HK1079955A - Luminary control system - Google Patents

Description

Luminary control system

Technical Field

The present invention relates to light sources.

Background

Light Emitting Diodes (LEDs) are promising candidates for replacing traditional light sources such as incandescent and fluorescent light sources. LEDs have higher light conversion efficiency and longer lifetime. Unfortunately, LEDs produce light in a relatively narrow spectral band. Therefore, in order to produce a light source having an arbitrary color, a hybrid light source having a plurality of LEDs is generally utilized. For example, an LED-based light source can be constructed by combining light from red, green, and blue emitting LEDs, which provide emissions that are perceptually consistent with a particular color. The intensity ratio of the various colors sets the color of light perceived by a human observer.

One way to vary the intensity of an LED is to vary the amount of time that the LED is turned on. In such a scheme, the LEDs are pulsed on a time scale that is too fast to be seen by a human observer. During each cycle, the LED is turned on for some fraction of the cycle time. Because the observer's eye accumulates the light received over a period of time that is long compared to the cycle time, the observer "sees" a light source whose intensity is proportional to the duty cycle, i.e., the proportion of time that the LED is on relative to the time that the LED is off. The intensity is a linear function of the duty cycle and therefore the control system is relatively simple. Alternatively, the intensity of the LED may be varied by varying the magnitude of the current through the LED.

To provide very accurate color reproduction, LED light sources often use sensors and feedback systems. The light source is made up of a combination of three LEDs emitting red, green and blue light and three photodetectors that observe the light generated by the LEDs and adjust the duty cycle of each LED to provide the exact color desired. The feedback circuit must generate a measurement of the average light generated at each color from the pulsed light signal produced by each LED. The average light signal is typically generated by filtering the output of each photodetector with a low pass filter. The low pass filter introduces a delay in the feedback system that is sufficient to cause the feedback system to become unstable when the light source is switched between different colors with a large variation in one or more duty cycles. While this instability can be reduced by using complex active filters, this solution increases the cost and complexity of the light source.

Disclosure of Invention

The invention comprises a light source and a method for controlling such a light source. The light source utilizes a light generator that generates a first light signal at a first wavelength at a first intensity set by a first control signal. The light monitor generates a first monitor signal having an amplitude determined by the first intensity. The target signal generator generates a first target signal having a magnitude indicative of a first target value. The difference circuit generates a first error signal having a magnitude related to a difference of the first monitor signal amplitude and the first target signal amplitude. The feedback controller generates a first control signal in response to the first error signal. The initial control signal generator causes the feedback controller to generate a preliminary first control signal in place of the first control signal, the preliminary control signal being independent of the first error signal and being generated for a first period of time after the first target value becomes a new first target value. In one embodiment, the preliminary first control signal is dependent on the new first target value. In another embodiment, the first control signal comprises a periodic signal that switches between a first value and a second value, wherein the first value causes the light generator to generate light at the first wavelength and the second value at which the light generator does not generate light at the first wavelength. The feedback controller varies a fraction of time that the first control signal is at the first value. In another embodiment, the light monitor includes a light detector and a low pass filter characterized by a delay time, the first time period being set to a value greater than the delay time. Embodiments comprising a plurality of light generators generating light in different spectral bands can also be constructed to provide a colored light source with an output color that can be controlled and rapidly changed.

Drawings

FIG. 1 is a block diagram of a prior art LED light source that utilizes a feedback system to control the duty cycle of the individual LEDs to produce an accurate output color.

FIG. 2 is a block diagram of a light source according to one embodiment of the present invention.

Fig. 3 is a more detailed block diagram of the feedback controller and duty factor predictor.

Fig. 4 is a block diagram of a circuit for generating an analog error signal to adjust the duty cycle.

Detailed Description

The manner in which the present invention provides its advantages can be more readily understood with reference to fig. 1. FIG. 1 is a block diagram of a prior art LED light source that utilizes a feedback system to control the duty cycle of the individual LEDs to produce an accurate output color. The light source 10 generates light of an arbitrary color using red, green, and blue LEDs 11. The LEDs are driven by a driver 12 which sets the current through each LED when it is "on". In the "on" state, each LED is driven at a predetermined current, which is independent of the color generated by the light source 10. The LEDs are driven in a pulsed manner with a cycle time having a period T. During each period, each of the LEDs is turned on for a time t that depends on the color of light to be generated by the light source 10.

To simplify the following discussion, the ratio T/T is referred to as the duty cycle. In principle, assuming that the period T is small enough, as seen by a human observer, the intensity of light from each LED is proportional to T for that LED. Unfortunately, LEDs are not instantaneously turned on and off, and the light output from any LED may also be a function of the duty cycle, as the operating temperature of the LED increases with increasing duty cycle. However, there is a fixed relationship between the desired output color and the duty cycle applied to the three LEDs. The relationship is continuously determined by measuring the actual generated light and adjusting the duty cycle using a servo loop.

Referring again to FIG. 1, the light source 10 includes three photodetectors 16-18 that receive a portion of the light exiting the LEDs. Ideally, each photodetector is configured to respond in the same manner as CIE (international association for illumination) color matching functions X (λ), Y (λ), and Z (λ), respectively. The output of an ideal photodetector will produce a measurement corresponding to the CIE standard color scheme. Alternative standard color schemes may also be used. In practice, light detectors that do not correspond to standard color schemes may be utilized, as long as there is a particular relationship between the light produced by the light source 10 and the light detector output.

Each photodetector has a corresponding interface circuit that is matched to the signal from the corresponding photodetector to the low pass filter. Interface circuits corresponding to the photodetectors 16-18 are shown at 13-15, respectively. An exemplary low pass filter is shown at 19. Each low pass filter is composed of a resistor 20 and a capacitor 21. The values of the resistors and capacitors are selected to equalize the on and off cycles such that the output of the low pass filter is a dc level representative of the intensity of light in each of the three wavelength bands. The output of the low-pass filter is digitized using ADC 22 and compared to a target value stored in register stack 24 in subtraction circuit 23, which subtraction circuit 23 generates a signal related to the difference between the output of the low-pass filter and the target value. The target values represent three intensities corresponding to the desired output color. The difference between the measured and target intensities provides three error signals that are used by the feedback controller 25 to adjust the three respective duty cycles until the measured output coincides with the target value.

When the output color is changed by providing a new target value to the register stack 24, in principle, the feedback loop operates to adjust the duty cycle to new values that will provide the new color. This desired color change is shown at 30. Unfortunately, low pass filters have a considerable time delay. When a new set of color values is introduced into the register stack 24, the error signal increases immediately, because the dc value generated by the low-pass filter, which coincides with the old value, will now differ from the new value. The feedback controller immediately changes the duty cycle. However, the change in duty cycle does not immediately change the output of the low pass filter. Thus, even if the feedback controller accurately infers the correct duty cycle, the error signal will remain non-zero for a period of time determined by the RC time constant of the low pass filter. These time constants are long compared to T because the low pass filter has to smooth the pulses in the drive signal to generate the dc value. Thus, a number of T periods have elapsed before the result of the first adjustment is seen at the output of the low pass filter. During this transition period, the feedback controller will continue to vary the duty cycle based on the now incorrect error signal. As a result, the feedback loop may become unstable.

These instabilities can be avoided by updating the duty cycle at more sparse intervals, or by using more complex active filter designs that increase cost and complexity of the feedback system. Typically, the feedback controller samples the error signal on a periodic basis. The feedback system will be stable if the time between samples is much longer than the RC time constant of the low pass filter. However, if the difference between the old duty cycle and the correct new duty cycle is too large, the time required to settle to the correct new duty cycle will be too long. In many applications, such long stabilization times are not acceptable.

The present invention avoids these problems by using a two-phase system to change the duty cycle when introducing new color values. In the first phase, a close approximation of the final duty cycle is used to immediately change the duty cycle. During this phase, the feedback loop is disabled. Thus, the output of the LED immediately changes to a reasonable approximation of the final duty cycle. After the low pass filter has had time to settle, the system enters a second phase in which the feedback loop is enabled and takes over the adjustment of the duty cycle to provide the final output color. Because the approximation introduced in the first phase is close to the final value, the error signal at the beginning of the second phase will be small. Thus, even in the case where the update cycle time is much longer than the RC time constant of the low pass filter, the time required to settle to the final value is reduced.

Referring now to fig. 2, fig. 2 is a block diagram of a light source 100 according to one embodiment of the present invention. To simplify the following discussion, those elements of light source 100 that serve the same function as those discussed above with reference to fig. 1 are assigned the same reference numerals and will not be discussed here. The light source 100 comprises a duty cycle predictor 126, which duty cycle predictor 126 is activated when a new color setting is fed into the color register 124. When the duty cycle predictor 126 is activated, it disables the feedback controller 125. The duty cycle predictor 126 then provides a pulse sequence to the LED driver 12 based on the new color value. After a sufficient period of time has elapsed, the duty cycle predictor 126 releases the feedback controller 125, and the feedback controller 125 takes over the generation of the pulse train and sets the duty cycle using a conventional feedback strategy.

As noted above, the duty cycle of each color LED is proportional to the intensity of light to be generated by that LED. The color input to the color register 124 is represented by a color triplet (R, G, B), where R is the light intensity in the red region, G is the light intensity in the green region, and B is the light intensity in the blue region. Corresponding duty factor estimation (D)_r，D_g，D_b) And (4) showing. The duty cycle estimate to be utilized with any particular color value may be obtained from experimental data collected prior to deployment of the light source into an application. Typically, this data is obtained during a photodetector calibration procedure. Such a process includes measuring a light source using a spectrometer that outputs three values of a standardized color scheme for each of three conditions: (D)_r，D_g，D_b)＝(1，0，0)、(D_r，D_g，D_b) (0, 1, 0) and (D)_r，D_g，D_b) Where 1 corresponds to the LED being "on" during the entire period T and 0 corresponds to the LED being "off" during the entire period T. The output of the spectrometer is represented by a triplet (X, Y, Z). Each of the three measurement conditions produces a spectrometer output triplet, which is: (Xr, Yr, Zr), (Xg, Yg, Zg), and (Zb, Yb, Zb). These three triplets may be combined to form a matrix S:

now, a relationship can be made between the output of the spectrometer and the duty cycle.

Alternatively, the first and second electrodes may be,

in this way, a duty factor triplet may be generated for a given color requirement (the color requirement must correspond to the same color scheme as the spectrometer). Ideally, the spectrometer and the (R, G, B) triplet input to the color register 124 correspond to the same standard color scheme. In such a case, any (R, G, B) triplet will possess the corresponding duty cycle estimate generated by the above equation. In the non-ideal case where the spectrometer and color register 124 correspond to different color schemes, a translation process is required to change one color scheme to another.

(X, Y, Z) and (D) given by the S matrix_r，D_g，D_b) The relationship therebetween may become increasingly inaccurate over the life of the light source 10. This relationship will also vary with operating conditions. However, the effect of this relationship is to generate an estimate of the duty cycle that is only a close, reasonable approximation of the final duty cycle. Changes caused by aging and operating conditions have no major effect on this function. However, the S matrix may be updated periodically throughout the lifetime of the light source, if desired.

The manner in which a light source according to one embodiment of the present invention operates can be more readily understood with reference to fig. 3, which is a more detailed block diagram of the feedback controller 330 and the duty factor predictor 340. The pulse train driving the LEDs is generated by red, green and blue pulse generators shown at 311-313. Each pulse sequencer has a corresponding duty cycle register containing digital values representing the duty cycle to be used by that pulse sequencer. The duty cycle registers corresponding to the pulse generators 311-313 are shown at 321-323, respectively. The pulse sequencer uses clock 329 to define the pulse period T shown in fig. 1. In the embodiment shown in fig. 3, at the beginning of each pulse cycle, the duty cycle value is input to a countdown register in the pulse generator, the output of which is set to a first value. At each clock pulse, the countdown register is decremented. When the count in this register reaches 0, the output of the pulse generator is set to a second value.

Periodically updating the controller 325 cycles the ADC shown in fig. 2 through the outputs of the low pass filters and reads the error signals generated when each digitized low pass filter output is compared to the corresponding RGB values from the register stack 124 shown in fig. 2. The error signal is then used to calculate updates, which are entered into duty cycle registers 321-323. The update period used by update controller 325 is preferably synchronized with the pulse period used by the pulse generator. The connection between the register defining the pulse period and the clock 329 is omitted from the figure for simplicity of illustration.

In this embodiment, duty cycle predictor 340 is connected to duty cycle registers 321-323 and register stack 124. When the contents of the register 124 change, the duty factor predictor 340 calculates duty factor estimates corresponding to the new values in the register stack 124 and causes these values to be loaded into the duty factor registers 321-323. The duty factor controller 340 then sends a signal to the update controller 325 causing the update controller 325 to stop updating the contents of the duty factor registers 321-323. For example, this control function may be implemented by inserting a series of gates between the duty cycle register input and the update controller 325, which are controlled by the duty factor controller 340. After the error signal has stabilized, the duty cycle predictor 340 returns the update controller 325 to a mode in which it provides periodic updates to the duty cycle registers 321-323.

As noted above, the duty cycle predictor 340 prevents the update controller 325 from updating the duty cycle registers 321-323 for some period of time after the register stack 124 receives the new value. The length of this period of time may be set by any of a number of criteria. The simplest approach is to wait a fixed period of time sufficient to ensure that the error signal value has time to establish a new value reflecting the duty cycle that was fed into the duty cycle registers 321-323 by the duty cycle predictor 340. Since the RC time constant of the low-pass filter is known, the time required for the error signal to be sufficiently stable in the worst case can be determined. However, if the content of the duty cycle registers 321-323 varies significantly less than this worst case, the first phase of the color change protocol may be longer than necessary.

Alternatively, the duty cycle predictor 340 may directly check the error signal to determine when the error signal has stabilized sufficiently to enable the update controller 325 to continue its normal operation. Such an embodiment assumes that during the first phase of the update process, update controller 325 continues to operate as follows: in preparation for the generation and reading of the error signal.

The above-described embodiments of the present invention utilize a digitization-based error signal generation scheme. Embodiments may also be constructed in which the error signal used by the update controller is an analog signal. Referring now to fig. 4, fig. 4 is a block diagram of a circuit for generating an analog error signal to adjust the duty cycle. In this embodiment, the outputs of the low pass filters 425-427 are compared to the corresponding analog target signals generated by the DACs 445-447 in the target register stack 440. There is one such DAC for each register in the stack. Registers corresponding to DACs 445-447 are shown at 441-443, respectively. In the subtraction circuits 451 to 453, the output of the low-pass filter is compared with the analog target signal. The resulting three error signals are then sent to a feedback controller.

The embodiments of the duty factor predictor described above utilize a particular functional relationship between the target RGB value and the duty factor. Alternatively, a look-up table may be used to map the relationship between the target RGB value and the duty factor.

It should be noted that the present invention can accommodate relatively long feedback loop update time periods. When the target color changes, the present invention immediately switches to a color that is close to the specified final color. When the feedback loop is released to fine tune the duty cycle, the difference between the predicted and final values is relatively small, so few updates are required to reach the final value.

The above-described embodiments of the present invention utilize 3 LEDs as the light generators. However, embodiments of the invention utilizing a different number of LEDs may also be constructed. Any number of LEDs may be used as long as there is a particular relationship between the LED light output and the photodetector output. The minimum number of LEDs is 1.

In addition, the present invention is not limited to the use of LEDs as light generators. Any light generator that provides an output spectrum that can be individually monitored may be used. For example, a laser may be used in place of the LED discussed above.

The above-described embodiments of the present invention utilize an intensity control scheme in which the duty cycle of the LEDs is varied to vary the intensity of the light generated by the light source. However, the present invention may also be applied to a light source in which the intensity of light generated by each LED is changed to change the intensity of the light source. In such an embodiment, the duty cycle predictor described above would be replaced by circuitry that sets the initial intensity values when the target color values are changed. Such an embodiment will settle to a new intensity value more quickly than an embodiment that does not utilize an initial value that depends on the new target value when the target value is changed.

Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the invention is to be limited only by the scope of the following claims.

Claims (12)

1. A light source, comprising:

a light generator that generates a first light signal at a first wavelength at a first intensity set by a first control signal;

a light monitor that generates a first monitor signal having an amplitude determined by the first intensity;

a target signal generator that generates a first target signal having a magnitude indicative of a first target value;

a differential circuit that generates a first error signal having a magnitude related to a difference of the magnitude of the first monitor signal and the magnitude of the first target signal;

a feedback controller that generates the first control signal in response to the first error signal; and

an initial control signal generator that causes the feedback controller to generate a preliminary first control signal in place of the first control signal, the preliminary control signal being independent of the first error signal and being generated for a first period of time after the first target value becomes a new first target value.

2. A light source according to claim 1, wherein the preliminary first control signal is dependent on the new first target value.

3. A light source as claimed in claim 1, wherein the light generator comprises a light emitting diode.

4. The light source of claim 1, wherein the light generator comprises a laser.

5. The light source of claim 1, wherein the first control signal comprises a periodic signal that switches between a first value that causes the light generator to generate light at the first wavelength and a second value at which the light generator does not generate light at the first wavelength.

6. The light source of claim 5, wherein the feedback controller varies a fraction of time that the first control signal is at the first value.

7. The light source of claim 1, wherein the light monitor comprises a light detector and a low pass filter.

8. The light source of claim 7, wherein the low pass filter is characterized by a delay time, and wherein the first time period is greater than the delay time.

9. The light source of claim 1, wherein

The light generator generates a second light signal at a second wavelength at a second intensity set by a second control signal,

the optical monitor generating a second monitor signal having an amplitude determined by the second intensity,

the target signal generator generating a second target signal, the second target signal having an amplitude indicative of a second target value,

the difference circuit generates a second error signal having a magnitude related to a difference of the magnitude of the second monitor signal and the magnitude of the second target signal;

the feedback controller generates the second control signal in response to the second error signal; and

the initial control signal generator causes the feedback controller to generate a preliminary second control signal in place of the second control signal, the preliminary control signal being independent of the first and second error signals and being generated for a second period of time after the first or second target value changes.

10. A method for controlling a light source that generates an optical signal of one wavelength at an intensity set by a control signal, the method comprising:

forming an error signal from a monitor signal and a target signal, the monitor signal having an amplitude determined by the strength, the target signal having an amplitude indicative of a target value, the error signal having a magnitude related to a difference of the amplitude of the monitor signal and the amplitude of the target signal;

the control signal is generated in response to the error signal, and a preliminary control signal is generated in place of the control signal, the preliminary signal being independent of the error signal and being generated for a period of time after the target value becomes a new target value.

11. A method according to claim 10, wherein the preliminary control signal is dependent on the new target value.

12. The method of claim 10, wherein the control signal comprises a periodic signal that switches between a value that causes the light source to generate light of the wavelength and a second value at which the light source does not generate light of the wavelength.