GB2201770A - Security sensors - Google Patents

Abstract

A passive infra-red sensor includes infra-red sensitive elements (2, 4) differentially coupled to a processing circuit (6, 8, 10, 12, 14, 16) in which the frequency spectrum of the differential infra-red signal is analysed. The frequency spectrum comprises a fundamental freqency F1 which depends on the zone spacing of the sensor and on the rate of crossing the zones by the intruder, and a range of harmonics F2, F3, etc the relative size of which depends on the element spacing and other factors. Whenever the processing circuit detects a frequency component which exceeds a threshold and is within the range appropriate to normal walking speeds and the zone spacing of the sensor, the amplitudes at equivalent frequencies for the harmonics are measured, and an alarm signal is produced only if the harmonic contents are within a defined range, thereby avoiding false alarms due noise spikes. <IMAGE>

Description

SECUR1 y gNSORS The present invention relates to security sensors of the type which detect the presence of an intruder by detecting fluctuations in infra-red radiation received from a protected area and directed onto an infra-red detector by means of an optical arrangement which defines a number of zones within the protected area.

Such passive infra-red sensors are well known. In some types, the optical arrangement is a multi-faceted mirror mounted behind an infra-red transmitting window in a housing, with a detector including one or more infra-red sensitive elements mounted between the window and the multifaceted mirror. Each facet of the mirror defines a zone from which infra-red radiation is directed onto each element. In other types of security sensor the optical arrangement is a series of Fresnel lens segments which are formed in the window itself. An infra-red detector is mounted in the focal plane of the Fresnel lens segments.

Each Fresnel lens segment defines a zone in the protected area from which infra-red radiation is focused onto the detector.

The infra-red sensitive elements of the detector are AC coupled to a processing circuit so that only variations in the infra-red radiation received from the protected area are transmitted to the processing circuit. In conventional processing circuits for use with such passive infra-red sensors, the circuit produces an alarm signal if the amplitude of the infra-red fluctuation, within a given range of frequencies, exceeds a predetermined threshold. However, it has been found that processing the output of the infrared sensitive elements in this way produces false alarms.

These result from signals due to such things as radio transmissions, line-borne interference, temperature fluctuations of the background environment, or the movement of small animals near to the detector. These, and other false signal sources, can produce fluctuations in output, within the required frequency range, of sufficient amplitude to exceed the threshold.

The present invention is therefore directed towards solving the technical problem of producing a processing circuit which is less likely to produce false alarms than the prior art circuits.

The inventors have discovered that the frequency spectrum of the output from the infra-red sensitive detector is not simply a pulse at a single frequency but has a definite structure consisting of a series of peaks for an intruder moving across the protected area. The frequency spectrum of the output from a false alarm source will in general have a different structure. Therefore, the present invention solves the technical problem by analysing the frequency spectrum to distinguish between human intruders and sources of false alarms so that an alarm signal is only produced when the frequency spectrum has the characteristics of a human intruder.

A passive infra-red sensor in accordance with the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a block diagram showing a first embodiment of a passive infra-red sensor in accordance with the invention; Figure 2 is a plot of the input signal to the processing circuit versus time for a dual-element detector as an intruder moves across the zones; Figure 3 is a typical frequency spectrum of the infra-red detector output signal when a human is present in the protected area; and Figure 4 is a block diagram of a second embodiment of a passive infra-red sensor in accordance with the invention.

Figure 1 shows diagrammatically a processing circuit for a passive infra-red security sensor. Infra-red radiation is focused by an optical arrangement from a number of discrete zones in a protected area onto two infra-red sensitive elements 2, 4 of an infra-red sensitive detector. The two elements are positioned relatively close together so that each facet of a multi-faceted mirror or each Fresnel lens segment focuses radiation from a slightly different zone onto each of the elements 2 and 4. The outputs from these elements are voltage signals, changes in the levels of which represent changes in the infra-red radiation incident on each element.

Buffer 6 provides a buffered differential signal, in which the common-mode signal of the two elements is cancelled out.

This is usually achieved by connecting the elements, with opposed polarities, to the input of a F. E. T. buffer.

The output from buffer 6 is amplified by amplifier 8, which is generally of the band-pass type, since the useful frequency components lie within a reasonably well-defined range. Frequencies outside this range include noise, drift and other unwanted signals. These are filtered out or otherwise ignored by the remainder of the processing circuit.

The remainder of the processing circuit is shown diagrammatically in accordance with the function it is to perform in analysing a frequency spectrum of the output of the amplifier 8. It will be appreciated that ~ these functions may be performed in a number of different ways.

Preferably the output signal is converted into a form in which it can be processed by a suitably programmed computer.

One pratical embodiment of the circuit is described later with reference to Figure 4.

In order to understand the function of the processing circuit, reference will first be made to Figure 2 which shows the voltage level of the output from the amplifier 6 versus time. It is assumed that an intruder is passing across the zones defined by the optical arrangement. Since the infra-red detector includes two elements, each time the intruder enters into a zone there is a double peak as infrared radiation is received first by one element and then the other. The two peaks are of opposite polarity because of the manner in which the elements are connected. The output from the amplifier then falls as the intruder moves out of the zone. The same pattern is repeated as the intruder moves in turn into and out of each pair of zones defined by each facet of the mirror or each Fresnel lens segment.

These pairs of zones are generally of equal size and equal spacing. Where the intruder moves in a reasonably nonerratic manner through two or more pairs of zones, the waveform of Figure 2 will be repeated the appropriate number of times with t1, approximately constant.

The frequency spectrum of the voltage signal of Fig. 2 is shown in Fig. 3. This consists of a series of more or less sharply defined peaks. The structure of the spectrum, like the signal of Fig. 2, is determined by the optical arrangement and the movement of the intruder.

Given the assumptions noted above, of reasonably non-erratic movement through at least two zone pairs, the structure is as follows.

The fundamental frequency, f1 is the inverse of t1, the repetition period. The other frequency components are multiples of f1, so that f2 is twice f1, f3 is three times fl and so on.

Where a large number of zone pairs are crossed by the intruder, the peak are sharply defined, that is, do is small relative to f1. However, even where only 2 zone-pairs are crossed, the shape of the harmonic structure will be sufficiently well-defined.

The ratio of the amplitudes of f2 to f1 is significant.

Where the spacing between the two parts of the zone pair is small relative to the spacing between two adjacent pairs, the amplitude of f2 will exceed that of 1* This relationship is primarily dependent on the optical arrangement, and therefore the time interval t2.

The whole pattern will shift together in frequency, depending on the speed of movement of the intruder through the protected area.

The structure of the spectrum resulting from many common sources of false alarm is quite different. For example single, short, spikes of noise have a flat spectrum, without the peaked structure shown in Fig. 3. By comparing the spectra, genuine intruders may be distinguished from many types of false signal.

The processing circuit of Figure 1 is adapted to distinguish between the frequency spectrum shown in Figure 3 and those resulting from false alarm sources. The output from the amplifier 8 is fed in parallel to three band pass filters 12, 14, 16. There may be more than three filters if desired.

The frequency bands of the filters 14 and 16 are variable in response to a control input from a control circuit 10. The filter 12 is tuned to have a band pass frequency response which includes the first component of the spectrum shown in Figure 3. The centre frequency of this filter varies in dependence on the range of speeds of a possible intruder to be considered. The output of the filter 12 is fed to a control circuit 10 which detects whether the output of the filter 12 exceeds a predetermined threshold. If the output exceeds a predetermined threshold, filters 14 and 16 (and any others used) are tuned to the expected harmonics of f1, as shown in Fig. 3.

The outputs of these filters are compared with the fundamental, f1, to assess whether the relative amplitudes of the various harmonics correspond to a genuine intruder.

As well as checking for the presence of peaks at multiples of f1, it is also possible to check for dips in the spectrum between the harmonics.

If the amplitude and frequency of the fundamental component f1 are within an acceptable range, and the other filter outputs fit the expected pattern, then the control circuit produces an output on line 18.

The actual harmonics of f1 examined, and the number of them, will depend on the shape of the expected signal in Fig. 2.

Fig. 2. If, for example, t2 was half of t1, giving a symmetrical waveform, then the even harmonics (f2, fq, 6 etc) would be very small, and the odd harmonics would be more important. Also, where a single element detector was used, Fig. 2 would be single sided, and a different set of harmonics would result.

A possible construction for the processing circuit, is shown in the sensor of Figure 4.

A multi-faceted mirror 20 is shown as the optical arragement for this sensor. The pair of zones defined by one facet of the mirror for the two elements 2, 4 are indicated. In this embodiment the output signal from the buffer 6 is fed through amplifier 8 to an analogue to digital convertor 22 which samples the output signal from the amplifier 8. The samples are passed to a Fourier transformer 24. The output of the Fourier transformer 24 represents the frequency spectrum and is stored in a store 26, the contents of which are accessed by a microprocessor 28. The microprocessor 28 is suitably programmed to carry out the functions described with respect to the embodiment of Figure 1. That is, the microprocessor scans the contents of the store over a predetermined frequency range corresponding to the first component of the spectrum of Figure 2 and, if the magnitude of any frequency within that range exceeds the predetermined threshold, the microprocessor then calculates from that frequency the expected frequencies of two or more further harmonic components of the spectrum. The contents of the store locations corresponding to these frequencies are then accessed and, if these frequencies are within the preset thresholds, an alarm signal is output on line 18.

Claims (6)

1. A security sensor comprising an infra-red detector and an optical arrangement such that the detector is responsive to infra-red radiation received from a plurality of zones defined within a protected area, and a processing circuit for analysing the structure of the spectrum of the output from the infra-red detector at at least two different frequencies.

2. A sensor according to claim 1, wherein the detector comprises two infra-red sensitive elements, the outputs of which are fed to a differential amplifier to provide the output to the processing circuit.

3. A sensor according to claim 2, wherein the processing circuit comprises at least two filters connected in parallel to the output of said differential amplifier, said filters being tuned to different frequency ranges, and a control circuit for comparing the outputs from each filter and outputting an alarm signal in response to a predetermined relationship therebetween.

4. A sensor according to claim 2, wherein the output of the differential amplifier is fed to an analogue-to digital converter, and the processing circuit comprises means for performing a Fourier transform to determine the spectrum of the signal output from the differential amplifier, and microprocessor means for identifying a structure of said spectrum indicative of the presence of an intruder in the protected area.

5. A sensor according to claim 3 or 4, wherein a further amplifier is connected between the output of the differential amplifier and the processing circuit.

6. A security sensor substantially as herein described with reference to the accompanying drawings.

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