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    • 专利摘要:
    The invention relates to a method for calibrating an LED of an RGB LED device, which is arranged for setting a PWM duty cycle and for supplying to the LED either a current with a first value or with a second value higher than the first value. The method comprises: - measuring a first series of tristimulus values for the LED when the current with the first value is applied to the LED with a first PWM duty cycle and calculating therefrom parameters for expressing color intensity as a function of the PWM - work cycle, - measuring a second and third series of tristimulus values for the LED when the second and third value current is applied with a second and third PWM duty cycle, - calculating parameters for expressing the color intensity from the second and third series of tristimulus values.
    公开号:BE1025914B1
    申请号:E2018/5735
    申请日:2018-10-24
    公开日:2019-10-31
    发明作者:Thomas Freitag；Raik Frost
    申请人:Melexis Technologies Nv；
    IPC主号:
    专利说明:

    Method and Device for Calibrating LED Lighting
    FIELD OF THE INVENTION The present invention relates generally to the field of solutions for controlling red-green-blue light-emitting diodes (RGB LEDs).
    Background of the Invention RGB light-emitting diodes (LEDs) are increasingly being used in a variety of lighting applications. Each LED can be individually controlled in intensity and color by an integrated circuit (IC) via a communication interface. The LEDs may need to be calibrated to a specific target value before use with the IC.
    RGB LEDs are typically controlled with pulse width modulation (PWM) to set a defined current for each color channel to adapt to specific color points and color intensities to obtain a specific light output from a specific RGB LED. For a given target light output, the currents in the different color channels should not change because this could cause a change in the spectrum, resulting in a different color point and a different color intensity. For optimum light control, a high PWM resolution of at least 18 bits on these streams is required. On the other hand, the PWM resolution is usually limited to 16 bits. This is because a high PWM resolution also requires a high-frequency clock generator. The LED itself is driven in a PWM frequency range up to 500 Hz, so that the human eye sees no flicker. This would be the case if lower PWM frequencies were used. As a result, a 16-bit PWM resolution requires a clock source of approximately 32,768 MHz (65535 * 500 Hz). Each additional bit increases the required clock signal by a factor of two, which can lead to increased electromagnetic emissions and is not cost-optimal. Furthermore, the current through the LED needs a certain amount of time to stabilize, rendering high PWM resolutions unusable at low PWM duty cycle values. That is why there is a need to perform light control with a PWM resolution that is limited to 16 bits.
    This problem has been addressed many times in the prior art. WO2015 / 061237 relates, for example, to controlling the brightness of an LED display by using a combination of current and PWM dimming of the LEDs.
    BE2018 / 5735
    The application offers a solution for dealing with the non-linear relationship between the current and the luminous flux, which is used for dimming the light output in a brightness control.
    EP3076758 presents a light control circuit. A solution for enabling the optimization of a driver for one or more light sources has been disclosed. A duty cycle value is selected from a table. The selected duty cycle corresponds to the intended output current of the driver and has a corresponding voltage. The selected duty cycle is applied to the driver. An output voltage at the light source is measured and compared with the corresponding voltage of the selected duty cycle to produce a voltage comparison result. The selection of the duty cycle is adjusted based on the comparison result. In addition, an output current from the light source is measured and compared with the intended output current to produce a current comparison result. An adjustment coefficient is applied to a feedback circuit of the driver based thereon, the feedback circuit adjusting a switching frequency of the driver based on the selected duty cycle.
    However, there is room for alternatives for performing LED light control over a wide dynamic intensity range with a limited PWM resolution.
    Summary of the Invention It is an object of embodiments of the present invention to provide a method for calibrating an RGB LED device. It is also an object of embodiments of the present invention to provide a method for operating the RGB LED device. It is a further object to provide an RGB LED device.
    The above object is achieved by the solution according to the present invention.
    In a first aspect, the invention relates to a method for calibrating a light-emitting diode, LED, of a red-green-blue, RGB, LED device, said RGB LED device being adapted to set a pulse width modulation, PWM, duty cycle and for supplying to said LED from
    BE2018 / 5735 either a current with a first value or a current with a second value that is higher than said first value. The method comprises:
    - measuring a first series of tristimulus values for the LED when the current with said first value is applied to the LED with a first PWM labor cycle,
    - calculating from the measured first series of tristimulus values parameters for expressing color intensity as a function of the PWM duty cycle when the current with said first value is applied,
    - measuring a second series of tristimulus values for the LED when the current with said second value is applied with a second PWM work cycle, said second PWM duty cycle being lower than the first PWM cycle, whereby the resulting current is lower than the current flowing arises when the current with the aforementioned first value is applied with the first PWM duty cycle,
    - measuring a third set of tristimulus values for the LED when the current with said second value is applied with a third PWM duty cycle, said third PWM duty cycle being higher than the first PWM cycle, the resulting current being higher than the current that arises when the current with the aforementioned first value is applied with the first PWM labor cycle,
    - calculating from the measured second and third series of tristimulus values for expressing color intensity as a function of the PWM duty cycle when the current with said second value is applied.
    The proposed solution makes it possible to control the RGB LED device. By determining calibration parameters associated with the use of the low-value current and calibration parameters associated with the use of the high-current value, all information is collected to take into account the possible operating point shift when switching between the two flows. In preferred embodiments, the process steps are repeated for each LED of the RGB LED device.
    In advantageous embodiments, the first PWM duty cycle is 50%. In advantageous embodiments, the third PWM duty cycle is 100%.
    BE2018 / 5735 In embodiments of the invention, the calculated parameters are stored in a memory.
    Preferably, the calculated parameters of the measured first series are used to express color intensity as a linear function of the PWM duty cycle when the current with the first value is applied.
    Preferably, the function expressing color intensity as a function of the PWM labor cycle when the current with said second value is applied using the calculated parameters from the measured second and third series is a linear function.
    In another aspect, the invention relates to a method for operating a red-green-blue, RGB, LED device, comprising
    - calibrating the RGB LED device as previously described,
    - receiving tristimulus value information about a target color point and intensity,
    - calculating during operation a PWM duty cycle value for each LED, while applying the current with the first value, using parameters obtained in the calibration step,
    - checking for each LED whether the calculated PWM duty cycle value exceeds a threshold level set to the first PWM duty cycle value, and if so, switching for that LED to the current with said second value and calculating the corresponding PWM labor cycle, using parameters obtained in the calibration step.
    In a particular embodiment, switching to the stream with said second value is performed after a certain time interval has elapsed.
    In a preferred embodiment, the threshold level is equal to the first PWM duty cycle.
    In a further aspect, the invention relates to an RGB LED device
    - a red, green and blue LED,
    - pulse width modulation, PWM, control means for setting a PWM work cycle value,
    - power supply means for supplying to the LEDs a current with a first value or a current with a second value that is higher than said first value,
    - a controller that contains a processing means adapted for
    BE2018 / 5735 receiving information about a target color point and target intensity, calculating parameters for expressing color intensity as a function of the PWM duty cycle from a measured first set of tristimulus values when said current with said first value is applied, it from a measured second and third series of tristimulus values calculating parameters to express color intensity as a function of the PWM labor cycle when said current with said second value is applied, said second series being measured when said current with said second value is applied with a second PWM labor cycle, wherein said second PWM duty cycle is lower than said first PWM cycle, the resulting current being lower than the current generated when said current with said first value is applied with said first PWM labor cycle, said third straight ks tristimulus values are measured when said current with said second value is applied with a third PWM work cycle, said third PWM duty cycle being higher than said first PWM cycle, the resulting current being higher than the current created when said current with said first value value is applied with said first PWM duty cycle, wherein said RGB LED device further includes storage means for storing said calculated parameters.
    In a preferred embodiment, the current supply means comprises a current selection switch for selecting either the current with said first value or the current with said second value that is higher than said first value.
    In other embodiments, the current supply means includes a single current selection switch for providing either the current with said first value or the current with said second value that is higher than said first value.
    In embodiments, the power supply means includes two power sources. In other embodiments, the power supply means includes a switchable element that is controllable by the PWM control means.
    BE2018 / 5735 To summarize the invention and the realized advantages over the prior art, certain objects and advantages of the invention have been described above. It goes without saying that all such objectives or advantages are not necessarily achieved in accordance with a specific embodiment of the invention. Thus, for example, persons skilled in the art will recognize that the invention can be embodied or implemented in a manner that achieves or optimizes one benefit or group of benefits as described herein, without necessarily realizing other goals or benefits described or suggested herein. .
    The above and other aspects of the invention will become clear and further explained with reference to the embodiment (s) described below.
    BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be further described, by way of example, with reference to the accompanying drawings.
    Fig. 1 illustrates an LED device according to an embodiment of the present invention.
    Fig. 2 illustrates a color gamut triangle for two different current values.
    Fig. 3 illustrates a possible behavior of the tristimulus values for a red LED as a function of the PWM duty cycle when a low current is applied.
    Fig. 4 illustrates an embodiment of operating an RGB LED device calibrated according to the present invention.
    Detailed Description of Illustrative Embodiments The present invention will be described with reference to specific embodiments and with reference to certain drawings, but the invention is not limited thereto, but is only limited by the claims.
    In addition, the terms first, second, etc. in the description and in the claims are used to distinguish between similar elements and not necessarily for describing a sequence, either in time, in space, in importance or in any other way. It must be understood that the used
    BE2018 / 5735 terms are interchangeable under proper conditions and that the embodiments of the invention described herein are capable of operating in sequences other than those described or illustrated herein.
    It is to be noted that the term comprising as used in the claims should not be interpreted as being limited to the means specified thereafter; it does not exclude other elements or steps. It must therefore be interpreted as a specification of the presence of the listed features, units, steps or components referred to, but it does not exclude the presence or addition of one or more other features, units, steps or components or groups thereof. Therefore, the scope of the term a device comprising means A and B should not be limited to devices that consist only of parts A and B. It means that with regard to the present invention, the only relevant parts of the device are A and B.
    References in this specification to one embodiment or an embodiment means that a particular feature, structure, or feature described in connection with the embodiment is included in at least one embodiment of the present invention. Mentions of the phrase in one embodiment or in an embodiment at different places in this specification do not necessarily all refer to the same embodiment, but it is possible. Furthermore, the specific features, structures or characteristics may be combined in any suitable manner in one or more embodiments, as will be apparent to those skilled in the art from this disclosure.
    In a similar manner, it should be noted that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped into a single embodiment, figure, or description thereof to streamline disclosure and understanding of one or more of the facilitate various inventive aspects. However, this method of disclosure should not be interpreted as an expression of an intention that the claimed invention requires more features than expressly stated in each claim. As shown in the following claims, the inventive aspects lie in less than all the features of a single preceding disclosed embodiment. Therefore, the conclusions following the detailed description are hereby explicitly included in this detailed
    BE2018 / 5735 description, wherein each claim stands on its own as a separate embodiment of the present invention.
    In addition, since some embodiments described herein include some, but not other, features included in other embodiments, combinations of features of different embodiments are intended to fall within the scope of the invention and form different embodiments, such as will be understood by someone skilled in this field. For example, in the following claims, any of the claimed embodiments can be used in any combination.
    It should be noted that the use of certain terminology in describing certain aspects of the invention does not imply that the terminology herein is redefined to be limited to any specific features of the features or aspects of the invention with which that terminology is associated.
    Numerous specific details are set forth in the description given here. However, it is understood that embodiments of the invention can be worked out without these specific details. In other cases, well-known methods, structures and techniques were not shown in detail in order not to obstruct the understanding of this description.
    The present invention proposes a calibration method for an RGB LED device for use over a wide dynamic intensity range with a limited PWM resolution.
    The LED device of Fig. 1 is considered. The LED device consists of a red-green-blue light-emitting diode (RGB LED) (1) and an RGB LED controller (2). In one embodiment, this is realized, for example, as an integrated circuit (IC).
    In the embodiment of Fig. 1, the RGB LED controller includes a voltage regulator (3) capable of controlling an external supply voltage VS down to an internal supply voltage for the integrated circuit. This external power supply may not be stable in car environments.
    An RC oscillator (4) supplies a system clock to a micro controller (5) and the power source and PWM control (6). The micro controller includes a central processing unit (CPU) (51), a random access memory (RAM) (52), a non-volatile data memory
    BE2018 / 5735 (e.g. EEPROM, NVRAM) (53) and a non-volatile program memory (e.g. ROM), Flash, OTP) (54).
    The microcontroller can receive information for a certain target color point and a certain target color intensity via a bus interface, e.g., a LIN interface, from a higher level unit (not shown in Fig. 1).
    In the approach according to the invention, the RGB LED is driven by means of two different continuous currents, i.e. a low current and a high current, which are selectable by a current selection switch (8). Note, however, that the proposed approach can easily be extended to the use of more than two streams. In the further explanation below, a current of a first, low value and a current of a second value higher than that low value are considered.
    In certain embodiments, e.g., those illustrated in FIG. 1, two independently controllable current sources (9, 10) are provided to adjust the currents. In preferred embodiments, there is only a single current source that can alternately provide the high and the low current as defined by the current selector. The selected current can also be controlled by a PWM via a switchable element (7) that allows a wide dynamic range in intensity per color channel. The setting of the current sources, the selection of the currents and the PWM settings are supplied by the microcontroller (5) in cooperation with the current source (s) and the PWM control (6).
    The low and high current can be controlled, for example, in steps of 3 mA between, for example, 0 mA and, for example, 30 mA. Preferably, the low current is kept at, for example, 3 mA or 6 mA or another current lower than the high current. The higher current is preferably maintained at, for example, 30 mA or 27 mA or another value that is higher than the low current.
    For a given light scene (e.g., color point and color intensity), a fixed low current value and a fixed high current value are programmed. In a given light scene, the two fixed currents together with the current selector switch and the PWM control on the PWM controllable switch element (7) are used to adjust the light output to a specific color point and a specific color intensity.
    The current change from a selected low current to a selected high current for one or more color channels of a given RGB LED results in a shifted color gamut triangle (from triangle (201) to triangle (202))
    BE2018 / 5735 as shown in Fig. 2 since the operating point of the one or more LEDs has shifted from point T1 (204) to point T2 (205) and therefore also shifts the light output spectrum of the RGB LED. Only color points within a common color gamut (203) constructed from the gamma diagrams of the two selected streams can be displayed when the two stream values are used. A gamma is the complete subset of colors that can be accurately represented by a specific device. A gamut of an RGB LED panel is represented by a triangle in a chromaticity diagram of the CIE 1931 color space.
    If the target color point falls outside of the general color gamut (203), the general color gamut can be used to calculate a closest color point of the target color point that can be represented by both selected current values. This closest color point is then used in a specific light scene.
    Because the change between a selected low current and high current shifts the spectrum, it is mandatory to calibrate the RGB LED for the two current values used for each color channel.
    The present invention proposes an alternative and novel way to perform the calibration.
    In a preferred embodiment, the calibration is performed for all three LEDs of an RGB LED at three characteristic operating points, i.e.:
    - at a selected fixed low current (I_low) at a given PWM labor cycle value PWMsp, e.g. 50%,
    - at a selected high current (I_high) at a PWM duty cycle PWMlow lower than PWMsp, the high current value and the applied PWM duty cycle PWMlow being selected such that the resulting current is smaller than the current that occurs when the low current (I_low ) is used, eg I_high * PWMlow <I_low * PWMsp,
    - on the selected high current (I_high) with a PWM duty cycle PWMmax, usually the maximum of 100%. The resulting current I_high * PWMmax is higher than the resulting current I_low * PWMsp when the low current is used.
    The color scene (color point and color intensity) can be described and measured as X, Y, Z values (CIE1931 color coordinates) during a spectral optical
    BE2018 / 5735 measurement. This is known in the art and is briefly explained here. The CIE XYZ color space includes all color sensations that are visible to a person with average vision. Therefore, CIE XYZ (the so-called tristimulus values) is a device-invariant color rendering. It serves as a standard reference on the basis of which many other color spaces are defined. A series of color matching functions, such as the spectral sensitivity curves of the LMS color space, but not limited to non-negative sensitivities, link physically produced light spectra to specific tristimulus values.
    During the calibration, the currents are selected and applied and different PWM duty cycles are used. The X, Y, Z values for the single red, green and blue LED of the RGB LED are measured spectrally by means of, for example, an optical array spectrometer. The measurements can be performed in a serial manner. This means that each LED (red, green and blue) is applied in a serial way with the currents with the PWM work cycles.
    All this results in 3 times three measurements for the calibration of the RGB LED:
    1. XLEDred, YLEDred, ZLEDred measured @ I_low and PWM duty cycle PWMsp (50%)
    2. XLEDgreen, YLEDgreen, ZLEDgreen measured @ I_low and PWM duty cycle PWMsp (50%) 3. XLEDblue, YLEDblue, ZLEDblue measured @PWMsp (50%) I low and PWM duty cycle 4. XLEDred, YLEDred, ZLEDred measured @PWMlow (<50%) I high and PWM duty cycle 5. XLEDgreen, YLEDgreen, ZLEDgreen measured @PWMlow (<50%) I high and PWM duty cycle 6. XLEDblue, YLEDblue, ZLEDblue measured @PWMlow (<50%) I high and PWM duty cycle 7. XLEDred, YLEDred, ZLEDred measured @PWM max (100%) I_high and PWM duty cycle 8. XLEDgreen, YLEDgreen, ZLEDgreen measured @PWM max (100%) I_high and PWM duty cycle 9. XLEDblue, YLEDblue, ZLEDblue measured @ I_high and PWM duty cycle
    PWM max (100%)
    BE2018 / 5735 The currents and PWM duty cycles during calibration are selected taking into account the fact that a specific target color point can be displayed by combining any fixed current value of the selected fixed low current or the selected fixed high current with an appropriate set PWM duty cycle value, as also illustrated in Fig.2.
    Fig. 3 shows the tristimulus values XLEDred, YLEDred, ZLEDred as a function of PWM duty cycles for, for example, the red LED. Since the tristimulus values behave almost linearly with respect to the current, linear comparisons can be made for each tristimulus value:
    XLEDred = mX_LEDred * PWMLEDred + bX_LEDred
    XLEDgreen = mX_LEDgreen * PWMLEDgreen + bX_LEDgreen
    XLEDblue = mX_LEDblue * PWMLEDblue + bX_LEDblue
    YLEDred = mY_LEDred * PWMLEDred + bY_LEDred
    YLEDgreen = mY_LEDgreen * PWMLEDgreen + bY_LEDgreen (11)
    YLEDblue = mY_LEDblue * PWMLEDblue + bY_LEDblue
    ZLEDred = mZ_LEDred * PWMLEDred + bZ_LEDred
    ZLEDgreen = mZ_LEDgreen * PWMLEDgreen + bZ_LEDgreen
    ZLEDblue = mZ_LEDblue * PWMLEDblue + bZ_LEDblue where the target color point / target color intensity has tristimulus values X, Y, Z given by the expressions
    X = XLEDred + XLEDgreen + XLEDblue
    Y = YLEDred + YLEDgreen + YLEDblue (12)
    Z = ZLEDred + ZLEDgreen + ZLEDblue The basis for starting the further calculations is the selected fixed low current I_low. As can also be seen in Fig. 3, the intersection for the y-axis is always zero for this case, so bX = bY = bZ = 0 for all red, green and blue LEDs in the equations (1) above. This means that the following variables can be derived from the measurements described above for the selected fixed low current:
    mX_LEDred = XLEDred / PWMsp, with XLEDred measured, PWMsp = 50% applied mY_LEDred = YLEDred / PWMsp, with YLEDred measured, PWMsp = 50% applied mZ_LEDred = ZLEDred / PWMsp, with ZLEDred measured, PWMsp = 50% applied XLREDPreen / PWMsp , measured with XLEDgreen, PWMsp = 50% applied
    BE2018 / 5735 mY_LEDgreen = YLEDgreen / PWMsp, with YLEDgreen measured and PWMsp = 50% applied mZ_LEDgreen = ZLEDgreen / PWMsp, with ZLEDgreen measured and PWMsp = 50% applied (13) mX_LEDblue = XLEDblue / PWMsp, 50Wmlue = 50WMsp, with XLEDblue applied mY_LEDblue = YLEDblue / PWMsp, measured with YLEDblue and PWMsp = 50% applied mZ_LEDblue = ZLEDblue / PWMsp, measured with ZLEDblue and PWMsp = 50% applied Therefore, for the fixed low current selected, all unknown variables (eg for the red LED: mX_LEDred, mY_LEDred, mZ_LEDred) of the equations (11) in this calibration step determined using measurements 1, 2 and 3.
    Because the transition from the selected fixed low current to the selected fixed high current can also result in a different operating point of the LED, the color spectrum can shift from T1 to T2 as shown in Fig.2. This also means that for the calibration on the selected fixed high current, the intersection of the y-axis in the tristimulus graph may no longer be zero. Therefore, it is necessary to perform the calibration on the selected high current using a current value (I_high) and two different PWM duty cycle values as previously described (see measurements 4 to 9).
    Since either the fixed low current or the fixed high current is used in the application and the selection criterion is set to a PWM work cycle value PWMsp, the calibration must be performed on the selected fixed high current below the switching point (i.e. <50% PWM duty cycle and a value for I high selected so that the resulting current is below I low and another reference point with, for example, a duty cycle of 100% The calibration below the current switching point ensures a smooth transition when the current is switched from the selected fixed low current to the selected fixed high current or vice versa The linear equations can be arranged in the following manner according to Fig. 3 between both measured points for all tristimulus values so that the unknown variables can be derived for the selected high current:
    mX_LEDred = (XmaxLEDred - XlowLEDred) / (PWMmax-PWMlow), bX_LEDred = XmaxLEDred - mX_LEDred * PWMmax with XmaxLEDred, XlowLEDred measured, PWMmax and PWMlow applied mY_LEDred = (YmaxLEDmed) * PWM max
    BE2018 / 5735 with YmaxLEDred, YlowLEDred measured, PWMmax and PWMlow applied mZ_LEDred = (ZmaxLEDred - ZlowLEDred) / (PWMmax-PWMlow), bZ_LEDred = ZmaxLEDred - mZ_LEDred * PWMmax with ZmaxWedX, PlowMedX, PlowMedX ) / (PWMmax-PWMlow), bX_LEDgreen = XmaxLEDgreen - mX_LEDgreen * PWMmax with XmaxLEDgreen, XlowLEDgreen measured, PWMmax and PWMlow applied mY_LEDgreen = (YmaxLEDgreen - YlowLEDgreen) / (PWMmax-PWMlow), bY_LEDgreen = YmaxLEDgreen - mY_LEDgreen * PWMmax with YmaxLEDgreen, YlowLEDgreen measured, PWMmax and PWMlow applied mZ_LEDgreen = (ZmaxLEDgreen - ZlowLEDgreen) / (PWMmax-PWMlow), bZ_LEDgreen = ZmaxLEDgreen - mZ_LEDgreen * PWMmax with ZmaxLEDgreen, ZlowLEDgax measured, PWMlow and PWMlow, PWMlow , bX_LEDblue = XmaxLEDblue - mX_LEDblue * PWMmax with XmaxLEDblue, XlowLEDblue measured, PWMmax and PWMlow applied mY_LEDblue = (YmaxLEDblue - YlowLEDblue) / (PWMmax -PWMlow), bY_LEDblue = YmaxLEDblue - mY_LEDblue * PWMmax with YmaxLEDblue, YlowLEDblue measured, and PWMmax PWMlow applied mZ_LEDblue = (ZmaxLEDblue - ZlowLEDblue) / (PWMmax-PWMlow), bZ_LEDblue = ZmaxLEDblue - mZ_LEDblue * PWMmax with ZmaxLEDblue, ZlowLEDblue measured, and PWMmax PWMlow applied
    Therefore, also for the selected fixed high current, all unknown variables (eg for the red LED: mX_LEDred, bX_LEDred, mY_LEDred, bY_LEDred, mZ_LEDred, bZ_LEDred) of the equations (1) in this calibration step were determined using measurements 4 to 9.
    The parameters derived in this calibration step can be placed as a data array in the non-volatile data memory (53).
    The calibration data can further be used to calculate any PWM value to be applied to achieve a particular color scene (color point, color intensity) described via X, Y and Z when using a fixed selected low current or a fixed selected high current. In other words, the RGB LED controller (2) can receive the target light scene information X, Y, Z from a higher level unit, e.g. via a LIN bus connection and could calculate the PWM settings via
    BE2018 / 5735 the comparison matrix shown at (11) for a selected fixed low or fixed high current using the parameters derived in the calibration step and stored in the non-volatile data memory (53).
    Current selection during operation Fig. 4 describes an embodiment of a method for setting the current source (100), i.e. the way in which during operation the current must be selected to be used, i.e. a low current or a high current. flow.
    After the processing is started (101), the RGB LED controller (2) receives the target color information X, Y, Z (102). First the fixed low current (103) is selected for all LEDs (red, green, blue) and the current source (s) is (are) switched accordingly. PWM values are calculated based on the calibration data and the comparison matrix displayed on (11) for the selected fixed low current (104).
    In a determination step (105), it is checked whether the PWM duty cycle for the red LED is> PWMsp, e.g.,> 50%. In the case that this is true, the fixed high current is selected (107), in the case that this is not true, the selected fixed low current is retained (106).
    In a subsequent step (108), it is checked whether the PWM duty cycle for another LED, e.g., the green LED, is> PWMsp. In the event that this is true, the fixed high current is selected (110); in the event that this is not true, the selected fixed low current is retained (109).
    In a subsequent determination step (111), it is checked whether the PWM work cycle for the third LED, e.g., the blue LED, is> PWMsp. In the event that this is true, the fixed high current is selected (113); in the event that this is not true, the selected fixed low current is retained (112).
    It is then checked whether one or more currents have been switched from a selected fixed low current to a selected fixed high current (114). In the event that this is true, all PWM values are calculated using the calibration data and the comparison matrix at (1) for the selected (115); in the case that this is not true, the PWM values of step 104 are retained. The selected PWM values are applied (116) and the routine is stopped (117).
    The routine can be used in the event that X, Y, Z values are updated from an applied light scene to a new light scene. Further, the
    BE2018 / 5735 new PWM values first calculated and applied synchronously in step 116. This ensures that no flicker can be received in the event that light scenes are updated very often.
    In another embodiment, the currents are not switched immediately after the determination steps. The selection is just remembered. The switch of the current can also be applied synchronously in step 116, when the new calculated PWM values are applied.
    The routine can also be applied if more fixed currents than just a selected fixed low and high current are used. This increases the calibration efforts, since also for these flows a calibration as described earlier must be applied. On the other hand, each additional stream again increases the resolution that can be achieved with a 16-bit PWM.
    The following resolution can be achieved using the approach presented here. With a selected fixed low current of, for example, 3 mA, a 16-bit PWM resolution leads to a current resolution of 3 mA / 65535 = 45.7 nA. This gives a good resolution for a specific light scene to be displayed. For comparison, if the proposed approach were not applied and only a maximum current of, for example, 30 mA was used, a 16-bit PWM resolution would result in a resolution of 0.457 pA per bit or PWM step. This corresponds to approximately 6556 PWM steps only or approximately 12 bits to represent a current of, for example, 3 mA. This resolution is not sufficient and can lead to an incorrect color point.
    An alternative would be to increase the PWM resolution. To obtain the same resolution as with a 16-bit PWM, it can be, for example, 30 mA / 45.7 nA = 656455 or about 19 bit. A disadvantage of using a 19-bit PWM, however, is the much higher clock frequency. This can lead to an increased electromagnetic emission and possibly also to higher costs.
    Although the invention has been illustrated and described in detail in the drawings and foregoing description, such illustrations and descriptions are to be considered as illustrative or exemplary and not restrictive. The foregoing description explains certain embodiments of the invention in detail. It should be noted, however, that no matter how detailed the foregoing is included in the text, the invention
    BE2018 / 5735 can be implemented in many ways. The invention is not limited to the disclosed embodiments.
    Other variations on the disclosed embodiments may be understood and performed by persons skilled in the art and by practicing the claimed invention, through a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps and the indefinite article does not exclude a plural. A single processor or other unit can perform the functions of different items in the claims. The mere fact that certain measures are enumerated in mutually different dependent claims does not mean that a combination of those measures cannot be used to benefit. A computer program can be stored / distributed on a suitable medium, such as an optical storage medium or semiconductor medium supplied with or as part of other hardware, but can also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems. Any references in the claims should not be construed as limiting the scope.
    权利要求:
    Claims (15)
    [1]
    Conclusions
    A method for calibrating at least one light-emitting diode, LED, of a red-green-blue, RGB, LED device, the method comprising:
    measuring a first series of tristimulus values for said at least one LED when a controller of said RGB LED device supplies a current with a first value to said at least one LED and applies a first PWM duty cycle, said current of said first value to which said first PWM labor cycle is applied, forms a first resulting stream,
    - calculating from said measured first series of tristimulus values, parameters for expressing color intensity as a function of the PWM duty cycle when said current with said first value is applied to said at least one LED,
    measuring a second series of tristimulus values for said at least one LED when said controller of said RGB LED device supplies a current with a second value greater than said first value and applies a second PWM duty cycle, said second PWM duty cycle being lower than said first PWM cycle, wherein said current of said second value to which said second PWM duty cycle is applied forms a second resultant stream, said second resultant stream being lower than said first resultant stream;
    measuring a third series of tristimulus values for said at least one LED when said controller of said RGB LED device supplies said current with said second value and applies a third PWM duty cycle, said third PWM duty cycle being higher than said first PWM cycle wherein said current of said second value to which said third PWM labor cycle is applied forms a third resultant stream, the third resultant stream being greater than the first resultant stream, and
    calculating parameters for expressing color intensity as a function of the PWM duty cycle from said measured second and third series of tristimulus values when said current with said second value is applied to said at least one LED.
    BE2018 / 5735
    [2]
    The calibration method of claim 1, wherein the method steps are repeated for each LED of said RGB LED device.
    [3]
    The calibration method according to claim 1 or 2, wherein said first PWM labor cycle is 50%.
    [4]
    The calibration method according to any of claims 1 to 3, wherein said third PWM labor cycle is 100%.
    [5]
    A calibration method according to any one of the preceding claims which includes storing the calculated parameters in a memory.
    [6]
    A calibration method as claimed in any preceding claim, wherein said calculated parameters of said measured first set are used to express color intensity as a linear function of the PWM duty cycle when said current with said first value is supplied to said at least one LED .
    [7]
    A calibration method as claimed in any preceding claim, wherein said calculated parameters of said measured second and third series are used to express color intensity as a linear function of the PWM duty cycle when said current with said second value is supplied to said at least at least one LED.
    [8]
    8. Operating method of a red-green-blue, RGB, LED device, comprising
    - calibrating said RGB LED device according to one of the preceding claims,
    - receiving tristimulus value information about a target color point and intensity,
    calculating during operation for each LED of said RGB LED device a PWM duty cycle value, while said current is supplied with said first value, using parameters obtained in said calibration step,
    - checking for each LED of said RGB LED device whether the calculated PWM work cycle value exceeds a threshold level, and if so, switching over for each LED from said RGB LED device to said current with said second value and calculating the corresponding PWM duty cycle, using parameters obtained in the aforementioned calibration step.
    BE2018 / 5735
    [9]
    The operating method according to claim 8, wherein switching to said current with said second value is performed after a certain time interval has elapsed.
    [10]
    Operating method according to claim 8 or 9, wherein said threshold level is equal to said first PWM duty cycle.
    [11]
    11. Red-green-blue light-emitting diode, RGB LED, device including
    - a red LED, a green LED and a blue LED,
    - pulse width modulation, PWM, control means (6) for setting a PWM work cycle value,
    - power supply means (8, 9, 10) comprising at least one current source for supplying to at least one of said red, green and blue LED, a current having a first value or a current having a second value higher than said first value,
    - a controller comprising a processing means (51) adapted to receive information about a target color point and target intensity, calculating from a measured first set of tristimulus values, parameters for expressing color intensity as a function of the PWM duty cycle when said current with said first value is applied to the at least one of said red, green and blue LEDs, said first series of tristimulus values measured when said current of said first value is supplied to the at least one of said red, green and blue LEDs with a first PWM duty cycle, wherein said current of said first value to which said first PWM duty cycle is applied forms a first resulting current, calculating parameters from a measured second and third series of tristimulus values to express color intensity as a function of the PWMarbeidsc yclus when said current with said second value is supplied to said at least one of said red, green and blue LEDs, said second series being measured when said current with said second value is supplied to said at least one of said red, green and blue LEDs with a second PWM duty cycle, wherein said second PWM duty cycle is lower than said first PWM cycle, wherein said current of said second value at which said second PWM duty cycle becomes
    BE2018 / 5735, forming a second resultant stream, said second resultant stream being lower than said first resultant stream, wherein said third set of tristimulus values is measured when said stream of said second value is supplied to said at least one of said red, green and blue LEDs with a third PWM duty cycle, wherein said third PWM duty cycle is higher than said first PWM cycle, wherein said current of said second value to which said third PWM duty cycle is applied forms a third resulting current, said third resulting current is higher than said first resulting current, and wherein said RGB LED device further comprises storage means for storing said calculated parameters to express color intensity.
    [12]
    The RGB LED device according to claim 11, wherein the power supply means comprises a current selector for selecting either said current with said first value or said current with said second value that is higher than said first value.
    [13]
    The RGB LED device according to claim 11 or 12, wherein said power supply means comprises a single power source for providing either said current with said first value or said current with said second value that is higher than said first value.
    [14]
    14. RGB LED device as claimed in claim 11 or 12, wherein the power supply means comprise two power sources.
    [15]
    RGB-LED device according to one of claims 11 to 14, wherein said current supply means comprise a switchable element (7) that is controllable by said PWM control means.
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    同族专利:
    公开号 | 公开日
    US10492256B2|2019-11-26|
    CN109729617A|2019-05-07|
    DE102017125405A1|2019-05-02|
    US20190132919A1|2019-05-02|
    FR3073118A1|2019-05-03|
    CN109729617B|2020-11-20|
    BE1025914A1|2019-08-07|
    DE102017125405B4|2021-03-18|
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    法律状态:
    2019-11-27| FG| Patent granted|Effective date: 20191031 |
    优先权:
    申请号 | 申请日 | 专利标题
    DE102017125405.6A|DE102017125405B4|2017-10-30|2017-10-30|Method and device for calibrating and operating RGB-LED lighting|
    DE102017125405.6|2017-10-30|
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