GB2339277A

GB2339277A - Analysing data from detector arrays in two or more modes

Info

Publication number: GB2339277A
Application number: GB9814831A
Authority: GB
Inventors: David Appleby; Stephen Henry Ellwood; John Fox; Bhajan Singh; Nicholas Frederick Stogdale; Stephen George Porter
Original assignee: Infrared Integrated Systems Ltd
Current assignee: Infrared Integrated Systems Ltd
Priority date: 1998-07-08
Filing date: 1998-07-08
Publication date: 2000-01-19
Anticipated expiration: 2018-07-08
Also published as: GB2339277B; GB9814831D0

Description

2339277 A Method of analysing the data from Detector Arrays in two or more

Modes Optical detectors operating in the infra-red, visible and ultra-violet bands are frequently used to monitor scenes for significant events. Often the detectors are used in surveillance, for example pyroelectric detectors may be used to detect intruders or the outbreak of fire, or to monitor pedestrians or traffic. Where only one or a few detectors are used the occurrence of a signal above a certain threshold may indicate a significant event, but further inforTnation may be required for satisfactory and reliable operation of the complete system. Analogue features of the signal may be examined, for example the occurrence of irregular low frequency changes characteristic of fixes, i.e. flickering, or other changes characteristic of waving curtains, or leaves or branches in the wind.

Alternatively arrays of detectors may be used, most commonly in surveillance by video cameras. Here it may also be possible to set thresholds for significant events, or to look at changes of signal from one frame to another. Such techniques can be used in automatic event detection as well as to alert a human operator.

It is well known that automatic detection of events can bring with it a high false alarm rate and a human operator is costly and has a limited attention span. This invention is concerned with means by which some of the analogue features used with one or a few detectors can be extended for use with arrays of detectors.

Detector arrays are commonly sampled to generate time multiplexed analogue data, such as a video pulse. train, whose characteristics may be examined for example for extremes of amplitude, spatial correlation within a frame, or spatial and temporal correlation between frames. Large arrays are also of course used for imaging. The time multiplexed analogue data trains may be derived from the whole frame or from individual rows or columns. Except for rather small arrays it is not practical or economic to process continuously the output from all the detectors and thus some of the techniques used with single detectors cannot be applied to the output from arrays. On the other hand the method of examining time multiplexed analogue data is simple and gives a prima facie case for the occurrence of a significant event as well as giving information about its location.

One aspect of the present invention provides a method of analysing signals from an array of detectors, said method comprising (a) deteftnining whether signals from at least a subset of the detectors are within one or more predetermined limits (the threshold mode), (b) selecting those detectors whose signals are outside the limit(s), and (c) determining the analogue values of the signals from at least the selected detectors. In the preferred embodiment of the invention there is an upper and a lower limit, but it is possible for the invention to operate with only one limit.

The invention enables a detector array on which a scene is focused, and its associated signal processing circuitry, to operate in two or more modes. In the threshold mode the outputs, usually from all the detectors, usually after amplification and filtering, are scanned, preferably under the control of a microprocessor. In the preferred method, each output is subject to a thresholding operation such that any output outside predetermined limits or thresholds generates a logical ONE. At one or more points in the scene which correspond to a logical ONE there is an event of possible significance. In the analogue mode, the signal from a number of the detectors is monitored directly.

In one analogue mode which may form part of the method of the invention, the output from a detector or detectors where a logic one has been generated is continuously connected to one or more signal processing channels containing amplifiers and/or filters, and the resultant signals analysed and compared with the 'signatures' of significant events such as the flickering of flames, the ramp function of slow fires, the movement of humans or vehicles, as well as events which have been defined as less significant such as the movement of curtains, leaves, or small animals and changes in temperature associated with clouds and sunlight. Signals from one or more detectors near detectors associated with above or below threshold signals, as well as one or more signals from remote detectors may also be continuously connected and the characteristics from the associated channels compared.

In an alternative analogue mode, the signals from a selected number of detectors are sampled in turn to give time multiplexed analogue data. Generally the number to be sampled will be much smaller than the number in the array, so that each detector may be sampled more frequently than when sampling the whole array. Programmable samplig may be used to extract frequency or magnitude information at optimum signal to noise ratios. The outputs sampled will include that from the detector or detectors above or below threshold and may include output from neighbouning detectors as well as output from one or more remote detectors for comparison. These analogue data are passed through further analogue signal processing circuitry, which may include amplifiers and filters. The use of analogue signal processing circuitry does not preclude the subsequent use of analogue to digital converters followed by digital processing of the resultant data. From this smaller number of detectors more information can be derived about the amplitude of the outputs, spatial correlations within a frame and the temporal and spatial correlations between frames. This in turn may also be compared with possible "signatures" for significant events. A prime purpose of the combination of the threshold mode with one or more analogue modes is to reduce the incidence of false alarms, by a knowledge of the signatures of genuinely significant events.

The operation is normally controlled by a microprocessor. The threshold mode is the standby mode to indicate the possibility of a significant event, for example an alarm. Where one or more logical ONES have been generated the processor will switch the equipment into one of the analogue modes to examine signals from the relevant detectors and from neighbours. Temporal and/or spatial correlations from this smaller number of detectors may be more easily sought and compared with signatures of particular classes of event. Subsequently another analogue mode may be employed. Later the threshold mode may be re-established to monitor the whole field of the array, which may allow the progress of such events as the movement of vehicles or the spread of a fire to be followed. This in turn may be followed by an analogue mode and so on. The microprocessor may also be programmed to allow different operation at different times of day, or different days of the week or between dusk and dawn. Some or all of these control functions may be incorporated in the same integrated circuit or circuits which perform the signal processing tasks.

Another aspect of the invention provides a device for analysing signals from an array of detectors comprising: means for comparing signals from the detectors with predetermined limits; means for determining the analogue values of signals from the detectors; and means for selecting those detectors whose signals are outside the limits and controlling the analogue value detem-dning means to determine the analogue values of signals from at least the selected detectors.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawing in which:

Figure I is a graph of the amplitude of a typical signal, versus time; and Figure 2 is a schematic diagram of the components of a device according to the invention.

Referring firstly to Figure 1, under normal conditions with no significant event occurring the signal from a detector will typically vary by small amounts above or below a mean level indicated by the dotted line, the variations mainly being due to background noise. A large departure from the mean will indicate the possibility of a significant event, in which case the analogue mode is used to examine the signal in more detail.

In the embodiment of the invention to be described below, a signal above threshold I or below threshold 2 triggers the analogue mode. It should be noted that the thresholds are not necessarily equidistant from the mean.

Figure 2 shows five out of a number of detectors I in an array. The output from the detectors passes through units 2 performing amplifying and filtering functions and then passes through a multiplexer 3 with an output 4. The multiplexer is driven by the outputs from an address decoder 5. In the standby mode initiated by a digital signal from the control unit 6, the address decoder operates as a shift register at a rate determined by clock pulses. This produces an analogue pulse train at 4 which is fed to a threshold unit 7. Pulses above threshold I or below threshold 2 (Fig. 1) give rise to a logic ONE at its output 8, associated with the sampling of any detector giving rise to a signal greater than the threshold. The resultant train of ONES and ZEROS is the digital output.

After a logic ONE has been recognised, the control unit 6 switches to one of the analogue modes. In one mode the control unit 6 receives the digital output 8 and determines the operation of the address decoder 5 so that a signal channel 1, 2 corresponding to a detector which has generated the logic ONE is switched by the multiplexer 3 so that its output appears continuously at 4. This output is fed to the amplifier and filter unit 9 whose output is at 10. Low pass filtering may be used to optimise the signal to noise ratio. The characteristics of this signal are examined, for example one or more of the following: amplitude, rate of change of amplitude, frequency and phase. These characteristics are compared with those of different types of event. As these characteristics pertain to only a part of the scene, they are better defined than those that would be given by a single detector monitoring the whole scene. This process may be repeated for other detectors which have generated signals outside the limits defined by the thresholds.

An alternative analogue mode involves sampling. The control unit 6 feeds signals to the decoder 5 so that detectors that have generated a logic ONE are sampled together with neighbouring detectors and possibly some remote detectors, to generate time multiplexed analogue data. These analogue data are then fed to an amplifing stage I I whose output is shown at 12. The output 12 is examined for spatial and temporal correlations. The process can be more effectively accomplished than if data from all of the detectors had been used.

One way of accomplishing the analysis of the analogue signals is to use digital to analogue converters 13 and 14 after the analogue outputs. These digital signals, 15 and 16, possibly after further processing, are fed back together with the digital signals from 8 to a microprocessor 17. The microprocessor processes these signals to establish temporal and spatial correlations which are characteristic of significant events. Some or all of the functions 5 - 16 may be incorporated into the microprocessor 17. Also, the functions 9 - 16 may be combined into a single channel with optional filtering.

In Figure 2 the outputs from the common point 4 are directly connected to the threshold unit 7 and the amplifier and filter unit 9 and the amplifier 11. Buffer amplifiers may be interposed to reduce the load at point 4 and if desired fiii-ther analogue switches can be interposed at the same places. These switches are activated when a given mode is to be established.

The system shown in Figure 2 applies directly to one dimensional arrays. It is modified for two dimensional arrays by using two decoders 5, one for rows and one for columns, each driven ftom a common control unit, in ways that are well known in accessing two dimensional detector arrays.

Signal analysis and event detection is carried out in the microprocessor 17. Analysis can use either the digital output 8, or the analogue outputs 10 and 12, or a combination of these.

The microprocessor 17 utilises internally prograrnmed algorithms to process the array outputs 8, 10 and 12 to determine the movement of objects or activity in the scene and controls the array to extract further information. The interactive nature of the microprocessor 17 and array of detectors I and control logic 5 provides a high probability of activity detection and a low probability of false alarm detection. Typical scenarios may be the detection of intruders or the early observation of flame and fire.

The simplest mode would utillse the digital output 8 to provide spatial and temporal information about moving objects in the scene. At the detection of an event in one or more elements, characterised by a 'one' or a series of 'ones' on the digital output 8, the microprocessor 17 would ufilise internal algorithms to control the array and to closely observe adjacent elements to determine whether the detected object exhibited behaviour, such as a trajectory and velocity through the scene, compatible with a moving object requiring detection. Extraction of the spatial extent of the object is indicated by the number of adjacent elements exhibiting events. The algorithms will allow discrimination of objects requiring detection from spurious events causing false alarms. Extraction of motion, spatial extent and trajectory will be the function of the simplest algorithms.

More sophisticated modes would make use of both the digital output 8 and analogue outputs 10 and 12. The analogue outputs are converted to digital informafion by the Analogue to Digital converters 13 and 14 and can be processed numerically by the microprocessor 17. At the detection of an event as indicated by a 'one' at the digital output, the microprocessor 17 can direct the array to observe the analogue output from one or more elements exhibiting an event and can utilise signal processing algorithms to extract any combination of frequency, amplitude, rate of change of amplitude, and phase informafion from the signal. This information can be used to discriminate flames, exhibiting random or chaotic behaviour, from false alarm sources with periodic behaviour. Observation of adjacent elements allows information about size and growth of events and signal coherence between elements to be extracted. -

Claims

1. A method of analysing signals from an array of detectors, said method comprising (a) determining whether signals from at least a subset of the detectors are within one or more predetermined limits, (b) selecting those detectors whose signals are outside the limit(s), and (c) determining the analogue values of the signals from at least the selected detectors.

2. A method as claimed in claim I in which step (c) includes determining the analogue values of the signals from detectors adjacent to the selected detectors.

3. A method as claimed in claim I or 2 in which the spatial characteristics of the analogue values are exammied.

4. A method as claimed in claim 1, 2 or 3, in which step (c) includes determining the analogue values of signals from one or more detectors which are remote from the selected detectors.

5. A method as claimed in any preceding claim in which, during step (c) the analogue values are determined at regular intervals in order to determine the variations of the signal over time.

6. A method as claimed in any claims I to 4 in which during step (c) the analogue values are continuously monitored in order to determine their variation over time.

7. A method as claimed in any preceding claim in which step (a) alternates with step (c).

8. A method as claimed in any preceding claim in which the signals represent amplitudes of a detected phenomenon and the limits represent predetermined amplitudes.

9. A method as claimed in any preceding claim in which the analogue values are frequency analysed.

10. A method as claimed 'in any preceding claim in which the amplitudes of the analogue values are analysed.

11. A method as claimed in any preceding claim in which the rates of change of amplitude of the analogue values are analysed.

12. A method as claimed in any preceding claim in which the analogue signals are analysed to identify known events viewed by the detector arTay.

13. A method as claimed in claim 12 in which characteristics of the analogue values are compared with those characteristics of known events.

14. A method as claimed in claim 12 or 13 including generating an alarm signal in response to the detection of certain known events.

15. A method as claimed in claim 13 or 14 including the step of discriminating between those events for which an alarm is required and other events for which an alarm is not required.

16. A method of detecting a flame using an array of detectors and including the method of any preceding claim.

17. A method of detecting the presence of a person using an array of detectors and includmig the method of any of claims I to 16.

18. A device for analysing signals from an array of detectors comprising:

means for comparing signals from the detectors with predetermined limits; means for determining the analogue values of signals from the detectors; and means for selecting those detectors whose signals are outside the limits and controlling the analogue value determining means to determine the analogue values of signals from at least the selected detectors.

19. A device as claimed in claim 18 including analysis means connected to the output of the analogue value determining means.

20. A device as claimed in claim 18 or 19 comprising means for identifying certain known events from the output of the analogue value determining means.

13-

21. A device as claimed in claim 20 in which a known event is the presence of a person.

22. A device as claimed in claim 20 in which a known event is the presence of a flame.

23. A device as claimed in any of claims 18 to 22 in which the detector array is an array of pyroelectric detectors.

24. Any of the methods substantially as heremibefore described with reference to the accompanying drawing-

25. A device substantially as hereuilbefore described with reference to the accompanying drawing 01 I

24. Any of the methods substantially as hereiribefore described with reference to the accompanying drawing.

25. A device substantially as hereinbefore described with reference to the accompanying drawing.

4.

Amendments to the claims have been filed as follows A method of analysing signals from a detector array compri'sing detector elements onto which the ima-e of a scene is focused, said method comprising (a) determining whether signals from at least a subset of the detector elements are within one or more predetermined limits, (b) selecting those detector elements whose signals are outside the limit(s), and (c) determining the analogue values of the signals from at least the selected detector elements.

2. A method as claimed in claim 1 in which step (c) includes determining the analoaue values of the signals from detector elements adjacent to the selected 0 detector elements.

A method as claimed 'in claim 1 or 2 in which the spatial characteristics of the analogue values are examined.

4. A method as claimed 'in claim 1, 2 or 3, in which step (c) includes determining the analogue values of signals from one or more detector elements which are remote from the selected detector elements.

5. A method as claimed in any preceding claim in which, durmig step (c) 0 0 the analogue values are determined at regular 'intervals 'in order to determine the variations of the signal over time, 6. A method as claimed Mi any claims I to 4 in which during step (c) the analogue values are continuously monitored in order to deterrnmie their variation over time.

16. A method of detecting a flame using a detector array and including the method of any preceding claim.

0 17. A method of detecting the presence of a person using a detector array and including the method of any of claims I to 15.

18. A device comprising: a detector array comprising detector elements onto which the image of a scene is focused; and means for analysing signals from the detector array compnismig: means for comparing signals from the detector elements with predetermined linuts,, means for determining the analogue values of signals from the detector elements; and means for selecting those detector elements whose signals are outside the lirru'ts and controlling the analogue value determining means to determine the analogue values of signals from at least the selected detector elements.

I F 19. A device as claimed in claim 18 including analysis means connected to the output of the analogue value determining means.

21. A device as claimed in claim 20 'in which a known event is the presence of a person. 22. A device as claimed 'in claim 20 in which a known event is the presence of a flame. 23). A device as claimed in any of claims 18 to 22 in which the detector array is an array of pyroelectric detector elements.