GB2142757A - Improvements in and relating to fire and explosion detection and suppression - Google Patents

Abstract

A fire and explosion detection system for responding to certain fires and explosion (e.g. hydrocarbon fires) but discriminating against other fires and explosions (such as an exploding ammunition round which does not cause a hydrocarbon fire) is described, such as for use in a military vehicle. A unit 5 sends signal A HIGH both in response to a hydrocarbon fire and also in response to the flash from an exploding ammunition round. This triggers a 100 mS monostable 52 and holds gate 56 closed through inverter 54. If the ammunition round ruptures the vehicle fuel tank a pressure or similar transducer 65 sends signal H HIGH and produces a fire alarm signal I which passes through OR gate 58 to an output 80. If the round does not rupture the fuel tank, gate 64 is held closed and no alarm is produced. However, if fire persists for more than 100 mS, even if the fuel tank has not been ruptured, a HIGH signal A causes a HIGH fire alarm signal F through gate 56 and thence to output 80. <IMAGE>

Description

SPECIFICATION Improvements in and relating to fire and explosion detection and suppression 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 fires and explosions which do not.

Systems to be described by way of example below and embodying the invention, may be used, for example, in situations where it is required to discriminate between the explosion of an ammunition round itself and a fire or explosion of combustible or explosive material which is or may be set off by that round so as to produce an alarm in response to the fire or explosion set off or about to be set off by the round but not to produce such an alarm in response to the exploding round itself. In this way, the system can initiate action so as to suppress the fire or explosion set off by the round but does not initiate such suppression action merely in response to the exploding round.

One particular application of the systems is for use in an armoured personnel carrier or battle tank which may be attacked by high energy anti-tank (H.E.A.T.) ammunition rounds. In such an application, the system is arranged not to alarm in response either to the exploding H.E.A.T. round itself (even when it has passed through the vehicle's armour into the vehicle) or the secondary nonhydrocarbon fire which may be produced by a pyrophoric reaction of the H.E.A.T.round with the vehicle's armour, but to produce an alarm in response to hydrocarbon fires (that is, fires involving the fuel carried by the vehicle) set off or about to be set off by an exploding H.E.A.T.round or by hot metal fragments produced from or by the round (or set off by other causes).

Various novel features of the invention will be apparent from the following description, given by way of example only, of fire and explosion detection systems embodying the invention, reference being made to the accompanying diagrammatic drawings in which: Figure 1 is a block circuit diagram of one of the systems; and Figures 2, 3, 4 and 5 are diagrams showing logic signals occurring in the circuit of Figure 1.More specifically to be described below is a fire or explosion detection system comprising fire or explosion sensing means operative to produce a first signal both in response to a fire or explosion of a first type which is intended to result in an alarm output and in response to a fire or explosion of a second type which is not intended to result in an alarm output, transducing means operative to produce a second signal in response to change in a physical parameter indicating a condition conducive to a fire or explosion of the first type, and logic means operative in response to the first and second signals to produce the alarm output.

The transducing means may comprise a transducer operative to produce the second signal in response to the rupturing of a container of a flammable substance. Such a transducer may for example be of the optical, temperature, pressure or strain gauge type.

The sensing means advantageously comprises means responsive to the electromagnetic radiation produced by the said fires or explosions to produce the first and second signals. For example, it may comprise radiation sensing means tuned to respond to radiation in two different and predetermined wavelength bands and to produce a said signal only when radiation having at least a predetermined intensity is received in each of the said bands. Advantageously, the sensing means is arranged so that a said signal is only produced when, in addition, the radiation intensity in at least one wavelength band is increasing at at least a predetermined rate.

There will also be more specifically described below a fire and explosion detection system of the discriminating type which is operative to produce an alarm output in response to an exploding ammunition round striking a hydrocarbon fuel tank but not in response to an exploding round not striking the tank, comprising radiation sensing means responsive to electromagnetic radiation produced by the exploding round or by a hydrocarbon fire to generate a first signal, transducing means responsive to the striking of the fuel tank by the round to produce a second signal, and logic means operative in response to the first and second signals to produce the alarm output.

The logic means may comprise means operative to produce an alarm output both when the first and second signals occur simultaneously and when they occur serially but with less than a predetermined time separation between them.

Advantageously, the logic means includes means operative to produce an alarm output in response to the first signal alone having persisted for at least a predetermined time period.

The logic means may also include means operative to produce an alarm output in the absence of the second signal, provided that at least two successive first signals are produced in at least a predetermined time.

The foregoing are exemplary of and not exhaustive of the various features of the systems now to be more specifically described with reference to Figures 1 to 5.

As shown in Figure 1, the system has a fire or flame detector unit 5. This may be of any suitable type for the purposes of detecting not only the fire or explosion in response to which the system is intended to produce an alarm but also the fire or explosion against which the system is to discriminatc that is, not produce an alarm. The unit 5 may operate by sensing any suitable parameter of such fires or explosions. Preferably, however, it operates by detecting the electromagnetic radiation em itted by the fires of explosions.

If the system is intended for installation in an armoured personnel carrier or battle tank (as will be assumed in the following description purely by way of example), the system is intended to discriminate in favour of hydrocar bon fires or explosions such as produced by an H.E.A.T. round striking and piercing the vehicle's fuel tank (that is, to produce an alarm) and to discriminate against (that is, not to produce an alarm) an exploding H.E.A.T.

round itself which pierces the vehicie's armour but does not rupture the vehicle's fuel tank or otherwise be followed by a hydrocarbon fire.

For such an application, one suitable form which the unit 5 can take is illustrated by way of example within the block 5. As shown, it comprises two sensors 6 and 8 which produce electrical outputs in response to electromagnetic radiation, the sensor 6 being arranged by means of a suitable filter to respond to radiation in a narrow wavelength band centred at 0.96 microns and the sensor 8 being arranged by means of a suitable filter to respond to radiation in a narrow wavelength band centred at 4.4 microns. The sensor 6 may be a suitable photo-detector while the sensor 8 may be a thermopile.The electrical outputs from the sensors 6 and 8 are fed through respective amplifiers 10 and 1 2 to threshold detectors 14 and 1 6. These are arranged to produce HIGH outputs on lines 18 and 20 respectively when the inputs which they receive from the sensors exceed predetermined thresholds. In addition, the amplified output of the sensor 8 is fed through a rate of rise threshold detector 22 which produces a HIGH output on a line 24 when the sensor output is rising at at least a predetermined rate. A hydrocarbon fire emits a substantial amount of radiation at 4.4 microns (which is the wavelength corresponding to hot carbon dioxide in the flame) while solar radiation is severely attenuated at this wavelength (because of atmospheric carbon dioxide).A hydrocarbon flame also emits radiation at 0.96 microns but thermal radiation from hot surfaces and the like is low at this wavelength. Therefore, a hydrocarbon flame will cause both sensors 6 and 8 to produce substantial electrical outputs simultaneously but radiation from sunlight and/or hot surfaces will not. By appropriately setting the thresholds of the detectors 14, 1 6 and 22, these detectors will therefore all produce HIGH outputs in response to a hydrocarbon flame but not to interfering radiation from the sun or hot surfaces.

Lines 1 8, 20 and 24 are connected to an AND gate 26 which therefore produces an output on a line 28 when all three input lines are carrying HIGH signals. The HIGH output from the AND gate 26 is passed to one input of an AND gate 30 after a one millisecond delay imposed by delay unit 31, but is also passed directly to the other input of the AND gate 30. Therefore, AND gate 30 will produce an output on a line 32 provided that lines 18, 20 and 24 are still producing their binary HIGH signals at the end of the one millisecond period.

In this way, therefore, unit 5 produces a HIGH output on line 32 in response to a hydrocarbon fire of sufficient magnitude but not in response merely to solar radiation and/or radiation from hot surfaces. However, an exploding H.E.A.T.round will also produce significant radiation at both of the wave lengths under consideration and the unit 5 itself is not therefore able to discriminate against the latter occurrence.

Line 32 is connected to three circuit paths.

First, by means of a line 34 it is connected to one input of an AND gate 36.

Secondly, it is connected directly to one input of an AND gate 38 and connected to the second input of this gate via a very short delay unit 40. The output of the AND gate 38 is connected to a retriggerable monostable circuit 42 having a two second period, and the output of the monostable is inverted by an inverter 44 and applied to the second input of the AND gate 36.

The third circuit path comprises a line 46 which feeds the output on line 32 directly to one input of an AND gate 48 and to the second input of this gate through a 0.5 millisecond delay unit 50.

The output of AND gate 36 is fed to a monostable circuit 52 having a 100 millisecond period and thence through an inverter 54 to one input of an AND gate 56. The second input of the AND gate 56 is fed directly from the output of the AND gate 48.

The output of gate 56 is fed to one input of an OR gate 58 via a line 60.

The output of the monostable 52 is also fed via a line 62 to one input of an AND gate 64.

The system also includes a transducer 65 fitted in, on or adjacent the vehicle's fuel tank and which is of a type which is capable of producing an electrical signal when the tank is punctured or ruptured as will of course happen when it is struck by the H.E.A.T.round.

The transducer 65 may be of any suitable type. It may, for example, be of the optical type or it may be a strain gauge, a temperature sensor, or a to sensor. Advantageously, however, it is a pressure sensor which produces an output in response to the increase of pressure within the tank which will occur when it is struck by the H.E.A.T.round, and the use of this particular form of sensor will be assumed by way of example in the following description. The output is passed via an amplifier 66 to a threshold detector 67 which produces a HIGH output on a line 68 when the pressure increase exceeds a predetermined threshold. The HIGH output on line 68 is fed to one input of an AND gate 70 directly and is fed to the second input of this gate through a 0.5 millisecond delay unit 72.The output of the gate 70 feeds a monostable 74 having a 50 millisecond period, and the monostable output on line 76 is fed to the second input of the AND gate 64. Finally, the output line 78 of the gate 64 is connected to the second input of the OR gate 58.

The vehicle may of course have several fuel tanks, in which case each will be fitted with a suitable transducer 65, their respective lines 68 being all connected together through suitable circuitry which feeds a HIGH signal onto line 68 when any one of the transducers reacts to rupturing of its respective fuel tank.

Similarly, there may be not one but several units 5 distributed around the interior of the vehicle and interconnected through suitable circuitry which produces a HIGH output on line 32 when any one of the AND gates 30 produces a HIGH output.

The operation of the system will now be considered in more detail under four conditions with reference to Figures 2 to 5: (a) A "dry shot" condition in which an exploding round enters the interior of the vehicle but does not rupture the fuel tank or otherwise cause a hydrocarbon fire; (b) a "wet shot" condition in which an exploding H.E.A.T.round penetrates into the interior of the vehicle and also pentrates the fuel tank; (c) a slow-propagating hydrocarbon fire having a duration of at least a predetermined period (100 milliseconds in this example) with no penetration of the fuel tank; (d) an intermittent slowly propagating hydrocarbon fire with no penetration of the fuel tank.

Each of Figures 2 to 5 is divided into two parts, a part ''A'' to a relatively large time scale and a part "B" to a much smaller time scale. The logic signals illustrated in Figures 2 to 5 are identified by reference letters corresponding to the letters shown on Figure 1.

It will be apparent that the system is required to produce an "alarm" output in response to conditions (b), (c) and (d), but is not to produce an alarm in response to condition (a). As will be explained in detail below, the system operates so that a fire or explosion which is intended to produce an alarm output makes either signal F or signal I go HIGH, thus causing the OR gate 58 to produce an ALARM output on line 80. In response to a fire or explosion which is to be discriminated against, so as not to produce an alarm output, the system holds both signals F and I at LOW.

The operation of the system under condition (a); the dry shot condition, will now be described with specific reference to Figure 2.

When the exploding H.E.A.T.round pierces the vehicle armour, the flash which it produces will be detected by the unit 5 and signal A will go HIGH after the one millisecond delay imposed by delay unit 31, assuming that the intensity and rate of rise thresholds imposed by the units 14, 1 6 and 22 are satisfied (the 1 millisecond delay is not visible in Fig.2, or Figs.3 to 5).

The HIGH signal A causes signal B to go HIGH after a very small delay imposed by delay unit 40 and signal B remains HIGH for two seconds as shown in Figure 2B. This of course switches signal C from HIGH to LOW at which it remains for two seconds.

Because of the small time delay imposed by delay unit 40, signal C will in fact be HIGH for a correspondingly short time after signal A has gone high and thus the output of gate 36 will go HIGH momentarily to trigger the 100 millisecond monostable 52, causing signal D to go HIGH for 100 milliseconds. Signal E therefore goes LOW for the same period.

After the 0.5 millisecond delay imposed by delay unit 50, the HIGH signal A will cause signal J to go HIGH. However, by this time signal E is LOW, and therefore gate 56 cannot switch signal F to HIGH on line 60.

Because the H.E.A.T.round is assumed not to rupture the fuel tank in this example, the pressure transducer 65 will not produce a HIGH output on line 68. Signal H will therefore remain LOW. Therefore gate 64 maintains signal I LOW on line 78.

Therefore, under this condition, neither signal F nor signal I goes HIGH and the OR gate 58 therefore produces no ALARM output on line 80.

The monostable 52, which holds signal E LOW for 100 milliseconds, ensures that gate 56 cannot send signal F HIGH for this period.

In the particular example being considered, experiment has shown that the flash from the exploding H.E.A.T.round will have dissipated by the end of 100 milliseconds and there is therefore no risk that signal F will go HIGH when signal E returns to HIGH at the end of the 100 millisecond period.

The operation under condition (b) will now be considered with reference to Figure 3.

Signals A, B, C, D and F are the same as for condition (a), as shown in Figure 2, except that signal A is shown as going HIGH for a shorter period (about 25 milliseconds) than in condition (a). This might, for example, be caused by the attenuating effect on the flash from the exploding round as it penetrates the fuel tank.

As in the case of condition (a), signal E will be held at LOW for the 100 millisecond period of monostable 52 and this will prevent signal F from going HIGH.

However, as soon as the round penetrates the fuel tank, signal G will go HIGH and signal H will therefore be held HIGH for the 50 millisecond period of monostable 74. This signal will therefore be HIGH during the 100 millisecond period for which signal D is HIGH.

Signal I will therefore go HIGH and gate 58 will therefore produce an ALARM output on line 80.

If the exploding round should penetrate the fuel tank first, that is, before emerging into the interior of the vehicle, signal H will go HIGH and be held HIGH for 50 milliseconds which will be sufficient for the exploding round to emerge from the fuel tank and send signal A HIGH, which will in turn send signal D HIGH, thus again making signal I HIGH and producing an ALARM output on line 80.

The operation of the system under condition (c) will now be considered with reference to Figure 4. Under this condition, it is assumed that a slowly propagating hydrocarbon fire starts but that the fuel tank is not ruptured.

The pressure transducer 65 does not therefore switch signal G to HIGH, signal H remains LOW and gate 64 is thus held closed. However, the slowly propagating fire causes the unit 5 to produce a HIGH signal A, and signals A, B, C, D and E are thus the same as for conditions (a) and (b), as shown in Figures 2 and 3, with the difference that, because a slowly propagating fire is being assumed, signal A remains HIGH for longer than 100 milliseconds. Therefore, when the signal D goes LOW at the end of the 100 millisecond period of the monostable 52, signal E goes HIGH and, because signal J is also HIGH at this time, signal F goes HIGH and the OR gate 58 produces an ALARM output. The delay provided by the delay unit 50 is necessary to ensure that signal J does not go HIGH before signal E goes LOW at the beginning of the 100 millisecond period of the monostable 52.

Condition (d) will now be considered with reference to Figure 5. This is a condition where a flickering slowly propagating fire causes the unit 5 to produce intermittent HIGH output signals A. In one particular example under test, it was found that the unit 5 responded to a slowly propagating fire by producing a HIGH signal A output once every 1.3 seconds on average, each such output having an average duration of 30 milliseconds. A single such HIGH signal could of course be the result of a flash from an exploding H.E.A.T.round, but this produces no alarm output because, as the fuel tank is not ruptured in this example, signal H is LOW, see Figure 5A Therefore, gate 64 is held closed.

Furthermore, signal D being switched HIGH, holds gate 56 closed for the 100 millisecond period of the monostable 52.

However, if signal A goes HIGH for a second time during the two second period of the retriggerable monostable 42, signal J will go HIGH (after the 0.5 millisecond delay in the delay unit 50) and, because signal E is still HIGH at this time, signal F will go HIGH and produce an ALARM output on line 80. It will be apparent that the monostable 42 holds gate 36 closed for its two second period and thus prevents the second HIGH signal A from retriggering the 100 millisecond monostable 52.

It will be apparent that the circuit arrangement described with reference to Figure 1 is merely one example of various different forms which the circuit can take.

It will be appreciated that the detector unit 5 described is particularly advantageous where the apparatus is required to be able to respond to hydrocarbon fires not necessarily produced by an H.E.A.T. round (e.g.conditions (c) and (d) discussed above) since the detector 5 is sensitive enough to detect small fires and yet does not false alarm to other ambient "noise" radiation sources. However in an application where this facility is not required or "noise" radiation sources are completely absent, the detector unit could be replaced by a simpler form having, for example, a single radiation or other suitable form of fire detector, appropriate modifications being made to the circuitry.

Claims (14)

1. A fire or explosion detection system, comprising fire or explosion sensing means operative to produce a first signal both in response to a fire or explosion of a first type which is intended to result in an alarm output and in response to a fire or explosion of a second type which is not intended to result in an alarm output, transducing means operative to produce a second signal in response to change in a physical parameter indicating a condition conducive to a fire or explosion of the first type, and logic means operative in response to the first and second signals to produce the alarm output.

2. A system according to claim 1, in which the transducing means comprises a transducer operative to produce the second signal in response to the rupturing of a container of a flammable substance.

3. A system according to claim 2, in which the transducer is a pressure transducer.

4. A system according to claim 2, in which the transducer is an optical transducer.

5. A system according to claim 2, in which the transducer is a temperature transducer.

6. A system according to claim 2, in which the transducer is of the strain gauge type.

7. A system according to any preceding claim in which the sensing means comprises means responsive to the electromagnetic radiation produced by the said fires or explosions to produce the first and second signals.

8. A system according to claim 7, in which the sensing means comprises radiation sensing means tuned to respond to radiation in two different and predetermined wavelength bands and to produce a said signal only when radiation having at least a predetermined intensity is received in each of the said bands.

9. A system according to claim 7 or 8, in which the sensing means is arranged so that a paid signal is only produced when, in addition, the radiation intensity in at least one wavelength band is increasing at at least a predetermined rate.

10. A fire and explosion detection system of the discriminating type which is operative to produce an alarm output in response to an exploding ammunition round striking a hydrocarbon fuel tank but not in response to an exploding round not striking the tank, com- prising radiation sensing means responsive to electromagnetic radiation produced by the exploding round or by a hydrocarbon fire to generate a first signal, transducing means responsive to the striking of the fuel tank by the round to produce a second signal, and logic means operative in response to the first and second signals to produce the alarm output.

11. A system according to claim 10, in which the logic means comprises means operative to produce an alarm output both when the first and second signals occur simultaneously and when they occur serially but with less than a predetermined time separation between them.

12. A system according to claim 10 or 11, in which the logic means includes means operative to produce an alarm output in response to the first signal alone having persisted for at least a predetermined time period.

1 3. A system according to any one of claims 10 to 12, in which the logic means also includes means operative to produce an alarm output in the absence of the second signal, provided that at least two successive first signals are produced in at least a predetermined time.

14. A fire or explosion detection system substantially as described with reference to the accompanying drawings.