Patent GB-1462913-A (GB)
Title: Dual spectrum infrared fire detection system with high energy ammunition round discrimination
IPC: G08B17/12
Filed: 1974-06-12
Abstract: 1462913 Detecting fires and explosions HUGHES AIRCRAFT CO 12 June 1974 [23 July 1973] 26150/74 Heading G1A In a fire detection device capable of recognising the difference between an explosion "per se" without a subsequent fire and an explosion producing a fire, the basic long and short wave infra-red detector circuit 14 and 12 of Specification No. 1432106 is used in conjunction with an explosion detection channel 18 and logic circuit to only produce an output when a fire is present. Explosion present inside a fuel tank causing a fire. When this happens there is a relatively slow build-up of radiation. The output of the thermopile 40 and the photo diode 30 of the long and short wave channels 14 and 12 are amplified 44, 32, inverted 46, 34, and passed to threshold gates 36, 48. Thus if a fire is present or an explosion is occurring, the gates 36, 48, will produce outputs that are delayed by delays 38 and 50 before passing to an AND gate 56. This gate also receives two logical 1 signals from the explosion detection channel 18 which, in the absence of an explosion "in open air", is in its quiescent state, so the AND gate 56 output can pass via an OR gate 110 to activate some kind of fire suppression system. An explosion outside the fuel tank but no fire. When an explosion occurs detectors 30 and 40 will be activated in the normal way and so will a further photodiode 64 whose processed output signal is supplied to first and second level threshold gates 70, 72. The threshold gates 70, 72 and the associated logic circuitry of monostable multi-vibrators 86 and 96, and gates 90 and 84, and delay circuits 88, 94 and 98, are designed to respond to the characteristic intensity pattern of an explosion in "open air" and operate a system of delays, including a 25 second delay introduced by multi-vibrator 96, to provide logical 0 signals on either of lines 58 or 60 during the explosion, so that in conjunction with the delays introduced into channels 12 and 14, the AND gate 56 does not produce a logical 1 output. A detailed explanation using waveforms Fig. 2 (not shown) is disclosed. An explosion outside the fuel tank - then a secondary fire. During the 25 second delay provided by the "Q" output of the monostable multivibrator 96, a logical 1 signal is provided via a delay, 65 millisecs, circuit 98 to an AND gate 102. Thus if a fire develops after the explosion and within the 25 second delay imposed by multi-vibrator 96, then the first level gate 20 will be triggered once again by the rising incident radiation intensity to provide another logical 1 to the AND gate 102. Thus there will be a logical 1 output from AND gate 102 which, on passing via OR gate 110, activates the fire suppression apparatus. An explosion partly in the tank and partly outside the tank causing a fire. This type of explosion is characterized by a narrower intensity peak Fig. 4a (not shown) than the "normal" explosion in "open air", and the signal passing through the second level threshold gate 72 is of a shorter duration than the delay imposed by a delay circuit 86, 5 millisecs, on the signal from a multi-vibrator 86 so that signal from the delay circuit is not present when the signal on line 82 from the gate 76 is present, and so the 25 second multi-vibrator 96 is not activated. Thus by the time that the signals from detectors 30 and 40 have passed through the delays 38 and 50, the channel 18 is back in its quiescent state wherein two logical 1 outputs are on lines 58 and 60. The description discloses several waveforms Figs. 2, 3 and 4 (not shown) showing the circuit operation.