US 3812484 A
Description (OCR text may contain errors)
United States Patent 1 Miller et al.
[ PER IMETER INTRUSION DETECTION SYSTEM Inventors: Robert C; Miller; Walter J. Carr,
Jr., both of Pittsburgh, Pa.
Assignee: Westinghouse Electric Corporation,
Filed: Oct. 10, 1972 Appl. No.: 296,376
US. Cl...... 340/258 C, 235/92 TC, 340/38 L Int. Cl. G08b 13/22 Field of Search 340/258 C, 258 D, 38 L;
 References Cited UNlTED STATES PATENTS 8/1973 Shtrikman 340/258 C 3/1973 Kardashian 340/38 L l2/l959 Mierendorf et al 324/41 Primary Examiner-John W. Caldwell Assistant'ExaminerGlen R. Swann, lll Attorney, Agent, or Firm-M. P. Lynch:
 ABSTRACT A perimeter intrusion detection system includes an AC EXCITATION ac MAGNETIC FIELD MEAS CIRCUIT  3,812,484 May 21, 1974 elongated magnetic core member with-a helical winding positioned about the magnetic core and a source of AC current applied to the helical winding. The combination of the magnetic core and helical winding is positioned adjacent to or below the surface of the earth about the perimeter of an object to be protected i.e., a building, an airplane, a military encampment, etc. Monitoring circuitry is attached to the helical winding and responds to a change in the earths magnetic field produced by the presence of a magnetically permeable object by generating an intrusion alarm signal. The sensitivity of the monitoring circuitry can be adjusted so as to produce an alarm signal in response to the presence of magnetically permeable material carried on the person of an intruder or magnetically permeable material associated with a vehicle approaching the protected object. The invention makes use of the theory of operation of a flux gate magnetometer to provide a continuous perimeter intrusion detection device capable of monitoring several hundred feet of perimeter and providing indication of the presence of a magnetically permeable object at a specific interval of the permimeter intrusion detection system.
5 Claims, 5 Drawing Figures aATENTEDHAY21 m4 EXCITATION SHEET 1 BF 2 DC. MAGNETIC FIELD MEAS CIRCUIT FIG. 3
PATENTEU IIAY 2 I 1974 SNEET 2 0F 21 AC. EXCITATION v FILTER 1 km 2 CIRCUIT Q T DETECTION CIRCUIT FILTER 42 CIRCUIT I Q DETECTION CIRCUIT A.C. EXClTATlON FILTER CIRCUrr 59 1' FIG. 4
DETECTION J4 CIRCUIT STEPPING SWITCH 72 74 R i n76 7,3
2 HARMONIC I DIFFERENTIAL AL RM i AMPLIFIER EEUR I CIRCUIT l i COMPARATOR so DRIVE 3 CIRCUIT -7| 7T TREND PERIMETER INTRUSION DETECTION SYSTEM BACKGROUND OF THE INVENTION There exists an ever increasing demand for security apparatus. A most serious security requirement is for detection systems capable of monitoring the movement of personnel and vehicles and particularly the move- SUMMARY OF THE INVENTION The perimeter intrusion detection system described herein with reference to the accompanying drawings is a perimeter device for detecting intruders or vehicles on the basis of the presence of magnetically permeable objects which produce a distrubance in the earths magnetic field. Inasmuch as most weapons, and guns in particular, are constructed from magnetically permeable material, a system capable of responding to the presence of such material provides an important security tool. A sensitive measurement of the earths field is obtained from asystem including a ferromagnetic core magnetized to saturation by a winding carrying alternating current and monitoring circuitry for measuring the second harmonic of the signal induced in the winding. This second harmonic is a measure of the DC field. The perimeter intrusion detection system described herein includes a long continuous core member which may be several feet or several hundred feet in length. The core can be a fine wire or a'thin ribbon of magnetically permeable material, such as a ferromagnetic material typically represented herein as Permalloy. A source of AC current is applied to a continuous helical winding which is wound about the elongated core to magnetize the core to saturation. Monitoring circuitry associated with the helical winding responds to the second harmonic of the signal induced in the winding as a result of the presence of a magnetically permeable material adjacent to the perimeter intrusion detection system. An alarm indication is produced when the signal is indicative of a class of objects of interest, i.e., weapons.
Several embodiments of the invention are disclosed and described to provide various degrees of sophistication in monitoring the perimeter of a protected object. While the simplest embodiment utilizes a single helical coil, the preferred embodiment utilizes a secondary coil operatively coupled to the elongated core to develop the signal indicative of the presence of a magnetically permeable object. Additional system accuracy is obtained by applying a bucking current to the system in order to cancel out the effect of the unperturbed earths magnetic field on the signal induced in the secondary winding.
Yet another variation of the invention utilizes numerous tap locations along the secondary winding in order to develop output signals indicative of specific intervals of the perimeter intrusion detection system in order to not only detect intrusion along the perimeter represented by the intrusion detection system but to further isolate the intrusion to a specific interval or intervals of the perimeter intrusion detection system.
DESCRIPTION OF. THE DRAWING The invention will become more 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 an electrical schematic of a basic configuration of the system of FIG. 1;
FIG. 3 is an alternate electrical schematic illustration of the system of FIG. 1;
FIG. 4 is an electrical schematic illustration of a particular winding arrangement of the secondary winding 7 of the embodiment of FIG. 1; and
FIG. 5 is an alternate schematic illustration of a tapped secondary winding for the embodiment of FIG.
I DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is illustrated schematically a perimeter intrusion detection system 10 comprised of four intrusion detection devices 20 positioned about the perimeter of a protected area PA herein represented as including a building in a substantially parallel relationship with the surface of the ground G. The intrusion detection device 20 is shown in three different installations, one with the device buried beneath the surface of the ground, a second with the device 20 positioned on the surface of the ground and a third configuration with the device 20 secured to a fence F. Inasmuch as each of the perimeter intrusion detection devices shown in FIG. 1 is substantially identical a description will be limited to the embodiment shown buried beneath the surface of the ground.
The intrusion detection device 20 is comprised ofan elongated core member 22 and a helical coil element 24 wound thereabout. The core element may be in the form of a tine wire or a thin ribbon of a magnetically permeablematerial, such as soft iron. The combination of the core member 22 and the helical coil element 24 is positioned within housing 26 which functions to isolate the core and helical coil from damaging contact resulting from adverse weather conditions. A typical embodiment of the intrusion detection device 20 consists of a core member of wire approximately 3/32 inches in diameter and 10 to feet in length in a helical coil of wire wherein the diameter of the wire is approximately 0.040 inches. Experimental analysis has shown that lengths varying from 10 to several hundred feet of continuous core and coil material will provide suitable signals for determining the intrusion of a perimeter by a magnetically permeable object.
An excitation circuit 30 comprised of an AC source is connected to the helical coil element 24 of the intrusion detection device 20. The excitation circuit 30 functions to'apply an alternating current to the helical coil element 24 sufficient to saturate core member 22. Monitoring circuitry 40 is attached to the helical coil element 24 to monitor the second harmonic content of the signal induced in the helical coil element 24 in response to the presence of a magnetically permeable object in the vicinity of the intrusion detection device 20. Monitoring circuit 40 is set to respond to a predetermined level of the second harmonic signal which is representative of a predetermined class of magnetically permeable objects. In the application of a perimeter inperimeter of the protected area PA are illustrated as having separate AC excitation sources, it is apparent that a single AC excitation source could be utilized to provide saturation of the core members of the various intrusion detection devices. Furthermore, the four separate intrusion detection devices of FIG. 1 could be replaced by a single intrusion detection device having a core and helical coil of sufficient length to extend about the perimeter of the protected area PA thus necessitating but one AC excitation source 30 and one monitoring circuit 40.
While the embodiment shown in FIG. 1 depicts the basic operation of the perimeter intrusion detection system 10, variations of this embodiment can be designed to eliminate the effect of changes in the earths magnetic field produced by remote conditions unrelated to the presence of magnetic permeable objects and to further provide identification of a particular interval or intervals of the perimeter intrusion detection system within which an intrusion is taking place.
Referring to FlG. 2, there is schematically illustrated the basic embodiment of the intrusion detection device of FIG. 1 including a variable DC source V a capacitor C and an inductor L connected to the helical coil element 24. The function of the voltage source V to establish a bucking or nulling current of sufficient value to essentially eliminate the effect of the unperturbed earths magnetic field on the signal transmitted to the monitoring circuit 40 in response to the presence of a magnetically permeable object in the vicinity of the intruding detection device 20. The level of the nulling current is established such that in the absence of an in trusion no second harmonic signal is transmitted to the detector circuit 44. Under these conditions, the small change in the magnetic field resulting from an intrusion is more readily detected. The capacitors C and inductors L function to provide paths for the AC and DC signals while isolating the AC and DC circuits.
The monitoring circuit 40 is shown as including a filter member 42 to effectively reject the fundamental of the voltage induced in the helical coil element 24 and a second harmonic detection circuit 44 for producing an output signal indicative of the level of the second harmonic signal developed in the helical coil element 24. The second harmonic detector circuit 44 may be implemented through the use of a phase detector circuit such as model HR8 for Princeton Applied Research. 7' I While a single winding configuration provides a workable embodiment, there is illustrated in FIG. 3 an embodiment of the intrusion detection device comprised of a primary winding 52, a secondary winding 54 and a bucking winding 56. The AC excitation source 30, typically an oscillator circuit, is connected-to the primary winding 52 and the bucking voltage source V,,,. is connected to the bucking winding 56. The combination of the filter circuit 42 and the second harmonic detection circuit 44 is connected to the secondary winding 54 and operates in a manner as described with reference to FIG. 2.
There is schematically illustrated in FIG. 4 a secondary winding 60 of the intrusion detection device illustrated in FIG. 3 wherein the secondary winding is comprised of a plurality of winding elements 60 serially connected in alternately opposed relationship. This serial arrangement of alternately opposed segments of the secondary winding functions to cancel the effect of changes in the earths magnetic field caused by remote conditions i.e., lightning, etc. This cancellation results from the fact that the oppositely connected adjacent segments of winding produce identical induced signals but of opposite polarity.
The secondary winding illustrated in FIG. 5 includes a plurality of spaced apart tap locations T. The induced signals produced in the secondary winding in the intervals defined by the adjacent tap locations T are transmitted by lead wires 62 to a sequential scanning mechanism, herein illustrated to be a stepping switch 70. The stepping switch is activated by drive circuit 71 to sequentially scan adjacent pairs of intervals of the secondary winding and applies the induced signals present in adjacent segments to differential amplifier 72.'A difference in the adjacent signals indicative of variation in the earths magnetic field in the respective intervals, as would be caused by the presence of a magnetically permeable object in the vicinity of one of the intervals, re-. sults in an output signal from the differential amplifier 72 to the second harmonic detection circuit 74. The second harmonic detection circuit 74 supplies an output signal proportional to the difference in the second harmonic signal content of the output signal from the differential amplifier 72 to the comparator circuit 76. Comparator circuit 76 compares the inputfrom the second harmonic detection circuit 74 to a reference R representative of the second harmonic signal attributable to predetermined class of magnetically permeable objects of interest i.e., weapons, and if the input from a second harmonic detection circuit 74 corresponds to the preselected class of objects the comparator circuit 76 activates alarm circuit 78.
The output signals from the comparator circuit 76 are also supplied as inputs'to the trend recorder 80 which sequentially plots the'intrusion information thus providing an indication as to the number of secondary coils segments affected by the intrusion of a magnetically permeable object, this plot represents information as to the size of the magnetically permeable object.
We claim: I
1. In a perimeter intrusion detection apparatus for providing an indication of the presence of a magnetically permeable object, the combination of, an elon gated magnetic core member and coil means wound thereabout, said combination of the core member and coil means being positioned relative to the surface of the earth to respond to changes in the earths magnetic field, an AC excitation source operatively connected to said coil means for establishing a current flow in said coil means to saturate said core member, said coil means responding to changes in the earths magnetic field caused by the presence of a magnetically permeable object by developing AC output signals, and first circuit means operatively connected to said coil means for responding to the content of said AC output signals indicative of the strength of the magnetic field.
2. In a perimeter intrusion detection apparatus as claimed in claim 1, wherein said coil means consists of a primary helical winding wound about said core mem- I bcr and a secondary helical winding wound about said core member, said AC excitation source operatively connected to said primary winding and said circuit means operatively connected to said secondary winding, said secondary winding comprised of a plurality of intervals, said circuit means monitoring the AC signals developed by the respective intervals to determine the interval or intervals closest to the magnetically permeable objects.
3. In a perimeter intrusion detection apparatus as claimed in claim 2 wherein said coil means includes a third helical winding wound about said core member and second circuit means operatively connected thereto to apply current of a polarity and magnitude sufficient to cancel the effect of the earths magnetic field.
4. In a perimeter intrusion apparatus as claimed in claim 1 wherein said first circuit means includes a second harmonic detection circuit, the second harmonic content of said AC output signals being a measure of the strength of the DC magnetic field, said first circuit means develping an output signal indicative of the second harmonic content of said AC output signals.
5. In a perimeter intrusion detection apparatus as claimed in claim 4 further including third circuit means operatively connected to said first circuit means for comparing the output signal of said first circuit means to a reference signal indicative of the strength of a magnetic field representative of a predetermined magnetically permeable object, said third circuit means generating an output signal when the output signal of said first circuit means corresponds to said reference signal.