US3147467A

US3147467A - Vibration detection vault alarm system

Info

Publication number: US3147467A
Application number: US136487A
Authority: US
Inventors: Laakmann Peter
Original assignee: American District Telegraph Co
Current assignee: American District Telegraph Co
Priority date: 1961-09-07
Filing date: 1961-09-07
Publication date: 1964-09-01
Anticipated expiration: 1981-09-01
Also published as: GB958865A

Description

Sept. 1, 1964 P. LAAKMANN VIBRATION DETECTION VAULT ALARM SYSTEM 4 Sheets-Sheet l Filed Sept. 7, 1961 Sept. 1, 1964 P. LAAKMANN 3,147,467

VIBRATION DETECTION vAuLT ALARM SYSTEM Sept. 1, 1964 P. I AAKMANN VIBRATION DETECTION VAULT ALARM SYSTEM 4 Sheets-Sheet 3 Filed Sept. 7, 1961 4 Sheets-Sheet 4 @aki P. LAAKMANN VIBRATION DETECTION VAULT ALARM SYSTEM Sept. 1, 1964 Filed Sept. 7, 1961 United States Patent O 3,147,467 VIBRATION DETECTION VAULT ALARM SYSTEM Peter Laakmann, Fort Lee, NJ., assignor to American District Telegraph Company, Jersey City, NJ., a corporation of New Jersey Filed Sept. 7, 1961, Ser. No. 136,487 14 Claims. (Cl. 340-261) The present invention relates to burglar alarm systems, and more particularly to burglar alarm systems of the type especially adapted for the protec-tion of vaults and similar enclosures.

Burglar alarm systems for vaults and the like operating on the principle of noise detection have been used for many years, and a variety-of such systems have been proposed. An early example of such a system is found in United States Patent 1,192,312, issued July 25, 1916 to R. M. Hopkins and J. F. D. Hoge. The present invention is concerned primarily with systems of this general type and in particular with systems responsive to vibrations produced in attacks on vaults and similar structures.

The construction of vaults varies from essentially monolithic, steel-reinforced, poured concrete structures to masonry structures of bricks or blocks bonded together with mortar. And an attack on a vault may vary from the extreme of a dynamite blast to the scraping away of mortar with a pointed tool. Modern tools, and especially tools such as portable core drills Wi-th diamond cutters, have been used widely and effectively by burglars in conducting attacks on vault structures, and such tools produce relatively little noise. Sledge hammers and similar noisy instruments have been used by many burglars in the past to gain entry to some types of vaults, but the more eicient burglars have found the hydraulic jack to be a more effective and far less noisy tool for gaining entry to Vaults.

It will be evident that providing an acoustical burglar alarm system usable with different types of vaults and effective in detecting the different possible modes of attack is a problem of some magnitude. Moreover, a satisfactory system must be operable at a relatively high sensitivity level without an undue occurrence of spurious alarms. Almost any of the systems heretofore used can be made sufficiently sensitive to detect the different modes of attack, but such high sensitivity has produced an intolerable false alarm rate. Thus, it has been the practice to compromise between sensitivity on the one hand and stability and freedom from false alarms on the other hand.

Ambient noise has been a serious problem in acoustical burglar alarm systems, and the problem of ambient noise has been greatly aggravated by noise producing machinery, trucks, and especially jet aircraft. In many cases, and especially in the weaker, more easily attacked vaults, the average level of sound of vibratory energy resulting from an attack may be substantially less than the average ambient sound energy. For example, if a vault structure were made of bricks or masonry blocks bonded together by a relatively weak mortar, an attack eifected by scraping away mortar to free one or more blocks might well produce less sound energy than present as a result of ordinary activities in the surrounding area.

The principal object of the present invention has been to provide a novel and improved vault alarm system of the acoustical type. While the invention may be applied with advantage in the protection of the various types of vault structures and under various ambient conditions, the invention has been found especially useful in the protection of the weaker type of vault structure and particularly where located in an area having a high airborne ambient noise level. Typical of such a vault strucice ture would be a fur storage vault made from brick or masonry blocks bonded together with mortar.

By weaker type of vault structure is meant those structures which do not present a physical barrier of the strength afforded by the monolithic structures usually used as bank vaults. The term vault is intended to refer to the various types of rooms and compartments used for the storage of valuables.

An important object of the invention has been the provision of an acoustical type of vault protection system which is effective even when the average ambient noise energy level exceeds the average noise energy level resulting from an attack.

Another object of the invention has been the provision of such a system which is highly responsive to the various possible modes of attack but which is resistant to spurious alarms due to ambient noises.

The electrical protection system of the invention detects physical attacks on a vault or like structure and comprises vibration transducer means physically disposed relative to the structure to be protected so as to produce a signal voltage proportional to a function of the energy expended in an attack on the structure to be protected, integrating means coupled to the transducer and arranged continuously to average the signal voltage over a predetermined relative long time interval, and alarm signalling means coupled to the integrating means and arranged to produce an alarm signal indication when the integrated voltage exceeds a predetermined value.

Other and further objects, features and advantages of the invention will appear more fully from the following description of the invention taken in connection with the appended drawings, in which:

FIG. l is a schematic diagram for use in explaining certain principles of the invention;

FIG. 2 is another schematic diagram for use in explaining certain principles of the invention;

FIG. 3 is a block diagram of a system embodying the invention;

FIGS. 4 and 5, when placed with FIG. 4 above FIG. S, provide a schematic illustration of one form of circuit arrangement embodying the invention; and

FIG. 6 is a schematic diagram illustrating a modification of the circuit of FIG. 5.

Acoustic vault protection systems in the past have been designed to initiate an alarm signal when the Power P of a noise source, which is a function of time, reaches some critical value; or, expressed in mathematical terms,

However, in so far as vault protection systems are concerned, there are two types of noise. One is the sound produced by normal activity in the vicinity of the protected area, termed ambient noise, and to which the alarm system should not respond. The other is the noise resulting from an actual attack upon the vault, to which the protection system should respond promptly. The problem, therefore, is to distinguish between the two types of noise so that false alarms will be avoided while no actual attack is undetected.

Analysis of a burglarious attack upon a vault reveals that the noise produced is a byproduct of the mechanical Work expended in the physical penetration of the wall. Most of the work is converted to heat by means of friction and only a small portion thereof is converted to sound. Thus, the sound energy resulting from an -attack may be expressed as N=(KW) (2) where W is the total energy expended and K is an unknown constant less than unity.

3 Dilferentiating Equation 2 with respect to time yields dN dw -K'a 3) Because N represents the total sound energy, dN/dt represents the energy per unit time or instantaneous power P(t).

Thus

dw P(t) K- (4) where dw/ dt represents the rate of Work expended by the intruder. As stated above, prior protection systems required the value of dw K to reach a critical level before 1an alarm could be initiated. Thus, through the use of a relatively slow-working tool, such as a diamond ltipped drill (slow as compared to the rate of energy expended in a sledge hammer attack), or by scraping away mortar, an intruder could avoid initiating an alarm by keeping his rate of work below the critical value.

However, while the intruder may vary the rate of work widely, the total energy expended, while somewhat dependent upon the type of tool employed, will not vary nearly as widely. Therefore, the basic energy relationships can be solved to determine the total work W by combining Equations 4 and 1 as follows:

K-=F(t)=P(t) (5) and, transposing,

dw=Po dr (6) then integrating both sides l t2 W-KL Pom Therefore, if the protection system is to successfully discriminate between the sounds produced -by the mechanical work of an actual attack on the vault and mere ambient noises in the vicinity of the vault, the circuit must be so designed as to be responsive to a predetermined value of the integral (7) rather than the simple function P(t). Hence, the output M of the detecting device expressed as a function of time should be where K is a constant to be determined for the particular device.

But when the time limits t1 and t2 are taken as zero and infinity, even a small amount of ambient noise would eventually build the integral up to the alarm level. Therefore, the output of the detector must be modified by the inclusion of a term to counteract its tendency to increase continuously. Therefore, some portion of detector output which is represented by QM (i) (9) (wherein Q is a constant less than unity) is continually deducted from the entire detector output, thus (8) becomes carded but the sound energy of a bona tide attack will accumulate to initiate an alarm when the predetermined level is reached.

The foregoing mathematics may be translated into electrical circuits by means of the analog shown in FIG. 1 wherein the noise function P(t) is impressed on an electrical network 20 in the form of a current i and the detector output has its analog in the voltage e.

The form of Equation 10 suggests an RC circuit and it has been found that the network of FIG. 2 has a solution similar to Equation 10. In FIG. 2, the network 20 of FIG. 1 is Iformed as the parallel combination of a resistor 21 and a capacitor 22, whose values are R and C, respectively.

Thus in FIG. 2:

Comparison of Equations 14 and l0 shows both equations to be of the same form and that the constant Q is resembled by the conductance l/R and the constant K by the capacitance C.

Consequently, the operation expressed by Equation l0 can be performed by an electrical circuit and the system of the invention has been designed to operate on this principle coupled with certain other features to be described below. However, the output of a mechanicalelectrical vibration transducer is a function of the vibration intensity and not a function of the instantaneous Vibration power. And the instantaneous noise power is proportional to the square of the noise intensity. Thus, in order to translate rigorously the theoretical concept of Equation 14 into a practical design, an electrical squaring circuit would be necessary. It has been found that such a circuit does not provide, significantly more information than that which is alreadyfp'resent in the output of the vibration transducers. Also, if the signal is squared, the output range of signals would be even larger than the range of input amplitudes. In View of the difficulties associated with amplifying a large range of input amplitudes, a squaring circuit is not illustrated in FIGS. 4

and 5 but could be used if considered desirable.

The system of the invention is illustrated in block diagram form in FIG. 3. The 'system input is derived from the interior surface of theAv it to be protected by means of vibration transducers 2'l vibration transducers, a number of which are preferably connected in parallel, are mounted in direct contact with the internal surface of the vault, preferably in an equally spaced pattern. Typically, twelve transducers might be used, although the number may be varied to suit particular requirements.

The vibration transducers are :preferably microphones of the piezoelectric type and my""each comprise direct actuated Rochelle salt crystal elements arranged in Bimorph construction and housed in a die-cast metallic casing. To ensure reliable sensitivity over large ambient temperature and humidity ranges, the vibration transducer case may be sealed in a suitable potting compound such as an epoxy resin with a hardening agent so that vibratory energy will be transmitted to the case and from the case to the crystal elements within the case. Enclosing the vibration detecting elements also serves to shield these elements from airborne sound except as the latter may produce vibrations in the vault structure.

Vibration transducers other than those of a crystal type may be used, although the crystal type is preferred. Contact microphones of the type used on electrical guitars, banjos or other stringed instruments with sounding boards are in general suitable for use in the system of the invention.

When any one or more f the vibration transducers are subjected to an acceleration, which will occurVwhen the surface to which the transducer is attached vibrates, an alternating voltage will be produced. This alternating voltage is applied to a variable attenuator 24 which may be set to adjust the system for the ambient noise level prevalent at a particular installation and to provide a predetermined system sensitivity level.

As mentioned above, one of the objects of the invention has been to provide successful operation when the ambient sound energy is of a higher average intensity than that of the attack level. This object has been achieved through frequency discrimination obtained by the natural characteristics of the vibration transducers selected and a high-pass filter.

It has been found that the ambient sound energy in vault structures is concentrated at frequencies below 1000 cycles per second whereas the maximum sound energy produced by an attack thereon lies between 1000 and 2000 c.p.s. A high signal to noise ratio is, of course, desirable and the ratio is dependent upon the frequencies of the sounds involved. The minimum signal to noise ratio generally is in the frequency range of 50 to 1000 c.p.s. because most of the ambient noise energy is concentrated in this region, whereas there is only a small portion of the attack energy present in this range.

As the frequencies increase, the signal to noise ratio improves constantly until it reaches a maximum in the region between 3 and 10 kilocycles, even though the attack energy in this region is relatively very small. The transducers and subsequent circuitry in the system of the invention provide peak performance in this region. Each of the transducers has an electrical output proportional to the magnitude of the acceleration to which it is subjected. Acceleration is measured in terms of the gravitation unit G(=981 cm./sec.2), and it has been found that a typical scraping attack on a brick wall produces a vibration therein of l0-4G between 3 and l0 kc., while the maximum vibrations produced by the same attack, regardless of frequency, are lin the order of -3G. Ambient wall vibrations usually vary between 10'2G and 105G, depending on frequency. Thus, it is evident that attack vibrations lie within the region of the ambient disturbances. However, in the system of the invention successful detection of an attack is accomplished through frequency discrimination to permit utilization of the maximum signal to noise ratio.

Thus, as shown by the block diagram of FIG. 3, the alternating output of the transducers is supplied to variable attenuator 24 for system sensitivity adjustment, then to a high-pass filter 25 which cuts off all frequencies below 3 kc. existing in the output of the transducers. The filtered transducer output signal is supplied to a low noise A.C. amplifier 26 having a flat response between 3 and 10 kc. This bandwidth minimizes the noise caused by thermal agitation of the resistors, transistors and like components. The amplifier generally will operate with an input signal only a few db above the inherent noise of the amplifier.

The amplifier output is rectified by a full wave rectifier Z7, producing a pulsating current which is a function of the instantaneous wall vibrations in the frequency region from 3 to 10 kc. The rectifier output is fed to an integrating network 28 which averages the rectifier output over a suitable time interval, preferably about 15-25 minutes. Thus the output of the integrating network is proportional to the average wall vibrations over a selected period. It should be understood that the longer the time interval over which averaging occurs the less likely it will be that an attack will be undetected, since evenan extremely slow attack will eventually produce a substantial integrated output. While for practical purposes an integrating time interval of from about 15 to 25 minutes is preferred, an integrating time interval as low as one-half minute will provide improved results over systems previously used, while an integrating time interval of more than 25 minutes and as much as several hours might be used to advantage in special cases. Such special cases would be ones where a very slow rate of work in an attack was possible.

The integrating network 28 is followed by a differentiating network 29 having a time constant which is long relative to the integrating network time constant. For example, the time constant of the differentiating network might be about 30 to 60 minutes. The network 29 may be omitted if a sufficiently high signal to noise ratio has already been achieved. The differentiating network 29 will pass all signals varying at a faster rate than its time constant and differentiates only those signals which exist over a long period of time. The differentiating network acts to cancel the effect of amplifier noise and does so because the contribution of such noise to the integrated output is a constant which can be eliminated by subsequent differentiation. Differentiation also serves to prevent ambient noises which vary over a long period of time from causing a false alarm, since such slow varying signals will not be passed.

The output of differentiating network 29 is supplied to a voltage sensitive detector 30 which actuates an alarm indicating device when the voltage reaches a predetermined level. The alarm indicating device is here suggested by conductors 31 and normally open relay contacts 32; when detector 30 is actuated, contacts 32 close, completing whatever alarm circuit may be connected to conductors 31. It is highly desirable that the alarm signal be transmitted to a central station, guard station or police headquarters, but a local audible or visual alarm may be provided as is well known in the art.

If the differentiating circuit is used, a slow varying ambient noise, while insufficient to produce an alarm, may eventually reach a sufficient amplitude to overdrive the amplifier with a consequent decrease in amplifier gain and system sensitivity. Such a situation might prevent detection of an attack. To avoid this possibility, the differentiating network is bypassed, as shown by the block 33, the bypass being effective when the output of the integrating network exceeds a predetermined value. This bypass may conveniently be effected by connecting a Zener diode across the differentiating network and selecting the diode to become conductive at the desired value of the integrating network output voltage.

There is a possibility that the short duration but high intensity of a dynamite blast or a similar impact could overdrive the amplifier so heavily that the contribution to the integrated output would be insufficient toproduce an alarm. To overcome this possibility an impact channel has been provided. As shown mathematically above, the integrating operation provided in the integrating channel will provide approximately equal sensitivity for different kinds of attacks even though they vary widely in instantaneous amplitudes and time duration. The mathematics, however, presume a completely linear system. But completely linear accelerometers, amplifiers and rectifiers cannot be constructed to operate over the wide range of amplitudes that is required. As an example, suppose that the lowest wall vibrations which are being detected are of the order of 104G and that these vibrations produce an output voltage at the amplifier which is below the maximum possible output of the amplifier by a factor of 10. If a dynamite blast now occurs, the wall vibrations will easily reach amplitudes of 10G or more resulting in a l change in vibrations and 1010 change in power. Obviously, no amplifier could handle such a variation in input signal amplitudes and its output will be considerably less than that of the theoretical completely linear amplifier. Consequently, the integrated signal will be less and not directly related to wall vibrations as is indicated by the mathematical analysis.

To avoid a defeat of the system by such attack, an impact channel has been added in parallel with the integrating channel. The impact channel comprises a variable attenuator 34 for system sensitivity adjustment, a low gain amplifier 35 operating in a frequency range such as 500 to 10,000 c.p.s. and a rectifier 36 which feeds signals directly to the voltage sensitive detector 30.

The voltage sensitive detector functions upon receipt of a signal of sufficient voltage from either the integrating channel or the impact channel to operate the usual alarm indicating devices at the central station or other remote place and/or local alarm devices.

Referring now to FIGS. 4 and 5, the vibration transducers 23 are connected in parallel across the winding of a potentiometer 37 and across the winding of a potentiometer 38. The potentiometer 37 serves as the variable attenuator for the impact channel while the potentiometer 38 serves as the variable attenuator for the integrating channel.

The slider and one end of potentiometer 38 are connected to respective input terminals of a high-pass filter formed by

capacitors

39, 40 and 41 and coils 42 and 43. The cutoff frequency of the high-pass filter is selected so as to pass the frequencies predominant in vibrations resulting from an attack and to suppress the frequencies predominant in ambient noise. As suggested above, this cutoff frequency will be of the order of 3 k.c. in most installations. The filter impedance is preferably selected as equal to the average impedance of the parallel connected vibration transducers, e.g., 3000 ohms.

The output terminals of the high-pass filter are connected to the respective sides of the primary winding of an input transformer 44 by

conductors

45 and 46. The secondary winding of transformer 44 is coupled to the input circuit of a six stage high gain amplifier comprising direct coupled

transistors

47, 48, 49, 50, 51 and 52. The transistors 47-52 might be, for example, of the 2N336 type.

The high gain amplifier input voltage from transformer 44 is supplied to a resistor 53 through a coupling capacitor 54. One end of resistor 53 is connected to the base of transistor 47 and the other end thereof is coupled to a negative supply conductor 55 through a capacitor 56. The capacitor 56 has a high capacitance, so as to effectively connect resistor 53 and conductor 55 at signal frequencies.

Negative supply conductor 55 and corresponding positive supply conductor 57 are coupled to voltage supply input terminals 58 and 59 through a rectifier circuit 60 which comprises rectifiers 61, 62, 63 and 64 arranged to maintain the polarities of

conductors

55 and 57 irrespective of the supply voltage polarity. A diode 65, preferably of the Zener type, is connected between

conductors

55 and 57 for protection in the event of voltage surges in the supply and a capacitor 66 is connected between

conductors

55 and 57 to smooth the supply voltage.

The collector of each of transistors 47-52 is coupled to positive supply conductor 57 through a respective one of

resistors

67, 68, 69, 70, 71 and 72. The collector of each of transistors 47-51 is also directly connected to the base of the following transistor. The emitter of each of transistors 47-51 is Connected to negative supply conductor 55 and the emitter of transistor 52 is coupled to conductor 55 through a resistor 73.

The amplifier output appears at the collector of transistor 52.

Stability of the transistor amplifier operating point is achieved by means of a first feedback connection, including a transistor 74, which might be of the 522A type. The base of transistor 74 is coupled to the collector of transistor 52 through a resistor 75, the emitter thereof is connected to the junction of resistors 76 and 77 forming a voltage divider between

conductors

55 and 57, and the collector thereof is coupled to the base of input transistor 47 through resistor 53. A capacitor 78 is connected between the base of transistor 74 and negative conductor 55.

If the D.C. operating voltage at the collector of transistor 52 should deviate from -its normal value, e.g., seven volts, the resulting change in voltage at the base of transistor 74 causes a change in the collector current of transistor 74. Since part of the collector output of transistor 74 supplies the base current for transistor 47, a change in the collector current of transistor 74 will change the operating point of transistor 47. The feedback loop is negative, so that a change in amplifier operating point will cause a change in the collector current of transistor 74 in a sense to return the amplifier operating point to its normal value. The corrective feedback loop has a large time constant compared to the operating frequencies so that signal voltages do not experience corrective feedback through this loop.

The amplifier gain is stabilized by a second feedback arrangement formed by

resistors

79, 80, 81, 82 and 83, each of which is connected between the collector and base of a respective one of transistors 47-51. The resistors 79-83 form individual feedback loops for the individual stages, which overcomes the tendency to regenerate at the higher frequencies which would exist were one overall feedback loop used With a six stage amplifier. Direct connection of the feedback resistors between collector and base is possible because the collector voltage of each stage is equal to the base to emitter voltage so that there will only be a very small cornonent of D.C. current fiow from the collector to the ase.

In order to keep the amplifier noise output voltage as low as possible, the amplifier band width should be designed to exclude so far as possible the major ambient noise frequency components. For example, the amplifier. from the base of transistor 47 to the collector of transistor 52 might have a band width of three kilocycles with a center frequency of 4.5 kc. The amplifier lower frequency cutoff is determined by a third negative feedback arrangement afforded by a feedback loop between the base of transistor 52 and the collector of transistor 48. This feedback loop comprises resistors 84 and 85 connected in series between the base of transistor 52 and the collector of transistor 48 and is frequency sensitive because of the low frequency suppression afforded by a capacitor 86 coupled between the junction of resistors 84 and 85 and negative supply conductor 55. The capacitor 86 tends to shunt high frequency components out of the negative feedback loop thus affording less negative feedback at the higher frequencies. High frequency cutoff is determined by a capacitor 87 coupled between the collector of transistor 49 and positive supply conductor 57.

The high gain amplifier output from the collector of transistor 52 is supplied through a capacitor 88 and a conductor 89 to a rectifier circuit formed by rectifiers 90 and 91 connected as a voltage doubler circuit.

The rectified voltage output of the voltage doubler appears across parallel connected capacitor 92 and resistor 93, which form the integrating circuit. Typically, resistor 93 might have a value of 5 megohms and capacitor 92 might have a value of 250 microfarads, yielding a time constant of about 2O minutes. The voltage across capacitor 92 and resistor 93 will be proportional to the integral function as discussed above in connection with Equations and 14. y

A differentiating network formed by a capacitor 94 and a resistor 95 is coupled across the integrating circuit. The differentiated signal output appearing across resistor 95 is supplied through a coupling diode 96 to one end of the coil of a relay 97. The differentiating circuit has a substantially higher time constant than the integrating circuit, preferably more than one and one-half times as high. For example, the capacitor 94 might have a value of 250 microfarads and the resistor 95 a value of megohms, yielding a time constant of about 83 minutes.

The integrating circuit produces an output signal in the form of a pulsating direct voltage whose value represents the average detected vibration over a period determined by the integrating circuit time constant. The differentiating network will pass all signals which Vary at a faster rate than its time constant. Slow varying signals will, however, be suppressed.

A diode 98, which is preferably of the Zener type, is shunted across capacitor 94 and resistor 95, i.e., across the differentiating circuit. Normally the Zener diode 98 does not contribute to the network output. However, if the integrating network output, i.e., the voltage across capacitor 92 and resistor 93, exceeds the Zener diode breakdown voltage, eg., 7 volts, the Zener diode will break down. With the Zener diode 98 conductive the differentiating network is bypassed and a contribution is made to the network output equal to the Zener diode breakdown voltage.

This bypassing of the differentiating network is provided to overcome the possibility that an ambient noise may reach such a large amplitude as to overdrive the amplifier, with consequent decrease in amplifier gain and system sensitivity, and yet vary so slowly as to be suppressed by the differentiating circuit. With the bypass afforded by the Zener diode 98, the signal output required for alarm registration is reduced by the amount of the Zener breakdown voltage. When the voltage across the integrating network drops below the Zener voltage, the Zener diode 98 will become non-conducting and the differentiating network will again be effective.

As mentioned above, the diiferentiating network output voltage is supplied to one end of the coil of relay 97. The other end of the coil of relay 97 is connected to the emitter of a unijunction transistor or double base diode 99, which might be of the 2N498 type. Base B2 of unijunction transistor 99 is connected to the positive supply conductor 57 and base B1 thereof is connected to negative supply conductor 55. The return path for the integrating channel output signal supplied to the emitter of unijunction transistor 99 through the coil of relay 97 is completed through a resistor 100 and a potentiometer 101, the slider of the latter being connected to the junction of rectifier 90, capacitor 92 and resistors 93 and 95.

Unijunction transistor 99 will not conduct until the voltage between its emitter and base B1 equals or exceeds a predetermined percentage of the voltage between bases B1 and B2 thereof, e.g., 60%. In other words, the unijunction transistor will not conduct until a predetermined signal voltage is supplied to the emitter of transistor 99 from the differentiating network or from the integrating network in combination with Zener diode 98. Assuming a 14 Volt supply potential, the B1-B2 voltage will be 14 volts and the signal voltage required to render unijunction transistor 99 conductive would be about 8.4 volts. The alarm signal voltage is that existing between the junction of capacitor 94 and resistor 95 and negative supply conductor 55, or, in other'words, the sum of the voltage across

resistors

95 and 119 and a portion of potentiometer 101.

When unijunction transistor 99 becomes conductive,

an energizing circuit for relay 97 is momentarily completed and the relay 97 picks up. The circuit is energized by the charge on capacitor 120. The current flows through

diodes

102, 103, relay 97, the emitter and base B1 of transistor 99, and back to conductor 55. A capacitor 120 connected between one side of the coil of relay 97 and conductor 55 provides a triggering action which assists in the initial discharge of capacitor 120. Energization of relay 97 results in closing of contact 104 thereof, completing the central station or other alarm signal circuit and transmitting an alarm signal.

Unijunction transistor 99 may also be energized by the impact channel. The impact channel derives its input from a transformer the primary winding of which is connected to potentiometer 37. One end of the secondary winding of Ytransformer 105 is connected to the base of a transistor 106 and the other end thereof is connected to the junction of

resistors

107 and 108 connected as a voltage divider between

supply conductors

55 and 57. This end of the secondary winding of transformer 105 is also coupled to the emitter of transistor 106 through a relatively large capacitor 109. The emitter circuit of transistor 106 is completed to negative supply conductor 55 through a resistor 110.

The collector of transistor 106 is coupled to positive supply conductor 57 through a resistor 111 and is coupled to the base of a transistor 112 through a coupling capacitor 113. A resistor 114 is connected from the base of transistor 112 to the junction of

resistors

107 and 108 to provide bias for transistor 112. The collector of transistor 112 is coupled to positive supply conductor 57 through a resistor 115. The emitter of transistor 112 is coupled to negative supply conductor 55 through the parallel combination of a resistor 116 and a capacitor 117.

Transistors 106 and 112 form a low gain broad band amplifier which will amplify the wide range of frequencies associated with heavy impacts such as those of a dynamite or other blast. The amplified output is supplied from the collector of transistor 112 through a capacitor 118 to the junction of

rectiers

102 and 103 which are connected as a voltage doubler. The voltage doubler circuit is completed by a resistor 119 and a capacitor 120 connected in parallel between one end of rectifier 102 and negative supply conductor 55.

The rectified output of the voltage doubler 102-103 is supplied through the coil of relay 97 to the emitter of unijunction transistor 99 and will cause the latter to conduct if this rectified output is sufficiently great, i.e., if this rectified output is sufficient to make the emitter-base B1 voltage of the unijunction transistor at least a predetermined percentage of the base Bl-base B2 voltage. Energization of unijunction transistor 99 results in momentary energization of relay 97, as described above, and hence results in transmission of an alarm signal.

The sensitivity of unijunction transistor 99 can be adjusted by changing the position of the slider of the potentiometer 101. This slider is connected to the junction of

capacitors

121 and 122 connected in series between

conductors

55 and 57.

The unijunction transistor 99 may conveniently be replaced with a field effect device such as a field effect transistor or a field effect tetrode. Such a field effect device affords the reliability and other advantages of a solid state device but permits a continuous output which is desirable for recording ambient sound level in a new installation.

A modification of a part of the circuit of FIG. 5 to use a field effect transistor is shown in FIG. 6. The field effect transistor is shown at 123 and might be of the C4612 type. The grid of field `effect transistor 123 is connected to the junction of capacitor and rectifier 103 and the cathode thereof is coupled to negative supply conductor 55 through a variable resistor 124. The anode of transistor 123 is coupled to positive supply conductor 57 through the coil of an alarm relay 125 and a jack 126.

A resistor 127 is coupled between the cathode and anode of transistor 123.

Relay 125 is arranged to become energized when the anode current of transistor 123 reaches a value corresponding to an input voltage at which unijunction transistor 99 of FIG. 5 become conductive. Energization of relay 125 closes contacts 128 thereof, transmitting an Yalarm signal as in the case of contacts 104.

By connecting a recorder to jack 126, a record may be made of the transistor 123 output. This record will be proportional to the integrating and impact channel outputs and hence to the ambient noise and provides use- Aful information in setting the channel sensitivities.

While the invention has been described in connection with a specific embodiment thereof and in a specific use, various modifications thereof will occur to those skilled in the art Without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

l. An electrical protection system for detecting physical attacks on a vault or like structure, comprising vibration transducer means disposed in close proximity to the inside surface of the structure to be protected so as to be subject to acceleration resulting from vibratory energy in the structure walls and arranged to produce an alternating signal voltage proportional to the magnitude of the acceleration to which it is subjected, means to filter said signal voltage to suppress frequency components therein below a predetermined value, a first amplifier, means to Vsupply the filtered signal voltage output of said filter means to the input circuit of said first amplifier, a rectifying circuit coupled to the output of said first amplifier and arranged to produce a pulsating direct current from the amplified signal voltage output of said first amplifier, an integrating network having a relatively long time constant and coupled to the output of said rectifier circuit to integrate said pulsating direct current, a differentiating network having a time constant substantially longer than said relatively long time constant and coupled to the output of said integrating network to differentiate the integrated pulsating direct current output of said integrating circuit, a detector arranged to produce an alarm signal indication when the input supplied thereto exceeds a predetermined value, means to derive an output from said differentiating network and to apply the same as an input to said detector, bypass means coupled to said integrating and differentiating networks and arranged to bypass said dierentiating network when the output of said integrating network exceeds a predetermined value thereby to apply said output of said integrating network to said detector as an input potential, a second amplifier, means to apply said signal voltage to the input of said second amplifier, means to rectify the output of said second amplifier, and means to apply the rectified output of said second amplifier to said detector as an input potential.

2. An electrical protection system for detecting physical attacks on a vault or like structure, comprising vibration transducer means disposed in close proximity to the inside surface of the structure to be protected so as to be subject to acceleration resulting from vibratory energy in the structure walls and arranged to produce an alternating signal voltage proportional to the magnitude of the acceleration to which it is subjected, means to filter said signal voltage to suppress frequency components therein below about 3000 cycles per second, a first amplifier, means to supply the filtered signal voltage output of said filter means to the input circuit of said first amplifier, a rectifying circuit coupled to the output of said first amplifier and arranged to produce a pulsating direct current from the amplified signal voltage output of said first amplifier, an integrating network having a relatively long time constant and coupled to the output of said rectifier circuit to integrate said pulsating direct current, a differentiating network having a time constant substantially longer than said relatively long time constant and coupled to the output of said integrating network to differentiate the integrated pulsating direct current output of said integrating circuit, a voltage-sensitive detector arranged to produce an alarm signal indication when the input potential supplied thereto exceeds a predetermined value, means to derive a voltage from said differentiating network and to apply the same as an input potential to said detector, a second amplifier, means to apply said signal voltage to the input of said second amplifier, means to rectify the output of said second amplifier, and means to apply the rectified output of said second amplifier to said detector as an input potential.

3. An electrical protection system for detecting physical attacks on a vault or like structure, comprising a plurality of vibration transducers disposed in close proximity to the inside surface of the structure to be protected so as to be subject to acceleration resulting from vibratory energy in the structure walls and each arranged to produce an alternating output voltage proportional to the magnitude of the acceleration to which it is subjected, means to combine said output voltages into a composite signal voltage proportional to the intensity of vibrations in said walls, means to filter said signal voltage to suppress frequency components therein below about 3000 cycles per second, a first plural stage amplifier having a substantially fiat response over a predetermined frequency range having a lower limit of about 3000 cycles per second, means to supply the filtered signal voltage output of said filter means to the input circuit of said first amplifier, a rectifying circuit coupled to the output of said first amplifier and arranged to produce a pulsating direct current from the amplified signal voltage output of said first amplifier, an integrating network having a time constant lying in the range of about l5 to 25 minutes and coupled to the output of said rectifier circuit to integrate said pulsating direct current, a voltage-sensitive detector arranged to produce an alarm signal indication When the input potential supplied thereto exceeds a predetermined value, means to apply the output voltage of said integrating network to said detector as an input potential, a second amplifier having a gain substantially less than the gain of said first amplifier, means to apply said signal voltage to the input of said second amplifier, means to rectify the output of said second amplifier, and means to apply the rectified output of said second amplifier to said detector as an input potential.

4. An electrical protection system for detecting physical attacks on a vault or like structure, comprising a plurality of vibration transducers disposed in close proximity to the inside surface of the structure to be protected so as to be subject to acceleration resulting from vibratory energy in the structure Walls and each arranged to produce an alternating output voltage proportional to the magnitude of the accelera-tion to which it is subjected, means to combine said output voltages into a composite signal voltage proportional to the intensity of vibrations in said Walls, means to filter said signal voltage to suppress frequency components therein lbelow about 3000 cycles per second, a plural stage amplifier having a substantially flat response over a predetermined frequency range having a lower limit of about 3000 cycles per second, means to supply the filtered signal voltage output of said filter means to the input circuit of said amplifier, a rectifying circuit coupled to the output of said amplifier and arranged to produce a pulsating direct current from the amplified signal voltage output of said amplifier, an integrating network having a time constant lying in the range of about l5 to 25 minutes and coupled to the output of said rectifier circuit to integrate said pulsating direct current, a differentiating network having a time constant lying in the range of' about 30 to 60 minutes and coupled to the output of said integrating network to differentiate the integrated pulsating direct current output of said integratingcircuit, a voltage-sensitive detector arranged to produce an alarm signal indication when the input po- Air tential supplied thereto exceeds a predetermined value, means to derive a voltage from said differentiating network and to apply the same as an input potential to said detector, and bypass means coupled to said integrating and differentiating networks and arranged to bypass the latter when the output voltage of said integrating network exceeds a predetermined value thereby to apply said output voltage of said integrating network to said detector as an input potential.

5. An electrical protection system for detecting physical attacks on a vault or like structure, comprising a plurality of vibration transducers disposed in close proximity to the inside surface of the structure to be protected so as to be subject to acceleration resulting from vibratory energy in the structure walls and each arranged to produce an alternating output voltage proportional to the magnitude of the acceleration lto which it is subjected, means to combine said output voltages into a composite signal voltage proportional to the intensity of vibrations in said walls, means to filter said signal voltage to suppress frequency components therein below about 3000 cyclesper second, a first plural stage amplifier having a substantially flat response over a predetermined frequency range having a lower limit of about 3000 cycles per second, means to 4supply the lfiltered signal voltage output of said filter means to the input circuit of said first amplifier, a rectifying circuit coupled to the output of said first amplifier and arranged to produce a pulsating direct current from the amplified signal voltage output of said first amplifier, an integrating network having a time constant lying in the range ofabout 15 to 25 minutes and coupled to the o-utput of said rectifier circuit to integrateV said pulsating direct current, a differentiating network having a time constant lying in the range of about 30 to 60 minutes and coupled to the output of said integrating network to difierentiate the integrated pulsating direct current output of said integrating circuit, a voltage-sensitive detector arranged to produce an alarm signal indication when the input potential supplied thereto exceeds a predetermined value, means to derive a voltage from said differentiating network and to apply the same as an input potential to said detector, bypass means coupled to said integrating and differentiating networks and arranged to bypass the latter when the output voltage of said integrating network exceeds a predetermined value thereby to apply said output voltage of said integrating network to said detector as an input potential, a second amplifier having a gain substantially less than the gain of said first amplifier, means to apply said signal Voltage to the input of said second amplifier, means to rectify the output of said second amplifier, and means to apply the rectified output of said second amplifier to said detector as an input potential.

6. An electrical protection system for detecting physical attacks on a vault or like structure, comprising a plurality of vibration transducers disposed in close proximity to the inside surface of the structure to be protected so as to be subject to acceleration resulting from vibratory energy in the structure walls and each arranged to produce an alternating output voltage proportional to the magnitude of the acceleration to which it is subjected, means to combine said output voltages into a composite signal voltage proportional to the intensity of vibrations in said walls, means to filter said signal voltage to suppress frequency components therein below a predetermined frequency, a first plural stage amplifier having a substantially flat response over a frequency range lying above said predetermined frequency, a frequency-sensitive negative feedback loop coupled to said first amplifier and having a frequency response tending to suppress amplification of frequencies lying outside said range, means to supply the filtered signal voltage output of said filter means to the input circuit of said first amplifier, a rectifying circuit coupled to the output of said first amplifier 4and arranged to produce a pulsating direct current from the amplified signal voltage output of said amplifier, an

integrating network having a time constant of many minutes and coupled to the output of said rectifier circuit to integrate said pulsating direct current, a differentiating network having a time constant substantially longer `than the time constant of said integrating network and coupled to the output of said integrating network to differentiate the integrated pulsating direct current output of said integrating circuit, a voltage-sensitive detector arranged to produce an alarm signal indication when the input potentional supplied thereto exceeds a predetermined value, means to derive a voltage from said differentiating network and to apply the same as an input potential to said detector, bypass means coupled to said integrating and differentiating networks and arranged to bypass the latter when the output voltage of said integrating network exceeds a predetermined value thereby to apply said output voltage of said integrating network to said detector as an input potential, a second amplifier having a gain substantially less than the gain of said first amplifier, means to apply said signal voltage to the input of said second amplifier, means to rectify the output of said second amplifier, and means to apply the rectified output of said second amplifier to said detector as an input potential.

7. An electrical protection system for detecting physical attacks on a vault or like structure, comprising a plurality of vibration transducers disposed in close proximity to the inside surface of the structure to be protected so as to be subject to acceleration resulting from vibratory energy in the structure walls and each arranged to produce an alternating output voltage proportional to the magnitude of the acceleration to which it is subjected, means to combine said output voltages into a composite signal voltage proportional to the intensity of vibrations in said walls, means to filter said signal voltage to suppress frequency components therein below a predetermined frequency, a first plural stage transistor amplifier having a substantially fiat response over a frequency range lying above said predetermined frequency, a first negative feedback loop intercoupling the output and input circuits of said amplifier and having a long time constant at the frequencies of said signal voltage whereby said first negative feedback loop stabilizes the operating point of said first amplifier, a second negative feedback loop coupled to said first amplifier and having frequency-sensitive components selected and arranged to suppress amplification of signal frequencies outside said range, a rectifying circuit coupled to the output of said first amplifier and arranged to produce a pulsating direct current from the amplified signal voltage output of said amplifier, an integrating network having a relatively long time constant and coupled to the output of said rectifier circuit to integrate said pulsating `direct current, a voltage-sensitive detector arranged to produce an alarm signal indication when the input potential supplied thereto exceeds a predetermined value, means to apply the output Voltage of said integrating network to said detector as an input potential, a second amplifier having a gain substantially less than the gain of said first amplifier, means to apply said signal Voltage to the input of said second amplifier, means to rectify the output of said second amplifier, and means to apply the rectified output of said second amplifier to said detector as an input potential.

8. A'n electrical protection system as set forth in claim 7 in which said voltage sensitive detector comprises a field effect device and in which said input potentials are applied to the input electrode of said field effect device.

9. An electrical protection system for detecting physical attacks on a vault or like structure, comprising a plurality of vibration transducers disposed in close proximity to the inside surface of the structure to be protected so as to be subject to acceleration resulting from vibratory energy in the structure walls and each arranged to produce an alternating output voltage proportional to the magnitude of the acceleration to which it is subjected, means to combine said output voltages into a composite signal voltage proportional to the intensity of vibrations in said walls, means to filter said signal voltage to suppress frequency components therein below a predetermined frequency, a first high gain plural stage amplifier having a substantially fiat response over a frequency range lying above said predetermined frequency, means to supply the filtered signal voltage output of said filter means to the input circuit of said first amplifier, a rectifying circuit coupled to the output of said first amplifier and arranged to produce a pulsating direct current from the amplified signal voltage output of said amplifier, an integrating network having a time constant lying in the range of about 15 to 25 minutes and coupled to the ouput of said rectifier circuit to integrate said pulsating direct current, a differentiating network having a time constant lying in the range of about 30 to 60 minutes and coupled to the output of said integrating network to differentiate the integrated pulsating direct current output of said integrating circuit, a unijunction transistor aranged as a voltage-sensitive detector to produce an alarm signal indication when the input potential applied between the emitter and one of the bases of said unijunction transistor exceeds a predetermined value, means to derive a voltage from said differentiating network and to apply the same as an input potential to said unijunction transistor emitter-base circuit, bypass means comprising a diode element connected across said differentiating network and arranged to bypass said differentiating network when the output voltage of said integrating network exceeds a predetermined value thereby to apply said output voltage of said integrating network to said unijunction transistor emitter-base circuit as an input potential, a second low gain amplifier, means to apply said signal voltage to the input of said second amplifier, means to rectify the output of said second amplifier, and means to apply the rectified output of said second arnplifier to said unijunction transistor emitter-base circuit as an input potential.

10. An electrical protection system for detecting physical attacks on a vault or like structure, comprising a plurality of vibration transducers disposed in close proximity to the inside surface of the structure to be protected so as to be subject to acceleration resulting from vibratory energy in the structure walls and each arranged to produce an alternating output voltage proportional to the magnitude of the acceleration to which it is subjected, means to combine said output voltages into a composite signal voltage proportional to the intensity of vibrations in said walls, means to filter said signal voltage to suppress frequency components therein below a predetermined value, a first amplifier having a substantially fiat response over a frequency range lying above said predetermined value, means to supply the filtered signal voltage output of said filter means to the input circuit of said first amplifier, rectifying means coupled to the output of said first amplifier to rectify the output thereof, means coupled to said rectifying means to integrate the output thereof over a relatively long time interval and to produce an integrated first output potential having a magnitude proportional to the detected vibration intensity in said walls over said relatively long time interval, a second amplifier circuit having a gain substantially less than the gain of said first amplifier circuit, means to apply said signal voltage to the input of said second amplifier, means to derive from the output of said second amplifier circuit a second output potential proportional to the instantaneous vibration intensity in said walls, signalling means responsive to an applied input potential greater than a predetermined value to transmit an alarm signal, and means t supply each of said first and second output potentials to said signalling means as an input potential.

l1. An electrical protection system for detecting physical attacks on a vault or like structure, comprising a plurality of vibration transducers disposed in close proximity to the inside surface of the structure to be protected so as to be subject to acceleration resulting from vibra- -16 tory energy in the structure walls and each arranged to produce an alternating output voltage proportional to the magnitude of the acceleration to which it is subjected, means to combine said output voltages into a composite signal voltage proportional to the intensity of vibrations in said walls, means to filter said signal voltage to suppress frequency components therein below about 3000 Vcycles per second, a first plural stage amplifier having a substantially fiat response over a predetermined frequency range having a lower limit of about 3000 cycles per second, means to supply the filtered signal voltage output of said filter means to the input circuit of said first amplifier, a rectifying circuit coupled to the output of said first amplifier and arranged to produce a pulsating direct current from the amplified signal voltage output of said first amplifier, an integrating network having a relatively long time constant and coupled to the output of said rectifier circuit to integrate said pulsating direct current, a field effect transistor, signalling means coupled to the anode of said field effect transistor and arranged to produce an alarm signal when the anode current of said field effect transistor exceeds a predetermined value, means to apply the output voltage of said integrating network to the grid of said field effect transistor, a second amplifier having a gain substantially less than the gain of said first amplifier, means to apply said signal voltage to the input of said second amplifier, means to rectify the output of said second amplifier, and means to apply the rectified output of said second amplier to the grid of said field effect transistor.

12. An electrical protection system for detecting physical attacks on a vault or like structure, comprising vibration transducer means physically disposed relative to the structure to be protected so as to produce a signal voltage proportional to a function of the energy expended in an attack on said structure to be protected, rectifying means coupled to said transducer means to rectify said signal voltage, integrating means coupled to said rectifying means and arranged continuously to average said rectified signal voltage over a predetermined relatively long time interval, differentiating means coupled to said integrating means and having a time constant substantially longer than said relatively long time interval, and alarm signalling means coupled to said differentiating means and arranged to produce an alarm signal indication when the integrated voltage exceeds a predetermined value.

13. An electrical protection system for detecting physical attacks on a vault or like structure, comprising vibration transducer means physically disposed relative to the structure to be protected so as to produce a signal voltage proportional to a function of the energy expended in an attack on said structure to be protected, rectifying means coupled to said transducer means to rectify said signal voltage, integrating means coupled to said rectifying means and arranged continuously to average said rectified signal voltage over a predetermined relatively long time interval, said integrating means comprising a capacitive element, a charging circuit for said capacitive element and a resistive element coupled in parallel with said capacitive element, differentiating means coupled to said integrating means and having a time constant substantially longer than said relatively long time interval, and alarm signalling means coupled to said differentiating means and arranged to produce an alarm signal indication when the integrated voltage across said capacitive element exceeds a predetermined value.

14. An electrical protection system for detecting physical attacks on a vault or like structure, comprising vibration transducer means physically disposed relative to the structure to be protected so as to produce a signal voltage proportional to a function of the energy expended in an attack on said structure to be protected, rectifying means coupled to said transducer means to rectify said signal voltage, integrating means coupled to said rectifying means and arranged continuously to average said 17 rectied signal voltage over a predetermined relatively long time interval greater than about one-half minute and less than about 25 minutes, differentiating means coupled to said integrating means and having a time constant lying in the range of about 30 to 60 minutes, alarm sig- 5 nalling means coupled to said differentiating means and arranged to produce an alarm signal indication when the integrated voltage exceeds a predetermined value, and means coupled to said transducer and to said alarm signalling means and responsive only to a signal voltage 10 above a selected level to cause said alarm signalling means to produce an alarm signal indication independently of the value of said integrated Voltage.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. AN ELECTRICAL PROTECTION SYSTEM FOR DETECTING PHYSICAL ATTACKS ON A VAULT OR LIKE STRUCTURE, COMPRISING VIBRATION TRANSDUCER MEANS DISPOSED IN CLOSE PROXIMITY TO THE INSIDE SURFACE OF THE STRUCTURE TO BE PROTECTED SO AS TO BE SUBJECT TO ACCELERATION RESULTING FROM VIBRATORY ENERGY IN THE STRUCTURE WALLS AND ARRANGED TO PRODUCE AN ALTERNATING SIGNAL VOLTAGE PROPORTIONAL TO THE MAGNITUDE OF THE ACCELERATION TO WHICH IT IS SUBJECTED, MEANS TO FILTER SAID SIGNAL VOLTAGE TO SUPRESS FREQUENCY COMPONENTS THEREIN BELOW A PREDETERMINED VALUE, A FIRST AMPLIFIER, MEANS TO SUPPLY THE FILTERED SIGNAL VOLTAGE OUTPUT OF SAID FILTER MEANS TO THE INPUT CIRCUIT OF SAID FIRST AMPLIFIER, A RECTIFYING CIRCUIT COUPLED TO THE OUTPUT OF SAID FIRST AMPLIFIER AND ARRANGED TO PRODUCE A PULSATING DIRECT CURRENT FROM THE AMPLIFIED SIGNAL VOLTAGE OUTPUT OF SAID FIRST AMPLIFIER, AN INTEGRATING NETWORK HAVING A RELATIVELY LONG TIME CONSTANT AND COUPLED TO THE OUTPUT OF SAID RECTIFIER CIRCUIT TO INTEGRATE SAID PULSATING DIRECT CURRENT, A DIFFERENTIATING NETWORK HAVING A TIME CONSTANT SUBSTANTIALLY LONGER THAN SAID RELATIVELY LONG TIME CONSTANT AND COUPLED TO THE OUTPUT OF SAID INTERGRATING NETWORK TO DIFFERENTIATE THE INTEGRATED PULSATING DIRECT CURRENT OUTPUT OF SAID INTERGRATING CIRCUIT, A DETECTOR ARRANGED TO PRODUCE AN ALARM SIG-