Abstract:
A low profile, low volume smoke chamber displaces a light source, such as a light emitting diode or laser diode, and a sensor, such as photodiode or phototransistor, to the exterior of the sensing volume. A symmetrical sensing volume results which can be coupled to symmetrical input/output ports via a generally U-shaped flow path. A two part sensing chamber housing is formed with a lower cylindrical base portion and an upper cylindrical cover portion which slideably engage one another. At least the upper cylindrical portion carries a plurality of grooves for suppression of reflections and collection of dust. An exterior end of the upper portion carries a plurality of spaced apart openings which can be filled with a screen. A reduced sensing volume in combination with a selected screen size produces an acceptable signal to noise ratio and response rate while still excluding insects and other non-smoke related particulate matter.

Description:
The benefit of the filing date of Apr. 29, 1999 of Provisional Application Serial No. 60/131,654 for Fast Miniature Photoelectric Sensing Chamber is hereby claimed. 
    
    
     FIELD OF THE INVENTION 
     The invention pertains to smoke sensors of a type used in fire detectors. More particularly, the invention pertains to such sensors having a reduced size and a low profile. 
     BACKGROUND OF THE INVENTION 
     Fire or smoke detectors have become widely used elements of fire alarm systems. Such alarm systems often incorporate large numbers of such detectors spread over substantial regions to detect and track the build-up of smoke. 
     Known detectors while effective for their purpose have at times been regarded as less than aesthetically pleasing due to their profile and over-all size. There thus continues to be an on-going need for smaller detectors having lower profiles and a smaller over-all size. 
     While small chamber size has been recognized as being preferable from an aesthetic and architectural point of view, it has also been recognized that as chambers become smaller the signal to noise, ratio can potentially drop and become less than optimal. As chamber dimensions have become smaller, background light levels detected in photoelectric smoke chambers by the respective light sensitive element (such as a photodiode or a phototransistor) can increase significantly. There continues to be a need for smoke sensors which while physically small exhibit appropriate signal to noise ratios while minimizing nuisance alarms. 
     SUMMARY OF THE INVENTION 
     A photoelectric sensing chamber has a cylindrical shape with a relatively low profile. A base element is formed with a cylindrical region and a closed end. A cover has a hollow cylinder which extends therefrom. The cover slideably engages the base such that the distal end of the cylinder is located adjacent to the base. Together they form a substantially enclosed, cylindrical symmetrical sensing chamber. The chamber encloses a symmetrical sensing region. 
     The cover carries a plurality of openings at an exterior, proximal, end displaced from the distal end of the cylinder. The openings permit ingress and egress of adjacent ambient atmosphere, which could carry smoke or particles of combustion. 
     An annular flow path extends between the base and the cylinder, coupled to the openings. This path, around the cylinder and extending to the base couples the openings to the sensing region. 
     The cylinder cooperates with the base to form an inflow/outflow region between the annular flow path outside of the cylinder and the internal sensing region. This produces a more or less U-shaped flow path which is symmetrical around the sensing region. 
     The symmetrical flow path and symmetrical internal sensing region are achieved by displacing a source of radiant energy, such as a light emitting diode or laser diode and a sensor of scattered radiant energy, such as a photodiode or a phototransistor, into the base of the chamber outside of the internal sensing region. With this configuration, the shape of the source does not distort and detract from the symmetry of the sensing region. Similarly, by displacing the sensor into the base, its shape does not distort the symmetrical shape of the sensing region. 
     Each of the source and the sensor can be, in one aspect of the invention, located in conduits displaced from the sensing region. One conduit, in addition to supporting the source, provides a focusing function for the radiant energy being projected into the sensing region. Another provides a collecting function for scattered incident light directed to the sensor. This increases optical gain of the chamber. 
     In another aspect of the invention, protrusions can be provided in the conduit for the sensor to block a first reflection of light from the source off of the internal side wall of the sensing chamber to provide an enhanced signal to noise ratio. Such protrusions for example could occupy 20 to 40 percent of the area of the respective conduit to produce the noise suppressing function. A preferred percentage is on the order of 27 percent. 
     A protrusion in the conduit for the source cooperates with the interior geometry of the conduit to block and reflect a portion of the light injected through the conduit by the source. This also contributes to the enhancement of the signal to noise ratio. 
     The conduits are located at an angle relative to one another which corresponds to the primary scattering angle for the sensing chamber. In this regard, for laser sources, an angle can be established in a range of 20 to 30 degrees. A 25 degree angle is preferable. For infrared light emitting diodes, an angle can be established in a range of 40-45°. 
     In another aspect of the invention, the orientation of the conduits directs the beam of light from the source and directs the field of view of the light sensitive element toward opposite sides of the grooved interior surface of the chamber. The source projects a spot of radiant energy, or light, onto the opposite wall of the sensing chamber, the internal grooved side wall of the cylinder. Preferably in this embodiment, no light will illuminate the fringe of the cover cylinder. However, if due to component variations, emitted radiant energy illuminates the cover fringe, the above-noted protrusion in the conduit for the sensor should block any resultant stray light from reaching the sensor. 
     The opposite side of the cover cylinder, which is intersected by the optical axis of the sensor does not receive any direct illumination from the source. As such, the sensor is directed to a region having low levels of stray background light or radiant energy. 
     Hence, the orientation of the conduits taken together reduces the degree of stray background light or radiant energy which can find its way onto or into the light sensor. This in turn contributes to an enhanced signal to noise ratio and a detectable level of scattered light in response to smoke permeating the sensing region. 
     In another aspect of the invention, the inner surfaces of the side wall and the bottom of the chamber can be formed with grooves to promote absorption of light and to provide depressed regions for accumulating dust that has drifted into the sensing chamber. 
     In yet another aspect of the invention, the cylinder which extends from the cover has a continuous closed peripheral surface without perforations therethrough. Ambient atmosphere including ambient smoke, flows up and down the continuous side walls to and from the sensing region. Consequently, the cover, in yet another aspect of the invention, can incorporate a screen or a mesh at an exterior end thereof. Mesh openings can have a length in a range of 0.013″ to 0.02″ long. 
     The mesh can be inserted into the mold before the cover/cylinder are molded. Alternately, the openings can be molded into the cover without a separate mesh or screen. 
     The nested cylinders, namely the cylinder carried on the cover and the cylinder formed by the base provide a substantially continuous annular flow path into the sensing region unlike known multiple vane labyrinths which result in several, restricted flow paths into the sensing region. A substantially continuous opening around the exterior perimeter of the cover of the housing can be provided for ingress and egress of smoke. 
     Taking into account the above-noted characteristics and features, results in a sensing chamber height on the order of 0.7 inches or less with a diameter of less then 1.5 inches. This produces a sensing volume of less than 1.24 cubic inches and an optical spacing on the order of 1.35 inches. 
     The smaller sensing volume reduces time to respond to incoming ambient smoke. Additionally, a smaller mesh size can be used, thereby improving exclusion of insects and dust, while at the same time, the chamber still exhibits an acceptably short response time to ambient smoke. 
     Increasing the size of the mesh or screening of the chamber will also shorten response time. Thus, sensing chambers in accordance with the invention produce increased signal to noise ratios as a result of a combination of reduced sensing region volume, and appropriately selected screen or mesh size in combination with the symmetry of the sensing region and the protrusions in the optical conduits which reduce background chamber noise. 
     Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective, exploded, view of a detector in accordance with the present invention; 
     FIG. 2 is a top plan view of the sensing chamber of FIG. 1 taken along plane  2 — 2 ; 
     FIG. 3 is an enlarged, side, sectional, exploded view of a sensing chamber of the detector of FIG. 1; 
     FIG. 4 is an enlarged, side, sectional, assembled view of the sensing chamber of FIG. 2; 
     FIG. 5 is a side elevational view of the sensing chamber of the detector of FIG. 1; 
     FIG. 6 is a bottom view of the sensing chamber of FIG. 5 taken alone plane  6 — 6 ; 
     FIG. 7 is a view of the interior of the cover of the sensing chamber of FIG. 1 taken along plane  7 — 7 ; 
     FIG. 8 is a perspective, exploded, view of the sensing chamber of FIG. 1; and 
     FIG. 9 is a different perspective, exploded, view of the chamber of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While this invention is susceptible of embodiment in many different forms, there are shown in the drawing and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated. 
     FIG. 1 illustrates a fire detector  10  in accordance with the present invention. The detector  10  includes an exterior enclosure  12  which might have a substantially cylindrical shape. 
     The enclosure  12  has a mounting base or mounting surface  12   a  and a central opening  12   b . A removable top  14  extends into the opening  12   b  and can be removably attached to the enclosure  12 . 
     The top  14  includes a plurality of open regions,  14   a ,  14   b  which permit the ingress and egress of ambient atmosphere into the enclosure  12 . It will be understood that the exact configuration of the enclosure  12  and the top  14  are not limitations of the present invention. 
     When the top  14  has been removed by moving it away from the enclosure  12  in a direction  14   c , access is provided to a fire sensor  20 . The fire sensor  20 , as described further below, includes a small, low profile sensing chamber which responds to the presence of airborne particulate matter which enters and leaves the sensor  20  via cover  14 . 
     Sensor  20  includes a generally cylindrical base section  22  and a removable cover section  24 . The cover section  24  extends through opening  12   b . Once top  14  has been removed, section  24  is readily removable for maintenance and service purposes. The section  24  slideably engages base section  20  as discussed in more detail subsequently. 
     Base section  20  is carried on a printed circuit board  26 . The printed circuit board  26  also carries electronic circuitry  28  for purposes of receiving signals from the fire sensor  20  and for carrying out control and communications functions of a type associated with fire sensors as would be known to those of skill in the art. It will be understood that the exact configuration of the control circuitry  28  is not a limitation of the present invention. A light emitting diode  28   a  coupled to circuitry  28  can be used to provide status information. 
     FIGS. 2-9 illustrate various features of the sensor  20 . As illustrated in FIGS. 3 and 4, base section  22  carries a cylindrical portion  30  with a side wall  30   a  which terminates at a planar end  30   b . As illustrated, the fire sensor  20  is implemented as a scattering-type photoelectric smoke sensor. Conduits  32   a  and  32   b  are molded into base section  22  and extend from end surface  30   b  away from the cylindrical side wall  30   a.    
     One of the conduits, such as conduit  32   a , can receive a source of radiant energy, which might be a light emitting diode or a laser diode without limitation,  34   a . When energized, the source  34   a  projects a beam of radiant energy  34   b , illustrated in phantom in FIG. 3, through conduit  32   a  and into a sensing region  50 . 
     Base section  22  also carries a sensor  36   a , which could be implemented as a photodiode or a phototransistor, in the conduit  32   b . It will be understood that the exact choices of source  34   a  and sensor  36   a  are not limitations of the present invention. 
     As a result of the conduit  32   b , the field of view of sensor  36   a  is directed toward a region formed in sensor  20  which is 180° away from the region of incidence of the radiant energy  34   b  from the source  34   a . By so-orienting the source and the sensor, stray reflections are minimized. 
     It will be understood that as a result of off-setting the conduits  32   a ,  32   b  from the base  30   b  of the cylindrical  30 , the cylinder  30  bounds, in part a symmetrical or cylindrical sensing region  50 . The region  50  is free from intrusion by either the source  34   a  or the sensor  36   a.    
     Extending from surface  30   b  are elongated support elements  40   a ,  40   b  which are substantially identical. Between the elements  40   a ,  40   b  is a support and engaging element  40   c.    
     The cylindrical cover element  24  includes an exterior top surface  24   b  which terminates at circumferential edges  24   c ,  24   d . The edges  24   c ,  24   d  bound a plurality of openings such as openings  42   a ,  42   b  which extend peripherally about the cover  24 . 
     The openings  42   a ,  42   b  permit the ingress and egress of ambient air which in turn may be carrying fire indicating gases or particulate matter. The openings  42   a ,  42   b  could be completely open or could be closed in part by mesh having openings of various sizes. 
     Smaller mesh sizes are known to more effectively exclude undesirable airborne material such as dust, airborne fibers, insects or the like. For example, screen openings on the order of 0.017 inches or 0.43 mm can be used without unduly delaying the response of the chamber  20 . Hence, the openings  42  which are circumferentially spaced around the entire upper edge of the cover  24  provide symmetrical access to the chamber  20  by ambient atmosphere as discussed in more detail subsequently. 
     The cover element  24  carries thereon a cylindrical section  46  which extends substantially perpendicularly from the exterior end surface  24   b . The cylindrical section  46  is hollow defining a grooved interior region indicated generally at  46   b.    
     As the cover portion  24  moves toward the base portion  22 , it ultimately becomes supported by and rests on upper surfaces  40   a - 1  and  40   b - 1 . Additionally, cover portion  24  slideably and lockingly engages upper latching member  40   c - 1 . Hence, the cover portion  24  is symmetrically supported and removably attached to body portion  22 . 
     In this configuration, as illustrated in FIG. 4, an annular conduit  48  exists between the side wall  30   a  formed in base member  22  and exterior peripheral surface  46   a  of cylindrical element  46 . Annular conduit  48  permits inflow and outflow of ambient airborne gases and smoke related particulate matter in a generally U-shaped flow pattern  48   a  in and out of the openings  42   a ,  42   b . Flow is along the channel  48  formed by surfaces  30   a  and  46   a  and into the sensing region  50 . 
     The flow regions for ingress and egress of ambient airborne gases and particulate matter are symmetrical about the chamber  20 . The sensing region  50  is also symmetrical about a centerline thereof without any distortion thereof or intrusion thereinto of the source  34   a  and the sensor  36   a . The nested cylindrical structure of the chamber  20  also contributes to the exclusion of stray exterior light. 
     Airborne particulate matter which enters the sensing region  50  will in turn cause scattering of the radiant energy  34   b . The scattered radiant energy will in turn be sensed by sensor  36   a  using electronics  28  in a known fashion. 
     The optical axis of the emitter or source  34   a  relative to the optical axis of the center  36   a  is oriented preferably on the order of 25° for a laser diode. Where the source  34   a  corresponds to an infrared light emitting diode, the relative angle between the axis is preferably in a range of 40 to 45°. 
     Each of the conduits  32   a ,  32   b  terminates in a respective overhang  60   a ,  60   b . The overhangs reduce noise in the chamber, as detected at sensor  36   a , more than they reduce the signal sensed thereby due to airborne particulate matter. Hence, they enhance the chamber signal to noise ratio. 
     The emitter conduit  32   a  in combination with overhang  60   a  contributes to focusing the beam  34   b  into the sensing volume or region  50 . This beam  34   b  will ultimately be incident on grooves  60   a  formed within cover  24 . 
     Preferably overhang  60   b  associated with sensor  36   a  will extend into the conduit  32   b  enough to prevent the sensor from directly receiving any scattered light from grooves  60   b′  that originated from the source  34   a . The overhang  60   b  blocks the first reflection of any such scattered light. The optical axis of sensor  36   a  impinges on grooves  60   a  180° away from where the beam  34   b  impinges thereon. This also enhances the signal-to-noise ratio. 
     Preferably, the overhangs in the conduits  32   a ,  32   b  will represent 20-40 percent of the cross sectional area of the respective conduit. A 27 percent intrusion into the respective conduit is preferred. 
     The chamber  20  benefits from relatively rapid response to inflowing airborne particulate matter due to its relatively small volume, on the order of 20 cc or less. 
     Representative chamber parameters are on the order of less than 1.5 inches in diameter with a sensing volume height of less than 0.7 inches to produce the noted 20 cc sensing volume. Compatible mesh sizes will be on the order of 0.013-0.02 inches. A preferred size is on the order of 0.017 inches. 
     Those of skill in the art will understand that the size of the openings of the mesh can be altered to effect chamber response. Somewhat larger openings will provide faster response to low energy fires at the cost of potentially permitting increased dust flow or insect problems in the chamber. 
     With respect to FIG. 4, a shield  26 - 1  is illustrated in phantom associated with sensor  36 a. Such shields could be formed out of a conductive material such as metal. Alternately, base portion  22  could be molded of conductive plastic to provide a shield about the sensing element  36   a . This will provide an AC ground about the chamber  22  and the sensor  36   a . In one embodiment, contacts might be molded into the conductive plastic to create connections to the shield. 
     One of the advantages of the chamber  20  lies in the fact that the side walls of cylindrical members  30  and  46  are continuous and unperforated. They do not exhibit labyrinth-type openings therethrough. These side walls block outside ambient light from reflecting into the interior of sensing region  50  and contributing to noise which might be incident upon sensing element  36   a . The mesh and the openings  42   a ,  42   b  can be molded into the cover portion  24 . The cylindrical peripheral openings  42   a ,  42   b  provide access to the symmetrical annular flow channel  48  between the cylindrical side walls  30   a  and  46   a  into and from sensing region  50 . 
     Additionally, internal grooves  60   a′  and  60   b′  can be provided in the side walls of the cylindrical member  46  as well as in the end portion. The grooves are very effective in absorbing light originating from the source  34   a  as well as any reflections from outside of the chamber. In addition, the number of required reflections for exterior light to enter the sensing region  50  is high enough so as to substantially eliminate such interference. The grooves also trap internal chamber dust and contribute to an enhanced signal-to-noise ratio. 
     As noted previously, the cover portion  24  extends through opening  12   b  of the enclosure  12 . Hence, cover portion  24  can be slideably removed from base portion  22  and replaced. This process will not only provide a dust free interior side wall  46   b  but it can be achieved without disturbing the source  34   a  or the sensor  36   a.    
     The out of phase orientation of the offset source  34   a  and sensor  36   a , the symmetrical annular inflow/outflow channel and non-perforated side walls with internal reflection suppressing grooves each contribute to a relatively low volume, symmetrical sensing region with an acceptable signal-to-noise ratio. Readily separable and replaceable cover  24  facilitates maintenance. The small chamber size results in an aesthetically acceptable, low profile detector. 
     Various sizes of mesh can be molded into covers  24  to vary chamber performance characteristics. The relatively small sensing chamber volume makes feasible the use of relatively small mesh sizes yet the chamber exhibits acceptable response levels and adequate signal-to-noise ratios. 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.