Method and system for optical communication

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/831,380, entitled “Method and System for Optical Communication in the Presence of Optical Scanners”, filed Jun. 5, 2013, the contents of which are incorporated by reference herein.

TECHNICAL FIELD

Embodiments of the present invention relate generally to light sources, including light-emitting diodes (“LEDs”), and, more specifically, to encoding information in the light emitted from the light sources.

BACKGROUND

LED-based lamps typically include a power converter (such as a transformer and/or rectifier) to convert an input AC mains voltage to a smaller DC voltage, driver circuitry to generate a constant current given the DC voltage, and LEDs that emit light when driven by the constant current. The driver circuitry may dim the LED by periodically and repeatedly “chopping” portions of the drive current down to zero amps; the nonzero portions of the drive current, however, remain at the constant level, effectively driving the LED with a series of constant-amplitude current pulses. By driving the LED with only one nonzero current value, the driver ensures the light output by the LED does not vary in color temperature.

Information may be encoded in the light output by the LED and transmitted therefrom in a similar fashion. The frequency or width of the constant-amplitude current pulses may be varied such that the changes in frequency and/or pulse width represent encoded information. One such technique, known as pulse-width modulation (“PWM”), samples an input data signal and represents its magnitude as a percentage of pulse width; maximum amplitude corresponds to 100% width, for example, while half amplitude corresponds to 50% width.

There are drawbacks to PWM-based LED driver circuits, however. The rapid switching may consume unnecessary power, and the frequency of switching may cause interference with other electronics. A need therefore exists for an improved method and system for transmitting information via light emitted by LEDs.

SUMMARY

Embodiments of the present invention include systems and methods for driving light sources with continuous-time data signals encoded with information such that the light output by the light source conveys this information. In one embodiment, the continuous-time data signal includes one or more frequencies; a driver circuit varies the current to an LED in accordance with these frequencies, thereby causing the light output to vary and transmit the data therein. A PWM signal that encodes the amplitude of a data signal as pulses of varying width may be filtered or converted to an analog signal and used to drive the LED; in other embodiments, the continuous-time data signal may be generated using oscillators or similar components and used directly to drive the LED. A power balancer may be used to match output power to input power, thereby eliminating or mitigating changes in light output due to the transmission of the information; a dimmer controller may be used to receive a dimming signal and adjust light output by the LED accordingly.

In one aspect, a system for transmitting data via light emitted by a light source includes a continuous-time data-signal source for generating a continuous-time data signal; a driver circuit for generating a drive signal, the drive signal having a drive voltage and a drive current, that provides power to the light source, thereby causing the light source to emit light, wherein variations in the amplitude of the drive signal represent information in the continuous-time data signal; and circuitry for (i) detecting a change in average power delivered to the light source as a result of the variations in the amplitude of the drive signal and for (ii) causing the driver circuit to vary the power delivered to the light source to compensate for the change in average power.

The light source may include a light-emitting diode. Variations in the amplitude of the drive signal may include frequencies less than 100 kHz, 10 kHz, or 1 kHz. The continuous-time data-signal source may include a pulse-width modulator generator for generating a pulse-width-modulated representation of the continuous-time data signal and a filter or digital-to-analog converter for removing a high-frequency component of the pulse-width-modulated representation of the continuous-time data signal to create the continuous-time data signal. The continuous-time data-signal source may include an oscillator and a mixer. The continuous-time data-signal may include a signal blending a plurality of frequencies and/or a plurality of symbols, each symbol represented by changes in frequency modulation. The circuitry for detecting the change in average power may include a comparator for comparing input power to power delivered to the light source. The circuitry for detecting the change in average power may include a current sensor to measure a current flowing in the light source. The circuitry for causing the driver circuit to vary the power delivered to the light source may include circuitry to change a DC offset of the drive signal or circuitry to change pulse widths of a pulse-width-modulated representation of the continuous-time data signal.

In another aspect, a method for transmitting data via light emitted by a light source includes generating, using a continuous-time data-signal source, a continuous-time data signal; generating, using a driver circuit, a drive signal, the drive signal having a drive voltage and a drive current, that provides power to the light source, thereby causing the light source to emit light, wherein variations in the amplitude of the drive signal represent information in the continuous-time data signal; detecting a change in average power delivered to the light source as a result of the variations in the amplitude of the drive signal; and causing the driver circuit to vary the power delivered to the light source to compensate for the change in average power.

The light source may include a light-emitting diode. A pulse-width-modulated representation of the continuous-time data signal may be generated and a high-frequency component of the pulse-width-modulated representation of the continuous-time data signal may be removed to create the continuous-time data signal. Detecting the change in average power delivered to the light source may include sensing a current flowing through the light source or sensing a voltage applied to the light source. Causing the driver circuit to vary the power delivered to the light source may include changing a DC offset of the drive signal or changing pulse widths of a pulse-width-modulated representation of the continuous-time data signal. A dimming signal may be received and the power provided to the light source may be adjusted accordingly.

These and other objects, along with advantages and features of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the same parts throughout the different views. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:

FIGS. 1A and 1B illustrates block diagrams of circuits for driving a light source with a continuous-time signals in accordance with embodiments of the present invention;

FIGS. 2A, 2B, 3A, 3B, and 3C illustrate examples of continuous-time data-signals in accordance with embodiments of the present invention;

FIG. 4 illustrates a block diagram of a circuit that generates a continuous-time data-signal based on a PWM signal in accordance with embodiments of the present invention;

FIG. 5 illustrates an example of a PWM signal;

FIG. 6 illustrates an example of a continuous-time data signal generated from a PWM signal;

FIG. 7 illustrates a block diagram of a circuit that balances power in and power out in accordance with embodiments of the present invention;

FIG. 8 illustrates a block diagram of a circuit that dims a light source in accordance with a dimming input in accordance with embodiments of the present invention; and

FIG. 9 illustrates a block diagram of a circuit that generates a continuous-time data-signal using an oscillator in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention include methods and systems for driving light sources, such as LEDs, using variable currents or voltages, wherein variations in the LED current or voltage are derived from a continuous-time data signal. Light output of LEDs may be linearly proportional to the current flowing in them over a wide bandwidth (e.g., 0-20 mA); an LED driven with a varying current will thus produce light having an intensity proportional to that current. Information encoded in the drive signal as changes in current amplitude or frequency is therefore transmitted as changes in light intensity. A remote receiver, such as a cellular telephone equipped with a camera or light sensor, may receive the emitted light and extract the transmitted information therefrom.

In various embodiments, the transmitter includes a power-balancer circuit that monitors received power and output power; if the output power changes as a result of the information-representative fluctuations in the LED drive current or voltage, the power-balancer circuit may adjust the output power accordingly until the output power matches the input power. In other embodiments, a dimmer controller receives a dimming signal and/or determines a desired dimming level by analyzing the phase of the input power signal (i.e., the amount of phase chopping) and increases or decreases output current accordingly.

FIG. 1A illustrates a block diagram of a circuit 100 for transmitting information via light emitted from a light source 102. A power converter 104 receives power from a power source, such as an AC mains, battery, solar panel, or any other AC or DC source. The power converter 104 may include a magnetic transformer, electronic transformer, switching converter, rectifier, or any other similar type of circuit. One of skill in the art will understand that any circuitry for converting an input power signal into a power signal suitable for other devices in the circuit 100 is within the scope of the present invention. In some embodiments, the power converter 104 is not necessary because the input power signal does not require conversion.

The circuit 100 includes a driver circuit 106 that provides power to the light source 102. The driver circuit 106 may include an AC or DC current source or voltage source, a regulator, an amplifier (such as a linear amplifier or switching amplifier), a buck, boost, or buck/boost converter, or any other similar type of circuit or component. The present invention is not limited to any particular type of driver circuit. The driver circuit 106 outputs a variable voltage or current to the light source 102. The variable voltage or current may include a DC offset, such that its average value is nonzero, and a variable portion, such as an AC ripple, that represents information. In one embodiment, the frequency of the AC signal represents the information. The driver circuit 106 may be controlled by a continuous-time data-signal source 108. The signal source 108 may be any analog, digital, or mixed-signal circuit capable of generating an output signal. As explained in greater detail below, the signal source 108 may include a pulse-width modulation (“PWM”) generator, a filter, a digital-to-analog converter (“DAC”), an oscillator, a mixer, or any other type of similar circuit. The signal source 108 may instead or in addition include a receiver, such as a network interface, for receiving the data signal from an external source. The continuous-time data signal may be any time- or frequency-varying signal having a value that corresponds at all or most points in time to assigned information; in other words, the continuous-time data signal is not modulated, pulsed, chopped, or otherwise altered from its original value(s). In one embodiment, the continuous-time data signal is a baseband signal that is not modulated with a high-frequency carrier signal.

In an alternative embodiment, as shown in FIG. 1B, a circuit 110 includes a discrete-time data-signal source 118, which may generate a PWM or similar signals. The driver 116 receives the PWM signal and generates a continuous-time driver signal to power the light source 112. In other words, the filtering is performed after the driver receives the data signal instead of before, as described above with reference to FIG. 1A.

FIGS. 2A and 2B illustrate two examples of continuous-time data signals. A first signal 200 comprises a DC offset (i.e., its average value is greater than zero) and a sine wave oscillating at a frequency. A second signal 202 also includes a DC offset; the variations in this signal are generated by mixing two sine waves of different frequencies. In other embodiments of the present invention, any number of periodic signals may be combined to produce the continuous-time data signal.

In these examples, current is shown to vary; in other examples, the voltage may similarly vary. The signals 200, 202 may represent the output of the signal source 108 or the output of the driver circuit 106. In one embodiment, the driver circuit 106 receives a time-varying voltage signal from the data source 108 and outputs a corresponding time-varying current signal to the light source 102.

FIGS. 3A, 3B, and 3C illustrate other embodiments of the continuous-time data signal. In these embodiments, the frequency and/or amplitude of the continuous-time data signal may be modulated to encode different information at different points in time. For example, as shown by the data representation 300 in FIG. 3A, three different data symbols may be sent at different points in time. The symbols may correspond to members of a library of symbols, such as binary digits, hexadecimal digits, or any other type of symbol. In one embodiment, as shown in FIG. 3B, a frequency-modulated signal 302 is used to represent the different symbols; a different frequency is assigned to each symbol, and the symbol is transmitted by selecting the appropriate frequency. In another embodiment, as shown in FIG. 3C, an amplitude-modulated signal is used to represent the different symbols.

The frequency of the signals 200, 202, 302, 304 may vary within a range; in one embodiment, the frequency range is 1-1000 Hz, but any frequency is within the scope of the present invention. The signal 200 may oscillate at 500 Hz, for example, and the signal 202 may be the combination of a 400 Hz signal and a 600 Hz signal. The different symbols 300 may correspond to different frequencies or frequency ranges, such as 100 Hz for a first symbol and 200 Hz for a second symbol. Any assignment of frequencies or combinations thereof is within the scope of the present invention. In one embodiment, the frequencies used are sufficiently high (greater than, e.g., 200 Hz or 300 Hz) such that a human eye cannot perceive any fluctuations in light intensity or temperature in light emitted from light sources driven at those frequencies.

FIG. 4 is a block diagram of a circuit 400 that generates the continuous-time data signal using a PWM generator 402. The PWM generator may be a digital processor, microcontroller, digital-signal processor, FPGA, ASIC, or any other type of digital circuit. The PWM signal output by the PWM generator 402 may include pulses that vary in width in accordance with an amplitude of an input data signal, such as the PWM signal 500 shown in FIG. 5.

The PWM signal may be received by a filter or a DAC 404 to remove high-frequency components therein. The filter may be, for example, a low-pass or notch filter, and may include capacitors, inductors, resistors, or any other such components. The DAC may be any type of circuit that samples the PWM signal at intervals and outputs a voltage or current indicative of the level, pulse width, or frequency of the PWM signal. An example of a continuous-time data signal 600 derived from the example PWM signal 500 is shown in FIG. 6. The circuit 400 may also include the power converter 406, driver/amplifier 408, and light source 410 discussed above.

In another embodiment, the filter/DAC 404 may filter and/or sample the PWM signal after it has been amplified by the driver/amplifier 408 (instead of, or in addition to, the filtering/sampling performed on the PWM signal output by the PWM generator 402). In this embodiment, the amplifier 408 is a switching amplifier and receives the PWM signal at one or more switch-control terminals. Then one or more inductors and capacitors are used to filter the output of these switches to remove the high-frequency components.

Referring now to FIG. 7, the light source 702 may be designed to output a certain amount of lumens of light energy given a certain, fixed input voltage or current. The light output of the light source 702 may be increased or reduced, however, if it is driven with a time-varying voltage or current, as described above. In one embodiment, the data source 704 selects a DC offset for the generated continuous-time signal such that the average value of the generated current or voltage matches that of an equivalent fixed voltage or current. For example, if the light source is rated to be driven with a constant current of 20 mA, the signal source 704 and driver 706 may be configured to generate a time-varying drive current that has a maximum value of 25 mA, a minimum value of 15 mA, and an average value of 20 mA.

In another embodiment, the light source 702 is driven with a voltage or current having an average value less than its rated or usual value. For example, if the light source 702 is rated to be driven with a constant current of 20 mA, the time-varying drive current may vary between 10 mA and 20 mA and have an average value of 15 mA. If the data source 704 is shut off for any reason (i.e., no data is being sent), the driver 706 may hold the drive current at the average value (e.g., 15 mA) instead of increasing it to the normal or rated value (e.g., 20 mA) to thereby avoid large swings in light output if the data source 704 is temporarily or permanently disabled.

In one embodiment, a power balancer 708 monitors input power (by, e.g., monitoring an input voltage or current or an output voltage or current of the power converter 710) and output power delivered to the light source 702 (by, for example, monitoring a voltage or current in the light source 702). The power balancer may include an averaging circuit, integrator, or similar circuit element for tracking the history of a signal. If the average or instantaneous output power is greater or lesser than the input power, the power balancer 708 may instruct the driver 706 and/or data source 704 to increase or decrease the drive current or voltage delivered to the light source. In one embodiment, the driver 706 includes a constant-current driver and a switching amplifier; the voltage of the light source 702 is measured and used to set a DC voltage level such that the voltage at the input to the switching amplifier equals the voltage of the light source 702. In another embodiment, the driver 706 includes a constant-voltage driver and the DC level is set using a voltage reference; the time-varying signal is added thereto. In another embodiment, the signal source 704 includes a PWM generator (as described above). The PWM signal may be modified to include not just the continuous-time data signal but also the pulse-width values necessary to provide an average DC current to the light source 702.

The power balancer 708 is depicted as a separate block in FIG. 7, but one of skill in the art will understand that some or all of its functionality may be implemented as part of the light source 702, driver 706, or data source 704. In particular, its functionality may be included in a PWM modulator, as discussed above.

FIG. 8 illustrates a circuit 800 that includes a dimmer controller 802. As one of skill in the art will understand, the light source 804 may be dimmed by an “upstream” phase-cut/chop dimmer circuit, which may set greater or lesser portions of each cycle of an input 60 Hz power signal to zero. The dimmer controller 802 may, in other embodiments, receive a discrete dimming signal. In these and other embodiments, the dimmer controller 802 may instruct the driver 806 and/or signal source 808 to increase or decrease an average power delivered to the light source 804 accordingly. For example, if an average current level delivered to the light source 804 is 20 mA and the dimmer controller 802 detects that 50% of the input power has been chopped by a phase-cut dimmer, the dimmer controller 802 may control the driver 806 to reduce the average power to 10 mA (e.g., 50% of nominal current). The drive current may similarly be adjusted to 5 mA if 75% of the input power has been chopped and to 15 mA if 25% of the input power has been chopped. Similarly, if the discrete dimming input includes a voltage in a range of voltages, the dimmer controller 802 may determine where in the range the voltage lies and adjust the output power level accordingly.

The dimmer controller 802 may work in tandem with the power balancer 708 mentioned above with reference to FIG. 7. Because the power balancer 708 may resist changes in output power, it may resist dimming even when a user intends for the light source 804 to dim. In one embodiment, when the dimmer controller 802 detects that an input dimming signal is active, the power balancer 708 temporarily shuts off in order for the power level delivered to the light source 804 to change. Once the input dimming signal has ceased changing, the power balancer 708 may then re-engage and continue monitoring output power. In other embodiments, the power balancer 708 performs some or all of the functionality of the dimmer controller 802 in that, if the input power is chopped by an upstream phase-cut dinner, the power balancer 708 may accordingly adjust the output power to match.

FIG. 9 illustrates another embodiment 900 of the present invention. In this embodiment, one or more oscillators 902 generate one or more oscillating signals, such as the signal 200 appearing in FIG. 2A, and a mixer 904 may combine the signals to form a multi-frequency signal such as the signal 202 appearing in FIG. 2B. The oscillator 902 and/or mixer 904 may be made using digital or analog components. Unlike the PWM-based embodiment 400 appearing in FIG. 4, the present embodiment 900 may not generate a high-frequency signal and later filter the signal to remove high-frequency components; instead, only low-frequency signals (e.g., 1-1000 Hz) are generated, and filtering may not be necessary.

Certain embodiments of the present invention were described above. It is, however, expressly noted that the present invention is not limited to those embodiments, but rather the intention is that additions and modifications to what was expressly described herein are also included within the scope of the invention. Moreover, it is to be understood that the features of the various embodiments described herein were not mutually exclusive and can exist in various combinations and permutations, even if such combinations or permutations were not made express herein, without departing from the spirit and scope of the invention. In fact, variations, modifications, and other implementations of what was described herein will occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention. As such, the invention is not to be defined only by the preceding illustrative description.