Field of the invention

The invention relates to LED airfield lights, like runway lights or taxi-way lights. In particular, the invention relates to a LED airfield light with at least two lighting modules, which are preferably separately replaceable and a method for driving LEDs of the lighting modules.

Description of the related art

In the present, LED airfield lights become more and more popular due to their high efficiency and low maintenance costs. Often LED airfield lights comprise two or more separately replaceable lighting modules.

W099/54854 A2 discloses an airfield lighting system comprising multiple LED airfield lights. These airfield lights each comprise a microprocessor which is connected by a data bus to central control unit. The microprocessor controls the LEDs of the respective airfield light. The microcontroller, the LEDs, switching and control components are integrated in a module, which is separately replaceable.

WO 2009/112318 Al discloses a lighting device with readable operating parameters. These operating parameters are provided to a controller, which buffers the operating parameters in a separate storage unit. The operating data can be accessed for an external analysis via an interface. WO 01/45471 Al discloses an arc lamp. Attached to the arc-lamp is a storage device for storing run time data of the arc lamp. This runtime data can be read by the manufacturer for defending a claim for free replacement, in case the arc- lamp was operated under conditions that did not comply with the arc-lamp's specification. WO2007/104137A2 discloses a light unit being interchangeably connected to a lighting device with a controller for controlling the activation of one or more lighting elements. The lighting unit has a carrier onto which at least one lighting element and a memory are arranged. In the memory operational characteristics of the lighting element are stored. The controller reads the information from the memory and controls the lighting element according to the information, to thereby avoid a calibration after replacement of a lighting unit of the lighting device.

WO 2007/006684 Al discloses a LED-traffic light having a light unit for emit- ting a light signal. The light unit comprises a controller for controlling and driving the LEDs. The controller is connected to a unit, which synchronizes signals being emitted by different light units. To better detect malfunctions of the LEDs the controller of the light unit is positioned in the proximity of the LEDs and not in the unit for synchronizing the signals. EP 1 696 707 A2 discloses a lighting unit for alighting device. The lighting unit has lighting element, e.g. a LED, and a memory for providing information about the lighting element to a controller via a signalling line. The memory is included in the controller.

EP 1 696 711 Al discloses a lighting device with a lighting element, a memory and a driver unit. The lighting device is configured for being included a network using the DALI-protocol (DALI means Digital Addressable Lighting Interface). For addressing the lighting devices in the network, a unique identifier of a driver unit of the lighting device is stored in the memory. Additional information, e.g. a type number may be encoded in the unique identifier or stored separately. The type number may be used for retrieving technical data from a data base and to control the lighting device accordingly. Summary of the invention

LED airfield lights often comprise two or more lighting modules. A close matching of the emitted luminances is required by ICAO standards. The problem to be solved by the invention is to permit replacement of single lighting modules of LED airfield lights with two or more lighting modules and to ensure, that the lighting modules emit light with at least substantially equal luminance even if the LEDs on the module have a different age or are of different types. At the same time a replacement of a single lighting module should be simple and should not stress the components to thereby enhance reliability of the LED-airfield light. The problem is solved by a method and a LED airfield light of the respective independent claims. The dependent claims relate to further improvements of the invention.

The LED-airfield light has a printed-circuit board carrying a controller for driving at least two LED-modules as light modules. The controller may comprise e.g. a microcontroller and a driver circuit, the latter being controlled by the microcontroller.

In one embodiment the LED-airfield light comprises at least a first and preferably a second LED-module. Each of the LED-modules may comprise at least one LED for lighting the airfield. Thus, on the first module is at least one first LED and on the second module is at least one second LED. The first and second LED-modules are preferably each electrically connected to the first printed-circuit board by a first and second cable, respectively. Thus there may be a first cable connecting the first LED-module with the printed-circuit board and a second cable for connecting the second LED-module with the printed-circuit board. Typically ribbon cables are used. At least one of the LED-modules may comprise a memory for storing status information about the respective LED(s). In a preferred embodiment a first memory for storing status information about the first LED is on the first LED-module and a second memory for storing status information about the second LED is on the second LED-module. This permits the controller to read the status information of the LEDs and drive the LEDs in accordance with the status information. If a LED-module is being replaced it may comprise a different type of LED. The information required for driving the replaced LED(s) may be comprised in the status information, permitting the controller to drive the LED in accordance with the LED's specification. The memory for storing status information about one or more LEDs of a LED-module is preferably part of the LED module and may thus be inserted or replaced together with the LED-module

Preferably, the first and second status information is processed by the controller to identify a LED with a lower luminosity at some given driving parameters, e.g. some standard driving parameters or the current driving parameters, and a LED with a higher luminosity from the first and second LEDs. Subsequently the driving parameters of the LED with the higher luminosity are adapted to emit the lower luminosity. This permits to replace the modules independently and nevertheless make sure that the LED-modules emit at least substantially the same luminosity. In a further embodiment the luminosity of the lower luminosity LED can be increased by providing more power to the respective LED. This permits to compensate for the loss of luminosity due to aging effects. Although one should consider that driving an older LED, i.e. with lower luminosity, with driving parameters that result in the luminosity of the younger LED, may significantly decrease the lifetime of the older LED.

Preferably the first and/or second cable is connected by at least one connector to the first printed-circuit board and/or first and/or second LED-module, respec- tively. The term connector preferably means an electrical connector that may be plugged to a complementary mating counterpart. A very simple embodiment of such connector is for example commonly referred to as strip. This permits an easy replacement of the LED module by simply unplugging the old LED-module und plugging in of the new LED-module. The cable may have e.g. two signalling lines for connecting the memory with the printed-circuit board and thus with the controller. The signalling lines are each connected to a connecting pin of the connector. Preferably the connecting pins to which the signalling lines are con- nected to are positioned symmetrically under a rotation of for example 180°C of the connector. Thus, the connector connects in any case the signalling lines with each other, however a signalling line mismatch may occur. Such mismatch can be resolved by a permutation of the signalling lines by the controller or some other permutation means. This permits to ignore the orientation of the connector, but instead avoid in any case a crossover over torsion of the cable and thereby reduce the probability of failure of the cable. The cable may have additional power lines for providing power to the memory and/or at least one LED. These lines are preferably as well symmetric under a rotation of the connector. For example may the cable comprise at least two groups of power lines for example for powering at least one LED by each of the groups. Preferably the connecting pins to which the groups of power lines are connected to are positioned symmetrically under a rotation of the connector. Means, the two groups are "mapped" onto each other under a rotation of the connector, taking into account the polarity of the power lines. Each group may in the simplest case consist of two lines (+, -). In a preferred embodiment at least one of the LED-modules comprises a printed- circuit board with a sheet metal layer. If a LED is mounted on such a printed- circuit board the heat dissipation from the LED is enhanced, permitting a higher luminosity at given LED-temperature and/or a reduction of LED temperature at a given luminosity, which yields a longer lifetime of the LED. The temperature of the LED(s) is preferably monitored by a sensor element for measuring the LED's temperature. A sensor an integrated sensor, a diode, a NTC (=negative temperature coefficient resistor) or a PTC (=postive temperature coefficient resistor) may be used. Such sensor element is preferably mounted on the first and/or second LED-module's sheet metal and may be connected directly to the controller and/or to the memory by analog or data bus lines.

In a preferred embodiment the memory is non volatile. Thus, the status information is kept without power supply. The memory may be an EEPROM. Another embodiment of the invention relates to a method for operating an airfield light like the one described above, e.g. with at least one controller being at least connected two LED-modules, wherein the LED-modules each comprise a memory. The method may at least comprise the steps of storing status information about each LED-module on the respective LED-module, reading the sta- tus information by the controller, and determining driving parameters for a first LED-module of the at least two LED-modules, considering the status information about at least one other LED-module. If one replaces only one of e.g. two LED- modules, the LEDs on the replaced modules are new. The other LEDs, however are older and typically provide less light at a given power setting and tempera- ture due to the LEDs degeneration. Thus, if the controller would drive the older and the younger LEDs with the same power, the younger LED-module would appear more bright. According to the invention, however, the younger module can be driven with less power, such that the LED-modules appear at least essentially equally bright. Alternatively one could enhance the power provided to the older LED-module, however, this may cause a reduction of its lifetime.

In a preferred embodiment the status information may comprise at least one of the values of the group formed by an emitted luminosity, a current, a voltage and a temperature and/or a value of use. This information permits to drive a LED with a predictable luminosity. If only one of these values is stored in the memory the other values define standard conditions under which the stored value is obtained. Thus the above values permit to parameterise a diode's characteristic. In a preferred embodiment the status information comprises at least one value of use representing the degeneration of the LED on the respective LED-module due to its use. This permits to consider the reduced luminosity due to the degeneration of the LED ("aging"), when driving the LED e.g. to ensure that even LEDs with a different "age" appear equally bright: If for example the second lighting module is replaced due to some maintenance, the first LED has a higher power- on time, resulting in a lower luminosity as the second LED at some given driving parameters. The method permits to adapt the luminosity of the second LED to the luminosity of the first LED by taking into account the reduced luminosity of the first LED and thereby to provide the LED-modules with driving parameters resulting in at least essentially the same luminosity.

The value of use may comprise the power on time count of the LED on the respective LED-module. The power-on time count is easy to measure and to store but provides already a good estimation of the LEDs aging. In preferred embodiment the power-on time count and the integrated current through the LED are comprised in the value of use.

In another embodiment the value of use comprises a function of

f(T(t'))dt', wherein T(t ^' ) is the temperature of the respective LED or LED- module measured at some time t ^' and f(T(t ^' )) a weighting function.

f(T(t'))dt' permits an even better estimation of the a LED's because the degeneration of a LED depends strongly on the temperature at which the LED is operated. This is accounted for by the weighting function. Thus a simple comparison of the value of use permits to identify lower and higher luminosity LEDs. A simple possibility of calculating the above integral can be obtained by storing power-on time counts for different temperature intervals, and summing the power-on time counts after multiplication with some factor, thereby taking into account the temperature dependence of the LEDs degeneration. The factor may be dependent on the respective temperature intervals and/or the current, because a power on time count at a higher operating temperature and/or current causes a faster generation than the same power on time at some lower temperature and/or current respectively. Based on the value of use one may determine an expected luminosity of e.g. the lower luminosity LED at some given driving parameters. Such luminosity could be stored in a table. By a reverse look-up of the luminosity one can obtain the driving parameters for the higher luminosity LED.

If the controller is connected to at least one of the memories by at least two sig- nailing lines it is preferred that the controller attempts to communicate with at least one of the memories and permutes the signalling lines if the communication with the memory chip fails. Subsequently the controller attempts to communicate again. Communication can typically be reading or storing information on the memory. Only for convenience, the term„LED" may be understood as plural or singular, i.e. as„LEDs" or„LED", wherein LED is the acronym for„ Light Emitting Diode".

Description of Drawings

In the following the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment with refer- ence to the drawings.

Figure 1 shows a simplified circuit diagram of an airfield light.

Figure 2 shows a circuit diagram of a LED-module.

In Figure 1 a component 10 comprises a controller 18 for driving LEDs 24, 34. The component may e.g. be a printed circuit board. The controller 18 is connected by some connection means to a main control unit (not shown). The LEDs 24 are on a first LED-module 20 and the LEDs 34 are on a second LED module 30. On each of the LED-modules 20, 30 is a memory 28, 38 for storing status information of the LEDs 24, 34 on the respective module. The status information can e.g. be the power-on time count at different operating temperatures of the LEDs, their type, manufacturer, max. current, luminosity as function of temperature coefficient, luminosity as function of value of use coefficient and the luminosity at some standard powers settings or any other parameter described herein.

The LED-modules 20, 30 are each connected by a ribbon cable 40, 50 to the printed-circuit board 10. The ribbon cables 40, 50 are connected to the printed- circuit board 10 and the LED-modules 20, 30 by complementary connectors 42, 22, 13, 43, 32, 52, 53. The memories 28, 38 are connected to the controller 18 by signalling lines 26, 27 and 36,37, respectively. Between the controller 18 and the memories 28, 38 are crossover switches 12, 14 for switching the signalling lines 26, 27. The connector pin assignment of the connectors is sym- metrically. Thus, if a connector is rotated by 180° the lines for driving the LEDs are exchanged, and the controller can however drive the LEDs 24, 34. If the connector is rotated, the signalling lines are interchanged, which can be corrected by the crossover switches 12, 14. If the communication between the controller 18 and one of the memories 28, 38 fails, the controller 18 activates the crossover switch 12, 14 between the controller 18 and the memory 28, 38. Thereby, a possible signalling line mismatch is resolved and information can be exchanged between the controller and the memory 28, 38.

In Figure 2 the LED-module 20 is depicted with some additional details: A NTC 29 is provided for measuring the LEDs 24 temperature. The two connectors of the NTC 29 are symmetrically under a rotation of the plug, thus the NTC 29 connects in any case with well defined connectors of the controller on the printed-circuit board 10. Further the power supply for the memory 28 is as well depicted. Power can be supplied via the symmetrically positioned pins 3 and 8 of the connector part 22 which are connected with the input side of a rectifier 21. The output side of the rectifier 21 is connected to the memory 28.

The LEDs 24 comprise each a series of LEDs. A first series of LEDs is connected to pins 1 and 2 of the connector 22. A second series of LEDs is connected to pins 9 and 10. The pins for connection of the first series are positioned symmetrically to the pins for connecting the second series of LEDs 24. The pin assignment for the signalling lines 27, 28 follows the same scheme. Briefly summarizing the pin assignment of the connector is symmetrical under a rotation of 180°. Thus the ribbon cable can in any case connect the component 10 with the LED-module 20 such that a twisted ribbon cable is avoided. All lines of the ribbon cable except the signalling lines will be connected correctly. A possible mismatch of the signalling lines can be corrected by the crossover switch 12. Although it is preferred to use ribbon cable other types of cable be used as well.

The method for operating the airfield light is depicted in figure3: The control- ler 18 attempts to read status information about the first LED-module (referred to as first status information) from the first memory 28. In case the reading fails, the controller 18 controls the crossover switch 12 to interchange the signalling lines 26, 27 and reads the status information from the first memory 28. Subsequently the controller 18 attempts to read status information about the second LED-module (referred to as second status information) from the second memory 38. In case the reading fails, the controller 18 controls the crossover switch 14 to interchange the signalling lines 36, 37 and reads the status information from the second memory 28. The controller evaluates the first and second status information to identify the LED-module with the LED emitting the lower luminosity. The status information comprises a value of use and a luminance of the LEDs under given conditions. The status information permits to obtain a characteristic of the respective LEDs. Based on these respective character- istics the first and second LEDs are driven such that they emit at least substantially an equal luminosity.

The invention and the method are explained in the context of airfield lighting. The invention can of course be used for light with at least two lighting modules that are controlled by a common controller.

List of reference numerals

10 compound / printed-circuit board

12 crossover switch

13 connector

14 crossover switch

18 controller

20 LED-module

21 rectifier

22 connector

24 LEDs

26 signalling line

27 signalling line

28 memory

29 temperature sensor

30 LED-module

32 connector

34 LEDs

36 signalling line

37 signalling line

38 memory

50 cable

52 connector

53 connector

40 ribbon cable

42 connector

43 connector