US3803548A

US3803548A - Fence tampering alarm system

Info

Publication number: US3803548A
Application number: US00172912A
Authority: US
Inventors: R Skujins
Original assignee: US Department of Army
Current assignee: US Department of Army
Priority date: 1971-08-18
Filing date: 1971-08-18
Publication date: 1974-04-09
Anticipated expiration: 1991-04-09

Abstract

The alarm system of this invention is intended primarily for use in conjunction with metallic fences and will detect the act of cutting the fence, climbing the fence, and certain forms of digging under the fence. The alarm system is useful in unattended applications, that is, visual monitoring of the system is unnecessary. The intrusion detection system of this invention embodies an external noise sensitive elongated transducer element which may be mechanically attached to a metallic fence along its length such that mechanical vibrations (noise) associated with the cutting of the fence may be conducted to the elongated transducer. The transducer is a two conductor device of coaxial design with a high impedance dielectric material, such as Teflon, separating the two conductors. The pulse signature output of the transducer, taken across the conductors, is processed in an electronic signal detection means to trigger an alarm.

Description

Skujins, Jr.

0R iateassme United St:

i 1 FENCE TAMPERING ALARM SYSTEM [75] inventor: Romans Skujins, Jr., Clifton, Va.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

22 Filed: Aug. 18,197]

21 Appl. No.: 172,912

[52] US. Cl. .L 340/15, 340/261 [51] Int. Cl. H04b 11/00 [58] Field of Search 340/261, 253, 254, 15

[56] References Cited UNITED STATES PATENTS 2,403,503 7/l946 Coulter. 340/261 3,610,808 10/1971 Horwinskin". 340/261 2,345,771 4/1944 Reynolds 340/261 Primary Examiner-Benjamin A. Borchelt Assistant ExaminerHt J. Tudor Attorney, Agent, or Firm-Edward J. Kelly; Herbert Berl; Glenn S. Ovrevik 1111 3,803,548 [455?] Apr. 9, i974 [57] ABSTRACT The alarm system of this invention is intended primarily for use in conjunction with metallic fences and will detect the act of cutting the fence, climbing the fence, and certain forms of digging under the fence. The alarm system is useful in unattended applications, that is, visual monitoring of the system is unnecessary. The intrusion detection system of this invention embodies an externauqise s e n sitive elongated transducer element which may be mechanically attached to a metallic fence along its length such that n eghanicgl vibrations (noise) associated with the cutting of the fence be conducted to the elongated transducer. The

transducer is a two conductor device of coaxial design with a high impedance dielectric material, such as Teflon, separating the two conductors. The pulse signature output of the transducer, taken across the conductors, is processed in an electronic signal detection means to trigger an alarm.

I 8 Claims, 6 Drawing Figures I -aiziifatfizaitziz DETECTOR 1 KT ELECTRONlCS MODULE U DETECTOR POWER steam/mus PM I ATENTEDAPR 9 IBM sum 1 [IF 3 P POWER PM TERMINATION DETECTOR MODULE DEVICE- 15 i 22; DETECTOR ELECTRONICS MODULE ELECTRONICS ALARM I TERMINATION DEVICE POWER PAK ELECTRONICS MODULE 34 DETECTOR ALARM 1N VENTOR 8 V .lv rU S germ m dr o V r wlnu FW udel HE G PATENTEUAPR 9 m4 SHEET 2 BF 3 MmIIm2 j N55 Q SEE F ESE PATENTED R 9 I974 SHEET 3 [IF 3 Fig. 6

INVENTOR Romans Skujms, Jr".

Harry M. Sara avifi Edward J. KelL Herbert Berl Glenn 6'. Ovrevik FENCE TAMPERING ALARM SYSTEM BACKGROUND OF THE INVENTION This invention relates to devices for the detection of intruders entering an enclosed area having a perimeter defined by a metallic fence structure. A wide variety of devices have been developed for such detection purposes including various crystal and electromagnetic transducer devices adaptedto be rigidly attached to the fence. In one systemembodying individual detectors, described in U.S. Patent No. 2,345,771,'the metallic fence is attached to metallic fence posts and the transducers sense vibrations transmitted to the posts. In another system, described in U.S. Patent No. 3,487,396, a pluralityof sensors are attached to fence elements in both rigid and semi-rigid fashion and electronic circuitry is provided to discriminate between the outputs ,of variously disposed sensors. In an alternate assembly which is more analagous to the present invention, reference U.S. Patent No. 2,403,503, a taut piano wire stringer is attached to the fence and the transducers are attached to the taut wire.

It will be appreciated that highly sensitive systems have been developed and that as the sensitivity of most systems is increased, the frequency of false alarm occurance also increases. Likewise, it will be recognized that the complexity and the expense of alarm systems which utilize a plurality of discrete point sensors generally increases with increased sensitivity.

It is an objectof this invention to provide a relatively simple and low cost alarm system with minimum sensor associated electronics, which is readily adaptable to existing metallic fences.

It is another object of this invention to provide a uniformly sensitive system for sensing sonic signal spectra generated. by acts such as cutting of the fence.

It is a further object of this invention to provide a highly sensitive. alarm system which permits discrimination between natural and man-made false alarm stimuli.

It is also an object of the invention to provide an alarm system which discriminates against acoustic background noise and noise of seismic origin.

SUMMARY OF THE INVENTION The fence tampering alarm system of. this invention embodies a uniformly sensitive transducer cable which serves to minimize the distance that the sonic impulse signal (generated by cutting action or the like) has to travel prior totranslation into an electrical signal. In this manner, the characteristics of the cutting signatures are preserved: sufficiently to permit discrimination between natural and deliberate man-made alarm stimuli with a minimum signal processing effort. The shielded aspect of the cable transducer construction minimizes the effect. of electromagnetic noise and the low air-to-metal coupling coefficient involved, minimize the effect of acoustic background noise.

The alarm system electronics are primarily designed to detect relatively high frequency signal pulses in the 500 Hz to 6,000 Hz frequency band. Such pulses are produced. during the act of cutting fence wires. The higher frequency pulse signal is generated as a result of a suddenrelease of: stresses in the wire at the moment thecut: is completed. The higher frequency pulse signal is especially pronounced in chain link fence applications. It has been found that this higher frequency pulse signal is attenuated in proportion to the distance the signal travels through the fence wire network. In accordance with the invention, the uniformly sensitive transducer cable is rigidly attached, at spaced intervals, to the fence wire network with each end of the cable transducer terminated in an appropriate termination means to stabilize sensitivity. The disposition of attachments, of course, determines the distance the high frequency must travel through the fence to reach the cable transducer. A signal processing electronics module responsive to pulse energy is attached to one end of the transducer cable.

A better understanding of the invention will be gained by a review of several embodiments of the invention for which reference is had to the description which follows and the illustrations depicted in the attached drawings wherein:

FIG. 1 depicts a single ended alarm system designed to monitor a fence segment.

FIG. 2 depicts a differential signal embodiment of the alarm system designed to monitor a large fence segment.

FIG. 3 is a block diagram of a preferred embodiment of the electronics module for use with the system of FIG. 2.

FIG. 4 is a cutaway view of a first form of uniformly sensitive transducer cable which may be utilized in this invention.

FIG. 5 is a cutaway view of a second form of uniformly sensitive transducer cable which may be utilized in this invention.

FIG. 6 is a graphical presentationv of various pulse information developed in the translation of noise to an electrical signal which is introduced to the electronics module for discrimination processing in the alarm system of this invention.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 depicts a chain link fence segment lll monitored in accordance with the system of this invention which embodies a single, uniformly sensitive, transducer cable 12 which is mechanically attached to the fence wire by ties 113 at spaced intervals. The cable 112 is terminated at the end remote from the detector electronics module 14 by an appropriate impedance termination means 15. The impedance termination means enables a tamper monitoring of the transducer cable as will be described hereinafter. A power pak I6 is connected to the detector electronics module as an energy power source therefor.

While the cable 12 is shown in an earth buried application, it will be appreciated that the cable may be attached to the fence above ground, if desired. Indeed, it has been found that the uniformly sensitive cable fence tampering system of this invention may be utilized advantageously irrespective of the fence environment, for example, the system may be used in above ground applications and in underwater metal fence barrier applications as well.

FIG. 1 also depicts a second fence tampering alarm system of the point sensor variety comprising a plurality of individual sensors 21, for example, xtal devices as shown in U.S. Patent No. 3,478,343, and a detector electronics module 22 interconnected by wiring, indicated at 23. It will be recognized that the second fence tampering alarm system is entirely independent of the basic uniformly sensitive transducer cable system and is not essential to the operation of the basic cable system. It has been found, however, that the two types of fence tampering alarm systems are compatible and may be utilized together as shown to optimize false alarm rejection due to noise sources other than an intrusion.

I embodiment serve to preserve the broad band characteristics of each transducer cable and to impedance match the two transducer cables, and a power pak 16 is connected to the detector electronics module 34 as an energy power source therefor.

It will be noted that FIG. 2 depicts an aboveground connection of the transducer cable to the fence. As in the embodiment of FIG. l,the precise disposition of the uniform impedance transducer cable is not critical to the invention. Likewise, the number of rigid connections of the transducer cable to the metallic fence may be varied to meet practical requirements. That is, the ties 13 may be applied to rigidly fasten the cable to the concertina fencing in the embodiment of FIG. 2 wherever a cable-fence wire crossing occurs. It will be recognized that the present invention is readily adaptable to a variety of different metallic fences and that fence supports are not essential to the operation of the device. In other words, the transducer cable may be. attached to the fence in a taut fashion or merely draped on the fence as shown in FIG. 2.

FIG. 3 shown a typical embodiment of the detector electronics module 34 in a block diagram schematic form. In the illustration of the typical embodiment, the electronic detection circuitry is connected to two uniformly

sensitive cables

32 and 33 of the coaxial variety and each coaxial cable is terminated by a resistive impedance termination means 15. Obviously, any appropriate type of simple or complex impedance termination means may be substituted for the resistive device shown.

The input of the exemplary detector electronic module may be a transformer, as shown at 41, with a center tapped input winding. As the outer conductor of each of the

transducer cables

32 and 33 is grounded, the center tap of the input winding may be DC grounded or AC grounded through a capacitor 42, as shown. For reasons which will become apparent hereinafter, the capacitor 42 and resistor 43 may be connected in series as a voltage divider across the battery 44.

The output winding of the transformer 41 is connected to audio amplifier 45 via switching means 46 and either high band-pass filter 47 or low band-pass filter 48. Typically, the

filters

47 and 48 might have bandwidths 3,0006,000 I-Iz (i3db) and 500l,500 Hz (i3db) respectively, with each having a band-pass selectivity skirt at a rate of attenuation of l8db/octave beyond its specified frequency range.

The signal input to the audio amplifier 45 which passes the

filter

47 or 48 has an envelope, as shown, and in processing through the amplifier 45, the ampliclip high amplitude short duration noise spikes such that the output thereof has a waveform envelope, as shown, of predetermined maximum amplitude and significant duration.

The output of the limiter 49 is applied to a rectifier circuitry 51 which serves as an envelope detector to produce a square wave pulse output, as shown, of significant amplitude and duration. The output of rectifier 51 is applied to an integrator 52, which may be a simple RC network, which produces a pulse with a constant rise time and an amplitude proportional to the amplitude and duration of the input signal thereto.

Thereupon, the pulse output of the integrator 52 is applied to a level detector 53 which produces an alarm pulse in the event the amplitude of the pulse output of the integrator exceeds a predetermined level. It will be appreciated that the level detector 53 may be a signal comparator means and may include a threshold reference signal source, not shown, for comparison purposes.

The alarm pulse may be utilized in any conventional manner to energize the alarm 54 which may be disposed in a remote location, if desired. Typically, the alarm pulse is applied through an intermediate alarm interface 55 which also affords means 56 for detecting any battery or case tampering which could inactivate the system and also may include a test command circuitry 59 for periodic checking of the operational state of the detector electronics module. It will be recognized that any form of battery or case tampering signal in voltage occurs at the centertap connection and this change in voltage may actuate the tampering detection means 57, as prescribed.

For optimum performance, the detector electronics module embodiment of FIG. 3 should be. relatively insensitive to variations in ambient temperature and power supply voltage deviations. It has been found that the effect of such deviations can be minimized by the selective use of therrnistors and/or other compensating circuit techniques. It will be recognized that recovery time, that is, the time required to reset to detect a second intruder should be short, i.e., within several seconds after a saturating signal. The fence tampering alarm system of this invention is intended for long term, maintenance free operation, for example, one year, and it will be recognized that a low current drain requirement which extends battery life is essential.

While an exemplary embodiment of the detector electronics has been shown in FIG. 3, it will be appreciated that various other detector circuitry which will accept a damped oscillation signal and reject other spurious signals, such as high amplitude, short duration noise spikes, may be substituted, if desired. In other words, the detector electronics should accept the signal produced by a cutting action and produce an alarm but should not be responsive to such action, for example,

as mechanical impact from dropping a regulation tennis ball on the detector electronics module from a height of one meter while the module is resting on a two inch thick moist cellulose sponge, nor responsive to placement within a Helmholtz coil test unit with 60 cycle current applied to coil unit.

Moreover, it will be seen that the switching means 46 is not essential to the preferred embodiment of FIG. 3, and that if continuous monitoring at both frequency levels is desired, each of the

filters

47 and 48 may be connected to its respective transformer and transducer cable pair assembly with the output of the filters connected to the input of the audio amplifier 45 through a conventional OR circuit, not shown.

The uniformly sensitive cable transducer shown in FIG. 4 is a conventional coaxial cable having a braided sleeve type outer conductor 61 covered by a weather resistant plastic sheath 62 which may be a plastic material, polyvinylichloride or the like. The center conductor 63 which may be a stranded wire, as shown, is separated from the outer conductor 61 by a high impedance dielectric 64. The dielectric 64, in most commercially available coaxial cable, is of the Teflon variety (Teflon is a trademark held by E. I. duPont de Nemeurs & Co. for identification of tetrafluoroethylene material.) The invention is not, of course, restricted to the use of transducer cable with this commercially available dielectric. The dielectric 64 may be formed on the center conductor by any conventional method such as extrusion, wrapping, coating or the like.

It has been found that any commercially available coaxial cable may be utilized as the uniformly sensitive cable transducer in the system of this invention. Some coaxial cable is more noise sensitive than other coaxial cable and for purposes of the present invention, the more noisy coaxial cable is preferred. The noise sensitivity of any particular coaxial cable is dependent generally on the method of manufacture of the cable. Most manufacturers attempt, with varying degree of success, to manufacture by methods which minimize the noise sensitivity characteristic. It has been discovered that the noise sensitivity of conventionally manufactured coaxial cable can be enhanced for the purpose of the system of this invention by subsequent cable processing. A useful method for subsequently processing the coaxial cable is described and claimed in the copending patent application, Ser. No. 111,291 which was filed Feb. 1, 1971 in behalf of Charles F. Burney, et al. The invention described and claimed in the aforesaid copending patent application was made under a Department of the Army contract.

The noise sensitivity characteristic of coaxial cable has been theoretically explored and appears to be either magnetostrictive or triboelectric in nature. Irrespective of the phenomenon involved however, it has been found that acoustic pressure incident upon the uniformly sensitive cable transducer at its points of rigid connection to the metallic fence produce a physical disturbance at such points which excites the transducer cable and thus generates a significant voltage deviation across the transformer connected end of the transducer cable. It will be appreciated that the uniformly sensitive transducer cable with a resistor termination is, in effect, a lightly charged transmission line which may be represented as a resistive impedance in parallel with a plurality of capacitors in parallel connection. When the acoustic pressure deforms the cable at the rigid connection points, the heretofore relatively quiescent transmission line is activated and current flow within the lightly charged transducer cable produces an output voltage across its transformer connected terminals. Enhancement ofthe noise sensitivity, in essence, involved increased development of an electrically charged condition in the dielectric separating the center and outer conductors. It will be appreciated that for use as a transducer in the present fence tampering application, the charged condition must be maintained over an extended period of time. It has been found that a wide variety of coaxial cable structures may be adapted for use in the system of this invention and that the particular dielectric utilized determines the degree and nature of the energization required to produce a charged condition which will be maintained in the dielectric over an extended period of time.

For example, FIG. 5 depicts a commercially available flexible delay line of the coaxial variety which has been successfully employed in the system of this invention. In FIG. 5, the center conductor 71 of the coaxial cable structure is would in helical form with minimum pitch about a flexible powdered iron core 72 and the outer conductor 73 consists of parallel wires wound about the dielectric 74 in spiral form with a high pitch to the spiral winding. The dielectric 74 may be, for example, a polyethylene or other low loss dielectric which is applied in the form of overlapping tapes, as shown. A strong, weather resistant polyvinlychloride jacket 74 is extruded over the outer conductor 75. The particularly described cable is commercially identified as a DELAX delay line and are produced by Kabel-und Metallwerke, 3000 Hannover, Postfach 260, a German manufacturer. DELAX cables are distributed-parameter delay elements and as such do not exhibit a cut off frequency typical of the conventional lumped parameter delay line design. Rather, the attenuation increases gradually with increasing frequency. It has been found that the impedance and bandwidth characteristics of this type of coaxial cable are suitable for use in applications involving cable transducer lengths of 50 feet. In installations utilizing DELAX type III-14000 cable, tests have been conducted in conjunction with a four foot high chain link fence which have demonstrated that the system will detect at least percent of all fence cuts with no false alarms in a high road noise and 40 mph wind environment. Such results were obtained with conventionally manufactured DELAX type HH4000 cable without additional processing of the cable to enhance the energization of the dielectric.

It will be appreciated that in both the braided outer conductor construction shown in FIG. 4 and the parallel wire spiral outer conductor construction shown in FIG. 5, the electrically energized dielectric of the transducer cable is omnidirectionally shielded from extraneous electrical noise. Thus, such extraneous noise does not physically disturb the internal structure. It has been found that the effect of substitution of one form of cen ter conductor for another, stranded, solid, helix wound or the like, is minimal.

FIG. 6 illustrates noise pulse signals typically encountered which must be discriminated and excluded in normal operation of the system of this invention.

The high level, short duration pulse indicated at 81 in FIG. 6 is generated by electrical transients in the vicinity of the fence. Integration of the pulse 81 by the system of this invention produces an output pulse of constant rise time, indicated at 84, which is insufficient in magnitude to trigger the alarm.

The initially high level, damped oscillation pulse indicated at 82 in FIG. 6 is generated by a strong cut of the wire fencing material. Integration of the pulse 82 by the system of this invention produces an output pulse of constant rise time, indicated at 85, which is of sufficient magnitude to trigger the alarm. Likewise, the weak cut, damped oscillation pulse, indicated at 83, produces, when integrated by the system of this invention, an output pulse of constant rise time, indicated at 86, which is of sufficient magnitude to trigger the alarm.

It will be recognized that in adapting the system of this invention to an existing fence installation, the cable transducer may be rigidly attached to the fence by a variety of connection means. For example, the cable may be attached by wire hog rings using pliers or by a simple wire twist which enables quick attachment of the cable transducer to the metal fencing at selected, spaced, intervals.

While each of the coaxial cable transducers described herein may be constructed by attachment of suitable termination means to coaxial cable which is readily available in the commerical market, it will be appreciated that the invention is not restricted to the use of conventional coaxial cable. For example, the cross-section configuration may be different and/or the cable may be adapted to serve additional purposes, if desired. Likewise, the electronics employed to discriminate between wanted and unwanted signals may be modified by simplification or refinement without departure from the purview of this disclosure. For example, other analog or digitaltechniques may be utilized instead of the disclosed RC integrator. In its simplest form, of course, the transducer cable may-be directly connected to a single stage voltage level sensitive device which is adapted to trigger an alarm. I claim: 1. An intrusion detection system for use in conjunction with a metallic fenced enclosure comprising:

at least one elongated transducer cable of the coaxial cable variety having an inner conductor and an outer conductor separated by a dielectric material wherein said dielectric material is characterized by a relatively high noise sensitivity in response to low level mechanical vibration disturbance of said outer conductor thereof; means for mechanically attaching said elongated transducer cable means to the metallic fence at selected space intervals along the fenced perimeter of said enclosure such that relatively low level mechanical vibratimliillmfildamql llic rWEFe condiicfed to said outer conducto r ofs'aid transducer cable means; alarm means; electronic signal detection means electrically interconnecting said transducer cable means and said alarm r ga rgs, said signal detection means having an input means electrically connected across said dielectric material, said signal detection means adapted to detect input pulse signals and to activate said alarm means in response to selected input pulse signals produced across said dielectric material by a cutting of the metallic fence, said selected pulse signal signals having at least a selected amplitude and at least a selected pulse duration.

2. An intrusion detection system as defined in claim 1 wherein said dielectric material has an inherent static electric charged condition of sufficient magnitude to produce a measurable voltage output in response to deviations in said charged condition;

said outer conductor of said cable means being a substantially continuous metallic surface which shields said dielectric material from' external electrical field effects.

3. An intrusion detection system as defined in claim 2 wherein a cutting of said metallic fence produces a pulse output of significant amplitude and duration and said electronic signal detection means includes integrator means which produces an output of sufficient magnitude to activate said alarm means in response to said cutting of the metallic fence.

4. An intrusion detection system as defined in claim 3 wherein impedance means interconnect said innner and outer conductors at each end of each transducer cable means, one of said impedance termination means associated with each transducer cable means is an inductive coupling means and said electronic signal detection means interconnect said inductive coupling means and said alarm means.

5. An intrusion detection system as defined in claim 4 wherein said electronic signal detection means includes band-pass filter means adapted to pass frequencies within a selected band.

6. An intrusion detection system asdefined in claim 5 wherein said selected band of frequencies is in the 3,000 to 6,000 Hz range.

7. An intrusion detection means defined in claim 6 wherein two balanced transducer cable means are rigidly attachedto the metallic fence, said inductive coupling means is a transformer means with a center tapped input winding, each of said transducer cable means is connected across a respective half of the center tapped input winding of said transformer and the input of said electronic signal detection means is connected across an output winding of said transformer means.

8. An intrusion detection means as defined in claim 7 wherein means are provided for detection of system tampering, the last said means include an energized DC voltage divider network which incorporates each transducer cable means and its respective impedance termination means, and means for monitoring the DC voltage level at a selected point on said voltage divider network such that DC voltage deviations activate said alarm means.

Claims

1. An intrusion detection system for use in conjunction with a metallic fenced enclosure comprising: at least one elongated transducer cable of the coaxial cable variety having an inner conductor and an outer conductor separated by a dielectric material wherein said dielectric material is characterized by a relatively high noise sensitivity in response to low level mechanical vibration disturbance of said outer conductor thereof; means for mechanically attaching said elongated transducer cable means to the metallic fence at selected space intervals along the fenced perimeter of said enclosure such that relatively low level mechanical vibrations in said metallic fence are conducted to said outer conductor of said transducer cable means; alarm means; electronic signal detection means electrically interconnecting said transducer cable means and said alarm means, said signal detection means having an input means electrically connected across said dielectric material, said signal detection means adapted to detect input pulse signals and to activate said alarm means in response to selected input pulse signals produced across said dielectric material by a cutting of the metallic fence, said selected pulse signal signals having at least a selected amplitude and at least a selected pulse duration.

2. An intrusion detection system as defined in claim 1 wherein said dielectric material has an inherent static electric charged condition of sufficient magnitude to produce a measurable voltage output in response to deviations in said charged condition; said outer conductor of said cable means being a substantially continuous metallic surface which shields said dielectric material from external electrical field effects.

6. An intrusion detection system as defined in claim 5 wherein said selected band of frequencies is in the 3,000 to 6,000 Hz range.

7. An intrusion detection means defined in claim 6 wherein two balanced transducer cable means are rigidly attached to the metallic fence, said inductive coupling means is a transformer means with a center tapped input winding, each of said transducer cable means is connected across a respective half of the center tapped input winding of said transformer and the input of said electronic signal detection means is connected across an output winding of said transformer means.