A combined microwave and infra-red detector device, particularly for anti-intrusion systems

Abstract

 The microwave unit includes a transmitting unit (4, 5) and a receiving unit (6, 7) with respective active surfaces (4, 6) which are close together. The infra-red sensor (8) is interposed substantially between said active surfaces (4, 6) so that any attempt to mask the infra-red sensor (8) causes a corresponding masking of the microwave unit (4 to 7) which can be detected externally.

Description

[0001] The present invention relates to combined microwave and infra-red detection devices which can be used, for example, in anti-intrusion devices.

[0002] A combined detector device including a microwave unit and an infra-red sensor is described, for example, in European patent application 0,255,812 in the name of the same Applicant.

[0003] The object of the present invention is to provide a detector device of the type specified above which is further improved as regards its resistance to masking operations (or obstruction) and as regards the possibility of providing a very compact device with dimensions such that its identification by an intruder and the recognition of the combined characteristics of the device itself are more difficult.

[0004] According to the present invention, this object is achieved by virtue of a detector device comprising a microwave unit and an infra-red sensor, characterised in that the microwave unit comprises a transmitting unit and a receiving unit with respective active surfaces which are close together, and carries associated means for the detection of the at least partial masking of the microwave unit, and in that the infra-red sensor is interposed substantially between the active surfaces so that any attempt to mask the infra-red sensor causes corresponding masking of the microwave unit which can be detected by the detection means.

[0005] The invention will now be described purely by way of non-limiting example, with reference to the appended drawings, in which:

Figure 1 is a perspective view of a detector device according to the invention,

Figure 2 is a perspective view if the front part of the detector device of Figure 1, in which a cover for protecting the front face of the device has been removed,

Figure 3 is a front view taken on the arrow III of Figure 2, and

Figure 4 shows, in the form of a block diagram, the circuit structure of the receiving and alarm-generating part of the detector device according to the invention.

[0006] In Figure 1, a detector device, generally indicated 1, is intended for use, for example, in an anti-intrusion system.

[0007] The casing of the device 1 is constituted essentially by a support bracket 2 which is intended to be fitted to a wall or other support and by a prismatic body 3 which is generally orientable relative to the support 2. A microwave detector unit and an infra-red sensor which can detect and indicate externally the movements of any intruders in the area surveyed by the device are mounted within the body 3.

[0008] According to a known solution, the microwave unit comprises essentially a transmitting unit or horn 4 supplied by a microwave generator 5 such as a Gunn diode operating at a frequency between, for example, 9.3 and 10.8 GHz, and a receiving unit (horn) 6 which directs the microwave radiation received to a receiver member usually constituted by a Schottky mixer diode 7.

[0009] An infra-red sensor, indicated 8, however, is mounted on the front face of the prismatic casing 3 in a position facing a Fresnel lens 9 fitted to a front cover 10 which has been removed for clarity in the views of Figures 2 and 3.

[0010] The main characteristic of the detector device according to the invention is the fact that the open ends (active surfaces) of the horns 4 and 6 are side-by-side and close together, whilst the infra-red sensor 8 is located between these active surfaces.

[0011] In practice, the open ends of the horns 4, 6 are rectangular in shape with dimensions, for example, of the order of 38 x 30 mm. The two horns 4 and 6 are side-by-side along two of their longer sides, and are separated by a solid central strip approximately 14.5 mm wide, or a little more, in which the infra-red sensor 8 is fitted.

[0012] The latter is not usually situated exactly in the centre of the strip separating the two horns 4, 6, but in a position slightly nearer the top. Thus, by the consistent definition (according to known criteria) of the geometry of the Fresnel lens 9, it is possible to arrange for the infra-red sensor 8 to have an asymmetrical reception lobe oriented generally downwards (with reference to the position illustrated in Figure 1). The detector device 1, which is usually intended to be mounted in a room, near its ceiling, can thus extend its action in an optimum manner over the whole of the floor area of the room.

[0013] The positioning of the two horns 4 and 6 side-by-side, and the positioning of the sensor 8 between them, enables the device 1 as a whole (that is, the casing 3) to have very small dimensions which facilitate its mounting and also make its identification by any intruder more difficult. Moreover, the fact that - due to obvious geometrical factors - the Fresnel lens associated with the infra-red sensor 8 extends over a substantial part of the front face of the casing 3 makes it more difficult for the intruder to realise that he is in front of a combined action sensor, in which, as well as an infra-red-sensitive element (that is, the sensor 8), there is also a microwave detector unit.

[0014] The fact that the sensor 8 is mounted on the strip which separates the two horns 4 and 6 also means that any attempt to mask (or obstruct) the infra-red sensor 8 with an object such as a sheet, a plate, etc. (see, for example, the outline of a rectangular sheet S shown schematically in broken outline in Figure 3) causes a corresponding, at least partial, masking (or obstruction) of the microwave unit with a consequent variation in the coupling voltage between the Gunn diode 5 associated with the transmitting horn 4 and the Schottky diode 7 associated with the receiving horn 6.

[0015] The two horns (that is, the microwave transmitting and receiving units) are coupled to a microstrip antenna 11 which is constituted, for example, by a glass-­fibre-reinforced plastics plaque applied (for example by gluing) to the front face in which the infra-red sensor 8 is situated, with a metallised strip 12 (for example of copper) applied thereto and having a length slightly greater than the wavelength ( λ ) at which the microwave system is operating, usually 38 mm, so as to flatten the energy-transfer characteristic as a function of the frequency (aperiodic operation).

[0016] According to a possible variant, the antenna 11, 12 may be produced in the form of a T-shaped microstrip element projecting like a cantilever from the front of the casing 6 in a generally symmetrical position relative to the two horns 4 and 6, and constituted by a leg part with a length ( λ /4) equal to one quarter of the operating wavelength of the microwave unit and a head part whose length ( λ ) is slightly greater than the operating wavelength.

[0017] The outputs of the mixer diode 7 and the infra-red sensor 8 are connected to respective signal-processing chains - of known type - including amplifier units (e.g. a Doppler amplifier for the microwave unit) and units 13, 14 for conditioning the output signal. These units usually include timing elements which make the device 1 sensitive only to disturbances which extend over a minimum time threshold. The outputs of the units 13 and 14 are connected to threshold circuits 15, 16 which can generate respective warning signals at their outputs. The signal generated by the threshold circuits 15, 16 are sent to a processing module 17 which in turn can activate a warning element (a siren, an acoustic alarm, a transmitter which sends an alarm signal to a surveillance post...) 18/in dependence on certain programming logic criteria.

[0018] As stated, these criteria are widely known in the art and will not therefore be described in detail below since they are not relevant for the purposes of an understanding of the invention.

[0019] By way of reference, the operating criterion known as "AND almost OR" operation may be mentioned.

[0020] According to this operating criterion, the processor module 17 activates the warning 18 under the following conditions:
- when both the alarm-signal-generating chains (13, 15; 14, 16) - that is to say, both the microwave unit and the infra-red sensor - have emitted respective alarm signals for a time interval of a predetermined duration (for example 40 seconds);
- when just the infra-red sensor 8 provides two successive alarm signals separated by a given interval, for example, at least 6 seconds within a predetermined time interval (for example 40 seconds); and
- when just the microwave unit sends two successive alarm signals at least twice in a predetermined time interval (for example 40 seconds).

[0021] In the detector device according to the invention, the function of protection against masking (or obstruction) is effected from the output signal of the microwave unit, by the detection of an increase in the envelope of the output signal of the mixer diode 7 beyond a given threshold level set on a comparator 19 (possibly adjustable in dependence on the mounting conditions).

[0022] Such an increase results from an equally anomalous increase in the coupling voltage between the Gunn diode 5 which acts as the transmitter and the Schottky diode 7 which acts as the receiver.

[0023] This increase may be caused, for example, by an attempt - by an intruder - to mask the infra-red sensor 8 by sliding a screen, such as a sheet or plate (S in Figure 3), in front of the latter.

[0024] In the device according to the invention, this attempt to mask the infra-red sensor 8 inevitably causes at least a partial masking or obstruction of the microwave unit. This latter phenomenon is immediately detected, with the emission, on an output line 20 of the threshold comparator 19, of an obstruction alarm which can immediately be communicated to the exterior by the activation of the alarm 18.

[0025] As stated at the beginning of the description, the compact dimensions of the device 1 as a whole usually make it difficult for an intruder to realise that he is in front of a combined detector device in which, as well as the infra-red sensor 8, there is also a microwave unit which will also be masked - with the corresponding emission of an alarm signal - when there is an attempt to mask the infra-red sensor 8.

Claims

1. A detector device including a microwave unit (4 to 7) and an infra-red sensor (8), characterised in that the microwave unit includes a transmitting unit (4, 5) and a receiving unit (6, 7) with respective active surfaces (4, 6) which are close together, and carries associated means (19, 20) for the detection of the at least partial masking of the microwave unit (4 to 7), and in that the infra-red sensor (8) is interposed substantially between the active surfaces (4, 6) so that any attempt to mask the infra-red sensor (8) causes a corresponding masking of the microwave unit (4 to 7) which can be detected by the detection means (19, 20).

2. A device according to Claim 1, characterised in that the transmitting unit (4, 5) and the receiving unit (6, 7) of the microwave unit (4 to 7) include respective horns (4, 6) which are side-by-side, and in that the infra-red sensor (8) is interposed between the horns (4, 6).

3. A device according to Claim 1 or Claim 2, characterised in that the transmitting unit (4, 5) and the receiving unit (6, 7) of the microwave unit (4 to 7) have active surfaces (4 to 6) which are generally quadrangular in shape, and in that the infra-red sensor (8) is interposed between the two facing sides of the active surfaces (4, 6).

4. A device according to any one of Claims 1 to 3, characterised in that electromagnetic coupling means are provided between the transmitting unit (4, 5) and the receiving unit (6, 7) of the microwave unit (4 to 7).

5. A device according to Claim 4, characterised in that the coupling means comprise a microstrip antenna (11, 12) extending between the active surfaces (4, 6).

6. A device according to Claim 5, characterised in that the microstrip antenna (11, 12) is a λ antenna, where λ is the wavelenght corresponding to the operating frequency of the microwave unit (4 to 7).

7. A device according to any one of Claims 1 to 6, characterised in that the detection means (19, 20) associated with the microwave unit (4 to 7) comprise a sensor element (19) which is sensitive to the coupling voltage between the transmitting unit (4, 5) and the receiving unit (6, 7) of the microwave unit (4 to 7), the sensor element (19) being able to generate a masking alarm signal (20) when the coupling voltage exceeds a predetermined threshold level (19).

8. A device according to any one of the preceding claims, characterised in that the transmitting unit (4, 5) of the microwave unit (4 to 7) includes a Gunn diode (5).

9. A device according to Claim 1 or Claim 8, characterised in that the receiving unit (6, 7) of the microwave unit (4 to 7) includes a mixer diode.

10. A device according to Claim 9, characterised in that the mixer diode (7) is a Schottky diode.

11. A device according to any one of Claims 1, 8 to 10, characterised in that the microwave unit operates at a frequency substantially in the range from 9.3 to 10.8 GHz.

12. A device according to any one of the preceding claims, characterised in that a Fresnel lens (9) is associated with the infra-red sensor (8).

13. A device according to Claim 12, characterised in that the Fresnel lens extends at least partly over the active surfaces (4, 6) of the transmitting unit (4, 5) and the receiving unit (6, 7) of the microwave unit (4 to 7).

Drawing