GB2080592A - Vibration and/or impact detection system - Google Patents

Abstract

An inertial sensor (31) and a pulse shaper (33) produce a pulse train in response to sensed vibrations and impacts. A reference pulse generator (37) compares the pulse periods in the train with a reference period (tr) which is adjustable by an adjuster (47). Pulses of periods less than (tr) are fed via a vibration pulse separator (37A) and an AND gate (38) to an integrator (39) having an adjustable integration time to produce a vibration alarm if vibrations have at least a 50% duty ratio and occur in a burst of preset duration. Pulses of periods greater than (tr) are fed via an impact pulse separator (37B) to an impact pulse counter (49) which in response to a first pulse is enabled by a unit (52) for a 5s window period to count the pulses. A delay (120) and a inhibit circuit (121) disable the counter (49) for an inhibit period of ls after a delay of 20ms from the first pulse. An impact alarm is produced if a preset, selectable, number of pulses is counted in the 5s window minus the inhibit period. Any pulses occurring in the inhibit period are not counted but extend the window. In a further embodiment, the vibration alarm is produced by an integrator without taking account of duty ratio. In the further embodiment the impact alarm is produced in response to the number of pulses first occurring in each of a succession of windows without counting any successive pulse in each window. <IMAGE>

Description

SPECIFICATION Vibration and/or impact detection system The present invention relates to a vibration and/or impact detection system.

The system has, as an illustrative application, the detection of out of balance conditions in machinery or the detection of intruders for an intruder alarm system.

United States Patent 3947835 (Layman - GTE Sylvania Inc). discloses a fence protection system in which a pulse train produced by an electret cable is analysed as to the width of pulses and as to spacing of pulses to determine whether a pulses should be fed to a pulse counter. The counter gives an alarm if 3 pulses are counted in 120s. An integrator gives an alarm if a continuous series of pulses occurs for 8 seconds, the pulses failing to satisfy the width and spacing tests.

United States Patent 4180811 (Matsushita) discloses a piezoelectric sensor buried in a wall to detect vibrations and impacts which are separated in circuits according to amplitude. Impacts are counted, whilst vibrations are integrated, to produce an alarm.

The present invention provides in one aspect a vibration and/or impact dection system comprising: sensor means responsive to vibrations and impacts to produce a pulse train, means for comparing the pulse periods of the train with a reference period to separate vibrational pulses having a period less than the reference period from impact pulses having a period greater than the reference period, and, means for producing a vibration indication signal in response to the vibration pulses alone and/orfor producing an impact indication signal in response to impact pulses alone the indication signal producing means being responsive to at least the number of pulses produced in a preset time to produce the indication signal.

An embodiment of the invention is provided to enhance the discrimination between ambient vibrations of traffic or normally running machinery and abnormal vibrations arising from machinery out of balance or intruders forcing doors or breaking windows etc.

Accordingly the embodiment of the invention designate sensed vibrations as alarm-significant if they (1) exceed a given threshold, (2) show a substantial, e.g. at least 50% duty ratio, and (3) form a burst af a specified minimum duration.

Sensed impacts to be designated dangerous may require a minimum duration of an individual pulse, longer than that typical of vibration, and will generally, but not always, exhibit totally irregular pulse or inter-pulse durations.

Further alarm status signals may result in the embodiment if the signal lead from the sensor gives open or short circuit behaviour. The lead would then not convey sensor signals and may have been tampered with by an intruder. Alternatively, the lead may have been accidentally broken, thus compromising the integrity of the detector system.

The embodiment further comprises indication signal producing means responsive to the impact pulses alone and operable to produce the impact indication signal only if a preset number of the impact pulses occur in a preset time.

A further embodiment of the system is provided to further enhance the discrimination between real and false alarms due to impacts.

A system is required to detect impacts indicative of alarm conditions, e.g. intruders breaking in or flywheels out of balance, but it may generate two sorts of signals liable to set offfalse alarms.

Excessive false alarms cause unnecessary trouble and expense, and lead people to ignore dangerous conditions which are correctly indicated. One sort of false alarm is inherent in typical transducers of impacts, in that they generate some internally derived noise pulses, which are relatively easy to discriminate against by amplitude and time thresholding. The other sort of false alarm originates from what one might term environmental impacts, e.g., slamming doors and widows, traffic on uneven roads near the safety installation etc.

It is the environmental sort of impact indication that the present further embodiment is aimed to designate innocent, while maintaining full integrity to warn of danger-indicating impacts.

The present embodiment in one aspect is based on the realisation that two strong single impacts having less than a short period, such as 20 milliseconds between them, will very likely indicate a first group of danger signals, e.g., a deliberately broken pane of glass, or fanlight, shop window etc. Therefore we propose that one impact pulse followed within a short time by another be caused to create an alarm condition. For instance, the leading edge of any detected pulse can be caused to create a 20 milliseconds, first "window" period after its termination edge, during which window period one or more further impacts cause an alarm, but after which period further impacts are ignored.

A second aspect of the present embodiment causes any pulse considered as possibly signifying a dangerous situation to set up in sequence first, a relatively short, wait-and-see or "window" period, then an intermediate time period during which all pulses are ignored, at least for the purpose of signifying an alarm condition in combination with the first pulse, and finally a second, relatively long, window period. A counter or integrator will sum the effects of all pulses detected throughout the two window periods. The total of the two window periods may be five seconds or so, and e.g. eight pulses therein signified as consistent with an intruder or out-of-balance machinery. Alternatively or additionally, as described for the first aspect, two pulses during the first window period alone may be used to raise the alarm.

The intermediate period during which pulses are ignored is based on our realization that any impact pulses occurring during a sizeable period following the first period are likely to be innocent, denoting e.g. heavy road traffic rattling a loose window frame.

Therefore during such an intermediate period, the embodiment causes inhibition of the impact signal counting, integration or consolidating, because such impact signals are assumed to originate from en vironmental impacts or other innocent factors.

Presently preferred are a first counting period, following the first impact pulse termination edge, of 20 milliseconds, during which a singe further pulse raises the alarm, an intermediate period of one second during which further counting is inhibited and a second window period, e.g. of perhaps nearly four seconds, during which pulses are again counted and assumed not to be environmental.

After the termination of the second period the counting or integration is reset, any total being wiped out.

A further preferred refinement is for any intermediate period pulse occurrence to have an effect, not for contributing to any total count, but for prolonging the total, window period by initiating another five second period. In effect it is the second counting period which is extended by five seconds from any reception of an impact-type pulse during the inhibit period (or indeed during the second "window" period). Thus the counting period in this refinement will be extended e.g. to run a full five second from reception of the impact during the intermediate period (or the second counting period), instead of the originally set up five seconds from the start of the original first pulse.

Resetting in this embodiment thus only occurs if the whole five second period after the original impact is clear of further impacts.

The foregoing further embodiment of the invention at least when used with an inertial sensor comprising a spherical mass which contacts spaced apart contacts and acts as a switch in response to impacts to produce the impact pulses, may suffer from "contact bounce". Thus a signal impact may produce several impact pulses as the mass bounces several times on the contacts. Thus in accordance with a yet further embodiment, the indication signal producing means is responsive to impact pulses alone to produce an impact indication signal and comprises means responsive to the first impact pulse to occur in each of a succession of present time periods to count that pulse and to not count any succeeding pulse in that period, the impact indication signal being produced in response to a, selectable predetermined number of counted pulses.Each preset time period is sufficiently long to allow contact bounce to die.

The yet further embodiment may also process the vibration pulses differentially from the foregoing embodiments in that it integrates the pulses over a preset time, being unresponsive to duty ratio; in this way it is more sensitive.

For a better understanding of the invention, reference will now be made, by way of example to the accompanying drawings in which: Figure la shows a vibration signal waveform, eg.

afterthresholding and limiting: Figure 1b shows an impact signal waveform: Figure 2 shows a block diagram of an inertial sensor system of the invention; and Figure 3 shows a circuit diagram of a system similar to that of Figure 2.

Figure 4A shows a typical signal from an inertial switch type sensor denoting a glass facture impact: Figure 4B shows on a similar time scale as Figure 4A a hammering impact on structures by an intruder and Figure 4C shows innocent or enviromental impact pulses sequence derived from sttiie'res or windows: Figure 5shows a total count window period: Figure 6shows a block diagram of an impact pulse analyserfordiscrimination against environmental impacts; Figure 7 shows a circuit diagram of such a-n analyser.

Figure 8 shows a modification of the impact pulse analyser of Figure 6 and 7, and shows a modification of the vibration pulse analyser of Figure 3 or Figure 7.

The circuit type numbers referred to herein and shown in the drawings are numbers used by RCA Corporation.

Referring to Figure 1a, there is shown a train or square wave negative - going pulses 21. They may appear when vibrations are transduced e.g. by an inertial sensor, into electrical signals which are compared continuously with high and low amplitudes, such that a fixed level output is given when incident signals exceed the high, or go below the low, amplitude. Lower sensor output amplitudes can play no part in pulses 21, but can indicate faults.

These vibration pulses may have pulse lengths as short as 1/2 ms.

The pulse train of Figure la shows the train of individual pulses to have, approximately, uniform durations t, and uniform spacings t2. The train lasts for a time oft3 as shown or longer.

The Figures 1,2 and 3 embodiments are based in part on the criterion that vibrations should be indicated only if t1 equals or exceeds t2 i.e., the duty ratio is at least 50%, and t3 has at least a minimum preset-table value.

Referring nowto Figure 1b, there is shown an irregular train or burst of square wave pulses, which may be derived by digitizing the output of a transducer in the same way as described for Figure 1a. Ip this case the transducer is subject to Impact forces about a certain minimum. These pulses may have, lengths of greater than 50 ms. The resulting pulses 22 are longer than the vibrational pulses 21. The pulses 22 may have similar durations t4, t6 or may exhibit differing ON times t4, teas shown. The OFF times e.g. 5 may be similar to the ON times t4, t6, or may be different therefrom and irregular. The total duration of a train, shown in Figure 1b as t7, may be designated as only being consistent with an alarm status if less than a certain maximum (t7) Figures 2 shows a block circuit diagram of a system sensing vibrations and impacts and minifesting only those that are deemed dangerous by the cirteria set out above. In Figure 2 an inertial sensor 31 has an output 32 coupled to a pulse shaper unit 33 to produce square pulses, The square pulses of FigursR 1e or lb may be the outputs of unit 33, but this unit is arranged also to give an output if the input hereto, e.g. sensor 31 or its connection to pulse shaper 33, has an electrical fault such as an open or short circuit.In the last case, the condition is recognised by a line monitor unit 34 to give a specific alarm output at a terminal 35 and a general alarm outputto a multi-alarm register 36.

If shorter pulses such as the vibration pulses of Figure la result from shaper 33, a reference pulse generator 37 gives an output to an AND gate 38 so that this passes the shorter pulses, but not longer impact pulses, to an integrator 39. This is arranged in this embodiment to integrate only such shorter pulses when of at least 50% duty cycle in a train of the pulses. Moreover, a sensitivity adjustable control 40, e.g. a series or a feedback device as shown, selects the sensitivity e.g., it may result in the integrator 39 only giving an output after a given integration period. The integrator output may result in an alarm signal, e.g. at a specific vibration alarm output terminal 41, only if pulses are still being received from pulses shaper 33 over a line 42 to an AND gate operating as a vibration alarm outputting gate 43.This ensures a minimum t3, burst period, for the alarm to operate. The multi-alarm register 36 which may well be at a distant point, will also receive this gated alarm output, over a line 44. The period t3 may vary according to vibration signal duty ratio.

The vibration pulses separated from impact pulses by the reference pulse generator 37 may be quite short, and may be lengthened by a pulse stretcher 45 to turn on a local visual indicator 46. This can be used to facilitate the initial setting of the various controls so as to minimise false alarms, For instance, a reference adjuster 47 sets the appropriate length of pulses which are to be selected by the reference pulse generator as signifying vibrations, and indicator 46 would check the setting. There would only be an output from vibration alarm output device 43 if all the criteria described above as designating dangerous vibrations were present also.

If the pulses from pulse shaper 33 are longer than vibration pulses, e.g., if they have lengths t4 or t6 in Figure 1 b, the reference pulse generator 37 will combine with an impact pulse separator 37B so that only impact-derived pulses are passed to an impact window determining device 48, which will determine if such pulses, i.e., recognized to be of the impact pulse type, form a train within a maximum duration, say t7 of Figure ib. If so, it will also be checked whether a sufficient number of the pulses occur in a burst, by an impact pulse counter-and-selector 49. If this is so, an impact alarm output device 50 will respond to simultaneous inputs from selector 49 and. over line 42, from the pulse shaper 33.Alarm device 50 will apply danger indication signals to an output point 51 and or the multi-alarm register 36.

As for the vibration pulses, the separated impact pulses from separator 378 are lengthened by a pulse stretcher 52 and fed to an indicator 53 so as to facilitate the setting up of the detector system.

Indicator 53, will record that impact pulses are being received, whereas window device 48 and counter 49 ensure that alarm is only registered or sounded when a predetermined number of pulses identified as impact pulses have occurred within a given time period or window.

The count selector in counter 49 can vary the predetermined number, when differing applications for the system are envisaged. For instance, in a bank vault there may be negligible ambient noise and maximum need for speed of reaction to the alarm, so that only one impact pulse may be considered sufficient cause to deploy counter measures against any intruders detected. Where machines are present, or are themselves being checked, consistent knocking would have to be present to cause alarm; it would therefore be necessary to select a high count within device 49 to set off the alarm output.

The line 42 it will be noted connects the output of pulse shaper 33 directly to both the vibrator and impact alarm logic. This prevents any risk of these alarms being given, in error, when the lines have been tampered with, the output of the pulse shaper 33 being conseqentially not high, and the correct indication of course being the alarm state only from the line monitor device 34. Whatever the type of alarm raised, register and indicator 36 preferably contains a latch to sustain the visual or other indication that an alarm state has occurred, until reset manually.

A further embodiment closely similar to that of Figure 2 is shown in more detail in Figure 3 in which an inertial sensor 31 comprising a low-mass sphere 62 having a conductive surface shorts two or more electric terminal points or rods 63 unless displaced by an impact or vibration threshold intensity. A resistance 64 local to the sensor and another similar resistor 65 on an integrated circuit board 66 containing the detector alarm evaluation circuitry and having input terminals A and B enable A to be 6 volts above ground when the ball 62 is at rest on the rods 63 or at nearly 12 volts at instants when the ball is displaced from the rods, following shockwaves or other inertial forces. Pulses are thus formed by vibrations etc.

The pulses are squared in the pulse shaper 33 by a comparator 67 using an open collector output e.g.

formed by one quarter of an amplifier chip type 339 which is arranged to deliver a negative going output e.g., a pulse 21 or 22 in Figure 1 whenever terminal A exceeds 8 volts. Another similar comparator 68 delivers a negative-going output to the same position 69 when terminal A falls between ground and 4 volts.

If the terminal A remains between ground and 4 volts, it will generally mean an open or short-circuit (e.g. by accident, failure or tampering therewith by intruders) in the line 32 from the inertial sensor 31.

Tampering may be detected by an anti-tamper switch 61 in the line 32. The resulting continuous output from position 69 will pass to the line monitor circuit 34 which includes a time constant circuit 70, and an exclusive OR device 71 (which gives an output only when its inputs differ) to give an output at the terminal 35 denoting a circuit fault, perhaps due to tampering with the line in an attempt to prevent a shock alarm being properly registered. This alarm condition is also registered via a line 73 at the multi-purpose alarm indicator 36 including a red L.E.D. 75, a NAND gate 76, and a bistable 77 reset by a manual reset 78 or an automatic reset 79. An alarm is also registered by circuit 36 in response to a sensed excess vibration alarm over a line 80, or an impact alarm over a line 81.

If ambient vibrations are not too excessive for abnormal vibrations to be distinguishable, a vibration select switch 82 is closed, to permit vibration pulses such as the train in Figure 1 a to reach an AND gate 38 e.g., type 4081. By the action of the reference pulse generator 37, which comprises a retriggerable monostable 83 e.g., a logic circuit type CD 4098, connected with time circuit components 47 valued to maintain its triggered state for a set time tr and an Exclusive - OR gate 84 as shown. The input pulses are analysed and separated according to whether their period is longer or shorterthan a reference period tr. The monostable is triggered by a negative- going edge to produce logic '0' at output 0 83 for the period tr.If a subsequent pulse (i.e., negative going edge) arrives within tr, the Q output 83 remains at logic '0'. This will occurforvibrational pulses only.

TheQoutput83A, is connected to one input of the Exclusive OR gate 84, type 4070 being prefered, the other input of which receives the comparator output from position 69 directly. Thus at the output 86 of Exclusive OR gate 84 there is produced the vibrational pulses which are fed to the other input of AND gate 38. Gate 38 will thus pass the vibration pulses.

The output of gate 38 is applied, via the pulse stretcher 45 to a "vibration pulses present" L.E.D.

indicator 46 which is typically used for setting up the vibration detection apparatus, and also passed, over a line 88 with time constant components 89 to a NAND gate 90 at the beginning of a vibration pulse evaluation channel 91.

The other input of gate 90 is connected via an inverter to the comparator and pulse shaper output position 69, when switch 82 is closed, and vibration pulses are checked for a duty cycle of at least 50% by an integrator circuit 39. The passed pulses are then NAND-ed in a gate 43 with the original signal from position 69 and fed to an output alarm point 41 and also over line 80 to the multi-alarm indicator L.E.D.

75 in alarm indicator circuit 36. An adjuster 40 selects the period over which vibration pulses are integrated in circuit 39. This period corresponds to t3 in Figure 1a. Thus the vibration pulses are passed only if exceeding a selected burst period and if of sufficient duty cycle. They will only have reached the channel 91 if of sufficient amplitude and of insufficient length to be impact pulses. The NAND gate 43 prevents false vibration signal responses to line faults, which are to be detected at output 72.

Impact pulses which are received by the reference pulse generator 37 have a period greater than the reference period t, set by circuit elements 83b. Thus in response to each negative-going impact pulse, the output Q will become logic 'I' before the monostable 83 is retriggered. Thus 'I' outputs appear at theQ output 83A in response to impact pulses. The output 86 of exclusive OR gate 84 will also produce logic 'I' pulses in response to the negative going impact pulses and the Q output. An AND gate 96 will feed these pulses via a time constant circuit 97 to apply stretched impact pulses to an L.E.D. "impact pulses present" indicator 53.The impact window t7 of Figure 1b will also be set by this time constant circuit 97 operating in conjunction with a logic and counter circuit 49, for instance a BCD counter of type CD 4520, which counts the number of pulses in a given window period. The pulse count in response to which an alarm is produced can be selected by a count selector switch 100 to determine perhaps whether eight pulses are received, e.g., in a window period often seconds, or whatever may be preselected for an alarm condition.

An alarm output circuit 50 constituted in this example by a NAND gate receives the count output selected by switch 100 and also receives the pulse train from the outut 69 of the pulse shaper/comparator circuit 33, to prevent pulse alarms as discussed hereinbefore. The output of the gate 50 is fed to alarm output 51 and to the general purpose alarm indicator circuit 36.

In summary, the above described illustrative embodiment of the present invention relate to a vibration and/or impact detector, which uses inertial sensor 31 and means to evaluate from its output whether an impact or vibration has been sensed of such magnitude as to warrent establishment of an alarm status. Minor shocks or vibrations are ignored.

Previously proposed vibration detectors tend not to be able to discriminate between ambient vibrations of traffic or normally running machinery and abnormal vibrations arising from machinery out of balance or intruders forcing doors or breaking windows etc.

Accordingly the above described embodiments of the invention separate vibrations from impacts in circuits 37, 37A, 37B, 38, 48 according to pulse period and designate sensed vibrations as alarm-significant if they (1) exceed a given threshold, (2) show a substantial, e.g., at least 50% duty ratio, and (3) form a burst of a specified minimum duration, the duration and duty ratio being sensed by circuits 39, 40 for example.

Sensed impacts to be designated dangerous require a minimum duration of an individual pulse, so as to be separated from vibrations by circuits 37 and 37b longer than that typical of a vibration, and will generally, but not always, exhibit totally irregular pulse or inter-pulse durations. Sensed impacts are also required to occur a predetermined number of g times in a window period as sensed by circuits 48 and 49 to produce an alarm. Further alarm status signals result if the signal lead 32 from the sensor 31 is open or short circuit. The lead would then not convey sensor signals and may have been tampered with by an intruder. Alternatively the lead may have been accidentally broken, thus compromising the integrity of the detector system. Such conditions are sensed by circuit 34.

There will now be described further embodiments of the invention which have, as compared to the above embodiments, additional analysis of the impact pulses. A system using inertial detectors and required to detect impacts indicative of alarm con die tions, e.g., intruders breaking in or flywheels out of balance generates two sorts of signals liable to set off false alarms. Excessive false alarms cause unnecessary trouble and expense, and lead people to ignore dangerous conditions which are correctly indicated. One sort of false alarm is inherent in typical transducers of impacts, in that they generate some internally derived noise pulses, which are relatively easy to discriminate against by amplitude and time thresholding.The other sort of false alarm originates from what one might term environmental impacts, e.g., slamming doors and windows, traffic on uneven roads near the safety installation, etc.

It is the environmental sort of impact indication that the following embodiment is aimed to designate innocent, while maintaining full integrity to warn of danger indicating impacts.

In the following, reference is made to Figures 4 to 7, which, as appropriate, use the same reference numberals as Figures 1 to 3.

Referring to Figure 4A a first impact pulse 103 of duration t1 is followed within a period t2 e.g., of 20 milliseconds from its trailing edge 104, by a second impact pulse 105 of duration t3. Afterwards follow weaker and/or shorter pulses 106 and 107 shown in dotted lines. These may be vibration of other pulses not of interest to us and are processed out of impact analysis by removing pulses shorter than a PRESET pulse duration minimum. The train of two pulses separated by less than a first period, e.g., 20 milliseconds denotes a danger-indicative impact, perhaps from a deliberately smashed window glass.

Figure 4B shows a first impact pulse 108 of length t4, perhaps like the previous first pulse 103, followed by a second impact pulse 109 of duration t6 a time tS later, e.g., at an interval or gap of greater than 1.02 sec. approximately. This may denote danger, e.g., hammering by an intruder; alternatively it may be decided that three or more impact pulses within e.g., a five second total "window" period be taken as the criterion for designating the pulse train as creating an alarm condition.

Thus this embodiment of our system should react whenever two pulses occur within 20 milliseconds, or whenever two, three or more impact pulses occur within 5 seconds, to sound or manifest the alarm or other counter measures.

As for Figure 4B pulses 110 and 111 are vibrational, i.e., weaker or shorter, and are not counted or in any way taken into consideration as impact pulses.

The third graph, in the series, Figure 4C, shows a pulse train of apparently impact-type pulses 112 and 113, but since they have gaps of duration t5 between twenty milliseconds and one second and twenty milliseconds (1.02 sec) in duration, we determine these to be environmental and innocent in character.

Again weak or short pulses 114,115,116,117 are ignored, by suitable initial processing. It is impor that, according to teaching herein, that such impactseeming pulses 112 and 113, should not be used in establishing an alarm-raising requirement. Circuitry described below enables such pulse sequences as 112 and 113 to be effectively ignored without losing ability to react to genuine danger indications from an inertial impact detector.

If pulse 112 is followed by a further strong pulse 118, say at an interval of t5 (see Figure 4B), then the train comprising impact-type pulses 112 and 118 should be recorded as a possible danger unless pulse 118 were one of a series, presumed environmental all spaced apart by shortish times such as t8.

The circuitry described below will either ignore or react to pulses such as 118 according to the spacing and nature of any neighbour pulses.

Referring now to Figure 5, and again to the above discussion of Figure 4, our preferred version of this embodiment discriminates between various apparently impact-type pulses by creating a first and second window period after a first detected pulse 119, for counting and assessing further pulses. The first and second windows are separated by an inhibit period of t12 duration (e.g., one second), wherein any pulses that might occur are not counted and thus cannot complete a count setting off the alarm.

However, such pulses occurring during the inhibit period of one second will be detected and be used to initiate a complete new window period (of five seconds).

It is pointed out that while the preferred version of this embodiment first creates a total window period of five seconds and then overrides a one-second period within the total period by inhibiting the window, the same result could be achieved by generating two separate window periods spaced by a one second non-window period, and such arrangements would still be within the scope of the invention.

Returning to the preferred version, the first win dowt11 is initiated by the final edge of first impact pulse 119, and we prefer an approximate period of 20 milliseconds as shown for the first window period.

The total window period t13 is initiated by the leading edge of pulse 119, although we do not take any steps to look for second pulses occuring before the final edge of pulse 119. Such facility would not presently appear to be desirable.

The count inhibit period t,2 is chosen to last one second after the first window period ceases t11, and is provided in order that environmental pulses, although resembling impact pulses, will not be counted and thus will not contibute to an alarm condition.

After the inhibit period finishes, there commences a second window period, of nearly four seconds (if no further pulses arrive). The four seconds is made of five seconds for the total window period tr3; minus the duration of the first pulse 119, 108, or 103, minus the first window period t11 20 milliseconds, and minus the inhibit period t12 of 1 second. The second window period will be extended if further pulses arrive that might be impact pulses. On elapse of the second window period, unextended or extended, without an alarm condition being sensed, the pulse count is reset to zero, and the system awaits a next "first impact pulse" such as 119.

Although digital counting is usually more practical than analogue summing, or integration of pulse "areas" by charge storage or the like, the latter is feasible and within the invention. The summation would be permitted during the first and second window periods, and would cease or be inhibited during an intermediate period, of one second or so.

Figure 6 shows a modification of the system described in Figures 1 to 3 in which impact-type pulses are recognised as such by a separator 378, which examines durations and amplitudes for minimum or threshold values. The negative-going separated pulses are fed to an impact pulse counter 49, which will count them under certain conditions, and which will cause an alarm circuit 50 to produce an alarm after a given count, e.g., two, is reached under these certain conditions.

The separated pulses are also fed through a 20 millisecond delay unit 120 to a one-second count inhibit unit 121. The latter triggers on a negativegoing input pulse to deliver on an output line 122 a positive, inhibit pulse to counter 49 for one second.

The delay unit 120 reacts with 20 milliseconds delay to a positive-going pulse edge, such as the trailing edge of the first pulse 103 or 108, Figures 4A and 4B.

The inhibitor 121 delivers its count-inhibiting output for the one second and then terminates the output, because of the time constant components in the inhibitor. The inhibitor cannot be triggered again while it is still inhibiting the count after a previous triggering. Summarising, the 20 millisecond delay unit 120 sets up the delay from the trailing edge of the impact pulse. The output of the delay unit triggers the count inhibit unit 121 so that the latter prevents counter 49 from counting, but the prevention is not actually effective until elapse of the 20 milliseconds delay.

When the one second of count inhibition is finished, an output 122 of the inhibit unit goes negative, and any subsequent negative edges from impact pulse separator 37B will again be counted by counter 49. The counter will continue to be enabled for the rest of the five seconds total window period defined by window unit 52.

The window defining unit 52 may be provided by a timed monostable triggered by the leading (i.e., negative-going in this embodiment) edge of the first impact pulse 103, 108 or 119 (see Figures 4 and 5).

The time monostable acts to create the total count window period of five seconds apparent in Figure 2, and hence has the legend "5 sec window" in the box referenced 52 in Figure 6.

The output 123 of unit 52 holds the counter 49 enabled for the five seconds, but this period is overridden for the aforementioned inhibit period of the inhibiting output 122 from inhibit unit 121.

The five second window unit 52, like the one second count inhibitor 121, is a triggered monostable. Window unit 52 differs from inhibitor 121, however, in that unit 52 is re-triggerable. In fact it is re-triggered by each further impact-type pulse that may arrive during any five second window created, whether or not such arrival is during the count inhibit period.

The last fact means that the system is put more on its guard by impact-type pulses which are probably innocent but of course may not be, when these occur during the inhibit period.

Afurther inhibit period will also be created by each of such newly arriving impact-type pulses, in each of the renewals of the five second period. Therefore the security system remains "on its guard", but still does notcountthe repeated impact-type pulses 112, 116 and 118 of Figure 1C, because their interval has been such as to indicate an environmental pulse train.

The remaining units in Figure 6 are similar to units in the embodiment of Figures 1 to 3 previously referred to hereinabove.

Referring to Figure 7, a circuit diagram is given '- detailing a realization of the Figure 6 arrangement. A NAND-gate 131 gives a negative-going output pulse of variable length whenever an impact pulse is sensed. The variable length arises due to the variable length of the impact pulse. A pulse is assumed to be an impact pulse if it has greater than a certain (presentable) minimum duration.

The NAND-gate 131 derives the impact style negative-going pulses by virtue of previous processing circuitry, operating as follows. An inertial sensor 31 delivers 12 volts at point A whenever a sphere 62 is subject to acceleration and six volts when the sphere is at rest, by virture of a series path from a 12 volt rail 133 through two resistors 65 and 64, and the terminals of the sensor in the series path to earth. I.C. comparators 67 and 68 of a comparator/ pulse shaper 33 deliver a low voltage (OV) at their connected outputs 69, due to a voltage source 139 feeding the connected outputs which comprise open collectors of the comparators, through a resistor 140, whenever input point A exceeds eight volts (or drops below 4V).The eight volts is set by amplifier 67 having a positive input terminal connected to eight volts; when the negative terminal (connected to A) exceeds 8V a low (0) output results. The OV, showing accelerations which may be due to impacts, triggers a monostable 83 deliver a low outut (0) at an output 83a for as long as a time constant circuit 47 holds the triggered state. Long duration O-inputs at 69 cause an exclusive OR-gate 84to have unlike input, because retriggerable multivibrator 83 will revert to having a 1 - output at 83a while the other input 143 to gate 84 is still low (due to the long pulse).

Under these conditions the NAND-gate 131 receives 1 - outputs from both point 142 and from gate 84 and the negative-going impact-type pulse results at its output. Exclusive OR-gate 84 will also receive dissimilar inputs and thus give a high output after; cessation of a, or each, shorter negative-going vibration pulse at 69, because output 83a will be held low by time-constant circuit 47. Thus a 1 - output also appears at the output 5 of gate 84 in response to the cessation of the or each shorter negative-going pulse 69 but because the output at 83a remains low, the output of 131 remains high and does not therefore respond to the short pulses at 138. Such short pulses are assumed to indicate vibrations rather than impacts, for instance see 106, 107, 116, 117 etc., in Figures 1A, 18 1C. The high-output from 84 in response to the short vibrational pulse at 69 enables those pulses to pass (inverted) through a NAND gate 38 which acts as-the vibration pulse separator of Figure 6. The pulses passed (inverted) through gate 38 are processed in similar manner as described with reference to Figure 3 in the circuits 39,40,43,45,46.

Considering again the negative-going impact-type pulse from gate 131 we have to decide if an impact is truly present, or whether a train of environmental pulses 112,113,118 as in Figure 4C is present (due to rattling window-frames in an old building etc).

Accordingly this embodiment teaches the inhibition, or ceasing or recording impact signals, after a short first window period, herein assumed to be 20 milliseconds.

This embodiment produces a negative-going output at an exit point 147 of 20 millisecond delay unit 120 when 20 mS has elapsed after a positive-going edge, i.e., trailing edge of a negative-going impacttype pulse has been input to unit 120 on line 148. The delay is defined by a NAND-gate 149, a capacitor 150 and a resistor 151. The previous negative-going output (leading edge of an impact pulse) from NAND-gate 131 immediately inhibits any negativegoing output at 147, due to a diode 152 then being directed to shunt the time constant resistor 151 for negative edges, otherwise the 20 mS would be uncertain in accuracy. The diode makes sure that the capacitor 150 is normally discharged fully, The negative-going output at 147 of time delay unit 20 triggers a monostable 158 of a count inhibit unit 121.

The negative-going output from gate 131 triggers a 5-second window-creating unit 52 so that the latter enables a counter 49 over a line 153 for five seconds only. This period is determined by time constant elements 154 and 155. The five seconds is extendible by another five seconds on each re-triggering of unit 52, which comprises a re-triggerable monostable wired as shown, the count in the counter 49 being unaffected by each re-triggering to extend the window.

Counter 49 is a BCD counter e.g., of type 4520 which has edge triggered exclusive-OR inputs indicated by the arrows at 156 and 157. Any negativegoing excursion on input 156 causes a count increment provided that input 157 is at a 0 or low. In its untriggered state, i.e., normally, the Q output of the non-retriggerable monostable 158 is a logic 0.

Therefore negative-going excursion at the output of gate 131,which it will be recalled represent the leading edges of impact-type pulses, will normally incrementthecounter49while it is enabled on its input 153. The direction of the edges triggered on-is unimportant, being an engineering design choice.

In order to inhibit counter 49 during part of its enabled, five-second window period, the negativegoing leading edge from gate 131, delayed 20 rOS by unit 120 triggers monostable 158 by application at a negative trigger input-tr thereof. This monostable forms part of the count - inhibit unit 121, which unit also comprises one-second time constant components 159 and 160. When triggered, monostable 158 delivers a logic 1 - output at 122 which performs two tasks. Firstly the logic 1 - output prevents the monostable from being re-triggered by another negative pulse at its input; for this purpose the logic 1 - output is led via a connection 161 to a positive trigger input of the monostable 158.Secondly the logic 1 - output at 122 is applied to the second input 157 of the counter, and this prevents negative going edges at the first input 156 of the counter from incrementing the count. The counter is thus inhibited from counting although still enabled by the windowcreating input from line 153.

Loosely speaking, counter 49 thus operates to count negative-going pulse edges from the impactselecting circuits ending in gate 131,when enabled by 5-second window unit 52 and when not inhibited by a logic 1 - input from monostable 158. In practice, the first negative-going pulse edge from gate 131 may not trigger unit 52 to enable the counter in time to record the same first edge; but this does not alter the operability because positive-going edges leading the one second inhibit input at 157 will in practice also increment the counter because input 156 will be high, the first impact pulse having terminated. The latter is because the counter has edge triggered exclusive-OR incrementing. The first impact pulse will thus often increment the counter by means of the positive-going edge from line 122 a matter of 20 mS later.Subsequent danger pulses, if any, will increment the counter by negative-going edges at 156 once the inhibit period has finished. A second pulse from gate 131 within the 20 mS delay period will be counted first, by its negative, leading edge.

The first pulse, creating a positive edge at 15720 mS laterwill then be recorded second. The result is that the count will increment to two, and the alarm will be raised as required. Alternatively it is easy enough to engineer the counter to register the first pulse after being enabled by it. This is also a design detail.

After the monostable 158 relaxes due to the action of the time constant components 159, 160, the inhibit unit output at 122 reverts to a logic 0, and the counter 49 can now again count negative-going pulses from NAND-gate 131. We are now entering the second window period during the five-second enabled period of the counter, since the count-inhibit unit 121 ceases to have effect.

The BCD outputs Q1 - Q4 of the counter are connected to a switch 100 which selects the number of counts in response to which an alarm output is given on line 162.

Counter 49 may deliver a logic one-output on line 162 when two (as shown) or more, pulses occur during the total window period, as chosen by the switch SW2.

A NAND gate constitutes the alarm output circuit 50 generates as described above with reference to Figure 3 to produce an alarm at output 51.

If a second impact-pulse occurs during the inhibited period, it is assumed environmental but nevertheless the window unit 52 triggers again, and the window period is extended by another full period.

This allows the system to ignore this second pulse, but to become altered for a longer time for a third pulse which would be recorded in general. However, a second inhibit period of one second may also be set up after another 20 mS from the trailing edge of the second pulse (depending on whether the first inhibit period had then expired).

It is emphasized that inhibiting an enabled counter is not necessary; the counter could be enabled twice, with an intermediate disabled period to provide two separate "window" portions. Also three or more window portions separated by off periods may sometimes be found preferable, to prevent false alarms. Furthermore more than two impact pulses may be decided as being the criterion to raise the alarm. Also the impact system may stand alone, vibration and line defect alarms may not be required.

The impact pulses need not originate from an inertial sensor, since other accelerometers, pressures transducers etc., will supply impact pulses after suitable processing. In general the 20 mS, I second and 5 second duration need be only approximately selected. Circuit element variations may vary these values by i 25% without affecting the facilities given and the principles involved.

Although the invention has been described by way of example, with reference to an inertial sensor, other sorts of sensor could be used with suitable processing to produce a similar pulsed output as is used in the above embodiments, for example an acoustic sensor could be used with suitable signal processing.

In a modification of the vibration pulse analyser (38, 45, 46, 89, 88, 90, 40, 39, 43 of Figure 8) or (38, 45, 46,89, 90,40, 39,43 of Figure 7) the analyser is no longer sensitive to duty ratio. Referring to Figure 8 (top half) a diode D90 is inserted into the adjuster circuit 40 and the feedback path around the NAND gate of circuit 39 (provided by a capacitor) is omitted.

Instead a series arrangement of a resistor 92 and a capacitor 91 is connected between +V and OV and the junction of the resistor 92 and capacitor 91 is connected to the junction of the NAND gate and the adjuster resistor 40. This arrangement integrates the vibration pulses fed to it from NAND gate 91 (without substantial sensitivity to duty ratio). The vibration pulses cause capacitor 91 to discharge via the diodes D90 and the output of gate 90 until the input to the NAND gate of circuit 39 goes low allowing it to latch and supply an alarm signal to NAND gate 43.

Furthermore NAND gate 43 is in Figure 8 arranged as an inverter (when switch 82 is closed to enable it).

Thus the vibration pulses are no longer gated by the alarm signal through gate 43, it having been found that the subsequent circuitry may respond in an undesirable manner to the gated vibration pulses.

A modification of the impact pulse analyser (52, 120, 121,49) of Figure 7 is shown in Figure 8. It has been found that the inertial sensor 62 of Figure 7 may respond to a single impact by bouncing several times on its contacts thus producing not one impact pulse but several. Thus the circuit of Figure 7 could then give an alarm in response to only one impact which produces many impact pulses.

In the circuit of Figure 8, the delay circuit 120 of Figure 7 is omitted entirely and various other changes are made as shown, the more significant ones being made more apparent in the following description.

Referring to Figure 8 (bottom half) a negative going output, indicative of an impact pulse, from NAND gate 131 triggers monostable 52's Q output for a 10s (c.f. 5s in Figure 7) window period into a state (low) which enables counter 49 to count. Any subsequently received impact pulse (including pulses due to bounces) retriggers monostable 52 to extend the window as before. With the counter 49 and the monostable 158 connected as shown, a first negative-going edge triggers monostable 158 so that its Q output produces a positive going edge and monostable remains with its Q output high for a preset time, in this example 100 ms. During this period the counter 49 is inhibited from counting. The Q output of monostable 158 is connected to input 156 of the counter (with OV connected to input 157) so that the counter responds to the negative going edge produced at Q of 158 at the end of the preset time to count one impact pulse. The preset time of 100 ms is shown to allow impact-like pulses due to sensor bounce to die away. The first impact pulse to be produced after the end of the 100 ms preset time, triggers a new 100 ms time at the end of which it is counted in counter 49.

When the count in the counter reaches a number selected by switch 100 an alarm output is fed to NAND gate 50 which in the example of Figure 8 is connected as an inverter for the same reason as gate 43 Figure 9 is connected as an inverter.

The output of gate 50 in response to an alarm is fed back to the monostable 158 to reset it.

Claims (16)

1. Avibration and/or impact system comprising: sensor means responsive to vibrations and impacts to produce a pulse train, means for comparing the pulse periods of the train with a reference period to separate vibratorial pulses having a period less than the reference period from impact pulses having a period greater than the reference period, and means for producing a vibration indication signal in response to the vibration pulses alone and/or for producing an impact indication signal in response to the impact pulses alone, the indication signal producing means being responsive to at least the number of pulses produced in a preset time to produce the indication signal.

2. A system according to claim 1, comprising the indication signal producing means responsive to the vibration pulses alone and operable to produce the vibration indication signal if the vibration pulses have at least a preset duty ratio and form a train ofat least a preset minimum duration.

3. A system according to claim 1 wherein the indication signal producing means comprises means for integrating the vibration pulses over a preset time.

4. A system according to claim 1, 2 or 3 comprising indication signal producing means responsive to the impact pulses alone and operable to produce the impact indication signal only if a preset number of the impact pulses occur in a preset time.

5. A system according to claim 1,2 or 3, comprising indication signal producing means responsive to two impact pulses occurring in a preset time with a preset period between them to produce the impact indication signal.

6. A system according to claiti- 1, 2, or 3 compris ing indication signal producing means responsive to the impact pulses alone to produce an impact indication signal and comprising means responsive to the first impact pulse to occur in each of a succession of preset periods to count that pulse and not count any subsequent pulse in that period, the indication signal being produced in response to a selectable predetermined number of counted pulses.

7. A system according to claim 6 comprising means responsive to each impact pulse to allow the counting of the said first pulses to continue for a preset time.

8. A system according to claim 1,2 or 3 wherein the indication signal producing means comprises means responsive to the impact signals alone during first and second preset periods and non-responsive to impact signals in a third preset period separating the first and second periods to produce the impact indication signal if at least a preset number of impact signals occur in the sum of the first and second periods.

9. A system according to claim 8, wherein the second period is extended in response to a further impact signal occurring in the third period to a preset time from the occurrence of that further impact signal.

10. A system according to any preceding claim comprising means for selecting from a range of numbers, the number of pulses in response to which the appropriate indication signal is produced.

11. A system according to any preceding claim, further comprising means for monitoring the sensor means to produce an alarm indication signal in response to tampering or a circuit fault.

12. A system according to any preceding claim, wherein the said reference period is adjustable.

13. A system according to any preceding claim wherein the sensor means comprises an inertial sensor.

14. A vibration and impact detection system substantially as hereinbefore described with reference to Figures 1-3 of the drawings.

15. Avibration and impact detection system substantially as hereinbefore described with reference to Figures 4-7 of the drawings.

16. A vibration and impact detection system substantially as hereinbefore described with reference to Figure 3 or 7 as modified by Figure 8 and or 9.