Lighting device

Abstract

A method for digital communication on a pulsed power supply having alternatingly high and low voltage, wherein a logic ZERO is communicated by a low voltage pulse having a first low voltage duration, tlow_zero, followed by a high voltage pulse having a first high voltage duration, thigh_zero, and a logic ONE is communicated by a low voltage pulse having a second low voltage duration, tlow_one, followed by a high voltage pulse having a second high voltage duration, thigh_one, the method comprising detecting a parameter value of at least one parameter affecting a transition time between high and low voltage, and adjusting said first low voltage duration, tlow_zero, and said second low voltage duration, tlow_one, based on said parameter value(s).

Description

[0001] LIGHTING DEVICE

[0002] FIELD OF THE INVENTION

[0003] The present invention generally relates to digital communication on power line (PLC, power line communication).

[0004] BACKGROUND OF THE INVENTION

[0005] In some applications, a power line can be used for digital communication. One way is to superpose high frequency data communication on a DC or AC line. Another approach is to code the logic values using voltage pulse with different pulse width. In this case, the power line voltage is alternated between a low voltage and a high voltage, and the logic value (ONE / ZERO) is determined by pulse width, e.g. of the low-voltage pulse. For example, a short low voltage pulse can represent a ZERO, while a longer low voltage pulse can represent a ONE.

[0006] EP1687898A2 relates to an electrical signaling system comprises a modulator arranged to accept information and encode that information in an alternating signal containing repeated rising and falling edges, the encoding being by way of the time between consecutive rising or consecutive falling edges, a transmission path for the signal from modulator to a demodulator, where the demodulator is arranged to detect the signal edge and store a record of the signal around that edge and compare a subsequent part of the signal with that record thereby to detect a like edge and detect the time difference between like edges.

[0007] G2442892A relates to an electrical signalling system comprising a modulator which accepts information from a plurality of sources and encodes that information as subsequent pulse widths, at least the information from one sensor so that an increase in value corresponds to an increase in pulse width and the information from at least one other sensor is encoded such that and increasing value corresponds to a decreasing pulse width.

[0008] In practice, however, the digital pulses will not be perfect when they are realized as voltage pulses. The transition between low and high voltage will not be instantaneous, but take a certain time, referred to as rise time (from low to high) and fall time (from high to low). The rise time and fall time will slightly reduce the pulse width (both low and high voltage pulses). A problem arises when this discrepancy between digital and voltage pulse width is not constant over time, but dependent on variable factors. If the voltage pulse width differs too much from the nominal values, for which the receiver logic is calibrated, the receiving side may experience difficulties in decoding the pulses.

[0009] SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to mitigate the above-mentioned problems, and to provide a more robust power line coding. This and other objects may be achieved by a method and system according to independent claims 1 and 12. Embodiments of the present invention are defined in the dependent claims.

[0011] According to a first aspect of the invention, there is provided a method for digital communication on a pulsed power supply having alternatingly high and low voltage, wherein a logic ZERO is communicated by a low voltage pulse having a first low voltage duration, tiowzero, followed by a high voltage pulse having a first high voltage duration, thigh zero, and a logic ONE is communicated by a low voltage pulse having a second low voltage duration, tiowone, followed by a high voltage pulse having a second high voltage duration, thigh one. The method comprises detecting a parameter value of at least one parameter affecting a transition time between high and low voltage, and adjusting the first low voltage duration, tiowzero, and the second low voltage duration, tiowone, based on the parameter value(s).

[0012] With this method, the duration (width) of the digital low pulses can be increased when the actual duration (width) of the low voltage pulses is expected to be shorter. For example, when the high voltage has increased (greater voltage gap => longer rise time and fall time) or when the ambient temperature has increased (slower slope => longer rise time and fall time).

[0013] In some embodiments, the first high voltage duration, thigh zero, and the second high voltage duration, thigh one, are also adjusted, so as to maintain a predetermined ratio, H / L, between high voltage pulse duration and low voltage pulse duration.

[0014] The method may include obtaining a nominal high voltage level, and a nominal voltage rate of change during transitions between low and high voltage (at nominal temperature). In this case, if the detected parameters include high voltage level, the step of adjusting the first and second low voltage duration can include calculating a change in pulse duration resulting from a change in high voltage level, given the nominal rate of change. Basically, if the low voltage level is assumed to be constant (e.g. determined by voltage requirements of the circuitry) then an increase in high voltage will cause longer rise and fall times. The low pulse width may thus be increased appropriately in response to an increase in high voltage.

[0015] The method can further include obtaining a relationship between ambient temperature and the voltage rate of change (slope). In this case, if the detected parameters include ambient temperature, the step of adjusting the first and second low voltage duration can include calculating a change in pulse duration resulting from a change in the voltage rate of change caused by a change in ambient temperature. Basically, if the temperature increases, the slope can be expected to be less steep, so that the rise and fall times increase. The low pulse width may thus be increased appropriately in response to an increase in high voltage.

[0016] In some embodiments, the adjusting steps are performed every time the detecting step is performed (at least every time a sufficient difference is detected), resulting in a more or less continuous adjustment of the voltage pulse timing. Alternatively, a set of timing profiles are defined, each timing profile comprising preset values for the first and second low voltage durations and optionally for the first and second high voltage durations. Each timing profile is associated with a parameter range for each of the detected parameters, and an appropriate timing profile may be selected based on detected parameter value(s).

[0017] In some embodiments, a change of timing profile is made by means of overlapping ranges for the parameter value(s), thereby avoiding too frequent changes of profile.

[0018] The pulsed power supply may be used to power a lighting device including at least one LED unit with at least one LED and a LED driver. In this case, the step of adjusting the first low voltage duration, tiowzero, and the second low voltage duration, tiow_one, may further be based on temperature dependence in circuitry in the LED driver(s). Also in this case, the step of adjusting the first low voltage duration, tiowzero, and the second low voltage duration, tiow one, is further based on operation of the LED unit(s).

[0019] According to a second aspect of the invention, there is provided a lighting device comprising at least one LED unit (4), each including a at least one LED (6) and a LED driver (5), a voltage supply (1), and a power line controller (3) configured to provide a pulsed drive signal to the string of LED units (4), and to encode digital communication on the pulsed drive signal for controlling the LED drivers (5), wherein a logic ZERO is communicated by a low voltage pulse having a first low voltage duration, tiowzero, followed by a high voltage pulse having a first high voltage duration, thigh zero, and a logic ONE is communicated by a low voltage pulse having a second low voltage duration, tiowone, followed by a high voltage pulse having a second high voltage duration, thigh one, The power line controller comprises a detector for detecting a parameter value of at least one parameter affecting a transition time between high and low voltage, and the controller is configured to said first low voltage duration, tiowzero, and said second low voltage duration, tiowone, based on said parameter value(s).

[0020] In embodiments, the controller is also configured to adjust said first high voltage duration, thigh zero, and said second high voltage duration, thigh one, so as to maintain a predetermined ratio between low pulse duration and high pulse duration.

[0021] The device may include several LED units, connected in a serial / parallel fashion. For example, the device may include at least one LED set including at least two LED units connected in parallel. Several such LED sets may be serially connected.

[0022] BRIEF DESCRIPTION OF THE DRAWINGS

[0023] This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiments of the present invention.

[0024] Fig. 1 schematically shows a lighting device employing power line communication.

[0025] Fig. 2 shows a diagram of voltage pulse coding.

[0026] Fig. 3 shows a flow chart of an embodiment of the present invention.

[0027] DETAILED DESCRIPTION

[0028] The lighting device 1 in Figure 1 has a power supply 2, e.g. for providing a suitable DC voltage. As an example, the power supply 2 may be configured to be plugged into the power mains (e.g. 110V or 230 V AC) and provide a 24 V DC output voltage. The power supply 2 is connected to a power line control unit 3, which in turn is connected to a string of LED units 4. Each LED unit includes a LED driver 5 and at least one group of different color LEDs 6, in the illustrated example four LEDs. In a typical implementation, each LED unit includes at least a red, a blue and a green LED, enabling the LED unit to emit a wide range of color light.

[0029] The power line control unit 3 is configured to provide a pulsed drive signal to the string of LED units 4, and to encode digital communication on the pulsed drive signal. The digital communication allows control of the LED units 4. The electrical power delivered to the LEDs 6 is determined by the voltage levels (high / low) of the pulsed drive signal and by the ratio between high and low voltage.

[0030] The control unit 3 is provided with detection circuitry 7, for measuring the input voltage from the power supply 2, and ambient temperature. Also other parameters may be detected.

[0031] In one implementation, the LED units are connected in serially connected groups of parallel connected LED units. Specifically, in one example, five groups are connected in series, and each group contains 50 LED units 4 connected in parallel, leading to in total 250 LED units 4. The high and low voltage levels outputted by the control unit 3 are typically 24 V and 12 V, which means that an individual LED unit 4 will receive high and low voltage levels of 24V / 5=4.8 V and 12V / 5=2.4 V. In fact, due to relatively long wires, causing a slight voltage drop, the actual voltage applied to LED units 4 may be slightly lower.

[0032] The high voltage (i.e. in our case 4.8 V) is high enough to activate the LEDs 6. The low voltage is too low to activate the LEDs 6. This means that the ratio H / L between high voltage and low voltage must be sufficiently high to ensure a desired light output. Further, the low voltage must be sufficiently high so that the circuitry in the LED drivers 5 is always powered ON.

[0033] The logical values (ONE / ZERO) of the digital communication are determined by the duration of the low voltage pulses. For example, a first, shorter duration, tiowzero, can indicate a logical ZERO, while a second, longer duration, tiow_one, may indicate a logical ONE. The pulse width is typically in the order of microseconds, and the durations tiow_zero and tiow one may be e.g. 3 ps and 6 ps, respectively. As mentioned, a ratio H / L between high voltage and low voltage duration is stipulated by the required brightness. Therefore, the corresponding high voltage pulse durations, thigh zero and thigh one will be selected to maintain this given ratio H / L.

[0034] Each drive unit 5 is configured to receive the pulsed drive signal, and decode data encoded in the drive signal. The drive unit 5 is further configured to control the LEDs 6 in accordance with the encoded data, e.g. temporal and color control. For example, the color of the LEDs 6 may be modified, or the LEDs 6 may be controlled to blink. At the same time, the pulsed drive signal provides power to drive the LEDs 6.

[0035] Figure 2 shows a digital coding signal 21, and the actual pulsed drive signal 22 received by a drive unit 3. The drive signal 22 varies between low voltage VL and high voltage VH, which, in the present example, are 2.4 V and 4.8 V, respectively. The coding signal 21 has a logic low pulse 23 which is converted to a low voltage pulse 24. Each drive unit 5 has circuitry or decoding the power line communication. In brief, the drive unit 5 determines when the voltage level falls below a predetermined detection threshold TH, and measures the duration until the voltage level rises above the threshold. In Figure 2, the threshold level TH has been indicated by dashed line 27. In the illustrated example the threshold level TH is around 3 V.

[0036] As is clear from Figure 2, practical limitations in the circuitry cause the ideal square pulse shape to be slightly deformed. In particular, the drive signal 22 exhibits a fall time 25 for decreasing the voltage from Vuto TH and a rise time 26 for increasing the voltage from VL to TH.

[0037] It is clear from Figure 2 that, as a consequence of the fall time 25 and rise time 26, the actual duration of the low voltage pulse 24, detected by the drive unit 5, is shorter than the logic low pulse 23. The actual pulse durations will depend on the voltage gap between low voltage and high voltage, and changes in the voltage rate of change (slope). This slope will in turn depend on the performance of the electrical components generating the voltage signal, and environmental factors, in particular temperature. Therefore, variations in voltage gap and factors such as temperature will cause variations in the voltage pulse durations. In worst case, this may lead to errors in decoding and errors in communication.

[0038] According to an implementation of the present invention, illustrated in Figure 3, the control unit is configured to adjust the pulse width, in order to avoid errors in communication.

[0039] First, in step SI, nominal high voltage VnOm (e.g. voltage level supplied by the power supply 2) and nominal voltage rate of change (rise and fall) dV / dt at nominal temperature, TnOm, e.g. 25 degrees C, are determined. These are design parameters, for which the system will operate satisfactorily.

[0040] In step S2, high voltage (voltage from the power supply) Vinand ambient temperature are detected by circuitry 7 in the control unit 3.

[0041] In step S3, it is checked if the high voltage is different than the nominal high voltage, e.g. more than a given amount. If this is true, in step S4 the low pulse duration representing a logic ZERO, tiowzero, and the low pulse duration representing a logic ONE, tiow one, are adjusted (increased or decreased) based on the detected high voltage and the nominal slope dV / dt.

[0042] In step S 5, it is detected if the detected temperature T is different than the nominal temperature T nom, C.g., more than a given amount. If this is true, in step S6, the low pulse duration representing a logic ZERO, tiowzero, and the low pulse duration representing a logic ONE, tiow one, are adjusted (increased or decreased) based on the temperature difference, and a known temperature dependence of the slope dV / dt. Generally speaking, the slope will decrease at higher temperature, causing the voltage change to take longer time. The adjustment of the low voltage durations should compensate for this.

[0043] The amount of adjustment due to change in temperature may additionally be influenced by (known) temperature dependence in the circuitry of the LED drivers 5, if it assumed that the temperature in the LED units 4 will typically be the same as in the controller 3. For example, a change in temperature may have an impact on any delay when detecting whether the line voltage exceeds the threshold TH.

[0044] Also the current operation of the LED units 4 may have an impact on the pulse width adjustments. For example, high light output (large current through LEDs) will result in a voltage drop at the LED driver 5.

[0045] Indeed, the operation of the LED units 4 may also have an impact on the ratio H / L between high voltage duration and low voltage duration. As mentioned above, there is typically a power requirement which stipulates this ratio. However, during periods of low power operation, e.g. a dimming of all LED units 4, it may be possible to temporarily adjust the H / L-ratio. Such adjustment can be effected by the controller 3, by decreasing (or increasing) the high voltage durations thigh zero and thigh one.

[0046] It is noted that the adjustments of pulse durations may be calculated at runtime or be obtained from pre-established look-up tables stored in memory.

[0047] In step S7, the high voltage pulse durations, thigh zero and thigh one are adjusted to maintain a required ratio between low pulse duration and high pulse duration, L / H.

[0048] Steps S2-S7 are performed during runtime, at regular intervals. In step S8, the control unit 2 provides encoded drive signal 22 on the power line, based on the adjusted pulse durations.

[0049] The pulse width adjustments may be calculated dynamically, for example according to the following expressions:

[0050] T low, zero [|1S] Tiow, zero * [& (Vin / Vnom) + b (T / Tnom)]

[0051] Tiow, one [|J.S] Tiow, one * [a (Vin / Vnom ) + b (T / Tnom)] where a and b are scaling factors. It is noted that the initial scaling factor a will be related to the nominal rate of change (slope) dV / dt, but may depend on changes in temperature. The second scaling factor b will be based on component specifications and possibly system calibration measurements. In another approach, pairs of low pulse widths (Ti ow, zero, Tlow, one ) are precalculated for various combinations of Vinand T. A set of such preset “timing profiles” may be stored in the controller 3, and an appropriate timing profile may be selected based in voltage and temperature measurements. The number of timing profiles is a system design choice, but already two different profiles may provide substantial advantages. Switching between profiles is preferably done using overlapping ranges (hysteresis) to avoid frequent changes.

[0052] The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the specific voltages and other parameters discussed above are only examples. Also, the LED units 4 may include additional circuitry, and have a larger number of LEDs 6. Also the LED units 4 may be connected in any manner, not restricted to the serial / parallel configuration described above.

Claims

CLAIMS:

1. A method for power line coding for digital communication on a pulsed power supply having altematingly high and low voltage, wherein a logic ZERO is communicated by a low voltage pulse having a first low voltage duration, tiowzero, followed by a high voltage pulse having a first high voltage duration, thigh zero, and a logic ONE is communicated by a low voltage pulse having a second low voltage duration, tiowone, followed by a high voltage pulse having a second high voltage duration, thigh one, the method comprising: detecting a parameter value of at least one parameter affecting a transition time between high and low voltage, and adjusting said first low voltage duration, tiowzero, and said second low voltage duration, tiowone, based on said parameter value(s).

2. The method according to claim 1, further comprising adjusting said first high voltage duration, thigh zero, and said second high voltage duration, thigh one, so as to maintain a predetermined ratio, H / L, between high voltage duration and low voltage duration.

3. The method according to claim 1 or 2, further comprising obtaining a nominal high voltage level, and a nominal voltage rate of change during transitions between low and high voltage.

4. The method according to claim 3, wherein said parameters include high voltage level and wherein the step of adjusting the first and second low voltage duration includes calculating a change in pulse duration resulting from a change in high voltage level, given the nominal rate of change.

5. The method according to claim 3 or 4, further comprising obtaining a relationship between ambient temperature and the voltage rate of change, wherein said parameters include ambient temperature, and wherein the step of adjusting the first and second low voltage duration includes calculating a change in pulseduration resulting from a change in the voltage rate of change caused by a change in ambient temperature.

6. The method according to one of the preceding claims, wherein the adjusting steps are performed every time the detecting step is performed, resulting in a continuous adjustment of the voltage pulse timing.

7. The method according to claim 1, further comprising: defining at least two timing profiles, each timing profile comprising preset values for said first and second low voltage durations and optionally for said first and second high voltage durations, associating each timing profile with a parameter range for each of said detected parameters, based on said detected parameter value(s), selecting an appropriate timing profile.

8. The method according to claim 7, wherein change of timing profile is made by means of overlapping ranges for said parameter value(s).

9. The method according to claim 7 or 8, wherein the parameters include high voltage level and ambient temperature.

10. The method according to any one of the preceding claims, wherein the pulsed power supply is used to power a lighting device including at least one LED unit (4) with at least one LED (6) and a LED driver (5), and wherein the step of adjusting said first low voltage duration, tiowzero, and said second low voltage duration, tiow_one, is further based on temperature dependence in circuitry in the LED driver(s) (5).

11. The method according to any one of the preceding claims, wherein the pulsed power supply is used to power a lighting device including at least one LED unit (4) with at least one LED (6) and a LED driver (5), and wherein the step of adjusting said first low voltage duration, tiowzero, and said second low voltage duration, tiowone, is further based on operation of the LED unit(s) (4).

12. A lighting device comprising: at least one LED unit (4), each including a at least one LED (6) and a LED driver (5), a voltage supply (1), and a power line controller (3) configured to provide a pulsed drive signal to the string of LED units (4), and to encode digital communication on the pulsed drive signal for controlling the LED drivers (5), wherein a logic ZERO is communicated by a low voltage pulse having a first low voltage duration, tiowzero, followed by a high voltage pulse having a first high voltage duration, thigh zero, and a logic ONE is communicated by a low voltage pulse having a second low voltage duration, tiowone, followed by a high voltage pulse having a second high voltage duration, thigh one, the power line controller comprising a detector (7) for detecting a parameter value of at least one parameter affecting a transition time between high and low voltage, wherein the controller (3) is configured to adjust said first low voltage duration, tiowzero, and said second low voltage duration, tiowone, based on said parameter value(s).

13. The lighting device according to claim 12, wherein the controller (3) is further4configured to adjust said first high voltage duration, thigh zero, and said second high voltage duration, thigh one, so as to maintain a predetermined ratio between low pulse duration and high pulse duration.

14. The lighting device according to claim 12 or 13, comprising at least one LED set including at least two LED units (4) connected in parallel.

15. The lighting device according to claim 14, comprising at least two serially connected LED sets.

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