US3543261A

US3543261A - Upper threshold circuit

Info

Publication number: US3543261A
Application number: US737229A
Authority: US
Inventors: Charles F Burney
Original assignee: US Air Force
Current assignee: US Air Force
Priority date: 1968-06-14
Filing date: 1968-06-14
Publication date: 1970-11-24
Anticipated expiration: 1987-11-24

Description

Nov. 24, 1970 C. F. BURNEY- UPPER THRESHOLD CIRCUIT Filed June 14, 1968 tates Patent ce Patented `Nov. 24, 1970 Us. Cl. 340--261 3 Claims ABSTRACT OF THE DISCLOSURE Two spatially separated signal detection systems sense with acoustic transducers impulse vibrations in their vrclnity. The electrical signals after passing predetermined threshold levels in the two systems are conducted to an AND gate and to an OR gate. Simultaneous signals in both systems above the predetermined levels pass the AND gate and initiate an alarm. Repetitive signals from either sensing system pass the OR gate and are stored as up-counts in an up-down counter from which counts are periodically removed by opposite polarity counts from a freerunning down-count generator. An accumulation of a predetermined number of stored counts also activates the alarm.

BACKGROUND OF THE INVENTION The field of this invention is in acoustic protective equipment that will sound an alarm to an intrusion into a protected area. The detection of an intrusion is by the sensing of vibrations that result from the impulses of the intrusion tools employed, or through the detection of the vibrations that result from the use of explosives.

Acoustic devices that will sound an alarm responsive to sounds and vibrations are well known. These current devices are prone to give false alarms due to ordinary en-1 vironmental or background noises, or if set to such a threshold level so as to be unresponsive to common background sounds and vibrations they become ineffectual to detect the intrusion of a relative quiet chipping hammer or similar device.

SUMMARY OF THE INVENTION An intrusion detecting device `is disclosed that will initiate an alarm when acoustic vibrations indicative of a mechanical intrusion have taken place within an area to be protected. The system will respond to high-level widespread vibrations such' as may be caused by blasting with explosives, as for instance in the blowing open of a door or the blasting of an entrance through a wall. It will also respond to sustained, even though intermittent, hammering or chiseling, yet be responsive to vibrations caused by normal working conditions.

lBRIEF DESCRIPTION OF THE DRAWING The drawing shows a 'block diagram of an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention detects acoustic vibration within a protected area. This area over which protection is provided is divided into at least two sub-areas. In each sub-area a vibration sensor is located. This invention is particularly suited to structures constructed of materials capable of conducting sound frequencies with small, known amounts of attenuation, such as steel structures and concrete structures or combinations of such materials When a pulse of energy is`coupled to such a structure, at any point, then vibrations will travel throughthe material with essentially known frequency characteristics, known attenuations, and known velocities. Thus, if two vibration sensing trans= ducers of similar characteristics are attached to the structure and physically separated a known distance, the level and Vfrequency characteristics of the vibration pulses ar rivinga't each transducer will be indicative of the amplitude of an impact pulse on the structure.

Referring to the drawing, acoustic transducers 1 and 2 are conventional transducers having a relatively wide frequency response for converting pulse vibrational energy into electrical signals. Piezoelectric, ceramic, magnetic, capacitance, and inductive devices all operate satisfactorily. Due to their slightly different response characteristics, a judicial decision may be made in the selection of the type of transducer employed with regard to the character of the normal background vibration signals present in the environment. Ceramic transducers, due to their relatively wide frequency response, relatively high output, and general insensitivity to environmental temperature and humidity changes, are suitable for most applications.

It is desirable that transducers 1 and 2 be physically disassociated so that the attenuation present in the construction medium results in a signal difference at the two transducers of 3 db or greater when the pulse input energy is applied in close proximity to one transducer.

The signals generated in the transducers are coupled to conventional audio amplifiers 3 and 4. Each of these amplifiers has an equal gain, with the level of the gain at least equal to that determined by the relation of the particular transducer employed to the furthermost boundary point of the protected area coming under its inuence, asthis point is excited by a pulse of energy equal to a representative minimal manual intrusion attempt. Of course, the absolute value of amplifier gain will depend upon the sensitivity and impedance of the transducer used.

It has been found generally desirable to filter the out puts of the amplifiers to remove the .signals generated by machinery that operates in the protected area. Electric motor-driven equipment will generally set up structural vibrations. The filters also remove the commonly called naturalvibrations, such as building rumble brought about by the movement of heavy trucks and railroad equipment in the nearby vicinity. These signals-.are of relative low frequency in relation to intrusion frequencies, thus 'the high pass filters 5 and 6 have a low frequency cutpif that attenuates .and effectively eliminates these noise sources. A cutoff frequency of approximately 100 cycles per second will generally be satisfactory; again, it will be understood that the low frequency cutoff frequency of the filter may desirably be tailoredto suit the particular application with frequencies from r*afew Hz. to several hundred Hz. being optimum for a .particular application. Such filters are well known and r'wilely used.

Rather than depend upon the :gain of the amplifiers to establish a minimum detectiona level of the system, it has been found to be highly desirable to use separate f sharp cutoif thresholds 7 and 8. These thresholds permit only signals abovea predetermined level to be passed to the AND and OR gates, and prohibit the passage of noise and low level signals to the gates. Ihese thresholds thus establish the minimum detection level of the system. They are normally adjusted so that a minimum manual intrusion pulse at the boundary limits (least sensitive.) portion of the protected area will always result in an output from the particular sensing circuit monitoring that sub-area. Such threshold circuits are well known and widely used in the electronic art.

The signal processing circuits 16 receive the outputs from the various signal detection circuits 15. It is to be understood that while only two signal detection circuits are shown, any number of additional detection circuits may be used with the increase of corresponding AND and OR gates. A manual intrusion results in generally only an output from one threshold circuit; the particular one having an output being determined by the physical relation of the transducer to the location of the pulse energy input. This single threshold output signal is coupled to both the conventional AND gate circuit 9 and the conventional OR gate circuit 10. No output will occur from the AND circuit for a manual intrusion since it requires simultaneous signal inputs to produce an output. The signal will pass the OR circuit and enter the updown counter 12. This counter is a conventional binary counter that provides an output after accumulating a fixed amount of stored counts. It has two inputs. It is driven in a positive direction (up-count) by inputs from the OR circuit, is driven in thenegative direction (down-count), so as to remove stored counts, by the free-running down-count generator 13. It is desirable that the total count capacity of the up-down counter 12 be relatively large (from 50 to 200 counts, with a count capacity of 100 being typical), since manual intrusion attempts generate a large amount of threshold crossing signals in a relatively short periodof time. When the determined count capacity is reached within the up-down counter, an output is provided to Ithev OR gate circuit 11, which is passed by it to the alarm indicator 14. This alarm may be an audio alarm such as a bell or a visual light or bothn The purpose of the down-count generator 13 is to subtract counts at a known rate "from those accumulated lwithin the up-down counter. This subtraction rate is generally determined empirically by the false alarm rate experienced at the installation. The false alarm being created by signals within the frequency band of the intrusion signals, but resulting from normal environmental vibrations or normal man-made working vibrations. Since these false signals occur at a slow rate, as opposed to the rate from a valid intrusion, the down-count generator time base is relatively long, depending on the environment. A time base of 20 minutes is typical, with values from il-O minutes to one hour being suitable to cover most applications of this invention. This determined time base count interval allows for signals that would tend to create a false alarm to be cancelled out periodically. In operation it has been found that generally actual intrusions result in a full count and an alarm in less than five minutes.

Explosive intrusions generate large signal inputs to both transducers. It has been found that the signal level Igenerated by a minimum explosive attempt will be 40 db or greater than the typical system threshold when set as previously outlined. This large signal travels throughout the structure and activates both (or all) transducers and simultaneous signals arrive at the input to the AND circuit 9. Simultaneous signals arriving at the AN4 circuit pass and provide an output signal to the OR circuit l11 which passes on through to alarm circuit 14 generating an immediate alarm.

Generally it has been found that the acoustic separation (that is, the attenuation through the structure between the locations of the transducers) be at least 3 db, but less than the minimum explosive threat expected, that is, in a typical installation, less than 40 db. The count capacity of the up-down counter is determined by the anticipated manual intrusion tools that will be employed. The down-count generator rate is determined by the frequency of signals from false alarm sources. The vulnerability of the system to be undetected manual intrusion is a function of the down-count generator time base to the capacity of the up-down'counter... In the embodiment of this invention just described, the signal from the downcount generator removes all stored counts in the up-down counter periodically over the determined time base and the up-down counter starts over with a new count from zero each period. In another embodiment the down-count generator periodically removes, at a much higher rate, a single count from the accumulated total stored count.

4 In this embodiment like in the previous one, when the total determined capacity of stored counts is obtained, a signal is generated which initiates the alarm.

In a two channel signal detection as shown in the drawing it is 'possible for a normal working impulse halfway acoustically between the two transducers to p rovide a pulse that passes both thresholds and thus initiates a false alarm. A threeor more, channel system practically eliminates this possibility of a false alarm, except for the unusual instance in which the transducers are equilaterally located from a source.

What is claimed is:

1. The vibration sensing intrusion detection system comprising:

(a) a plurality of vibration signal detection means each having a threshold level, for detecting vibrationsv'and providing independent output signals of vibrations higher in level than the said threshold .level;

(b) AND gate circuit means; cooperating with the said plurality of detection means for providing an output signal responsive to simultaneous outputs from a plurality of the said detection means; t

(c) a first OR gate circuit means cooperating with the said plurality of detection means for providing an output signal responsive to outputs from each of the'said detection means;

(d) up-down counter means cooperating with the said first OR circuit means for storing the outputs of the said first OR circuits means and providing an output signal after a predetermined accumulation of stored counts;

(e) down count generator means cooperating with the said up-down counter means for periodically at predetermined time intervals removing all the stored counts in the said up-down counter means;

(f) a second OR gate circuit means cooperating with the said AND gate circuits means and with the said up-down counter means for providing an output signal responsive to output signals from either the said AND circuit means or the said updown counter means; and

(g) alarm means responsive to the said second OR circuit means for providing an alarm indication.

2. The intrusion detection system as claimed in claim 1 wherein each of the said plurality of vibration signal detection means includes amplification means and high pass filter means for attenuating low frequency signals.

3. The intrusion detection system comprising:

(a) a plurality of vibration pulse signal detection means, each means including,

(1) acoustic vibration transducer means for providing a pulse signal output responsive toacoustic vibration pulses,

(2) amplification means for amplifying the said transducer signal output,

(3) high pass filter means cooperating withthe said amplification means for removing low frequency signals, and

(4) threshold means cooperating with the high pass filter means for passing only pulse signals above a predetermined level;

(b) AND gate circuit means cooperating with the said plurality of detection means for providing an output signal responsive to simultaneous outputs from a plurality of the said vibration detection means;

(c) a first OR gate circuit means cooperating with the said plurality of vibration detection means for providing an output pulse signal responsive to any output from any of the said vibration detection means;

(d) up-down counter means cooperating with the said first OR circuit means for storing counts of said pulse signal and providing an output signal after a predetermined accumulation of stored counts;

(e) down count generator means cooperating with the said up-down counter means for periodically at p're= determined time intervals removing a predetermined number of the said stored counts on the said updown counter means;

(f) a second OR gate circuit means cooperatingwith the said AND gate circuit means and with the said iup-down counter means for providing an output signal responsive to output signals from either the said AND circuit means or the said up-down counter means; and

(g) alarm means responsive to the said output signals from the said second OR circuit means for pmviding an alarm.,

References Cited UNITED STATES PATENTS.

Patterson et ala S40-261K l0 ALVIN H. WARING, Primary Examiner P PALANs Assistant Examiner l5 S40-25S