsed by an
underdamped sinusoidal current of an initial peak amplitude of 180
amperes.
Referring now to FIG. 4 there is shown a "three-dimensional" field
producing apparatus. In View A, a schematic plan view, the
apparatus appears as a grid 39. This grid is a composite of three
coils. In a preferred embodiment, these coils lie in closely
spaced, parallel planes. Each of the coils is individually shown in
one of Views B, C and D. The grid is indicated as having three
vertical components 40, 42 and 44 and five horizontal components
46, 48, 50, 52 and 54. As will become apparent following a
discussion of the individual components, each of these components
comprises at least one segment of a coil; most comprise coincident
segments of several coils. It should be noted that the fields
produced by the coil segments forming components 40 and 44 are not
required. They will be produced, however, and do, if the components
40 and 44 are sufficiently close to component 42, reduce the
"length" of the zone. Also, in those cases where a component
comprises a segment of more than one coil, only the field of one
segment is required. Therefore, in the following discussion of the
individual coils, only those fields which are required shall be
discussed. Referring now to View B, there is shown an up-right
generally rectangular figure-8 coil 55. The coil comprises a
conductor wound in a figure-8 fashion to form two generally
rectangular series connected portions of nearly equal size. The
portions are indicated generally as 58 and 60; each has
substantially parallel top and bottom lengths. Portion 58 is shown
to have a top length 72 and a bottom length 74 and portion 60 is
shown to have a top length 76 and a bottom length 78. The bottom
length 74 of loop 58 and the top length 76 of loop 60 are
coincident to form the center segment of the figure-8 coil 55. The
other top and bottom lengths of portions 58 and 60, top length 72
and bottom length 78, form the end segments of the figure-8 coil
55. Terminals 80 and 82 permit coupling of a pulse of energy to the
figure-8 coil. The arrows within the two loops, one of which is
indicated as 84, indicate the direction of current flow through the
various segments of the loop for a current of one polarity. It is
apparent from View B that length 72 corresponds to horizontal
component 46, that lengths 74 and 76 correspond to horizontal
component 50 and that length 78 corresponds to horizontal component
54. Referring now to View C there is shown an o coil 57 the
horizontal segments of which are indicated as 86 and 88 and which
respectively correspond to horizontal components 48 and 52 of grid
39 (View A). View D illustrates a figure-8 coil turned on its side.
The lengths of the coil identified as 94 and 96 correspond to
vertical component 42 of grid 38 (View A) and lengths 90 and 92
correspond to components 40 and 44 respectively.
Vector representations of the magnetic force lines of the fields
produced by the three field producing means (coils 55, 57 and 59)
of FIG. 4 are illustrated in FIG. 5. View A is a view of field
vectors in a plane both parallel to the coil 59 "vertical" lengths
(e.g., lengths 94 and 96) and normal to the plane of the coils 55
and 57 showing an end view of their respective horizontal lengths
72, 74, 76, 78, 86, and 88. Coil 59 is not shown. An area of a
cross-section is drawn in short dashed lines and indicated as 39.
Field vectors are illustrated at three arbitrarily selected points
110, 112 and 114. The vectors identified as the a vectors
correspond to the field produced by the lengths 72, 74, 76 and 78
of coil 55; the vectors identified by the character b correspond to
the field produced by the conductors 86 and 88 of coil 57. The
field vectors of the field produced by the third coil, coil 59, at
points 110, 112 and 114 are illustrated in View B wherein the field
is viewed in a plane both parallel to the horizontal lengths of the
coils and normal to the plane of the coils. Coils 55 and 57 are not
shown. Area 10 is again shown by dashed lines and the vectors of
the field of coil 59 are identified by the lower case character c.
In View B, an end view of the center lengths 94 and 96 and the end
lengths 90 and 92 of coil 59 is shown. The dimension 21 of area 10
extends only a part of the way from the pair of center segment
lengths 94 and 96 to the end conductors 90 and 92. This is because
end conductors 90 and 92 carry current in a direction opposite to
that carried by conductors 94 and 96. Conductors 94 and 96 carry
current in the same direction and can be treated as a single
conductor. It will be appreciated that in such a figure-8 coil, the
separation between the center segment conductors 94 and 96 and end
conductors 90 and 92 can be controlled to control dimension 21. The
field vector diagrams of View A of FIG. 5 illustrate that the coils
55 and 57 produce "two-dimensional" fields at points 110, 112 and
114 and those of View B illustrate that the field of coil 59 adds a
"third dimension" to the fields.
In an embodiment of the apparatus of FIG. 4 specifically intended
for use in conjunction with an anti-pilferage system for detecting
markers such as those described in the afore-identified pending
application, and wherein the interrogation zone was an area leading
to a doorway, an interrogation zone of at least 5 feet high by 2
feet wide by 21/2 feet long (dimensions 34, 19 and 21) was
produced. The grid 39 formed by coils 55, 57 and 59 had the
following dimensions: vertical components 40, 42 and 44 were 6 feet
long and horizontal components 46, 48, 50, 52 and 54 were 4 feet
long. The o coil was centered about the intersection of the center
segments of the figure-8 coils and had its lengths 86 and 88 spaced
21 inches respectively above and below lengths 74 and 76 of coil
55. Coils 55, 57 and 59 all were made of No. 10 vinyl-covered
stranded wire (Belden No. 30610). Coil 55 comprised 6 turns, coil
57 comprised 10 turns, and coil 59 comprised 7 turns.
It should be noted that the planar construction of FIG. 4 provides
a structure which is inexpensive to produce and lends itself to
packaging in a variety of aesthetic forms. Such a planar structure
may be easily installed along either side of a passageway either
leading to a doorway or an otherwise channeled traffic pattern.
Another advantage of the structure is that the coils can easily be
"balanced" resulting in little or no mutual inductance between an
energized coil and the other two unenergized coils. In this way,
virtually all of the energy produced by a coil is available for
production of a field within the interrogation zone.
In FIG. 6, there is shown another three-dimensional field producing
apparatus. An apparatus 100 is shown to comprise a pair of
ferro-magnetic electromagnets 102 and 104, orthogonal and crossing
each other at approximately their mid-points. The apparatus further
comprises an air core loop 106, the periphery of which passes
proximate the magnetic poles of electromagnets 102 and 104. In an
embodiment of the apparatus of FIG. 6 suitable for producing an
interrogation zone in a space such as a doorway, an apparatus 100
is placed on one or both sides of the doorway. For such an
embodiment the electromagnets 102 and 104 may each be formed of
solenoids surrounding a 5.1 centimeter square bar of iron. The bar
is conveniently a laminate 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 0.259 centimeter diameter enameled wire at each
end of the bar. Both additional 60-turn windings are uniformly
wound in a bifilar-aiding fashion with the end-most 60 turns of the
corresponding 125-turn winding. These windings are schematically
shown in FIG. 7 where the 60-turn windings are shown as 108 and
110, and the 125-turn windings are shown as 112 and 114. The
60-turn windings are shown wound in series with each other and in
series with the parallel combination of the 125-turn windings. This
combination of windings shall hereafter be referred to as the coil
"winding" and the terminal shown as 116 shall be referred to as the
coil "input" terminal and the terminal designated 118 shall be
referred to as the coil "output" terminal. For a construction such
as that described and illustrated in FIGS. 6 and 7, we have found
that the poles of the linear electromagnets are located
approximately 7.5 centimeters in from the ends of the steel bars.
The air core loop 106 has a diameter equal to the pole separation
of electromagnets 102 and 104. The loop was made of 80 turns of
enameled wire 0.205-centimeter in diameter. A circuit for
sequentially switching a source of energy of 60 hz. sinusoidal
waveform among each of the afore-described coils is disclosed in
the afore-identified pending application.
In another embodiment of an apparatus 100 for providing a a
plurality of fields alternating at a frequency of about 800-1,000
hz., electromagnets 102 and 104 each comprised a 142-centimeter
long, 5.1-centimeter thick and 5.1-centimeter wide lamination of
short pieces of transformer steel about 0.0457 centimeters thick,
5.1 centimeters wide and not more than 60 centimeters long. The
pieces were adhered to form the lamination with an approximately 50
micron thick layer of viscoelastic transfer adhesive such as 3M
Company No. 467. For this embodiment, each of electromagnets 102
and 104 were provided with one 18-turn winding about 2 feet long
approximately centered along the length of the laminate. These
windings and the winding of air core loop 106 each were of a vinyl
insulated number 10 A.W.G. stranded wire commercially available as
Belden No. 30610. The air core loop comprised ten turns. We found
that the vinyl insulation, short pieces, stranded wire, and
viscoelastic adhesive each reduced (compared to the corresponding
materials used in the previously described embodiment) the
accoustical noise generated by the apparatus.
FIGS. 8 and 9 respectively illustrate free space characteristic
magnetic fields of an electromagnet and an air core loop such as
those of FIG. 5. (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 axis, 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.
FIG. 10, View A, illustrates two sets of magnetic field lines from
each of means 102, 104 and 106 of an apparatus 100; the force
vectors of these lines at their points of intersection are shown in
Views B and C. For the sake of clarity, the lines representing the
field produced by means 102 are shown uniformly dashed; the lines
representing the field produced by means 104 are shown in
alternately long and short dashes; and the lines representing the
field produced by means 106 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. 10 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 104, 106 and 102; 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 substantially
mutually perpendicular even though the r, s, t component directions
are different from the r', s', t' directions. Inspection of FIG. 10
makes it readily apparent that the vector components of
intersections of other sets of lines will be nearly perpendicular,
too.
A circuit for sequentially impressing electrical energy on each of
a plurality of field producing means which each have a coil such as
the coils of either FIG. 4 or FIG. 6 is shown in FIG. 11. The
circuit shown is particularly useful for applications requiring a
field having a very high signal to noise ratio. An example of such
an application is an antipilferage system employing a responder (or
"marker") which produces a very low intensity signal. The circuit
is shown to comprise an energy source section, shown generally as
170, three energy transfer sections 172A, 172B and 172C and a
sequence-control signal generator 183. The circuit is not limited
to having only or exactly three transfer sections 172; three are
shown to facilitate description of the circuit relative to the
fields producing apparatus including three coils such as those of
either FIG. 4 or FIG. 6. Only one of transfer sections 172 is shown
in detail as they all may be identical. Obviously, if the
electrical properties of the coils are not identical, e.g., if they
had different inductive properties, and if it is desired to
transfer to each coil energy identical in duration and alternation
frequency, some component values of the sections would differ in a
well known manner. Conveniently, source section 170 may comprise a
source of A-C voltage 174 such as ordinary 117 volt line voltage
and a conventional voltage doubler rectifier 176. The voltage
doubler 176 provides a D-C output potential of approximately zero
and minus 200 volts, on leads 178 and 180 respectively, to each of
energy transfer sections 172A, 172B and 172C.
Energy transfer section 172 includes a switch, or, more
specifically, a transistor 182A which is actuated or rendered
conductive when a control signal from a sequence-control signal
generator 183 is impressed on node 184A. When transistor 182A
conducts, energy transfer section 172A is enabled to pass a pulse
of energy to an associated field producing means coil. In FIG. 11,
the coil associated with energy transfer section 172A is
schematically shown as an inductor 186. A capacitor 188 stores a
pulse of energy from the energy source and in response to
conduction of transistor 182 passes the pulse to the field
producing means as a damped pulse of energy. The circuit for
charging capacitor 188 and subsequently causing transfer of this
charge to field producing means or inductor 186 is believed to be
unique and comprises inductor 190, a silicon controlled rectifier
(SCR) 192, and a circuit, shown generally as 194, for triggering
SCR192. This circuit is the subject of a copending patent
application of Donald A. Wright, one of the co-inventors of this
application. Trigger circuit 194 comprises normally non-conducting
transistor 196 the base lead of which is coupled to one end of each
of resistors 198 and 200. The other end of resistor 198 is also
coupled in series with a blocking capacitor 204 and diode 206 to
the collector of transistor 182A. One plate of capacitor 204 and
the cathode of diode 206 are also common to one lead of a resistor
208 the other lead of which is connected to the zero volt reference
lead 178. Normally, with transistor 182 in a non-conducting state,
the potential across capacitor 204 is about 200 volts and thus
current to the base of transistor 196 is cut off, holding the
transistor in its normally non-conducting state. When transistor
182 conducts, capacitor 204 charges toward 212 volts and the
charging current through capacitor 204 turns on transistor 196
until the potential across capacitor 204 reaches 212 volts. The
remainder of trigger circuit 194 comprises the emitter and
collector resistors, 210 and 212 respectively, of transistor 196, a
capacitor charging network formed by resistor 214 and zener diode
216 and a capacitor 218. The gate lead of SCR192, is common to the
emitter of transistor 196 and its emitter resistor 210 so that,
with both the other end of resistor 210 and the cathode lead of
SCR192 held at approximately minus 200 volts, the SCR will be
rendered conductive or triggered only whenever transistor 196
conducts. Transistor 196 conducts when a pulse applied to node 184
forward biases transistor 182. Current flows from the emitter to
collector of transistor 182, through diode 206, capacitor 204 and
resistor 198 to turn on transistor 196. With transistor 196 on,
capacitor 218 discharges through resistor 212 and transistor 196 to
provide a pulse of current to the gate of SCR192 thereby triggering
the SCR into conduction. Upon conduction of SCR192, current flows
through capacitor 188, inductor 190 and SCR192 to begin storage of
energy in capacitor 188. To insure a sufficiently rapid storage, it
has been found necessary to select the value of inductor 190 to be
not more than about one-fifth that of inductor 186. In this way
most of the current from capacitor 188 flows through inductor 190
rather than through inductor 186. When capacitor 188 is charged to
approximately -200 volts the current in inductor 190 is at a
maximum. This current continues to flow until the energy in
inductor 190 is dissipated, and as a result capacitor 188 charges
to a voltage of about -300 volts. This final voltage depends
somewhat on the ratio of the inductances of inductors 186 and 190,
and also on resistive losses in inductor 190 and other components
through which the current passes. When the voltage across capacitor
188 has reached its peak and current has stopped flowing in
inductor 190, SCR192 stops conducting. Capacitor 188 now discharges
into field-producing inductor 186, generating a characteristic
damped sinusoid waveform of current within the coil. A resistor 191
of very small resistance is provided between one end of inductor
186 and the zero volt reference lead 178. The voltage developed
across resistor 191 is exactly proportional to the current in
inductor 186 and may thus conveniently be employed as a
synchronizing signal for a detector system. In such a system, a
detector indicates the presence of a responder upon detection of a
characteristic signal at a particular time specified relative to
the time base of the electromagnetic field.
For an application not requiring as great a signal to noise ratio
as the afore-described anti-pilferage system, the circuit of FIG.
11 could be greatly simplified. Each field producing means could be
provided with its own capacitor and a single supply voltage coupled
to each of the capacitors. FIG. 12 is a circuit diagram
illustrative of the basic components of such a system. Referring
now to FIG. 12, DC supply voltage 230 is adapted to be coupled to
each of capacitors 232, 234 and 236 through, respectively, switches
238, 240 and 242. These switches are each controlled by a pulse
sequence-control device not shown but which would perform the same
function as the like device previously discussed. Each switch is
adapted to switch an associated capacitor into circuit with either
the D-C voltage or its associated coil. The coils associated with
switches 238, 240 and 242 are shown respectively as inductors 244,
246 and 248.
The function of sequence-control signal generator 183 is to provide
a control signal to each of nodes 184A, 184B and 184C in sequence
to produce a field pulse from each of the coils associated with
energy sections 172A, 172B and 172C such as the coil schematically
illustrated as inductor 186. Sequence-control signal generator 183
could conveniently comprise a plurality of monostable
multivibrators coupled in a ring such that switching of each
multivibrator from its unstable to its stable state effects a
converse switching of states of the succeeding multivibrator in the
ring. The individual multivibrator outputs would be provided to
nodes 184. Another way of providing control signals to nodes 184
would be by connection of individual stages of a simple flip-flop
counter driven by a free-running multivibrator.
Components for an embodiment of the circuit shown in schematic in
FIG. 11 and which has been used at different times to sequentially
energize both the apparatus of FIG. 4 and the "18-turn" apparatus
of FIG. 6 are given below in Table I.
capacitor 25 microfarad, 230 diode D1 silicon rectifier, C1 VAC 1
amp, 600 volt diode silicon rectifier, resistor 15 ohm 2 watt D2 1
amp, 600 volt R1 Resistor 1000 ohm 100 watt capacitor 4500
microfarad, R2 C2 250 volt, electro- lytic resistors 1000 ohm 1/4
watt transis- PNP Transistor R4A, R4B R4C ors 182A, 2N5139 182B,
182C diode 206 silicon diode 1N914 capacitor .1 microfarad, 204 400
volts resistor 470 ohms 1/4 watt resistor 220 ohms 1/4 watt R198
R200 resistor 1000 ohms 1/4 watt resistor 1 megohm 1/4 watt R208
R214 resistor 47 ohms 1/2 watt transis- NPN Transistor R212 tor 196
2N3414 resistor 100 ohms 1/4 watt zener Zener diode, R210 diode 216
400 mw, 18 volts capacitor .22 microfarad, capacitor 80 microfarad,
C218 100 volts C188 230 VAC resistor 47 ohms 1 watt capacitor .22
microfarad, R3 wirewound C3 600 volt resistor .02 ohm 10 watt
inductor Field Producing R191 wirewound 186 Inductor; 300
microhenry inductor 60 microhenry; silicon SCR 2N3898 190 formed of
8 turns of control- A.W.G. No. 10 stranded wire led recti -fier
192
While the invention has been described in certain preferred forms
and particular areas of the invention have been illustrated other
modifications and uses of the invention will be readily suggested
to others with the foregoing discussion before them.
* * * * *