GB2067749A

GB2067749A - Improvements in and Relating to Fire and Explosion Detection

Info

Publication number: GB2067749A
Application number: GB8041508A
Authority: GB
Original assignee: Graviner Ltd
Current assignee: Graviner Ltd
Priority date: 1980-01-17
Filing date: 1980-12-30
Publication date: 1981-07-30
Also published as: GB2067749B

Abstract

A fire and explosion detection system for discriminating between radiation produced by a source of fire or explosion to be detected and radiation produced by a source of fire or explosion not to be detected, comprises two radiation detectors 10, 12 respectively responsive to the intensity of radiation in different wavelength bands to produce respective electrical outputs. Detector 12 responds to a wavelength band including 4.4 microns (to detect hydrocarbon fires). In one embodiment, the latter is relatively narrow and included by the band to which detector 10 responds which is relatively broad. In another embodiment, the bands of both detectors are relatively narrow, the band of detector 10 being at 1 micron. A rate of rise unit 22 and a threshold unit 24 responsive to detector 12 produce signals of one type when the rate of rise of, and the value of, the intensity of the radiation received by that detector exceed predetermined values. A ratio unit 16 measures the ratio of the intensities of the radiation respectively received by detectors 10 and 12 and produces a signal of the opposite type when the ratio indicates that the source of radiation is a fire or explosion to which the system is not to respond. An AND gate 36 produces a fire and explosion indicating output only when the signals of the first type exist in the absence of the signal of the opposite type. <IMAGE>

Description

SPECIFICATION Improvements in and Relating to Fire and Explosion Detection The invention relates to fire and explosion detection systems and more specifically to systems which are able to discriminate between fires and explosions which need to be detected and those which do not.

Systems embodying the invention are particularly for use in situations where it is required to discriminate between the explosion of an ammunition round and a fire or explosion of combustible or explosive material which is set off by that round-so as to detect the fire or explosion set off by the round but not to detect the exploding round itself. In this way, the systems can initiate action so as to suppress the fire or explosion set off by the round, but do not initiate such suppression action merely in response to the exploding round.

One particular application of the systems is for use in armoured personnel carriers of battle tanks which may be attacked by high explosive antitank (H.E.A.T.) ammunition rounds. In such an application, the systems are arranged to respond to hydrocarbon fires (that is, fires involving the fuel carried by the vehicle) such as set off by the exploding H.E.A.T. round or set off by hot metal fragments produced from or by the round (or set off by other causes), but not to detect either the exploding H.E.A.T. round itself (even when it has passed through the vehicle's armour into the vehicle itself) or the secondary non-hydrocarbon fire which may be-produced by a pyrophoric combustion of the armour initiated by the H.E.A.T.

round.

According to the invention, there is provided a fire and explosion detection system for discriminating between radiation produced by a source of fire or explosion to be detected and radiation produced by a source of fire or explosion not to be detected, comprising first and second radiation detection means respectively responsive to radiation in different wavelength bands to produce first and second electrical signals respectively, the band of the radiation detection means including a wavelength characteristic of a source to be detected, the two bands being each narrow and spaced apart from each other, signal processing means responsive to at least the first electrical signal for producing, unless inhibited, a fire or explosion indicating signal when that electrical signal indicates the presence of a fire or explosion to be detected, and inhibiting means responsive to the ratio of the first and second electrical signals to produce an inhibiting signal to inhibit the production of the fire or explosion indicating signal when the ratio indicates that the source of the radiation received by the detection means is a fire or explosion source not to be detected.

According to the present invention, there is also provided a fire and explosion detection system for discriminating between radiation produced by a source of fire or explosion to be detected and radiation produced by a source of fire or explosion not to be detected, comprising first and second radiation detection means respectively responsive to radiation having wavelengths lying between different limits to produce first and second electrical signals respectively, the band of the first radiation detection means including a wavelength characteristic of a source to be detected, the wavelength limits applicable to the second radiation detection means being relatively widely spaced and being respectively above and below the upper and lower wavelength limits applicable to the first radiation detection means, signal processing means responsive to at least the first electrical signal for producing, unless inhibited, a fire or explosion indicating signal when that electrical signal indicates the presence of a fire or explosion to be detected, and means responsive to the ratio of the first and second electrical signals to produce an inhibiting signal to inhibit the production of the fire or explosion indicating signal when the ratio indicates that the source of the radiation received by the detection means is a fire or explosion source not to be detected.

According to the present invention, there is further provided a fire and explosion detection system for discriminating between radiation produced by a source of fire or explosion to be detected and radiation produced by a source of fire or explosion not to be detected, comprising first and second radiation detection means respectively responsive to the intensity of radiation in different wavelength bands so as to produce first and second electrical signals respectively, the band to which the said second radiation detection means is responsive being relatively broad and including the other which is relatively narrow, means responsive to one or both of the direction means for producing a third electrical signal when the intensity of the radiation received by that, or each detection means and/or the rate of rise of its intensity, exceeds a predetermined value, means responsive to the first and second electrical signals to measure the ratio of the intensities of the radiation respectively received by the two detection means whereby to produce a fourth signal when the ratio indicates that the source of the radiation is a fire or explosion source not to be detected, and output means having a first, inoperative, state and a second state in which it produces a fire or explosion indicating output, and connected to receive the third and fourth signals and capable of being switched into the second state by the third signal only when the fourth signal is absent.

According to the present invention, there is still further-provided a fire and explosion detection system for discriminating between radiation produced by a source of fire or explosion to be detected and radiation produced by a source of fire or explosion not to be detected, comprising first and second radiation detection means respectively responsive to the intensity of radiation in different and spaced apart narrow wavelength bands to produce first and second electrical signals respectively, means responsive to one or each of the first and second electrical signals for producing a third electrical signal when the intensity of the radiation received by that, or each, detection means, and/or the rate of rise of its intensity, exceeds a predetermined value, means responsive to the first and second electrical signals to measure their ratio whereby to produce a fourth electrical signal when the ratio indicates that the source of radiation is a fire or explosion not to be detected, and output means having a first, inoperative, state and a second state in which it produces a fire or explosion indicating output and connected to receive the third and fourth signals and capable of being switched into the second state by the third signal only when the fourth signal is absent.

Fire and explosion detection systems embodying the invention will now be described by way of example and with reference to the accompanying diagrammatic drawings in which: Figure 1 is a block circuit diagram applicable to two of the systems; Figure 2A is a graph of relative spectral intensity against wavelength for a fire source to be detected by the systems of Figure 1 and showing how the first of those systems responds thereto and Figure 2B is a similar graph but for a source of fire and explosion which is not to be detected by the systems of Figure 1 and showing how the first system responds thereto; Figure 3A corresponds to Figure 2A in that it refers to the same fire source but shows how the second of the systems of Figure 1 responds thereto;; Figure 38 corresponds to Figure 28 in that it refers to the same source of fire and explosion but shows how the second of the systems of Figure 1 responds thereto; Figure 4 is a block diagram applicable to two further ones of the systems; and Figure 5 is a block diagram applicable to another two of the systems.

As shown in Figure 1, one form of the system comprises two radiation detectors 10 and 12 each of which produces an electrical output in response to radiation received. Detector 12 is made to be sensitive to radiation in a narrow wavelength band and approximately 4.4 microns.

The detector 10 is arranged to be sensitive to radiation in a broad wavelength band, again centred at 4.4 microns.

The detectors may, for example, each be a thermopile sensor arranged to receive radiation through a filter having the required wavelength transmitting band. However, other forms of sensor may be used; thus, either or each detector could be a photoelectric cell such as a lead selenide cell, combined with an appropriate filter.

Detector 10 is connected to feed its electrical output to an amplifier 14 in a channel 1 5 and thence to one input of a ratio unit 16 by means of a line 17.

Detector 12 feeds its output through an amplifier 1 8 into a second channel 20. In the second channel 20, the output amplifier 1 8 is fed to a rate of rise detector 22. The rate of rise detector 22 produces a "1" output when its input indicates that the intensity of the radiation sensed by detector 12 is rising at at least a predetermined rate; otherwise, it produces a "0" output.

The output of amplifier 1 8 is also fed to one input of a threshold comparator whose other input receives a reference signal from a reference source 26 representing a predetermined level of radiation intensity. If the intensity of the radiation sensed by detector 12 exceeds this level, the comparator 24 produces a "1" output; otherwise, it produces a "0" output.

In addition, the output of amplifier 1 8 is fed to the first channel 1 5 by means of a line 28 which connects to the second input of the ratio unit i 6.

In the first channel 1 5, the output of the ratio unit 1 6 is a "1" when the signals received by the unit 1 6 correspond to the case when the ratio of the intensity of the radiation sensed by detector 10 to the intensity of the radiation sensed by detector 12 is below a predetermined value (unity, say) and is "0" when the signals correspond to the case when the ratio is above this value. This output is fed through a delay unit 34 to one input of AND gate 36. It is also fed to one input of a NOR gate 38 through a second delay unit 40 and fed directly to the second input of the NOR gate 38 on a line 42. The delay unit 40 may have a delay of say, 10 milliseconds or only 1 or 2 milliseconds. The output of the NOR gate triggers a monostable circuit 44 whose output feeds an input > of the AND gate 36.Until triggered, the circuit 44 produces a "1" output; when triggered, it produces a "0" output for a predetermined period in the range 10 to 100 milliseconds, say, and in this example 100 milliseconds is chosen.

In the second channel 20, the output of the threshold comparator 24 feeds a third input of the AND gate 36 on a line 48 while the fourth or last input of the AND gate 36 is fed from the output of the rate of rise unit 22 on a line 50.

The operation of the system will now be described in the three situations (referred to as Case I, Case II and Case lil) explained in detail below.

Figure 2A shows the relative spectral intensity of the radiation produced by a hydrocarbon flame plotted against wavelength, and Figure 2B shows the comparable plot for the flash emitted by an exploding H.E.A.T. round. In Figures 2A and 2B, the wavelength ranges to which the detectors 12 and 10 are sensitive are shown at A and B respectively.

Case I This is the case where an H.E.A.T. round hits the fuel tank of the vehicle and causes an explosive fire. In such a case, the H.E.A.T. round explodes inside the fuel tank and the resultant explosion of the H.E.A.T. round itself is "quenched" and it does not emit significant radiation. However, the burning and exploding hydrocarbon fuel causes radiation of high intensity to be emitted at 4.4 microns (corresponding to CO2 emission) in the wavelength range A (Figure 2A) as compared with the intensity of the radiation in the larger wavelength range B.

The system is arranged so that under these conditions, the ratio unit 1 6 (Fig. 1) receives a relatively lower input from the detector 10, on line 17, than from the detector 12 on line 28. It therefore produces a "1" output which, after delay of 0.5 milliseconds imposed by the delay circuit 34, is passed to one input of the AND gate 36. Because the ratio unit 1 6 is producing a "1" output, the monostable circuit 44 is not activated and continues to feed a "1" output to its associated input of AND gate 36.

The output from the detector 12 will also be passed to the channel 20. It is assumed that the fierceness of the fire is such that the detector output rises at a greater rate than the threshold rate of the rate of rise unit 22, and therefore the latter will produce a "1" output which is fed to the AND gate 36. It is also assumed that the intensity of the radiation is such that the threshold set by the reference source 26 is exceeded, and the threshold comparator 24 will therefore also feed a "1" output to the and gate 36.

Therefore, the AND gate 36 has all its inputs energised with "1" signals and consequently it produces a "1" output at a terminal 54-- which can be used to produce a fire and explosion warning signal and to initiate fire and explosion suppression.

Case II This is the case where the H.E.AT. round explodes in air but causes no fire. Therefore, Figure 2B, and not Figure 2A, applies, and the detector 12 will sense less radiation than detector 10.

Consequently, the ratio unit 1 6 will be switched to produce a "0" output which will be fed to the AND gate 36 through the delay unit 34.

Therefore, the AND gate 36 is disabled and cannot produce a "1" output even if detector 12 receives sufficient radiation at 4.4 microns to cause the rate of rise unit 22 and the threshold comparator 24 to produce a "1" output.

If the exploding H.E.A.T. round produces such radiation as to cause the ratio unit 16 to maintain its "0" output for longer than the delay period (10 milliseconds) of the delay unit 40, then the latter will activate the NOR gate 38 which will trigger the monostable unit 44 to produce a "0" output which will be held for the period (100 milliseconds) of the monostable unit. Therefore, for the whole of this 100 millisecond period, the AND gate 36 is held disabled and the AND gate is thus positively prevented from initiating fire or explosion suppression even if, during this period, the energy inputs to the detectors 10 and 12 change in such a manner as to cause all the other inputs of the AND gate to be switched to "1 ".As the exploding H.E.A.T. round fragments cool, the relative intensities of radiation emitted in the two wavelength bands A and B of Figure 2B will change and could produce inputs to the ratio unit 1 6 such as to cause it to produce a "1" output, but false fire suppression, which might otherwise occur, is prevented during this 100 millisecond period by the output of the monostable circuit 44.

The latter also prevents fire suppression being initiated by the ratio unit 1 6 producing a "1" output in response to momentary "blinding" of the detector 10 by the fragments passing in front of it or excitation of detector 12 as the fragments pass close to the detector giving the detector an increasing signal to which to respond, and additionally the signal may be large enough to trigger both the threshold comparator 24 and the rate of rise circuit 22.

Case III This is the case where the H.E.A.T. round explodes in conditions in which its radiation is partially "quenched", for example by the products of a hydrocarbon fire caused by the round itself.

In this case, the exploding H.E.A.T. round would emit radiation having the characteristics shown in Figure 28, and consequently the ratio unit 16 would be switched to produce a "0" output which would disable the AND gate 36 through the delay unit 34 in the manner explained. Fire suppression would therefore be initially prevented. However, in this case the partial quenching of the exploding H.E.A.T. round would cause its radiation to fall away rapidlybefore the end of the delay period (1Q milliseconds) of the delay unit 40. Therefore, if a hydrocarbon fire started subsequently, the AND gate 36 would receive all "1" inputs and would initiate fire suppression.

The system is advantageous in that the ratio unit 1 6, which controls inhibition of fire suppression, is, as explained, responsive to the ratio of intensities at narrow and broad bands based on 4.4 microns and the variation between the value of this ratio for an H.E.A.T. round and the value for a hydrocarbon fire can be significantly higher than, for example, systems where the ratio is taken between intensities at two or near infra-red wavelengths much closer to each other.

The variation between the value of the ratio for an H.E.A.T. round and the value for a hydrocarbon fire can be increased further, by making the detector 10 insensitive to radiation in a narrow band corresponding to that to which the detector 12 is sensitive. This can be done, for example, by placing a narrow band absorption filter (e.g. CO2) in front of the detector 10. Instead, an appropriately weighted signal from detector 12 could be subtracted from the signal output of detector 10.

Another system will now be described, again with reference to Figure 1. In this system, detector 10 is made to be sensitive to radiation in a narrow wavelength band in the range 0.7 to 1.2 microns, e.g. at approximately 1 micron, in contrast to the first system described (where detector 10 is sensitive to radiation in broad band centered at 4.4 microns). For example, the detector 10 may be a silicon diode detector arranged to view radiation through a filter transmitting radiation only within the required wavelength band. Again, detector 1 2 is arranged to be sensitive to radiation in a narrow wavelength band centred at 4.4 microns. For example, the detector 12 may again be a thermopile sensor arranged to receive radiation through a filter having the required wavelength transmitting band.However, other forms of sensor could be used and preferably they should either have substantially the same time constant or the sensor used for detector 12 shouid have a slower response than that used for detector 10.

For example, both sensors could be thermopiles receiving radiation through appropriate filters.

Instead, both could be photoelectric sensors; thus detector 10 could be a silicon diode sensor and detector 12 could be a lead selenide sensor, each receiving radiation through an appropriate filter.

Another possibility is for each detector to be a lead selenide sensor, again receiving radiation through an appropriate filter.

Apart from the changes to the sensitivity ranges of the detectors, the system is otherwise as previously described.

The operation of the system will now be described in the three situations, Case I, Case II and Case Ill, defined above.

Figure 3A, like Figure 2A, shows the relative spectral intensity of the radiation produced by the hydrocarbon flame plotted against wavelength, and Figure 38, like a figure 28, shows the comparable plot for the flash emitted by an exploding H.E.A.T. round. In Figures 3A and 3B, the narrow wavelength ranges to which the detectors 12 and 10 are sensitive are shown at A and B respectively.

Case I This is the case where the H.E.A.T. round explodes inside the fuel tank and the resultant explosion of the H.E.A.T. round itself is "quenched" and it does not emit significant radiation. However, the burning and exploding hydrocarbon fuel causes a significant amount of radiation to be emitted at 4.4 microns (corresponding to CO2 emission) and a relatively smaller amount of radiation at 1 micron. The system is arranged so that under these conditions the ratio unit 1 6 receives a relatively higher input from the detector 12, on line 28, than from the detector 10 on line 1 7. It therefore produces a "1" output which, after the delay of 0.5 milliseconds imposed by the delay circuit 34, is passed to one input of the AND gate 36.Because the ratio unit 1 6 is producing a "1" output, the monostable circuit 44 is not activated and continues to feed a "1" output to its associated input of AND gate 36.

The output from the detector 12 will also be passed to the channel 20. It is assumed that the fierceness of the fire is such that the detector output rises at a greater rate than the threshold rate of the rate of rise unit 22, and therefore the latter will produce a "1" output which is fed to the AND gate 36. It is also assumed that the intensity of the radiation is such that the threshold set by - the reference source 26 is exceeded, and the threshold comparator 24 will therefore also feed a "1" output to the AND gate 36.

Therefore, the AND gate 36 has all its inputs energised with "1" signals and consequently it produces a "1" output at a terminal 54-which can be used to produce a fire and explosion warning signal and to initiate fire and explosion suppression.

Case II This is the case where the H.E.A.T. round explodes in air but causes no fire. Therefore, Figure 38, and not Figure 3A, applies, and the detector 10 will thus receive a relatively higher amount of radiation than the detector 12.

The system is arranged such that the ratio unit receives a higher value signal on line 17 than on line 28.

Consequently, the unit 16 will be switched to produce a "0" output which will be fed to the AND gate 36 through the delay unit 34.

If the exploding H.E.A.T. round produces such radiation as to cause the ratio unit 1 6 to maintain its "0" output for longer than the delay period (10 milliseconds) of the delay unit 40, then the latter will activate the NOR gate 38 which will trigger the monostable unit 44 to produce a "0" output which will be held for the period (100 milliseconds) of the monostable unit, as already described. Therefore, again the AND gate is positively prevented from initiating fire or explosion suppression even if the cooling H.EA.T.

round fragments change the inputs to the ratio unit 16 such as to cause it to produce a "1" output.

In this case, the exploding H.E.A.T. round would emit radiation having the characteristics shown in Figure 38, and consequently the ratio unit 16 would be switched to produce a "0" output which would disable the AND gate 36 through the delay unit 34 in the manner explained. Fire suppression would therefore be initially prevented. However, in this case the partial quenching of the exploding H.E.A.T. round would cause its radiation to fall away rapidlybefore the end of the delay period (10 milliseconds) of the delay unit 40. Therefore, if a hydrocarbon fire started subsequently, the AND gate 36 would receive all "1" inputs and would initiate fire suppression.

The system just described is advantageous in that the ratio unit 16, which controls inhibition of fire suppression, is, as explained, responsive to the ratio of intensities at 1 and 4.4 microns. The variation between the value of this ratio for an H.E.A.T. round and the value for a hydrocarbon fire is high (it can be between 200 and 1000 for example) and much higher than, for example, systems where the ratio is taken between intensities at two near infra-red wavelengths much closerto each other.

It will be noted that channel 15, the channel which inhibits the production of the fire or explosion indicating output at terminal 54 when the radiation detected is produced by an exploding H.E.A.T. round, operates by measuring the ratio of the detector outputs and is therefore independent of the actual level of intensity of either detector output. The systems thus contrast with systems in which inhibiting action occurs when the intensity of radiation received by a detector exceeds a relatively high threshold and is thus assumed to originate from an exploding H.E.A.T. round.

Figure 4 will now be used to describe two further systems.

In one of thesewsystems, detector 12 is made sensitive to radiation in a narrow wavelength band centred at 4.4 microns and detector 10 is made sensitive to radiation in a broad wavelength band also centred at 4.4 microns, that is, the same bands as for the first system described with reference to Figure 1.

The output from detector 10 is fed through an amplifier 1 00A to a rate of rise unit 1 02A which produces a "1" output to an AND gate 104 when the output from detector 10 is rising at at least a predetermined rate. The output of amplifier 1 00A is also fed to a threshold unit 1 06A which compares it with a reference signal on a line 108A and produces a "1" output when the input to the unit 1 06A is such as to indicate that the intensity of the radiation sensed by detector 10 has at least a predetermined value set by the reference.

Finally, the output of amplifier 1 00A is fed to one input of a ratio unit 110.

Detector 12 feeds corresponding components which are identified by reference numerals with the suffix "B" instead of the suffix "A".

The ratio unit 110 produces a "1" output when the ratio of the intensity of the radiation sensed by detector 12 to the intensity of the radiation sensed by detector 10 is above a predetermined value (unity, say), and produces a "0" output when the ratio is below this value. The output is fed to one input of an AND gate 114 and thence to a delay unit 116 having a delay of, say 0.5 milliseconds. The delay unit 11 6 feeds one input of an AND gate 18 whose other input is directly connected to the output of the AND gate 114.

AND gate 118 feeds the second input of AND gate 104.

The output of the ratio unit 110 is also fed to a NOR gate 120. The other inputs of this NOR gate are fed with the output of inverters 1 22A and 1 22B which are energised by the outputs of amplifiers 1 06A and 1 06B respectively. The output of the NOR gate 120 feeds a delay circuit 124, having a delay of 10 milliseconds. This delay unit feeds one input of an AND gate 126 whose other input is fed directly with the output of the NOR gate 120. The output of AND gate 126 triggers a monostable 1 28 whose output feeds the fourth input of the AND gate 104. When triggered, the monostable changes its output from "1" to "0" for a period of 100 milliseconds.

The operation of the arrangement of Figure 4 will now be described with particular reference to Case I, Case II and Case lil (as defined above).

Case I In this case, the H.E.A.T. round explodes inside the fuel tank of the vehicle and the explosion of the round itself is quenched and does not emit significant radiation. However, the burning fuel produces a significant amount of radiation at 4.4 microns.

Therefore, the waveform of Fig. 2A applies and the ratio unit 110 produces a "1" output.

Assuming that, at the same time, the levels of radiation produced by the detectors 10 and 12 are above the predetermined (relatively low) thresholds of the threshold units 1 06A and 106B, AND gate 114 passes a "1" output to the delay unit 116 and the AND gate 118. After the delay of 0.5 milliseconds (to ensure that the signals are not being produced by a transient phenomenon), the AND gate 104 receives the "1" output.

Because the ratio unit 110 is producing a "1" output, NOR gate 120 will not be enabled and the monostable 128 will therefore remain in its stable state, thus maintaining its "1" output to AND gate 104.

Assuming that the rate of rise of the intensity of the radiation sensed by the detectors is above the predetermined levels set in the rate of rise units 102A and 1028, AND gate 104 will also receive "1" inputs from them.

Therefore, the AND gate 104 has all its input energised with "1" signals and consequently it produces a "1" output at terminal 130-which can be used to produce a fire and explosion warning and to initiate fire and explosion suppression.

Case II In this case, Figure 2B, and not Figure 2A, applies and the detector 12 will receive a relatively lower amount of radiation than detector 10.

Consequently, the ratio unit 110 produces a "0" output which is fed to AND gate 104 through AND gate 114 and through AND gate 118 (the delay unit 11 6 does not delay a "0" signal).

Therefore AND gate 104 is disabled and cannot produce a "1" output and fire and explosion suppression is prevented.

If the exploding H.E.A.T. round produces such radiation that the ratio unit 110 maintains its "0" output for longer than the 10 millisecond period of delay unit 124, monostable unit 128 is triggered and produces a "0" output which it holds for its period of 100 milliseconds. As for the circuit of Figure 1, therefore, fire and explosion suppression is prevented during this 100 millisecond period (and for the same purpose as explained above), but can take place at the end of this period.

Case III This is the case where the H.E.A.T. round explodes in conditions in which its radiation is only partially quenched. Initially, radiation is produced having the characteristic shown in Figure 2B, and the ratio unit 110 produces a "0" output which disables the AND gate 104 in the mannerexplained above, and fire suppression is initially prevented. However, provided the radiation from the exploding H.E.A.T. round falls away rapidly, before the 10 millisecond delay period of delay unit 124, subsequent starting of a hydrocarbon fire (if the intensity level and rate of rise thresholds are met) cause AND gate 104 to receive "1" inputs and thus to initiate fire and explosion suppression.

In the alternative system to be described with reference to Figure 4, detector 10 is made sensitive to radiation in a narrow wavelength band centred at approximately 1 micron and detector 12 is made sensitive to radiation in a narrow wavelength band centred at 4.4 microns, that is, the same bands as for the second system described with reference to Figure 1. The operation of the ratio unit 110 produces a "1 " output when the ratio of the intensity of the radiation sensed by detector 10 to the intensity of the radiation sensed by detector 12 is below a predetermined value (unity, say), and produces a "0" output when the ratio is above this value.

The operation of this alternative system will now be described with particular reference to Case I, Case li and Case III (as defined above).

Therefore, the waveform of Fig. 3A applies and the ratio unit 110 produces a "1" output.

Assuming that, at the same time, the levels of radiation produced by the detectors 10 and 12 are above the predetermined (relatively low) thresholds of the threshold unit 1 06A and 106B, AND gate 114 passes a "1" output to the delay unit 116 and the AND gate 118. After the delay of 0.5 milliseconds (to ensure that the signals are not being produced by a transient phenomenon), the AND gate 1 04 receives the "1" output.

Because the ratio unit 110 is producing a "1" output, NOR gate 120 will not be enabled and the monostable 128 will therefore remain in its stabile state, thus maintaining its "1" output to AND gate 104.

Assuming that the rate of rise of the intensity, of the radiation sensed by the detectors is above the predetermined levels set in the rate of rise units 1 02A and 102B, AND gate 104 will also receive "1" inputs from them.

Therefore, the AND gate 104 has all its input energised with "1" signals and consequently it produces a "1" output at terminal 130-which can be used to produce a fire and explosion warning signal and to initiate fire and explosion suppression.

Case II In this case, Figure 3B, and not Figure 3A, applies, and the detector 10 will receive a relatively higher amount of radiation than detector 12.

Consequently, the ratio unit 110 produces a "0" output which is fed to AND gate 104 through AND gates 114 and 118. Therefore AND gate 104 is disabled and cannot produce a "1" output and fire and explosion suppression is prevented.

If the exploding H.E.A.T. round produces such radiation that the ratio unit 110 maintains its "0" output for longer than the 10 millisecond period of delay unit 124, monostable unit 128 is triggered and produces a "0" output which it holds for its period of 100 milliseconds. As for the circuit of Figure 1, therefore, fire and explosion suppression is prevented during this 100 millisecond period (and for the same purposes as explained above), but can take place at the end of this period.

Case III This is the case where the H.E.A.T. round explodes in conditions in which its radiation is only partially quenched. Initially, radiation is produced having the characteristic shown in Figure 2B, and the ratio unit 110 produces "0" output which disables the AND gate 104 in the manner explained above, and fire suppression is initially prevented. However, provided the radiation from the exploding H.E.A.T. round falls away rapidly, before the 10 millisecond delay period of delay unit 124, subsequent starting of a hydrocarbon fire (if the intensity level and rate of rise thresholds are met) causes AND gate 104 to receive "1" inputs and thus to initiate fire and explosion suppression.

In the circuit of Fig. 4, the outputs of the inverters 1 22A and 1 22B prevent the monostable 128 being triggered by a noise output from the ratio unit 110 in the event that the detector outputs are below the thresholds of the threshold units 1 06A and 106B. Were this to be allowed to happen, then a true fire developing during the period of the monostable could not trigger the AND gate 104 until the end of that period. The circuit of Fig. 1 could be modified similarly if required, by feeding an inverted output from the threshold unit 24 to the NOR 38.

Figure 5 shows a simplified form of circuit which may be used instead of the circuit of Figure 4 in order to implement the two systems described in connection therewith. Items in Figure 5 corresponding to items in Figure 4 are similarly referenced. The basic difference between the circuits of Figures 4 and 5 is that the circuit of Figure 5 omits the delay units 116 and 124 and the monostable unit 128. The operation is otherwise as described with reference to Figure 4.

Delay unit 11 6 can be omitted because of the inherent small delays in the circuitry.

As far as the delay unit 1 24 is concerned, this is provided mainly to cope with Case Ill for the form of the system in which detector 12 is sensitive to radiation in band A of Figures 3A and 3B and detector 10 is sensitive to radiation in band B of Figures 3A and 3B. In such a system, delay 124 (in Figure 4) prevents false fire suppression which might arise if the cooling fragments of an exploding H.E.A.T. round are "seen" by the ratio unit 110 as a fire. However, it is found that the difference in the value of the ratio measured by the ratio unit 110 for an H.E.A.T. round and the value for a hydrocarbon fire is so large that by the time the relative intensities of radiation sensed by the detectors 10 and 12 from the cooling fragments reach such values as to cause the ratio unit output to switch from "0" to "1", the actual levels of the intensities will have fallen below the thresholds of the threshold units 1 06A and 106B. False fire suppression will therefore not take place, and delay unit 124 can therefore be omitted.

Claims

1. A fire and explosion detection system for discriminating between radiation produced by a source of fire or explosion to be detected and radiation produced by a source of fire or explosion not to be detected, comprising first and second radiation detection means respectively responsive to radiation in different wavelength bands to produce first and second electrical signals respectively, the band of the radiation detection means including a wavelength characteristic of a source to be detected, the two bands being each narrow and spaced apart from each other, signal processing means responsive to at least the first electrical signal for producing, unless inhibited, a fire or explosion indicating signal when that electrical signal indicates the presence of a fire or explosion to be detected, and inhibiting means responsive to the ratio of the first and second electrical signals to produce an inhibiting signal to inhibit the production of the fire or explosion indicating signal when the ratio indiates that the source of the radiation received by the detection means is a fire or explosion source not to be detected.

2. A system according to claim 1, in which the wavelength band to which the said second radiation detection means is responsive includes a wavelength characteristic of a source of fire or explosion not to be detected.

3. A system according to claim 1 or 2, for use where the source of fire or explosion which is not to be detected is a high explosive anti-tank round (H.E.A.T. round), in which the wavelength band to which the second detecting means is responsive is a narrow wavelength band near to the infra-red region.

4. A fire and explosion detection system for discriminating between radiation produced by a source of fire or explosion to be detected and radiation produced by a source of fire or explosion not to be detected, comprising first and second radiation detection means respectively responsive to radiation having wavelengths lying between different limits to produce first and second electrical signals respectively, the band of the first radiation detection means including a wavelength characteristic of a source to be detected, the wavelength limits applicable to the second radiation detection means being relatively widely spaced and being respectively above and below the upper and lower wavelength limits applicable to the first radiation detection means, signal processing means responsive to at least the first electrical signal for producing, unless inhibited, a fire or explosion indicating signal when that electrical signal indicates the presence of a fire or explosion to be detected, and means responsive to the ratio of the first and second electrical signals to produce an inhibiting signal to inhibit the production of the fire or explosion indicating signal when the ratio indicates that the source of the radiation received by the detection means is a fire or explosion source not to be detected.

5. A system according to claim 4, in which the wavelength limits applicable to the first radiation detecting means are relatively closely spaced and in which the second radiation detection means is substantially unresponsive to radiation within those limits.

6. A system according to claim 4, in which the second radiation detection means includes means for producing an electrical signal in dependence on the radiation received and means for subtracting from that electrical signal a quantity substantially corresponding to the radiation received by that detecting means and lying between the wavelength limits of the first detecting means, whereby to produce the said second electrical signal.

7. A system according to any preceding claim, in which the means responsive to at least the first electrical signal includes means responsive to the absolute magnitude of that electrical signal and means responsive to its rate of change.

8. A system according to any one of claims 1 to 6, in which the signal processing means comprises means responsive to at least the first electrical signal to produce a threshold signal when the intensity of the radiation received by that detection means exceeds a predetermined value, and output means having a normal, inoperative, state and a second state in which it produces the fire or explosion indicating signal and connected to receive the threshold and inhibiting signals so as to be switched into the second state by the threshold signal only in the absence of the inhibiting signal.

9. A system according to claim 8, including means responsive at least to the first signal to produce a rate of rise signal when the rate of rise of the first signal exceeds a predetermined value, and in which the output means is connected to receive the rate of rise signal so as to be switched into the second state only when the threshold signal and the rate of rise exist simultaneously and the inhibiting signal is absent.

10. A system according to claim 8, including means responsive to the said second signal to produce a further threshold signal when the intensity of the second signal exceeds a predetermined value, and in which the output means is connected to receive the further threshold signal so as to be switched into the second state only when the two threshold signals exist simultaneously and the inhibiting signal is absent.

11. A system according to claim 9, including means responsive to the said second signal to produce a further threshold signal when the intensity of the second signal exceeds a predetermined value, and in which the output means is connected to receive the further threshold signal so as to be switched into the second state only when the two threshold signals and the rate of rise signal exist simultaneously and the inhibiting signal is absent.

12. A system according to claim 8, including means responsive to the second signal to produce a rate of rise signal when the rate of rise of the second signal exceeds a predetermined value, and in which the output means is connected to receive this rate of rise signal so as to be switched into the second state only when the threshold signal and that rate of rise signal exist simultaneously and the inhibiting signal is absent.

13. A system according to claim 9, including means responsive to the second signal to produce a rate of rise signal when the rate of rise of the second signal exceeds a predetermined value, and in which the output means is connected to receive this rate of rise signal so as to be switched into the second state only when the threshold signal and the two rate of rise signals exist simultaneously and the inhibiting signal is absent.

14. A system according to claim 10, including means responsive to the second signal to produce a rate of rise signal when the rate of rise of the second signal exceeds a predetermined value, and in which the output means is connected to receive this rate of rise signal so as to be switched into the second state only when the two threshold signals and this rate of rise signal exist simultaneously and the inhibiting signal is absent.

1 5. A system according to claim 11, including means responsive to the second signal to produce a rate of rise signal when the rate of rise of the second signal exceeds a predetermined value, and in which the output means is connected to receive this rate of rise signal so as to be switched into the second state only when the two threshold signals and the two rate of rise signals exist simultaneously and the inhibiting signal is absent.

1 6. A system according to any one of claims 8 to 15, including means responsive to the inhibiting signal and operative when the inhibiting signal has existed for at least a first predetermined duration to produce an additional inhibiting signal having a second predetermined duration, and in which the output means is adapted to be incapable of being switched to the said second state during the existence of the additional inhibiting signal.

17. A fire and explosion detection system fqr discriminating between radiation produced by a source of fire or explosion to be detected and radiation produced by a source of fire or explosion not to be detected, comprising first and second radiation detection means respectively responsive to the intensity of radiation in different wavelength bands so as to produce first and second electrical signals respectively, the band to which the said second radiation detection means is responsive being relatively broad and including the other which is relatively narrow, means responsive to one or both of the detection means for producing a third electrical signal when the intensity of the radiation received by that, or each detecting means and/or the rate of rise of its intensity, exceeds a predetermined value, means responsive to the first and second electrical signals to measure the ratio of the intensities of the radiation respectively received by the two detection means whereby to produce a fourth signal when the ratio indicates that the source of radiation is a fire or explosion source not to be detected, and output means having a first, inoperative, state, and a second state in which it produces a fire or explosion indicating output. and connected to receive the third and fourth signals and capable of being switched into the second state by the third signal only when the fourth signal is absent.

1 8. A fire and explosion detection system for discriminating between radiation produced by a source of fire or explosion to be detected and radiation produced by a source of fire or explosion not to be detected, comprising first and second radiation detection means respectively responsive to the intensity of radiation in different and spaced apart narrow wavelength bands to produce first and second electrical signals respectively, means responsive to one or each of the first and second electrical signals for producing a third electrical signal when the intensity of the radiation received by that, or each, detection means, and/or the rate of rise of its intensity exceeds a predetermined value, means responsive to the first and second electrical signals to measure their ratio whereby to produce a fourth electrical signal when the ratio indicates that the source of radiation is a fire or explosion not to be detected, and output means having a first, inoperative, state and a second state in which it produces a fire or explosion indicating output and connected to receive the third and fourth signals and capable of being switched into the second state by the third signal only when the fourth signal is absent.

19. A system according to claim 18, for use where the source of fire or explosion not to be detected is a high explosive anti-tank round (H.E.A.T. round), in which the wavelength band to which the second detection means is responsive is a narrow wavelength band near to the infra-red region.

20. A system according to any preceding claim, for use where the fire or explosion source to be detected is a hydrocarbon fire, in which the wavelength band to which the first detection means responds is a band including the wavelength of 4.4 microns.

21. A fire and explosion detection system substantially as described with reference to Figures 1, 2A and 2B of the accompanying drawings.

22. A fire explosion detection system substantially as described with reference to Figures 1, 3A and 3B of the accompanying drawings.

23. A fire and explosion detection system substantially as described with reference to Figures 2A, 2B and 4 of the accompanying drawings.

24. A fire and explosion detection system substantially as described with reference to Figures 3A, 3B and 4 of the accompanying drawings.

25. A fire and explosion detection system substantially as described with reference to Figures 2A, 2B and 5 of the accompanying drawings.

26. A fire and explosion detection system substantially as described with reference to Figures 3A, 3B and 5 of the accompanying drawings.