FIRE DETECTOR

Abstract: A fire detector, comprising a printed circuit board on which at least one optical transmitter and at least one optical receiver are surface-mounted with a method known commercially as SMD and arranged so as to avoid a line-of-sight configuration, their respective emission and reception fields intersecting each other. The board is associated with a box-like optical chamber which is composed of a fluid detection portion and a fluid circulation portion. The detection portion is arranged through a corresponding through hole formed in the printed circuit board and is provided with openings which are formed in the perimetric wall for the corresponding transmitters and receivers, whose respective emission and reception fields mutually intersect inside it. The circulation portion is arranged on the opposite side with respect to the detection portion and is formed perimetrically by a plurality of partitions which from access ducts for the fluid.

Description: FIRE DETECTOR TECHNICAL FIELD The present invention relates to a fire detector. BACKGROUND ART Commercially available fire alarm systems generally comprise a control unit which manages peripheral units, usually sensors/detectors, over a two- wire line or by means of radio systems. There are various kinds of fire detector which use optical, thermal, optical-thermal means, et cetera. In particular, optical-thermal detectors ensure better coverage of a broader range of types of fire. It is in fact known that fires that develop quickly, such as those caused by refined hydrocarbons, usually produce a kind of smoke which is poorly detectable by an optical sensor but generate a considerable amount of heat. Vice versa, a fire that develops slowly generally produces smoke which is easily detectable by an optical sensor. Optical sensors, particularly photoelectric ones, use the principle according to which an electromagnetic radiation which is incident to a particle suspended in a fluid is partly absorbed and partly diffused by the particle. The structure of these detectors comprises an optical chamber which has a box-like structure and is perimetrically formed by access ducts for the fluid; a light source, advantageously an LED, and a light receiver, advantageously a photodiode, are arranged in said chamber. The fluid access ducts are provided so as to block as much as possible the penetration of ambient light in the optical chamber, in order to avoid affecting the sensing of the photodiode. The transmitter is constituted by an LED which emits a light pulse in which the ratio between the width and the period is very low. When smoke enters the optical chamber, the light emitted by the LED is partially diffused by the particles suspended in the fluid, is detected by the photodiode, and is converted into an electrical signal which is then amplified and processed by an electronic circuit. The LED and the photodiode are arranged so as to avoid a line-of-sight configuration but are arranged so that their respective emission and reception fields mutually intersect in a point which lies inside the optical chamber. One problem which is usually observed with these detectors is the high sensitivity of the system to radio-frequency noise. This is usually due to two factors, namely the high transimpedance gains required by the amplification circuits and the antenna effect due to the pins and the connection tracks of the receiving device, i.e., of the photodiode. Moreover, in most smoke detectors of the photoelectric type the emitter- receiver pair is fitted on the printed circuit board by through-hole mounting. This solution entails considerable additional costs, both for the reworking that is required with respect to the "surface-mounted" production stream known industrially as SMD (Surface Mounted Device) adopted by most currently commercially active manufacturers and for the manual insertion of the LEDs and of the photodiode in the corresponding seats. Another drawback observed in conventional detectors is that the cover of the optical chamber is too close to the active volume of said chamber, i.e., the region where the emission field of the LED and the reception field of the photodiode mutually intersect. This structural characteristic causes considerable drawbacks, because owing to the fact that the detectors are usually placed on the ceilings of the rooms in which the presence of smoke is to be monitored, the probability of sedimentation of dust is highest on the cover, inside the optical chamber. Accordingly, a slow increase in the background signal of the detector in clean air conditions has been observed which affects the efficiency of the optical chamber and can lead to false alarms in extreme cases. DISCLOSURE OF THE INVENTION The aim of the present invention is to eliminate or substantially reduce the problems of conventional fire detectors. Within this aim, an important object of the invention is to provide a fire detector which ensures manufacturing and assembly at high speed and low cost. Another object is to provide a detector characterized by low sensitivity to radio-frequency noise. Another important object is to provide a fire detector whose overall quality at least equals or exceeds that of conventional detectors. Another object is to provide a detector by virtue of which the presence of dust inside the optical chamber, particularly dust settled on the cover, has a marginal effect on efficiency. Another object is to provide a fire detector whose total height is modest and comparable with that of conventional ones. This aim and these and other objects which will become better apparent hereinafter are achieved by a fire detector, characterized in that it comprises a printed circuit board on which at least one optical transmitter and at least one optical receiver are surface-mounted with a method known commercially as SMD, said transmitter and said receiver being arranged so as to avoid a line-of-sight arrangement, their respective emission and reception fields intersecting each other, said board being associated with a box-like optical chamber which is composed of a fluid detection portion and a fluid circulation portion, said detection portion being arranged through a corresponding through hole formed in the printed circuit board and being provided with openings which are formed in the perimetric wall for the corresponding transmitters and receivers, whose respective emission and reception fields mutually intersect inside it, said circulation portion being arranged on the opposite side with respect to the detection portion and being formed perimetrically by a plurality of partitions which form access ducts for the fluid. BRIEF DESCRIPTION OF THE DRAWINGS Further characteristics and advantages will become better apparent from the description of a preferred and nonexclusive embodiment of the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: Figure 1 is a view of a fire detector according to the invention; Figure 2 is a sectional view of the fire detector of Figure 1; Figure 3 is a perspective view of the assembled internal components of the detector; Figure 4 is an exploded view of the components of Figure 3; Figure 5 is a view of one of the components shown in Figures 3 and 4; Figures 6, 7 and 8 are various views of another one of the components shown in Figures 3 and 4. WAYS OF CARRYING OUT THE INVENTION With particular reference to Figures 1 and 2, a fire detector according to the invention is generally designated by the reference numeral 10. In this embodiment, the detector 10 comprises a box-like body 11 which is constituted by two complementary components 11a and lib connected by a bayonet coupling and has a substantially cylindrical shape which tapers upward. The tapering upper portion 12 is provided with lateral openings 13. An assembly, generally designated by the reference numeral 14, is contained in the box-like body 11. The assembly 14 comprises a printed circuit board 15 on which a transmitter 17, advantageously constituted by an LED, and a receiver 16, advantageously constituted by a photodiode, are surface-mounted; said receiver and said transmitter operate in the infrared part of the spectrum. The transmitter 17 and the receiver 16 are arranged on the board 15 so as to avoid a line-of-sight configuration but so that the emission and reception fields mutually intersect. In this embodiment, the emission and reception fields are arranged at 45° with respect to the line-of-sight position. A second LED 18, which is visible from outside, and a thermistor 19 are further arranged on the printed circuit board 15. The board 15 is associated, by means of a removable interlocking coupling, with a first cover 20 for protecting the electronic components which lies perimetrically and is monolithic with an optical chamber 21. The optical chamber 21 is constituted by a lower portion 22 for detection of the fluid and by an upper portion 23 for circulation of the fluid. The optical chamber 21 is substantially cylindrical. The detection portion 22 extends downward beyond the dimensions of the first cover 20, through a corresponding through hole 24 formed in the printed circuit board 15. The detection portion 22 is further provided with two openings 25 provided in the perimetric wall 26 for the LED 17 and the photodiode 16, whose emission and reception fields accordingly intersect inside the portion 22. The internal perimetric wall 26 of the lower portion 22 has tooth-like projections 27 so as to minimize the signal produced by internal reflections collected by the photodiode 16. While having a substantially cylindrical configuration, the lower portion 22 is shaped with lateral recesses 28 at the openings 25 for advantageously accommodating the LED 17 and the photodiode 16. The upper circulation portion 23 lies on the opposite side to where the lower portion 22 lies with respect to the first cover 20, is substantially cylindrical and is formed perimetrically by a plurality of access ducts 29 for the fluid. The ducts 29 are formed by partitions 30 having a substantially V-shaped cross-section, which are arranged so that their vertices are arranged one after the other along the circular perimeter of the optical chamber 21 and lie in a direction which is substantially perpendicular to the plane of arrangement of the first cover 20. The upper ends 31 of the partitions 30 are accommodated in corresponding seats 32 formed in the flat inner portion of a second cover 33 for the upper portion 23. The second cover 33 has a perimetric portion 34 which lies at right angles to its plane of arrangement and is made of mesh, as required by the standards, adapted to cover the plurality of ducts 29 for controlled access of fluid inside the optical chamber 21. The flat inner portion of the second cover 33 is also provided with toothlike projections 35. As mentioned, the printed circuit board 15 is associated, with a removable interlocking coupling, with the first cover 20; the removable interlocking coupling is allowed by flexible elastic teeth 36 which protrude monolithically with respect to the first cover 20 and at right angles to its plane of arrangement. As mentioned, the printed circuit board 15 has, in addition to the photodiode 16 and the LED 17, a second LED 18 and a thermistor 19. The second LED 18 does not face the inside of the optical chamber 21 but is visible from the outside of the detector 10 because it is arranged at through holes 37 formed in the first cover 20 and in the box-like body 11. The thermistor 19 is constituted by a sensitive portion 38, which in this embodiment is made of a material having a negative temperature coefficient, and is associated with a support 39 which lies at right angles to the printed circuit board 15 and passes through the optical chamber 21 and finally through a through hole 40 of the second cover 33. In practice, the sensitive portion 38 is external to the optical chamber 21 but is contained within the box-like body 11 at its upper portion 12 provided with lateral openings 13. As regards operation, the above-described detector 10 is a detector of the optical-thermal type. The sensor detects the presence of the smoke by means of the emitter- receiver pair, which operates in the infrared part of the spectrum. The transmitter 17 (LED) and the receiver 16 (photodiode) are arranged in the optical chamber 21 so that they are not in a line-of-sight configuration: when smoke enters the volume that corresponds to the intersection of the beam emitted by the LED 17 and of the viewing field of the photodiode 16, it diffuses part of the incident light emitted by the transmitter 17, which is detected and converted into an electrical signal (current) by the photodiode 16. The thermistor 19, supported by a support 39, is arranged outside the profile of the optical chamber 21. In this position, the sensitive part 38 of the thermistor 19 is exposed to the air flow, improving the efficiency of ambient temperature monitoring on the part of the detector 10. Advantageously, the LED 17 and the photodiode 16 can be replaced with a transmitter and a receiver which operate in the visible part of the spectrum. In practice it has been observed that the present invention has achieved its intended aim and objects. In particular, it is evident that the mechanical configuration ensures a high production and assembly rate and low-cost production, using standard electronic components. Moreover, this particular embodiment also allows to obtain a detector whose sensitivity to radio-frequency noise is particularly low, since the length of the connections of the various electronic components is very limited. Moreover, the efficiency of the production process and the low cost of the procedure do not affect the overall quality of the detector. Finally, the total height of the optical chamber has been kept as great as possible, to the extent allowed by the bounding mechanical constraints, so as to space the active volume from the second cover of said chamber. Operating tests conducted in very dusty environments have verified that the presence of dust inside the through chamber, particularly accumulated at the cover, has a marginal effect on the efficiency of the detector. The present invention is subject to numerous modifications and variations, all of which are within the scope of the inventive concept. All the details may be replaced with other technically equivalent elements. In practice, the materials employed, as well as the dimensions, so long as they are compatible with the contingent use, may be any according to requirements. The disclosures in Italian Patent Application No. PD2000A000083, from which this application claims priority, are incorporated herein by reference.

Claims: CLAIMS 1. A fire detector, characterized in that it comprises a printed circuit board on which at least one optical transmitter and at least one optical receiver are surface-mounted with a method known commercially as SMD, said transmitter and said receiver being arranged so as to avoid a line-of- sight arrangement, their respective emission and reception fields intersecting each other, said board being associated with a box-like optical chamber which is composed of a fluid detection portion and a fluid circulation portion, said detection portion being arranged through a corresponding through hole formed in the printed circuit board and being provided with openings which are formed in the perimetric wall for the corresponding transmitters and receivers, whose respective emission and reception fields mutually intersect inside it, said circulation portion being arranged on the opposite side with respect to the detection portion and being formed perimetrically by a plurality of partitions which form access ducts for the fluid. 2. The detector according to claim 1, characterized in that said at least one optical transmitter and at least one optical receiver operate in the infrared part of the spectrum. 3. The detector according to claim 1, characterized in that said at least one optical transmitter and at least one optical receiver operate in the visible part of the spectrum. 4. The detector according to claim 1, characterized in that said printed circuit board is associated by removable interlocking with a first cover for protecting the electronic components which lies perimetrically and is monolithic with said optical chamber. 5. The detector according to claim 1, characterized in that said detection portion of the optical chamber has walls provided with tooth-like projections. 6. The detector according to one or more of the preceding claims, characterized in that said detection portion is substantially cylindrical, with longitudinally arranged lateral recesses at said openings for said transmitter and said receiver. 7. The detector according to claim 1, characterized in that said circulation portion of the optical chamber is substantially cylindrical and has a second openable cover. 8. The detector according to claim 7, characterized in that said second cover has a perimetric portion which lies at right angles to its plane of arrangement and is at least partially made of mesh, said perimetric portion being adapted to cover the plurality of ducts for the access of the fluid to the optical chamber. 9. The detector according to claims 7 and 8, characterized in that the flat internal portion of the second cover is at least partially provided with toothlike projections. 10. The detector according to one or more of the preceding claims, characterized in that said partitions have a substantially V-shaped cross- section, are arranged so that their vertices lie one after the other along the circular perimeter of the optical chamber and substantially at right angles to the plane of arrangement of the second cover, the upper end of said partitions being accommodated in corresponding seats formed in the flat inner portion of said second cover. 11. The detector according to claim 4, characterized in that said removable interlocking coupling between the printed circuit board and the first cover is provided by means of flexible elastic teeth which protrude monolithically with respect to the first cover and at right angles to its plane of arrangement. 12. The detector according to one or more of the preceding claims, characterized in that said first cover has at least one through hole for a corresponding LED which is mounted on the printed circuit board and is visible from the outside of said structure. 13. The detector according to one or more of the preceding claims, characterized in that the circuit printed on said board comprises a thermistor. 14. The detector according to claim 13, characterized in that the sensitive part of said thermistor lies outside the optical chamber, at the second cover, and is rigidly coupled to a support which passes through corresponding holes formed in said chamber and said second cover. 15. The detector according to one or more of the preceding claims, characterized in that it comprises a box-like containment body for the printed circuit board and the optical chamber, constituted by two detachably interlocking complementary components, which is substantially cylindrical and tapers upward, the tapering upper portion being provided with lateral openings at said fluid inlet ducts and at the sensitive part of the thermistor, and with at least one hole for said LED which is visible from the outside.

Inventors: LUTEROTTI LORENZO, LUTEROTTI, LORENZO
Applicants: LUTEROTTI LORENZO, SYSTEM SENSOR DIVISION OF PITT, LUTEROTTI, LORENZO, SYSTEM SENSOR DIVISION OF PITTWAY CORPORATION
Application year: 2001
Main classification: G08B17
IPC codes: G08B17