US3487396A - Intrusion vibration detector with ambient noise compensator - Google Patents

Description

Dec. 30, 1 F. M. PERELMAN 3, 7, 6

INTRUSION VIBRATION DETECTOR WITH AMBIENT NOISE COMPENSATOR Filed March 11, 1966 2 Sheets-Sheet 1 ENVELOPE DETECTOR INTEG RHTOR TRIGGER A LARM 18 L I9 2o ENVELOPE A DETE O \NTEGRATOR TRIGGER LHRM ENVELOPE DETECTOR INTEGRHTOR TRIGGER ALARM as 70 INVERTER 72 6O b ENVELOPE l DETECTOR 5 62 .TA/VEMY'O fiZw/waw M. PEEELMRN Dec.

F. M. PERELMAN INTRUSION VIBRATION DETECTOR WITH AMBIENT NOISE COMPENSATOR Filed March 11, 1966 2 Sheets-Sheet 2 7 TO OUTSIDE NOISE SENSOR To BODY SENSOR 73 L olsis incs ENVELOPE SEQ w E $23 DETECTOR mmarr GATE coMPosrrE ENVELOPE FILTER DETECTOR A E; I 60 25 START SIGNAL.

2/ smw azsz'r 29 L 4% \LL 24 26 28 92 CONTROL RESET I 9 30 27 CONTROL 20 \23 ALARM f fvvs/vroe.

W M .PEEEL MHN United States Patent Ofilice 3,487,396 Patented Dec. 30, 1969 US. Cl. 340261 9 Claims ABSTRACT OF THE DISCLOSURE The alarm system detects tampering vibrations in a substantially rigid body by a vibration sensor attached to the body, an envelope detector connected across the rectified output of the sensor, and an integrator driven by the envelope detector that cumulates the rectified signals to operate an alarm trigger. The envelope detector includes a capacitor arranged to be charged by the rectified output and discharged through a large resistor whereby under constant or slowly changing ambient noise conditions, the noise signal is blocked. In other forms of the invention, paired sensors are provided in a subtracted network, with one of the sensors responsive only to ambient noises and the other sensor responsive not only to ambient noises but characteristic tampering vibrations.

BRIEF SUMMARY OF THE INVENTION This invention relates to an alarm system for detecting vibrations in solid material occasioned by unauthorized tampering. The invention will be described with reference to an automobile; however, the alarm system is equally applicable to safes, doors, fences, files, etc.

Various alarm systems have been proposed. In general, such devices are either relatively insensitive or they respond spuriously to ambient noises or vibrations due to thermal contractions. The primary object 'of this invention is to provide an alarm system that is sensitive only to vibrations characteristic of tampering even though high level vibrations of similar frequencies may be generated by other sources.

Another object of this invention is to proivde a system of this character incorporating a novel system of time delays in which the system can be armed and disarmed by key means located within the secured compartment of the automobile Without causing the alarm to operate. Accordingly, the disarming mechanism is itself protected from tampering.

Another object of this invention is to provide an alarm system of this character incorporating novel circuitry for preventing vibrations due to thremal contraction or the like from sounding an alarm. Another object of this invention is to provide novel circuitry by which operation of the alarm is inhibited by sudden or continual external noises without aflecting the sensitivity of the system to physical tampering.

This invention possesses many other advantages, and has other objects which may be made more clearly apparent from the consideration of several embodiments of the invention. For this purpose, there are shown a few forms in the drawings accompanying and forming part of the present specification. These forms will now be described in detail, illustrating the general principles of the invention; but it is to be understood that this detailed description is not to be taken in a limiting sense, since the scope of the invention is best defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a block diagram of an alarm system incorporating the present invention;

FIGS. 2 and 3 are circuit diagrams showing components illustrated in FIG. 1;

FIGS. 4 and 5 are block diagrams illustrating modified forms of the present invention;

FIG. 6 is a circuit diagram of one of the components illustrated in FIG. 5;

FIG. 7 is a block diagram of still another modified form of the present invention; and

FIG. 8 is a diagram of the trigger circuit.

DETAILED DESCRIPTION The medium intended to be protected against unauthorized tampering includes certain substantially solid or relatively rigid parts 10, 11 or 12 that would likely be contacted should unauthorized tampering occur. These parts in an automobile thus may be the doors, trunk lid, etc; Sensors 13, 14 and 15- may be attached to each of these parts, depending upon the degree of security desired or the characteristics of the particular installation. The sensors may be contact microphones, such as made for example by Monarch Electronics of North Hollywood, Calif. (Part No. M.C.51), which have provisions for being bolted to the solid body. Optionally, the microphone could be encapsulated in epoxy material, and clamped in contact with the rigid part. By encapsulating the sensor, external noises are attenuated. Diiferent types of sensors could be used, as for example strain gauges or floating magnet assemblies, etc. The sensors may be connected in parallel or series. The signal level of the sensors 13, 14 and 15 is elevated by an amplifier 16. Preferably the amplifier has a gain characteristic maximized at the prime disturbance frequency, that is, the frequency at which noises due to tampering tend to peak. The prime disturbance frequency or frequencies may be determined by testing. In a number of cases, the prime disturbance frequency is about 300 or 400 cycles per second or 900 or 1000 cycles per second.

The signal is then operated upon by an envelope detector 17 and an integrator 18, both hereinafter to be described, and applied as a unidirectional pulse to a trigger 19 that controls an alarm 20. As shown in FIG. 8, the alarm 20 derives energization from leads 91 and 92. One of the leads is controlled by a key switch 22 which may connect the lead 91 to an available power source such as the automobile battery. When the key switch 22 is closed, suitable power is made available to the amplifier 16 and other components of the system which derive energy from the lead 91. The key switch may be located within the protected space, as for example on the automobile dashboard. The alarm 20, which may be the automobile horn, radio transmitter or other suitable device, is operated only when a control relay relay 21 is operated. Normally open contacts 23 of the control relay are in series circuit with the alarm 20. The control relay 21 has an energization circuit that serially includes, first, normally open contacts 24 of a start relay 25; second, normally closed contacts 26 of reset relay 27; and third, .a controllable silicon switch 28, multilayer diode or equivalent device. The start relay 25 is connected directly across the leads 91 and 92 and thus closes the contacts 24 after the key switch 22 is closed.

The switch 28 has a gate lead 29 to which the detected signal is applied. When a detected signal corresponding to unauthorized tampering is applied to the gate lead 29, the switch 28 fires and operates the control relay 21 which, in turn, through contacts 23 operates the alarm 20. Once, fired, the switch 28 remains closed until current is interrupted,

By using as the start relay 25 a device having a time delay characteristic, it is possible for the authorized user to close the key switch 22 in the protected compartment and then lock the automobile, all before the start relay contacts 24 close. The time delay of the start relay also precludes warm-up transients of the amplifier 16 or the other components from generating a false signal that otherwise might trip the alarm.

The control relay 21 also is of the time delay type so that the alarm operates ten or more seconds following the disturbance. This allows the authorized user to unlock the automobile and quickly operate the key switch 22 before the alarm 20 operates.

If the alarm is once operated, it continues to operate only for a predetermined time, as for example thirty seconds, whereupon the system is reset. The reset relay 27 accomplishes this purpose. It likewise has a time delay characteristic.corresponding .to the intendedperiod of operation of the alarm. The time delay reset relay is initially energized through a circuit that includes normally open contacts 30 of the control relay 21. Accordingly, the time delay period of the reset relay 27 is initiated just as soon as the alarm 20 begins to operate. After the set time period, the reset relay 27 opens the contacts 26. Relay 21 is denenergized, and the alarm circuit is opened. The reset relay 27 is furthermore deenergized by opening of the control relay contacts 30. This causes the reset contacts 26 immediately to reclose. If the signal persists or recurs, the alarm will operate for another cycle.

In order to convert the amplifier signal to pulse form, a rectifying diode 31 is provided (FIG. 1) that forms a part of the envelope detector 17.

There may be certain background noises in the alarm system itself. The object intended to be secured against unauthorized tampering may be situated in the locality Where there is substantial background noise. Accordingly, the envelope circuit is arranged to pass the signal only when its level sharply changes. The envelope detector 17, shown in detail in FIG. 2, includes two parallel branches fed by the signal. Each branch includes a condenser 32 or 33 and charging resistors 34 and 35. The condenser 32 has a leakage path provided by a large resistor 36. The condensers 32 and 33 become equally charged under substantially steady noise conditions. The terminal 37 acquires a DC. bias, and only small peaks of the signal pass to the condenser 32 to balance the leakage current. But this bias cannot suddenly change since the time constant of the charge circuit for the condenser 32 is large. Accordingly, if the signal level increases suddenly over the noise level, a signal corresponding to the excess is transmitted through the condenser 33 and appears at the output terminal 38 for operation of the alarm. However, if the noise level changes slowly, the bias increases correspondingly, and no signal of significance is transmitted through the condenser 33. In practice, the condenser 33 is substantially smaller than the condenser 32, and its charge circuit has a relatively short time consant.

Creaks or strains in the solid body, due to thermal effects or high power impulse noises in the surrounding medium such as backfire, will create pulses at the output terminal 38 of the envelope detector. However, such noises subside quickly as contrasted with tampering signals. To distinguish between tampring and such noises, the integrator 18 is provided. The pulses applied to the integrator 18 from the envelope detector (FIG. 3) charge the condenser 39 through a resistor 40. The input terminal 41 of the integrator is operatively connected to the resistor through bufiering transistor amplifiers 42 and 43 in a conventional manner. A single pulse is produced at the output terminal 44 only when a number of pulses have been applied to the input terminal 41 sufficient to cause the condenser 39 to charge to a preset value.

For this purpose, a controled rectifier 45 is provided that compares the voltage on the condenser 39 to a reference value. The reference value is derived by the aid of voltage dividing resistors 46 and 47. The mid-terminal 48 is connected to the anode gate 49 of the rectifier 45. The condenser terminal 50 remote from ground is connected by a lead 51 to the anode 52 of the'switch 45. When the condenser reaches a value adequate to pass gate current, the switch 45 fires and the condenser charge is passed to the cathode 53 and to ground via resistor 54. The output terminal 44 is connected to the cathode 53 and thus transmits the pulse corresponding to firing of the switch 45. The output terminal 44 of the integrator may be applied to the gate lead 29 of the triggering device 28 (FIG. 8). Other integrating circuits may be provided.

In the form of the invention illustrated in FIG. 4, sudden sustained external noises are prevented from operating the alarm. In order to accomplish this objective, a second sensor 60 is provided that is mounted adjacent the main sensor 61. However, acoustic damping material 62 is interposed between the sensor 60 and the body 63, a

whereas the sensor 61 is intimately connected to the body 63. Accordingly, the sensor 60 reponds primarily to outside noises while the sensor 61 responds both to external and internal noises.

By suitably scaling and subtracting the signals, the resultant signal corresponds to internal noises only. For this purpose, the output of the sensor '61 is applied to the gain control of the amplifier 64 to reduce the signal by an amount corresponding to noise. Only internal disturbance signals pass to the envelope detector 65, integrator 66, etc.

A preferred method for subtracting sudden sustained external noises is shown in FIG. 5, Where the amplifier gain remains at set value whatever the noise level may be. Accordingly, maximum sensitivity to internal disturbances is retained. In this arrangement, the subtraction process is accomplished in the integrator circuit 70-. Sensors 60 and 61 are provided as in the previous form, but each has its own amplifier 71 and 72. The junction is shown in FIG. 6.

The signal from the body sensor 61 is applied to the base of an amplifying transistor 73 whereby charges are applied to a cumulating condenser 74 similar to the condenser 39. A second transistor 75 controls a leakage path for the condenser 39, and is operated by the signal from the outside noise sensor 60. Thus the condenser charges at a rate corresponding to the sum of the external noise and internal disturbances, and discharges at a rate corresponding to the external noises. The result is that the condenser charges in accordance with internal disturbances alone. The condenser terminal is connected via a lead 76 to a comparison switch as in the form illustrated in FIG. 3.

The form of the invention illustrated in FIG. 7 is essentially similar to that shown in FIG. 5 except that a single sensor 80 is used that is rigidly connected to the body. The output of the amplifier 81 is split into two branch leads 82 and 83. A filter 84 tuned to the prime disturbance frequency conducts the restricted signal to an envelope detector 85. Outside noises may also contain the prime disturbance frequency. If the outside noises are sufliciently constant, or of sufiicient short duration, the envelope detector 85 and an integrator will adequately distinguish the unauthorized disturbance signal. However, if outside noises suddently increase and dwell, a signal indicative of such fact is applied to the inhibit gate of 21131 giffgrential integrator 86 similar to that shown in In order to provide such signal, a composite filter 87 is provided that cooperates with lead 83 to pass frequency components that normally would be produced by outside noise and which are significantly different from the selected prime disturbance frequency. These noise signals are also operated upon by an envelope detector 88 so that only the sudden ones are passed. By suitably scaling the circuit components, the output of the integrator corresponds to internal sustained disturbances. Thus the sudden sustained outside noise components passing through the filter 84 are balanced out by the equivalent signals from the composite filter 87.

The inventor claims:

1. In an alarm system for detecting characteristic tampering vibrations in a substantially rigid body: a sensor attached to said rigid body; an envelope detector having an input connected to said sensor; an integrator driven by said envelope detector and having cumulating means for providing an output signal only after repeated or continued signals have been applied to said integrator; a trigger circuit operated by said integrator; said sensor being responsive to outside noises as well as tampering signals; said integrator including a cumulating device; a controllable subtracting circuit for said cumulating device; a controllable adding circuit for said cumulating device; means operatively connecting one of said circuits to said sensor for storing positive information in said cumulating device in accordance with the existence of characteristic tampering signals and outside noises; means producing a signal corresponding to outside noises substantially free of tampering signals; and means operatively connecting said outside noise signal means to the other of said circuits for storing negative information in said cumulating device in accordance with the existence of outside noises; said integrator providing an output signal when the store in said cumulating device reaches a predetermined level.

2. The alarm system as set forth in claim 1 in which said outside noise signal means comprises a filter system operated by said sensor, and an envelope detector, said filter system passing signals corresponding to the characteristic tampering frequency as well as other frequencies characteristic of outside noise.

3. The alarm system as set forth in claim 1 in which said outside noise signal means comprises a second sensor attached to said body by the aid of acoustic damping means.

4. In an alarm system for detecting characteristic tampering vibrations in a substantially rigid body: a sensor attached to said rigid body; an envelope detector having an input connected to said sensor; an integrator driven by said envelope detector and having cumulating means for providing an output signal only after repeated or continued signals have been applied to said integrator; a trigger circuit operated by said integrator; said sensor being responsive to outside noises as well as tampering signals; said integrator including a cumulating condenser; a controllable discharge circuit for said condenser; a controllable charge circuit for said condenser; means operatively connecting one of said circuits to said sensor for changing the charge on said condenser in one direction in accordance with the existence of characteristic tampering signals and outside noises; means producing a signal corresponding to outside noises substantially free of tampering signals; and means operatively connecting said outside noise signal means to the other of said circuits whereby the charge on said condenser changes in the other direction in accordance with the existence of outside noises; said integrator providing an output signal When the charge on said condenser reaches a predetermined level.

5. The alarm system as set forth in claim 4 in which said outside noise signal means comprises a filter system operated by said sensor, and an envelope detector, said filter system passing signals corresponding to the characteristic tampering frequency as well as other frequencies characteristic of outside noise.

6. The alarm system as set forth in claim 4 in which said outside noise signal means comprises a second sensor attached to said body by the aid of acoustic damping means.

7. In an alarm system for detecting characteristic tampering vibrations in a substantially rigid body; a sensor attached to said rigid body; an envelope detector having an input connected to said sensor; an integrator driven by said envelope detector and] having cumulating means for providing an output signal only after repeated or continued signals have been applied to said integrator; a trigger circuit operated by said integrator; a second sensor attached to said body by the aid of acoustic damping means; and an amplifier for said first sensor; said amplifier having a variable gain control operated by said second sensor to reduce the gain of said amplifier as the level of signal from said second sensor increases.

8. In an alarm system: means subject to external noise for producing a signal corresponding to unauthorized tmapering; means producing a signal corresponding to external noise and substantially insensitive to unauthorized tampering; a condenser having a charge circuit operated by said first named signal; a transistor paralleling said condenser to form a discharge path therefor; and means controlling said transistor by said second named signal whereby said condenser acquires a charge corresponding to unauthorized tampering independently of external noise; and a trigger circuit for operating an alarm, including a comparison device that fires upon the existence of a predetermined charge on said condenser.

9. The alarm system as set forth in claim 8 in which said device serves to discharge said condenser and reset said condenser for further signals.

References Cited UNITED STATES PATENTS 2,216,452 10/1940 Owen. 2,365,218 12/ 1944 Rogers. 3,109,165 10/1963 Bagno 340-261 X 3,147,467 9/1964 Laakmann 340261 3,201,776 8/1965 Morrow et al. 340-261 3,276,006 9/1966 Hansen 340261 JOHN W. CALDWELL, Primary Examiner D. L. TRAFTON, Assistant Examiner US. Cl. X.R. 340276

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