Transient suppression device and method of packaging the same

A transient suppression device for limiting to desirable levels current and energy entering a fuel tank of an aircraft over interface wiring that penetrates a wall of the fuel tank comprises: an elongated, hollow, conductive housing capped at one end and open at another end, the housing being disposed external and in proximity to the fuel tank, and supported physically from a structure of the aircraft; a transient suppression circuit disposed within the housing and connected in series with the interface wiring; and a shield braid of conductive material disposed over the open end of the housing and covering the interface wiring over the distance between the housing and tank wall, the shield braid electrically coupling the housing to the tank wall. A method of packaging the transient suppression device is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

U.S. patent application Ser. No. 10/695,040, entitled “Float Switch Transient Suppression Device”, filed on Oct. 27, 2003, which is assigned to the same assignee as the instant application.

U.S. patent application Ser. No. 10/427,460, entitled “Apparatus and Method For Protecting The Safe Side Wiring of a Protective Barrier Against Transferring Fault Energy Into a Potentially Explosive Environment”, filed May 1, 2003, which application being assigned to the same assignee as the instant application.

BACKGROUND OF THE INVENTION

The present invention relates to transient suppression devices, in general, and more particularly, to a transient suppression device (TSD) being located external and in proximity to an aircraft fuel tank for limiting to desirable levels the current and energy of signals that may enter the fuel tank over interface wiring that penetrates a wall of the fuel tank, and a method of packaging the same.

An example of a float switch application in a combustible liquid is in connection with a transport aircraft on which one or more float switches is or are disposed within a fuel tank for monitoring the fuel level thereof. The float switch may comprise a magnetic reed switch that is sealed within a tube, for example. A float within the switch tube moves with the level of fuel and renders an electrical connection when the fuel rises above or drops below a predetermined fuel level. Each float switch is electrically connected to an interface circuit external to the tank by electrical wiring which passes through a wall of the tank. The electrical wiring interconnecting the float switch with its interface circuit is disposed within a sealed conduit which keeps the wiring from making contact with the fuel.

The float switch interface circuits vary in function with the aircraft. On some aircraft, the interface circuit acts as a safety device or back-up to prevent overfilling the tank during a fueling or re-fueling process. In this application, the interface circuit comprises a shut-off valve which typically uses currents on the order of two amps maximum, for example. On other aircraft, the interface circuit functions to initiate a fuel transfer between internal tanks of the aircraft. In this application, the interface circuit comprises a fuel transfer valve which typically uses currents on the order of sixty to one-hundred milliamps, for example. In some cases, the float switch may also drive a low fuel level indicator lamp. Such drive current levels are passed into the fuel tank through the interconnecting wiring and float switch.

Under normal operating conditions, a clear separation is maintained between the switch and wiring and the fuel to avoid the possibility of fuel vapor ignition. However, there are possible internal fault conditions in which fuel vapor may make contact with the wiring and/or switch contacts. For example, the conduit seal or the sealed tube or the connection between the conduit and tube may leak permitting fuel vapor to enter the sealed chambers. Other potential fault conditions include: chafing of the wires within the conduit resulting in potential exposure to fuel vapor; and hot short to the aircraft wiring. With the current and energy levels of the present float switch designs, any one of these fault conditions poses the risk of an explosive reaction. Another area of concern is with external threats, like lightning, high voltage wiring shorts, and high intensity radiated fields, for example, passing unsafe current and/or energy levels into the tank through the interconnecting switch wiring.

The Federal Aviation Administration or FAA has been considering these potentially threatening conditions and is on the verge of promulgating mandatory regulations to limit the current and energy levels permitted to pass into an aircraft fuel tank under any conditions, including normal operating conditions. These regulated levels of current and/or energy are intended to prevent an ignition of in-tank fuel vapors under all possible conditions. Under the anticipated FAA regulations, current levels of less than ten milliamps are considered safe, but current levels from ten to thirty milliamps will require an explanation of safeness. RMS current levels greater than thirty milliamps are considered unsafe under the anticipated regulations. In addition, the anticipated FAA regulations limit the energy permitted to enter the tank to less than two-hundred microjoules.

Accordingly, in order to comply with the anticipated FAA regulations, it may be necessary to modify the present float switch system on aircraft to reduce the normal in-tank operating current and energy of such systems while maintaining the current and energy needed to drive the interface electronics for all possible applications, and to protect against potentially threatening conditions as noted above. The present invention is intended to provide for these modifications.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a transient suppression device for limiting to desirable levels current and energy entering a fuel tank of an aircraft over interface wiring that penetrates a wall of the fuel tank comprises: an elongated, hollow, conductive housing capped at one end and open at another end, the housing being disposed external and in proximity to the fuel tank, and supported physically from a structure of the aircraft; a transient suppression circuit disposed within the housing and connected in series with the interface wiring; and a shield braid of conductive material disposed over the open end of the housing and covering the interface wiring over the distance between the housing and tank wall, the shield braid electrically coupling the housing to the tank wall.

In accordance with another aspect of the present invention, a method of packaging a transient suppression device on board an aircraft to limit to desirable levels current and energy entering a fuel tank of an aircraft over interface wiring that penetrates a wall of the fuel tank comprises the steps of: disposing circuitry of the transient suppression device into an elongated, hollow conductive housing; capping one end of the housing; passing interface wiring that is exposed to potential threats into the housing through an aperture in the capped end thereof; connecting the exposed interface wiring to an unprotected side of the transient suppression circuitry; passing interface wiring that penetrates a wall of the fuel tank into the housing through an open end thereof; connecting the wall penetration interface wiring to a protected side of the transient suppression circuitry; covering the wall penetration wiring between the housing and the tank wall with a conductive shield braid; securing the shield braid to the housing and tank wall to form an electrical coupling therebetween; and physically supporting the housing from a structure of the aircraft at a location external and in proximity to the tank wall.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a block diagram schematic of a float switch system10including a transient suppression device in accordance with one aspect of the present invention. Referring toFIG. 1, a conventional float switch12, which may be a magnetically operated reed switch, for example, is disposed within a container14containing a combustible liquid for monitoring the level thereof in the container. For the present embodiment, the container14may be a fuel tank of an aircraft, for example, but it is understood that the present invention is not limited in application to aircraft fuel tanks and may extend to any tank containing a combustible liquid that may be ignited by an electrical spark or the like. In any event, the float switch12includes contacts16and18which are electrically coupled to a transient suppression device (TSD)20over wires22and24, respectively. The float switch12is operative to render an electrical connection between the contacts16and18when the liquid within the container14rises above or drops below a predetermined liquid or fuel level.

In one embodiment, the TSD20is disposed in close proximity to a wall26of the container, preferably on the order of twelve inches away, for example. As noted above, within the tank or container14, the wires22and24are contained within a sealed conduit (not shown) to maintain separation from the fuel or liquid. This conduit provides a sealed passage of the wires22and24from the switch12to a sealable opening in the wall26through which they pass to the TSD20. Between the tank wall26and TSD20, the wires22and24may be contained within a conductive shield27which is connected at one end to the wall26and at the other end to a conductive enclosure28of the TSD20. The enclosure28may be connected to an electrical ground at30, which may be a frame or structure of an aircraft, for example. Shield27acts as an electrical barrier against potential threats coupling or attaching to the wires22and24by diverting any such threat energy to the aircraft frame through which it is sufficiently absorbed and mitigated.

Within the enclosure28, the wires22and24are series coupled through a current limiting circuit or circuits32and a voltage limiting circuit34is coupled across the wires22and24. The current limiting circuits32are designed for the present embodiment to limit the current entering the tank14over wire22and24to current and energy levels considered safe by the FAA. The voltage limiting circuit34is designed for the present embodiment to protect the circuitry of the TSD20against damaging voltage levels due to short duration threats, like lightning, for example. Upstream of the current limiting circuits32is a circuit36which is operative to monitor the operation of the float switch12and simulate the contact status to drive an interface circuit40accordingly. The interface circuit40, which may be a liquid flow control valve, a relay and/or an indicator lamp, for example, may be disposed a substantial distance from the TSD20and coupled to the circuit36thereof over wires42and44, for example. The drive current of the interface circuit40is considered unsafe by the FAA for entering the tank14. Note that in operation, the drive current of the interface circuit40is conducted through the circuit36of the TSD20and does not enter the tank14through the float switch12as will become better understood from the following description.

A circuit schematic of a circuit suitable for embodying the TSD20is depicted by way of example inFIG. 2. Referring toFIG. 2, in series with each wire22and24is an isolation resistor network50and52, respectively. These resistor networks50and52afford passive isolation and voltage limiting to absorb and thus, prevent undesired or unsafe current and energy levels from entering the fuel tank14over wires22and24which are on the “safe side” of the TSD20. An exemplary embodiment of a suitable resistor network for50and52is shown inFIG. 3. In the embodiment ofFIG. 3, four pairs of parallel connected resistors are connected in series to form the resistor network. If the worst case in-tank fault condition is presumed to be a fifty ohm resistance between either or both wires22and24to ground, then all of the resistors of the network may be valued at approximately six thousand ohms, for example, in order to provide sufficient current limiting to the presumed in-tank fault condition. Note that at six thousand ohms per resistor, the lumped or effective resistance of each network50and52is twelve thousand ohms.

The parallel/series network embodiment ofFIG. 3was chosen based on several limitations such as, for example,: (1) the series resistance path should have a minimum physical separation to prevent arcing across a resistor body due to voltage potential, which separation being enhanced by the addition of more than one resistor to achieve the desired maximum potential voltage present; (2) a parallel path enables redundant separation to achieve an extremely improbable shorting condition; and (3) the parallel/series impedance network should survive and provide the desired safety levels when exposed to aircraft and environmental threat conditions.

Referring back toFIG. 2, the simulation circuit36of the present embodiment offers a high side drive connection (H) or a low side drive connection (L) to the interface circuit40and a return or ground connection (G). An example of a high side drive application is shown by the schematic diagram ofFIG. 4in which the interface circuit is embodied by an inductive coil54which may be part of a relay circuit or a solenoid for driving a flow valve, for example. In the high side drive case, a DC voltage supply which may be +28 V may be coupled to the H connection and the coil54may be coupled across the L and G connections. An example of a low side drive application is shown by the schematic diagram ofFIG. 5in which the interface circuit is embodied by the inductive coil54which may be also part of a relay circuit or a solenoid for driving a flow valve, for example. In the low side drive case, the coil54is coupled between a DC voltage supply which may be +28 V and the H connection and the L and G connections are coupled together.

Referring back toFIG. 2, in both foregoing described examples, the circuitry of the TSD20is powered by the DC voltage supply provided through the H connection to a supply bus56and a current return is provided by a ground bus58which is connected back to the supply through the G connection. Capacitors C1and C2are coupled from wires22and24, respectively, on the safe side of the TSD20to the ground bus58. One side of network50is connected to wire22and the other side is connected to a current mirror circuit comprising PNP transistors Q1and Q2which are coupled together through their base junctions. The collector of Q1is coupled to its base and the other side of network50. The emitter of Q1is coupled to the supply bus56through a resistor R1which may be on the order of 100 ohms for the present design. A zener diode Z1is coupled cathode to anode across the supply bus56and collector junction of Q1for protecting the base-emitter junction of Q1against voltage breakdown. In addition, the emitter of Q2is coupled to the supply bus56through a resistor R2which may also be approximately 100 ohms.

Further, the collector of Q2is coupled to another current mirror comprising NPN transistors Q3and Q4which are coupled together through the base junctions thereof. More specifically, the collector of Q2is connected to the collector of Q3and the emitter of Q3is coupled to the ground bus58through a resistor R3which may be approximately 100 ohms. The collector of Q4is coupled to its base and to the supply bus56through a resistor R4which may be on the order of50K ohms, for example. The emitter of Q4is coupled to the ground bus58through a resistor R5which may be approximately 100 ohms. The collector of Q2is also connected to a base of another NPN transistor Q5which is coupled to the ground bus58through a zener diode Z2which protects the base-emitter junction of Q5against voltage breakdown. The emitter of Q5is connected to the ground bus58and the collector of Q5is coupled through a parallel connection of resistors R6and R7to a gate of a field effect transistor (FET) Q6which may be a metal oxide semiconductor FET for the present design.

The gate of Q6is also coupled to the supply bus56through the parallel connection of resistor R8and zener diode Z3. The source of Q6is connected to the supply bus56. The drain of Q6which is the L connection is coupled to the ground bus58through a parallel connection of a capacitor C3and a pair of series connected transorbs T1and T2which may be of the type manufactured by International Semiconductor under the part no. SMLJ40A, for example. Moreover, the supply bus56which is the H connection is coupled to the ground bus58through a parallel connection of a capacitor C4and a pair of series connected transorbs T3and T4which may be of the same type as transorbs T1and T2. Capacitors C3and C4are operative to bypass electromagnetic threats to ground potential.

In operation, current of the current mirror of Q3and Q4is set by the resistor R4and the voltage of the supply bus56. For the present design, this current may be approximately one-half a milliamp. Current of the current mirror of Q1and Q2is set by the series resistance of the two networks50and52and the voltage of the supply bus56. For the present design, this current may be approximately a milliamp. Accordingly, when the float switch12is closed, current flows through Q1and the float switch12limited by the resistor networks50and52. A like valued current flows through Q2by the current mirror effect. However, since the current set to flow through Q3is only one-half of the current flowing through Q2, the remainder of Q2current is conducted to the base of Q5and renders Q5conducting.

With Q5conducting, the voltage at the gate of Q6is dropped sufficiently below the source voltage thereof to force Q6into conduction. In the high side drive connection, when Q6is conducting, the supply voltage is applied to the L connection, thus energizing the coil54(seeFIG. 4). In the low side drive connection, when Q6is conducting, current is permitted to flow through coil54from the voltage source through the H and L connections to ground return. In either case, the driving current for the interface circuit flows only through the FET Q6and the current flowing to the float switch12is limited by the resistor networks50and52to less than one milliamp in the present design.

If the float switch12is open-circuited, no current may flow through Q1or Q2as a result of the current mirror effect. Since no current flows through Q2, there is no current to drive Q5into conduction and it remains open circuited. Accordingly, the gate to source voltage of Q6is insufficient to cause conduction thereof, and therefore, Q6is rendered open circuited. Thus, when switch12is open circuited, no drive current is provided to the interface circuit40by the TSD20. In this manner, under normal operation, the TSD20monitors the status of the float switch12with a minimal amount of current which is considered well within the safe levels of the anticipated FAA regulations and drives the interface circuit40at sufficient current levels which do not pass into the tank14.

In the event a short duration threat, like a lightning strike or high intensity radiated pulse, for example, is coupled to one or both of the wires42and44at the unprotected side of the TSD20, the resistor networks50and52and corresponding capacitors C1and C2will absorb most of the current and energy of the coupled threat. These short duration threats are not expected to last more than around70microseconds with a peak voltage of 1200 volts in the worst case. The current and energy which may pass through the resistor network-capacitor combination as a result of the short duration threat is expected to be well within the energy levels of the anticipated FAA regulations. The transorbs T1–T4and capacitors C3and C4at the unprotected side will protect the circuitry of TSD20against over voltage damage as a result of and permit the circuitry to survive these short duration threat conditions.

Also, should the one or both of the wires on the unprotected side of the TSD20be shorted to a high voltage source, like 115 VAC, for example, for a sustained period of time, the resistor networks50and52would keep the current passed to the tank over wires22and24to levels considered safe by the anticipated FAA regulations. However, under these conditions, the circuitry of the TSD20may not survive.

The advantages of this aspect of the present invention are: (1) an embodiment of the float switch TSD may use inexpensive and reliable components, like resistor isolation networks, transorbs and capacitors, for example, for current and voltage limiting to protect the tank of combustible liquid from undesirable and unsafe current and energy levels under all possible conditions; (2) normal operating currents are maintained well within levels considered safe on the protected side of the TSD for monitoring the status of the float switch; and, in turn, (3) sufficient currents are provided through a simulation circuit of the TSD for driving the interface circuitry in accordance with the monitored status of the float switch.

In some existing aircraft installations, as shown by way of example in the illustration ofFIG. 6, a single wire70connects the coil72of an existing aircraft interface to one contact16of the float switch12. Wire70passes through a pressure seal or bulkhead74of the aircraft and the wall26of the tank14. The other contact18of the float switch12is connected to a ground76of the aircraft by a wire78, the ground connection76being between the pressure seal74and tank wall26. To install the foregoing described embodiment of the float switch TSD20to the existing float switch system may require adding additional wires through at least the pressure seal74which is a timely and costly procedure. Referring toFIG. 7, one solution to avoid this procedure and use existing aircraft wiring is to add a passive TSD80in series with the wires70and78between the pressure seal74and tank wall26for protecting the tank against undesirable current and energy levels. For example, the passive TSD80may comprise the resistor network50in series with wire70, resistor network52in series with wire78, and capacitors C1and C2connected to the ground76, as described herein above in connection withFIG. 2.

However, for the coil72to operate, the voltage drop across the float switch12should be very small. Adding the passive TSD80in series with the wires70and78as proposed in the embodiment ofFIG. 7will increase this voltage drop when the switch12is closed, possibly to the point of rendering the interface coil72inoperative. So, merely adding the passive TSD80using the existing aircraft wiring will not solve the problem completely. To resolve this issue, an active circuit, referred to as a signal translation unit (STU),82may be disposed in proximity to the existing aircraft interface device72and powered by the same power source which may be +28 VDC, for example. The STU82may function to sense the status or impedance change of the float switch and TSD80over existing signal line70and in response, drive the coil72with a low impedance switch. Thus, by providing the drive to operate the coil72, the STU82reduces the normal operating current to the passive TSD80to within desirable limits, allows for greater operating voltage drop across the TSD80and obviates the need for a pressure seal penetration to add additional wiring.

In one embodiment as shown by way of example inFIG. 8, the STU82may comprise a micro relay90, for example. The relay90may be connected between the power source +28 VDC and the existing wire70and include a normally open contact92as shown and/or a normally closed contact. The relay contact92may be disposed in series with the connection between the coil72and aircraft ground94in proximity to the coil72. A drive current to operate the micro relay90is typically around a milliamp, for example. Thus, when the float switch12closes, the voltage drop across the passive TSD80will not defeat the operation of the relay90. For example, if the resistance of the passive TSD is around twenty-four thousand ohms, then the voltage drop thereacross at one milliamp is approximately twenty-four volts, leaving around four volts to drive the relay90which is more than sufficient.

Accordingly, when the float switch12closes, the relay90is energized and contact92is closed permitting the coil72to be energized through the low impedance of the switch contact92. When switch12open circuits, little or no current is conducted over wire70and relay90is de-energized, thus opening contact92and de-energizing the coil72. The passive TSD80will limit the current and energy coupled over wire70to the tank14to safe and desirable levels under all conditions as described herein above in connection with the embodiment ofFIG. 2and the relay90will be protected against pulsed high energy threats over wire70by appropriate selection of the relay such that it is not susceptible to pulsed high energy threats.

In an alternate embodiment of the STU82as shown by way of example inFIG. 9, current mirror circuitry similar to the circuitry described in connection with the embodiment ofFIG. 2may be included. Circuit elements common to the embodiments ofFIGS. 2 and 9will retain there reference numerals and not be described again here. Referring toFIG. 9, the collector of Q1may be connected to the passive TSD80, preferably the resistor network50, over the existing wire70at the connection node100. The connection node100may be coupled to the aircraft ground94, which is in proximity to the coil interface72, in series with two transorbs T5and T6which may be of the same type as transorbs T1–T4, for example. A capacitor C5is coupled between node100and ground94in parallel with the transorbs T5and T6. The power source +28 VDC, for example, is connected to the supply bus56at a circuit node102to provide power to the active circuit elements of82. In addition, the source of MOSFET Q6is connected to the coil72.

In operation, the active circuit elements of82perform the same functions as described for the embodiment ofFIG. 2. Accordingly, the MOSFET Q6conducts current to energize coil72when the float switch is closed and open circuits to de-energize coil72when the float switch is open. Current that flows over wire70under normal float switch operation will be limited to the resistance of the passive TSD80which may be approximately one-half milliamp as described supra. Transorbs T5and T6and capacitor C5protect the circuitry of82against transient threats coupled over wire70, like lightning and pulsed high intensity radiated fields, for example. However, while the current and energy levels to the tank are maintained at safe and desirable levels from a steady state short of wire70to a high voltage like 115 VAC, for example, by the passive TSD80, the circuitry of82may not survive such a threat.

In accordance with yet another aspect of the present invention, apparatus for packaging the TSD electronics is secured in place at the tank wall penetration where the float wires pass through the tank wall to the float switch disposed within the tank as described supra. An illustration of an exemplary package suitable for embodying this aspect of the present invention is shown inFIG. 10. Referring toFIG. 10, the package comprises a conductive hollow enclosure or housing110, which may be a tube machined from brass, for example. The tube110is capped at one end112and open at the other end114. In the present embodiment, the diameter and length of the tube110are approximately one inch and five inches, respectively. However, it is understood that the housing110is not limited to a tube shape, but may take upon other shapes without deviating from the broad principles of this aspect of the present invention.

A small aperture is provided through the capped end112to permit passage of the wire70into the housing110and connected to the unprotected side thereof. Once connection is made to wire70, the TSD electronics, which may be fabricated on a printed circuit (PC) card of suitable dimensions, for example, is disposed into the tube110through the open end114. Prior to insertion into the housing110, the PC card may be coated with a protective layer, like a conformal coating, for example, to protect the TSD electronics from moisture and other contaminates, which may affect the circuits thereof. Wires22and24from the contacts of the float switch are connected to the safe side of the TSD as described above. Once wires22and24are connected to the TSD electronics, the inside of the housing110is fully encapsulated with an insulating compound, like an RTV™ compound, for example, to provide structural integrity to the TSD electronics.

Also in the present embodiment, a shield braid116encloses the wires22and24between the safe side of the TSD and tank penetration to maintain the protection thereof. One end118of the shield braid116is passed over the conductive housing110to form a snug fit therebetween. The end118of the shield braid116may be soldered or brazed to the conductive surface of the housing110to form a distributed electrical contact around the perimeter of the conductive surface of the housing110. The shield braid116may be on the order of a foot to a foot and a half in length. The wires22and24penetrate the tank wall26through a conduit as mentioned above. As shown in the illustration ofFIG. 11, a portion120of the conduit protrudes out externally from the tank wall26. The other end122of the shield braid116may be slid over the conduit portion120and clamped in place with a conventional C clamp124, for example, to form a distributed electrical connection around the perimeter of the conduit120. In an aircraft, the tank wall is in contact with and supported by the aircraft frame which forms a solid and distributed ground throughout the aircraft. Thus, the shield braid116is in direct electrical contact with the airframe via the tank wall26. Accordingly, with the present embodiment, current and energy coupled from the unprotected side of the TSD by a fault or threat will be dissipated to the aircraft ground via the conductive housing110, shield braid116and tank wall26, and not be coupled to the wires22and24.

In addition, the housing110is supported in place from the tank or aircraft frame by a mounting clamp. In the present embodiment, a P type clamp130may be fitted over the outside mounting surface of the tube110and mounted to the tank wall26utilizing a cantilevered mounting structure132which may be secured to the wall26by bolts134, for example. The clamp130and support structure132may offer a further electrical pathway to aircraft ground for threat and fault energy dissipation. Test point leads from the TSD PC card may be provided from the housing110through additional apertures in the capped end112, for example, for testing the functionality, conductivity and system fault isolation of the TSD electronics, in situ.

Accordingly, the foregoing described packaging configuration provides for dissipative electrical shielding and mechanical structure for protection of the internal TSD electronics and an attachment surface for external mounting support. The configuration of the present embodiment offers many advantages such as: (1) the tubular housing allows the package to be structurally supported using conventional supporting hardware which permits greater flexibility in choosing mounting locations and obviates the need for unique mounting brackets; (2) it is inexpensive to manufacture; (3) it offers physically no opportunity for threats to couple into sensitive safe side wires; and (4) it provides for speedy installation with minimal aircraft modification.

While the present invention has been described herein above in connection with one or more embodiments, it is understood that such embodiments were provided by way of example and not intended to limit the invention in any way. Accordingly, the present invention should be construed in breadth and broad scope in accordance with the recitation of the claims appended hereto.