Abstract:
A smoke detector system including two projected beams operating in parallel, the outputs of which are processed to distinguish between outputs caused by smoke and outputs caused by other conditions affecting either or both beams. By comparing both the occurrence and the duration of reduced intensity outputs from the two beams, discrimination against false-alarm conditions is achieved.

Description:
FIELD OF THE INVENTION 
     This invention is concerned with smoke detection systems, and more particularly with projected-beam smoke detection systems containing circuitry for discrimination against false alarms. 
     BACKGROUND OF THE INVENTION 
     Of many devices available for the detection of smoke, the projected-beam type of smoke detector has several advantages over the more common types of smoke detector in which smoke is sensed within a chamber. A projected-beam detector provides very high sensitivity to even low smoke densities because of its ability to integrate over a relatively large volume of smoke; it provides the capability for covering a large area per device; and it provides quick response time because of the lack of resistance to smoke admittance which is inherent to chamber-type smoke detectors. 
     In spite of these advantages, projected-beam systems have disadvantages which have prevented their widespread acceptance. Projected-beam detectors are sensitive to alignment and must be carefully aimed on installation. Additionally, projected-beam detectors are more prone to false alarms caused by accidental beam blockage, such as by carelessly carried ladders, moths, or large buildups of dust on the optical components thereof. 
     SUMMARY OF THE INVENTION 
     The present invention provides a smoke-detection system including two projection-type detectors operating in parallel with one another. Broad transmitted beams and receivers with a wide angle of acceptance are used to reduce alignment problems. In the preferred embodiment the two transmitters are squarewave modulated in a quadrature relationship, and interference between the two beams is minimized by properly detecting the outputs from the two receivers. Alarm signals from the two receivers are processed by logic which compares the magnitudes and the durations of alarm signals from each of the beams with predetermined limits to sense and distinguish between alarm conditions, potential false-alarm conditions and trouble conditions. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     These and other advantages of the present invention will become more apparent upon reading the detailed description of the invention below and with reference to the accompanying drawings of which: 
     FIG. 1 is a block diagram of the invention; and 
     FIG. 2 is a truth-table useful in explaining the operation of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown a block diagram of the invention. Central to the operation of the invention are two smoke-detection channels including projected-beam smoke detectors 10a and 10b and the electronics associated with each channel described below. Corresponding components of each channel are identified in the figures by the same numeral, the two channels being distinguished by the use of &#34;a&#34; or &#34;b&#34; following the numeral. Since the operation of each channel is essentially identical, the following description of operation is applicable to either channel, and the &#34;a&#34; and &#34;b&#34; designations will be used in the description only where necessary to distinguish between the two channels. 
     Each of the projected-beam smoke detectors 10 contains a source of illumination 12 which, as shown in FIG. 1, may typically be implemented using a gallium arsenide infrared diode. The illumination sources 12 each project beams of infrared radiation which traverse a path through the area to be protected by the smoke detector and which impinge upon receiver sections 13 of the projected-beam detectors 10. The receiver sections include lenses 14 which concentrate the received radiation. The outputs from the lenses 14 next pass through infrared filters 16 which eliminate variations in the transmitted radiation due to changing levels of visible radiation. The outputs from filters 16 are applied to photocells 18 in each of the parallel smoke detectors 10. 
     The maximum allowable distance between the infrared sources 12 and infrared receivers 13 in each of the detector channels is typically 200 feet. The present invention is provided with a relatively wide transmitted beam from source 12 coupled with a relatively wide received field of view in receiver 13, as determined by lenses 14, to obtain a measure of protection against misalignment. The two projected-beam smoke detectors 10a and 10b operate in parallel and are typically separated by approximately three feet. The use of such a dual-beam system in conjunction with the transmitting and receiving electronics described below serves to minimize false alarm signals which have been a major drawback to the use of projected-beam types of smoke detectors. 
     By properly modulating the two infrared sources 12a and 12b of each of the projected-beam channels 10a and 10b, and by properly demodulating the outputs from photocells 18, effects from stray radiation transmitted by one channel which impinges upon the receiver 13 of the other channel may be reduced or eliminated. This allows both the sources of infrared radiation 12 and the infrared receiving optics 13 to have a wider angle of transmittance and acceptance than would be otherwise possible, resulting in an increased tolerance to misalignment. 
     In a preferred embodiment, infrared sources 12a and 12b are modulated by quadrature signals from a multivibrator 20. The output from multivibrator 20 is typically a squarewave having a frequency of 2 kHz, although practically any frequency may be used. Two outputs 21a and 21b are provided by multivibrator 20, one applied to each of the infrared sources 12a and 12b. As indicated in FIG. 1, the signals to infrared sources 12a and 12b  are separated by 90° of the modulation frequency and are thus in quadrature phase relationship with one another. By modulating the two infrared sources 12a and 12b of each of the projected-beam channels 10a and 10b in quadrature, stray infrared radiation from one channel which impinges upon the receiver 13 in the other channel may be cancelled by properly detecting the output from the photocell 18, as described below. Other methods of modulating the beams in the two channels, such as frequency modulation, may be used to provide similar cross-channel rejection performance. The angle of the beam transmitted by sources 12 is typically 5° or larger, and the angle of acceptance of receivers 13 is typically 5° or larger. 
     The outputs from photocells 18 are respectively applied to bandpass amplifiers 22. These amplifiers 22 serve to amplify the output signal from photocells 18 and to reduce extraneous noise from the output thereof via bandpass filtering. The outputs of bandpass amplifiers 22 are applied to demodulators 24. 
     The detection signal applied to demodulator 24a is the output signal 21a from multivibrator 20 having a phase angle of 0°. The modulation signal 21b of infrared source 12b and, hence, the radiation transmitted by source 12b are 90° out-of-phase with respect to signal 21a; and the radiation transmitted from infrared source 12a is in-phase with respect to signal 21a, as described above. By synchronously detecting the output of photocell 18a with signal 21a in demodulator 24a, any signals produced in photocell 18a due to stray illumination from infrared source 12b are 90° out-of-phase with the detection signal 21a and tend to be cancelled, while variations in the output of photocell 18a due to variations in the received illumination from infrared source 12a are in-phase with the detecting signal and are enhanced. 
     Similarly to the operation of demodulator 24a, demodulator 24b detects the output from photocell 18b with signal 21b from multivibrator 20 having a phase angle of 90°. In the same manner as described above, the operation of detector 24b tends to enhance components in the output of photocell 18b caused by changes in illumination in infrared source 12b and tends to cancel components in the output of photocell 18b caused by illumination from infrared source 12a. 
     The outputs from the demodulators 24 are applied to processing circuitry 26-42 for each channel, which operates in the following manner. The output from synchronous detector 24 is first amplified by a DC amplifier 26. The output from amplifier 26 is applied to a threshold circuit 28, the output of which is applied to an integrator 30; and the output of integrator 30 is in turn applied to a second threshold circuit 32. In response to a sufficiently large change in the illumination from source 12 detected by photocell 18, the output from DC amplifier 26 decreases by an amount sufficient to trigger threshold circuit 28. In response to this, threshold circuitry 28 starts the integration of integrator 30. If integrator 30 is not reset within 20 seconds, the output level of integrator 30 triggers threshold circuit 32; and the output of threshold circuit 32, which is applied to trouble alarm logic 34, goes high. If the output from DC amplifier 26 rises above the threshold determined by the threshold circuit 28 prior to the completion of the 20-second integration period of integrator 30, integrator 30 is reset, and the output from threshold circuit 32 remains low. 
     The output from DC amplifier 26 is also applied to AC amplifier 36. The output from AC amplifier 36 is applied to a threshold circuit 42 in a manner similar to the circuitry 28-32 following amplifier 26. This circuitry 36-42 produces an output which is applied to trouble and alarm logic 34 when the output from AC amplifier 36 exceeds the threshold set by threshold circuit 38 for a period greater than two seconds, as set by integrator 40. 
     The gains of the DC amplifier 26, AC amplifier 36, and threshold circuits 38 are such that the signal level from the synchronous detectors 24 required to trigger threshold circuits 38 is much lower than that required to trigger threshold circuits 28. Typically, a blockage of 2% or less per foot of the beam for beams of up to 22 feet and less than 1% per foot for long beams of up to 200 feet will trigger threshold circuit 38 while the level of threshold circuits 28 is typically set for 75% beam blockage or higher. Since the gain of the signal path through DC amplifier 26 and AC amplifier 36 is much higher than the gain through DC amplifier 26 alone, an output from the threshold circuits 32 associated with the outputs of DC amplifiers 26 is always preceded by an output from threshold circuits 42 associated with AC amplifier 36. 
     Threshold circuits 38 are AC coupled to the outputs from demodulators 24 via AC amplifiers 36 so that long-term changes in the output levels from receivers 13, caused, for example, by changes in the gain of the electronics or dust obscuring the detector optics, are rejected and do not cause false alarms. The time constants of AC amplifiers 36 are very long, being on the order of several minutes or longer. 
     Power for the circuitry is provided from a DC output voltage 44 which is filtered by a filter 46 and maintained at the desired level by a regulator 48. The output from regulator 48 is applied to the circuitry of the invention for the provision of power thereto. The output from regulator 48 is also applied to a level sensor 50 which serves to monitor the level of the output from regulator 48 and to provide a signal to trouble and alarm logic 34, should the output from regulator 48 fall below a predetermined limit. 
     Trouble and alarm logic 34 produces two outputs, an alarm signal 52 and a trouble signal 54. The presence of a positive alarm signal 52 indicates that the smoke detection system has determined that there is a high probability that smoke is actually present. The presence of a positive trouble signal 54 indicates that the smoke detection system has detected signal conditions indicative of a persisting potential false-alarm condition. Accidental blockages of the beam in detectors 10a and 10b, which are transitory in nature, are distinguished by trouble and alarm logic 34 and produce no alarm or trouble signal therefrom. As the vast majority of false alarm signals are transitory in nature, most conditions which otherwise would produce a false alarm will be disregarded by trouble and alarm logic 34 and thus do not even result in a trouble signal 52 being provided. 
     Referring to FIG. 2, there is shown a truth table describing the functioning of trouble and alarm logic 34 in producing the two output signals therefrom in response to the inputs applied thereto. Circuitry to perform such functioning may easily be constructed by one of ordinary skill in the art. Trouble and alarm logic 34 may be implemented, for example, by digital logic gates, a read-only memory, a properly programmed microprocessor, or by other means. 
     The column labels across the top of FIG. 2 denote the inputs to and outputs from trouble and alarm logic 34; and the &#34;1&#39;s&#34; and &#34;0&#39;s&#34; designate the presence or absence of the corresponding signal, a &#34;0&#34; indicating a low or negative signal and a &#34;1&#34; indicating the presence of a high or positive signal. The first column in FIG. 2, labeled 56, denotes the presence of a signal to trouble and alarm logic 34 from threshold circuitry 42a, indicating that the output from the &#34;A&#34; detector channel has exceeded the threshold set by threshold circuit 38a for longer than two seconds. The column labeled 58 denotes the presence of a signal from threshold circuit 32a, indicating that the output from channel A has exceeded the threshold set by threshold circuit 28a for longer than 20 seconds, as determined by integrator 30a. Similarly, columns 60 and 62 in FIG. 2 denote the corresponding signals from circuitry 26b-42b of channel B. Column 64 denotes the output signal from level sensor 50. The states of alarm signal 52 and trouble signal 54 in response to the corresponding inputs to trouble and alarm logic 34 shown in columns 56-64 is indicated in columns 66 and 68. 
     As indicated in row 70, in the absence of any positive inputs to trouble and alarm logic 34, both the alarm signal 52 and the trouble signal 54 are low. Disturbances detected by AC circuitry 36-42 and producing a &#34;two-second&#34; signal in one channel only are usually caused by an accidental blockage of the beam and are ignored by trouble and alarm logic 34, as shown in rows 72 and 74 of FIG. 2. The occurrence of such a distrubance in both channels simultaneously is indicative of the presence of smoke and produces an alarm signal 52, as shown in row 76. 
     Smoke blockages are usually on the order of a few percent of the beam, and the occurrence of both a two-second signal from threshold circuitry 42 and a 20-second signal from threshold circuitry 32 in the same channel indicates a solid object blocking the beam for a period of time that is more than transitory. In response to such an input indication, trouble and alarm logic 34 produces a trouble signal 54, as shown in rows 78 and 80. 
     While smoke detected by the projected beam detectors 10a and 10b is usually not sufficiently thick to cause activation of the circuitry 28-32 following the DC amplifiers 26, occasionally such thick smoke is produced by certain types of fires. The presence of such smoke would cause all four input signals to trouble and alarm logic 34 to be active simultaneously, as indicated in column 82. However, it is more frequently the case that such an input condition to trouble and alarm logic 34 is caused by an accidental blockage of both of the beams in the detectors 10a and 10b by some large object. Accordingly, in response to such an input condition wherein all four inputs are high, trouble and alarm logic 34 causes both trouble signal 54 and alarm signal 52 to be activated, as shown in row 82. In the event that both two-second and twenty-second signals are present in one channel and a two-second signal is present in the other channel, the alarm logic produces both alarm and trouble signals, as shown in rows 84 and 86. 
     Should the DC power to the circuitry of the invention fall below a predetermined limit, trouble and alarm logic 34 causes trouble signal 54 to be activated, regardless of the state of the other inputs, as shown in row 88. 
     The above description has presented an improved projected-beam type of smoke detector in which false alarm conditions to which previous projected-beam types of smoke detectors have been susceptible either are rejected or produce a trouble signal rather than an alarm signal. Various modifications will be obvious to those of ordinary skill in the art in applying the teachings of the present invention to different requirements. Accordingly, the present invention is to be limited only as indicated in the appended claims.