MOTION DETECTOR WITH MASKING DETECTION

Abstract

A motion detector (1) for a security system includes a primary sensing system including a PIR sensor (5) configured to detect movement of a person within a monitored environment (2), and an ancillary sensing system including a first emitter (7), a second emitter (8) and a receiver (9) configured to detect masking of the primary sensing system. Masking by an object (10) in the monitored environment (2) is determined using the first emitter (7) and the receiver (9) based on a time-of-flight of an optical signal emitted into the monitored environment (2) and reflected by the object (10). Masking by obscuring of a window (4) of the motion detector (1) is determined using the second emitter (8) and the receiver (9) based on the intensity of light transmission from the second emitter (8) through the window (4) to the receiver (9).

Claims

1. A motion detector for a security system, the motion detector comprising: a primary sensing system configured to detect movement of a person within a monitored environment; and an ancillary sensing system configured to detect masking of the primary sensing system, the ancillary sensing system comprising an optical time-of-flight sensor.

2. A motion detector according to claim 1, wherein the primary sensing system comprises one of an infrared sensor, a microwave sensor and an ultrasound sensor.

3. A motion detector according to claim 1, wherein the primary sensing system comprises a passive infrared sensor.

4. A motion detector according to claim 1, wherein the optical time-of-flight sensor comprises an infrared time of flight sensor.

5. A motion detector according to claim 1, wherein the optical time-of-flight sensor comprises an emitter and a detector, wherein the emitter of the optical time-of-flight sensor is configured to emit light into the monitored environment, and wherein the detector of the optical time-of-flight sensor is configured to receive the light after reflection from the monitored environment.

6. A motion detector according to claim 5, further comprising: a housing including a window or lens, wherein the primary sensing system is disposed inside the housing and is configured to monitor the monitored environment through the window or lens, and wherein the emitter of the optical time-of-flight sensor is configured to emit the light into the monitored environment without passing through the window or lens.

7. A motion detector according to claim 6, wherein the detector of the optical time-of-flight sensor is configured to receive the light reflected from the monitored environment after passing through the window or lens.

8. A motion detector according to claim 6, wherein the optical time-of-flight sensor is a first ancillary sensor, and wherein the ancillary sensor system further comprises: a second ancillary sensor configured to detect a change in transparency of the window or lens.

9. A motion detector according to claim 8, wherein the second ancillary sensor comprises an emitter and a detector, wherein the emitter of the second ancillary sensor is configured to emit light directly through the window or lens to the detector of the second ancillary sensor.

10. A motion detector according to claim 9, wherein the detector of the first ancillary sensor and the detector of the second ancillary sensor are the same detector.

11. A motion detector according to claim 1, wherein the motion detector is configured to take a first action responsive to detection of movement within the monitored environment by the primary sensor, and wherein the motion detector is configured to take a second action responsive to detection of masking of the first sensor by the ancillary sensor.

12. A method of detecting masking of a motion detector, wherein the motion detector is configured to detect movement of a person within a monitored environment, the method comprising: emitting an optical signal from the motion detector into the monitored environment; receiving a reflection of the optical signal at the motion detector, the reflected optical signal having been reflected by an object within the monitored environment; and determining that the object is masking the motion detector when a time-of-flight of the reflected optical signal is below a predetermined threshold.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Certain preferred embodiments of the present invention will now be described, by way of example only and with reference to the accompanying drawings, in which:

[0036] FIG. 1 is a front view of a motion detector;

[0037] FIG. 2 is a cut-away side view of the motion detector of FIG. 1;

[0038] FIG. 3 is a schematic diagram showing operation of an ancillary sensor of the motion detector of FIG. 1;

[0039] FIG. 4 is a schematic diagram showing an alternative configuration of the ancillary sensor;

[0040] FIG. 5 is a front view of an alternative motion detector;

[0041] FIG. 6 is a cut-away side view of the motion detector of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

[0042] FIGS. 1 and 2 illustrate a first embodiment of a motion detector 1 for monitoring an environment 2. The motion detector 1 is of the type intended for use within an intrusion detection security system (not shown).

[0043] The motion detector 1 comprises a housing 3 with a window 4. The window 4 is transparent to at least infrared light. Such windows are often opaque or translucent to visible light. In the illustrated embodiment, the motion detector 1 is configured to monitor an environment 2 forwards of the motion detector 1, and hence the window 4 is on a front face of the motion detector 1.

[0044] The motion detector 1 comprises a primary sensing system for detection of motion within the monitored environment 2, and an ancillary sensing system for detection of masking of the motion detector 1.

[0045] The motion detector 1 comprises a controller 14 in the form of a printed circuit board comprising control logic 13. For example, the controller 14 may comprise a memory and a processor, where the memory stores computer readable instructions to control operation of the processor. The controller 14 controls operation of the motion detector 1, including the primary sensing system and the ancillary sensing system, as described herein.

[0046] The primary sensing system of the motion detector comprises a primary sensor 5 mounted within the housing for detecting motion within the environment 2. The primary sensor 5 comprises a passive infrared (PIR) sensor 5 configured to detect infrared light from within the environment 2.

[0047] A mirror 6 is provided within the housing 6 to act as a focusing element, and the mirror 6 is configured to focus light received through the window 4 from the environment 2 onto the primary sensor 5. The mirror 6 may, for example, comprise a Fresnel mirror.

[0048] The primary sensor 5 is configured to detect infrared light emitted by a live subject (not shown) within the environment 2. Motion of the subject within the environment 2 will cause changes in the luminous flux received by the primary sensor 5, and when a rate of change of the detected luminous flux exceed a predetermined threshold then the motion detector 1 will take an appropriate action, such as triggering an intrusion alarm or sending an alert to a controller of the intrusion detection system.

[0049] Optionally, an optical filter, such as a band-pass optical filter, may be provided between the primary sensor 5 and the environment 2 to restrict which wavelengths of light are received by the primary sensor 5.

[0050] The primary sensor 5, the focusing element 6 and the filter, if present, collectively provide the primary sensing system of the motion detector 1.

[0051] The ancillary sensing system of the motion detector 1 comprises a first emitter 7, a second emitter 8 and a receiver 9. The first emitter 7 and the second emitter 8 are each configured to emit infrared light, and the receiver 9 is configured to receive the emitted infrared light.

[0052] The first emitter 7 is provided with a first emitter lens 7a configured to act as a focusing element to direct infrared light emitted by the first emitter 7 into the monitored environment 2. The first emitter 7 is provided within the housing 3 of motion detector 1, and the first emitter lens 7a is positioned such that the emitted infrared light is emitted from the housing 3 but does not pass through the window 4 of the housing 3.

[0053] The second emitter 8 is provided with a second emitter lens 8a configured to act as a focusing element to direct infrared light emitted by the second emitter 8 onto the surface of the window 4. The second emitter 8 is provided within the housing 3 of motion detector 1.

[0054] The receiver 9 is provided within the housing 3 of the motion detector, and behind the window 4. The receiver 9 is configured to receive light emitted by the first emitter 7 after reflection by an object within the monitored environment 2, and to receive light emitted by the second emitter 8 without reflection by an object within the monitored environment.

[0055] In order to prevent cross-talk, one or more light barriers 15a, 15b, 15c are provided within the housing to isolate the receiver 9 from each of the first and second emitters 7, 8. The light barriers 15a, 15b, 15c are substantially opaque to the infrared light emitted by the first and second emitters 7, 8. In the illustrated embodiment, the light barriers 15a, 15b, 15c also isolate the first and second emitters 7, 8 from one another.

[0056] The first emitter 7 and the receiver 9 provide a first ancillary sensor for detection of masking of the motion detector 1 by an object 10 within the monitored environment 2, whilst the second emitter 8 and the receiver 9 provide a second ancillary sensor for detection of masking of the motion detector 1 by obscuration of the window 4.

[0057] The first ancillary sensor is configured to operate on a time-of-flight principle, as illustrated in FIG. 3, and is configured to detect masking of the motion detector 1 by an object 10 within the monitored environment 2.

[0058] The first emitter 7 is configured to periodically emit a predetermined signal 11, such as a single pulse of infrared light or a chain of pulses of infrared light. Typically, the or each pulse would be of a very short duration, such as less than 10 nanoseconds.

[0059] The emitted signal 11 may be a laser signal and the first emitter 7 may be a laser emitter, such as a laser diode. Thus, the emitted signal 11 may comprise light at substantially a single wavelength.

[0060] When the emitted signal 11 reaches an object 10 within the monitored environment 2, a reflected signal 12 will be generated by the scattering of the emitted light by the object 10. This reflected signal 12 is detected the receiver 9.

[0061] By comparing the emitted signal and the received signal, for example using processing logic 13, it is possible to determine a time of flight ΔT of the reflected signal 12, i.e. a time between emission of the signal by the first emitter 7 and detection by the receiver 9. The time of flight ΔT can be used to approximate a distance between the motion detector 1 and the object 10 based on the speed of light, as given below.

d=(c.Math.ΔT)/2

[0062] where d is the distance between the motion detector 1 and the object 10, c is the speed of light in a vacuum (which is also approximately the speed of light in air), and ΔT is the time-of-flight of the reflected signal 12.

[0063] In order to simplify processing, the time-of-flight is determined as the time delay between the emission of a pulse in the emitted signal and the first detection of a reflected pulse above a threshold. The threshold may be a fixed, predetermined threshold or may be a dynamic threshold, for example based on detected background infrared levels. The time-of-flight detected in this manner corresponds to a distance between the motion detector 1 and the closest object 10 within the field of view of the first emitter 7.

[0064] As a result of this processing, it is important not to have both of the first emitter 7 and the receiver 9 provided behind the window 4 because reflection of the emitted signal 11 by the window 4 (and within the housing 3) could be detectable, and may require more complex processing to cancel.

[0065] Based on the distance to the closest object 10 within the monitored environment, the motion detector 1 is able to determine whether it has been masked by an object. If the distance to the closest object 10 within the monitored environment is below a predetermined threshold, such as less than 2 meters, the motion detector 1 may trigger an alert. Such an alert may comprise triggering an alarm, and/or may comprise alerting an operator to inspect the motion detector 1.

[0066] The first ancillary sensor may have a limited range, typically up to about 4 m in good conditions, but falling to about 2 m in bright sunlight. Consequently, in order to maximize the range of the first ancillary sensor, the first emitter 7 preferably has a relatively narrow field of view.

[0067] With reference to FIG. 4, in an optional further embodiment, a plurality of first emitters 7, 7′, 7″ may be provided and configured to illuminate different regions of the monitored environment 2. A single receive 9 may be used to detect reflected signals 12 from each of the first emitters 7, 7′, 7″, as illustrated, or alternatively multiple receivers 9 may be provided depending upon the geometry of the motion detector 1. In such an embodiment, the first emitters 7, 7′, 7″ may be configured to emit signals 11 in a time-spaced manner such that only a single first emitter 7, 7′, 7″ illuminates the monitored space 2 at any particular time to enable the receiver 9 to discriminate between the reflected signals 12 from each first emitter 7, 7′, 7″.

[0068] Turning back to FIG. 2, the second ancillary sensor comprises the second emitter 8 and the receiver 9, and is configured to determine masking of the motion detector 1 by obscuration of the window 4.

[0069] During operation, the second emitter 8 periodically emits a predetermined signal, such as a single pulse of infrared light or a chain of pulses of infrared light. For the second emitter 8, the or each pulse does not need to be of particularly short duration, and could be up to several milliseconds or longer in duration.

[0070] The second emitter 8 may be a laser emitter, such as a laser diode, or may simply be an infrared LED.

[0071] The second emitter lens 8a is configured to direct the emitted light across the window 4 of the housing 3, and it is subsequently detected by the receiver 9, for example after scattering within the interior of the housing 3.

[0072] The intensity of light emitted by the second emitter 8 and detected by the receiver 9 should remain substantially constant. Therefore, by comparing the detected intensity of light detected by the receiver 9 to a predetermined value, it is possible to detect masking of the window 4. Such masking would typically result in a reduction in the detected intensity.

[0073] Optionally, the reference value may comprise a dynamic reference value, such as a rolling average of recent measurements. This could be used to account for dust or other fouling accumulating on the window 4, which would reduce its infrared transparency.

[0074] Based on the intensity of light detected by the second ancillary sensor, the motion detector 1 is able to determine whether the window 4 has been masked by an object. If the intensity of detected light is below a predetermined threshold, such as less than 80% of the expected intensity, the motion detector 1 may trigger an alert. Such an alert may comprise triggering an alarm, and/or may comprise alerting an operator to inspect the motion detector 1.

[0075] With reference to FIGS. 5 and 6, a second motion detector 21 is shown.

[0076] The second motion detector 1 is of similar construction to the first motion detector 1, and operates in substantially the same manner. Therefore, only the differences between these motion detectors 1, 21 will be described.

[0077] Elements of the second motion detector 21 that correspond to elements of the first motion detector 1 are numbered with the same reference signs, but incremented by 20. The descriptions of these elements above apply also to the second motion detector 21, unless otherwise stated.

[0078] In the second motion detector 21, the locations of the second emitter 28 and the receiver 29 have been reversed compared to the first motion detector 1. Thus, the second emitter 28 is provided within the housing 23 and behind the window 24, and the receiver is provided with a receiver lens 29a that is exposed from the housing 23 separately from the window 24.

[0079] The receiver lens 29a in this embodiment is configured to receive light emitted by the first emitter 27 and scattered by an object within the monitored environment 22 without passing through the window 24.

[0080] The receiver lens 29a in this embodiment is also configured to receive light emitted by the second emitter 28 after it has passed through the window 24 and without scattering by an object within the monitored environment 22.

[0081] The first emitter 27, second emitter 28 and receiver 29 in this embodiment form first and second ancillary sensors, which operate in substantially the same manner as the first and second ancillary sensors of the first motion detector 1.