Abstract:
An embodiment of an LED warning light includes a plurality of LED banks, each including a series of light emitting diodes. LED failure in one or more banks can result in a warning light that appears to be functional, but may not meet relevant standards for light production. The LED warning light monitors current flow through each LED bank to detect failure of an LED and produce a failure signal. The LED warning light includes a microcontroller programmed to evaluate the failure signals and take one or more pre-determined failure mode actions. Failure detection and failure mode actions are defined by program steps taken by firmware running in the microcontroller.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates generally to external lighting for aircraft and more particularly to an LED aviation warning light incorporating redundancy and fault detection features.  
         [0003]     2. Description of the Related Art  
         [0004]     Civilian air traffic control agencies such as the FAA specify standards for aircraft external lighting. Aircraft operating at night in civilian airspace are required to display lights to attract the attention of other aircraft operating in the same airspace. These external aircraft lights include flashing anti-collision lights mounted on the aircraft&#39;s upper and lower fuselage, as well as position/navigation lights on the tail and the wing tips. The location, color, intensity and light radiation pattern for each particular light is typically specified by the relevant regulation.  
         [0005]     Aircraft external lighting have previously been provided by “strobe” lights or incandescent lamps. Incandescent and strobe lamps suffer from relatively high power consumption and relatively short service life.  
         [0006]     With advances in the efficiency of light output from light emitting diodes (LEDs), it is now possible to replace incandescent and strobe lamps with LED light sources. LED light sources are attractive because of their extremely long service life and relatively low power consumption. High-output LEDs, such as the Luxeon™ emitter from LUMILEDS™ of San Jose, Calif., in certain configurations can achieve the required light output and radiation pattern for an aircraft position light.  
         [0007]     Strobe and incandescent light sources typically employ a single light source, making failure of that light source readily apparent upon inspection. An LED light source may employ multiple LED light sources due to the relatively low quantity of light produced by each LED. If all of the LEDs are arranged in series, failure of any individual LED would extinguish the entire light. Alternatively, subsets of LEDs could be arranged in a series/parallel configuration such that failure of any one LED would extinguish only the LEDs in that series branch, with the remaining LEDs continuing to operate. This type of redundancy prevents total failure of the light assembly. However, partial failure of an aircraft warning light may result in a light that fails to meet the requirements of the relevant regulation, while appearing to function normally to the typical observer/inspector. There is a need in the art for an LED aviation warning light configured to continue to function after failure of one or more LEDs that is also configured to detect failure of the LED light sources and provide some indication of that failure.  
       SUMMARY OF THE INVENTION  
       [0008]     It is accordingly an object of the invention to provide an LED warning light that detects failure of one or more LEDs in the warning light and is programmable to take one or more predetermined failure mode actions.  
         [0009]     An embodiment of an LED warning light includes a plurality of LED banks, each including a series of light emitting diodes. A bank driver circuit is arranged to apply a drive current to each LED bank in response to a first input and to interrupt current to each LED bank in response to a second input. A bank sense circuit is connected to sense current flow through each LED bank and produce a first output indicative of normal current flow through said LED bank or a second output indicative of abnormal current flow through said LED bank. A microcontroller is programmed to produce the first and second inputs to said bank driver circuits and responsive to the presence of the second output to take a predetermined failure mode action.  
         [0010]     Failure detection and failure mode actions are defined by program steps taken by firmware running in the microcontroller. For example, the microcontroller firmware can be configured to compensate for noise or spurious signals in the warning light to reduce the likelihood of false failure indications. Failure mode of the warning signal light is flexibly configurable by modification of the firmware. Examples of failure mode actions are shutting off the LEDs, changing the pattern of inputs to the bank driver circuits or generating a local failure indication or remotely detectable failure signal.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a functional block diagram of an embodiment of an LED aviation warning light according to aspects of the present invention;  
         [0012]      FIG. 2  is a schematic of the LED aviation warning light of  FIG. 1 ;  
         [0013]      FIG. 3  is a software flow chart for a microcontroller program for use in the LED aviation warning light of  FIGS. 1 and 2 ;  
         [0014]      FIG. 4  is a flowchart of a subroutine for use in conjunction with the microcontroller program of  FIG. 3 ; and  
         [0015]      FIG. 5  is a flowchart of a subroutine for use in conjunction with the microcontroller program of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]     A preferred embodiment of an LED aviation warning light will now be described with reference to  FIGS. 1-5 , wherein like numbers refer to similar parts.  FIG. 1  is a functional block diagram of an exemplary LED aviation warning light  10  according to aspects of the present invention. The LED aviation warning light  10  is connected to a power bus  12  of an aircraft through a power bus filter  14 . Filtered electrical power is delivered to a “N” LED bank circuits  16 , one for each series bank or branch of LEDs in the warning light. Electrical power is also delivered to a subsystem power circuit  18  that produces regulated low voltages (5VDC, 8VDC) for use by the microcontroller  20  and associated circuitry.  
         [0017]     Each bank circuit  16  may include a bank filter  22 . Filtering between the electrical system and the aviation warning light  10  and/or bank circuits  16  protects the warning light from voltage spikes in the aircraft electrical system and also prevents noise from the warning light from feeding back into the aircraft electrical system.  
         [0018]     Each bank circuit  16  includes a bank driver  24  configured to provide a constant current sinking path from each LED bank  26  to ground. The bank drivers  24  are controlled by the microcontroller  20 . Bank sense circuits  28  are arranged to produce a first input to a microcontroller indicating normal current flow through each LED bank  26  or a second input to the microcontroller corresponding to a failure detected in an LED bank  26  and/or bank driver  24 . Upon detection of a failure in an LED bank  26  and/or bank driver  24 , the microcontroller  20  is programmed to take one of several possible actions, including providing a diagnostic indicator of the failure.  
         [0019]     A schematic of an exemplary LED aviation warning light  10  is shown in  FIG. 2 . LED banks B 1 , B 2 , B 3 , B 4  are shown connected between filtered  28  VDC aircraft power and a bank circuit. Each bank driver  24 , includes a voltage regulator (U 1 , U 2 , U 3 , U 4 ) arranged in a constant current configuration through respective Darlington pair transistors (Q 1 , Q 2 , Q 3 , Q 4 ). Each Darlington pair transistor (Q 1 , Q 2 , Q 3 , Q 4 ) is controlled by a transistor (Q 6 , Q 8 , Q 10  and Q 12 , respectively), which are in turn controlled by a common signal from the microcontroller  20  (U 8 ). The current flow path through each LED bank  26  passes through the input and output of the voltage regulator (U 1 , U 2 , U 3 , U 4 ), a 4 resistor network and the Darlington pair transistor (Q 1 , Q 2 , Q 3 , Q 4 ) to ground.  
         [0020]     Bank sense circuits  28  employ transistors (Q 5 , Q 7 , Q 9  and Q 11 ) to sense current flow in the current flow path and provide a fault indication to microcontroller  20  (U 8 ) in the absence of current flow through an LED bank. It will be understood that failure of an LED in the series of LEDs of an LED bank will result in an open circuit and the current flow through the bank having a failed LED will drop to zero. In the illustrated circuit, a logic level high at microcontroller inputs RC 0 , RC 1 , RC 2 , and RC 3  indicates normal functioning of LED banks B 1  through B 4 . Absence of current flow through the 4 resistor network of a respective bank driver will turn off the failure detection transistor (Q 5 , Q 7 , Q 9  or Q 11 ) corresponding to the failed bank and result in a logic level low at the corresponding input of the microcontroller.  
         [0021]     The microcontroller is provided with 5 VDC power from the sub-system circuit  18 . 8 VDC is provided to a synchronization circuit  30 . The synchronization circuit allows the flash pattern of multiple LED aviation warning lights to be synchronized. The synchronization feature does not form part of the present invention and will not be described in any greater detail herein. Microcontroller  20  (U 8 ) is provided with clock pulses at a low frequency of 32.768 KHz to minimize production of high frequency RF noise.  
         [0022]     Relevant portions of the software of microcontroller  20  (U 8 ) will now be discussed with reference to  FIGS. 3-5 . The software routine is of the polling type, running in a continuous loop.  FIG. 3  illustrates a flowchart of the main polling routine  40 . On power up, the software initializes the RAM and hardware registers of the microcontroller. The main routine  40  then checks a polling interval and resets a polling interval counter. The software then checks whether the LEDs are off. If the LEDs are off, the answer at  46  is yes and the software proceeds to check whether the interval counter equals the off period at  48 . If not, the software proceeds to increment the interval counter and reset the timer to zero. If the interval counter equals the off period at  48 , the main routine resets the on/off interval counter and turns on the LEDs. At the next polling increment the answer at  46  is no, i.e., the LEDs are on. The main routine proceeds to check if the fault detected equals true at  50 . If fault detected equals true at  50 , the microcontroller is programmed to take a predetermined action.  
         [0023]      FIG. 3  illustrates several alternative actions such as turning off the LEDs  52   a , turning on a fault indicator  52   b , or altering the flash timing or period  52   c . Failure mode of the present LED aviation warning light will be discussed in greater detail below. If the fault detected is not true at  50 , the routine queries whether it is time to read LED status at  54 . If the answer at  54  is yes, the main routine proceeds to subroutine A 1  illustrated in  FIG. 4 . Subroutine A 1  includes the step of debouncing the status lines (RC 0 , RC 1 , RC 2 , RC 3  inputs to microcontroller  20  (U 8 )). Debouncing is a software routine that checks the condition of the status line over a period of time to eliminate spurious inputs such as voltage spikes or noise. Subroutine A 1  then reads the status lines. At  56  the subroutine checks whether the read value indicates proper functioning of each LED bank. If the answer at  56  is yes, a failure counter is set to zero and the subroutine returns to the main routine at A 4 . If the answer at  56  is no, the subroutine increments the failure counter and returns to the main routine at A 4 . The main routine then checks at  58  to see if it is time to test the bad cycle count. If the answer at  58  is yes, the main routine enters subroutine A 2  shown in  FIG. 5 .  
         [0024]     Subroutine A 2  compares the fail counter incremented in subroutine A 1  to a predetermined consecutive bad cycle count at  60 . The consecutive bad cycle count allows the failure detection function of the present invention to ignore momentary or spurious conditions affecting an LED bank by setting the consecutive bad cycle count in excess of 1. Typically, the consecutive bad cycle count will be set between 2 and 10. When the fail counter is incremented in subroutine A 1  to the point where it equals the predetermined consecutive bad cycle count at  60 , subroutine A 2  sets the fault detected to true and returns to the main routine at A 5 . If the answer at  60  is no, the subroutine returns to the main routine at A 5 .  
         [0025]     The main routine checks whether the interval counter equals the on period at  62 . If yes, the on/off counter is reset, the LEDs are turned off and the main routine returns to step  44 . It can be seen that the outcome of the fault detection query at  50  is determined by subroutines A 1  and A 2  which are in turn responsive to the condition of microcontroller inputs RC 0  RC 1 , RC 2  and RC 3 . The bank sense circuits  28  determine the status of inputs RC 0 , RC 1 , RC 2  and RC 3 , as shown in  FIG. 2 .  
         [0026]     For many practical reasons, it is desirable to configure an LED aviation warning light to drive multiple series strings, or banks of LEDs. One result of driving individual banks of LEDs is that failure of an LED in one bank will not extinguish the LEDs of the remaining banks. Thus it is possible for one or more banks of such an LED aviation warning light to fail, resulting in a reduced light output. To most observers, the LED aviation warning light will appear to be functional, but the warning light may not meet the specified light output.  
         [0027]     To avoid the situation where such a partially failed LED aviation warning light continues in service for an extended period, the present invention includes a failure detection circuit as discussed above. It is possible to configure failure detection circuits using discrete components. However, a failure detection circuit having the capability shown in subroutines A 1  and A 2  would be exceedingly complex. Further, employing a programmable microcontroller permits alteration of the subroutines, main routine, and/or failure mode function without alteration of the LED aviation warning light hardware. For example, the consecutive bad cycle count could be raised or lowered to allow the failure detection to ignore anomalies present in the circuit.  
         [0028]     One or more failure mode actions may be programmed into the microcontroller. One possible failure mode action would be to turn off all the banks of LEDs, giving a positive indication of failure. An alternate failure mode action would be to produce an electronic failure signal. Currently, aviation wiring systems are not equipped to receive such a fault indication. Such a fault indication might be converted to a visual signal by turning on a fault indicating LED located on the LED aviation warning light housing or otherwise visible to an inspector. A further alternative failure mode may include altering the flash timing or period of the aviation warning light as an alternative positive indication of failure. Currently, it is contemplated that the failure mode will be to turn off all the banks of LEDs.  
         [0029]     While the invention has been described in terms of specific embodiments, those skilled in the art will recognize that modification of the invention can be made without departing from the spirit and the scope of the appended claims.