WO2008124718A2

WO2008124718A2 - Light emitting diode controller, methods of light emitting diode control, and components for same

Info

Publication number: WO2008124718A2
Application number: PCT/US2008/059632
Authority: WO
Inventors: Tomislav J. Stimac
Original assignee: Lumination Llc
Priority date: 2007-04-10
Filing date: 2008-04-08
Publication date: 2008-10-16
Also published as: WO2008124718A3

Abstract

A light emitting diode controller comprises a transform processor receiving an illuminance level input and outputting a pulse frequency control signal accounting for a non-linear relationship between illuminance and pulse frequency, and a pulse frequency modulator that drives one or more light emitting diodes using pulse frequency modulation at a pulse frequency corresponding to the pulse frequency control signal. A light emitting diode control method comprises transforming an illuminance level input to a pulse frequency control signal accounting for a non-linear relationship between illuminance and pulse frequency, and driving one or more light emitting diodes using pulse frequency modulation at a pulse frequency corresponding to the pulse frequency control signal. Lookup tables for such controllers and methods are also disclosed.

Description

LIGHT EMITTING DIODE CONTROLLER, METHODS OF LIGHT EMITTING DIODE CONTROL, AND COMPONENTS FOR SAME

BACKGROUND

[0001] The following relates to the lighting and illumination arts. It especially relates to control and operation of light emitting diode based illuminators. However, the following will find application more generally in conjunction with control and operation of substantially any light emitting diode based light-generating device.

[0002] Control of light emitting diode intensity is typically done using pulse width modulation (PWM). For example, Chliwnyj et al, U.S. Patent No. 5,924,784 discloses independent microprocessor-based PWM control of two or more different light emitting diode sources of different colors to generate light simulating a flame. Such PWM control is well known, and indeed commercial PWM controllers have long been available specifically for driving LEDs. See, e.g., Motorola Semiconductor Technical Data Sheet for MC68HC05D9 8-bit microcomputer with PWM outputs and LED drive (Motorola Ltd., 1990). In PWM, a train of pulses is applied at a fixed frequency, and the pulse width is modulated to control the time-integrated power applied to the light emitting diode. Accordingly, the time-integrated applied power is directly proportional to the pulse width, which can range between 0% duty cycle (no power applied) to 100% duty cycle (full power applied).

[0003] PWM control produces non-linear light output intensity respective to pulse width. That is, a linear increase in pulse width produces a non-linear increase in intensity. In a digital system, the input current is typically represented by a binary value. For an N-bit binary drive current representation, the number of discrete current levels is 2^N. Thus, for example, an 8-bit binary current representation allows for 256 discrete drive current levels, and a corresponding 256 discrete light intensity values. For PWM using 256 levels, "stepwise" intensity variations are observed when the intensity is ramped up or down. These problems are exasperated when independently controlled different-color light emitting diodes are blended to form a composite color, such as blending red, green, and blue light emitting diodes. [0004] To address this problem, some PWM light emitting diode systems employ a large number of bits, such as a 24-bit current representation providing 2²⁴=16,777,216 discrete drive current levels. With this large number of drive current levels, it is possible to make the corresponding stepwise intensity variations so small as to be unnoticeable. However, using a 24-bit current representation substantially increases computational load.

[0005] Application Note 008 - Frequency Modulation Techniques for the control of LED Colour Mixing and Intensity (Artistic Licence, 2002) proposes pulse frequency modulation for light emitting diode intensity control. In pulse frequency modulation, the pulse width is constant, but the frequency of applied pulses is varied to vary the intensity. As with PWM, in pulse frequency modulation the input current is typically represented by a binary value. For an N-bit binary drive current representation, the number of discrete pulse frequencies is 2^N. Thus, for example, an 8-bit binary pulse frequency representation allows for 256 discrete pulse frequencies, and a corresponding 256 discrete light intensity values. However, Note 008 suggests that pulse frequency modulation provides increased resolution over the low intensity end of the control range as compared with PWM.

[0006] The following contemplates improved apparatuses and methods that overcome the above-mentioned limitations and others.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting.

[0008] FIGURE 1 diagrammatically shows S-curve relationships between illuminance and frequency modulation for red, green, and blue light emitting diodes, respectively, powered by pulse frequency modulation.

[0009] FIGURE 2 diagrammatically shows a color controllable illumination system employing pulse frequency modulation. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0010] The following discloses improved light emitting diode intensity control apparatuses and methods, which provide smooth intensity control using a low-bit digital control input representation, for example a representation of 12 bits or less, such as an 8-bit representation. The approach employs an illuminance control signal that ensures uniform illuminance increments, and an inverse S-curve mapping between the illuminance control signal and a pulse frequency of the pulse frequency modulated power input to the light emitting diodes. As disclosed herein, this approach provides substantially uniform and smooth illumination transitions, which is valuable for any illumination application and is especially valuable in multi-color applications in which illumination from light emitting diodes of two or more different colors are combined to generate a selected blended-color illumination.

[0011] As used herein, a pulse frequency control signal represents the frequency of a pulse frequency modulated control signal. The frequency F corresponds to a repetition period T=l/F. For example, if F=5 kHz, then T=l/5000=0.2 milliseconds. The pulse width is of constant duration, denoted herein as duration C, with the condition that C<T. The condition C=T corresponds to a DC signal. The time-integrated input current is proportional to C/T=CxF. For example, doubling the pulse frequency results in a doubling of the time-integrated current. Another way to see this is that if the pulse frequency is doubled, then the number of pulses in a time period much greater than T is doubled, which doubles the time-integrated current. Thus, the average current is linear with the frequency in pulse frequency modulation.

[0012] Application Note 008 discloses the concept of pulse frequency modulation as a method for controlling light emitting diode output intensity. Control of the pulse frequency modulation by direct manipulation of the pulse frequency control signal is advocated in Note 008 as providing a non-linearity that opposes the inherent non-linear light emitting diode response, resulting in increased resolution over the low intensity end of the control range. Such an approach has the advantage of being straightforward to implement, since the input pulse frequency control signal can be directly used to control the pulse frequency modulation. [0013] However, it is recognized herein that control approaches that employ a direct input pulse frequency control signal produce substantially non-linearity of the illuminance respective to the input control signal. This non-linearity can be problematic as it can produce visually perceptible abrupt intensity jumps during intensity changes, and in the case of color blending can produce aesthetically disconcerting color transitions.

[0014] With reference to FIGURE 1, the inventors have investigated the relationship between illuminance and the frequency of pulse frequency modulation of red, green, and blue light emitting diodes. The inventors have found that an S-curve relationship describes the dependence of illumination on the frequency. To determine this, a pulse frequency modulated input was applied to a light emitting diode at a selected pulse frequency, and the illuminance generated by the driven light emitting diode was measured. This process was repeated at different driving pulse frequencies and it was found that the aforementioned S-curve relationship holds. It was found that at low illuminance levels, a large change in frequency of the pulse frequency modulated input was required to produce a small change in illuminance. A similar result was observed at high illuminance. At intermediate illuminance levels, however, smaller changes in the pulse frequency were sufficient to produce a commensurate change in illuminance. FIGURE 1 diagrammatically depicts the S-curve relationship for red, green, and blue light emitting diodes. The parameters of the S-curve, such as the lower and upper inflection point positions and the absolute illuminance and frequency values, were found to depend upon the specific light emitting diode.

[0015] In view of these results, the inventors have concluded that direct manipulation of a pulse frequency control signal, as advocated in Note 008, has certain disadvantages. The conclusion of Note 008 that a pulse frequency control signal provides increased resolution in the low illuminance range is confirmed. This increased resolution results from the relatively flat portion of the S-curve at the low illuminance range. Increased resolution is also provided at high illuminance levels, again due to the relative flatness of the S-curve at high illuminance. However, control using an input pulse frequency control signal provides relatively poorer resolution at the mid-illuminance levels corresponding to the transitional portion of the S-curve. The larger slope of the S-curve in that region translates into the input pulse frequency control signal providing relatively poor resolution.

[0016] Unfortunately, it is precisely this central region of the S-curve that is regularly traversed as illumination intensity is ramped up or down. The result is that, similar to PWM, employment of pulse frequency modulation using a direct pulse frequency control signal results in a choppy illuminance ramp that produces visually perceptible abrupt illuminance jumps unless the pulse frequency control signal is of high resolution (e.g., greater than 8 bit resolution).

[0017] Employment of pulse frequency modulation using a direct pulse frequency control signal is also disadvantageous for color control. As with intensity ramps, color changes regularly involve ramping the illuminance levels of one or more sets of light emitting diodes up or down across the central region of the S-curve. For example, if a smooth color change from more reddish to more yellowish is desired, the intensity of the red light emitting diodes is ramped down while the intensity of the green light emitting diodes is ramped up. Because the resolution provided by input pulse frequency control signals is relatively low in the central region of the S-curve, such a color transition may appear aesthetically poor.

[0018] With reference to FIGURE 2, the inventors have constructed an improved controller 8 for modulating light intensities of light emitting diode strings 10, 12, 14 which overcomes these difficulties. The illustrated controller 8 independently controls three flexible light emitting diode strings, namely a red light emitting diode string 10, a green light emitting diode string 12, and a blue light emitting diode string 14. In other embodiments, one, two, three, four, or more strings may be similarly controlled, and the independently controlled string or strings can be of any color, where "color" as used herein is intended to encompass "white" as a color. In some embodiments, a single string of light emitting diodes may be controlled. In some embodiments, two or more strings of light emitting diodes all of the same color may be independently controlled. Moreover, it is to be appreciated that the illustrated light emitting diode strings are illustrative examples, and that light emitting diode assemblies other than flexible string assemblies can be similarly controlled, such as light emitting diode spot module assemblies, rigid light emitting diode linear border illumination assemblies, single LED packages, and so forth.

[0019] The controller 8 receives red, green, and blue illuminance level inputs 20, 22, 24 that are indicative of (such as proportional to) the desired red, green, and blue illuminance levels. It is emphasized that the illuminance level inputs 20, 22, 24 are not pulse frequency control signals, in that the illuminance level inputs 20, 22, 24 do not directly specify pulse frequencies. Rather, a red S-curve transform 30 converts the red illumination level input 20 into a pulse frequency 31 for driving the red light emitting diode string 10. A green S-curve transform 32 converts the green illumination level input 22 into a pulse frequency 33 for driving the green light emitting diode string 12. A blue S-curve transform 34 converts the blue illumination level input 24 into a pulse frequency 35 for driving the green light emitting diode string 14.

[0020] In a suitable embodiment, the illuminance level inputs 20, 22, 24 are digital signals having a relatively low resolution, such as an 8-bit resolution, while the output pulse frequencies 31, 33, 35 are suitably represented as analog or higher resolution digital signals. A red pulse frequency modulator 40 drives the red light emitting diode string 10 at the pulse frequency output by the red S-curve transform 30. A green pulse frequency modulator 42 drives the green light emitting diode string 12 at the pulse frequency output by the green S-curve transform 32. A blue pulse frequency modulator 44 drives the blue light emitting diode string 14 at the pulse frequency output by the blue S-curve transform 34. In one approach, for example, the pulse frequency outputs 31, 33, 35 of the S-curve transforms 30, 32, 34 are analog d.c. voltages that control the frequencies of oscillation of an oscillator component (not shown) of each of the respective pulse frequency modulators 40, 42, 44.

[0021] In an illustrative example, the red S-curve transform 30 suitably applies the following transformation:

PR (1)»

where I_R denotes the red illumination level input 20, S_R ^l denotes a red inverse S-curve transformation that is the inverse of the S-curve S_R diagrammatically depicted in FIGURE 1 , D_R denotes a density of the red light emitting diodes, and F_R denotes the red pulse frequency output 31. The symbol A_R denotes an optional scaling factor for converting the output of the red S-curve transform 30 to a frequency control input signal level 31 suitable for driving the downstream red pulse frequency modulator 40. For example, the red pulse frequency modulator 40 may be configured to receive an analog d.c. input in the range 0-10 volts that scales with an output frequency range 0-20 kHz, and the scaling constant A_R=( 10 V/20 kHz). While a linear scaling factor A_R is illustrated, it will be appreciated that more complex scaling can be used, for example including a d.c. offset value or so forth. The terms "scaled", "scalable", and the like as used herein are intended to encompass scaling of the form "ax" (that is, linear), "ax+b" (that is, linear with an offset), and the like. (Similarly, the red, green, and blue illuminance level inputs 20, 22, 24 that are indicative of the desired red, green, and blue illuminance levels are typically scaled versions of the actual desired illuminance levels, and such scaling may include linear scaling, linear-with-offset scaling, or so forth). For the illustrated flexible linear string of red light emitting diodes 10, the density D_R is suitably in units of light emitting diode packages per unit length. For a focused spotlight, the density may suitably be in units of total number of red light emitting diode packages in the spotlight. The inverse S-curve S_R ^l is used because the input is the illuminance level input 20, and the output is the pulse frequency 31. In contrast, the S-curves of FIGURE 1 take as input the pulse frequency (the x-coordinate) and output the illuminance level (the y-coordinate). In an actually constructed embodiment, the function S^¹ was implemented as an empirically derived look-up table pairing pulse frequencies with corresponding measured illuminance levels. A frequency range was selected for setting up the look-up table, and (frequency, illuminance) pairs were selected to ensure substantially uniform coverage of the current range. This results in relatively few (frequency, luminance) pairs at the low end. Optionally, the upper low-slope region of the S-curve is not included in the look-up table and operation in the upper low-slope region of the S-curve is not supported. In some embodiments, the lower end of the look-up table extends to zero frequency, so that the "no illuminance" operating condition corresponds to zero frequency and no power is applied to the light emitting diode string 10 in the "no illuminance" operating condition. While a look-up table was used in the actually constructed embodiment, it is also contemplated to implement the inverse S-curve in other ways, such as using a computational algorithm implemented, for example, by a suitably programmed microcontroller or microprocessor. In analogous fashion, the green S-curve transform suitably applies the following transformation:

where I_G denotes the green illumination level input 22, S^¹ denotes the green inverse

S-curve transformation that is the inverse of the S-curve S_G diagrammatically depicted in FIGURE 1, D_G denotes a density of the green light emitting diodes, A_G denotes an optional scaling factor for converting the output of the green S-curve transform 32 to a frequency control input signal level 33, and F_G denotes the green pulse frequency output 33. The blue S-curve transform suitably applies the following transformation:

where /g denotes the blue illumination level input 24, S_B ^l denotes the blue inverse S-curve transformation that is the inverse of the S-curve S_B diagrammatically depicted in FIGURE 1 , D_B denotes a density of the blue light emitting diodes, A_B denotes an optional scaling factor for converting the output of the blue S-curve transform 34 to a frequency control input signal level 35, and F_B denotes the blue pulse frequency output 35.

[0022] With reference back to FIGURE 1, the use of the S-curve transforms optimizes usage of the available resolution. To illustrate, in FIGURE 1 the parameter ΔI denotes a change in illuminance corresponding to a one-bit change in the red illuminance level input 20. The pulse frequency change ΔF\_OW denotes a pulse frequency change corresponding to a one-bit change ΔI of the red illumination level input 20 for a low illumination level. The pulse frequency change ΔF[_0W is large due to the relatively flat lower portion of the red S-curve transformation S_RQ. By comparison, the pulse frequency change ΔF_m,d denotes a pulse frequency change corresponding to a one-bit change ΔI of the red illumination level input 20 for a mid-level illumination level. The pulse frequency change ΔF_md is substantially smaller due to the larger slope of the middle portion of the red S-curve transformation S_RQ. While not expressly shown in FIGURE 1, it can be seen that the relatively flat upper portion of the red S-curve transformation S_RQ will produce a large pulse frequency change corresponding to a one-bit change at higher illumination levels. The effect is have higher pulse frequency resolution in the middle region where it is best utilized to map the relatively rapid increase in illuminance, and lower pulse frequency resolution at the low and high ends where the illuminance is less sensitive to the precise pulse frequency level.

[0023] In the calibrations performed by the inventors, an inverse S-curve transformation relating illuminance to pulse frequency was found to be advantageous. However, in actually constructed embodiments of the controller of FIGURE 2, the transform processors 30, 32, 34 implemented the illuminance-to-pulse frequency conversion using an empirical look-up table. Accordingly, a differently shaped (e.g., other than inverse S-shaped) non-linear illuminance-to-pulse frequency transform curve is readily implemented merely by using an appropriate look-up table. It is anticipated that non-linear relationships other than the illustrated inverse S-curve relationships may be appropriate for certain types of light emitting diodes.

[0024] The transform processors 30, 32, 34 can be implemented in various ways. In one approach, each of Equations (l)-(3) are implemented wholly as lookup tables, so that, for example, the transform processor 30 has a lookup table with paired entries of the form (I_R, F_R). In other embodiments, the lookup tables provide partial conversions and additional analog, digital, or mixed circuitry completes computation of the pulse frequencies. For example, in the illustrated embodiment the transform processor 30 includes a look-up table 50 providing paired entries of the form (I_R, S_R ^] (I_R) ), and the scaling factor — — or other desired scaling is suitably implemented by additional

digital, analog, or mixed circuitry. Similarly, the illustrated transform processor 32 includes a look-up table 52 providing paired entries of the form (I_G, S^¹ (I₀) ), and the illustrated transform processor 34 includes a look-up table 54 providing paired entries of the form (I₈, S_B ^l(I_B)).

[0025] In some suitable embodiments, the controller 8 is used in conjunction with a regulated DC class II power supply, and is connected with the secondary side of the power supply, electrically in between the class II power supply and the light emitting diode strings 10, 12, 14, to perform the illuminance output control through pulse frequency modulation. It is contemplated for the frequency modulation controller 8 to be built into a lighting fixture design or to be implemented as a separate component electrically connectable into the system. The pulse frequency modulation controller 8 can be disposed in a damp location rated box with mounting tabs for mounting to the back or side of an illumination enclosure, with leads coming from the box, or other electrical connectors provided, to connect the light emitting diode strings 10, 12, 14 with the controlled outputs of the controller 8. The illuminance level inputs 20, 22, 24 can be in substantially any suitable format, such as DMX, DALI input, an analog voltage proportional to desired illuminance level, or so forth, and the transform processors 30, 32, 34 convert the respective illuminance level inputs or requests 20, 22, 24 to appropriate pulse frequencies 31, 33, 35 for pulse frequency modulated driving of the respective light emitting diode strings 10, 12, 14. If a class II power supply is used, then each output from the pulse frequency modulation controller 8 is independently controllable and part of a class II system, as part of a control box with power and illuminance requests input and DC voltage as output. This has substantial regulatory advantages with respect to outdoor installations and other harsh environment installations.

[0026] In the foregoing, it has been recognized that the control apparatuses and methods disclosed herein are particularly advantageous when used in conjunction with an illuminance control signal input or inputs that are digital signals each having a low-bit digital illuminance control input representation, for example a representation of 12 bits or less, such as an 8-bit representation. However, the disclosed control apparatuses and methods are also contemplated to be used with higher-bit digital illuminance control input representations, such as 16-bit representations, 24-bit representations, or so forth. In such embodiments, the higher resolution provides substantial smoothing of the intensity transitions, but the control methods and apparatuses disclosed herein continue to have advantages including additional smoothing of intensity transitions, aesthetically improved color transitions, and so forth.

[0027] The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

[0028] The appended claims follow:

Claims

1. A light emitting diode controller comprising: a transform processor receiving an illuminance level input and outputting a pulse frequency control signal accounting for a non-linear relationship between illuminance and pulse frequency; and a pulse frequency modulator that drives one or more light emitting diodes using pulse frequency modulation at a pulse frequency corresponding to the pulse frequency control signal.

2. The light emitting diode controller as set forth in claim 1, wherein the transform processor comprises: a lookup table relating the received illuminance level input with the pulse frequency control signal.

3. The light emitting diode controller as set forth in claim 1, wherein the transform processor comprises: a lookup table relating the received illuminance level input with a value scalable to generate the pulse frequency control signal.

4. The light emitting diode controller as set forth in claim 1, wherein the transform processor accounts for an inverse S-curve relationship between illuminance and pulse frequency.

5. The light emitting diode controller as set forth in claim 1, wherein the illuminance level input is a digital signal with a resolution of twelve bits or less.

6. The light emitting diode controller as set forth in claim 1, wherein the transform processor and pulse frequency modulator comprise a plurality of transform processor/pulse frequency modulator units each controlling one or more light emitting diodes of a different color.

7. The light emitting diode controller as set forth in claim 6, wherein the transform processors each account for an inverse S-curvε relationship between the illuminance of the light emitting diodes of the respective color and the respective pulse frequency.

8. The light emitting diode controller as set forth in claim 6, wherein each illuminance level input is a digital signal with a resolution of twelve bits or less.

9. A lookup table for use in a light emitting diode controller as set forth in claim 1.

10. A light emitting diode control method comprising: transforming an illuminance level input to a pulse frequency control signal accounting for a non-linear relationship between illuminance and pulse frequency; and driving one or more light emitting diodes using pulse frequency modulation at a pulse frequency corresponding to the pulse frequency control signal.

11. The light emitting diode control method as set forth in claim 10, wherein the transforming comprises: retrieving the pulse frequency control signal from a lookup table relating the received illuminance level input with the pulse frequency control signal.

12. The light emitting diode control method as set forth in claim 10, wherein the transforming comprises: retrieving a value scalable to generate the pulse frequency control signal from a lookup table relating the received illuminance level input with said value scalable to generate the pulse frequency control signal.

13. The light emitting diode control method as set forth in claim 10, wherein the transforming accounts for an inverse S-curve relationship between illuminance and pulse frequency.

14. The light emitting diode control method as set forth in claim 10, wherein the illuminance level input is a digital signal with a resolution of twelve bits or less.

15. The light emitting diode control method as set forth in claim 10, wherein the transforming and driving are performed independently for two or more sets of light emitting diodes each of a different color.

16. The light emitting diode control method as set forth in claim 15, wherein the transforming operations each account for an inverse S-curve relationship between the illuminance of the one or more light emitting diodes of the respective color and the respective pulse frequency.

17. The light emitting diode control method as set forth in claim 15, wherein each illuminance level input is a digital signal with a resolution of twelve bits or less.

18. A lookup table for use in a light emitting diode control method as set forth in claim 10.