EP0726548A1 - Sonic detector for monitoring intrusions - Google Patents

Abstract

The detector has a human body noise pick-up (M), essentially a microphone, and the electronic evaluating unit (E). The signals go through an impedance converter (2) to an amplifier (3), another amplifier (4) and then a mixer (5), for mixing with a signal from the oscillator (7). Then, there is a sensitivity regulator (8), and an IF amplifier (9), an A/D converter, a comb filter (11), an integrator (12) and then an alarm relay (14) if the threshold of a comparator (13) is exceeded. The comb filter has two filters (18, 18') forming the mirror arrangement, with the output passing to a minimum stage (19) and its output to the integrator (12).

Description

The invention relates to a structure-borne noise detector for monitoring cash register cabinets, armored cabinets, vaults and cash dispensers, with a sensor connected to the object to be monitored for recording structure-borne noise, and with evaluation electronics connected to the sensor, in which a mixture of the amplified sensor signal with a Carrier frequency and a filtering of the mixed signals in a narrow frequency range.

These detectors, also known as noise detectors, are used to detect attacks on protective objects made of steel or concrete and on armored cabinets with plastic-reinforced protective coatings. The function of the structure-borne noise or noise detector is based on the fact that when hard materials such as concrete or metal are processed, mass accelerations occur and mechanical vibrations are generated, which spread out in the material as structure-borne noise. The preferably piezoelectric sensor picks up these vibrations and converts them into electrical signals. The detector electronics analyze the signals and trigger an alarm if the result is appropriate.

As with all automatic monitoring devices, it is also very important for structure-borne noise detectors that false alarms should not be triggered if possible, that interference signals should be suppressed. In the case of a structure-borne noise detector described in US Pat. No. 4,290,058, this interference signal suppression essentially occurs in that the vibrations picked up by the sensor are mixed with a carrier frequency which periodically and continuously runs through a specific frequency range, and in that the mixed signals are filtered in a narrow-band frequency range .

Although the evaluation electronics of this known structure-borne noise detector has the advantage that the evaluated frequency band is much more limited than when a bandpass filter is used, an interferer lying within the evaluated frequency band can of course still trigger a false alarm. The structure-borne noise generated during a break-in attempt lies in a characteristic frequency range, preferably in the kHz range near the upper hearing limit, between approximately 12 and 20 kHz, while typical interference noises are significantly lower-frequency or also higher-frequency. Experience shows that in the frequency range characteristic of structure-borne noise during a break-in attempt, sustained vibrations occur over and over again, so that false alarms are triggered.

The invention is now to provide evaluation electronics for a structure-borne noise detector, with which interference signals lying within the frequency range mentioned are also suppressed and thus the reliability and false alarm security of appropriately equipped structure-borne noise detectors is decisively improved.

The object is achieved according to the invention in that the evaluation electronics have a comb filter circuit, which consists of two parallel arranged, mirror-image comb filters, the outputs of which are led to a minimum level, of which only the smaller of the output signals of the two comb filters is fed to further processing.

A preferred embodiment of the structure-borne noise detector according to the invention is characterized in that each comb filter has a filter period of not more than 500 Hz. The filter period is preferably 200 Hz.

A normal attack or intrusion signal is relatively broadband and becomes an approximately equal signal at the outputs of both comb filters of the comb filter circuit deliver so that it does not matter which of the two signals is processed further. The smaller signal will only be insignificantly smaller than the larger one and will therefore trigger as quickly and as safely as this alarm. If, however, a relatively narrow-band interference radiation occurs in the frequency band under consideration, then because of the short filter period, it will certainly pass through one comb filter and not through the other, so that the occurrence of a certain difference between the output signals of the two comb filters is an indication is a jammer. If, as is proposed according to the invention, only the smaller of the two signals is processed further, the interference radiation is automatically suppressed and need not be analyzed in more detail.

Incidentally, practical experience has shown that unexpectedly a lot of interference radiation has appeared in the narrow frequency range between 12 and 20 kHz, especially in recent times. It can be assumed with almost certainty that this interference radiation is caused by electronic devices, for example by clocked power supply units or by screen devices standing on the object to be monitored. Higher frequency interference radiation in the frequency range of approximately 25 kHz can be caused, for example, by ultrasonic penetration detectors.

Fig. 1

ein Blockschema eines erfindungsgemässen Körperschallmelders mit einer Kammfilterschaltung,

Fig. 2

ein Schema der Kammfilterschaltung von Fig. 1,

Fig. 3

die Übertragungscharakteristik eines Kammfilters der Schaltung von Fig. 2,

Fig. 4

das Frequenzspektrum eines normalen Angriffs- oder Einbruchssignals; und

Fig. 5

das Frequenzspektrum eines Störsignals.

The invention is explained in more detail below with the aid of an exemplary embodiment and the drawings; show it:

Fig. 1

1 shows a block diagram of a structure-borne noise detector according to the invention with a comb filter circuit,

Fig. 2

1 shows a diagram of the comb filter circuit of FIG. 1,

Fig. 3

the transmission characteristic of a comb filter of the circuit of Fig. 2,

Fig. 4

the frequency spectrum of a normal attack or intrusion signal; and

Fig. 5

the frequency spectrum of an interference signal.

The structure-borne sound detector M shown in FIG. 1 contains a microphone 1 acting as a structure-borne sound pick-up and evaluation electronics E. The microphone is used to pick up the vibrations that are generated by mass accelerations that occur during the processing of hard materials and to convert these vibrations into electrical signals. The evaluation electronics E is known, for example, from US Pat. No. 4,290,058 and from the structure-borne sound detectors of the types GM31, GM35 and GM36 from Cerberus AG, and is only to be described briefly here. With regard to the microphone 1, reference is made to CH Patent Application No. 0 172/94 from Cerberus AG.

The output signal of the microphone 1 is fed to a preamplifier 3 via an impedance converter 2. The preamplified signal passes through a further amplifier 4 to a mixer 5, where the amplified signal is mixed with the signal from an oscillator 7. The signal mixing product is fed via a sensitivity controller 8 to an intermediate frequency amplifier 9, which also contains a low-pass filter. The amplified IF signal passes into an A / D converter 10 and from there into a comb filter circuit 11, the output signal of which is fed to an integrator 12, in which the output signal of the comb filter circuit 11 is numerically integrated. As soon as the value at the integrator 12 exceeds the threshold of an alarm comparator 13, an alarm relay 14 triggers the alarm.

The alarm comparator is connected as a Schmitt trigger. The switching thresholds are selected so that in the event of an alarm by the integrator 12, the alarm self-holding time is set to approximately 1 s via a timer 15. In the event of strong blows or in the event of a blast, a trigger stage 16 is triggered, which charges the integrator 12 in a very short time and triggers an alarm. If the time interval between two successive noises is greater than approximately 5 to 10 s, then the integrator 12 is quickly discharged by a stage 17. The functions of comb filter circuit 11, integrator 12, alarm comparator 13 and stage 17 are calculated in a microprocessor μP.

FIG. 2 shows a somewhat more detailed illustration of the comb filter circuit 11 from FIG. 1. According to the illustration, it consists of two comb filters 18 and 18 ', which are arranged in parallel and have a mirror image, the outputs of which are led to a minimum stage 19, of which only the smaller of the output signals the comb filter 18, 18 'is forwarded to the integrator 12 (FIG. 1) and the larger signal is suppressed. A comb filter is known to be a filter with a periodic frequency response in which the pass and stop bands alternate. Comb filters are used, for example, in video signal processing in the color decoder of television receivers (see, for example: H. Schönfelder "Bildkommunikation" ', p. 188f, Springer Verlag, Berlin, Heidelberg, New York, 1983). The mirror-image structure of the two comb filters 18, 18 'means that where the blocking areas are in one filter, the passband areas in the other, and vice versa. This has the consequence that a narrow-band signal occurring within a frequency band with a bandwidth corresponding to half a filter period is passed through by one of the two comb filters 18 or 18 'and is not passed through or at least strongly suppressed by the other.

3 shows the transmission characteristic of one of the two comb filters 18, 18 'over a frequency range of 800 Hz. As mentioned in the introduction, the typical structure-borne noise vibrations generated in an intrusion test lie in a frequency range between 12 and 20 kHz. This frequency range is mixed down in the evaluation electronics E to a band between 0 and 4 kHz, over which band the transmission range of the two comb filters 18, 18 'also extends. According to the illustration, the comb filters are each permeable for a frequency band of 100 Hz width and impermeable for an equally wide frequency band. The filter period P is 200 Hz and each of the two comb filters 18, 18 'each has 20 blocking and pass ranges, these being shifted by half a filter period in the two filters.

FIGS. 4 and 5 show the frequency spectra of a normal attack or intrusion signal (FIG. 4) or an interference signal (FIG. 5), the signal curve being shown over a frequency range of 10 - 25 kHz and the frequency range of interest in the case of structure-borne sound detectors between 12 and 20 Hz is highlighted by two dash-dotted lines with hatching.

The normal attack or intrusion signal shown in FIG. 4 is so broadband that the output signals of the two comb filters 18, 18 '(FIG. 2) will always be approximately the same size, so that it is necessary for deciding whether an intrusion or intrusion attempt is present and an alarm is to be triggered, it does not matter which of the two output signals is processed further.

The situation is different for the interference signal from FIG. 5: Here one recognizes two components from which the signal shown is composed: on the one hand a relatively small and quiet basic signal in which all frequencies from the area under consideration are represented approximately equally, and on the other hand a striking one , very narrow interference signal at around 16 kHz. This interference signal is so narrow that it is highly likely that only one of the two comb filters 18 or 18 'will let it through and block it by the other. Since the filter blocking the interference signal delivers the smaller output signal, the interference signal is therefore not taken into account in the further processing.

The comb filters are dimensioned such that in the great majority of all cases the interference signal is suppressed by one of the two filters 18 or 18 '. The two comb filters 18, 18 'are designed in such a way that interference signals which are precisely in the transition region A (FIG. 2) between the pass and the blocking region of the comb filters are reliably designed so that the blocking region is always somewhat wider than that Passband, so that both filters block in this transition area A and thus a possible interference signal is suppressed by both filters 18 and 18 '.

As already mentioned, the comb filter stage 11 is calculated in the microprocessor µP, either as a FIR (= Finite Impulse Response) - or as an IIR (= Infinite Impulse Response) filter.

Claims (8)

Structure-borne noise detector for monitoring cash registers, armored cabinets, vaults and cash dispensers, with a sensor for recording structure-borne noise connected to the object to be monitored, and with evaluation electronics connected to the sensor, in which a mixture of the amplified sensor signal with a carrier frequency and filtering of the mixed signals takes place in a narrow frequency range, characterized in that the evaluation electronics (E) has a comb filter circuit (10) which consists of two parallel arranged, mirror-image comb filters (18, 18 '), the outputs of which are connected to a minimum stage (19) are guided, of which only the smaller of the output signals of the two comb filters is fed to further processing. Structure-borne sound detector according to claim 1, characterized in that each comb filter (18, 18 ') has a filter period (P) of not more than 500 Hz. Structure-borne sound detector according to claim 2, characterized in that the filter period (P) is 200 Hz. Structure-borne sound detector according to claim 3, characterized in that the blocking and pass areas of the two comb filters (18, 18 ') are shifted against each other by half a filter period (P). Structure-borne sound detector according to one of claims 1 to 4, characterized in that the pass areas of the two comb filters (18, 18 ') are somewhat narrower than the blocking areas, so that both comb filters block at the same time in the transition areas between the pass and blocking areas. Structure-borne sound detector according to one of claims 1 to 5, characterized in that the evaluation electronics (E) contains a microprocessor (µP) and that the comb filters (18, 18 ') are calculated in this microprocessor. Structure-borne sound detector according to claim 6, characterized in that the comb filter circuit (11) in the microprocessor (µP) is calculated as an FIR filter. Structure-borne sound detector according to claim 6, characterized in that the comb filter circuit (11) in the microprocessor (µP) is calculated as an IIR filter.

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