Strain sensitive cable

A strain sensitive coaxial cable is disclosed as comprising a centrally dosed conductor about which is disposed a first insulating layer, and a second conductor disposed coaxially about the center conductor, configured as a helix and dimensioned to loosely fit with respect to the first dielectric layer and to permit relative axial movement generally between the second conductor and the first dielectric layer, whereby more uniform signals, and elastic-inelastic strain discrimination are provided and spurious outputs eliminated. An outer protective sheath is disposed about the helical turns of the second conductor. Another form includes using spaced mechanical means such as collar members at spaced intervals frictionally affixed to both the dielectric layer around the center conductor and the second helical conductor to prevent movement of those frictionally engaged spaced portions of the second conductor with respect to the dielectric layer around the center conductor, thereby reducing the possibility of providing false or spurious outputs due to strain relief of the cable.

BACKGROUND OF PRIOR ART 
The present invention pertains to intrusion detection systems and in 
particular to coaxial strain sensitive cables responsive to energy applied 
thereto to provide an output signal indicative of the presence of an 
intruder when used in such systems. 
Intrusion detection is becoming more important, both in military and 
civilian applications. Military bases and other installations, e.g. 
factories, power stations, stores and even private homes, are being 
protected by sophisticated electronic surveillance systems which are 
capable of monitoring the periphery of such a facility to provide a 
manifestation or warning indicative of the presence of an intruder. One 
type of such an intrusion detection system incorporates a sensing element 
in the form of a coaxial cable concealed in the ground and disposed about 
the outer edge or perimeter of the area to be protected. The sensing 
element is sensitive to the presence of the intruder of whatever type and 
is responsive to the movement of a person or object approaching the 
periphery, to produce a manifestation indicating such an intrusion. 
In the prior art, intrusion detection sensors have been developed which 
recognize magnetic field surface disturbances to intercept and detect 
non-metallic objects moving across the periphery of the area to be 
guarded. An example of the patents disclosing such sensors in the form of 
a cable is U.S. Pat. No. 4,001,745 of Goodman. Such a magnetic field 
surface disturbance sensor typically includes a central core extending 
along an axis having a plurality of wires formed as coils disposed about 
the core with a jacket disposed to protect the entire aforementioned 
assembly. It is disclosed that the coils are wound with a predetermined 
tightness about the core dependent upon the expected seismic sensitivity 
to prevent extraneous activity, such as thunder or explosion, from 
producing a magnetostrictive signal. Similar sesnors are also disclosed in 
U.S. Pat. No. 4,166,264 of Starr and U.S. Pat. No. 3,747,036 of Erdmann. 
Further, U.S. Pat. No. 4,206,451 of Kurschner discloses a system adapted to 
receive the output of a sensor as disclosed in U.S. Pat. No. 3,747,036 to 
filter and supply the output to a logic circuitry, which detects the 
amplitude of the input signal and its zero crossing history to determine 
whether the magnetically detected signal is that of an intruder. A further 
type of cable sensor is disclosed in U.S. Pat. No. 3,610,808 of Horwinski. 
This cable sensor includes a coaxially configured wire braid made of close 
woven wires, each such wire being insulated from the other and connected 
in a grid to form a continuous trigger circuit to be connected to a 
detection device providing an indication of any alteration of the trigger 
circuit characteristics. Thus, if excessive stress breaks one of the wires 
of low ductility of the trigger circuit, the detection device provides an 
indication of an abnormal condition. 
In addition to those sensors which sense a change of the magnetic field or 
the establishment of an open or short circuit condition within a cable, 
there are strain sensitive coaxial cables comprising a center conductor, 
and a first dielectric layer and a second conductor in the form of a 
braided wire or a solid cylindrically shaped conductor both disposed 
coaxially about the center conductor. This strain sensitive cable is 
provided with a protective shield and is responsive to the application of 
stress as by an intruder to provide a signal upon the central conductor 
which may be detected to provide an alarm indicating the intruder's 
presence. Although the precise nature of the operation of such cables is 
not fully understood, it is theorized that the application of a pressure 
or force to such a coaxial cable results in relative movement between the 
second coaxial conductor to establish a charge upon the first dielectric 
layer. The charge is capacitively coupled to the center conductor to 
produce a signal therein that may be detected by a detection circuit to 
provide an alarm indicating the presence of an intruder. Problems have 
developed with such coaxial cables, whereby spurious output signals from 
the center conductor may be produced even in the absence of an intruder. 
It is believed that due to the structure of the solid or braided second 
conductor, that the second conductor does not move freely with respect to 
the first dielectric layer with the result that as the cable or associated 
structure relaxes with age, i.e. strain relief, relative movement occurs 
between the second conductor and the first dielectric layer to produce an 
undesirable spurious output signal. 
U.S. Pat. No. 3,963,854 of Fowler and U.S. Pat. No. 4,131,758 of Felkel 
each relate to coaxial, shielded cables for transmitting data or power as 
opposed to sensing strain to provide an output signal indicative thereof. 
The significance of each of these patents is that it relates to cables 
having a center conductor about which there are disposed helically wound 
conductors. However, neither of these references disclose a cable that is 
adapted to sense the imposition of strain thereto and to permit relatively 
free movement between its helically wound conductors and its dielectric 
layer covering the center conductor. 
BRIEF SUMMARY OF THE INVENTION 
It is an object of this invention to provide new and improved strain 
sensitive coaxial cables in which the problems associated with the strain 
relief as the cable relaxes, are substantially overcome. 
It is a more particular object of this invention to provide new and 
improved strain sensitive coaxial cables having a higher output level and 
improved signal to noise ratio, as well as an improved uniformity in the 
linearity of its output signal with respect to the application of a force 
to the cable. 
It is a more particular object of this invention to provide a new and 
improved strain sensitive coaxial cable wherein the second or outer 
conductor is so configured and formed to move relatively freely in the 
axial direction but relatively stiff in the radial direction or plane of 
the center conductor and cable as a whole whereby an improved amplitude 
and uniformity of output from the cable is obtained. 
In accordance with these and other objects of the invention, there is 
provided a strain sensitive coaxial cable comprising a centrally disposed 
conductor about which is disposed a first insulating layer, and a second 
conductor disposed coaxially about the center conductor, configured as a 
helix and dimensioned to loosely fit with respect to the first dielectric 
layer and to permit free relative axial movement between the second 
conductor and the first dielectric layer. An outer elastomeric sheath is 
disposed about the helical turns of the second conductor. 
In one aspect of this invention, retaining means are disposed at regular 
intervals along the length of the coaxial cable to prevent movement of the 
second conductor with respect to the dielectric layer due to strain 
release as the coaxial cable ages. One form of the retaining means 
illustratively shown in FIG. 2 comprise a cylindrical member or collar 
disposed as spaced intervals between the first dielectric layer and the 
second helically shaped conductor to prevent movement of the spaced 
portions of the second conductor with respect to the first dielectric 
layer and therefore reduce the possibility of providing false outputs due 
to strain relief. Another form of retaining means is illustratively shown 
in FIG. 2A, wherein the cylindrical collar members are omitted, and spaced 
portions of the second or outer conductor are crimped in a retaining 
relationship against the dielectric layer of the center conductor. 
In a still further aspect of this invention, detection means are provided 
to detect and distinguish the presence of an intruder as opposed to a 
false output as would be produced by the normal aging or strain relief 
action of a coaxial cable or the surrounding media. More specifically, the 
detection means senses an elastic strain induced signal as produced by an 
intruder momentarily exerting a force upon the strain sensitive coaxial 
cable of this invention, i.e. that bipolar signal as produced by a first 
axially directed movement between the helically shaped second conductor 
and the first dielectric layer and a second return movement to the initial 
position. The signal is applied to the detection means comprising 
amplifying means for converting the input current signal to a voltage 
signal and amplifying the voltage signal to a level that is capable of 
being detected, first threshold detection means in the form of a threshold 
detector for detecting a positive going signal in excess of a 
predetermined level, second threshold detection means for detecting 
negative going signals above a predetermined level, and means for 
providing an output when the input bipolar signal exceeds both a negative 
and positive threshold level as sensed by the first and second threshold 
detection means. In this manner, a more accurate indication can be given 
of whether an intruder is present or the output in due to an inelastic 
stimulus such as a strain relief of cable or surrounding media.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings and in particular to FIG. 1, there is shown a 
strain sensitive coaxial cable 10 in accordance with the teachings of this 
invention and comprising a center conductor 12 about which is disposed 
concentrically a first layer 14 of a suitable solid dielectric material 
such as a polytetraflourethylene known under the trademark of DuPont as 
TEFLON. The center conductor 12 may illustratively be made of any 
electrically conductive material such as copper or steel. In accordance 
with the teachings of this invention, there is disposed a second helically 
configured conductor 16 that is disposed loosely about the dielectric 
layer 14 and coaxially of the center conductor 12. The second conductor 16 
is loosely spaced with respect to the dielectric layer 14 within the 
di-electric space between it and an outer sheath to permit the lateral or 
axial non-binding movement of the helically configured conductor 16 with 
respect to the dielectric layer 14 thus increasing the sensitivity, the 
signal to noise ratio and the uniformity of the resultant output signal 
that appears upon the center conductor 12, as will be explained. In a 
further aspect of this invention, a third conductor 17 may be wound in a 
second direction opposite to that of the first helical conductor. The 
third helically configured conductor 17 spaces the first helically 
configured conductor 16 from an outer sheath 20, thus further ensuring 
freedom of axial movement of the second helically shaped conductor 16 with 
respect to the dielectric layer 14. The second and third helically 
configured conductors are made of an electrically conductive material 
having a high degree of elasticity such as stainless steel. The sheath 20 
may be made of a suitable dielectric material such as polyurethane. The 
first helically shaped conductor 16 is formed with a pitch which is not 
critical but typically is in the range of 11/8D to 11/4D (D being the 
diameter of the conductor 16) to maintain its radial dimensions under 
normal stress while providing a high degree of flexibility in the axial 
direction. By maintaining the pitch, the axial spacing between successive 
turns of the helically configured conductor 16 is maintained in the range 
of 1/8D to 1/4D, where D is the diameter of the conductor 16. If the 
conductor 16 is configured with a lesser pitch, there results an 
insufficient axial movement between the conductor 16 and the dielectric 
layer 14. The outer diameter of the first dielectric layer 14 is made 
slightly less, typically 0.005 to 0.01 of an inch, then the inner diameter 
of the helically configured second conductor 16. Care is taken during the 
forming either by winding or helically slitting a solid tube to maintain 
smooth the inner surface of the conductor 16 to ensure its relative 
movement with respect to the dielectric layer 14. 
It is contemplated that the strain sensitive coaxial cable 10 may be 
disposed along the perimeter of the area to be protected. Illustratively, 
a V-shaped trench is dug into the earth to a depth in the order of eight 
inches and thereafter an extended length of the cable 10 disposed therein. 
Though only a short segment of the cable is illustrated, it is 
contemplated that an extended length of the cable 10 may be used. The soil 
is placed about the cable 10 and is packed to eliminate any voids. When 
extended lengths of the strain sensitive coaxial cable 10 are used, a 
current type of amplifier presenting a relatively low impedance to the 
output of the cable 10 as appears at terminals 24 and 26, is used to 
amplify the small currents typically in the order of 10.sup.-7 amps and to 
provide a voltage output in the order of 1 volt. In addition, the 
contemplated current type amplifier prevents the added coupled capacity of 
an extended length of the cable 10 from adversely affecting the current 
output signal. 
There is provided by the structure of the strain sensitive coaxial cable 10 
as described with respect to FIG. 1, a more effective and reliable means 
of detecting an intruder. First, there is less static friction created 
between the conductor 16 and its dielectric layer 14 thus creating a 
higher sensitivity to the pressure imposed by an intruder and increasing 
the signal to noise ratio of the cable's output signal. Secondly, the 
sheath 20 may shrink longitudinally with age imposing a further pressure 
on the conductor 16; even so, the structure of the cable 10 permits the 
relatively free movement of the conductor 16 with respect to its 
dielectric layer 14 thus assuring the relatively high sensitivity and 
uniformity of output signal. Further, the effects of strain imposed on the 
second conductor 16 induce erratic movement between the conductor 16 and 
its dielectric layer 14 as occur when these elements are not free to move 
with respect to each other and the resultant strain on the second 
conductor 16 builds up to a large level causing an erratic movement to 
impose a spurious output. To prevent this erratic movement, the coaxial 
cable is configured and dimensioned to permit the relative movement 
between the first dielectric layer 14 and the second conductor 16. In 
addition, the cable 10 is highly resistant to crushing radially directed 
forces, while providing a high degree of flexibility in bending and 
tensile modes. 
In FIG. 2, there is shown a further embodiment of this invention, where 
similar elements are identified by similar numbers except in the one 
hundred series. Additionally, there is shown mechanical retainers in the 
form of cylindrically shaped members 128 disposed between the dielectric 
layer 114 and the helically wound conductor 116. In particular, the 
members 128 are formed as by clamping about the first dielectric layer 114 
and may be made of a suitable dielectric material such as polyvinyl. The 
second helically shaped conductor 116 is formed tightly about the members 
128 to prevent the axial movement of the conductor 116 with respect to the 
dielectric layer 114 at the points where the members 128 are placed. Thus, 
the strain imposed on the cable 110 as it ages, is suppressed, and erratic 
movement of the conductor 116 with respect to its dielectric layer 114 is 
inhibited from producing a false output indicative of the presence of an 
intruder. An alternative modification of this embodiment is illustrated in 
FIG. 2A, wherein primed reference numbers are used for the parts 
designated by unprimed counterpart reference numbers in FIG. 2. In this 
FIG. 2A embodiment, the aforesaid members 128 may be deleted and the 
spaced portions 115 of the conductor 126' crimped in a retaining 
relationship against the dielectric layer 114'. 
In FIG. 3, there is shown a functional block diagram of means in the form 
of a circuit for detecting the presence of bipolar signals appearing upon 
the output terminals 26 and 24 of the cable 10 as shown in FIG. 1. As 
briefly indicated above, in the presence of an intruder, a force is 
exerted upon the ground surrounding the cable 10 whereby the second 
helically configured conductor 16 tends to move in a first direction and 
upon release of the force, the conductor 16 tends to move in a second, 
opposite direction to its initial position. Thus, due to what is believed 
to be a tribo-electric effect, a first electrostatic charge is induced in 
the dielectric layer 14 of a first polarity and upon the return of the 
conductors 16 to its first position, a charge of an opposite polarity is 
induced therein. The charges of opposite polarity in turn induce a bipolar 
signal in the conductor 12 which is coupled as shown in FIG. 3 to a charge 
amplifier 30. The frequency of such a signal as effected by human 
intrusion is in the order of 0.2 to 2.5 Hz. Though not described in detail 
herein, it is contemplated that the frequency of the signal may be 
detected to provide an indication of the nature of the intruder. The 
output of the charge amplifier 30, typically in the order of 1 volt, is 
serially applied to amplifiers 32 and 34 each imparting a gain 
illustratively in the order of 40 db. As shown in FIG. 3, the output of 
amplifier 34 is a bipolar voltage signal and is applied to each of a first 
positive threshold detector 36a and a second negative threshold detector 
36b. Upon detection of a positive going signal above a pedetermined 
variable level, the threshold detector 36a provides an output to a first 
one-shot multi-vibrator 38a which provides an output of a predetermined 
interval, e.g. 3 seconds. Similarly, if a negative going signal of a 
predetermined variable amplitude is detected by the threshold detector 
36b, an output is applied to trigger a second one-shot multi-vibrator 38b, 
which provides an output of a duration similar to that of the first 
one-shot multi-vibrator 38a. 
As shown in FIG. 3, both outputs of the multi-vibrators 38a and 38b are 
applied to an AND gate 40 which detects the coincidence of the 
multi-vibrator outputs to provide its output indicating the detection of a 
bipolar signal as derived from the strain sensitive coaxial cable 10. The 
AND gate output is in turn applied to a one-shot multi-vibrator 42 which 
provides its output of a predetermined duration, e.g. 0.5 seconds, to an 
emitter follower 44 which performs a buffer function between the 
aforementioned circuit and a suitable alarm utilization device 46. 
Typically, the alarm utilization device 46 may take the form of an audible 
or light emitting device that is activated upon the detection as described 
above of the bipolar signal to alert suitable personnel to the presence of 
an intruder. Alternatively, the output of the emitter follower 44 could be 
applied to a distant station as by telephone lines to provide the warning 
manifestation. 
It is contemplated that the subject system may be used in other 
applications other than for detection of physical stimuli produced by 
cultural or natural sources. For example, in earth structures such as 
water dams (dykes), built up roads, banks, fault areas, etc., it is often 
desirable to monitor the amplitude and shifts in critical portions to 
determine structure degradation to predict incipient failures. The unique 
capability of this system to detect and discriminate elastic and inelastic 
strains permits collecting data from which this vital information can be 
extracted. For example, in the case of an earthen dam, high incidence of 
inelastic earths shifts of significant amplitude would indicate an 
incipient catastrophic failure. Likewise the subject system is applicable 
of monitoring fixed and mobile structural elements including aircraft, 
bridges, pipelines, towers, cranes, roads, stairways, walls or other 
structural elements possibly subject to failure due to excessive loading 
or intrusions. This system is capable of detecting the amplitude and 
frequency of elastic strains which permits prediction of fatigue 
weakening, and of detecting the amplitude and frequency of inelastic 
strains, which permits prediction of structure degradation which could 
lead to catastrophic failure. Further, this system is also capable of 
monitoring normal structural functions including pressure changes in 
hydraulic systems, vibrational functions of machinery, seismic/sonar 
activity, speed/weight of vehicles, frequency and amplitude of events of 
moving masses, and acceleration. 
Thus, there has been described a strain sensitive cable for sensing the 
presence of a physical stimuli comprising a center conductor, a first 
insulator disposed concentrically thereabout and a helically configured 
conductor spaced therefrom to provide relatively free movement between the 
dielectric layer and the helically configured conductor whereby 
inadvertent movement due to strain relief and aging will not produce 
erratic signal outputs from the cable. Physical stimuli due to human 
intrusion produce bipolar signals which are sensed by first and second 
detector means responsive to the negative and positive going swings of 
such bipolar signals. AND gate means is used to detect the coincidence of 
the outputs of such detector means to provide a manifestation indicative 
of the bipolar signal and thus human intrusion. 
While specific embodiments have been illustrated and described herein, it 
is theorized that modifications and changes will occur to those of skill 
in the art. It is therefore to be understood that the appended claims are 
intended to cover all such modifications and changes which fall within the 
true spirit and scope of the invention.