GB2631283A

GB2631283A - A component for a motion detection system

Info

Publication number: GB2631283A
Application number: GB2309530.0A
Authority: GB
Inventors: Galbraith Colin
Original assignee: Orisec Ltd
Current assignee: Orisec Ltd
Priority date: 2023-06-23
Filing date: 2023-06-23
Publication date: 2025-01-01
Anticipated expiration: 2043-06-23
Also published as: GB202309530D0; GB2631283B

Abstract

A method of generating a motion detection state in an infra-red motion detector comprises: receiving a sensor signal from an infra-red sensor module 32; comparing the received sensor signal to a first band 34a, having a high limit and low band limit; comparing the signal to a second band 34b, having a high and low band limit; initialising a first and second band counter; and incrementing counters when the sensor signals cross the high and low thresholds 40 and 42. A motion detection state is generated if the counters meet a predetermined threshold 46. The approach enables greater flexibility in applying detection thresholds. Additional inputs may comprise an ambient temperature signal and an anti-cloak element which may be a microwave sensor.

Description

A COMPONENT FOR A MOTION DETECTION SYSTEM

The present invention relates to an infra-red motion detector and particularly but not exclusively to a passive infra-red (PIR) motion detector for use in an intruder detection system.

BACKGROUND TO THE INVENTION

Infra-red (IR) detectors, such as passive infra-red detectors, for use in intruder alarms systems or other systems which use motion detection, such as automatic lighting systems, are well known. PIR detectors typically use a dual-element pyroelectric sensor, and a lens or other optical arrangement to focus infra-red radiation onto the pyroelectric sensor. The lens or other optical arrangement is typically in the form of a plastic window having multiple facets. Each individual facet can be a Fresnel lens. The lens, or other optical arrangement, will produce a set of discrete detection zones separated by neutral zones.

The signal from the dual element pyroelectric sensor is filtered and amplified before it is processed. The filter is in effect a band-pass filter and so the signal which is processed is decoupled from the DC level of the pyroelectric sensor. This technique is well-known and is not described in detail here. The effect is that the signal being processed is a signal which responds to change in incident IR radiation.

As a person enters a detection zone the infra-red energy emitted by their body is focused onto the pyroelectric sensor through the lens or other optical arrangement, causing an increase in the amplitude of the sensor signal above its quiescent value. When the person is between or leaves the detection zone the infra-red energy emitted is not focused onto the pyroelectric sensor, causing the sensor signal to return to its quiescent value. As a person moves across the monitored area, their emitted energy is alternately focused onto one or the other element of the dual-element pyroelectric sensor. The elements are arranged with opposite polarities, and hence the signal from the pyroelectric sensor caused by a person moving across the room will typically be roughly sinusoidal.

Because the signal being processed is decoupled from the DC level of the pyroelectric sensor, if a person enters a zone and then stands still in the zone for some time, the amplitude of the sensor signal will initially increase above its quiescent value, and then gradually decrease to its quiescent value. Then, when the person leaves the zone, the amplitude of the sensor signal will drop below the quiescent value.

In some example IR detectors, a detection limit value is set, and a pulse is counted when the sensor signal exceeds the detection limit value. Typically, the pulse will last until the sensor signal returns under the detection limit value, i.e. a rising edge of the pulse corresponds with the sensor signal exceeding a detection limit value and the falling edge of the pulse corresponds with the sensor signal falling below the detection limit value.

The detector will have a pulse counter which will produce an alarm or detection state once the number of pulses meets or exceeds a threshold value. Typically, multiple pulses are needed within a time period before the alarm or detection state is produced.

The pulse count threshold may be changed so as to increase or decrease the sensitivity of the detector, and this may be done by a settable element such as a movable conductive jumper or a switch. It can be seen that setting a pulse count threshold in this way is a fairly basic way of trying to ensure that intruders are detected while false alarm sources are rejected. Setting the pulse count threshold is a basic trade-off between accepting false positives and accepting false negatives.

However, a "multilevel signal processing" technique was developed to provide a detector with both improved sensitivity and a lower false detection rate. In this technique multiple detection limit values are set, and a pulse is generated for each detection limit value exceeded by the sensor signal. US 5444432 is an example of a multilevel technique. The pulses generated are counted by a single counter, with the count being compared to an alarm threshold. In essence this technique is a way of making a large amplitude sensor signal count for more by increasing the total number of pulses.

While the multilevel technique is advantageous in certain circumstances, it does obscure important and useful information since there can be multiple pulses for one traversal of a detection zone. The pulse count in these multilevel devices does not therefore correspond to the number of zone boundaries triggered.

US5444432 teaches rectification of the PIR signal so that its absolute value can be compared to the multiple limit values. This halves the number of comparators required -for example, in the embodiment described in US5444432 there are four comparators, whereas eight would be required if the signal were not rectified, to detect each limit crossing both at a limit level above and a corresponding limit level below the quiescent level. However, by rectifying the signal, again information is obscured. A typical signal from a two-element pyroelectric sensor, caused by an intruder walking across a room, would be roughly sinusoidal with peaks above the quiescent level and troughs below the quiescent level. However, when the signal is rectified this common signal becomes indistinguishable from a series of peaks without the corresponding troughs, which would be an unusual signal from a real intruder, but could be caused for example by sunlight shining through a window, especially on a day where there is patchy cloud cover.

Although some modern microprocessor implementations of a similar technique do not rectify the signal, and instead compare the signal to both upper limits above the quiescent level and lower limits below the quiescent level, the known techniques count a lower limit crossing exactly the same as an upper limit crossing, and so the information about the shape of the original signal is still lost.

The invention of GB2598238 and GB2553133 sought to address some of this obfuscation by maintaining the concept of pulses representing traversal of detection zones while achieving the same advantages of the multilevel techniques discussed above. However, in the course of generating an alarm condition a single value for the amplitude of the PIR signal is used. In practice, the amplitude of the signal may change as an intruder walks across a detection zone, and so again these techniques fail to take full advantage of all the information available in the PIR signal. The techniques taught in these documents also treat an upper limit crossing as exactly the same as a lower limit crossing at all times.

The prior art described above is primarily useful for PIR detectors in the context of intruder alarm systems, where the main objects are to ensure intrusions are detected (i.e. reduce false negative rate) and to ensure that false alarms are minimised or eliminated (i.e. reduce false positive rate). However, PIRs are also used in other contexts, for example to control automatic lighting for security, convenience, and power savings. Using PIRs in this context can raise different considerations.

It is therefore an object of the invention to reduce the problems described above, and in particular to provide a more flexible and readily tuneable PIR detector.

STATEMENT OF INVENTION

According to the present invention there is provided an infra-red detector as set out in claim 1.

The claimed detector is an infra-red motion detector, preferably a passive infra-red motion detector (PI R).

The infra-red sensor module outputs a sensor signal. The sensor signal is based on received infra-red radiation, for example measured by a dual-element pyroelectric device. The sensor signal is preferably amplified and filtered, for example with a band-pass filter.

The sensor signal is monitored and is compared to a first band and second band. In other words, at any particular time, the sensor signal may be within the first band and within the second band (i.e. between the high first band limit and low first band limit, and also between the high second band limit and the low second band limit), or it may be within one band but outside the other band, or it may be outside both bands (i.e. higher than both the high first band limit and the high second band limit, or lower than both the low first band limit and the low second band limit).

The band counters count the number of times that the sensor signal crosses the respective band. A band is "crossed" when the signal moves from an amplitude higher than the relevant high band limit, to an amplitude lower than the relevant low band limit, i.e. moves from a value above the band to a value below the band, or vice versa.

Accordingly the sensor signal is considered to meet the band limit when it crosses from a value on the inside of the band to a value on the outside of the band. Bearing in mind that in many embodiments the sensor signal may be sampled and quantised for digital processing, "meeting" the band limit does not necessarily imply that any particular sample of the sensor signal will be exactly equal to the band limit.

In one embodiment, the counters may be initialised at zero, and on changing a counter it may be incremented by one. The first band detection threshold and second band detection threshold may accordingly be set to positive integers, and the detection state is entered when a counter meets or exceeds its corresponding threshold. In other embodiments, the counters may be initialised at positive integers and then changing the counter will involve decrementing the counter, with the first band detection threshold and second band detection threshold being set to zero so that the detection state is entered when either counter reaches zero. In other words, the counters may count up or count down. It will easily be seen that other equivalent arrangements will work just as well.

The present invention provides for an infra-red (IR) detector that has greater flexibility since the trade-off between sensitivity and false detection rate can be more easily tailored. Having a separate counter associated with each band ensures that information relating to a crossing of a detection zone by a person is not lost.

Furthermore, an individual detection threshold associated with each band allows for different signal strengths, i.e. higher or lower amplitude sensor signals, to trigger a detection with a different number of detection zone crossings.

False detections are reduced by monitoring for the sensor signal meeting a first band limit (either high or low) of one band and subsequently meeting a second band limit of the same band (e.g. high band limit of the first band and then low band limit of the first band) since the crossing through the band is indicative of movement through a detection zone. This passing through one band limit and subsequently passing through the other band limit may be considered monitoring for a change in polarity (in the sense of above and below a quiescent value) of the signal. Typically sources of false detections will only produce a brief increase in the amplitude of the sensor signal before returning to the quiescent state. This would cause one band limit to be met but not the other.

Preferably the first band counter is also changed when the first band counter is equal to the first initial value and the received sensor signal meets either one of the high first band limit or the low first band limit. The second counter is preferably also changed when the second band counter is equal to the second initial value and the received sensor signal meets either one of the high second band limit or the low second band limit.

In other words, the counter is incremented or decremented the first time the sensor signal goes outside the relevant band. However, the counter will only subsequently increment / decrement when the signal crosses through the band and goes outside the band on the other side (i.e. crosses from a level above the band to a level below the band, or vice versa).

By monitoring the received sensor signal for crossings of different bands it is possible to improve the sensitivity of the detector. The band limits can be set closer to the quiescent value than they otherwise would be. This is possible to do without increasing the false detection rate because only changing the counter when a band is crossed helps to reject some common false alarm sources. It may also be advantageous as it may more readily pick up an individual moving through detection zones in a way which would create a receding sensor signal in which the amplitudes decreases.

The first band may be an inner band and the second band may be an outer band. That is to say that the high and low limits of the inner band may be closer to the quiescent value of the sensor signal than the high and low limits of the outer band.

The band limits may be settable or modifiable. The band limits may be pre-set.

The high band limits and the low band limits may be set to be equidistant from the quiescent value of the sensor signal.

Each band may be modified by an additional input. That is to say that the high and low band limits may be modified based on an additional input.

The additional input may be an external input or a predetermined input. An external input may be an ambient temperature reading, for example from a temperature sensor provided with the infra-red detector. The ambient temperature reading allows for temperature compensation. A predetermined input may correspond to an additional element, such as an anti-cloaking element. The anti-cloaking element may be an additional detector of a different type, such as a microwave detector. In a dual-technology device, if the microwave detector detects movement but the PIR is not detecting, then the sensitivity of the PIR can be temporarily increased, by moving the limits closer to the quiescent value.

Temperature compensation can be achieved by adjusting the sensitivity of the PIR (by changing the limits) to increase the sensitivity when the background temperature is at a level where the changes in incident radiation caused by an intruder are likely to be small. The pyroelectric element detects changes in incident radiation, and so at a background temperature of about 31°C a human intruder will tend to produce the weakest signal. Accordingly the sensitivity of the PIR can be set to the highest level at this temperature, and is adjusted (by moving the limits further away from the quiescent value) for temperatures higher or lower than this.

Anti-cloaking is a feature designed to detect intruders who have deliberately acted to reduce their heat signature, for example by wearing multiple layers of clothing. This will cause a smaller change in incident radiation as the intruder walks across the zones, in exactly the same way as a background temperature of about 31°C will cause a weak signal. Accordingly it can be tackled in the same way -by increasing the sensitivity of the detector by moving the limits closer to the quiescent value.

In a detector according to the invention, the limits can be moved closer to the quiescent values than would otherwise be possible without creating an unacceptable false detection rate. In some embodiments, dynamic adjustment of the limits to increase the sensitivity of the detector for temperature compensation or anti-cloaking may be accompanied by an increase in the detection thresholds. Hence the possibility of false alarms by moving the limits closer to the quiescent value is mitigated by requiring a larger pulse count (more band crossings) to raise an alarm.

Being able to set the band limits provides for a way of adjusting the sensitivity of the detector to incident infra-red. It also allows for the band limits to be dynamically adjusted, especially when there is an additional input such as a temperature sensor or a microwave detector.

The band detection thresholds may be different. In some embodiments, the first band detection threshold may be less than the second band detection threshold. In other embodiments the first band detection threshold may be greater than the second band detection threshold. It will be understood that here a first band detection threshold which is less than a second band detection threshold really means that the difference between the first initial value and the first band detection threshold is less than the difference between the second initial value and the second band detection threshold, since in some embodiments counters may start at different values and count down (for example) to zero. Similarly, the difference between the initial value and respective threshold simply means the number of times that the counter needs to be changed (incremented or decremented) to reach the threshold.

An inner band detection threshold greater than the outer band detection threshold would be the usual case for a PIR used in an intruder alarm system -a weak (low amplitude) signal would then need to generate more pulses than a strong (high amplitude) signal to cause an alarm. However, in other applications it could be that this pattern is reversed. For example, in a PIR used to control lighting, it may be desirable to set the inner band detection threshold to less than the outer band detection threshold. This is because a low amplitude signal is likely to indicate a more distant (from the detector) target. That target will have to walk further to move through a zone, since the zones fan out from the detector and are larger when at a distance from the detector. Accordingly a low amplitude signal which only generates one pulse may indicate a person walking the same distance as a high amplitude signal which generates two or three pulses -a person close to the detector will cross many zones and also create a high amplitude signal. Hence the thresholds and limits can be tuned to create a detection state roughly based on the distance walked by a detected person.

Each band detection threshold may be settable or modifiable. Each band detection threshold may be set by an installer. Each band detection threshold may be set by a user or installer of the detector.

The infra-red detector may validate that the band limit has been met. In one embodiment, the band limit is validly met if the sensor signal remains outside the band (above the high limit or below the low limit, as the case may be) for a predetermined period. In other words, the band limit is only considered to have been met if after a predetermined time period the sensor signal is still outside the band. The predetermined time period may be for example around a few tens of milliseconds.

By checking the validity of a limit being met or exceeded it is possible to substantially mitigate the possibility of false detections.

If the total of the two counters ever exceeds a failsafe threshold, a detection state may be generated.

Initialisation may set the first band counter and/or the second band counter. The first and second band counters may be re-initialised when a re-initialisation condition is met. Typically, a re-initialisation condition is met when the received signal has remained inside both bands continuously for a pre-determined time period. In other words, the PIR has not detected anything exceeding the inner band limits for a period of time. The pre-determined time period may be for example between around 10 seconds and a few minutes.

Re-initialising the counters after a period of inactivity avoids false detections caused by infrequent but benign sources of infra-red radiation. In other embodiments, rather than re-initialising the counters, there may be a slow decay, or automatic changing of the counter away from the threshold, irrespective of any activity. For example, suppose the initial value of the counter is zero and the relevant threshold for a detection condition is 3. The counter will increment by one every time the relevant band is crossed. In one embodiment, once the counter has incremented to 2, if the band (or any band) is not crossed for 30 seconds, then the counter may be reset to 0. Three further crossings would then be needed to cause an alarm. The counter may be reset based on a timer expiring, but in embodiments the timer may be started either when the counter is first incremented from 0 to 1, or in other embodiments the timer may be re-started every time the counter is incremented, so the timer would start on the increment from 0 to 1 and then re-start on the increment from 1 to 2. In yet other embodiments, instead of re-initialising the counter the counter may be decremented (changed away from the threshold and towards the initial value, but not necessarily reset) on expiry of the timer.

The first band counter may be changed, for example by incrementing or decrementing, when the received sensor signal meets the first band limit. Preferably, the first band counter is initially changed when the received sensor signal meets either one of the high first band limit or the low first band limit. The first band counter is changed again only if the received sensor signal subsequently meets the other one of the first band limits. For example, the first counter may be incremented by one upon the received sensor signal meeting the high first band limit and incremented again if the received sensor signal subsequently meets the low first band limit. The first band counter is not changed if the received sensor signal subsequently meets the same first band limit, e.g. high band limit followed by high band limit. That is to say that the counter is only changed when it crosses the band.

The second band counter may be changed, for example by incrementing or decrementing, when the received sensor signal meets a second band limit. Preferably, the second band counter is initially changed when the received sensor signal meets either one of the high second band limit or the low second band limit. The second band counter is changed again only if the received sensor signal subsequently meets the other one of the second band limits. For example, the second counter may be incremented by one upon the received sensor signal meeting the high second band limit and incremented again if the received sensor signal subsequently meets the low second band limit. The second band counter is not changed if the received sensor signal subsequently meets the same second band limit, e.g. high band limit followed by high band limit. That is to say that the counter is only changed when it crosses the band.

The motion detector may comprise an optical element configured to produce a pattern of detection zones. The optical element may be a Fresnel lens. The Fresnel lens may have a plurality of Fresnel facets. Other optical elements suitable for producing a pattern of detection zones may be provided.

A typical signal received from a sensor, in the case where a person is walking in front of the detector, may be a roughly sinusoidal signal.

The motion detector may be for intruder detection. For example, in an alarm system.

The detection state may be transmitted to an alarm panel and produce an alarm.

The infra-red motion detector may be provided in a multimode motion detector, preferably a dual technology motion detector. The multimode motion detector may comprise the infra-red motion detector and at least one other motion detector comprising a sensing mode different from infra-red. The multimode motion detector may comprise an infra-red motion detector and a microwave detector.

The multimode motion detector may require a single detection state from at least one of the detectors in order to generate an overall detection state, for example to transmit to an alarm panel to produce an alarm. Alternatively, the multimode motion detector may generate an overall detection state only when multiple of the detectors generate a detection state.

The infra-red motion detector may comprise a processor and a memory. The processor and memory may be configured to perform the relevant features of the or each independent claim. That is to say that the processor and memory are configured to compare the received sensor signal to the bands, determine the band crossings and generate a motion detection state.

The infra-red motion detector may comprise a plurality of analogue or digital electronic components formed into a plurality of circuits. The plurality of electronic components and circuits may be arranged and configured to perform the relevant features of the or each independent claim. That is to say that the plurality of electronic components may be arranged and configured into a plurality of circuits which compare the received sensor signal to the bands, determine the band crossings and generate a motion detection state.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example only to the accompanying drawings, in which: Figure 1 shows a schematic representation of an infra-red detector according to an embodiment of the present invention; Figure 2 shows a schematic representation of an infra-red detector according to an embodiment of the present invention; and Figures 3a and 3b show schematic representations of a received sensor signal in a detector according to an embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring firstly to Figure 1, an infra-red (IR) detector is generally indicated at 10.

The detector 10 is operable to detect infra-red radiation within a field of view 12. A sensor 14 is provided behind an optical element 16. In some embodiments, multiple sensors and/or multiple optical elements may be provided.

In the current embodiment, the optical element 16 comprises a plurality of Fresnel lenses. A plurality of detection zones 18 are provided in the field of view 12 by the plurality of Fresnel lenses. Infra-red radiation emitted in a detection zone 18 is focused onto the sensor 14 by one of the Fresnel lenses in the optical element 16. Each detection zone 18 is separated by a neutral zone 20. Infra-red radiation emitted in the neutral zones 20 are not focused onto the sensor 14.

The sensor 14 generates a sensor signal in response to the infra-red energy focused onto the sensor 14. A pyroelectric material is provided in the sensor 14, and the pyroelectric material produces an electrical signal as an output in response to infra-red radiation emitted from the detection zones 18. As a person moves into a detection zone 18 the infra-red emitted by the person is focused onto the sensor 14 through the optical element 16. The sensor signal outputted increases in amplitude, i.e. above a quiescent level, as a result of the infra-red radiation and is proportional to the size of the change in infra-red energy received at the sensor 14.

The sensor signal is filtered and amplified, as is well known. Typically, the raw signal from the sensor is filtered by a bandpass filter with a passband of for example 0.1Hz -10Hz.

Where there is no change in incident IR detected by the sensor 14, the signal outputted may be a quiescent signal. It will be appreciated that the output may be subject to noise and similar interference which will cause the quiescent signal to fluctuate around a quiescent value. Typically, the quiescent signal after amplification has a DC voltage level of around 2.0V, and on a change in incident IR the sensor signal may go as low as for example around 0.5V or as high as for example 3.5V.

As an object moves across the field of view 12, passing through detection zones 18 and neutral zones 20, the signal outputted from the sensor 14 will oscillate above and below the quiescent value. The amplitude of the sensor signal will generally represent the infra-red energy emitted.

Referring to Figure 2, a schematic representation of the detection method is generally indicated at 30. In the current embodiment, the IR detector 10 of Figure 1 comprises a processor and memory configured to implement the detection method described.

The sensor signal is received 32 from the sensor 14. As discussed above, the sensor signal will generally fluctuate around a quiescent value until a source emitting IR energy moves across the field of view 12 of the detector 10. Example received sensor signals are shown in Figures 3a and 3b.

The received sensor signal will be compared to a first band 34a and compared to a second band 34b. The band limits are set during initialisation of the IR detector 10, but may be adjusted during operation based on different factors, for example ambient 15 temperature.

Each band comprises a first band limit and a second band limit, these may be referred to as high band limit 36a, 38a and low band limit 36b, 38b. The high band limit and low band limit of each band are set equidistantly from the quiescent value.

In Figures 3a and 3b, the first band is an inner band comprising a high band limit 36a and a low band limit 36b. The second band is an outer band and comprises a high band limit 38a and low band limit 38b, the high second band limit 38a and low second band limit 38b are further from the quiescent value than the band limits of the inner band 33.

The processor monitors for when the received sensor signal meets one of the band limits of one of the bands, i.e. when the received sensor signal goes outside the band, above the upper limit or below the lower limit. A counter is changed when the first band limit of the band is met for the first time. The processor monitors for when the received sensor signal subsequently meets the other band limit for the same band. Upon meeting the other band limit, the same counter is changed again. The processor determines that a crossing of the first band 40a or second band 40b has happened when both conditions have been met.

The number of band crossings is counted in step 42a, 42b. A counter may be incremented or decremented.

Taking Figure 3a as an example, the received sensor signal is at a quiescent value until an increase in the infra-red energy emitted in one of the detection zones causes an increase in the amplitude of the sensor signal, this may be from a person entering one of the detection zones 18. This causes the signal to meet and exceed the high band limit 36a of the first band.

In some embodiments, the first time the high band limit is met or exceeded a counter is changed, for example by incrementing by one, and/or a band value flag is set. A timer may start to determine a re-initialisation condition. It will be appreciated that this can equally apply when the low band limit is met or exceeded first.

In the example in Figure 3a, the received sensor signal starts to move back towards the quiescent value, for example because the person begins to move out of the initial detection zone 18 or simply remains standing still for a period of time (bearing in mind that the signal is filtered by a bandpass filter and decoupled from the DC level of the pyroelectric sensor). When the person moves into another detection zone 18 adjacent the first detection zone 18, the sensor signal increases in amplitude away from the quiescent value in an opposite direction. The sensor signal than meets or drops below the low band limit 36b.

The subsequent meeting of the low band limit causes the relevant counter to be changed and/or the relevant band flag status to be changed.

The band flags may allow the processor to more easily determine when the other band value has been previously met or exceeded.

When both band limits 36a, 36b are met sequentially, e.g. high band limit and then low band limit or low band limit and then high band limit, the processor determines that the first band has been crossed 40a and changes a count 42a on a first band counter, e.g. by incrementing or decrementing.

In some embodiments, the processor may validate that the band limit has been met. The validation process involves monitoring for the received sensor signal remaining outside of the band, i.e. above the high band limit or below the low band limit for a validation time period. This should reduce false detections. The validation time period may be predetermined based on a range of expected frequencies, for example in some embodiments the validation time period may be about 10ms.

In the Figure 3a example, the processor determines that there is a first band crossing since the sensor signal has met / exceeded the high band limit 36a and subsequently met / dropped below low band limit 36b. This increments the first band counter.

In Figure 3a, neither the second / outer high band limit 38a or second outer low band limit 38b has been met when compared 34b to the sensor signal. As a result the second counter remains at its initial value.

A detection threshold for each threshold band is provided. These may be a predetermined value, an adjustable value in response to other factors or sellable through a physical input.

The number of determined band crossings 40a, 40b for each threshold band is compared to the detection threshold 44a, 44b associated with that band.

The provision of a separate detection threshold for each band, i.e. a first band detection threshold and a second band detection threshold, allows for a flexible detection method which may be more easily tailored to different situations.

A detection state 46, or detection signal, may be generated based on the comparison between the count on the counter and the associated detection threshold.

In the example shown in Figure 3a, a single first band crossing has been determined 40a and as a result the first band counter changed 42a. The value of the first band counter 42a is compared to the first band detection threshold 44a. The first band detection threshold may be set to a value, for example, 2, 3 or more. In the current embodiment, the first band detection threshold is set to 2 using a jumper or switch. In the example shown in Figure 3, a detection state would be generated, since the counter would increment for the first time when the high band limit 36a is met, and then increment for the second time when the low band limit 36b is met. If the first band detection threshold was set to a higher value than 2, there would need to be another meeting of the high band limit 36a. Note that another meeting of the low band limit 36b would not further increment the counter, since after the first increment the counter is only incremented on subsequently meeting the other of the high and low band limits.

Turning to the example of a received signal shown in Figure 3b, the received sensor signal is at a quiescent value until an increase in the infra-red energy emitted in one of the detection zones causes an increase in the amplitude of the sensor signal. This may be from a person entering a detection zone. In this example the increase in amplitude is greater than that shown in Figure 3a.

The received sensor signal firstly meets and exceeds the high band limit 36a of the first band and then the high band limit 38a of the second band. A first band counter is changed and/or a first band flag is set since the high first band limit has been at least met, and a second band counter is changed and/or a second band flag is set since the high band limit 38a of the second threshold band is met or exceeded. It will be appreciated that this can also apply when the low band limits are met or exceeded first.

The received sensor signal starts to move back towards the quiescent value, for example because the person begins to move out of the initial detection zone. When the person moves into another detection zone adjacent the first detection zone, the sensor signal increases in amplitude away from the quiescent value in an opposite direction. In the current example, the sensor signal meets or drops below the low band limit 36b of the first band but not the low band limit 38b of the second band. The subsequent meeting or dropping below of the low band limit 36b of the first band causes the first band counter to be changed again and/or the first band flag status to be changed; however, the second band counter is not changed and/or the second band flag is not changed.

A first band crossing is determined 40a since both the high band limit 36a and low band limit 36b have been met or exceeded.

However, a second band crossing 40b is not determined, since while the high band limit 38a has been met there has been no subsequent meeting of the low band limit 38b.

In some embodiments, a third band may be provided, and the processor will generally perform the same steps in relation to the third threshold band as it does in relation to the first and second threshold bands.

The embodiments described above are provided by way of example only, and various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims

CLAIMS1. An infra-red motion detector for detecting movement of a person, the detector comprising: an infra-red sensor module for outputting a sensor signal based on received infra-red radiation; and a motion detection module configured to: compare the received sensor signal to a first band, the first band comprising a high first band limit and a low first band limit; compare the received sensor signal to a second band, the second band comprising a high second band value and low second band limit; initialise a first band counter to a first initial value and initialise a second band counter to a second initial value; change the first band counter when: a) the received sensor signal meets either one of the high first band limit or the low first band limit, and b) the received sensor signal subsequently meets the other one of the high first band limit or the low first band limit; change the second band counter when a) the received sensor signal meets either one of the high second band limit or the low second band limit, and b) the received sensor signal subsequently meets the other one of the high second band limit or the low second band limit; generate a motion detection state if either the first counter crosses a first band detection threshold or the second counter crosses a second band detection threshold.
2. An infra-red motion detector as claimed in claim 1, in which the first band counter is also changed when: a) the first band counter is equal to the first initial value, and b) the received sensor signal meets either one of the high first band limit or the low first band limit, and in which the second band counter is also changed when: a) the second band counter is equal to the second initial value, and b) the received sensor signal meets either one of the high second band limit or the low second band limit.
3. An infra-red motion detector as claimed in claim 1 or claim 2, in which the first counter is re-initialised to the first initial value and the second counter is reinitialised to the second initial value when a re-initialisation condition is met.
4. An infra-red motion detector as claimed in claim 3, in which the re-initialisation condition is met when the received signal has remained between the high first band limit and the low first band limit continuously for a pre-determined time period.
5. An infra-red motion detector as claimed in any preceding claim, in which the difference between the first initial value and the first band detection threshold is greater than the difference between the second initial value and the second band detection threshold.
6. An infra-red motion detector as claimed in any of claims 1 to 4, in which the difference between the second initial value and the second band detection threshold is greater than the difference between the first initial value and the first band detection threshold.
7. An infra-red motion detector as claimed in any preceding claim, in which the first band detection threshold and the second band detection threshold are settable.
8. An infra-red motion detector as claimed in any of the preceding claims, in which the high first band limit and the low first band limit are equidistant from a quiescent value of the received sensor signal, and the high second band limit and the low second band limit are equidistant from the quiescent value.
9. An infra-red motion detector as claimed in any preceding claim, in which the first and/or second band limits are further set based on an additional input.
10. An infra-red motion detector as claimed in claim 9, in which the additional input comprises at least one of an ambient temperature signal and an anti-cloak element.
11. An infra-red motion detector as claimed in any preceding claim, in which the first band is an inner band and the second band is an outer band.
12. A method of generating a motion detection state in an infra-red motion detector comprising: receiving a sensor signal from an infra-red sensor module, the sensor signal being based on received infra-red radiation; comparing the received sensor signal to a first band, the first band comprising a high first band limit and a low first band limit; comparing the received sensor signal to a second band, the second band comprising a high second band limit and a low second band limit; initialising a first band counter to a first initial value and initialising a second band counter to a second initial value; changing the first band counter when: a) the received sensor signal meets either one of the high first band limit or the low first band limit, and b) the received sensor signal subsequently meets the other one of the high first band limit or the low first band limit; changing the second band counter when: a) the received sensor signal meets either one of the high second band limit or the low second band limit, and b) the received sensor signal subsequently meets the other one of the high second band limit or the low second band limit; generating a motion detection state if either the first counter crosses a first band detection threshold or the second counter crosses a second band detection threshold.
13. A method as claimed in claim 12, in which the first band counter is also changed when: a) the first band counter is equal to the first initial value, and b) the received sensor signal meets either one of the high first band limit or the low first band limit. and in which the second band counter is also changed when: a) the second band counter is equal to the second initial value, and b) the received sensor signal meets either one of the high second band limit or the low second band limit.
14. A method as claimed in claim 12 of claim 13, in which the first counter is re-initialised to the first initial value and the second counter is re-initialised to the second initial value when a re-initialisation condition is met.
15. A method as claimed in claim 14, in which the re-initialisation condition is met when the received signal has remained between the high first band limit and the low first band limit continuously for a pre-determined time period.
16. A method as claimed in any of claims 12 to 15, in which the difference between the first initial value and the first band detection threshold is greater than the difference between the second initial value and the second band detection threshold.
17. A method as claimed in any of claims 12 to 15, in which the difference between the second initial value and the second band detection threshold is greater than the difference between the first initial value and the first band detection threshold.
18. A method as claimed in any of claims 12 to 17, in which the first band detection threshold and the second band detection threshold are settable.
19. A method as claimed in any of claims 12 to 18, in which the first and second band limits and further set based on an additional input.
20. A method as claimed in claim 19, in which the additional input includes an ambient temperature sensor.
21. A method as claimed in claim 19, in which the additional input includes an anti-cloaking detector.
22. A method as claimed in claim 21, in which the anti-cloaking detector is a microwave motion sensor.