US3406389A

US3406389A - Fire warning systems

Info

Publication number: US3406389A
Application number: US481849A
Authority: US
Inventors: James C Nailen
Original assignee: Whittaker Corp
Current assignee: Whittaker Corp
Priority date: 1965-08-23
Filing date: 1965-08-23
Publication date: 1968-10-15
Anticipated expiration: 1985-10-15
Also published as: DE1285920B; GB1148952A

Description

Oct. 15, 1968 J. C. NAILEN 3,406,389

I FIRE WARNING SYSTEMS Filed Aug. 23. 1965 3 Sheets-Sheet l d PO -i LL .9

2| F I I- I I I g; I I I I I I I. I v Q c s I 2 8 5 I 1? I I M m I N I N I 3 a I I I O I I MMHJ L I g a I I I II w F a I 3 N I I N I qh I I o al I I James C. Noilen, I INVENTOR. BY. L W M- u ATTORNEY.

J. C. NAILEN FIRE WARNING SYSTEMS Oct. 15, 1968 3 Sheets-Sheet 3 Filed Aug. 23, 1965 INVENTOR.

.ATTOR .l

Un'itecl States Patent 3,406,389 FIRE WARNING SYSTEMS James C. Nailen, Santa Ana, Calif., assignor, by mesne assignments, to Whittaker Corporation, Los Angeles, Calif., a corporation of California Filed Aug. 23, 1965, Ser. No. 481,849 17 Claims. (Cl. 340-411) ABSTRACT OF THE DISCLOSURE A fire warning system for use with heat sensors of the type wherein a heat sensitive salt insulates coaxial conductors under normal conditions and reduces in resistance when heated. The system discriminates between the relatively slow resistance decrease representing heat, and also the rapid resistance drop of short circuit producing faults in the salt, and actuates an appropriate alarm. If the system senses a short circuit, it applies a plurality of electrical pulses across the salt which, in most instances, will cure the fault and restore the heat sensor to an operational condition.

This invention relates in general to fire warning systems and relates in particular to fire warning systems for aircraft.

More specifically, this invention is an improvement in aircraft fire warning systems so that the crew is reliably informed when the aircraft is, in fact, on fire or that an area is overheating; that this warning is trustworthy and can be relied upon; and that this warning is not a false alarm. In order to accomplish this, the invention is able to detect and, in almost all cases, cure the system of defects which cause, by far, the largest number of false alarms.

In addition to the incorporation of reliability and certitude into existing fire warning systems, this invention also includes other improvements in existing fire warning systems to give the crew still more information, such as, malfunctioning of the system due to a grounded wire; but first, in order to understand the problem solved by this invention and how this invention works, an explanation of one type of existing fire warning systems is in order.

The existing fire warning systems in aircraft comprise one or more heat sensing units or sensors located where overheating or fire might be expected to occur, for example, they are located adjacent to or wrapped around each engine, or located in wheel well areas, or in any other location which has combustibles fed thereto. These heat sensors vary in length and are connected in series with a pilot test button and a fire warning light both of which are conventionally located in the cockpit in such a position that the pilot and/0r engineer can cut the flow of combustibles to the area indicated by the lighting of the fire warning light to reduce the danger to the aircraft. The pilot test button permits the crew to test the continuity of this system before the flight or during flight in order to determine whether the system is in operation.

Thus, in the existing systems, the crew members have two bits of information fed to them: (I) the system is in operation (by depressing the pilot test button to light the fire warning light) and (2) the fire warning light which indicates (theoretically) that there is fire and/or overheating in some particular area.

'ice

Under law, if a fire warning light becomes lit and the aircraft is on the ground, the pilot must not take off or, if in flight, must make a turn around or seek the nearest adequate airport and abort the flight to avoid the possibility of a disaster.

Unfortunately, however, the existing fire warning systems are subject to a serious defect. This defect is the fact that these systems will often indicate a fire when, in fact, none exists and thus give out a. false alarm. Unfortunately, too, experience has shown that about 90% of the fire warnings have been false alarms with the result that flight abortions and the cost thereof to the airlines are unduly high. For example, it is reliably estimated that the cost of each flight abortion is between $10,000 and $25,- 000 to say nothing of passenger ill-will incurred toward the airline due to the inconvenience caused.

Thus, when it is realized that about 90% of the total cost of flight abortions in any given period is unnecessary simply because present fire warning systems give out an excessively high number of false alarms, it can be appreciated that a solution to this problem becomes quite paramount. It should also be borne in mind that when the pilot is required by law to seek the nearest airport, he is required to land unscheduled at an unfamiliar field which, itself, is dangerous. Equally as important, however, is the fact that, since 90% of the fire warnings are false alarms, pilots, knowing this, will unfortunately tend to ignore or become complacent about the fire Warning light.

Numerous attempts in the past have been made to solve this problem of unnecessary high incidence of false alarms in fire warning systems and the typical solution suggested is the use of alternate circuits between the heat sensors and the fire warning light and coupled in such a manner that at least two of such circuits must be in operation before the fire warning light will go on. This system assumes, of course, that the heat sensor will short both circuits and thereby light the fire warning light and assumes that it is unlikely that shorts, such as grounded wires, in a number of circuits will occur at the same time. This system, of course, does increase the reliability of fire warning systems somewhat but does not really solve the problem of false alarms because experience has shown that a short due to the grounding of a wire in the circuit accounted for only about 1% of the total number false alarms; the remaining number (89%90%) of false alarms is due to a fault, i.e., a defect or short, occurring in the sensor itself which cannot be distinguished by this system. In other words, the suggested system, at best, would only decrease the number of false alarms by about 1%.

The present invention, on the other hand, not only will detect a short due to a fault which has occurred in the heat sensor and which would normally cause the fire warning light to go on, but will attempt to and will (in nearly all cases) cure this fault, and will do this automatically before the fire'warning light now has a chance to go on thereby not alarming the crew. If unable to cure pect of this invention, the reliability of fire warning sys-. terns is increased by the elimination of 90% of the fire warnings which have false alarms leaving only about 1% of the fire warnings due to false alarms due to short circuits, such as grounded wires, to be detected. These latter false alarms cannot be cured by this aspect of the invention, but this invention does include a detector which identifies these shorts and advises the crew that there is such a short in the system and that they can no longer rely on this system to detect a fire. As a matter of fact, with this invention not only is there a warning of a short in the system, but the fire warning light is prevented from lighting when this occurs so that the crew will not mistake the fact that a short exists in the system and the crew can no longer rely on the system to warn of a fire.

Now, in order to understand how the present invention solves the aforementioned problems and overcomes the deficiencies of the present fire warning systems and obsoletes the numerous attempts in the past to solve these problems, an explanation of the existing systems, and particularly the heat sensors in more detail is in order at this time.

A heat sensor comprises a hollow cylindrical tube or shell with a coaxial conductor separated from the wall of the outer tube by a salt. These sensors can vary in length from about three feet to 25 feet and have a diameter of about 0.1 inch with the radial distance from the center conductor to the wall of the outer tube being about 0.02 inch. The shell is usually made of Inconel, a very high temperature nickel-iron alloy, and the center conductor is nickel.

The salt between the center conductor is eutectic in that it is a compound designed to undergo a structural change at some predetermined temperature and is of a type chosen to electrically insulate the center conductor from the outer shell at low temperatures, and drop sharply in resistance, i.e., conduct current at some predetermined higher temperature. Thus, when the center conductor is connected to a source of current and an impedance measuring device, any drop in resistance due to the heating of the sensor caused by the salt losing its ability to insulate the center conductor from the outer shell will be detected or sensed by the impedance measuring device. In the present fire warning systems, the detection or sensing of a drop in resistance is visually referenced for the pilot by the energization of the aforementioned fire warning light coupled into the circuit.

Thus, as can be appreciated, much depends on the salt in the heat sensor for the correct operation of existing fire warning systems and heretofore the real problem of false alarms, namely, the malfunctioning of the salt was left unsolved. It was discovered, however, that if small amounts of electrical energy (usually one to three pulses of such energy) are applied to the center conductor, the defects or faults can be removed from the sensor. It was also found that in nearly all cases such faults were cured permanently.

The discovery and cure of such faults forms a major aspect of this invention, as previously stated, and the 'following is an explanation as to why such faults can be cured in the manner taught by this invention.

It is reasoned that if the salt is pure in form and voidof any defects, no conduction will occur from the center conductor to the outer shell unless heat is applied to the sensor. It is also reasoned that the salt has a very definite lattice structure and in such lattice structure there are a number of defects which may occur and which will affect the insulative properties thereof, because the current carr'iers in the crystalline structure (i.e., electrons, protons,

cations, anions, or any combination thereof) are affected.

These defects are well known; for example, there are' (l) Frenkel defects where either a cation or anion is removed from its interstitial position, to a point where it can no longerreturn to that position leaving a hole in the lattice structure and a free ion, (2) Schottky defects where both the anion and the cation lattice points are deserted, thus preserving the electrical neutrality of the crystalline interior, (3) planar slippage of the interstitial ions of the lattice structure, and (4) a dendr1t1c formulation of the lattice interstitial ion alteration.

Of these defects, the Frenkel and Schottky defects can be due to temperature, impurities, voltage gradients, and extreme sonic vibration. The other defects are also due to the same factors plus a mechanical stress such as caused by the difference in the coefficient of expansion of the material used in the sensors upon the application of heat thereto, or by mechanical shocks such as vibrations or twistin of the sensors.

Thefefore, with dendritic and planar faults created by physical stresses and mechanical shocks, 1t becomes clear that heating and cooling of the sensor as well as twisting, etc., during installation could create such faults. It is even suspected that some faults may be created during the manufacture of the sensor and which are subsequently cleared but which are reintroduced during the operation of the aircraft by reason of vibration. It seems also clear that the heating or the cooling of the whole sensor as a complete body is of no avail since the same faults Wlll reoccur when the sensor is heated or is cooled as the case may be. This also may explain why it is difficult to determine the cause of a false alarm because the fault maydisappear when the sensor is cooled. f

Accordingly, since these dendritic faults conslst o fern-like formations protruding from the inner .center conductor and from the inner wall of the outer shell through the crystal thus causing current circuits, and since the unstable energy state caused by these faults are extremely minute, they are disturbed by very small amounts of external energy appropr ately applied, that is, applied only through the fault region, leaving the surrounding crystalline structure undisturbed.

It is believed, therefore, that by passing a sufficient electrical current through the center conductor to cause an Internal thermal heating sufficient to overcome the valence bonds of the lattice structure, the crystal is allowed. to melt and resolidify into its lowest energy state which 1s that of the pure crystalline lattice structure. Since the pure crystalline lattice structure precludes the existence of free ions, electrons and other current earners, the crystal is once again a perfect dielectric and insulator.

It was found that the energy required to cure these faults are rather small which also bears out the above theoretical explanation of these fault shorts. In fact, it was determined that in most cases the fault can be cured by discharging 20 volts from a 0.5 mfd. capacltor, and in a few stubborn cases a discharge of volts from a 1 mfd. capacitor was required. Furthermore, it was found that the energy introduced into the sensor was so minute that it did not disturb the balance of the crystalline structure or other adjacent electrical devices, and the more times the sensor was cleared, the harder it became to make a fault occur. Thus, an essentially faultless sensor.may be obtained after several clearings.

Thus, from the foregoing, it can be seen that the high occurrence of false alarms can be eliminated by the teachings of this invention by the introduction of a clearing or curing pulse through the sensor when the short occurs and is reflected into the remainder of the fire warning system and it is to this end that this invention is primarily directed.

In addition to the foregoing principal object of this invention being accomplished, it is still another object of this invention to provide a system by which the decrease in resistance due to a fault occurring in the sensor may be sensed or detected and the sensor pulsed by the supply of DC voltage to cure the fault. This is accomplished in the" present invention by providing a circuit which is responsive' to the decrease in resistance in the sensor and which will act to pulse the sensor with the supply of DC voltage.

Furthermore, in order to make the existing fire warning paths or short.

systems more reliable and to give the crew still more in formation, this invention further includes a means by which true short circuits, that is, shorts occurring by reason of a grounded wire or the like, may be detected and the crew advised thereof.

As a further refinement to the existing fire warning systems, this invention further includes means which enable the crew to rely on the fact that when they are advised of a short existing in the system, there is in fact a short and the system can no longer detect a fire. In other words, with this invention, the crew can be reliably informed that a fire actually exists or, alternatively, that a short has occurred in the system which is not due to a fault in the sensor.

Other additional features and advantages of this invention will become apparent to those skilled in the art from the following specifications and drawings which form a part thereof and wherein:

, FIG. 1 is a schematic illustration of a present fire warning system;

FIG. 2 illustrates the present invention schematically in block diagrams and incorporated in, and forming part of, the present fire warning system of FIG. 1;

FIGS. 3 and 3a are schematic illustrations of the circuit involved in the block diagrams of FIGS. 1 and 2; and

FIG. 4 is a graph of the temperature-resistance and time relationship in a fire warning system.

The present fire warning system is shown in FIG. 1, in block diagrams and separated in FIG. 2 to illustrate clearly how this invention may be simply coupled into (and form part of) the present fire warning system without interference to the latters performance or operation.

Turning now specifically to FIG. 1, it can be seen that the existing system comprises a plurality of heat sensors; one being illustrated herein simply for the purpose of describing the system and designated as 10. This heat sensor comprises outer metallic tube or shell 12 having a coaxial center conductor 14 separated by a heat sensitive insulative material, usually eutectic salt, which at low temperatures electrically insulates the center conductor 14 from the shell 12 and thus acts like a very high resistance, almost an open circuit at the current level of this system, but which at elevated temperatures will lose its high resistance and conduct current therethrough substantially shorting the center conductor 14 to ground through tube 12. The existing system is also provided with a detector comprising in this embodiment a low impedance generator or a magnetic amplifier 18 connected between the center conductor 14 of the sensor 10 by a main sensing line 20 and a fire warning light 22. The detector loop for the fire warning system comprises magnetic amplifier 18, which applies AC current through line 20 and line 24 and to both ends of center conductor 14; contact 26 of a normally closed single pole double throw pilot test relay 28, being closed at this time. The coil 30 of the pilot test relay 28 is coupled to a pilot test switch 32 which in turn is connected to the ships supply of DC voltage (+28 v.). The closing of the pilot test switch 32 throws the relay swinger 34 to contact 36 which connects line 24 to ground energizing fire warning light 22.

In the existing system, as herebefore mentioned, the fire warning light 22 and the pilot test switch 32 are located in the cockpit adjacent the handle connection to the supply of all combustibles fed to each of the engines and to the landing gear. Usually, the system also has a buzzer connected in series with the fire warning light 22 to call the crews attention to the fact that the fire warning light is on.

Under normal condition, the magnetic amplifier 18, supplying AC current through the main sensing line 20 and sensing the reflected impedance therein will not energize the fire warning light 22, but normal procedure calls for the pilot to test the system by closing pilot test switch 32 to close contact 36 and thus ground the magnetic amplifier 18 which will, of course, temporarily energize fire warning light 22. However, while testing the continuity of the system, this test assumes that the magnetic amplifier 18 is working normally, that none of the connections between the various sensors are broken and the test is indicative that the center conductor 14 is unbroken. Nonetheless, since the sensing line 20 is shorted to ground (thus energizing the fire warning light 22), a short to ground, a short in relay 28, or a fire cannot be distinguished. This means that, if the fire warning light 22 were to go on at any time, the pilot cannot tell by this continuity test whether the plane is in fact on fire or whether the short is caused by some other means. Unfortunately, since sensors often develop faults or shorts, as discussed previously, which are reflected in the magnetic amplifier 18 and in the fire warning light 22 even when in fact there is no fire in the aircraft, the information available to the crew, (i.e., fire warning light 22 and the continuity test by closing pilot switch test 32) cannot inform them whether there is a short due to a fault in the sensor, a short due to grounding a Wire, or a fire. Thus, as required by law, the pilot must either not take off, or if in flight, must make a turn around (or seek the nearest suitable airport) and abort his flight. As hereinabove mentioned, this costs the airlines from $10,000 to $25,000 for each abort as well as much passenger ill-will and often causes the pilot to land unscheduled at an unfamiliar field. T00, and perhaps equally as important, is the fact that since of the fire warnings are false alarms, pilots knowing the unreliability of the system, will unfortunately tend to be complacent about the fire Warning light.

As hereinabove mentioned, this invention will detect and cure faults in the sensors without affecting the performance or operability of existing systems but necessarily incorporating more reliability into existing systems.

How the reliability is incorporated into the existing systems (either already installed in aircraft or forming part of newly installed systems) by the disclosed embodiment oFfIinvention will now be described in connection with In this figure, for purposes of clarity, most of the existing system, i.e., the sensor 10 and pilot test relay 28, have been placed within the phantomized block diagram in the upper left hand corner in FIG. 2 above the magnetic amplifier 18.

Taking first the means for curing a short due to a fault in the sensor itself, it can be seen that this cure is accomplished in the embodiment disclosed by applying one or more pulses of high DC voltage from a clearing supply 40 through a clearing relay 42 through the main sensing line to the center conductor 14 of the sensor 10. This pulse, or pulses, of high DC voltage returns the eutectic salt to its origlnal state of high resistance, as discussed supra. (It is to be noted parenthetically at this point that, while DC voltage is applied in this embodiment, an AC voltage could be used as, for example, from a blocking or square wave oscillator.)

Secondly, this figure shows the means to detect or sense the occurrence of the fault so as to actuate the clearing relay 42.

This means for detecting a fault and to actuate the clearmg relay 42 comprises an amplifier 50, a rectifier 52, a filter 54; the output of which is connected to a clearing trlgger circuit '56. Clearing trigger circuit 56 is connected to a clearing cycle relay limiter 58. Although considered as part of the fault sensing means, the clearing cycle limiter 58 is to limit the action of the clearing relay 42 to a predetermined number of cycles so that the sensor is pulsed not more than, for example, 3 times should a single pulse be insutficient to cure the fault. The reason for this limitation on number of times the clearing relay 42 is cycled is to allow the continued decrease in resistance in the sensor due to heat to thereafter continue to be reflected in the system to warn of a fire or a short as the case maybe; it being known by experiment that if the resistance in the main sensing line 20 cannot be restored to its high level, i.e., the fault cured, by three or four pulses of high DC voltage, the decrease is not due to a fault but due to a fire or a short other than a fault. An inherent advantage in the limitation of the number of cycles of operations of this switching relay 42 is, of course, the preservation of the life of the relay and thereby increase the reliability of present invention, as part of a fire warning system. In other Words, while a decrease in resistance due to a fire, a fault, or a short (other than a fault) will be sensed by the fault sensing means, if the decrease is caused by a fire or a short, the decrease is continual and this invention will automatically attempt to cure the decrease as if it were due to a fault; but after the sensor is pulsed the predetermined number of times, the continued decrease in this main sensing line cannot be cured by this invention. It can be appreciated, therefore, that while this invention will act automatically to attempt to cure a fault, it will not interfere 'with a true fire indication as sensed by the sensor, nor will it interfere with a short (a true grounding short) indicated by this invention to be in the system as will now be described.

It is to be understood that this short warning indication informs the crew that a short exists in the system, which short is not due to a fault in the sensor or due to a fire.

This short indicator means is illustrated in FIG. 2 and comprises a short indicating light 60 and the means to energize and control the light comprising the aforesaid amplifier 50 and rectifier 52, a second filter 62 connected to rectifier 52 and a short trigger circuit 64. The short trigger circuit 64 is coupled to a gate circuit 66, a short detector 68 and a relay driver 70 for actuating switching relay 72. Gate circuit 66 is also coupled to a time delay 74 which prevents the short trigger 64 from operating the short detector 68 unless the short trigger 64 is operated within a predetermined time. If the short trigger 64 is unable to operate the short detector 68 and the relay driver 70 within this short time delay, the magnetic amplifier will energize the fire warning light to advise the crew that a fire in fact exists.

In order to fully explain the function and operation of the clearing pulse circuit and the short light energizing control means, reference is now directed to FIG. 4.

This figure is a graph which shows the relationship between the sensor temperature and resistance as a function of time. Curve 80 shows the resistance drop in a sensor such as heated by a relatively cool fire (700 degrees F. to 1000 degrees F.) and curve 82 shows a resistance drop in a sensor heated by a relatively hot fire (1600 degrees F. to 2400 degrees F.). These curves are typical of the resistance drop to be expected to be reflected in the main sensing line and in the magnetic amplifier 18. It is to be noted that at the temperature of about 100 degrees F. the resistance of the sensor, that is, the resistance of the center wire 14 in series with the eutectic salt to ground, is not less than 100 ohms and can be as high as 1 megohm and that when the sensor is fully heated to about 1000 degrees F. the resistance will be about ohms and at about 2400 degrees F. the resistance of the sensor is approximately 5. ohms. Since the

curves

80 and 82 approach the lesser resistance asymptatically, in no event does the resistance of the sensor go below the 5 ohm level due to a fire even under the worst conditions, that is, a fire near the very end of a sensor connected in parallel (such as shown in FIGS. 1 and 2). v

It should be noted also that with a hot fire (curve 82), the resistance drop from maximum level to the 100-70 ohm level has a transition time of from 100 to 200 milliseconds, but thereafter the drop in resistance becomes less and less rapid taking about one second to reach the 30-5 ohm level. The drop in resistance to the cooler fire (curve 80) takes much longer and a further drop in resistance can vary from about 300 milliseconds to five minutes, depending upon the distance of the sensor from the source of radiation. The leveling off of the resistance drop belo wthe -70 ohm level is believed to be due to re-radiation of the sensor; the hotter sensor being a white hot body. Thus, even with the hottest fire, the transition time to the lower resistance from the maximum level to the 100-70 ohm level is from 100 to 200 milliseconds, but thereafter the transition time to drop to the 5 ohm level is about 300 milliseconds.

This graph also shows that about 580 degrees F. the resistance of the sensor is about 30 ohms and the transition time to that resistance level varies from milliseconds to 300'milliseconds or more and it is at this resistance level that the magnetic amplifier 18 isset to light the fire warning light 22 in the existing fire Warning systems. Note in the graph, that curve 84 representing a short will reflect a drop in resistance to the 10070 ohm level much the same as the drop in the resistance due to a very hot fire, but thereafter the resistance continues to drop rapidly until it reaches a level between 2 to 6 ohms. The rapid drop in resistance due to a short, as shown in this graph, will have a transition time from the 100-70 ohm level to the almost zero resistance level of about one millisecond.

From the foregoing it can be seen that, if the sensitivity of the fault curing sensing means to pulse the sensor to cure a fault is set at 100-70 ohm level, the sensor will be pulsed in an attempt to cure the fault before the resistance will drop to the fire warning level-30 ohms. If the fault is cured, of course, the level in resistance returns to its maximum level but, if there is actually a fire, the resistance will continue to drop to some level below the 580 degree F. level. This means, of course too, that the sensor will be pulsed in an attempt to cure a fault if in fact a short due, for example, to a grounded wire is reflected in the magnetic amplifier. Thereafter, whether the fire warning light is energized or whether the short indicating light is energized, depends upon the transition time sensed by this invention.

Turning now to the problem of a short, this graph clearly shows that, if a short has in fact occurred, the drop in resistance to the 2-6 ohm level will occur within 130 milliseconds; whereas, even with the hottest fire, the drop in resistance to the 25-5 ohm level requires at least 3 00 or more milliseconds. Therefore, if the time delay 74 and gate 64 are arranged to delay energizing of the fire warning light by the magnetic amplifier for some period exceeding 130 milliseconds but less than the 300 milliseconds, then the short indicating circuit time would have time to energize the short warning indicator light 60. If the latter occurs, the fire warning light 22 would never be energized. On the other hand, remembering that the drop in resistance to the 6-2 ohm level Will occur, if at all, before 130 milliseconds, and if the gate circuit 66 and time delay circuit 74 are arranged to prevent the energizing of the short indicating light 60 after 130 milliseconds, the time delay and gate circuit thus represent a lockout for the short light 60 such that the short light 60 will not be energized unless it is energized prior to the 130 millisecond transition time. With the foregoing, it can be seen that this inventiontwill attempt to cure, or will cure, a fault in the sensor and the output winding which lights the fire warning light 22.

Since the foregoing is a description of a generally conventional magnetic amplifier, no further description thereof is deemed necessary herein. It should be made clear, however, that for the purposes of the present invention, the magnetic amplifier is utilized to explain the operation of the fire warning system since it is one type of impedance detector utilized to detect the decrease in resistance in the sensors due to heat and to energize a fire warning light. Other systems may have some other means for obtaining a reflected impedance and it is to be understood that the present invention will be compatible with such systems once the description of the operation of the invention and its operation as well as the other refinements herein disclosed are understood by those skilled in the art.

Attention is now directed in particular to the clearing relay 42, which connects the DC clearing supply voltage 40, to apply a pulse of high voltage to the sensor 10 when the sensing means is cycled in response to a decrease in resistance in the sensor 10.

Clearing relay 42 is a double pole, double throw, relay and shown in the'position of normal operation in the existing system, i.e., in a position in which a source of DC voltage from supply 40 is disconnected from the sensing line 20, but with one swinger 10 6 closed on contact 108 closing the sensing line 20 between magnetic amplifier 18 and sensor 10. Contact 110 connected to the DC voltage supply 40 is shown open but, upon actuation of the relay 42 to throw swingers 106 and 112, DC power supply 40 will apply a pulse of DC voltage through swinger 112, contact 110, line 116, and swinger 106 which closed onto previously open contact 118. The other contact 120 for swinger 112, shown closed, 'is connected through line 116 to the other swinger 106 and to sensing line 20 to insure operativeness of the existing fire warning system. That is to say, if the clearing relay 42 becomes inoperative, as for example, if one of the swingers should stick in either of its positions and thus be non-responsive to the relay coil 124, sensor 10 will still be coupled into the magnetic amplifier 18. This is a refinement in this invention to make it completely compatible with the present fire Warning system.

Thus, upon actuation of the relay coil 124 to connect contact 112 to line 20, a pulse of DC voltage is applied to the sensor 10 from clearing supply 40. This supply 40 comprises capacitor 130 (1 mid, 150 v.) supplied with energy from terminal 132 and diode rectifier 134 connected to the ships AC supply (115 v., 400 c.p.s.) through filter capacitor 136 and current limiting resistor 138.

The clearing relay 42 which supplies the fault clearing pulse of DC current from clearing supply 40 to sensor 10 is monitored by the sensing means which comprises aforementioned amplifier 50, rectifier 52, filter '54, clearing trigger circuit 56 and clearing cycle limiter 58 which will now be described.

The level of AC voltage (3.5 volts) from the control winding 90 of the magnetic amplifier 18 is monitored by the sensing means at relay contact 108 by the capacitance coupled amplifier 50.

Amplifier 50 is of the emitter follower type selected to have high impedance and for high current amplification and comprises a transistor 146 whose emitter is coupled to an output transformer 148 and which is provided with suitable collector and base resistance coupled in the conventional manner. The output of the transformer 148 is fed into a rectifier and voltage doubler

network comprising diodes

160, 162 and capacitor 164 to rectify the output level of voltage to about 25 v. which is then filtered before being coupled into the clearing trigger circuit 56 and into the short trigger circuit 64. The filter circuit for the clearing trigger 56 is coupled to the output of the voltage-doubler rectifier network 52 at tap 170 and com-prises a pair of

resistors

172 and 174 and capacitor 176 which is coupled to an isolation resistor 178 of the clearing trigger 56.

The clearing trigger circuit 56 as disclosed herein is a Schmitt trigger of conventional design having a pair of

NPN transistors

190 and 192. The operation of this type of trigger, a bistable one-shot multi-vibrator, depends on the amplitude of the voltage from the isolation resistor 178 coupled to the base of the normally on transistor 190 (operated at saturation). The collector of the transistor 190 is connected to the ships supply of DC voltage (+28 V. DC.) at tap 194 and to the base of the second transistor 192 through suitable collector and base resistors. The collector of the second transistor is in turn also connected to the ships supply of DC voltage through a suitable collector resistor and is also connected to line 200 which couples the clearing trigger 56 to the clearing cycle limiter 58, the time delay circuit 74 and to the short detector 68.

The clearing cycle limiter 58 comprises a limiting capacitor 210 and a capacitor discharge resistor 212 in parallel therewith, both of which are in parallel with the output of transistor 192 of the clearing trigger. Capacitor discharge resistor 212 permits the capacitor 210 which is only partially charged by one pulse from the clearing trigger circuit 58 to discharge, provided the clearing trigger circuit does not pulse the capacitor 210 again within a predetermined time.

With the actuation of the clearing relay 42, applying the clearing pulse from capacitor to the sensor 10 to cure the fault, the sensing line 20 returns to itsnormal AC voltage (3.5 v.) since the fault short in the sensor is then cut out. This rise in voltage in sensing line 20 is reflected in the clearing trigger 56 in a decrease to 25- 26+ in voltage bias on the base of transistor 190 which again returns to saturation and the second transistor 192 is returned to cut off, its original state. If a clearing of the fault does not occur, the clearing relay 42 will again be actuated by the clearing trigger 56 because of the increase in voltage again appearing at the base of the on transistor 190 to flip the trigger and to again discharge another pulse of DC from power supply 40 through the sensor. This cycling of the relay would go on, ad infinitum, without the limiting capacitor 210 in the circuit which limits the cycling of the relay 42. It can be understood that one pulse from trigger circuit 56 only partially charges limiting capacitor 210 but the size of the latter and the capacitor discharge resistor 212 are selected so that after a limited number of charges, i.e., three, the capacitor is fully charged and will not pass a pulse from the clearing trigger 56 to the relay coil 124 to actuate the relay. Once fully charged, the resistor 212 will discharge the capacitor 210 but only after either the fire warning light 22 or the short light is energized. Experience has shown however that upon the discharge of the high voltage to the sensor 10 three times, it is an indication that a true fire or permanent short exists in the system and, consequently, either the fire warning light 22 or the short light 60 will be energized in reaction to the continued decrease in resistance in the system. The resistance 212 has still another advantage and that is, it acts to place the invention in operation substantially immediately after the pilot test switch is actuated by discharging the condensor 210 only partially charged at that time.

More specifically, if the pilot switch 32 were actuated, the relay 42 would respond three times in accordance with the preselected setting of the clearing cycle limiter. Since the capacitor discharge resistor 212 allows the charge on the capacitor 210 to bleed 01f, if the pilot test button is again immediately depressed, the system will again operate three times. This is a safety feature in that it could be coincidental that the pilot test button is depressed a the same time either a short or a fire is occurring, in which case, the pressing of the ,pilot test button does not cause any delay in the actuation of the system.

Turning now to the short indicator light 60 and the means for energizing and controlling the same, which as previously mentioned comprises the aforementioned am plifier 50, rectifier and voltage doubler 52, a filter 62 coupled to the output of the rectifier, a short trigger circuit 64, a gate circuit 66 which is coupled to a time delay circuit 74 and to the short detector 68. The short detector 68 is in turn coupled to a relay driver 70 to operate the relay switch 72.

It can be seen that the short trigger 64 is coupled to the rectifier and voltage doubler 52 at tap through the second filter 62, which is substantially identical to the other filter 54 coupled to the input of the clearing trigger 56 and for that reason no further description of this filter is deemed necessary. The short trigger 64 is also identical to the clearing trigger 56 and no further description of the elements thereof is deemed necessary except that it should be noted that the output of the short trigger 64 is coupled to the collector of the first stage transistor 220 (as distinguished from the clearing trigger whose output is coupled to the second stage transistor collector). The output of this first stage is then coupled into the gate circuit 66. Gate circuit 66 is an AND or coincidence gate of conventional design, and comprises

tWo diodes

230 and 232; diode 230 being connected to the output of the short trigger 64 and diode 232 being connected to the output of the time delay circuit 74. Characteristic of this type of gate, both inputs must be brought to a predetermined level of voltage before an output is provided from the gate. The outputs of the two

diodes

230 and 232 are connected to a third or clamping diode 234 which in turn is connected to the input of the short detector 68.

The short detector 68 is a conventional bistable multivibrator comprising a pair of common emitter coupled

NPN transistors

240 and 242 whose collectors and bases are respectively coupled together and which is provided with suitable base and load resistors. In this circuit the output from the clamping diode 234 is coupled to the base of the first stage transistor 240. The first stage transistor 240 is biased to cut off or off, and the second transistor 242 is biased to saturation or on.

The short detector 68 is operated only when the two

transistors

240 and 242 are simultaneously gated by the short trigger 64 and time delay 74. The short detector 68 is maintained in an on condition by the short trigger 64 via the steering diode network, resistor 244 and diode 246; however, this path through this network does not initiate the short detector operation. On the other hand, if the short detector is operated, it pulses this relay driver 70 which is a conventional Darlington amplifier comprising a pair of transistors 250 and 252; the base of the latter being coupled to the emitter of the first stage and the collector of the second stage being coupled to relay 72.

As mentioned before, the output of the time delay circuit 74 is coupled to the gate circuit 66 as also is the short trigger output.

It should also be clear that the time delay 74 also operates to permit the magnetic amplifier to indicate a fire from the energization of the fire warning light 22 because it will prevent the short detector from energizing the relay driver and hence the short light 60 unless it does so within a predetermined time (i.e., 100 milliseconds or as required). As mentioned before, a short in the system will be reflected by an immediate drop in resistance where as the drop in resistance due to a fire takes from 300 milliseconds to minutes. At this latter time, the time delay will prevent the operation of the short trigger by reason of the bias applied to the output diode 232 in the gate 66.

The time delay 74 comprises a monostable multivibrator circuit comprising a pair of common emitter coupled

transistors

260 and 262. The collector of the first stage is coupled to a time constant network comprising capacitor 264, fixed resistance 266, and a variable resistance 268. The capacitor 264 is also coupled to a diode 270 which in turn is connected to the base of the second stage 262. As thus arranged, when the base of the first stage receives a pulse from the output from the clearing trigger, the normally off transistor 260 is rendered conducting causing a spike in the collector. circuit which is reflected into the time constant network. With the first stage 260 operating the second stage is rendered nonconducting causing a voltage increase in the collector circuit of the second stage 262 which in turn is reflected into the gate circuit 66. This voltage increase, however, is delayed by the time constant circuit the time of which is fixed by the variable resistor 268.

While the above invention has been described in connection with a magnetic amplifier, it is to be understood that this is but one example of the type of impedance which is used in a typical fire warning system. Other impedance devices may be used, for example, a transistor circuit to pulse a DC current through the sensor 10 in which case the invention may be modified to match the circuitry by one skilled in the art having understood the present invention. For example, parts of the present invention may be moduled apart from other parts (for example the amplifier and rectifier in one module and the remainder in other modules), to make the invention readily interchangeable with different fire warning systerns; it being important to understand that the major aspect of this invention is in the discovery that the pulsing of a DC voltage (or a steep wave front AC) through the sensor clears the shorts due to faults and thus reduce the high incident of false alarms and that the other modifications and refinements thereof are to make the system compatible with existing systems as well as to give the crew more information about the condition of the system than heretofore available to them. In connection with this latter, for example, it is within the purview of this invention to include a modification to the pilot test switch 32 to make it a three position switch with different resistance levels at each position which match the resistance levels prescribed in the graph (FIGURE 4) in order to test the reaction of the circuitry at each level.

Although the invention has been described with reference to a particular embodiment thereof, it is to beappreciated that many modifications are possible without departing from the scope and spirit of the invention.

What is claimed is:

1. An improvement for use in a fire warning system having a heat sensor and a detector to sense impedance change and to actuate a warning indicator at predetermined impedance level due to an increase in temperature of the heat sensor, comprising:

means coupled to said system and responsive to a decrease in impedance across the sensor for applying a plurality of electrical pulses to said heat sensor for clearing said sensor of any fault occurring therein which is reflected in said detector by a decrease in impedance.

2. An improvement for use in a fire warning system having a heat sensor and a detector to sense impedance change and to actuate a warning indicator at predetermined impedance level due to an increase in temperature of the heat sensor, comprising:

means coupled to said system for applying a plurality of electrical pulses to said heat sensor for clearing said sensor of any fault occurring therein which is reflected in said detector by a decrease in impedance, and

means for sensing and for actuating said pulsing means upon a predetermined impedance decrease.

3. An improvement for use in a fire warning system having a heat sensor and a detector to sense impedance changes and to actuate a warning indicator at predetermined impedance level due to an increase in temperature of the heat sensor, comprising:

pulsing means coupled to said system for applying a plurality of electrical pulses to said heat sensor for clearing said sensor of any fault occurring therein which is reflected in said detector by a decrease in impedance,

means for sensing and actuating said pulsing means upon a predetermined impedance decrease, and means for limiting the actuation of said pulsing means to a preselected number of pulses.

4. An improvement for use in a fire warning system having a heat sensor and a detector to sense impedance changes and to actuate a warning indicator at predetermined impedance level due to an increase in temperature of the heat sensor comprising:

means coupled to said system and responsive to a decrease in impedance across said sensor for applying a plurality of electrical pulses to said heat sensor for clearing said sensor of any fault occurring therein which is reflected in said detector by a decrease in impedance, and

means for discriminating between a decrease in impedance due to a short in the system and a decrease in impedance due to an increase in temperature of said heat sensor.

5. The improvement claimed in claim 4 further including means for actuating a short warning indicator in the event a short is detected by said discriminating means.

6. The improvement claimed in claim 5 further including means for preventing operation of said short warning indicator if the heat warning indicator is actuated and for preventing operation of said heat warning indicator if said short warning indicator is actuated.

7. In combnation with a fire warning system having at least one heat sensor and a detector to apply electrical energy to said heat sensor to detect a decrease in resistance in said system due to an increase in temperature of said heat sensor and to actuate a fire warning indicator when said resistance drops to a predetermined low level, the improvement comprising:

means responsive to a decrease in impedance across said sensor for clearing said heat sensor of any short circuits which may occur in said sensor due to factors other than heating of said sensor and which would otherwise be detected by said detector as an increase in temperature in said sensor and thus actuate said warning indicator falsely.

8. In combination with a fire warning system having at least one heat sensor and a detector to apply electrical energy to said heat sensor to detect a decrease in resistance in said heat sensor due to an increase in temperature of said heat sensor and to actuate a fire warning indicator when said resistance drops to a predetermined low level, the improvement comprising:

and means responsive to a decrease in impedance across said heat sensor for clearing said heat sensor of any short circuits which may occur in said sensor due to factors other than heating of said sensor and which would otherwise be detected by said detector as an increase in temperature in said sensor and thus actuate said warning indicator falsely, said means comprising means for applying said sensor with one or more pulses of electrical energy.

9. In combination with a fire warning system having at least one heat sensor and a detector to apply electrical energy to said heat sensor to detect a decrease in resistance in said system due to an increase in temperature of said heat sensor and to actuate a fire warning indicator when said resistance drops to a predetermined low level, the improvement comprising:

means responsive to a decrease in impedance across said heat sensor for clearing said heat sensor of any short circuits which may occur in said sensor due to factors other than heating of said sensor and which would otherwise be detected by said detector as an increase in temperature in said sensor and thus actuate said warning indicator falsely, and

means for actuating a short warning indicator in the event the resistance in said system continues to drop to a resistance level at or below the aforesaid predetermined resistance level but reaches said level within a preselected time.

10. In combination with a fire warning system having at least one heat sensor and a detector to apply electrical energy to said heat sensor to detect a decrease in resistance in said system due to an increase in temperature of said heat sensor and to actuate a fire warning indicator when said resistance drops to a predetermined low level, the improvement comprising:

means for clearing said heat sensor of any short circuits which may occur in said sensor due to factors other than heating of said sensor and which would otherwise be detected by said detector as an increase in temperature in said sensor and thus actuate said warning indicator falsely,

means responsive to a decrease in impedance across said heat sensor for actuating said clearing means,

means for limiting the number of times said actuating means operates as the resistance in said system continues to drop,

means for actuating a short warning indicator if the resistance in the system drops to a predetermined level within a predetermined time, and

means for locking out said short warning indicator actuator after said predetermined time to enable said detector to actuate said fire warning indicator.

11. In combination with a fire warning system having at least one heat sensor and a detector to apply electrical energy to said heat sensor to detect a decrease in resistance in said heat sensor due to an increase in temperature of said heat sensor and to actuate a fire warning 0 indicator when said resistance drops to a predetermined low level, the improvement comprising:

a source of voltage for clearing said heat sensor of any short circuits which may occur in said sensor due to factors other than heating of said sensor and which would otherwise be detected by said detector as an increase in temperature in said sensor and thus actuate said fire warning indicator falsely, and

means responsive to a decrease in resistance across said heat sensor for intermittently applying a source of voltage to said heat sensor.

12. A fire warning system including:

a heat sensor comprising an outer tubular shell of conductive material, an inner coaxial conductor, and a heat sensitive material between said inner center conductive material, an inner coaxial conductor, and material having high electrical resistance and insulat ing said center conductor from said outer conductor at a predetermined low temperature level and having a low resistance and shorting said center conductor from said outer conductor when said heat sensitive material reaches a predetermined higher temperature level,

means coupled to said center conductor and said outer shell of said heat sensor for applying electrical energy across said heat sensitive material,

means for sensing the drop in resistance of said heat sensitive material to a predetermined low level and for actuating a warning indicator when said resistance reaches said low level to warn of an increase in heat at said sensor, said heat sensitive material being subject to faults which also short the center conductor to the outer shell and thus when sensed by said sensing means would also actuate said warning indicator but which would be interpreted roughly as an increase in heat at said sensor, and

means for discriminating and clearing said heat sensor of faults before said warning indicator is actuated by said detector.

13. The fire warning system claimed in claim 12 wherein said last mentioned means comprises:

a source of electrical energy, and

means for intermittently coupling said source of electrical energy across said heat sensitive material to cure said heat sensitive material of faults.

14. The fire warning system as claimed in claim 13 wherein said means for intermittently coupling said source of energy to said center conductor comprises a clearing trigger, circuit and a switching relay, said clearing trigger circuit actuating said relay so that the latter will connect said source of energy to said center conductor.

15. The fire warning system as claimed in claim 14 wherein means are provided for preventing further actuation of said relay after a predetermined number of times.

16. The fire warning system claimed in claim 15 further including means for discriminating between further re- 7 l5 sistance drop in said system due to a'short other than a fault and a fire.

17..The fire warning system as claimed in claim 16 wherein said means for discriminating between a short due other than to a fault comprises:

a short trigger circuit connected to said system, a time delay circuit, a coincidence gating circuit, and a bistable multivibrator, the output of said trigger circuit being connected to said time delay and the output of said short trigger circuit being connected to said gating circuit and to said bistablemultivibrator so that when said clearing trigger and short trigger are triggered, said time I delay prevents operation of said bistablerfiult ivibrator unlessthe resistance drop in said system falls to a predetermined level within a predetermined time.

References Cited UNITED STATES PATENTSY, I

D. L. TRAFTON, Assistant Examiner.