Abstract:
An improved illuminated airfield guidance sign of the type used for marking ground traffic routes on runways and taxiways employs LED-based lamps for even illumination of the legend panels. The lamps each employ an array of white LEDs mounted on a generally cylindrical turret, and oriented with the turret axis horizontal, so that the LEDs aim their beams up, down, or laterally, and not towards the legend panels. The illumination has to bounce at least once on an interior reflective surface before reaching the legend panels. These can receive power from storage batteries that are charged from solar panels or wind turbines, with back-up power coming from the standard runway-taxiway power.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to illuminated airfield signs of the type used for marking ground traffic routes on runways and taxiways. The invention is more particularly concerned with an improved airport guidance sign having reduced power consumption, greater reliability and more even illumination across its legend panel(s) than the guidance signs of the prior art. 
     Illuminated guidance signs are commonly used at airports to identify runways and taxiways and provide ground route marking information for air crews. These guidance signs have transparent or translucent legend panels supported in a frame or case, with the case being supported on legs that provide a minimum ground clearance. Airport runway and taxiway guidance signs are an essential element of airport operation and safety, and are essential for managing ground traffic for airplanes and for maintenance personnel. The sign needs to be clearly visible at all hours, both by air crews and by maintenance crews, and non-dimming, so the illumination needs to be even so that the message on the sign can be clearly distinguished in all ambient lighting conditions and at all viewing angles. 
     The standards for airport guidance signs in the United States are established by the Federal Aviation Administration. The FAA requires a wind load and frangibility test to ensure that the sign will perform its function under severe wind conditions, but can break off under conditions of extreme force, for example if an aircraft rolls into or over the guidance sign. A frangible connector employed in the mounts for the guidance signs to provide a safe failure mode for the signs so that the danger to the aircraft is minimized. The FAA also requires that the guidance sign meets specifications for photometry, and power source specifications when drawing power from connections to the airfield circuit, i.e., runway and taxiway power. 
     In double-width, multiple-width, or larger guidance signs, and in those that have curved legend panels, the internal support structure can block the light from the guidance sign lamps, and can create undesirable shadows and bright spots in the legend panels, and this can affect their readability adversely. There have been some attempts to remedy this unevenness of lighting by including additional reflective structure within the frame of the guidance sign. One such attempt is discussed in Hansler et al. U.S. Pat. No. 6,946,975. In the approach that is discussed in that reference, there is a multiple-element light source, with the lamps producing downwardly directed beams. There are prism refractors disposed at the base of the sign, with V-shaped grooves, which are intended to diffuse the incident light so that it is more or less even when it reaches the guidance sign legend panels. While this rather complex design does improve the evenness of lighting somewhat, it does not provide even lighting across the entire panel. Moreover, the power consumption is of that design is rather high. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide an illuminated guidance sign with a lamp arrangement that creates even lighting across the legend panels of the sign, and which avoids the drawback of the prior art. 
     Another object is to provide a guidance sign with evenly illuminated legend panels, and at the same time with reduced power consumption. 
     It is yet another object to provide for even illumination of guidance signs that employ either flat or curved legend panels. 
     It is a further object to provide an illuminated guidance sign that can be powered by stored solar-generated power, or stored wind-generated power, but which can switch over to the normal runway-taxiway power at times when the stored solar-generated or wind-generated energy is insufficient for sign operation. 
     In accordance with an aspect of the present invention, an illuminated airport guidance sign includes a pair of translucent or transparent legend panels formed of a sheet of a sturdy rigid material, e.g., a clear or translucent acrylic, and a sign frame that supports the legend panels in a symmetric oppositely-facing orientation. The frame has a base that closes off the bottom of the sign; a top cover that closes off a top of the sign; and end portions that close off ends of the sign. The panels and frame form a hollow interior, in which a plurality of lamps are supported on structure between the pair of legend panels. Electrical power and control equipment disposed within the guidance sign provide electrical power to the lamps so as to produce even illumination that falls onto the panels and passes through them to make the message clearly visible. The are support legs supporting the base of the guidance sign upon a support surface alongside the runway or taxiway. 
     The guidance sign of this invention incorporates the improvement in which each of lamps is formed as an array of white LEDs supported on a cylindrical turret. These lamps are constructed so that the LEDs emit light in directions that are radially outward in respect to the central axis of the cylindrical turret. Then each multiple LED lamp is mounted on the support structure such that the cylindrical turret axis is horizontal and disposed in the direction that is generally towards the associated pair of legend panels. In this fashion, the array of LEDs emits light in the directions that is generally parallel to the legend panels, that is, towards the base and top panel, and towards the end panels, but not directly at the guidance sign legend panels. The LED arrays direct the light up, down, and parallel to the panels, so that the light bounces at least once on the internal structure before reaching the panel. A preferred guidance sign employs four LED arrays, i.e., two pairs of two, in the smaller signs. The larger signs can employ multiple LED arrays per sign module. Thus, the light from said LEDs reflects off at least one interior surface of the frame of the sign before being incident upon one of said panels. Each of the legend panels has an interior surface that has a reflective coating, and the interior portions of the sign frame also are provided with a highly reflective coating, to ensure maximal bouncing of light within the sign. The multiple reflections of the illuminating light within the guidance sign means that there are no bright spots and no dark spots over the length and height of the legend panel, and that the light impinges from all angles so that the sign is clearly visible at all angles of view. 
     These LED lamps have a much smaller power draw than the incandescent lamps. This makes it possible to power the signs with a solar array and battery, or a small wind turbine and battery, using the standard runway/taxiway electrical system to power the signs only when the solar and battery system or wind turbine and battery system does not have enough charge. 
     Preferably, each of the lamps is formed with a flat circuit board, with means for mounting onto the support structure, and with the cylindrical turret disposed on one side of the circuit board so that it projects out towards one of the legend panels. In a preferred arrangement, there are two pairs of these lamps, with two oriented towards the one panel and two towards the other. The array of LEDs on the turret can be configured as a plurality of axial rows of LEDs, with the rows being arranged at successive predetermined angles around the axis of the turret. In a preferred arrangement, there are fifty-six LEDs, in fourteen rows of four, each row angled at about 25.7 degrees from the next. The LEDs are most favorably arranged so they are electrically connected in groups of LEDs in which each LED of each group is in a different row. Then, in the case of a failure of a group (in which the LEDs thereof are extinguished and go dark) the dark LEDs of that group are distributed at different angular positions about the axis of the turret. The lamp can also be arranged so that when some predetermined number of these groups fail, e.g., four groups, then the entire lamp goes off, so it can be replaced. An additional LED placed on the circuit board lights up when there is a problem, such as a partial failure of the LED array of the lamp. This can be a colored LED, e.g., red, and allows a maintenance technician to identify a problem lamp, despite the brightness of the remaining LEDs of the lamp. 
     Some preferable features of the LED arrays include having the device connected into fourteen groups of four LEDs, not all the LEDs of each group at the same angular position, and having the device shut down when 25% of the LEDs were not lighting up. There is also a red (or green) LED on the device which comes on in the case of a failure or partial failure, to indicate which unit needs to be replaced. 
     The electrical power means for the guidance sign preferably includes a storage battery within the frame and circuitry for providing electrical power to the LED lamps from the storage battery. A solar panel or a small wind turbine generator disposed outside the sign or onto the sign, but in the near vicinity of the sign, provides power to charge the battery. There is circuitry that couples the solar panel to said rechargeable battery for charging the battery during daylight hours. A wind powered generator would not be limited to daylight hours. Also, power conductors carry runway-taxiway power to the guidance sign for back-up, and the sign includes switching circuitry, with means to detect the amount of charge on the storage battery, as well as means to switch from the rechargeable battery to the runway-taxiway power when the electrical charge falls below some predetermined threshold. This can also be used for charging the battery on days when it is too cloudy or dark for the solar panel to provide sufficient electrical power for that purpose (or with the wind turbine, when there is insufficient wind). 
     Should the battery power become insufficient for the sign to maintain photometric requirements according to the applicable FAA directive, the sign will switch its power source to draw from its connection to the airfield circuit. The guidance sign of this invention has the benefits that it imposes a near-zero load on the runway and taxiway power circuit (CCR) so long as battery power is sufficient. In the case of circuits powering only signs, the CCR could alternatively be controlled by a communications interface between the signs and the CCR, which would energize the CCR when the battery in any sign on the circuit had too low a charge, and de-energize the CCR when the battery on the circuit having the least charge became charged by the CCR to a predetermined sufficient level. The sign turns on and off under control of the runway and taxiway power circuit, as a solely CCR-powered device would do. The guidance sign function is never compromised by battery failure, i.e., when the battery power is too low to meet photometric performance requirements. 
     The above and many other objects, features, and advantages of this invention will become apparent to persons skilled in the art from the ensuing description of a preferred embodiment, which is to be read in conjunction with the accompanying Drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is an exploded assembly view of a guidance sign according to one preferred embodiment of this invention. 
         FIGS. 2 and 3  are front and side elevational views of this embodiment. 
         FIGS. 4 ,  5  and  6  are a perspective view of an LED lamp employed in this embodiment, an axial end view thereof, and a side elevation thereof, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the Drawing, and initially to  FIG. 1 , a curved-face airport guidance sign  10  of this invention is of the illuminated type, which has curved transparent or translucent legend panels  12  supported in a metal case or frame  14 . The panels  12  may be formed from sheets or panels of a sturdy plastic material, i.e., a substrate such as acrylic, with a letter, number or symbol appearing on the outer surface. Other substrates, e.g., polycarbonate or a hybrid, could be used. These legend panels are used to identify a runway or taxiway, and/or to give directional information to assist the air crew in ground navigation. The size and visibility of the legends is governed by FAA directives. The inward side of each panel is coated with a reflective material, which assists in the internal reflecting of the incident light, the importance of which is discussed below. 
     As shown in  FIG. 2 , the frame or case  14  has a flat floor or base member  16 , side members or end panels  18 , and a top cover  20 , which fit together and retain the legend panels  12 . In this case there are horizontal slots or recesses  22  on front and rear edges of the base member  16  and curved slots or recesses  24  on the rising edges of the end panels  18 . There are similar slots or recesses in the top cover  20 , but these are not visible in this view. The legend panels  12  are inserted into the frame so that the edges are held snugly in the slots  22  and  24 , and then the cover  20  is installed at the top of the frame. Turn fasteners  26  secure the cover  20  to the tops of the end panels  18 . While the curved legend panel design is preferred, the invention could be practiced in guidance signs that have flat panels. A multi-module insert  28 , in the form of an open frame, is employed to accommodate two guidance sign legend panels  12  per side. 
     Within the frame  14 , there are a plurality of LED based lamps  30 , here arranged as two pairs of two lamps  30  per pair of legend panels  12 . These LED lamps are supported on a lamp support channel  32  that extends transversely across the mid part of the guidance sign, and supports a power supply and control circuit device  34  for each set of lamps. In this embodiment, there is at least one storage battery  36 , here a pair of storage batteries  36 , situated on the base  16 . 
     The frame  14  is supported above the ground surface, upon tubular legs, each with a floor flange  38  that mounts onto a ground surface, and a frangible coupling  40  of sufficient length to provide the required ground clearance, e.g., six inches, in the form of a metal tube or nipple provided with an annular cutout to weaken the coupling  40  sufficiently so that it will shear at a given stress level, e.g. if an aircraft strikes it or rolls over it, yet withstand the required wind load. 
     The runway or taxiway sign  10  is shown in a side elevation or profile in  FIG. 2  and in an end view in  FIG. 3 . These views show a wiring that passes through in-ground conduit and upwards through each of the support legs or couplings  40 , respectively. This permits recharging current from a solar panel or wind turbine generator (not shown) to pass into the guidance sign for charging up the storage batteries  36 , and also shows conductors from the traditional runway/taxiway lighting power to pass through the other one of the couplings  40 . This can be connected to the power supply and control circuit devices  34  and to the storage batteries  36 . A switching circuit  42  is shown here, which is sensitive to the charge level on the storage battery or batteries, and serves to switch over from solar power to runway/taxiway power when needed. That is, the runway/taxiway power is directed to the LED lamps  30  when the stored energy on the batteries  36  is detected to be insufficient, and can be directed to the batteries  36  when the solar power voltage is insufficient for charging up the batteries. The switching circuit is also operable to turn on the guidance sign lamps  30 , by sensing the circuit activity of the runway-taxiway power. When that power circuit is not active, the sign will not be lighted. When battery power is sufficient, this connection is used only to sense circuit activation. When the battery power is low, the operating current can come from the runway-taxiway power circuit. The wind turbine can be a low-profile device that does not interfere with airplane operations. 
     As shown in  FIG. 3 , and in more detail in  FIGS. 4 ,  5 , and  6 , each of the LED based lamps  30  is formed of an array of white LEDs  44  which are mounted on a generally cylindrical turret  46 . Here, the turret  46  is in the form of fourteen strips, each holding a row of four LEDs, such that each row is facing radially out at an angle. The angular spacing between rows is thus approximately 25.7 degrees. The generally cylindrical turret  36  has a main turret axis A, and the axis A for each lamp is situated horizontal and directed towards each of the associated pair of panels, as seen in  FIG. 3 . The light from the array of LEDs  44  beams up, down, sideways, but generally parallel to each of the panels, so that the light reflects off interior surfaces of the sign, at least once, and preferably numerous times, before passing through the legend panels  12 . This action results in uniform and consistent lighting of each legend panel  12 . 
     Each lamp device  30  has a main circuit board  48 , with the turret  46  being mounted centrally on one side of the board  48 . The board  48  holds various current control electronics, which will not be described in detail here. The board  48  is also used to mount the lamp device  30  on the support channel  32 . The lamps  30  are mounted, between each pair of legend panels  12 , with two lamps above the support channel, and two beneath, and with the turrets  46  thereof oriented to one side and the other alternately, as shown. 
     Each lamp device  30  has its array of LEDs grouped into a plurality of groups of individual LEDs, with the LEDs of each group on different strips of the turret  46 , so that the LEDs of that group are at different angular positions. In the preferred embodiment as shown, the array  44  includes fifty-six individual LEDs, arranged as fourteen groups of four LEDs per group. The successive LEDs of each group are located at successive angular positions. In this fashion, if there is a failure of any given group, the dark or unlit LEDs will be distributed at different locations around the turret, and the failure of that group will not result in unevenness of lighting. In practice the lamp  30  can lose up to three groups of four LEDs without needing to be taken out of service. The circuitry on the lamp includes a provision to cause the entire lamp to go dark if there are four or more groups (i.e., sixteen LEDs) that fail, which is 25% of the array  44 . There is an additional or auxiliary LED  50  positioned on the circuit board  48  apart from the turret  46 . This may be a distinct color, e.g., red, and this LED glows when there is a failure among any of the groups of LEDs of the array  44 . The red LED makes if possible for the service technician to identify which of the lamps  30  may need service or replacement. 
     In this embodiment, the array of LEDs uses the so-called white LEDs, which emit light over the visible spectrum. In some cases, the LEDs may have more light in some wavelengths than others, and in some alternative embodiments the lamps may employ a mix of red, green and blue LEDs to achieve the effect of white light. The term LED as used here should be interpreted to cover solid state light emitting devices, including not only light emitting diodes but also solid state laser devices or other illuminating devices of that general type. 
     As shown in  FIG. 6 , there can be an optional diffusion plate  52  mounted on the axis of the turret  46  at the end remote from the circuit board. The diffusion plate  52  can be of a standard translucent or frosted material, e.g., plastic of a type used in road signs. The diffusion plate can be used in larger signs to prevent rings of light from appearing on the legend panels  12 . 
     A main advantage of the guidance sign of this invention with solid state based illumination is the inherent low power draw, long life, and low maintenance of such devices. Also, because of the array of LEDs of these lamps being oriented in planes that are generally parallel to the legend panels, the beams of light undergo multiple reflections on surfaces inside the sign, including the reflective interior surfaces of the frame and the reflective inside surfaces of the legend panels, which results in high diffusion, with extremely even lighting and without hot spots or dark spots. 
     While the invention has been described in detail with respect to a preferred embodiment, it should be recognized that there are many alternative embodiments that would become apparent to persons of skill in the art. Many modifications and variations are possible which would not depart from the scope and spirit of this invention, as defined in the appended claims.