Discriminating fire sensor with thermal override capability
This is a continuation of United States patent application Ser. No. 419,872, filed Sept. 20, 1982. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to fire and explosion sensing and suppression systems, and more particularly, to such systems in which radiation is detected in at least two different spectral bands. 
     2. Background Art 
     Certain types of fire suppression systems utilize fire sensors having multiple signal processing channels which respond to fire- or explosion-produced electromagnetic radiation to generate a fire suppression command output signal. The fire suppression command output signal is used to trigger the release of a fire suppression agent, such as halon gas. 
     Such systems employ more than one signal processing channel in order to discriminate against radiation which is not associated with a fire or explosion requiring suppression. For example, hydrocarbon fires produce long wavelength infrared radiation in the 6 to 30 micrometer spectral band, and also short wavelength radiation in the 0.7 to 1.2 micrometer spectral band, at characteristic relative intensities. Multiple channel fire sensors have been designed which produce an output signal only when radiation is detected in each of the aforementioned spectral bands above predefined levels associated with a fire of a predetermined size, and in relative amounts associated with a hydrocarbon fire. Such systems are resistant to false triggering from, for example, direct exposure to a high intensity lamp, heater, flash light or the like. One such multiple channel fire suppression system of the above type is disclosed and claimed in U.S. Pat. No. 3,931,521, which issued to R. J. Cinzori and which is assigned to the present assignee. 
     While the aforementioned fire suppression systems operate satisfactorily in many environments, nonetheless certain adverse conditions may occur which interfere with the operation of one or more of the radiation channels. For example, if an area being monitored by such a fire sensor system becomes filled with smoke, while detection in the long wavelength region may be substantially unaffected, short wavelength radiation from a potentially dangerous fire may be obscured from the sensor system by the smoke. 
     Another, quite serious problem which can occur in the operation of multi-channel fire suppression systems is the failure to operate because of contamination on the sensor windows. For example, multi-channel fire sensor systems are used in armored personnel carriers to protect the occupants from fires which may start in the engine compartment of the vehicle. The sensors are placed physically within the engine compartment in such instances, thus affording as early as possible detection of an engine compartment fire. Such a armored carrier may be put to considerable use, and go for a considerable length of time before a fire occurs requiring the activation of the suppression system. Over such an extended period of time, the windows of the sensors of a fire suppression system located within the engine compartment are likely to become covered over with a film of contaminates including grease, sand, dust, and other components frequently found in such a location. A sufficiently thick build-up of such contaminants will prevent the effective operation of the typical multi-channel fire suppression system, primarily due to blockage of the short wavelength channel thereof. 
     The failure of a fire suppression system to operate properly, for example, to suppress a fire in the engine compartment of an armored personnel carrier, could be disastrous to the personnel the fire suppression system is designed to protect. There is therefore a need for an improved multiple channel fire sensor system which overcomes the aforementioned problems. In particular, there is a need for an improved fire sensor system which provides adequate discrimination against false triggering signals, while at the same time, which provides for the timely production of a fire suppression command output signal even if radiation obscuring conditions occur which tend to interfere with the proper operation of the system. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above-described problems associated with such radiation blocking conditions in a multiple channel fire suppression system. The present invention provides a discriminating fire sensor for detecting fire in a predefined area by detecting radiation in at least two different spectral bands associated with a fire. The discriminating fire sensor provides an output signal in response to predetermined amounts of radiation in those spectral bands, associated with a fire of the type and size to be detected. A special heat sensor channel is also provided which generates an output signal in response to a predetermined amount of heat in the area. 
     The present invention represents a significant advance in the field of optical fire sensor systems. In particular, the present invention provides the advantages of prior art multi-channel fire sensor systems in discriminating between fire- or explosion-produced radiation and radiation associated with events other than fires or explosions to be detected, while at the same time, providing protection against fire conditions which would otherwise go undetected because of the occurrence of radiation obscuring phenomena in the environment of the fire sensor system. Other features and advantages of the present invention will become apparent from a consideration of the following detailed description of the present invention, taken in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram representation of the fire and explosion system according to the invention; 
     FIG. 2 is a schematic diagram of a portion of the detection system shown in FIG. 1; 
     FIG. 3 is a partial block diagram of a further embodiment of a fire and explosion detection system according to the invention; and 
     FIG. 4 is a partial block diagram of a still further embodiment of a fire and explosion detection system according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment of the present invention utilizes an existing multi-channel fire and explosion detection system, and also provides an additional special channel capable of providing a thermal override protection capability to the system. FIG. 1 illustrates this embodiment. Many of the elements of the embodiment of FIG. 1 are disclosed and described in detail in prior U.S. Pat. No. 3,931,521 (the &#39;521 patent), in connection with FIG. 1 thereof. The &#39;521 patent is incorporated herein by reference in its entirety. 
     Briefly summarizing the description of those elements of the embodiment shown in FIG. 1 herein which are common with the embodiment shown in FIG. 1 of the &#39;521 patent, the multi-channel fire detector 10, includes a short wavelength radiation responsive channel 12 and a long wavelength radiation responsive channel 14 coupled respectively to receive radiant energy 16 from a nearby or remote fire or explosion 18. The system 10 is typically designed so that it is highly responsive to high energy fuel-type explosions out to distances on the order of six yards. The radiant energy 16 of interest in channel 12 is that radiation in the near infrared region of the electromagetic frequency spectrum, whereas the radiant energy from source 18 of interest in channel 14 lies in the far infrared region of the electromagnetic frequency spectrum. 
     The short wavelength channel 12 includes a suitable conventional optical filter 20 for passing radiation wavelengths only in the spectral band of interest, for example, in the 0.7 to 1.2 micron range. The filtered radiation impinges on a detector 22, such as a silicon photodetector, which generates an output detection signal which is provided to the input of an amplifier 24. The amplifier 24 has its output connected as shown to one input 26 of a NOR and threshold gate 28. 
     The long wavelength channel 14 includes a conventional optical filter 30 for passing radiation wavelengths in a range different from that of optical filter 20, for example, in the 7 to 30 micron range. The filtered radiation impinges on a thermal detector 32. This detector may, for example, be a thermopile, and has its output connected to the input of a frequency compensating amplifier stage 34. 
     Alternatively, the detector may be heat wire, pneumatic heat detector, or thermocouple. Amplifier 34 has its output connected to a second input 36 of the NOR and threshold gate 28. Gate 28 is operative in response to input signals on lines 26 and 36 to generate an output pulse on line 38 when predetermined amounts of radiation are detected by detectors 22 and 32 in predetermined relative proportions, as explained in detail in the &#39;521 patent. The output pulse on line 38 triggers a monostable multivibrator 40 to generate an output pulse of a desired time duration sufficient to ensure the triggering of a subsequent stage, such as a suitable fire suppressant release mechanism. 
     A signal will thus appear on line 42 only when both long and short wavelength energy is detected at levels above the predetermined threshold levels. These threshold levels are controlled internally in the electronics of amplifiers 24 and 34 and NOR and threshold gate 28. Thus, the gain of amplifier 34 is selected such that the known threshold level required to activate the input of NOR gate 28 is reached by the output of thermal detector 32 when the selected level of radiation is detected. Similar considerations apply to channel 12. 
     It will be appreciated that the spectral ranges associated with channels 12 and 14 need not be in the 0.7 to 1.2 micron and 7 to 30 micron ranges, respectively. Other spectral ranges may be selected as desired without departing from the spirit or scope of the invention. Such considerations are considered well within the scope of one having ordinary skill in this art. 
     In accordance with the present invention, an additional channel 50 is also provided. This additional circuit channel 50 comprises a further amplifier 52, a threshold device 53 and an OR gate 54, one input of which is connected to the output of threshold circuit 53, and the other input of which is connected to line 42 which is the output of monostable multivibrator 40. The output of OR gate 54 is connected to a signal line 56 which is the output of the system 10. 
     The gain of amplifier 52 and the threshold voltage of threshold device 53 are selected such that the signal level at the output of thermal detector 32 activates the input of threshold circuit 53 at a selected level greater than that at which line 36 triggers the input of NOR gate 28. In the preferred embodiment, this selected threshold level is 10 times greater than the level which causes NOR gate 28 to be triggered. Other levels for the triggering of channel 50 may be selected in accordance with the present invention. In some circumstances, for example, the speed at which a fire is detected may be a far more important consideration than immunity from false triggering. In such cases, a level less than the level described above may be selected. On the other hand, under other circumstances, the protection of a limited amount of fire suppressant from release due to false triggering, and the maximization of the probability that such material will be released only in response to a fire may be the overriding considerations. In such cases, the system designer may choose a greater triggering level than that described above. Such considerations and selections are well within the scope of one having ordinary skill in this art. 
     It will be appreciated that channel 50 need not depend upon the output of detector 32 for a signal. In fact, a separate detector may be utilized to provide a signal to amplifier 52, if desired. 
     FIG. 2 is a schematic diagram of that portion of the system shown in FIG. 1 comprising channel 50, plus selected additional circuit elements to aid in explaining the interconnections of channel 50 to other parts of the circuits of FIG. 1. The schematic diagram of FIG. 2 herein should be considered in conjunction with the specification of the &#39;521 patent, and particularly in connection with FIG. 3 thereof which is a schematic diagram of the circuit of FIG. 1 thereof. 
     Referring now to FIG. 2 herein, it will be noted that amplifier device 68&#39;, resistors 92&#39; and 93&#39;, diode 116&#39;, and circuit reference potential points 72&#39; and 82&#39; are common with the circuit shown in FIG. 3 of the &#39;521 patent. 
     Amplifier device 58 is a conventional differential amplifier. Resistors 60, 62 and 64 and capacitor 70 are selected according to known principles to provide the aforementioned selected amount of gain for amplifier 52 and to provide a frequency response of approximately 0.3 Hz. This frequency response of 0.3 Hz is designed into the amplifier 52 of channel 50 of the preferred embodiment to suppress the AC component of the composite waveform applied to the input of amplifier 52. 
     Threshold circuit 53 is based upon a further differential amplifier device 67 having resistors 59, 61, 63 and 65 connected in conventional fashion, as shown, to provide a comparator function so as to provide an input signal to OR gate 54 when the output of amplifier 52 exceeds the selected threshold level, described above, which is determined by the values of resistors 63 and 65 which are connected together in a voltage divider configuration between reference potential point 82 and 72. 
     The output of threshold circuit 53 is connected to one input of OR gate 54 as shown. The other input of OR gate 54 is connected to line 42 (FIG. 1). 
     The present invention is readily adaptable for use in connection with many different multi-channel fire and explosion sensor systems to provide the novel thermal override protection provided by the present invention. For example, the present invention can be implemented in two somewhat different ways in connection with a multi-channel fire detection system such as that disclosed in U.S. Pat. No. 3,825,754 which issued on July 23, 1974 to Robert J. Cinzori and Gerald F. Stapleton, and which is assigned to the assignee of the present invention. These two implementation are discussed below in connection with FIGS. 3 and 4. 
     FIG. 3 is the first such implementation in connection with the &#39;754 patent. The circuit shown in FIG. 3 is based on FIG. 1 of the &#39;754 patent, and includes all of the elements contained therein, substantially as disclosed therein, except as modified as described herein. Circuit elements in FIG. 3 herein which are common to FIG. 1 of the &#39;754 patent are designated in FIG. 3 herein with a primed reference character having the same number value as the corresponding element in FIG. 1 of the &#39;754 patent. Thus, for example, circuit blocks 12&#39;, 14&#39;, and 16&#39; in FIG. 3 herein correspond to blocks 12, 14 and 16, respectively, in FIG. 1 of the &#39;754 patent. 
     Briefly, the circuit shown in FIG. 1 of the &#39;754 patent is a dual spectrum infrared fire detection system having two main channels 12, 14, which provide a discriminating fire detection capability, and having a third &#34;Round Channel&#34; (not shown herein). The Round Channel provides further discrimination against high energy exploding rounds of ammunition, by temporarily disabling the main detection channels in response to detected radiation from an exploding round of ammunition, and thus protects against false triggering from such exploding rounds. The circuit also has fail-safe logic and detection circuitry to override the temporary disablement in the event a delayed fire is produced which would otherwise escape detection. 
     Turning now to FIG. 3, the two main channels 12&#39; and 14&#39; are shown, as are AND gate 56&#39; which outputs a signal in response to the outputs of main channels 12&#39; and 14&#39;, subject to the aforementioned high energy ammunition round discrimination logic function. AND gate 102&#39; outputs a signal pursuant to the implementation of the aforementioned fail-safe logic. The outputs of AND gates 56&#39; and 102&#39; are applied to the respective inputs of OR gate 110&#34;, which has as an output line 114&#39;. Note that OR gate 110&#34; has three inputs, while OR gate 110 in the &#39;754 patent has only two, hence the double prime reference. A detailed description of the interconnection of and the operation of the aforementioned circuit elements of FIG. 3 can be found in the specification of the aforementioned &#39;754 patent. 
     In accordance with the present invention, the input of a further amplifier 120 is connected to the output of amplifier 44&#39;. The output of amplifier 120 is connected to the input of an inverter 122, the output of which is connected to the input of a threshold gate 124. The output of threshold gate 124 is connected to a third input of OR gate 110&#34;. 
     In operation, the gains of amplifier 120 and inverter 122 are set in conjunction with the threshold level of threshold gate 124 so as to cause the triggering of threshold gate 124 when a predetermined level of long wavelength radiation is received by main channel 14&#39;, so as to implement the thermal override function of the present invention. 
     FIG. 4 shows an additional implementation of the present invention in connection with the circuit shown in FIG. 1 of &#39;754 patent. As in FIG. 3, those circuit elements common to FIG. 1 of the &#39;754 patent are shown in FIG. 4 herein with primed reference numerals. However, since AND gate 56&#34; has three inputs as compared with four inputs in &#39;754 patent, it is shown with a double prime designation herein to show that it differs slightly from the &#39;754 patent. 
     In accordance with the present invention, the input of a further amplifier 126 is connected to the output of amplifier 44&#39;, as is the case in FIG. 3. The output of amplifier 126 is connected to the input of an inverter 128, the output of which is connected to a threshold gate 130. The output of threshold gate 130 is connected to the input of a time-delay stage 132, the output of which is connected to a first input of an OR gate 134. The outputs of main channels 12&#39; and 14&#39; are connected to the respective inputs of an AND gate 136, the output of which is connected to the second input of OR gate 134. The output of OR gate 134 is connected to the third input of a three input AND gate 56&#34;. The other two inputs of AND gate 56&#34; are connected to lines 58&#39; and 60&#39;, further details of which can be found in the aforementioned &#39;754 patent. 
     In operation, the gains of amplifier 126 and inverter 128 are set in connection with the threshold level of threshold gate 130, as described above in connection with FIG. 3. The timing of delay device 132 is set to be substantially the same as the timing of delay devices 38&#39; and 50&#39;, details of which can be found in the aforementioned &#39;754 patent. In the preferred embodiment according to this implementation, time-delay stage 132 provides at its output the same signal as that applied to its input, however, delayed by 4 milliseconds. This delay of 4 milliseconds permits the circuit to implement the high energy ammunition round discrimination function by way of AND gate 56&#34;, in an analagous fashion to the function of timing delay stages 38&#39; and 50&#39;, as described in detail in the &#39;754 patent. 
     Thus, it will be appreciated, that the implementation of the present invention shown in FIG. 4 herein utilizes a thermal override channel according to the present invention, which thermal override channel is subject to a high energy ammunition round discrimination logic. This implementation is suitable for applications wherein immunization of the fire detection system from false triggering is an important consideration. Nonetheless, the thermal override channel of the present invention additionally provides protection against the blockage of the fire detection system due to the build-up of contaminants on the windows of the detectors physically located within the vehicle to be protected. 
     Other embodiments of the present invention may readily be designed by one having ordinary skill in this art once the principles of the present invention disclosed herein are understood.