U.S. patent number 3,825,920 [Application Number 05/307,096] was granted by the patent office on 1974-07-23 for laminated security window system.
This patent grant is currently assigned to The Sierracin Corporation. Invention is credited to Clyde L. Lucky, Roger E. Nelson.
United States Patent |
3,825,920 |
Nelson , et al. |
July 23, 1974 |
LAMINATED SECURITY WINDOW SYSTEM
Abstract
A security window system having a transparent structure having
high resistance to penetration is described. A transparent
conductive layer is provided over most of the area of the window
and the resistance of the layer is monitored for sensing
penetration. Preferably the layer is subdivided into a number of
conductive regions for substantially increasing the sensitivity of
the system to minor interruptions in the layer. Temperature and
stress effects can be minimized by connecting different conductive
areas of the layer as arms of a resistance bridge. An alarm may be
sounded when a small steady state change in resistance is sensed or
when a rapid change in resistance is sensed.
Inventors: |
Nelson; Roger E. (Northridge,
CA), Lucky; Clyde L. (Santa Susana, CA) |
Assignee: |
The Sierracin Corporation
(Sylmar, CA)
|
Family
ID: |
23188223 |
Appl.
No.: |
05/307,096 |
Filed: |
November 16, 1972 |
Current U.S.
Class: |
340/550; 52/306;
109/42; 346/17; 52/786.11; 109/21; 324/98 |
Current CPC
Class: |
B32B
17/10091 (20130101); B32B 17/10036 (20130101); B32B
17/10761 (20130101); B32B 17/10174 (20130101); G08B
13/04 (20130101); B32B 17/10005 (20210101); B32B
2369/00 (20130101); B32B 17/10005 (20210101); B32B
2367/00 (20130101) |
Current International
Class: |
G08B
13/04 (20060101); G08B 13/02 (20060101); G08b
013/04 () |
Field of
Search: |
;340/274,285
;109/21,10,49.5 ;52/171,616 ;161/192,404 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Caldwell; John W.
Assistant Examiner: Swann, III; Glen R.
Attorney, Agent or Firm: Christie, Parker & Hale
Claims
What is claimed is:
1. A security window system comprising: a transparent electrically
conductive layer extending over most of the area of the window;
a set of bus bars in electrical contact with opposite edge portions
of the conductive layer;
means connected to the bus bars for detecting an analog change in
resistance of the conductive layer; and
means for providing an output signal in response to a detected
change in resistance in excess of a predetermined magnitude.
2. A security window system as defined in claim 1 further
comprising means for resetting the system after a change in
resistance for providing an output signal in response to a
subsequent analog change in resistance.
3. A security window system comprising:
a transparent electrically conductive layer extending over most of
the area of the window;
a set of bus bars in electrical contact with opposite edge portions
of the conductive layer;
means connected to the bus bars for detecting an analog change in
resistance of the conductive layer;
means for providing an output signal in response to a detected
change in resistance in excess of a predetermined magnitude;
means for resetting the system after a change in resistance for
providing an output signal in response to a subsequent analog
change in resistance; and
means for recording cumulative changes in resistance.
4. A security window system comprising:
a transparent electrically conductive layer extending over most of
the area of the window and divided into at least two conductive
areas;
a set of bus bars in electrical contact with opposite edge portions
of the conductive layer;
means connected to the bus bars for detecting an analog change in
resistance of the conductive layer comprising means for detecting a
change in the relative resistances of the two areas; and
means for providing an output signal in response to a detected
change in resistance in excess of a predetermined magnitude.
5. A security window system as defined in claim 4 wherein the means
for detecting comprises a bridge and the two conductive areas are
in adjacent arms of the bridge.
6. A security window system as defined in claim 1 wherein the means
for detecting comprises a bridge and wherein the conductive layer
is in one arm of the bridge.
7. A security window system comprising:
a transparent electrically conductive layer extending over most of
the area of the window;
a set of bus bars in electrical contact with opposite edge portions
of the conductive layer;
means connected to the bus bars for detecting an analog change in
resistance of the conductive layer comprising a resistance bridge;
and
means for providing an output signal in response to a detected
change in resistance in excess of a predetermined magnitude; and
wherein
the conductive layer is divided into at least four conductive areas
each in an arm of the bridge.
8. A security window system as defined in claim 1 comprising:
a frangible layer; and
a resilient layer isolating the electrically conductive layer from
the frangible layer.
9. A security window system as defined in claim 1 wherein the
electrically conductive layer is laminated between a pair of rigid
face plies and including a resilient layer between the conductive
layer and each of the rigid plies.
10. A security window system comprising:
a transparent sheet;
an electrically conductive layer extending over most of the area of
the transparent sheet for making an analog variation in an
electrical signal applied thereto in response to penetration of the
transparent sheet;
means for applying an electrical signal to the conductive layer;
and
means connected to the conductive layer for detecting a variation
in electrical signal greater than a predetermined limit.
11. A security window system as defined in claim 10 further
comprising means for resetting the system after a change in
resistance for detecting a subsequent change in resistance greater
than a predetermined limit.
12. A security window system comprising:
a transparent sheet;
an electrically conductive layer laminated within the transparent
sheet and extending over most of the area of the transparent sheet
for making an analog variation in an electrical signal applied
thereto in response to penetration of the transparent sheet;
means for applying an electrical signal to the conductive layer;
and
means connected to the conductive layer for detecting a variation
in electrical signal greater than a predetermined limit; and
wherein
the conductive layer is divided into at least two conductive areas
and the means for detecting is connected thereto for detecting a
change in the relative resistances of the two areas.
13. A security window system comprising:
a transparent sheet;
an electrically conductive layer laminated within the transparent
sheet and extending over most of the area of the transparent sheet
for making an analog variation in an electrical signal applied
thereto in response to penetration of the transparent sheet;
means for applying an electrical signal to the conductive layer;
and
means connected to the conductive layer for detecting a variation
in electrical signal greater than a predetermined limit; and
wherein
the conductive layer is divided into at least two conductive areas
in adjacent areas in adjacent arms of a resistance bridge.
14. A security window system comprising:
a transparent sheet;
an electrically conductive layer laminated within the transparent
sheet and extending over most of the area of the transparent sheet
for making an analog variation in an electrical signal applied
thereto in response to penetration of the transparent sheet;
means for applying an electrical signal to the conductive layer;
and
means connected to the conductive layer for detecting a variation
in electrical signal greater than a predetermined limit; and
wherein
the conductive layer is divided into at least four conductive areas
each in an arm of a resistance bridge.
15. A laminated security window system comprising:
an electrically conductive layer within the laminated window and
extending over most of the area of the window;
means for making electrical contact with edge portions of the
conductive layer for applying an electrical signal thereto;
means for detecting a change in resistance of the conductive layer
from a first value to a second value; and
means for providing an output signal when the change in resistance
from the first value to the second value is in excess of a
predetermined magnitude.
16. A security window system as defined in claim 15 further
comprising means for resetting the system after a change in
resistance for providing an output signal in response to a
subsequent change of resistance from the second value to a third
value.
Description
BACKGROUND
This application is related to copending U.S. Patent Applications
Ser. No. 307,095, entitled "Laminated Security Window"; by Berton
P. Levin et al.; Ser. No. 307,090 entitled "Impact Sensitive
Secuirty Window System" by H. Gordon Laidlaw, Jr. et al.; and Ser.
No. 307,089 entitled "Improved Security Window" by Clyde L. Lucky;
each of which was filed 11/16/72 and claims subject matter
disclosed herein and which is assigned to Sierracin Corporation,
assignee of this application.
In many situations it is desirable to have a transparent window
that is relatively impenetratable. Such windows may be used in
prisons, hospitals, museums, zoos, computer rooms, laboratories, or
in store fronts where theft or vandalism may be a problem. They are
useful any place where maximum natural lighting, visual access and
physical security are requisite. Jewelry counters and laboratory
hoods are other suitable locations. Bars can be added adjacent
ordinary glass; however, this is in many cases undesirable for a
variety of reasons. Thus, in a prison or hospital or similar
institutions, bars may have a significantly undesirable effect on
persons within the institution. Bars detract from the pleasure of
visitors to zoos or museums. In stores and the like where
protection is desired against entry, the presence of bars is highly
undesirable because of the adverse effect on potential customers.
Collapsible window grates are little better.
Windows that are highly resistant to penetration can be formed with
thick layers of glass or preferably with laminated glass sandwiches
which may include layers of tough, impact resistant plastics, such
as the polycarbonate plastics. Tempered glass is desirable in some
situations in case of breakage. Thus, for example, in some
institutions persons may deliberately break windows to obtain
slivers of glass to use as weapons or to ingest in a suicidal act.
Tempered glass is desirable for such situations since it does not
shatter like ordinary glass but breaks into relatively small
fragments substantially free of all sharp edges.
In addition to resistance to penetration it is often highly
desirable to provide sensing of efforts to penetrate so that an
alarm can be sounded locally or at some remote station. Thus, for
example, penetration of a prison window indicates either an escape
attempt or an effort to convey contraband. Sensors in the
individual prison windows can be monitored in a central location
for detection of such unlawful activities. Similarly, in stores or
the like, breakage of a window commonly precedes a burglary
attempt. For this reason, burglar alarm systems commonly include
means for sensing breakage of the window.
A very common technique for sensing breakage of a window is to
adhere a conductive tape such as thin aluminum or lead foil
directly to the glass around the periphery of the window. Such
strips are unsightly and are preferably avoided, particularly in
store windows and the like where an attractive appearance is highly
desirable. Omission of the obvious alarm strips may also be
desirable in some institutional windows. The alarm strips have
another disadvantage in that they are essentially a binary device
that is either intact or broken. When such a tape is broken the
alarm system is inoperative until someone gets to the window and
bridges a break in the tape. There is no way of resetting such an
alarm from a remote location.
It is therefore desirable to provide a security window system
having an alarm built into it which is sensitive to attempts to
penetrate the window and which can be reset from a remote location.
It is desirable to have a signal from the window that is related to
the degree of penetration, which in this context can be considered
to be an analog change as compared with the binary change that
occurs upon complete interruption of an electrical path. Preferably
such a security window is substantially free of apparent visual
indications of the presence of the alarm. For most uses the window
is preferably resistant to penetration with the impact resistance
of polycarbonate and the resistance to sawing that is
characteristic of glass.
BRIEF SUMMARY OF THE INVENTION
There is, therefore, provided in practice of this invention
according to a presently preferred embodiment a security window
system having a transparent electrically conductive layer over most
of the area of the window. Means are provided for making electrical
contact with edges of the conductive layer so that an analog change
in resistance can be detected. An output signal can then be given
in response to a change in resistance in excess of a predetermined
magnitude.
DRAWINGS:
These and other features and advantages of the present invention
will be appreciated as the same becomes better understood by
reference to the following detailed description of the presently
preferred embodiments when considered in connection with the
accompanying drawings wherein:
FIG. 1 is a face view of a security window including an alarm
sensor;
FIG. 2 is a fragmentary cross section of the window of FIG. 1;
FIG. 3 is a schematic diagram of a sensing circuit for the window
of FIG. 1;
FIG. 4 is a face view of another embodiment of security window;
FIG. 5 is a block diagram of a sensing circuit for the window of
FIG. 4;
FIG. 6 is a face view of another embodiment of security window;
FIG. 7 is a face view of still another embodiment of security
window;
FIG. 8 is a partially cut-away section of the window of FIG. 7;
FIG. 9 is a fragmentary cross section of another embodiment of
security window;
FIG. 10 is a fragmentary cross section of another embodiment of
security window;
FIG. 11 is a schematic diagram of another penetration sensing
system;
FIG. 12 is a schematic diagram of another resistance sensing
technique; and
FIG. 13 illustrates another embodiment of security window and a
schematic circuit connected thereto.
DESCRIPTION
FIG. 1 is a face view of a security window and FIG. 2 is a
fragmentary cross section showing the laminated layers thereof. In
face view the security window appears much like an ordinary
transparent window except that it may appear somewhat tinted or
have slightly less light transmission than an ordinary clear glass
window. In addition, very narrow isolation lines 20, described in
greater detail hereinafter, may be seen in the face of the window.
Ordinarily these lines are very minute and not noticeable except on
close examination. Metallic bus bars 21 are imbedded along opposite
side edges of the security window. A short tab 22 from each of the
bus bars typically extends beyond the edge of the window for making
electrical contact. The bus bars are preferably imbedded corrugated
copper strips as described in U.S. Pat. No. 3,612,745. Other
suitable bus bar arrangements will be apparent to one skilled in
the art, such as the external bus bars of U.S. Pat. No. 3,529,074.
Typically, when the security window is used it is mounted in a
frame so that the edge portions are all hidden and the bus bars 21
are thereby hidden by the opaque frame. This is desirable so that
the appearance of the security window essentially matches the
appearance of an ordinary glass window.
The various laminations forming the cross section of the security
window are illustrated in the fragmentary view of FIG. 2. A sheet
of tempered glass 23 forms one face of the window. As will be
apparent hereinafter it is preferred that this face be the one from
which penetration is most likely to occur. Typically the tempered
glass layer is about one quarter inch thick. A transparent
resilient plastic interlayer 24 is securely bonded to the glass
sheet 23. This interlayer is the same as that typically employed in
laminated automobile glass, for example. A layer 0.030 inch thick
of polyvinyl butyral makes a suitable interlayer that is
conveniently bonded to the other layers of the laminated window by
conventional heat and pressure laminating techniques.
A carrier film 25 having a metal layer 26 on one face thereof is
bonded to the plastic interlayer 24. It is relatively unimportant
which face of the carrier film has the metallic layer thereon. The
carrier film is, for example, a film of polyethylene terephthalate
about 0.005 inch thick. The metal layer 26 is an extremely thin
layer of a metal such as nickel, gold, silver, aluminum, copper or
the like which can be vacuum metallized onto the carrier film. Such
vacuum deposition of thin metal films is a conventional process
widely used for preparing electrically heatable windows. The metal
coating is deposited in a sufficiently thin layer that it is
transparent and absorbs relatively minor amounts of incident light
so that the overall transmission characteristics of the window are
not substantially diminished. The metal layer is sufficiently
continuous to have a substantial electrical conductivity.
By employing a thin carrier film the metal layer may be vacuum
deposited on the carrier film by a continuous process whereby large
sheets of carrier film are coated and subsequently cut to a desired
size. Relatively uniform resistance throughout the metal layer can
be achiever by such a vacuum metallizing treatment. The conductive
layer 26 extends over most of the area of the security window. If
desired for inhibiting environmental access to the metal layer it
may be deleted from the peripheral areas of the carrier film. About
the only requirement is that the conductive layer extend near
enough the edges of the security window to make good electrical
contact with the bus bars 21 (FIG. 1) adjacent the edges of the
sheet and extend over most of the area of the window where
penetration may be likely to occur. Ths bus bars are imbedded in
the laminate between the interlayer 24 and the carrier film 25 so
as to be in electrical contact with the metal film.
Another interlayer 27 is bonded to the opposite side of the carrier
film 25 from the first interlayer 24. These interlayers are
substantially identical. An impact resistant plastic ply 28 is
bonded to the second interlayer 27. A variety of transparent impact
resistant plastics are suitable for use in such a security window.
Methyl methacrylate resin may be employed, for example. It is
preferred, however, to employ a polycarbonate resin for the plastic
ply. This material is commercially available under the trademark
Lexan from General Electric and under the trademark Merlon from
Mobay Chemical Company. The polycarbonate sheet is extremely impact
resistant and has a high transparency. Thus, even if the tempered
glass layer 23 is broken the polycarbonate layer 28 normally
resists impact penetration. Such a polycarbonate sheet, for
example, may be about one quarter inch thick.
Another polyvinyl butyral layer 29 is bonded to the other side of
the impact resistant ply 28. This interlayer is substantially
identical to the first two. Finally a second sheet 30 of tempered
glass is bonded to the third interlayer 29 and forms the other face
of the laminated security window. This second layer of tempered
glass is also about one quarter inch. The glass and plastic layers
have differing coefficients of thermal expansion and when
temperature cycling is expected it is desirable to provide stress
relief around the periphery of the window. A suitable edge
separator technique is provided in copending U.S. Patent
Application Ser. No. 111,993 by Jan B. Olson, entitled "Interlayer
Stress Reduction in Laminated Transparencies" and assigned to
Sierracin Corporation, assignee of this application. The
polycarbonate layer may be subject to attack by plasticizers in the
polyvinyl butyral layer and it is usually desirable to employ a
polycarbonate sheet with a barrier layer on its faces. Such a
coated polycarbonate material is available from General Electric
under their trade designation MR-4000. Any of a variety of
conventional transparent melamine, phenoxy or urethane resins form
suitable barrier layers.
The security window illustrated in FIG. 1 is highly resistant to
penetration since the tempered glass has substantial impact
resistance. Even if the glass layer is broken by scratching or
sharp impact the polycarbonate layer has much higher strength and
ordinarily has sufficient impact resistance to prevent penetration.
Tempered glass breaks into a large number of relatively small
particles and these particles remain bonded to the interlayer. The
presence of such a mass of glass fragments on the surface of the
window does a great deal to inhibit sawing or other cutting of the
polycarbonate plastic.
There are substantial advantages to having a security window formed
with a glass face layer and a polycarbonate plastic layer laminated
together in combination with an alarm as herein described. It will
be noted that the carrier film where the conductive layer forming
the analog sensor of the alarm circuit is located is separated from
the glass and polycarbonate layers by a relatively soft and
flexible polyvinyl butyral interlayer. If the frangible glass layer
is broken, as by a sharp localized blow or a deep scratch which may
trigger fracture of tempered glass, the glass fragments are largely
held in place by adhesion to the interlayer. The cracks from the
glass seldom penetrate the resilient interlayer and hence do not
interrupt the thin metal film. Thus the mere fact that the glass is
broken does not necessarily trigger an alarm. The same is not true
of a system wherein the alarm sensor comprises a lead tape around
the periphery of the window or an electrically conductive film
applied directly to the glass. In such a system a crack propagating
to the edge of the window normally results in breaking of the lead
tape or conductive film and triggering of an alarm.
The resistance sensor extending over most of the area of the window
therefore serves to detect penetration of the window. When a hole
is made in the window of a sufficient size to interrupt a portion
of the conductive layer, the alarm will be triggered. Mere cracking
of the window or surface damage will not ordinarily trigger the
alarm. Detection of penetration is what is sought and this is
provided by the composite laminated window with a conductive layer
embedded therein.
Referring again to FIG. 1, the conductive layer is in electrical
contact with the bus bars 21 along opposite edges of the window. A
resistive connection is thereby provided between the two bus bars.
The isolation lines are actually extremely fine scribe lines made
in the face of the carrier film on which the conductive layer is
deposited. Since this conductive layer is extremely thin a scribe
line that is nearly invisible to the naked eye is sufficient for
interrupting the electrical continuity of the film. A scribe line
can be made with a shallow sharp groove that extends into the
carrier film a tiny distance, but not even this is needed. The
metal layer is so thin that almost any abrasion is enough to
interrupt it without marring the carrier film.
If an effort is made to penetrate the security window the
electrically conductive layer must also be penetrated. Any
interruption of the conductive layer having a component in a
direction parallel to the bus bars will cause an increase in the
resistance of the conductive layer. As pointed out hereinafter the
resistance between the two bus bars 21 can be monitored and any
significant change in resistance employed for triggering an alarm.
Any such sensing system has a predetermined sensitivity. If the
sensitivity threshold for triggering an alarm is too small, a
significant number of false alarms may be sounded. On the other
hand if the threshold of sensitivity for triggering the alarm is
too high, a rather large penetration of the window may occur before
an alarm is sounded. It has been found that a sensitivity threshold
in the area of about one to two percent change in resistance is
suitable for triggering an alarm, although higher or lower changes
are also suitable thresholds.
If a security window is made with a continuous conductive layer
over most of the area of the window without any electrical
isolation lines subdividing it into a plurality of conductive
areas, the change in resistance as a function of the magnitude of
the interruption of the conductive layer may be unduly low. When
the entire window between the two bus bars constitutes a continuous
conductive layer, each point on each bus bar is in direct
electrical contact with every point on the other bus bar. Current
flow between the two bus bars can therefore occur over a
substantial area and destruction of a minor portion of the
conductive layer may have a relatively minor affect on the total
resistance. Thus, for example, in one test wherein the distance
between the bus bars was 1.67 times the width of the conductive
layer in a direction parallel to the bus bars, a straight line cut
was made through the conductive layer in a direction parallel to
the bus bars. A cut extending more than 20 percent of the way
between the side edges of the conductive layer increased the
resistance less than 1.1 percent. A circular interruption in the
conductive layer having a diameter of about 17 percent of the width
of the conductive layer caused an increase in resistance of only
about 2.6 percent.
When isolation lines are scribed through the conductive layer in a
direction extending between the bus bars the conductive film is
divided into a plurality of conductive areas that are electrically
in parallel with each other. Then when a sufficient cut is made
parallel to the bus bars to completely sever one of such parallel
conductive areas a jump in resistance occurs. Thus, for example, a
conductive layer was subdivided into six conductive areas by five
scribed isolation lines. As a straight line cut proceeded across
one of the parallel conductive areas a nominal gradual change in
resistance occurred. When one of the six parallel conductive areas
was completely severed between adjacent isolation lines, an
increase in resistance of about 20 percent was observed. Since a
similar length cut in a film without isolation lines would produce
a resistance change of less than about 1 percent the value of the
parallel conductive areas can be readily seen. In addition to
increasing the sensitivity of the security window to relatively
small penetration, the sensitivity of the circuit for detecting a
change in resistance can be readily correlated with the resistance
change that may occur when one conductive area of a selected width
is severed.
Since the isolation lines are substantially invisible the security
window may have its conductive layer subdivided into any desired
width of conductive area for predetermined sensitivity. In a
typical window 30 inches wide the conductive layer is divided into
four segments, each 71/2 inches wide. Addition of only two more
isolation lines cuts the width of each area to only 5 inches for
very high sensitivity to penetration. If desired a large number of
electrical isolation lines can be extended between the bus bars so
that the conductive layer is divided into a number of narrow
parallel conductors. A penetration of the window interrupts a
number of such narrow conductors and the resistance change is the
usual change due to deleting some of the resistors in a parallel
array of resistors.
The array of resistors in electrical parallel is considered to
extend over most of the area of the window since penetration at any
point will interrupt one or more resistors. This may be true even
when the resistors become narrower than the electrical isolation
lines between them. Thus, if desired, one could form narrow strips
of conductive material on the carrier film with clear areas between
the strips and have a structure differing only in scale from the
arrangement illustrated in FIG. 1, for example.
FIG. 3 is a schematic illustration of a system for detecting
penetration of the security window. Very broadly the penetration of
the conductive layer causes an analog change in the resistance of
the window which is a function of the extent of penetration, and
the magnitude of this change may be used for triggering an alarm.
The resistance of the conductive layer 26 is represented by the
resistor 26' in the schematic illustration of FIG. 3. This
resistance is connected to the detecting circuit by electrical
leads 33 including the window bus bars and whatever additional
leads may be desired for conveying signals to a remote location.
The thin film resistor 26' is connected in a bridge with a resistor
34 as an adjacent arm of the bridge. A power supply 36 applies an
electrical signal to the resistances 26' and 34.
The electrical signal is also applied to a fixed resistor 37 and a
variable resistor 38 connected in series with a tapped resistor or
potentiometer 39. These additional resistors 37, 38 and 39 form the
other two arms of a bridge. The variable resistor 38 may be
employed for a coarse adjustment of the bridge balance. Resistor 37
can be adjustable for bridge balance, too, or both resistors 37 and
38 may be coupled for coarse bridge balance.
An amplifier 41 is connected between the adjacent bridge arms 26'
and 34 and is also connected to the tap on the potentiometer 39.
Adjustment of the tap can serve as a fine adjustment of the bridge
balance.
The bridge excitation provided by the power supply 36 can be either
a voltage or current arrangement. Similarly the power supply can be
either AC or DC as may be desired in a particular application.
Similarly the signal applied by the bridge to the amplifier 41 can
be either a differential signal, that is, with neither bridge tap
grounded or connected to a circuit common, or it may be a single
ended signal with either of the bridge connections grounded or
connected to a circuit common. Many variations in the bridge
excitation and unbalance detection will be apparent to one skilled
in the art.
The output of the amplifier 41 is applied to a conventional
threshold detector 42 which senses when the null balance of the
bridge is outside of a predetermined limit. A wide variety of
threshold detectors may be suitable, depending on the signal
selected from the amplifier in a particular embodiment. When the
threshold detector notes that the bridge is out of balance beyond
the preset limit an alarm 43 is triggered. Any desired alarm may be
used such as a bell, klaxon, light or the like. The alarm can be
adjacent the window or remotely located. One can even dispense with
the threshold detector and apply the amplifier output directly to
an audio alarm, such as, for example, a loudspeaker. When the sound
of the loudspeaker reaches some arbitrary level as noted by an
individual in the vicinity this can also serve as an alarm.
Once an alarm has sounded, and it has been determined that the
signal is erroneous or one decides to presently ignore the
unbalance, the bridge can be readjusted by means of the resistors
38 and 39 to bring it back into balance. This is quite feasible
since the signal output from the bridge is analog. A change in the
resistance of the conductive layer modifies the electrical signal
in an analog manner. The alarm system can therefore be reset by
rebalancing the bridge, all of which can be done from a remote
location if desired. Such is infeasible in a window fitted with
conventional lead tapes or with a conductive layer directly on the
glass since rupture of the tape or layer on glass is a binary
output and the circuit cannot be restored without access to the
window and repair of the tape.
Some changes in the resistance of the conductive layer may occur
gradually even when penetration of the window is not attempted,
thus for example, temperature changes in the window may cause
resistance changes in the conductive layer of a sufficient
magnitude to unbalance the bridge. One can therefore provide an
automatic balance reset 44 which senses an unbalance and brings the
bridge back to null by adjusting the potentiometer 39. The fact of
resetting of the bridge or the magntidue of the resetting may be
recorded with a conventional recorder 45. The cumulative change in
resistance recorded by the recorder 45 could be used to trigger an
alarm if desired. It will be apparent, of course, that the balance
reset 44 may be operated by the amplified null balance signal from
the amplifier 41 so as to operate in a more analog fashion and
accommodate slow drifts in the resistance balance of the bridge.
Similarly, if desired the balance reset or bridge adjustment may
simply control operation of the amplifier 41 to remain below the
threshold signal. Electrical balance of the amplifier can
substitute for actual bridge resistance balancing for alarm
actuation as well. The automatic reset 44 and recorder 45 are not
essential to the functioning of the system.
It will also be apparent that a high degree of sophistication may
also be incorporated in the balance detection system so that, for
example, a single transient of resistance can be ignored and a more
permanent change employed for triggering the alarm. Means may also
be provided for triggering the alarm in case the bridge leads are
shorted or cut, or if the power is cut off, or if any of a variety
of techniques are employed for circumventing the alarm system.
As mentioned above, the resistance of the conductive layer in the
window may vary with temperature and cause an unbalance of the
bridge. Although this can be readily accounted for with an
automatic balance resetting system, it is also quite easy to simply
compensate for the temperature change by making the fixed resistor
34 in the adjacent arm of the bridge to the resistor 26' also be a
conductive layer in a window. If the temperature pattern in the two
resistors is similar, any changes in resistance will be equivalent
and the balance of the bridge will not be upset. The second
conductive layer in a window may be in a separate window located in
a position subject to similar temperature conditions or it may
simply be another portion of the same window in which the layer
resistor 26' is located.
FIG. 13 illustrates in face view another embodiment of security
window having a conductive layer extending over most of the area of
the window. In this embodiment, there is a bus bar 102 extending
along one side edge of the window for making electrical contact
with one entire edge of the conductive layer in the window. An
electrical isolation line 103 extends across the window transverse
to the bus bar 102 and divides the conductive area of the window
into two conductive regions 104 and 106. A bus bar 107 extends part
way along the side edge of the window opposite from the full length
bus bar 102 and makes electrical contact with the conductive layer
of the first region 104. A second similar bus bar 108 extends the
balance of the way across the window and makes electrical contact
with the conductive layer in the second conductive region 106. Each
of the bus bars extends beyond the edge of the window for making
electrical contact with an external circuit.
FIG. 13 also illustrates schematically a typical external circuit
connected to the bus bars of the window. A power supply 109 is
connected to the two similar bus bars 107 and 108. Resistors 111
and 112 are also connected to the power supply. A first tap 113 is
connected between the resistors 111 and 112 and a second tap 114 is
connected to the bus bar 102 that makes electrical contact with
both conductive regions 104 and 106. The taps 113 and 114 may be
connected to any conventional null balance detection circuitry as
desired for triggering an alarm in response to unbalance of
resistance. It will be noted that the window and external circuit
illustrated in FIG. 13 are connected as a conventional bridge with
the two conductive regions of the window as adjacent arms of the
bridge. Either or both of the resistors 111 or 112 can be variable
for balancing the bridge, or balancing can be achieved in the
additional circuits (not shown) to which the window may be
connected.
The two conductive regions of the window will both be subjected to
similar temperature conditions and any changes in resistance in the
two regions will be similar. Being in adjacent bridge arms, the
resistance drift due to temperature change balances out and no
bridge unbalance results. It will be apparent that if desired the
conductive layer in each of the conductive regions 104 and 106 can
be subdivided by isolation lines extending between the bus bars to
any desired extent for enhancing sensitivity of the window to
penetration.
FIG. 4 illustrates in face view another embodiment of security
window having an electrically conductive layer over most of the
area of the window. In this embodiment, the conductive layer is
subdivided into four conductive areas 46, 47, 48 and 49 by
isolation lines 50. A first bus bar 51 is in electrical contact
along an edge of the first conductive area 46. A similar short bus
bar 52 is in electrical contact with an edge of the other outside
conductive area 49. A third bus bar 53 is in electrical contact
with the edges of both of the remaining two conductive areas 47 and
48 spanning one of the isolation lines 50. Along the opposite edge
of the security window from the first three bus bars is a fourth
bus bar 54 in electrical contact with the edges of the two
conductive areas 46 and 47. Another bus bar 55 on this same edge of
the window is in electrical contact with the edges of the remaining
two conductive areas 48 and 49. Suitable conductive tabs 56 extend
from the bus bars to and beyond the edges of the window for making
electrical contact to external circuits. If desired, the conductive
areas between the isolation lines 50 can also be subdivided into
parallel resistive areas in the same manner as the window of FIG. 1
for enhanced sensitivity.
FIG. 5 illustrates schematically the interconnection of the
conductive areas. The bus bars and conductive areas of FIG. 4 are
represented schematically with the same reference numerals bearing
a prime in FIG. 5. The two bus bars 51 and 52 are externally
interconnected at a point 51', 52'. This same piont is connected to
a suitable power supply 57 which is in turn connected to the center
bus bar 53 at a point 53' in the schematic illustration of FIG. 5.
Electrical connection is made to the opposite bus bars 54 and 55 at
the points 54' and 55' leading to a detector 58 of changes in the
electrical resistance. The detector 58 can be connected for
triggering an alarm 59 when a predetermined liminal change in
electrical resistance occurs in the bridge formed by the resistors
(conductive areas) 46', 47', 48', and 49'.
Since the four resistors or conductive areas are connected as the
four arms of a resistance bridge, any resistance changes occurring
in all of the conductive areas will not unbalance the bridge and no
net change in resistance will be noted. As mentioned above, the
conductive layer within the security window changes resistance
somewhat with changing temperature. Similarly, stresses on the
conductive layer which may be generated by bending of the window,
for example, may change resistance. When the conductive areas are
interconnected as arms of a bridge such thermal or stress changes
in resistance do not cause false alarms. It will be apparent to one
skilled in the art that if desired more than one window may be
interconnected as arms of a resistance bridge and the conductive
areas may sufficiently balance to compensate for thermal changes
and the like. This is generally less desirable since the thermal
changes or changes in stress between two windows is usually of much
greater magnitude than similar changes within two different areas
of the same window. It will also be apparent that the resistance
change detector 58 should be adjustable for resetting the alarm
system in case of a permanent change in the resistance balance
between the arms of the bridge.
FIG. 6 illustrates in face view another embodiment of security
window. In the embodiment of FIG. 1, the conductive areas were
electrically connected in parallel. In the embodiment of FIG. 6,
the conductive areas are connected in series. Thus, as illustrated
in this embodiment, a plurality of isolation lines 61 extend
between opposite edges of the security window and subdivide the
area into a plurality of conductive areas 62. Electrical contact is
made along a side edge of one of the outside conductive areas by a
bus bar 63. A tab 64 permits electrical connection of this bus bar
to an external circuit. At the opposite end of the first conductive
area from the bus bar 63 is a second bus bar 65 which makes
electrical contact along the side edge of the first conductive area
and also along the side edge of the second conductive area adjacent
the first. That is, the second bus bar 65 spans the isolation line
61 between the two adjacent conductive areas. Another bus bar 66
electrically connects the opposite side edge of the second
conductive area with the side edge of the third conductive area.
These additional bus bars 65 and 66 do not have an external tab for
electrical connection to circuits outside the window.
A similar series of additional bus bars connect adjacent conductive
areas clear across the window. In the final conductive area a bus
bar 68 makes electrical connection to both the edge of the
conductive area and a tab 69 permitting electrical connection to an
external circuit. Thus, all of the conductive areas within the
window are electrically connected together in series. Clearly a
penetration that extends through the full extent of one of the
conductive areas will interrupt the continuous circuit and provide
a substantially infinite increase in resistance. Such an electrical
connection is not resettable from a remote location.
If desired a tab 70 may be provided on a central bus bar for making
contact to an external circuit thereby permitting half of the
conductive areas to be in one arm of a bridge and the other half in
another arm of a bridge for temperature and stress compensation.
Such a series connected security window is very sensitive to small
penetrations. The same effect can be obtained without the large
number of bus bars by simply ending alternate isolation lines a
substantial distance from each of the opposite edges of the window
respectively. A pattern of isolation lines for a series-parallel
connection of conductive areas can also be used.
As mentioned hereinabove the security window is highly sensitive to
penetrations that have a component extending in a direction
parallel to the bus bars. If the interruption in the conductive
layer is primarily in a direction between the bus bars, that is,
for example, parallel to the isolation lines, little if any change
in resistance is observed. Thus, for example, if the object of
penetration of the security window is the passage of contraband, a
narrow slit extending between the bus bars may be sufficient for
the unlawful purpose without causing a sufficient change in
resistance to trigger the alarm. This possibility is effectively
forestalled with a security window of the type illustrated in FIG.
7.
As illustrated in this embodiment at least the central portion of
the security window has two spaced apart conductive layers
extending over most of the area of the window. A first pair of bus
bars 72 are provided along the opposite side edges of the security
window in electrical contact with the edges of one of the
conductive layers. Orthogonal to this first set of bus bars is a
second pair of bus bars 73 in electrical contact with the edges of
the second conductive layer within the window. The isolation lines
in the conductive layers between the opposed bus bars have been
deleted from FIG. 7 for enhancing clarity of the drawing. It will
be readily apparent that no penetration of the window can be made
that does not have a component parallel to one or the other of the
two pairs of bus bars. It is therefore substantially impossible to
penetrate such a window with any reasonable size hole without
triggering an alarm.
The arrangement of bus bars in the window illustrated in FIG. 7 is
as simple as possible and, if desired, arrangements such as
illustrated in FIGS. 4 and 6 may be employed. The two conductive
layers can be employed as a pair of arms in a bridge, or portions
of the two conductive layers may be used as the four arms of a
bridge. There is a possibility, although remote, that penetration
of both layers could cause compensating resistance changes in the
two layers when they are used as adjacent arms of a bridge. The two
layers can be used as opposite arms of the bridge so that
penetration of both layers causes an increase in sensitivity.
FIG. 8 illustrates in fragmentary cross section a laminate security
window having two conductive layers therein. In this illustration
successive layers are cut away to best shown the location of the
bus bars. In this particular example the arrangement of successive
layers is symmetrical from the center of the laminate, however, it
will be apparent that asymmetrical arrangements are also
suitable.
Each face of the laminated security window comprises a glass ply
75. A plastic interlayer 76 about 0.03 inch thick is bonded to each
of the glass plys 75. A carrier film 77 of polyethylene
terephthalate about 0.005 inch thick and having a thin conductive
metal coating 78 thereon is bonded to each of the interlayers 76.
Centrally located in the laminate is a third interlayer 79 which
bonds the two carrier films together. One of the bus bars 72 is
imbedded in one of the plastic interlayers 76 so as to be in
electrical contact with one of the conductive layers 78. The bus
bar 72 is illustrated schematically rather than show the
corrugations of the preferred bus bar hereinabove mentioned. The
other bus bar 73 is imbedded in the opposite interlayer 76 so as to
be in electrical contact with the other conductive layer 78. During
fabrication of such a laminated window initially flat sheets of
polyvinyl butyral for the interlayers are assembled in a sandwich
and during the heat and pressure cycle of lamination this
relatively soft material deforms so that the respective bus bar
imbeds therein. The effect of plural layers for detecting
penetration can also be obtained by depositing thin metal films on
both faces of the carrier film and laminating that carrier in a
window with bus bars in contact with both metal layers.
FIG. 9 illustrates in fragmentary cross section another embodiment
of security window suitable for use in locations where customary
access is almost entirely on one side of the window. This laminated
structure has a glass ply 81 on one face, such as, for example, one
quarter inch tempered glass. A polyvinyl butyral interlayer 82 is
bonded to the glass. A carrier film 83 having a thin conductive
metal layer (not shown) thereon is bonded to the plastic interlayer
82. A second interlayer 84 bonds the carrier film 83 to a
relatively thick ply of transparent polycarbonate plastic 86. The
exposed face of the plastic ply 86 is coated with a protective
layer 87 such as chemically deposited silica, titania or the like,
which affords a substantial degree of abrasion resistance and
protection against chemical attack on plastic.
Such an asymmetrical laminated security window may be used, for
example, in an institution wherein the glass layer 81 is used on
the inside where the inhabitants have access to the window. The
plastic layer would be used on the outside where there is no
regular day-to-day contact. Similarly in a store window or the like
the glass layer may be employed on the outside with the plastic
layer 87 on the inside where only store personnel may have access
to it. This is desirable since the plastic ply is softer than the
glass and can be scratched.
FIG. 10 illustrates a security window such as might be used for
temporary purposes. In this embodiment a pair of carrier films 89
having thin conductive metal layers 90 thereon are bonded together
with a plastic layer 91 which may be a polyvinyl butyral interlayer
as hereinabove described or may be other suitable adhesive bonding.
The relatively thick interlayer is not generally needed in such a
situation since its ability to conform to the rigid glass and
polycarbonate plys of the other embodiments is not a requirement.
Care must be taken, of course, to insulate the two metal layers 90
from each other if both are used in active alarm circuits.
Similarly the bus bars (not shown) making contact to the conductive
layers 90 must be insulated. It will also be apparent that if
desired a carrier film having a metal coating thereon can be
adhesively bonded to a similar film which serves to protect the
delicate metal layer from damage and a security window suitable for
temporary use may be very inexpensively provided. Bus bars are
needed to make contact with the metal layer. Such a security window
made with thin plastic films has considerable flexibility and is
light in weight making it quite suitable for temporary use. Such a
lamination of plastic films with a conductive layer therein can be
bonded on a window and connected to suitable detection and alarm
circuits for forming a security window. Preferably the conductive
layer in such a security window is scribed with electrical
isolation lines and electrically connected in a bridge circuit in
one of the manners hereinabove described.
There is a distinct advantage in the arrangement illustrated in
FIG. 2 wherein the conductive layer 26 is separated from the
tempered glass ply 23 by at least the interlayer 24 and, if
desired, the carrier film 25. The plastic isolates the conductive
film from the glass layer so that if the glass is merely broken
most, if not all, of the conductive layer remains intact. It is a
characteristic of tempered glass that a break propagates over the
entire extent of the glass breaking it into a very large number of
small fragments. If the conductive layer were on the glass or
closely coupled thereto such breakage of the glass would completely
rupture the delicate metal layer and it would appear that
penetration was being attempted. With the metal layer decoupled
from the glass by a relatively soft resilient intervening plastic
layer mere breakage of the glass does not disrupt the conductive
film to more than a minor extent. Even if an alarm might be sounded
when the tempered glass is broken the alarm system can be reset to
indicate when an attempt is made to penetrate the window.
The resistance of the thin metal film embedded in the laminated
security window is also sensitive to strain. That is, as the film
is strained, the resistance changes. Thus, for example, when the
conductive layer is located off of a neutral axis of the cross
section of the laminated window, a bending of the window will
induce strain in the conductive layer and change its resistance.
This discovery gives one an opportunity to employ the deformation
of the window prior to penetration for providing an alarm signal.
More particulary a transient change in resistnace can be detected
with a pulse or rate of change greater than some predetermined
magnitude.
FIG. 11 illustrates in block diagram a system for ulitizing the
strain sensitive properties of the conductive film in a laminated
window for providing a security alarm. A power supply 93 applies
power to the conductive metal layer in a security window 94. The
excitation applied to the window by the power supply may be AC or
DC, and may be either current or voltage as desired. The window 94
is also connected to an AC amplifier 96 with gain control 97. The
AC amplifier may also have a band width control, if desired, for
limiting the AC range amplified. The output of the AC amplifier is
applied to a threshold detector 98 which applies an out-of-limits
signal to an alarm 99.
If someone should commence striking or otherwise deforming the
window in order to effect penetration, the resultant time varying
signal is amplified. When the signal is within the frequency band
of the AC amplifier and beyond the preset threshold, an alarm will
sound. Such a system is also responsive to the rate of change of
penetration through a window as represented by the changing
resistance. In an impact sensitive system one can use the magnitude
of the pulse of changing resistance to detect a penetration or
attempt at penetration. If desired the rate of change or rise time
of the pulse or pulse width can be selected for triggering an
alarm. The circuitry for detecting any such characteristic of
changing resistance is conventional.
The sensitivity of the threshold detector 98 can be set so that the
strain required to activiate the alarm 99 is a large fraction of
the strain that would occur in the window before breakage. Gain
control 97 may effect this function. Thus, relatively minor blows
on a window which are far short of causing breakage can be ignored
and a pulse representing a sufficient blow to be quite near
breakage of the window can be detected. One can monitor strain
amplitude and detect pressures that are a large fraction of the
force required to break the window. With AC amplification in the
system a blow that is sufficient to raise the strain level to
breakage will activate the alarm and indicate penetration. It has
been noted in penetration tests of a window having a thin metal
conductive film that during the penetration event very high
excursions in resistance occur and thereafter the resistance
settles to an equilibrium value characteristic of the response to
severance of the film. Thus, for example, penetration of the
security window by a high speed projectile will cause a large
change in resistance as penetration occurs which can be detected by
the AC amplification system. Thus the penetration can be detected
by the rapid pulse of resistance change, even though the steady
state resistance may not be significantly different from the
original resistance.
It may be desirable to employ a strain sensitive detection system
as set forth in FIG. 11 with a penetration sensing system as
illustrated in FIG. 3. In such an arrangement the input to the AC
amplifier 96 may be either the output of the null balance bridge of
FIG. 3 or the output of a DC amplifier 41 which may reduce the gain
requirements of the AC amplifier 96.
If desired, a continuous surveillance monitor 101 may be connected
to the output of the AC amplifier 96. This continuous surveillance
monitor may have a visual or aural output so that an attendant can
perceive signal changes, such as, for example, due to someone
pounding on a security window. The continuous surveillance monitor
can also be some means for recording the output signal for review
at a later time.
A resistance bridge is of course not the only way of detecting a
change in the resistance of the conductive layer. A simple and
inexpensive technique is illustrated in FIG. 12. As illustrated in
this arrangement a voltage e.sub.i is applied to a resistor 102
connected to an input of an operational amplifier 103. A second
resistor 104 is connected across the amplifier. The output voltage
e.sub.o is proportional to the input voltage and the ratio of the
resistance of resistor 104 to the resistance of resistor 102. The
output voltage is thus quite sensitive to any change in the
relative values of the two resistors. A conductive layer in a
security window can be used for either of the two resistors in this
schematic diagram, or if desired two conductive areas in a window
could be used as both resistors 102 and 104 for temperature
compensation. The operational amplifier can be either a single
input amplifier, or can be a differential amplifier with two
inputs.
One can also use ohmmeter circuits or a variety of current or
voltage comparison circuits for noting a change in resistance due
to window penetration. It will also be noted that the resistance
values can be digitized at any point in the electrical circuit and
digital techniques used for balancing, comparing and the like. If
the resistance of a conductive layer is digitized it can be
compared to a digital reference number and the "balance" can be
maintained by changing the reference number. As much or as little
sophistication as desired can be achieved with digital techniques
for comparison, correction and avoidance of false alarms or
tampering with the system. Many other arrangements will be apparent
to one skilled in the art for detecting a variation in resistance
of the security window.
Although limited embodiments of security window and alarm systems
associated therewith have been described and illustrated herein,
many modifications and variations will be apparent to one skilled
in the art. It is, therefore, to be understood that within the
scope of the appended claims the invention may be practiced
otherwise than as specifically described.
* * * * *