Method and apparatus for networked illumination devices

An intelligent light source converts color and luminous flux data to luminous flux levels of individual color sources and automatically compensates for variations in operating conditions.

TECHNICAL FIELD

The present invention relates to the control of one or more illumination devices and, in one embodiment, to a communication protocol that transmits data encoded with color information.

BACKGROUND

Light-emitting diode (LED) technology has advanced to the point where LEDs can be used as energy efficient replacements for conventional incandescent and/or fluorescent light sources. One application where LEDs have been employed is in ambient lighting systems using white and/or color (e.g., red, green and blue) LEDs. Like incandescent and fluorescent light sources, the average luminous flux of an LED's output is controlled by the average current through the device. Unlike incandescent and fluorescent light sources, however, LEDs can be switched on and off almost instantaneously. As a result, their luminous flux can be controlled by switching circuits that switch the device current between two current states to achieve a desired average current corresponding to a desired luminous flux. This approach can also be used to control the relative intensities of red, green and blue (RGB) LED sources (or any other set of colored LED sources) in ambient lighting systems that mix colored LEDs in different ratios to achieve a desired color.

One approach to LED switching is described in U.S. Pat. Nos. 6,016,038 and 6,150,774 of Meuller et al. These patents describe the control of different LEDs with square waves of uniform frequency but independent duty cycles, where the square wave frequency is uniform and the different duty cycles represent variations in the width of the square wave pulses. U.S. Pat. Nos. 6,016,038 and 6,150,774 describe this as pulse width modulation (PWM). This type of control signal has high spectral content at the uniform frequency and its odd harmonics, which can cause electromagnetic interference (EMI) to sensitive devices, components, circuits and systems nearby.

U.S. Pat. Nos. 6,016,038 and 6,150,774 also describe a conventional networked illumination system that utilizes a DMX512 protocol to address network data to multiple individually addressed microcontrollers from a central network controller. Using the DMX512 protocol, the relative luminous flux of each individual color in a light source is transmitted from a lighting controller to a light source, as illustrated inFIG. 1A.

In solid-state (LED) lighting, the luminous flux output of each LED at a given operating current decreases as the junction temperature of the LED increases. LED junction temperature can increase due to power dissipation in the LED and/or increases in ambient temperature. This effect, illustrated in the curves ofFIG. 1B, can create both luminous flux errors and errors in color mixing because the magnitude of the effect is different for LEDs of different colors.

Another temperature effect in LEDs is a shift of the dominant wavelength of an LED as the junction temperature of the LED changes. Typically, the dominant wavelength increases as junction temperature increases, causing a red shift. This effect can cause additional color distortion independent of the luminous flux effects.

Another effect in LED lighting networks is LED aging. In general, the luminous flux of an LED decreases with accumulated operating time. The rate of decrease is different for different color LEDs and is affected by the operating current and temperature of the LED. This effect can cause luminous flux errors and color distortion independent of the other effects mentioned above.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques are not shown in detail or are shown in block diagram form in order to avoid unnecessarily obscuring an understanding of this description.

In the following description, reference may be made to colorimetry and lighting systems based on a red-green-blue (R/G/B) primary color system for convenience and ease of explanation. It will be appreciated that the methods and apparatus described herein are equally applicable to any other color system.

A networked lighting apparatus is described that, in one embodiment, includes a receiver to receive a commanded color point and/or luminous flux value, sensors and circuitry for determining 1) the junction temperatures of the LEDs, and/or 2) the output intensities of the LEDs, and/or 3) the output wavelengths of the LEDs, and/or 4) the accumulated operating time of the LEDs, and a processing device that controls the luminous flux of each LED using the commanded color point and/or luminous flux value, and the junction temperatures of the LEDs and/or intensities of the LEDs and/or the output wavelengths of the LEDs and/or the accumulated operating time of the LEDs.

In one embodiment, the method and apparatus described herein uses a lighting controller to transmit (command) a particular color point specific to a standard color chart and/or a luminous flux value. The luminous flux value may be expressed, for example, as an absolute luminous flux (e.g., in lumens) or as a relative luminous flux (e.g., 50% of a maximum level).

An example of a standard color chart200(rendered in gray-scale for convenience of illustration) is illustrated inFIG. 2for the CIE 1931 RGB chromaticity standard where every visible color point (represented by area201) within a color gamut determined by a set of RGB LEDs, can be specified by an x-y coordinate pair corresponding to a specific R/G/B ratio. The luminous flux of a particular color point can be represented by a third coordinate (e.g., a z coordinate) normal to the plane of the color chart. InFIG. 2, the numbers around the perimeter of area201(ranging from 380 to 700) indicate the wavelengths (in nanometers) of monochromatic light sources corresponding to the respective point on the perimeter. The color point and luminous flux values may be represented by digital words of predetermined bit length that may be transmitted by a lighting controller and received by an intelligent light source device as described herein.

FIG. 3illustrates one embodiment of an intelligent light source device300. In one embodiment, the intelligent light source device is composed of one or more illumination devices (e.g., red LED module301, green LED module302and blue LED module303), a receiver and processing device310including illumination device drivers (e.g., red control module311, green control module312and blue control module313). In one embodiment, the illumination devices may include environmental and/or optical sensors as described in greater detail below. In one embodiment, the processing device may be a microcontroller and may include a central processing unit (CPU)371to control the operations of the processing device310. Alternatively, other types of processing devices may be used.

The intelligent light source device300may be capable of monitoring/measuring environmental variables such as junction temperature(s) and/or ambient temperature and/or LED operating age, and LED output parameters such as wavelength and/or luminous flux, and setting the correct luminous flux of each LED (or group of like-colored LEDs) to compensate for the environmental changes and output variations to achieve the desired color point and/or luminous flux commanded by the lighting controller. These monitoring/measuring and control functions may be achieved with various feedback control networks as described below.

The intelligent light source device300receives network data from a controller on the lighting network (not shown inFIG. 3) that specifies the coordinates of a color point in a standardized color space, and/or a corresponding total luminous flux of the light source. The color point and total luminous flux data are converted to control signals for different color LEDs such that the combined outputs of the LEDs produce the desired color and/or luminous flux. In one embodiment of the present invention, the lighting controller may be coupled to the intelligent light source device300using a wired communication protocol. Standard wired lighting protocols that may be used include, for example, DMX512 and RDM. In one embodiment, the intelligent light source device300may be coupled to the lighting controller using a wireless communication protocol such as Zigbee® or Wireless USB (WUSB), for example.

The network data (color point and/or total luminous flux) is received by an input register361in the intelligent light source device. The data is decoded in a decoder351and sent to respective color register341and total luminous flux register342. The data in the color register is further separated into x and y coordinate values (in registers332and331, respectively) of a standard color space, such as the CIE 1931 RGB color space. The x coordinate from register332, the y coordinate from register331, and the total luminous flux value from the total luminous flux register342may be sent to a lookup table (LUT) and/or color mixing stage (e.g., RGB color mixing)321, which may be implemented as a firmware algorithm in the above referenced processing device. The x and y coordinates may provide entry points into the LUT as is known in the art. In the exemplary case of RGB color mixing, the LUT may generate digital RGB values, in a ratio (i.e., R::G::B) that corresponds to the color point, that may be used by the firmware algorithm in conjunction with the total luminous flux value to generate digital dimming values (e.g., absolute or relative luminous flux values) for each color LED module (e.g., RDIM, GDIMand BDIM), such that the combination will produce the desired overall color and total luminous flux. The digital dimming values are converted to analog control signals (e.g., RCONTROL, GCONTROLand BCONTROL) in a respective LED control module (e.g., red control module311, green control module312and blue control module313) that control the peak and average currents of the LED Modules (e.g., red, green and blue LED modules301,302and303) as described below. Current and voltage sensing in each LED module may provide feedback (e.g., RSENSE, GSENSEand BSENSE) to its respective control module to maintain the required peak current values. As described in greater detail below, a setup firmware module100may provide setup values to control modules311-313that configure the operation of the control modules. The setup values may include peak current values for the LED modules and initial values for pseudorandom number generators in the LED control modules as described below. In one embodiment, the setup firmware may be a one-time programmable (OTP) module that may be externally programmed during an initial setup operation of the intelligent light source device300. In one embodiment, the setup firmware may be reprogrammable.

FIG. 4Aillustrates an exemplary LED module30and an exemplary LED control module31with associated circuitry of an intelligent light source device according to one embodiment of the present invention. In this embodiment, the LED control module31includes a stochastic signal density modulation (SSDM) dimming circuit (“SSDM Controller”)32that receives a digital dimming value from the LUT and R/G/B firmware321and generates a waveform that controls the average LED current of LED39in LED module30. In this embodiment, the LED control module31also includes a pseudorandom sequence (PRS) current control circuit (“PRS Controller”)33that generates a waveform to control the peak current in LED39. The SSDM Controller32and the PRS Controller33are programmed with waveform setup values by the setup firmware100during an initial setup operation as described in greater detail below. The LED control module also includes a Proportional Controller36that may be programmed with a peak current value by the setup firmware100during the initial setup operation. The Proportional Controller36may be connected to an analog-to-digital converter (ADC)35that digitizes a SENSE signal (e.g., RSENSE, GSENSEand/or BSENSE) from the LED module30, which may be an LED current sensing voltage, in a feedback control circuit, described below.

The outputs of the SSDM Controller32and the PRS Controller33may be logically AND'd by AND gate34and passed through a lowpass filter/biasing network (LPF)37to generate a control signal that is used to control and modulate current in LED39. The SSDM controller32may be clocked at a ‘slow’ clock frequency (e.g., a kilohertz rate) that is below a nominal cutoff frequency of the LPF37. The PRS Controller may be clocked at a ‘fast’ clock frequency (e.g., a megahertz rate) that is above the nominal cutoff frequency of the LPF. As described below, the outputs of both the SSDM Controller32and the PRS Controller33may be stochastic and characterized by spread-spectrum (i.e., non-uniform frequency) waveforms.

FIG. 5is a functional block diagram illustrating one embodiment of the SSDM Controller and associated circuitry of the intelligent light source device300. The SSDM Controller32includes an n-bit stochastic counter51, an n-bit signal density register52and a comparator53.FIG. 6is a functional block diagram illustrating one embodiment of stochastic counter51. The stochastic counter51includes the n-bit polynomial register54that receives a setup value (polynomial value) from the setup firmware100, as described above. Stochastic counter51may also include an n-bit linear feedback shift register (LFSR)55configured as an n-bit pseudorandom number generator that generates pseudorandom numbers between 1 and 2n−1 at a rate equal to fclock(where fclock=slow clock). Stochastic counter51may also include an output register56to hold the pseudorandom number outputs of LFSR55for comparison with an output of signal density register52as described below.

The polynomial value in the polynomial register54configures the linear feedback shift register (LFSR)55and initializes (seeds) the pseudorandom sequence that is generated by the LFSR55. The polynomial value may be programmed into the setup firmware100during an initial setup operation.

The value in the polynomial register54corresponds to the coefficients of a polynomial equation that configures the LFSR55. A linear feedback shift register, in one embodiment, is a shift register with tap points and one or more exclusive-or (XOR) gates that determine the next value in the shift register when the register is clocked by a clock signal, such as fclock.

For example, a 4-bit LFSR is characterized by a polynomial equation of the form ax4+bx3+cx2+dx+1, where a, b, c, and d are equal to either 1 or 0. A coefficient of 1 for the xnterm indicates that the nthbit position in the shift register is tapped. Conversely, a coefficient of 0 indicates that the corresponding bit position is not tapped.FIG. 7illustrate an example of a 4-bit LFSR configured with the polynomial equation 1x4+1x3+0x2+0x+1 (=x4+x3+1), according to one embodiment of the present invention. The output of the XOR gate is a function of its inputs from Bit3and Bit4, according to the truth table illustrated inFIG. 8.

Each time the LFSR is clocked, bits1and2are shifted right, bit3is shifted to the XOR gate, the XOR value is shifted to bit4and bit4is fed back to the XOR gate and to bit1. The pseudorandom number generator is initialized with a seed value that is provided by the setup firmware100through the signal density register52.FIG. 9is a table illustrating the sequence of register values (Bits1-4) in the example 4-bit LFSR with the exemplary configuration and initial conditions corresponding to a polynomial value of [0,0,1,1] and a seed value of [0,0,0,1].

Each time the state of the LFSR55changes, the new value is transferred to the output register56, where it is compared with an n-bit dimming value in the signal density register52. The signal density register52in the SSDM Controller receives the n-bit dimming value between 0 and 2n−1 from the LUT and R/G/B firmware321, which represents a desired average value (e.g., in the range of 0% to 100%) of the output waveform of the SSDM Controller32corresponding to an average LED current (e.g. through LED39). The n-bit dimming value in the signal density register52is compared with the n-bit output of the stochastic counter51. When the output value of the stochastic counter51is at or above the output value of the signal density register52, the output of the comparator53is in a first state (e.g., a logical “1). When the output value of the stochastic counter51is below the output value of the signal density register52, the output of the comparator is in a second state (e.g., a logical “0”). It will be appreciated that different definitions of “first state” and “second state” are possible, depending on a particular choice of logic notation, without affecting the principles of operation of the present invention. As a result, the output of the comparator is a stochastic (pseudorandom) waveform with a code length of 2n−1, a clock rate of fclock=slow clock, and a period of 2n/fclock. For the exemplary 4-bit LFSR described above, the output1000of the stochastic counter51over one full period of operation is illustrated inFIG. 10A. Assuming, for example, that the dimming value in the signal density register52is 7 (binary 0111, shown as a line inFIG. 10A), the output of the SSDM Controller32would have the values over one period as illustrated by waveform1050inFIG. 10B.

As illustrated inFIG. 10A, the output of the stochastic counter51has a uniformly distributed pseudorandom output and the SSDM output waveform of the SSDM Controller32has a multiplicity of pseudorandom pulse widths within each period. As a result, the spectral content of the output of the SSDM Controller32is distributed (i.e., has a non-uniform frequency). The average value of the SSDM output1050is determined by the dimming value in the signal density register52.

FIG. 11is a functional block diagram illustrating one embodiment of PRS Controller33and associated circuitry in the intelligent light source device300. The PRS Controller33includes separate instances of an n-bit stochastic counter51, an n-bit signal density register52and a comparator53as described above with respect to the SSDM Controller32, which may have equivalent structure and function, but may generate different values depending on, for example, a particular polynomial or other programming variable (e.g. a peak current value as described below). It will be appreciated that while the stochastic counter51in SSDM Controller32may be structurally equivalent to the stochastic counter in PRS Controller33, the two stochastic counters may be clocked at different clock rates (e.g., “fast clock” and “slow clock”) and can be independently programmed with different polynomial values and seed values during the initial setup operation.

The signal density register52in the PRS Controller33contains an n-bit current value between 0 and 2n−1, which represents a desired average value (e.g., 0% to 100%) of the output waveform of the PRS Controller33corresponding to a peak LED current (e.g., through LED39). The n-bit current value in the signal density register is set by the Proportional Controller36, which is programmed with a peak current value by the setup firmware100during the initial setup operation, as one of the setup values. In one embodiment, as in the case of the polynomial values, the peak current is only programmed into the Proportional Controller a single time.

In operation, the Proportional Controller36compares a programmed n-bit peak current value with the output of ADC35. The output of ADC35is a digital representation of an analog sense voltage from LED Module30that is proportional to LED peak current. If the output value of the ADC35is below the n-bit peak current value in the Proportional Controller36, the Proportional Controller36increases the n-bit current value in the signal density register52. If the output value of the ADC35is above the n-bit peak current value, then the Proportional Controller36decreases the n-bit current value in the signal density register52.

The n-bit current value in the signal density register52(which is set by Proportional controller36) is compared with the n-bit output of the stochastic counter51. When the output value of the stochastic counter51is at or above the value in the signal density register52, the output of the comparator53is in a first state. When the output value of the stochastic counter51is below the value in the signal density register52, the output of the comparator53is in a second state. Note that the comments above with respect to the SSDM Controller and the arbitrary definition of logic states applies equally to the PRS Controller, here.

As a result, the output of the comparator53in PRS Controller33is a stochastic (pseudorandom) waveform with a code length of 2n−1, a clock rate of fclock=fast clock, and a period of 2n/fclock. As in the case of the SSDM Controller32, the waveform has a multiplicity of pseudorandom pulse widths within each period and a distributed, non-uniform frequency content with a higher average frequency due to the increased clock rate.

As described above (referring toFIG. 4A), the output waveform of the SSDM Controller32and the output waveform of the PRS Controller33are logically AND'd by AND-gate34to generate an LED control signal. In one embodiment, the LED control signal may be connected to the gate of a MOSFET38(or other current control element as is known in the art) through a lowpass filter (LPF)37. The drain of the MOSFET38is connected to a power source VDD, and the source terminal of the MOSFET38is connected to the anode of LED39(LED39may be a single LED in one embodiment or some series-parallel combination of LEDs having an anodic terminal connected to the source terminal of MOSFET38). The cathode of the LED (or the cathodic terminal of a combination of LEDs) is coupled to ground through a current sensing resistor RSENSE40.

The voltage developed across RSENSE40is proportional to the current through LED39and may be used in a feedback loop through ADC35. The MOSFET38that controls the LED current is located between the power supply and the anode of LED39. In this configuration, the MOSFET38operates as a current source for LED39and the cathode of LED39can be referenced to ground through the sense resistor RSENSE40. As described above, this allows the peak current through LED39to be controlled via the feedback of a sense voltage to ADC35in LED Control Module31in intelligent light source device300.

In one embodiment, a thermistor RTEMP41may be thermally coupled with LED39(via a known thermal resistance and/or thermal time constant) such that the temperature of RTEMP41and the junction temperature of LED39have a known relationship. The resistance of RTEMP41is proportional to the temperature of LED39and may be used, with the LED current and forward voltage sensing, to determine the junction temperature of the LED. A signal (TH) from RTEMP41may be used by the R/G/B Firmware321to compensate the R, G and B SSDM dimming values for luminous flux changes due to temperature to maintain the required intensities of the LED modules (i.e., at the correct ratio to maintain color and at the correct luminous flux levels to maintain total luminous flux).

In one embodiment, as illustrated inFIG. 4B, LED Module30may include a Color Sensor42, which may be optically coupled to LED39(e.g., via a fiber optic light pipe or other means for optical coupling as is known in the art). Color sensor42may be, for example, a single-channel or multi-channel color sensor (e.g., a TCS230 color sensor manufactured by Texas Advanced Optoelectronic Solutions), which may have one or more color filtered channels (e.g., red, green, blue and clear) that may generate a COLOR signal indicative of the red, green and blue content of an LED output, as well as an unfiltered channel to generate an LUMINOUS FLUX signal indicative of the luminous flux of the LED output irrespective of color. The COLOR/LUMINOUS FLUX signal output of color sensor42may be fed back to the LUT and RGB firmware321for correction of the dimming values provided to SSDM controller32to compensate for temperature and aging effects on the dominant wavelength and luminous flux of the LEDs described above.

In operation, the control signal output of LED control Module31may be viewed as the superposition of the output waveform of PRS Controller33(“PRS waveform”) and the output waveform of SSDM Controller32“SSDM waveform”). The PRS waveform is lowpass filtered by LPF37to produce a control voltage that sets the linear operating point of MOSFET38to establish the peak current through the LED. The SSDM waveform, being below the cutoff frequency of LPF37, passes through LPF37without filtering and operates as a switching control voltage that at the gate of MOSFET38that sets the average current through LED39. Additionally, the location of MOSFET38with respect to LED39makes it easier to turn the MOSFET38on and off because the control voltage at the gate of the MOSFET switch does not have to overcome a large source voltage.

FIG. 12A-12Dillustrate representative waveforms1201of the PRS Controller33(FIG. 12A),1202of the SSDM Controller32(FIG. 12B),1203of the resultant LED control signal at the input to the LPF (FIG. 12C) and 1204of the LED current waveform (FIG. 12D), according to one embodiment of the present invention. It should be noted that the relative frequencies and pulse widths of he waveforms are not shown to scale for ease of illustration.

FIG. 13is a flowchart illustrating a method1300for controlling a networked illumination device in one embodiment. The method begins by receiving data (e.g., from a network controller) encoded with a color point and a total luminous flux value of the color point (operation1301). Next, the data is converted to a plurality of stochastic control signals, where each of the plurality of stochastic control signals controls the luminous flux of one of a plurality of light sources, and where the combined output of the plurality of light sources generates the color point at the total luminous flux value (operation1302). Next, each of the plurality of light sources senses operating conditions (e.g., temperature) and generates a feedback signal indicative of the operating condition (operation1303). Lastly, in response to the feedback signals from the light modules, each of the plurality of control modules modifies its stochastic control signals to compensate for changes in the operating conditions, such that the color point and luminous flux value are maintained (operation1304).