Device for protecting components of security systems against obstruction

• US 5,243,326 A
• Filed: 10/15/1991
• Issued: 09/07/1993
• Est. Priority Date: 10/19/1990
• Status: Expired due to Fees

• Current Assignee: Elkron SpA (Urmet SpA)
• Original Assignee: Elkron SpA (Urmet SpA)
• Inventors: Disabato, Vito
• Primary Examiner(s): Peng, John K.
• Assistant Examiner(s): MULLEN, THOMAS J
• Application Number:
  US07/776,808
• Time in Patent Office: 693 Days
• Field of Search: 340/555, 340/567, 340/568, 340/550, 340/521, 340/506, 340/603, 340/693, 340/309.15, 250/221, 250/342, 250/353, 324/706
• US Class Current: 340/555
• CPC Class Codes: G08B 29/046 prevention of tampering with detection circuits

Abstract

An element of an intrusion-alarm system, for example a passive infra-red sensor, has two subsystems which protect it against obstruction. The first subsystem can detect attempts at obstruction by the removal of the lens or front screen of the element or by the spraying of the element with a spray or a lacquer. The second subsystem projects, towards the area under surveillance, radiation which can be reflected by a masking screen. The detection of this reflected radiation gives rise to the emission of a warning signal.

Claims

1. A device for protecting against obstruction of an element of a security system, said device comprising a housing disposed generally about the element, and an external surface adapted for propagation therethrough of radiation between the element within said housing and an area external of said housing, said external surface having an outer side open to obstruction, said device further comprising:

first radiating means disposed for projecting radiation through said external surface,

second radiating means for projecting radiation towards said area external of said housing of said device, in front of said outer side of said external surface,

first sensing means disposed for sensing radiation emitted by said first radiating means that is projected through said external surface, said first radiating means and said first sensing means disposed on opposite sides of a plane of said external surface, and

second sensing means for sensing the radiation emitted by said second radiating means and reflected towards said outer side of said external surface when an obstruction is placed in front of said outer side of said external surface, whereby, when such an obstruction is present, said second sensing means causes generation of a corresponding warning signal,

said housing further comprising an appendage which projects generally beyond said plane of said external surface into a region generally at the outer side of said plane of said external surface, and said first radiating means and said second radiating means are mounted in said appendage, within said housing, whereby a change in the characteristics of the propagation of the radiation through said external surface causes generation of a corresponding warning signal.

2. A device according to claim 1 wherein said external surface comprises an external surface of a lens of said element.

3. A device according to claim 2, wherein the lens is a Fresnel lens.

4. A device according to claim 1, wherein said element comprises a pyroelectric member sensitive to electromagnetic radiation in the infra-red range.

6. A device according to claim 1, wherein said first radiating means and said second radiating means each comprise at least one radiation-emitting diode operating in the infra-red range.

7. A device according to claim 1, wherein said first radiating means are disposed within said housing, generally above said external surface.

8. A device according to claim 1, wherein said first radiating means are disposed to project radiation obliquely relative to a plane of said external surface.

9. A device according to claim 1, wherein said first sensing means are located within said housing in a position which is generally screened from receiving radiation from outside the housing and passing through said external surface.

10. A device according to claim 1, wherein said first radiating means are associated with second sensing means, said second sensing means being located within said housing and relative to said first radiating means so as to directly receive and detect radiation generated by said first radiating means, without said radiation passing through said external surface, whereby the difference between the respective signals output by said first sensing means and by said second sensing means causes generation of a warning signal corresponding exclusively to changes in the characteristics of the propagation of the radiation through said external surface.

11. A device according to claim 10, wherein said device further comprises a screen disposed between said first radiating means and said second sensing means, said screen defining a hole and said second sensing means face the hole in alignment with said first radiating means.

12. A device according to claim 1, wherein said device comprises circuitry for activating the first sensing means for a selectively predetermined time interval after the first radiating means have been activated.

13. A device according to claim 1, wherein said device comprises circuitry for activating the second sensing means for a selectively predetermined time interval after the second radiating means have been activated.

14. A device according to claim 1, wherein said second radiating means include two separate sources which project radiation towards the area external of said housing, in front of said external surface, from different directions.

15. A device according to claim 14, wherein one of the sources projected radiation in a direction generally perpendicular to said plane of said external surface, and the other of the sources projects radiation in a direction generally inclined to said plane.

16. A device according to claim 14, wherein said one of the sources comprises a pair of sources which project radiation in a direction generally perpendicular to said plane.

17. A device according to claim 1, wherein the appendage has a front window disposed generally above and extending obliquely relative to said plane of said external surface and facing said first radiating means and said second radiating means.

18. A device according to claim 1, wherein said device comprises timing means for activating the first radiating means selectively at predetermined intervals.

19. A device according to claim 1, wherein said device comprises timing means for activating the second radiating means selectively at predetermined intervals.

20. A device according to claim 1, wherein said device comprises piloting means for activating the first radiating means and the second radiating means alternately.

21. A device according to claim 1, wherein said element comprises a sensing member for the surveillance of an area external of said housing, wherein the device includes timing means for activating said first radiating means and said second radiating means selectively at predetermined intervals and the sensing member is kept in operation whilst each of said first radiating means and said second radiating means are activated so that protection against obstruction is achieved without interrupting the surveillance.

5. A device according to claim 5, wherein the member is sensitive to radiation whose wavelength is substantially within the range of from 5 to 15 μ

.

22. A device for protecting against obstruction of an element of a security system, said device comprising a housing disposed generally about the element, and an external surface adapted for propagation therethrough of radiation between the element within said housing and an area external of said housing, said external surface having an outer side open to obstruction, said device further comprising:

first radiating means disposed for projecting radiation through said external surface, and

first sensing means disposed for sensing radiation emitted by said first radiating means and projecting through said external surface, said first radiating means and said first sensing means disposed on opposite sides of a plane of said external surface, whereby a change in the characteristics of the propagation of the radiation through said external surface causes generation of a corresponding warning signal, and

said device further comprising;

second radiating means for projecting radiation towards said area external of said housing of said device, in front of said outer side of said external surface, said second radiating means comprising two separate sources which project radiation towards the area external of said housing, in front of said external surface, from different directions, and

second sensing means for sensing the radiation emitted by said second radiating means and reflected towards said outer side of said external surface when an obstruction is placed in front of said outer side of said external surface, whereby, when such an obstruction is present, said second sensing means causes generation of a corresponding warning signal.

23. A device according to claim 22, wherein one of the sources projects radiation in a direction generally perpendicular to a plane of said outer side of said external surface, and the other of the sources projects radiation in a direction generally inclined to said plane of said outer side of said external surface.

24. A device according to claim 22, wherein said one of the sources comprises a pair of sources which project radiation in a direction generally perpendicular to a plane of said outer side of said external surface.