IL29843A - Intruder detecting apparatus - Google Patents

Description

This invention relates to security signal apparatus and, more particularly, to apparatus for detecting the penetration of a boundary of a predetermined region. A typical purpose of such apparatus is to sense that persons or objects are trespassing across the perimeter or barrier of a restricted area and are intruding into the area.

Prior art security signaling apparatus with electric ally or electromagnetically operating detectors included detectors of the photoelectric type and of the microwave type but also seismic detectors. These apparatus do not satisfactorily meet the following conditions. 1. A security signal apparatus should be capable of detecting a muffled attempt to cross the protected perimeter; for example, a person crawling, walking or attempting in any way to cross over a specific strip of ground at the perimeter or the movement of an inanimate object either automatically or by remote control over this ground. 2. It should include a detector which is completely concealed. 3. The detector should trip an alarm in the alarm signaling device when an attempt is made to destroy or disable the detector or any of its connections to the alarm signaling device. 4. The detector should require a minimum of maintenance .

. The apparatus should be such that detection over a large area with a minimum number of detectors and minimal signaling apparatus is feasible. 6. The apparatus should be immune to disturbances should at the same time have high sensitivity in this strip. 7. The apparatus should be of the fail-safe type and should indicate, by an alarm, failure of any detectors or of any other parts of the apparatus.

It is the principal object of this invention to overcome the deficiencies of the prior art apparatus and to provide security signaling apparatus which meet the above-listed conditions.

With this object in view, the present invention resides in an apparatus for detecting penetration of a predetermined boundary, including fluid filled compliant tube means burred along said boundary, and a receiving transducer connected to said tube means to sense any impulses received by said fluid, characterized in that said fluid filled tube means comprises two tubes disposed alongside each other, but spaced sufficiently to respond differently to pressure waves resulting from a source moving across said tubes ; each of said tubes being provided with at least one receiving transducer which is connected to means for indicating an alarm whenever different impulses are transmitted by the fluid in said tubes to the respective receiving transducers.

In a preferred embodiment of this invention the receiving transducer includes a piezoelectric crystal which produces an electrical potential across output terminals when pressure is applied to it. Typical crystals are composed of lead zironate-titanate , barium titanate, or quartz or related materials. Within the broader aspects of this invention, transducers of other types may be used. For example, the fluid-filled tube may terminate in a linear-variable-differen- highly sensitive capacitor whose plates are affected by the fluid. The unit including the fluid-filled tube and the transducer operates acoustically. The sound or mechanical pressure of the steps of an approaching intruder causes pressure impulses to flow along the fluid in the tube. These impulses produce an electrical potential which may serve to actuate an alarm signal. This detector is capable of detecting the very faint muffled sounds or pressure produced by quiet attempted penetration of a boundary as of a person crawling near or across a boundary. The frequencies of these sounds may be very low, or infrasonic, or the impulses may be aperiodic.

The detector unit may be provided with facilities for checking its integrity or its capability of operating.

For this purpose an interrogating transducer is connected to the opposite end of the fluid-filled tube in communication with the fluid. On this latter transducer, an interrogating signal potential is impressed. When subjected to such a potential the interrogating transducer impresses pressure impulses on the fluid and these are transmitted along the fluid to the receiving transducer at the other end which is to pick up the noise of an intrusion. The latter responds to the interrogating signal to produce a manifestation indicating that it is operational which continuously informs an attendant that the detector apparatus is in operating condition.

With the arrangement of this invention, sensitivity to local sounds or other disturbances and insensitivity to remote sounds or disturbances is achieved by providing a pair of fluid-filled tube detector components at each section of the boundary or perimeter to be detected. The assembly in- of the tube is referred to herein as a detector unit. The tubes are spaced a relatively short distance typically of the order of 2 to 5 feet. Preferably the tubes may be displaced horizontally but they may also be displaced vertically or along a plane at an angle other than 90° or l80° to the surface. The intruder pick-up transducers at one end of each of the tubes are connected with reversed polarity, or opposite phasing to the alarm indicator. The connection is such that pressure disturbances transmitted to the two tubes simultane-ously with substantially equal intensity produce no alarm indication. An alarm indication is only produced when the pressure disturbances transmitted along the fluid of the two tubes produce a net disturbance in the transducers, that is, the effect of the pressure disturbances impinging on each of the transducers is to cause one transducer to produce an electrical signal greater or smaller than the other.

The interrogating transducers at the other ends of the tubes may also be connected with opposite phasing. A common small interrogating signal is impressed on these trans-ducers which causes pressure impulses to be transmitted along the respective fluids. These pressure signals produce a small net output from the pick-up transducers which serves to monitor the integrity of the complete fluid-transducer assembly.

In accordance with this invention, the detector unit is under the surface of the perimeter being protected. In the usual situation when the perimeter is outside of a building or buildings, the detector units including the tubes and the transducers and any preamplifiers are buried in the ground and the output cables from the preamplifiers are underground.

The detecting unit is thus completely sealed from intruders.

The detector units may, for example, be buried about 6 inches underground with the ground around the detector units tightly packed so as to transmit the pressure impressed on the surface. In areas in which the temperature may be relatively low, the fluid within the resilient or pliant tubes has a low freezing temperature and may be, for example, a solution of glycerin or its derivatives in water.

For a perimeter of substantial length a number of detectors are used. It has been found that for a detector having a length of about four hundred feet, the response to a disturbance at the end of the detector remote from the intrusion responsive transducer is only about *J db below the response near the transducers. Each of the detector units for long perimeters may then be, for example, between about 300 and 500 ft. in length. The detector units should be so spaced around the periphery that there are no gaps between detectors through which undetected intrusion may occur. In certain situations there may be advantages in overlapping the detector units.

The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, in which: Figure 1 is a diagrammatic view showing the principle arrangement of the apparatus according to this invention; Fig. 2 is a view in perspective showing a detector unit in accordance with this invention including the fluid-filled tubes and their associated transducers; Fig. 3 is a view in perspective showing the manner in which the detector unit is buried in the practice of this Pig. Ί is a view in section showing the connection of the fluid of the detector unit to the transducers; Pig. 5 is a view in section taken along V-V of Pig. Ί; Fig. 6 is a schematic of an equivalent electrical circuit corresponding to the detector unit for situations in which the frequency of the impulses is of such magnitude that the mass of the fluid is appreciable; Pig. 7 is a corresponding equivalent circuit where the frequency of the impulses transmitted along the fluid is very low; Pig. 8a shows the reaction of a transducer diaphragm due to an increase in fluid pressure in one of the tubes as in detection of intrusion; Pigs. 8b and 8c show the deflections and the reaction on the transducer elements for accelerations of the entire detecting assembly, due to earthquakes or other large-scale vibrations.

Figs. 9a and 9b are views, enlarged and in section, of the transducer elements showing the relationship of the diaphragm and the piezoelectric crystal; Figs. 10 and 11 are schematic showing the manner in which the transducer elements may be electrically balanced; Fig. 12 is an oscillogram showing the response of the apparatus according to this Invention to movement parallel to a buried detector unit in accordance with this invention; Fig. 12a shows the direction and character of movement which produced the oscillogram shown in Fig. 12; Fig. 13 is ah oscillogram produced when a buried Fig. 13a shows the movement which resulted in the oscillogram of Fig. 13; Fig. I is a schematic showing the preamplifier and the amplifier in the alarm indicating apparatus; Fig. 15 is a schematic similar to Fig. 14, but showing the actual components in apparatus used in the practice of this invention which has been found to operate satisfactorily; Fig. 16 is a block diagram illustrating a preferred embodiment of this invention; and Fig. 17 is a sectional view of the interrogating transducer incorporated in Fig. 16.

Fig. 15 is self-explanatory and is presented here not with any intention of in any way limiting the scope of this invention but for the purpose of aiding those skilled in the art in practicing this invention.

The apparatus shown in the drawings includes a Detector Unit, a Preamplifier, an Alarm Indicating Unit, an Interrogating Signal Generator and a Monitoring Unit.^ This apparatus is divided into these components for the purpose of explaining this invention; in the actual apparatus according to this invention certain of the above-listed components may be eliminated or physically combined. In providing security apparatus for any Protected Region, a plurality of the Detector Units each including a Preamplifier is disposed around the region. The extent and disposition of the Detector Units depends in each case on the extent of the boundary which can be reasonably identified. In the practice of this invention, an Alarm Indicating Unit and a Monitoring Unit are con- connected to actuate, a plurality of Detector Units. The number of units with which an Alarm Indicating Unit, a Monitoring Unit and an Interrogating Signal Generator are connected to cooperate is determined by the extent to which an attendant may be taxed without becoming so fatigued as to operate ineffectively. In the practice of this invention, the Monitoring Unit and the Alarm Indicating Unit may be included in a single console with adequate light or other indicators to enable an attendant to monitor the security of the protected region.

The interrogating signal may be an attenuated potential derived from a commercial alternating-current supply.

In the use of this apparatus one or more attendants is positioned to observe the panel of the console and is provided with communicating equipment to take prompt action in the event of an alarm. Any actuation of a Detector Unit produces continuous signals and alarms and in the usual practice of this invention the alarms continue to be energized until corrective action is taken.

The Detector Unit includes a pair of fluid-filled tubes 31 and 33 · The tubes 31 and 33 are composed of compliant material such as rubber and filled with a fluid 35 (Pigs. 4 , 5 ) which may be a liquid such as water or water and glycerin or other freezing temperature depressant.

A Detector Unit made for use in the practice of this invention is shown in Pig. 2 . At each end, each tube 31 and 33 is connected to a metallic union 37 and 39 and 41 and 43 respectively. Each union 37 , 39 , 41 , 43 is provided with a manual valve 45 which is closed during shipment or repair. At one end, the unions 37 and 41 connect through a T-junction cate with the intrusion pick-up transducers 53 and 55 (Pigs. , 5). The remaining arm of each T-junction 57 is connected to a common overflow reservoir 71 through a relief valve 73. At the other end each union 39 and 43 is connected to a container 61 within which they communicate with interrogating transducers 63 and 65.

The containers 51 and 6l at each end of the Detector Unit are composed of abutting sections 75 and 77 (Figs. 4, 5) which defines chambers 79 and 81 between them. Each chamber includes a transducer 53 and 55 and 63 and 65 (Figs. 8a, b, c). Each chamber 79 and 81 is separated into two parts by the diaphragm 83 of the transducer which it contains. One part of each chamber 79 and 8l contains the fluid 35; the other part contains air. The diaphragm 83 forms a fluid-tight seal between the parts. Each transducer 53, 55, 63, 65 includes in addition to the diaphragm 83 a piezoelectric ceramic disk 91. The disk 91 is secured centrally to the diaphragm 83 on the side of the air section of each chamber 79, 8l.

The fluid in the tubes 31 and 33 exerts a pressure against the diaphragm 83 flexing the diaphragm 83 and the disk 91 secured to it. The flexing may be caused by an impulse moving towards the diaphragm 83 or away from it and its direction depends on the direction of the pressure impulse. The flexing causes the disk to produce an electrical potential whose polarity depends on the direction of flexing. This potential is derived through a conductor 93 secured to the disk 91 and a terminal in section 77 of the container 51 or 6l. A pressure impulse transmitted through the fluid 35 is reflected from the disk 91 on which it impinges producing a stress of tion in which it is dished or concave as viewed from the fluid to a position in which it is dished convex towards the fluid and potentials of opposite polarity may be produced.

Conversely a potential impressed on the ceramic disk 91 of a transducer causes the transducer to flex. Such a potential is impressed on the interrogating transducers causing them to flex and produce impulses on the fluid.

The relative thicknesses of the piezo-ceramic disk 91 and of the diaphragm 83 of each transducer 53, 55, 63, 65 are such that during flexing the neutral plane of the transducer is in the diaphragm 83. This relationship is illustrated in Pigs. 9a and 9b. Pig. 9a shows the situation which arises when the fluid 35 causes the transducer to be concaved as viewed from the direction of the fluid. In this case both the diaphragm 83 and the ceramic disk 91 are bent. Both the ceramic disk 91 and the portion 101 of the diaphragm 83 on the side of the neutral plane or the ceramic disk 91 are in tension, this tension having a direction parallel to the plane of the disks 91, while the part 103 of the diaphragm on the opposite side of the neutral plane is in compression also parallel to the plane of the disk. For circular disks this tension and compression are radial. Since the whole ceramic disk is in compression, the maximum electrical potential is derived by the flexing of the ceramic disk. A corresponding situation occurs when the transducer is deflected so that it is convexed towards the fluid. In this case, the ceramic disk 91 as a whole is in compression and the maximum electrical effect is again achieved.

In the practice of this invention the fluid in the to the adjacent transducers 53 and 55 suQh that in the standby condition of the apparatus, the disks 91 of the transducers 53 and 55 are subjected to slight tensional stress.

In the practice of this invention the disks 91 at each end are connected bucking, that is, so that substantially the same stress impressed on the disks at each end would produce a zero potential. The connection is illustrated in Pigs. 8a, 8b and 8c. In Fig. 8a, the relationship of the disks when subjected to tensional stress in one of the tubes is shown.

In this case a net signal is produced at output terms 95· In Figs. 8b and 8c the effect of equal stresses in the respective opposite directions are shown. In all cases the output is zero when the stresses are alike. Only when the stresses are different of the two plates either in polarity or in magnitude net potential appears at the output as in Fig. 8a.

For the purpose of checking the operation of the intrusion pickup transducers 53 and 55 units, an interrogating signal is impressed on the interrogator transducers at the end remote from the reservoir 71. This signal may be of the alternating current type and typically may be a 60-cycle potential derived from an available commercial source. The interrogator signal is impressed in common on the two transducers 63 and 65. The effect of the interrogator signal potential on one transducer is to produce a deflection of one polarity and the effect on the other is to produce a deflection of the opposite polarity so that pulses of opposite polarity are transmitted through the fluid to the intrusion-pickup transducers 53 and 55· The pickup transducers then produce a signal potential at The transducers 53 and 55 and 63 and 65 are shown in Figs. 8a through 8c as connected respectively bucking In parallel. They may also be connected bucking in series.

It is important that the transducers 53 and 55 and 63 and 65 respectively at each end of the fluid-filled tubes 31 and 33 be appropriately balanced. The balancing may be affected by a variable capacitor 102 connected across one of the transducers 53 and 63 of each pair 53 and 55 and 63 and 65, respectively (Pigs. 10 and 11), the balancing may be omitted if the transducers are selected in matched pairs as part of the manufacturing process.

The Preamplifier includes field-effect transistors Fl and F2 connected in cascode, transistor Q3 and integrated circuit or monolithic unit MO. The common input from transducers 53 and 55 is impressed on the base of Fl through a circuit including diodes Dl and D2 which protect the transistor from overvoltage. The transistor F2 compensates for the effects of temperature changes in transistor Fl, and serves as a constant-current load for Fl. The output of transducers 53 and 55 is composite including the response to the interrogating signal impressed on transducers 63 and 65 by the Interrogating Signal Generator. The interrogating signal is typically a 60 cycle signal derived from a commercial alternating supply. The output of transistor Fl is impressed on the base of transistor Q3 , the output of which is supplied to the monolithic element MO. The diodes D5 and D6 protect the capacitor CI which serves to suppress high-frequency disturbances. The output of the Preamplifier is derived from the output conductor LO of the unit MO.

Alarm Indicating Unit. This Unit includes the transistors Ql, Q2, Q , Q5 and Q6. The output from the Preamplifier is impressed on the base of the transistor Ql through capacitors C2 and C3, C2 being of high capacity. The output of the Preamplifier is also impressed on the base of transistor Q5 through filter PI. The filter PI suppresses the 60-cycle interrogating signal so that the potential impressed on capacitor Q5 is only the potential derivable from detection of intrusion. In the absence of intrusion only bias derivable from the resistor Rl is impressed on Q5.

The output of Ql, which is principally the 60 cycle interrogating signal is impressed on the base of Q2 through resistor R2 and capacitors C4 and C5. Diodes D3 and D4 are connected oppositely across the input to transistor Q2. These diodes maintain the alternating potentials impressed on the base of Q2 no greater than a predetermined level. Typically the potential Impressed on Q2 in the usual practice of this invention is limited and has a peak-to-peak amplitude of about 1 volt by reason of the operation of the diodes D3 and D4, The output of Q2 is derivable through capacitors C7 and C8 and resistor R . The output of Q2 is also impressed through capacitors C7 and C8, resistor R and capacitor C9 on the base of transistor Q6. The output of Q6 is impressed through capacitor CIO, rectified by D20 and D21, and applied through resistor R5 to the base of Q4. Q4 is connected to supply the coil of a relay R. In the stand-by condition of the apparatus Q4 is conducting and relay R is actuated so that its contact Ra is closed and its back contact Rb is open. The contacts Ra and Rb are connected to the Alarm Indicator. With Ra closed, the proper operating condition.

The Alarm Indicating Unit includes a discriminator DR which is formed of a plurality of Zener diodes Zl through Z5 and Z6 through ziO connected in series pairs respectively across supply1 conductors LI and L2, Z6 , Z2 and Z7, Z3 and Z8, Z and Z9 , and Z5 and ZIO are respectively connected in series. The pairs of diodes breakdown for different and progressively increasing voltages. A selector switch CW is provided for selecting any pair of series connected diodes Zl through Z5 and Z6 through ZIO. The signal from the output of Q2 is impressed through capacitors C7 and C8 and resistor R4 on the discriminator DR through the movable arm 103 of selector switch CW.

The selected diodes are normally non-conducting. When a voltage higher than the breakdown voltage of the selected diodes is impressed on the discriminator DR one diode of the selected pair breaks down for a_ portion of one half wave of one polarity of this voltage and the other diode of the pair breaks down for a portion of the other half wave. As shown in Fig. 14, the diodes Z3 and Z8 are connected to control the discriminator DR; typically these diodes break down for 8.2 volts. If the voltage during the negative half period of the interrogating potential impressed is of such magnitude that the voltage between the upper potential conductor LI and the movable arm 104 of the switch CW exceeds 8.2 volts, diode Z3 breaks down. If the potential during the positive half period exceeds 8.2 volts, above ground the diode Z8 breaks down. In the stand-by condition of the apparatus when the boundary is not being penetrated the tude to cause the diodes Z3 and Z8 to break down.

In the stand-by condition of the apparatus, when there is no intrusion at the boundary the only forces being impressed on the Detector Unit are those derived from remote sounds such as traffic or seismic disturbances. These sounds produce compressive waves of the fluid in the tubes 31 and 33 but the compressive waves are simultaneous and of equal amplitude in the two tubes because of the remoteness of the sources of the sounds. Under these circumstances the only signal at the output of L0 is the 60 cycle interrogating signal. This signal is impressed on the base of Ql but, because of the filter PI, not on the base of Q5. Q2 then delivers a flat-topped alternating-current signal to DR then selector switch CW and a like signal to the base of Q6. The signal on DR is insufficient to break down the diodes Z3 or z8.

The signal on Q6 produces a limited alternating-current signal which is impressed through CIO to charge capacitor C20 through voltage-doubler diodes D20 and D21. The potential on C20 maintains Q4, conducting, maintaining relay R actuated and contact Ra closed to indicate that the transducers 53 and 55 are in proper operating condition.

On intrusion in the boundary, a signal is impressed on Q5 through the resistor R8 and the filter PI. Potentials of alternate polarity but usually of low frequency then appear at the movable arm 102 of the switch CW through the resistor R9. Depending on the polarity of the potential impressed through Ra, Z3 or Z8 is rendered conducting. The 60 cycle alternating-potential impressed on the base of Q6 is then suppressed and Q6 no longer passes the 60 cycle signal. Capa- and D23 and after a short time interval Q4 becomes non-conduct ing and relay R drops out opening Ra and closing Rb and producing the alarm.

In considering maloperatlon of the apparatus, it may be assumed that the transducers 53 and 55 are connected in parallel. Under such circumstances , if a transducer is short circuited a short circuit is in effect connected across the transducer The interrogator potential then disappears and the relay R is deenergized so that its back contact Rb closes providing an alarm indicating the deficiency of the apparatus. If one of the transducers 53 or 55 is open-circuited, an unbalance is produced in the intrusion pickup transducer in the output. A remote sound would then immediately actuate the alarm.

Intrusion of the boundary can be distinguished from a defect in any transducer by the operation of relay R. As an object moves through the boundary or towards and away from the boundary the relay R is repeatedly energized and deenergized. Provisions can be made in the alarm indi-cator for energizing a characteristic alarm responsive to repeated operation of the contacts Ra and Rb to characterize an intrusion. A defect in a transducer is continuous so that the relay R remains continuously actuated.

Pigs. 12 and 13 show the response of the Detector Unit in the actual use of this invention. In producing Fig. 12 the response was observed for a person walking as disclosed in Pig. 12a. The tubes 31 'and.33 are spaced a distance of about 2 to 5 feet. The person followed a path as shown by the broken lines in Pig. 12a. The person walked then reversed his direction and moved along tube 33 a distance 2 feet from 33, and then again reversed his direction and walked parallel to tube 33 a distance 3 feet from tube 33, then again reversed his direction and walked a distance 4 feet from tube 33 and finally reversed his direction and walked parallel to 33 a distance 5 feet from 33. The corresponding signals are presented in Fig. 12. It is seen that at one foot from 33 a signal of substantial amplitude is produced and as the distance, from the tube 33 decreases the amplitude of the signal is reduced until a relatively low signal is produced with the person walking parallel to 33, 5 feet from tube 33. As shown in Pig. 12, signals at more remote distances than 5 feet from tube 33 are balanced out by the respective phasing of the transducers 53 and 55· Pig. 13 shows the response of the transducers to a person crossing the fluid-filled tubes. The person crossed the tube 33 first and then the tube 31. It is seen that two wave trains are produced, one as the person crossed tube 33 and the other as the person crossed tube 31. The wave trains are of appreciable magnitude and would readily trigger the alarm indicator.

So that the invention may be more thoroughly understood, the following summary is presented: The Detector Unit in its simplest form consists of two compliant hoses or tubes coupled to pressure-sensing transducers 53 and 55. The electrical outputs of the transducers 53 and 55 are combined in such a manner that the differential signal which they produce is capable of triggering the Alarm Indicator. The output signals of the transducers 53 and 55 are functions of pressure rate-of-change where the sure variations resulting from earth shrinkage, temperature variations, and the like are not detected. When an object enters the field of the Detector Unit over Section "A" (Fig. 3) an output of a positive polarity appears at the terminal 107 of the transducers 53 and 55. An object entering Section "B" produces an output of negative polarity at the terminal 107. A mechanical disturbance set up at some dis-. tance from both of these fields tends to cancel out because it excites both transducers 53 and 55 approximately the same. In practice, the transducers do not have equal sensitivities, and balancing may be effected with the capacitors CI and C2 (Figs. 10 and 11).

An object entering the field of the Detector Unit compresses the soil and sets up a stress field within the nearest tube 31 or 33· The resulting stain field, even through almost infinitesimal at the tube, shrinks its diameter thus increasing the fluid pressure. This change in pressure is then sensed by the transducers.

Fig. 6 is the electrical network equivalent to the mechanical system of one transducer and tube. The following mechanical to electrical analogs are employed in its derivation.

Force, Newtons Voltage, Volts Velocity, Meters/Second > Current, Amperes Displacement, Meters > Charge, Coulombs Mass, Kilograms ; ^ Inductance, Henrys Compliance, Meters/Kilogram—7 Capacitance, Farads Mechanical Resistance, MKs Units 7 Resistance, Ohms A noise source "N" has a pressure variation of force variation (product of pressure and area) at some point along the line. The. pressure at the point would be the result to the tube by the impedance characteristics of the earth. The representation of this situation is a "force" source and its associated mechanical impedance is zfi. For the general case the disturbing force accelerates the coupling fluid, creates dissipation losses by the viscous shear of the fluid, and expands the tube in other regions because the earth and tube are compliant members. When the force reacts on the electromechanical transformer of the transducer, a voltage appears at the electrical terminals.

In the general case, the tube has a propagation velocity, c, determined approximately by the following equation when the fluid viscosity can be neglected c = / Mass of fluid per unit length ' bulk compliance of tube per unit length At very low frequencies, the inertial effects of the fluid may be neglected and the equivalent circuit reduces to that shown on Fig. 7. For low frequencies, it is desirable to employ a low viscosity fluid since the series resistance shown is directly proportional to the dynamic viscosity of the fluid.

The labelled components in Figures 6 and 7 have the following significance.

Cf = free capacitance of transducer, CQ = open circuit transducer compliance 0 = electromechanical factor = mechanical viscous damping per unit length Mw = mechanical fluid mass per unit length = compliance of hose per unit length N = disturbing force F = reference force generator Cf = compliance of reference force generator The reference-force generator P shown on the far right in Figs. 6 and 7 is a low-frequency electromechanical driver which transmits a signal to the receiving pressure transducer for the purpose of making the system fail-safe. The frequency of this signal (60 cps.) is far removed from the expected alarm signal frequency (.01 cps. to 10 cps.) .

Failure to receive it constitutes a system failure, with a resulting continuous alarm or other indication. The alarm set off by intrusion would now be continuous and the operator would be able to distinguish between a system failure and a genuine alarm.

The electromechanical transducer housing shown in Figs. 4 and 5 contains two fluid compartments 79 and 8l which couple the fluid 35 respectively to a pair of diaphragms 83. On each diaphragm 83 a piezoelectric disk 91 is bonded such that the output polarity of one disk 91 is positive for a positive applied pressure and the other disk 91 negative for a positive applied pressure. Thus if equal pressure is applied to the fluid 35 each tube 31 and 33, the output potentials are equal and opposite so that for parallel electrically connected elements the output signal is zero. The output is a function of the pressure differential within the tubes, and localized sensitivity characteristic is obtained.

One of the unique features of the apparatus according to this invention is that it is insensitive to any accel-erational motions in the three principal directions. Fig. 8a shows the output signals for a given situation in which a show the output response for an acceleration applied normally to the diaphragms. Acceleration response to acceleration In directions in the plane of the diaphragms 83 is negligible because in this case the ceramic disks 8l are in shear and these elements are insensitive to shear for the electrode configurations shown. The configurations shown in Pigs. 8b and 8c require that the tubes be connected to the same side and be in the same plane for differential operation and in-ertial balancing.

The arrangement shown in Pigs. 8a, 8b, and 8c is necessary for obtaining both pressure differential operation and inertial balancing. The inertial balancing reduces the direct pickup from the earth and is highly desirable because it cannot be eliminated by the balancing feature of the array.

Typically, the sensor elements or transducers may be connected In parallel. In this case, external balancing of the elements is achieved as shown in Pig. 10. The elements can be operated in series provided suitable insulation is employed between the metallic diaphragm and piezo-ceramic disk. Balancing for this connection is shown in Fig. 11.

The electrical signal from the transducer element is amplified by a, Preamplifier and for test purposes may be applied to a Sanborn recorder having an alarm circuit.

Figs. 12 and 13 show recordings obtained on a Sanborn chart recorder with the transducers described, the tubes being buried at a depth of Waif a foot and five feet apart. The recording in Fig. 12 shows the recording obtained for normal walking parallel to one of the tubes 33, at various distances. -The weight of the person was 155 lbs. The spacing transducer voltage of 500 microvolts.

The recording in Fig. 13 shows an attempt to sneak across the tubes 31 and 33· Special effort was made not to step directly upon either tube, yet a significant signal was generated. The calibration was the same for each recording. It is probable that a low frequency cut-off (or a high pass filter) could be used to eliminate the very low frequency drift shown on the recordings without destroying useful information.

Fig. 16 shows a preferred embodiment of the apparatus according to the invention.

Again the Detector Unit includes a pair of fluid filled tubes 131 and 133 each being provided with a valve 13^ for purposes of filling. A receiver transducer 137 is connected to one end of hose 131 and an interrogating transducer 139 is connected to the opposite end of the tube 131. A receiver transducer l4l is connected to one end of the tube 133 and an interrogating transducer 143 at the opposite end of the tube 133.

The receiver transducers 137 and l4l are similar and may be of the piezoelectric type such as described in the previously mentioned copending application. The transducers 137 and 1*11 include a diaphragm 136 against which the fluid 135 bears. A piezoelectric crystal 138 is located on the opposite side of the diaphragm 136 of the transducers 137 and ΙΊΙ and responses to deflection of the diaphragm 136 to generate an electric signal. The crystals 138 are oppositely poled and balanced so that an equal pressure on both hoses 131 and 133 will cancel in the output circuit. similar nature and the transducer 139 is illustrated in Fig. 17. The transducer 139 comprises a diaphragm 145 of a suitable flexible material such as neoprene. The thickness of the diaphragm 45 may be about l/l6 inch. The fluid 135 bears against one surface of the diaphragm 145. A stiff back-up plate 146 of a suitable material such as steel is provided on the opposite side of the diaphragm 1 . A centrally located aperture 148 is provided in the back-up member 46. The driving mechanism for the diaphragm 145 is a solenoid 147 which con-sists of a coil 149 and a plunger 151. The excursion of the plunger 151 is controlled between limits to keep the change in fluid volume constant with fluid pressure. In the event that the end of the hose 131 where the interrogator transducer 139 is located is in a low altitude compared with the other end of the hose 131, a spring 153 is used to offset part of the pressure. The amount of change in fluid volume is controlled primarily by the stroke length, however, it is also a function of the inside diameter of the diaphragm 145, at least of the part that moves.

An additional feature of the transducers 139 and 143 is that the fail-safe or interrogating transducers 139 and 14 appear stiff due to back-up plate 146 to the fluid 135 when the solenoid 147 is at rest. This guarantees maximum intruder pressure change at the receiving transducers 137 and 141. In other words, a pressure increase in the hoses 131 and 133 produces no volume change in the cavity of the interrogating transducers 139 and 143. Only a decrease in pressure can have this 'effect which does not occur during an intrusion. Input terminals 155 and 157 are provided on the transducer 139 for actuate the transducer 139· In operation, the application of a pulse to one of the interrogating transducers 139 or 143 results in a pressure wave in the fluid 135 in the respective hose 131 or 133 which is in turn received by the respective transducer 137 or 141. This signal is fed through a suitable preamplifier and an amplifier circuit as previously described.

With the above arrangement, the use of a periodic pulse is provided to confirm the present of fluid 136 in the hoses 131 or 133. The interrogating transducers 137 and 141 also provide diaphragms which are restrained between failsafe tests to insure a stiff fluid termination for proper functioning of the device.

The output from the Detector Unit is connected into the Alarm Indicating Unit. The output from the Detector Unit is derived from the two piezoelectric crystals provided in the transducers 137 and 141. The crystals 138 are oppositely and the output is normally fed through a suitable preamplifier.

The output of the preamplifier is fed to the Alarm Indicating Unit. The Alarm Indicating Unit may include a low pass filter with an approximately 1 Hz cut. off. The output, of the filter may be fed into a suitable amplifier. This circuitry is normally of the solid state type and may be located above ground at the observer or console point. The output of the amplifier is coupled to a suitable alarm indicator such as a light system which in response to a signal obtained from the Detector Unit will cause the alarm light to go on. In addition if so desired a sonic signal may also be derived from the alarm detector in order to alert the observer.

The transducers 139 and 143 are driven by an Interro- gating Signal Generating Unit. The Interrogating Signal Generator is illustrated in the drawing for manual operation to avoid complexity of suitable electronic circuitry. A switch 160 is provided in series with a battery 162 between the terminals 155 and 157 of the transducer 139. A similar interrogating signal generator may be connected to the transducer 143. By closing the switch l60, a pulse of current passes through the solenoid 1 7 causing the plunger 151 to deflect the diaphragm 145. The deflection of diaphragm 145 imparts a pres-sure or energy pulse into the fluid 135 which the receiver transducer 137 detects. The electrical signal from the receiver transducer 137 actuates the Alarm Indicating Unit.

The Interrogating Signal Generating Unit may be electronic and include a suitable multivibrator circuit for generating a signal every ten minutes. A pulse from the multivibrator triggers either one of two one-second timers which are connected through suitable solenoid drivers to the transducers 139 and 143. The length of this interrogating pulse is determined by the time constants of the one-second timers and should be around one second. The pulse from the Interrogating Signal Generating Unit may also be connected to inhibit operating of Alarm Indicating Unit during the period of the test pulse .

The invention has the following advantages : 1. The apparatus embodying the invention is of relatively low cost; the cost is less than the cost of a microwave or like apparatus. 2. The Detector Unit, being completely below ground, is completely out of sight, an important consideration where visible structures or objects would lend themselves to tampering and might be objected to. 3. With the Detector Unit below ground, the apparatus is immune to damage from rain, wind, lawnmowers , and the like. 1». The Detector Unit has a strip sensitivity instead of area sensitivity as in the case of a single seismic sensor, making the described apparatus especially suitable for perimeter guarding.

. By reason of the cancelling effect of the two tubes 31 and 33 for pressure signals originating from a distance, the described apparatus is relatively immune to noise such as might be encountered if a street or sidewalk runs nearby to the guarded enclosure. 6. The apparatus, being sensitive for several feet each side of each tube 31 and 33, is not subject to disabling without first triggering the alarm. Draining of the tubes or removal of electric power also triggers the alarm.

Claims (10)

What we claim is:

1. An. apparatus for detecting penetration of a predetermined boundary, including fluid filled compliant tube means burred along said boundary, and a receiving transducer connected to said tube means to sense any impulses received by said fluid, characterized in that said fluid filled tube means comprises two tubes disposed alongside each other but spaced sufficiently to respond differently to pressure waves resulting from a source moving across said tubes, each of said tubes being provided with at least one receiving transducer which is connected to means for indicating an alarm whenever different impulses are transmitted by the fluid in said tubes to the respective receiving transducers.

2. An apparatus as claimed in claim 1, characterized in that said transducers are electrically connected such that pressure impulses of the same polarity produce electrical signals of opposite polarity, said alarm indication means being responsive to a combined signal of a predetermined strength depending on the difference in magnitude between said electrical signals.

3. An apparatus as claimed in claim 1 or 2 characterized in that each of said receiving transducers includes a flexible diaphragm exposed atone side to the fluid of the respective tube and having a piezoelectric crystal mounted on the other side of said diaphragm such that the neutral stress plane of the diaphragm-crystal unit is external to said crystal thereby to subject said crystal to stress when the diaphragm is deflected by the fluid in the respective tube.

4. An apparatus as claimed in claim 1, 2 or 3, characterized in that an interrogating transducer is connected to the end of each tube opposite said receiving transducer and an interrogating signal generator is connected to said interrogating transducers for transmitting interrogating signals through said tubes thereby to test operativeness of said apparatus .

5. An apparatus as claimed in claim ^ , characterized in that each of said interrogating transducers includes a diaphragm having one side exposed to the fluid in the respec tive tube and a rigid back-stop member disposed on the opposite side of said diaphragm with respect to said fluid.

6. An apparatus as claimed in claim 55 characterized in that said back-stop member has an aperture formed therein and that a plunger member extends through said aperture and is operatively associated with said diaphragm for imparting a driving action to said diaphragm in response to driving energy applied to said plunger.

7. An apparatus as claimed in claim 4, 5 or 6, characterized in that said interrogating signal generator is adapted to energize said interrogating transducers at periodic intervals and alternately said first and second transducers to initiate an energy pulse in said first and second tubes similar to that initiated by penetration of said boundary.

8. An apparatus as claimed in any of claims 4 to 6, characterized in that said interrogating signal generator is adapted to apply said interrogating signal continuously to response indicating operativeness of said apparatus.

9. An apparatus as claimed in any one of claims 1 to 8, characterized by a plurality of said tube means each extending over a predetermined length of said boundary, and each being provided at least with said receiving transduce

10. An apparatus for detecting penetration of the boundary of a predetermined region substantially as hereinbefore described with reference to, and as shown in the accompanying drawings . For the Applicants DR. RE/NHOi.0 COHN AND PARTNERS