U.S. patent number 3,846,780 [Application Number 05/382,264] was granted by the patent office on 1974-11-05 for intrusion detection system.
This patent grant is currently assigned to Westinghouse Electric Corporation. Invention is credited to Heinz Gilcher.
United States Patent |
3,846,780 |
Gilcher |
November 5, 1974 |
INTRUSION DETECTION SYSTEM
Abstract
A wire comprised of an electrical conductor enclosed within
insulation, is loosely positioned relative to an electrically
conductive member. This combination is positioned to monitor
disturbances caused by the intrusion of the protected area. The
relative movement of the wire and the electrically conductive
member caused by intrusion disturbances produces an electrical
signal which is attributable to a capacitance change. The signal
can be realized from either the electret characteristic of the wire
or it can be realized by the application of a d-c bias voltage to
the combination. A signal developed by the combination in response
to a mechanical disturbance includes a low frequency and a high
frequency component. The low frequency component, attributable to a
"strain mode" of operation, developed by the displacement of the
wire relative to the electrically conductive member by mechanical
deflection of the conductive member whereas the high frequency
component, attributable to an "acceleration mode" of operation,
results from the vibration of the wire relative to the conductive
member. The high frequency component code is comparable to a
"ringing" condition much the same as is experienced by an
accelerometer.
Inventors: |
Gilcher; Heinz (Export,
PA) |
Assignee: |
Westinghouse Electric
Corporation (Pittsburgh, PA)
|
Family
ID: |
23508197 |
Appl.
No.: |
05/382,264 |
Filed: |
July 24, 1973 |
Current U.S.
Class: |
340/562; 361/280;
367/181; 361/278; 367/177 |
Current CPC
Class: |
G08B
13/26 (20130101) |
Current International
Class: |
G08B
13/26 (20060101); G08B 13/22 (20060101); G08b
013/16 () |
Field of
Search: |
;340/261,17 ;179/1.41B
;317/246 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trafton; David L.
Attorney, Agent or Firm: Lynch; M. P.
Claims
I claim:
1. Apparatus for monitoring disturbances caused by mechanical
shock, pressure, impact, etc., comprising in combination, a
substantially rigid first electrically conductive member adapted to
respond to disturbances, a second electrically conductive member
disposed in a loose fitting, intermittent contacting relationship
with said first electrically conductive member and being
mechanically free to move with respect to said first electrically
conductive member, electrical insulating material disposed between
said first and second electrically conductive members to establish
a capacitance therebetween, disturbance of said substantially rigid
first electrically conductive member resulting in mechanical
displacement of said second electrically conductive member with
respect to said first electrically conductive member, the
displacement producing a change in said capacitance.
2. Apparatus as claimed in claim 1 wherein said insulating material
exhibits an electret characteristic, said electret characteristic
providing an electrical charge.
3. Apparatus as claimed in claim 1 further including a d-c voltage
source operatively connected to said electrically conductive
members to establish an electrical charge on said insulating
material.
4. Apparatus as claimed in claim 1 wherein the change in
capacitance produced due to the displacement of the second
electrically conductive member with respect to the first
electrically conductive member is manifested by the develoment of
an electrical signal, said electrical signal consists of a low
frequency component corresponding to the capacitance change
produced by separation of said first electrically conductive member
from said second electrically conductive member and a high
frequency component corresponding to the vibration of said second
electrically conductive member.
5. Apparatus as claimed in claim 1 wherein said first electrically
conductive member includes a sheet of electrically conductive
material, and said second electrically conductive member is a wire
resting loosely on said sheet of electrically conductive material
with said electrical insulating material disposed therebetween.
6. In an intrusion detection system for responding to disturbances,
the combination of, a substantially rigid tubular member exhibiting
electrical conductivity characteristics, a flexible, loose fitting
electrical conductor having an effective diameter which is
substantially less than the inside diameter of said tubular member
and being disposed within said tubular member in an intermittent
contacting relationship with an interior surface of said tubular
member so as to be substantially closer to one interior surface of
the tubular member than to the opposite interior surface of the
tubular member, electrical insulating material disposed between
said tubular member and said flexible electrical conductor to
establish a capacitance therebetween, a disturbance causing
displacement of the loosely positioned flexible electrical
conductor relative to said substantially rigid tubular member
causing a change in said capacitance, said change in capacitance
resulting in the development of an electrical signal indicative of
the disturbance.
7. Apparatus as claimed in claim 6 wherein said electrical
insulating material is a coating on said flexible electrical
conductor, the effective diameter of the combination of said
flexible electrical conductor and the coating of electrical
insulating material being substantially less than the inside
diameter of said tubular member.
8. Apparatus as claimed in claim 7 wherein the insulating material
coating exhibits an electret characteristic.
9. Apparatus as claimed in claim 7 including a d-c voltage source
operatively connected to said flexible electrical conductor and
said tubular member to establish a charge condition in said
insulating material.
10. Apparatus as claimed in claim 6 wherein said electrical signal
is comprised of a low frequency and high frequency component,
wherein said low frequency component corresponds to the change in
capacitance produced by the displacement of said wire element
relative to said tubular member in response to said disturbance and
said high frequency component corresponds to the vibration of said
wire element relative to said tubular member in response to said
disturbance.
11. Apparatus as claimed in claim 6 wherein said flexible
electrical conductor is a wire.
Description
BACKGROUND OF THE INVENTION
A typical prior art intrusion detection system for monitoring a
protected area is described in detail in U.S. Pat. No. 3,438,021
issued Apr. 8, 1969. In this system a pair of fluid filled
resilient tubes are buried beneath the surface of the ground and
associated pressure transducers develop output signals indicative
of pressure changes representing the movement of the intruders.
While this system provides a valuable and reliable technique for
detecting intruders, it is relatively expensive and difficult to
install, and requires treatment of the earth to preserve
sensitivity under frozen ground conditions.
Recent efforts to utilize coaxial cables in this buried
configuration have also failed due to the lack of sensitivity of
the coaxial cables resulting from the tight packaging of the wire
within the cable which effectively prevents movement of the
conducting elements relative to one another in response to
intrusion disturbances. The inherent compactness and tightness of
the fabrication of a coaxial cable thus limits the output of the
coaxial operation to a low frequency signal thus limiting the
sensitivity of such a system and rendering it unacceptable in
frozen ground applications.
SUMMARY OF THE INVENTION
There is described herein a preferred embodiment of a novel
detection system utilizing an insulated electrical wire loosely
positioned within an electrically conductive tube member having an
inside diameter substantially greater than the diameter of the
wire. The primary mechanism of operation is a change of capacity
between the wire and the tube produced by mechanical disturbance of
the wire within the tube. The change in capacitance is transformed
into a voltage signal which can be the result of an inherent
electret characteristic of the wire developed during fabrication of
the wire or can be provided or enhanced by the further application
of a bias voltage to the wire. A charge is developed by the
electret characteristic of the insulation or the d-c bias voltage
and a change in capacitance produced by a mechanical disturbance
produces an electrical signal. The loose positioning of the wire
within the tube member causes the wire to contact the wall of the
tube member at random and intermittent locations. In the event the
combination of the tube member and wire is positioned beneath the
surface of the ground, the wire will be displaced relative to the
tube in response to surface disturbances and an output signal will
be produced which includes both a low frequency and high frequency
component. Due to the fact the wire is significantly closer to one
wall of the tube than the opposite wall of the tube, the variation
of capacitance produced by displacement of the wire relative to the
near wall will be significantly greater than the variation in
capacitance due to displacement of the wire relative to the far
wall.
This wire-in-tube detection configuration provides advantages over
prior art detection configurations for buried systems in that it
does not require intimate contact with the soil to insure
sensitivity.
DESCRIPTION OF THE DRAWINGS
The invention will become readily apparent from the following
exemplary description in connection with the accompanying
drawings:
FIG. 1 is a schematic illustration of an embodiment of the
invention;
FIG. 2 is a section on the line II--II of FIG. 1;
FIG. 3 is an electrical equivalent circuit of the embodiment of
FIG. 1; and
FIG. 4 is an alternate embodiment of the invention disclosed in
FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
It has been determined experimentally through extensive testing
that a significant improvement in intrusion detection systems can
be achieved through the use of commercially available insulated
electrical wire loosely positioned in contact with an electrically
conductive member. A loose contact between the wire and the
electrically conductive member permits free movement of the wire
relative to the electrically conductive member in response to
mechanical disturbance of the conductive member produced by the
intrusion. Change in capacitance caused by the movement of the wire
relative to the electrically conductive member is transformed into
an electrical signal exhibiting a low frequency and/or a high
frequency component of sufficient magnitude to permit
identification of an intrusion. The electrical signal is developed
by either applying a bias voltage to the wire or by utilizing an
inherent electret characteristic of the insulation which is
developed in the fabrication of commercially available wire.
The electret characteristic found in dielectric insulation which
covers some commercially available electrical wire is a
characteristic produced during the fabrication of the wire which
establishes an electrical charge condition in the dielectric
material. A common use of the electret characteristic is in the
electret-foil capacitor microphone. In this microphone the electret
is a MYLAR film that is metalized on its outer side. The electret
characteristic eliminates the need for a d-c bias voltage.
The high and low frequency components capable of being developed by
the combination of the loosely positioned wire relative to the
electrically conductive member in response to an intrusion
disturbance, are both produced as a result in a change in
capacitance developed in the vicinity of the intrusion.
The high frequency component, attributable to an acceleration mode
of operation, results from the vibration of the wire relative to
the electrically conductive member while the low frequency
component, which is attributable to a "strain mode" of operation
results from the deflection of the electrically conductive member
relative to the wire in response to the weight or pressure of an
intrusion.
While the preferred embodiment illustrated in FIG. 1 discloses a
wire loosely positioned within an electrically conductive tubular
member which lends itself to applications wherein the system is
buried beneath the surface of the ground, it is apparent from the
illustration of FIG. 5 that the invention concept is equally
applicable to a flat electrically conductive member with a wire
loosely positioned thereon.
Referring to FIGS. 1 and 2 there is illustrated a disturbance
detector 10 in an embodiment for use in buried applications
comprised of a wire 12 loosely positioned within a tubular member
14. The wire 12 consists of an electrical conductor 16 and an
insulating coating 18. The selection of the wire for use in the
disturbance detector 10 can be such as to take advantage of the
electret characteristic of some commercially available wire. Some
insulating materials used in coating electrical wire exhibit an
electret effect. Evaluation of numerous commercially available wire
material indicates that the most significant electret signal is
derived from a TEFLON coated electrical conductor. The tubular
member 14 is constructed from an electrically conductive material
such as a metal or a metalized plastic. The tube member 14 is
connected to ground and the conductor 16 of the wire 12 is
electrically connected to amplifier 20 which in turn is connected
to a signal processing circuit 30.
An electrial equivalent circuit of the embodiment of FIG. 1 is
illustrated in FIG. 3. The signal source voltage source, be it the
electret characteristic or a d-c bias voltage, is represented as a
generator G, and the tube 14 and wire 12 capacitances are
represented as capacitors C1 and C2.
Assuming the positioning of the disturbance detector 10 of FIG. 1
at a distance beneath the surface of the ground g, the occurrence
of a disturbance on the surface of the ground resulting from
personnel or vehicular movement will cause the wire 12 to be moved
within the tube member 14. Two types of wire movement are
represented in FIG. 1 occurring as a result of a disturbance. The
wire movement represented by the dashed lines corresponds to
displacement of the wire 12 from the tube contacting points P
produced by deflection of the tube 14 in response to the pressure
applied to the surface of the ground. The second movement of the
wire 12 in response to a disturbance is represented by the dotted
lines of FIG. 1 wherein the portions of the wire 12 between the
contacts points P undergoes an acceleration mode or vibratory mode
of movement due to the mechanical shock introduced by the
disturbance.
The inside diameter of the tube member 14 is selected to be
significantly larger than the outside diameter of the wire 12 such
as would be represented by a quarter-inch copper tube and a No. 22
gauge wire. The positioning of the wire 12 within the tube 14 as
illustrated in FIG. 1 locates the wire in a near wall relationship
with one portion of the tube 14 such that a portion of the wire 12
adjacent to the contact point P establishes minimum distance D1
between the wire 12 and the near wall of the tube member 14. Since
the minimum distance (air gap) provides a maximum capacitance, very
small displacement of the wire relative to the near tube wall
provides maximum capacitance modulation and in turn maximum signal
voltages which are applied to the amplifier 20. In areas along the
wire where the air gap is significantly larger, the same mechanical
displacement of the wire 12 provides a much smaller capacitance
change since the wire to the tube wall capacitance is much smaller
and thus the proportional change in this capacitance is also much
smaller. It is therefore obvious that for this detector
configuration, the capacitance between the wire 12 and the far wall
of the tube wall 14 is negligible. The distance between the wire 12
and the tube wall 14 as defined by D1 corresponds to the wire in a
rest condition whereas the distance D2 corresponds to a typical
displacement of the wire in response to a disturbance.
The signal produced as a result of a relative movement of the
insulating material of the wire 12 relative to the conductive
members corresponding to the tube 14 and electrical conductor 16
exhibits a low frequency and high frequency component. The low
frequency component is a function of the displacement of the wire
as illustrated by the dashed lines while the high frequency
component corresponds to the dotted portion of the illustration of
FIG. 1. The low frequency component is typically in the range of
0.1-10 hz. while a typical high frequency range corresponds to
10-100 hz. The output of the amplifier 20 is applied to a filter
circuit 22 which effectively isolates the low frequency and high
frequency components and applies the respective components through
signal attenuating circuit 24 and 26 to a monitoring device herein
represented as recorder 30. The high frequency component of the
output signal of the detector circuit 10 is determined by the
distance between the contact points P and the mass per unit length
of the wire.
The isolation of the low frequency and high frequency component of
the output signal permits independent monitoring of the components.
It has been determined experimentally that the low frequency
component provides characteristic information under certain
operating conditions while the high frequency component provides
information under other operating conditions. The use of the
attenuating circuits 24 and 26 provides control of the relative
weight attributed to the low frequency and high frequency
components depending on the operating conditions to which the
detector 10 is subjected.
For instance, it may be desirable to vary the relative sensitivity
of the recorder 30 to the low frequency and high frequency
components of the output signal under varying weather conditions.
It has been determined experimentally that in underground
installations of the detector 10, wherein the ground is frozen, the
high frequency component reflects too wide a sensitive range to be
effective while the low frequency component provides a relatively
clear indication of intrusion disturbance. Under these conditions
therefore the circuits 24 and 26 would be adjusted such that
recorder 30 would respond primarily to the low frequency component
of the signal developed by the detector 10. On the other hand,
under high wind conditions it has been determined experimentally
that less false alarms are recorded when monitoring the high
frequency component of the detector signal. Under high wind
conditions, detector systems utilizing a low frequency output
signal such as the system in the above-identified patent, are
sensitive to wind pressure changes on the ground which adversely
affect the accuracy of the detector signal.
While the signal processing circuit 30 has been shown to consist of
basic components which clearly indicate a typical utilization of
the detector 10, it is apparent that the signal processing circuit
illustrated can be replaced with a far more sophisticated
electronic system which could cause automatic adjustment of the
sensitivity of a monitoring device to the low and high frequency
components of the detector device in response to changing
conditions.
As mentioned earlier the mechanism for developing the detector
signal can be based solely on the electret characteristic of the
wire or solely on the application of a remote d-c bias signal or on
the combination of the electret characteristic and the d-c bias
signal depending on the particular application of the detector
device. For instance in a buried application requiring extensive
length of the detector device it may be necessary to include an
external d-c bias source to provide an adequate signal output. A
typical application of a d-c bias voltage source is illustrated in
FIG. 1. A voltage source S is applied to the conductor 16 through a
high impedance device 40. Isolation of the bias voltage from the
amplifier 20 is provided by capacitor 42.
Referring to FIG. 4, there is illustrated an alternate embodiment
of the detector device of FIG. 1. In place of the tube member 14
illustrated for the buried application of the detector device 10 of
FIG. 1, there is illustrated a flat electrically conductive plate
42 positioned relative to the entrance to the door D and having
disposed thereon an electrical wire 42. Once again the wire 42 as
in the case of the wire 12 of the embodiment of FIG. 1 is loosely
positioned relative to the electrically conductive member 40 such
that impact by an intruder on the surface of the conductive member
40 will cause displacement of the wire 42 relative to the plate 40
thus producing an electrical signal in accordance with the
operation described above. Amplifier 44 responds to the electrical
signal by activating alarm circuit 46. The embodiment of FIG. 4 is
disclosed to clearly indicate that the inventive concept is not
limited to the use of a tubular member as illustrated in FIG. 1,
but rather extends to the basic concept of using an electrical wire
loosely positioned relative to a conductive surface to develop an
electrical signal in response to the change in capacitance produced
by displacement of the wire relative to a conductive member.
Results of impact sensitivity tests of detectors utilizing wires of
different sizes and insulation and tubes of different diameters are
presented in the following tabulation. It appears from the
tabulation that clear insulated TEFLON wire is probably better for
a detecting device utilizing electret characteristics of the wire
than a pigmented wire of a similar type. It also appears that there
is a minimum inside tube diameter that will accommodate a
particular wire size and still generate acceptable electrical
signals.
__________________________________________________________________________
Wire Capacitance Normalized Tube Description Pf/ft. Signal
Diameters MVPP
__________________________________________________________________________
No. 20 Teflon flex wire blue 20 275 MV No. 20 P.V.C. do. green 26
50 MV No. 24 Teflon do. blue 16 275 MV .25" O.D. No. 22 Teflon do.
red 18 220 MV .176" I.D. No. 30 Teflon do. -- -- 275 MV No. 22
Teflon do. clear 18 330 MV No. 22 Teflon do. red 25.5 26 MV No. 30
Teflon do. -- 15 275 MV .125" O.D. No. 20 Teflon do. blue -- 31 MV
.085" I.D. No. 24 Teflon do. blue -- 275 MV
__________________________________________________________________________
* * * * *