Abstract:
Described herein is a self-powered system for detecting a current spike. The current spike is delivered through a current return network that energizes a resonant circuit to produce an alternating electrical output. The output is rectified by a rectifier into direct output that is then transferred to an integrator circuit. The integrator circuit slowly builds to and dissipates from a voltage threshold corresponding to an output transistor. When the output transistor is triggered by the voltage threshold this trigger is communicated to a fault monitoring software that recognizes the current spike.

Description:
BACKGROUND 
       [0001]    The current disclosure relates to a current spike detector and more specifically to an electrical circuit for detecting lightning strikes in aircraft. 
         [0002]    Lightning strikes on aircraft are relatively rare events, yet occur with sufficient frequency and are sufficiently damaging to mechanical and electrical systems that lightning strike prevention and mitigation are important components within an aircraft. 
         [0003]    Reporting of lightning strikes, their location, intensity, and effects are presently handled by the aircraft and ground crews. The aircraft crew is responsible for determining whether the aircraft has been struck, typically through visual confirmation or short-lived electrical interference of instrumentation or lighting. The ground crew is then tasked to determine the severity, location, and impact of the lightning strike on the flightworthiness of the aircraft. However, because this system is based on an initial determination by the aircraft crew, this system of reporting may result in over or under reporting of strikes. 
         [0004]    When lightning strikes an aircraft a large amount of current passes through the aircraft. In a metal skinned aircraft, this current is predominantly carried over the exterior surfaces of the aircraft. However, aircraft using composite construction often incorporate a lightning or current return network of electrical connectors, installed within the aircraft, to carry the large currents which result from lightning strikes. In such composite aircraft, this current may be directed through a current return network that reduces the chance of electrical systems being damaged by the current spike. However, some temporary electrical system failures may still occur. When these systems recover, they issue a warning, typically to the cockpit although they may be recorded elsewhere, that must be checked by a maintenance crew when the aircraft is next grounded. 
         [0005]    Some of these electrical system warnings may be so-called “nuisance warnings” that occur due to the system resetting because of the lightning strike rather than any problem with the system. However, these systems will still require manual resetting by a maintenance worker to reset the warnings and determine if the fault was due to the lightning strike event or a problem in the affected system. 
         [0006]    Therefore, there is recognized a need in the art for a lightning detection system. 
       SUMMARY 
       [0007]    Described herein is a self-powered system for detecting a lightning strike on aircraft that include a current return network. A resonant circuit is in communication with the current return network and provides an alternating current electrical output. A rectifier receives this output and transforms it into a direct output that is transferred to an integrator circuit. As the integrator circuit is charged, it reaches or crosses a voltage threshold that in turn triggers an output transistor. The actuation of this transistor communicates a signal to a fault monitoring software. 
         [0008]    According to further embodiments, the resonant circuit may include a transistor and capacitor while the integrator circuit may include a capacitor that is charged at or above the threshold voltage for several seconds. 
         [0009]    Further described is a method for detecting a current spike in a current return network. A ferrite core is in inductive communication with the current return network and an inductor that forms a part of a detection circuit. The detection circuit includes a resonant circuit, an integrator circuit, and a transistor. When a current spike is passed through the current return network a current is provided in the resonant circuit through induction. The resonant circuit creates an alternating current that is rectified and transferred to the integrator circuit. When the integrator circuit builds to a threshold level the transistor&#39;s state is changed. This change in state is therefore indicative of the voltage spike. 
         [0010]    According to further improvements, the rectifier may be a half-wave rectifier and the resonant circuit may include an inductor and capacitor. Further, the ferrite core may be either permanently or removably coupled to the current return network. According to one embodiment, the transistor is a normally open n-MOSFET that allows a current to pass through when the system is activated. The passage of this current is used to determine the change in state. 
         [0011]    Also disclosed is a method for determining whether lightning has struck an aircraft. This method includes the steps of utilizing an existing current return network throughout the aircraft and providing a passive electrical circuit in inductive communication with the current return network. When a lightning strike passes through the current return network, an electrical current is induced in the passive electrical circuit. This current provides a voltage differential to a transistor that is triggered when the voltage differential reaches a threshold level. The state of this transistor is therefore indicative of whether the aircraft has been struck by lightning. 
         [0012]    According to further embodiments, the transistor may be normally-open so that current passing through the transistor indicates a lightning strike. Alternatively, the transistor may be normally closed so that a lightning strike is indicated by current no longer passing through the transistor. 
         [0013]    The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a perspective view of an aircraft showing a current return network. 
           [0015]      FIG. 2  is a schematic view showing the attachment between the current return network and a detection circuit. 
           [0016]      FIG. 3A  is a schematic view of the detection circuit. 
           [0017]      FIG. 3B  is a schematic view of an alternative embodiment of the detection circuit. 
       
    
    
     DESCRIPTION 
       [0018]      FIG. 1  shows a perspective view of an aircraft  100  cutaway to show a current return network  102  that may include longitudinal  104  and lateral  106  electrically conductive elements extending along a substantial portion of the aircraft  100 . The current return network  102  also includes current carrying paths  108  extending through the wings and tail of the aircraft  100 . The longitudinal  104  and lateral  106  elements as well as the current carrying paths  108  may be low resistance electrical wires, metal, or other conductive material including but not limited to aircraft structural elements, hydraulic lines, or dedicated current return components. These elements  104 ,  106 ,  108  of the current return network  102  may be connected to one another to provide a number of redundant electrical pathways that may be adapted to carry fault current, provide grounding, carry lightning current, provide electromagnetic shielding, minimize resistance and voltage differentials and provide a bleed path for electrostatic charge. 
         [0019]    As shown in  FIG. 2 , a lightning strike detection device  112  may include a clamp-on magnetically permeable core with windings  114  that is secured about a portion of the current return network  102  and a lightning strike detection circuit  116  in communication with the ferrite core  114 . The clamp-on ferrite core  114  is a closed loop of high-magnetic permeability material, such as iron, ferrous-oxide coated ceramics, or other material. The ferrite core  114  may be a removable or permanent device attached to the current return network. 
         [0020]      FIG. 3A  illustrates the lightning strike detection circuit  116  in further detail. As shown in this figure, the circuit  116  may include a resonant circuit  118 , an integrator circuit  120 , and a transistor  122  connected to external monitoring equipment  124 . The resonant circuit  118  may include an inductor  126  and first capacitor  128  in parallel that is in parallel with and coupled to the integrator circuit  120  by a rectifying diode  130 . The integrator circuit  120  includes a resistor  132  and second capacitor  134  in parallel. The integrator circuit  120  is tied to the gate  136  of the transistor  122  and the transistor source  138  goes to ground. The transistor itself  122  is shown as a normally-open enhancement-mode n-MOSFET that provides a voltage controlled current source between the external monitoring equipment  124  and ground. Electromagnetic interference (EMI) reduction elements are provided in the form of a zener diode  142  and drain diode  144  that reduce the chance of current feedback or voltage spikes that may damage the circuit  116 . 
         [0021]    According to one embodiment, a number of lightning strike detection devices  112  are positioned about the current return network  102  so as to capture a lightning strike event. With reference to  FIG. 2 , the devices  112  would preferably be positioned in the Zone  3  areas and may be positioned in Zone  1  or  2  areas such as on or near the engine nacelles or along the aircraft fuselage. 
         [0022]    With reference to  FIG. 2 , the operation of the lightning strike device  112  will be described in further detail. As will be appreciated, when there is no lightning strike event, the lightning detection circuit  116  will remain in an unpowered state. However, when there is a lightning strike event, the circuit will be energized to indicate the event. After the event has been indicated, the circuit will reset to an unpowered state. 
         [0023]    When lightning strikes the aircraft, a current spike lasting from 1-50 μs is transferred to the current return network  102 . The current return network  102  forms at least one loop around the ferrite core  114  and the inductor  126  forms a number of loops about the ferrite core  114 , thus forming a transformer so that when a current pulse passes through the current return network  102 , the inductor  126  generates a complementary current. 
         [0024]    The inductor  126  and first capacitor  128  that form the resonant circuit  118  will create an alternating current output that energizes the circuit  116 . The current output from the resonant circuit  118  is rectified by the rectifying diode  130  to a half-wave output before being transferred to the integrator circuit  120 . 
         [0025]    The integrator circuit  120  provides a slow charge and discharge for the second capacitor  134 , which preferably maintains the voltage difference across the capacitor at a threshold level for several seconds. The integrator circuit provides hold time and automatic resetting for the lightning indication to remain active even if the external monitoring equipment  124  is itself upset by the event. 
         [0026]    When the second capacitor  134  is at a threshold level measured by the gate threshold of the transistor  122 , the circuit between the source  138  and drain  140  is closed, allowing current to flow through the transistor from the external monitoring equipment  124  to ground. The external monitoring equipment  124  is an external monitoring apparatus that may be installed in the aircraft, and may be a standard aircraft equipment interface, such as an open/ground discrete which senses the electrical open/ground discrete signal made by the circuit  116  and then uses this for fault or maintenance indication logic. 
         [0027]    The zener diode  142  ensures that the voltage from the transistor gate to source does not reach a level that might damage the transistor  122  or other components. 
         [0028]    The drain diode  144  is positioned between the external monitoring equipment  124  and transistor  122  and allows current to flow from the external monitoring equipment  124  through the transistor. This arrangement ensures that current cannot flow from the transistor  122  to the external monitoring equipment  124  and cause damage in case of a current spike in the ground. 
         [0029]    Other improvements to the above-described electrical circuit are also contemplated. According to the embodiment illustrated, the circuit includes an n-MOSFET transistor  122  that is normally open and closes when a positive voltage is applied at the gate  136 , allowing current to flow between the source  138  and drain  140 . However, it is contemplated that a normally closed depletion mode MOSFET may be substituted for the normally open enhancement mode transistor  122 . 
         [0030]    The rectifying diode  130  is shown as a single diode in series between the tank circuit and integrator circuit. This half-wave rectifier only passes half of the resonant waveform generated by the resonant circuit  118  and therefore the amount of energy passed is reduced. However, this diode may be replaced with a full wave rectifier, such as a diode bridge, or other type of rectifier if additional energy is required. 
         [0031]    The circuit  116  has also been described as including EMI protection elements such as the zener diode  142  and drain diode  144 . These elements are included to provide protection against voltage overload of the transistor  122  (zener diode  142 ) or current feedback to the external monitoring equipment  124 . However, these elements are not necessary to operation of the circuit and may be omitted. Alternative protective elements may be included either in lieu of or in addition to these protective elements. 
         [0032]    The resonant circuit  118  provides an alternating current based on the lightning strike waveform, but is not necessary to provide a voltage differential to the integrator circuit  120 . A current spike in the current return network  102  would produce a corresponding voltage spike in the inductor  126  that could be used to drive the transistor  122 . However, the resonant circuit  118  provides the additional advantage of providing a bandlimit function to reduce the sensitivity of the circuit to radio frequency (RF) noise, for example from precipitation static or other RF noise. 
         [0033]    As a passive element with a single wire connection (current return network  112 ), the addition of a built in test for the detection circuit  116  may not be appropriate. The test function may be accomplished by the addition of a second set of windings on the ferrite core  114  that can provide a pulse to the circuit  116  to simulate a lightning strike. This would serve as an effective test to determine that the system is functioning properly. 
         [0034]    As described with reference to  FIG. 1 , the current return network  102  may serve as a ground for the electrical components of the aircraft. However, the surge from a lightning strike through the current return network  102  is often what causes electrical failures in various systems of the aircraft. Therefore, it may be undesirable to use the current return network  102  as a ground for the lightning strike detection circuit  116 . According to one embodiment shown in  FIG. 3A  the external monitoring equipment  124  is connected to a ground, such as the current return network  102 , and the lightning strike detection circuit  116  is connected to an independent ground.  FIG. 3B  shows an alternative arrangement where the lightning strike detection circuit  116  and external monitoring equipment share a common ground  146 , which may be an independent ground. 
         [0035]    While the method and forms of apparatus disclosed herein constitute preferred aspects of the disclosed lightning detection apparatus and method, other methods and forms of apparatus may be employed without departing from the scope of the invention.