Fire alarm systems

Abstract: An alarm system which incorporates a plurality of highly sensitive, early warning, smoke detectors (22) incorporates functionality for distinguishing between detector signals in response to ambient smoke and detector signals in response to the presence of non-smoke, fibrous materials. Detectors (22) are spatially arranged in predetermined regions. Information concerning the arrangement of detectors is stored in a common control unit. Additionally, a performance history for each of the detectors is also stored in the control unit (12). If one of the detectors exhibits a relatively large output which is large enough to indicate a possible fire, a previously stored history from the outputs of that detector is analyzed. If the previously stored history indicates a fire related profile, such as a relatively gradual increase in smoke level over a period of time, the signal from that detector is regarded as being indicative of smoke and an alarm is indicated. If the signal from the detector shows a relatively fast increase, from a very low level to an alarm level in a short period of time, fibrous material may have entered the detector. The output from at least one other detector in the same region is analyzed. If the second detector confirms the presence of smoke, at least the first detector is regarded as indicating an alarm condition. If the second detector does not indicate the presence of smoke, even a very low level of smoke, the output from the first detector is regarded as being due to a non-smoke condition, such as an intrusive fibrous material. The control unit indicates the presence of a trouble or maintenance condition with respect to that detector.

Description: Field of the Invention: The present invention relates to fire alarm systems, and in particular to fault detection of electrical signals received from ambient condition sensors. More particularly, the invention relates to processing apparatus and methods for minimizing false alarms due to non-smoke variations in electrical signals indicative of ambient conditions such as smoke or fire. Background of the Invention: Various systems are known for the detection of alarm conditions. One particular form of such a system is a smoke or fire detecting system of a type generally illustrated in previously issued Tice et al. U.S. Upon receipt of inputs from one or more of the detectors of the system, a control unit associated with the system is able to make a determination as to whether or not a fire condition is present in one or more regions of interest. A variety of techniques have been used in the past for the purpose of making this determination. Sensors of smoke such as photoelectric smoke detectors or ionization-type smoke detectors are intended to provide outputs indicative of sensed levels of ambient smoke. Environmental noise, such as dust particles or insects which may enter the respective detector can produce variations in output signals from the sensors which are not in any way correlated with the presence of smoke. These noise outputs can produce false alarms if the sensitivity of the respective detector is high enough. Such false alarms are undesirable. US 5483222 discloses an alarm system with a large number of sensors. The indicators from the sensors are each raised to a predetermined exponent and summed together. The resultant sum is compared to a predetermined threshold to determine whether or not the alarm condition is present. Photoelectric smoke sensors used for early warning typically use a light source and a light sensitive receiver. The design and placement of the light source, receiver, and baffling are such that no significant light from the source normally reaches the receiver unless smoke or other particles are present in the area of the light beam. If smoke or other particles are present in this area, they will scatter the light photons, and cause some of the light to reach the receiver. In non-early warning smoke detection systems, the density of smoke required at a sensor to cause an alarm is relatively large compared to the density of dust, fibers and other non-smoke particles normally existing in the environment, therefore these systems are not susceptible to false indications caused by such particles. In early warning smoke detection systems, the signals given by low levels of smoke may be comparable to that given by non-smoke airborne particles in the environment that this type of system is typically used. In prior art early warning systems, filters were used to remove non-smoke particles in the air present in the smoke sensors. The presence of a filter usually requires that the sensor include a fan or other means to draw air through the filter. The mechanical fans and filters used in prior art detectors are expensive, subject to failure, and require regular maintenance. Thus there continues to be a need for detectors which can be used in early warning systems without requiring the presence of fans or filters. Preferably minimizing false indications could be accomplished without significantly increasing the expense of such systems while avoiding any need to incorporate additional mechanical components. Summary of the Invention: A fire detection and alarm system in accordance with the present invention includes a control unit and multiple early warning smoke sensors. Each of these smoke sensors measures the density of smoke particles in its area. Each of the sensors then sends a signal to the control unit which is an electrical indication of that smoke density. The control unit processes the signals from at least some of the sensors and determines if an alarm condition exists. False indications caused by airborne particles that are not smoke need to be rejected since the system is designed to detect very low levels of smoke. Discrimination between smoke and fiber particles, such as lint or human hair, is a significant benefit of a system than embodies this invention. The design of the smoke sensors, combined with signal processing software in the control unit, permits the described system to detect these fiber particles. This detection feature enables the system to minimize false alarms caused by the presence of such fibers. The system requires that at least two smoke sensors be installed in each room or enclosed space. The probability that a fiber particle, large enough to cause a false reading, will enter a single smoke sensor is small, but significant. The probability that such a particle will enter two sensors at the same time is so small as to be insignificant. When the control unit identifies a signal from a first sensor that could be indicative of smoke alarm, it then analyzes the signal and determines if the reading could also be indicative of fiber particle. If the reading from the first sensor could be indicative of a fiber particle, the control unit then analyzes a reading from a second detector known to be in the same room. If, during a predetermined period of time, no readings from the second sensor are received that could be indicative of even a small level of smoke, then the control unit will provide an indication that the signal at the first sensor has been caused by a fiber particle or some other non-smoke phenomenon. A maintenance or trouble signal can then be generated. Brief Description of the Figures: FIG. 1 is a block diagram of an alarm system; FIG. 2 is a block diagram of a detector usable with the alarm system of Fig. 1; FIG. 3 is a sectional view of a prior art photoelectric detector; FIG. 4 is a sectional view of a photoelectric detector; FIG. 5 is a schematic representation of a detector containing a fibrous element; 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 block diagram of a system 10 in accordance with the present invention. This system 10 includes a control unit 12, which can be implemented with a programmable processor 14 and a storage unit 16. The storage unit 16, can include both control programs and data storage regions for use by the processor 14. The control unit 12 is coupled by a bidirectional communication link 20 to a plurality of ambient condition sensors or detectors generally indicated at 22. The members of the plurality 22, such as sensors 22a, 22b---22n are intended to detect a particular ambient condition in an adjacent region. The system 12 can also include an operator display unit with an output visual display device 15a and an operator control or input device such as keyboard 15b. The control unit 12 also includes a plurality of system outputs. The outputs can be used to activate audible or visual alarms. In addition, the unit 12 can be coupled to ventilation or air handling systems in the building so as to control smoke migration. Representative types of detectors include ionization-type or photoelectric-type smoke detectors. Temperature sensors as well as other types of ambient condition sensors could be used in the system 10 in accordance with the present invention. More particularly, the system 10 is intended to monitor one or more regions, for example regions R1, R2 which might or might not be contiguous. Two or more detectors 22-1, 22-2 - 22-k are located in region R1. Detectors 22-1' -- 22-k' are located in region R2. The regions R1, R2 can be substantially closed rooms for example. FIG. 2 is a block diagram representation of a detector 22i useable with the system 10. The detector 22i, includes a sensor element 30. The element 30 is intended to sense a particular ambient condition, such as smoke, temperature, infrared radiation or the like and it generates an electrical system indicative thereof on a line 32. Referring again to Fig. 2, output from the sensor 30, on the line 32 is coupled to a local detector control element 40. The control element 40 could be implemented with either digital or analog circuitry. If in digital form, the control element 40 could be implemented as either hard wired logic or could incorporate a programmed microprocessor. The control element 40, via interface circuitry 42 is capable of carrying on bidirectional communication with the system control unit 12, via the communication link 20. A method in accordance with the present invention, to be described subsequently, could be implemented in either the system control unit 12 or the detector local control element 40 without limitation. Implementation can be by either hardwired circuitry or by means of a programmed microprocessor also without limitation. Figure 3 illustrates in cross-section, a prior art photoelectric chamber PA-10. This chamber includes a housing PA-12 with an internal sensing volume PA-14. A light emitting source, PA-16 is carried on the housing and oriented to emit a beam of light PA-18 into the internal light sensing region PA-14. As is illustrated in FIG. 3, the emitted light beam PA-18 exhibits a somewhat conical expanding shape as it traverses the region PA-14. The light beam PA-18 is directed toward and absorbed on the housing PA-12. Offset from the axis of the beam PA-18 is a photoelectric sensor PA-20. The sensor PA-20 is oriented such that light from the beam PA-18 which has been scattered by particulate matter in the volume PA-14 will be incident thereon thereby generating an output electrical signal. Elements PA-22 and PA-24 limit the amount of light which can fall upon the sensor PA-20. The effective sensing light volume, which is the region in which smoke particles can be detected. for the geometry of the chamber PA-10 is on the order of .064 cubic inches. Figure 4 is a cross-sectional drawing of a smoke sensing chamber 30 of a representative smoke detection device such as 22i in accordance with the present invention. The housing 30 could, for example, have a diameter on the order of three inches or less. For example, a housing with a diameter on the order of two and one-half inches or less could be used. A high intensity source of coherent light 30-1, such as a laser or a laser diode, is carried by the housing or chamber 30. The light source is pulsed to cause it to emit a short pulse of light at periodic intervals (every few seconds). A lens 30-2 focuses the light into a small but intense beam 30-3. The light beam 30-3 continues through the detector chamber until it strikes a light trap 30-4 at the opposite end of the chamber. The light trap absorbs most of the light, and reflects a small amount away from the central chamber area. Preferably, source 30-1 in combination with the lens 30-2 will produce a beam 30-3 having an effective beam or light sensing volume on the order of .0022 cubic inches. This beam volume is on the order of 3% that of prior art detectors. Hence, dust particles are large compared to the diameter and volume of the beam 30-3. The dimensions of light beam 30-3 as well as those of the sensing beam volume are smaller than a typical distance between ambient dust particles. As described subsequently, this reduced volume makes the detector 30 less likely to produce dust induced output signals which appear to be due to the presence of smoke. Suitable early warning detectors were discussed previously. As illustrated in FIG. 5 such smoke detectors can also include a collector or baffle of scattered radiant energy 30-8. As discussed above, the volume of the light beam in which scattered light particles can reach the sensor 30-7 is small in comparison to the volume of the sensor. This small volume is called the Effective Scattering Volume (ESV). Smoke particles are small and numerous compared to dust and fiber particles, which are relatively large and sparse. The ESV is designed so its dimensions are small relative to the typical distance between large airborne dust particles, yet large relative to the distance between smoke particles in a true fire. In this way is very unlikely that more than one large dust particle (large enough to give a significant signal at the sensor 30-7) will occupy the ESV at the same time. Since the airborne particles are in constant motion, the occasional dust particles will cause a transient signal at the sensor 30-7 as the dust particles pass in and out of the ESV. Smoke particles generate a relatively constant signal at the sensor because many are in the ESV, and as some pass out of the ESV, others move in. Fiber particles may perform similarly to dust (i.e. pass through the ESV and cause only a transient signal). However, since they are very long in one dimension, it is possible that one end of the fiber may touch a surface in the sensor and the other end encroach on the ESV. This situation is illustrated in FIG. 5. Fiber particle F has entered the detector illustrated therein. Since the fiber F is not airborne, it may remain in this position for a long period of time and provide a constant signal to the sensor 30-7 and control unit 12. Since fiber particles are typically very large compared to smoke particles, their presence can cause a false alarm unless steps are taken to detect their presence. The present system and method discriminate between smoke and fiber particles. When a signal received from a first detector is large enough to indicate a possible fire, the control processor 14 via software first analyzes previous measurements stored in memory 16 for that detector. If the previous stored readings exhibit a profile indicative of a fire condition, such as a relatively gradual increase over time, the signal from that detector is indicative of smoke and an alarm is indicated by and at the control unit 12. It will be understood that other fire profiles can be used. For example, the slopes of the output signals from the first detector can be compared to a preset value. Alternately, pattern recognition techniques could be used without departing from the spirit and scope of the present invention. If the signal received from that detector shows a relatively sharp increase, from a very low level to an alarm level in a few seconds, this could possibly be a fiber, and the alarm indication is delayed for further analysis. If the signal received from that detector is determined, as above, to be possibly indicative of a fiber, the control unit 12 then analyzes the signals received from a second detector known to be located in the same room or physical space. For example, in FIG. 1, if a possible fiber or smoke alarm indication is received from detector 22-k, the control unit 12 will examine the output from detector 22-1, not detectors 22-1' or 22-k'. If no significant signal, even a very low signal, is received from detector 22-1, (which is in the same room R1), for a predetermined time period, this is further evidence that the signal at the detector 22-k is caused by a fiber particle and not smoke. If this lack of signal at the second detector 22-1 occurs, the control unit 12 does not indicate an alarm but instead indicates on its display 15a that a fault condition exists in the detector 22-k and that detector must be checked or cleaned. If instead, during the predetermined time period, a small analog signal is being sent from the second detector 22-1, the control unit 12 will indicate an alarm condition for the first detector 22-k. It will be understood that the outputs from other detectors, 22-2, 22-3 in the region R1 can also be analyzed in this process. A preferred analysis time is in a range of 5 to 60 seconds.

Claims: An alarm system having a control unit (12), a communications link (20) coupled to the control unit, at least first and second smoke detectors (22) coupled to the communications link (20) wherein the detectors (22) transmit signals indicative of conditions sensed in regions adjacent to the respective detectors (22), characterized in that : the first and second detectors have a common such region, and in that the control unit (12) includes: circuitry for determining if a signal received from said first detector in the common region is indicative of non-smoke airborne material having entered the detector; additional circuitry in the control unit (12) for determining whether said second detector in the common region is transmitting signals to the control unit (12) indicative of a possible alarm condition; and circuitry for indicating the presence of an alarm condition. A system as claimed in claim 1 wherein the control unit (12) includes circuitry for storage of prior signal values from the selected detector. A system as claimed in claim 2 wherein the determining circuitry analyzes the stored prior values for the detector and, in response to a trend indicating a fire condition, enables the circuitry for indicating the alarm condition. A system as claimed in claim 3 which includes delay circuitry in the event that the trend does not indicate a fire condition. A system as in claim 1, wherein said control unit (12) includes circuitry that responds to said signals from said first detector and to pre-stored trend information in order to reach a decision that a non-fire condition, indicative of non-smoke airborne material, may be present at that detector. A system as in claim 1, wherein said control unit (12) includes circuitry that responds to said signals from at least said first and second detectors in order to reach a decision that non-smoke airborne material has intruded into one of said first and second detectors. A system as in claim 1 wherein said control unit (12) uses said signal received from said first detector to determine if a possible alarm condition exists, and if the signal from said first detector has a sharp increase with respect to time, said control unit (12) performs further processing of the signals from said second detector before making a decision that a fire alarm condition exists at said first detector or making a decision that a special non-fire condition exists at said first detector. A system as in claim 7 where said control unit (12) indicates that the special non-fire alarm condition exists at said first detector if the signal from said second detector remains below a predetermined level for a predetermined time. A system as in claim 7, wherein said control unit (12) indicates that the special non-fire condition exists at said first detector and that maintenance action is necessary if the signal from said second detector fails to rise above a predetermined level during a predetermined time. A method of assessing the presence of an alarm condition in one or more regions being monitored with an alarm system (10), having a control unit (12) linked to a plurality of detectors, wherein the alarm condition is the presence of smoke and the detectors are smoke detectors, the method comprising: establishing at the control unit (12) records of the detectors associated with a plurality of pre-defined, regions being monitored by the alarm system; receiving at the control unit (12) signals from the detectors indicative of a sensed level of smoke at the respective detectors; for at least the signal from a first detector, determining if a possible fire condition may be present in the vicinity of the first detector; responsive to said possible fire condition, determining if the record of the first detector exhibits a predetermined trend during a selected time interval, and, in response to the presence of the predetermined trend, producing a signal indicative of an alarm condition, but in the absence of the predetermined trend, evaluating the signal from another detector, located in the same region as the first detector and in the absence of a predetermined signal from the another detector, indicating a fault condition at the first detector. A method as claimed in claim 10 wherein at least some of the regions are substantially enclosed. A method as claimed in claim 10 wherein the predetermined trend indicates an increasing level of smoke over a predetermined period of time. A method as claimed in claim 10 including: in the presence of a predetermined signal from another detector, indicating a fire alarm. A method of determining an alarm condition in response to signals received from a plurality of displaced smoke detectors, using an alarm system as claimed in claim 1, the method comprising: receiving signals from at least two detectors in a selected region being monitored; in response to one of the received signals changing in a way indicative of a possible fire, analysing the one received signal and then other received signals to differentiate between a fire condition and a non-fire condition. A method as claimed in claim 14 which includes storing a history of signals received from at least one of the detectors. A method as claimed in claim 15 wherein the stored history is used during the analysing step. A method as claimed in claim 16 wherein if the stored history includes a profile which indicates that a fire is probable, then an alarm is indicated. A method as claimed in claim 16 wherein if the stored profile does not indicate that a fire is probable, indication of an alarm is delayed. A method as claimed in claim 18 in the absence of a fire profile, analysing the other received signal to determine if it is indicative of a fire condition and if not, indicating that a selected non-alarm fault condition may be present at the one detector.

Inventors: ANDERSON DONALD D, ANDERSON, DONALD D.
Applicants: PITTWAY CORP, PITTWAY CORPORATION
Application year: 1998
Main classification: G08B29
IPC codes: G08B17, G08B29, G01N21