FIELD OF THE INVENTION

The present invention generally relates to the field of controlling illumination in a room, and more particularly to a lighting device arranged to perform an automatic calibration of its light sensor unit.

BACKGROUND OF THE INVENTION

In a room where it is desirable that the light output level of lighting devices is controlled, for instance, in order to save energy and/or to keep a constant illumination level at working areas when persons are present, lighting devices can be dimmed or switched off when a large amount of daylight enters the room, or when no one is present in a working area, and lighting devices may be dimmed up or switched on when it gets darker and/or a person enters a working area. In order to perform such an illumination control there is provided a lighting control system using a light sensor unit for measuring the total light level of the sensed light, which typically includes artificial light from lighting devices as well as natural light, i.e. daylight, entering the room through windows and other light inlets. The lighting control system can be applied for several lighting devices in common or for each individual lighting device. Such a lighting control system is also known as a daylight harvesting system.

US 2010/0045191 discloses a sensor system capable of automatic calibration. The system is arranged for recognizing when it is dark outside in order to define a measurement point for a minimum illumination level. This is done by monitoring the total light level and detecting when it reaches a minimum, which is assumed to occur when all lighting devices are switched off and it is dark outside at night. When the minimum total light level is sensed and all lighting devices are off a first measurement is performed, then all lighting devices are switched on at maximum output level, and a second measurement is performed. This prior art sensor system has a drawback in that the calibration cannot take place at an arbitrary time of the day and night. SUMMARY OF THE INVENTION

It would be advantageous to provide an automatic calibration which can be performed on a more arbitrary basis.

To better address this concern, in a first aspect of the invention there is presented a lighting device comprising a dimmable light source, a driver unit for the light source, a light sensor unit, and a controller. The lighting device is arranged to perform an automatic calibration sequence, wherein the controller is arranged to control the automatic calibration sequence. The controller is arranged to gradually increase a control signal to the driver unit from a minimum level; to control the light sensor unit to perform a plurality of measurements of total light level, comprising at least a first measurement of the total light level taken when the control signal to the driver unit is at the minimum level, and a second measurement of the total light level taken when the level of the total light has stopped to increase; and to use the plurality of measurements to calibrate the light sensor unit. Provided that the ramp-up of the dimming level is done appropriately fast, it can be assumed that the measurements are affected by the same amount of daylight, and therefore a relative range which is independent of the level of daylight for the control of the lighting device has been obtained. This has the additional advantage that the risk of interference by persons present with the calibration process, be it on purpose or accidentally, can be minimized.

According to an embodiment of the lighting device the controller is arranged to start the automatic calibration sequence when the lighting device is turned on. By such an off-on trigger event no particular conditions have to apply to perform the calibration sequence, and no person has to be involved to start the calibration.

According to an embodiment of the lighting device, the maximum output level of the driver unit is individually adjustable by an external limiting signal, and the output level is adjustable by the controller between a minimum output level and the maximum output level. Since the calibration sequence involves a detection of when the measured total light level has stopped to increase, the upper limit of the control range is adapted to the actually obtainable light output level of the lighting device determined by the external limiting signal, and not to the maximum level of the control signal to the driver unit.

According to an embodiment of the lighting device, the light sensor unit is arranged to sense an increasing total light level during the gradual increase and to sense a stop of that total light level increase. Thereby, there is no need to communicate the external limiting signal as such to the controller, which simplifies the lighting device.

According to an embodiment of the lighting device, the light sensor unit comprises the controller and a light sensor circuit connected with the controller. Thereby, the driver unit can be made simple and inexpensive.

According to an embodiment of the lighting device, the lighting device comprises a mains input for receiving external mains power, and the driver unit comprises a limiting input for receiving an external limiting signal adjusting the maximum output level of the driver unit, a dimming input, connected with the controller for receiving a dimming signal controlling the output level of the driver unit between a minimum output level and the maximum output level, and an output connected with the light source.

According to an embodiment of the lighting device, the driver unit comprises a logic circuit connected with the limiting input and with the dimming input, and a drive circuit connected with the logic circuit and with the output, wherein the logic circuit is arranged to provide a control signal to the drive circuit corresponding to the lowest signal level of the limiting signal received at the limiting input and the dimming signal received at the dimming input.

According to an embodiment of the lighting device, the controller is arranged to decide whether or not to use the measurements. Thereby, a too small range of the control signal, which can be the result if the maximum output level of the driver unit is too low, is avoided.

In a second aspect of the present invention there is provided a method of performing an automatic calibration sequence at a lighting device comprising a dimmable light source, a driver unit for the light source, a light sensor unit, and a controller, the method comprising: - performing a plurality of measurements of a total light level, comprising at least setting a control signal to the driver unit to a minimum level and performing a first measurement of the total light level, increasing the level of the control signal while monitoring the total light level, detecting that the level of the total light stops to increase, and performing a second measurement of the total light level when this level has stopped to increase; and

- using the plurality of measurements to calibrate the light sensor unit. According to an embodiment of the method, said using the plurality of measurements to calibrate the light sensor unit comprises deciding whether or not to use the measurements at all.

In a third aspect of the present invention there is provided a computer program product for a lighting device, comprising executable program portions for performing an automatic calibration sequence, comprising the following operations:

- performing a plurality of measurements of total light level and a control signal, comprising:

- receiving a trigger input trigging the sequence;

- setting a dimming signal to a driver unit to a minimum level;

- sampling an output of a light sensor circuit and storing the sample together with an associated value of the dimming signal as first increasing the level of the dimming signal while monitoring the output of the light sensor circuit;

- detecting that the output of the light sensor circuit stops to increase; and

- sampling the output of the light sensor circuit and storing the sample together with an associated value of the dimming signal as second values; and

- using the plurality of measurements to calibrate a light sensor unit comprising the light sensor circuit. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail and with reference to the appended drawings in which:

Fig. 1 is a block diagram of an embodiment of a lighting device according to the present invention;

Fig. 2 is a flow chart of an embodiment of an automatic calibration sequence;

Fig. 3 and 4 are timing diagrams for different operation examples of an embodiment of the automatic calibration sequence according to the invention;

Fig. 5 is a timing diagram for another embodiment of the automatic calibration sequence.

DESCRIPTION OF EMBODIMENTS

A first embodiment of the lighting device 1 according to the present invention comprises a dimmable light source 2, a driver unit 3 connected with the light source for powering it, and a light sensor unit 4 connected with the driver unit 3 for providing it with a control signal for controlling its power input to the light source 2. In other words, the light sensor unit 4 controls the dimming level of the light source 2. The light sensor unit 4 comprises a light sensor circuit 5 arranged to sense the total light level, which includes daylight and reflected artificial light from the lighting device 1 itself and possibly from other nearby lighting devices, which has been reflected by an object 17 in the vicinity of the lighting device 1 . The light sensor unit 4 further comprises a controller 6, which is connected with the light sensor circuit 5 and with the driver unit 3. The driver unit 3 comprises a drive circuit 7, which provides the very drive current to the light source 2, and a logic circuit 8, which is connected with the drive circuit, and which provides the drive circuit 7 with the drive current level signal. Furthermore, the driver unit 3 comprises a limiting input 9, which is also an input of the lighting device 1 , for receiving an external limiting signal adjusting the maximum output level of the driver unit 3, and a dimming input 10, which is connected with a corresponding dimming output 1 1 of the light sensor unit 4. The driver unit 3 has a drive current output 12 connected with a corresponding drive current input 13 of the light source 2. The lighting device 1 has a mains input 14 for powering the lighting device 1 .

The light sensor circuit 5 has an output 15 for providing the controller 5 with a sensed total light level signal at a corresponding input 16 thereof.

The lighting device 1 is arranged to output light at a dimming level which is dependent on the sensed level of total light level, including daylight. Within the present field of technology this is commonly referred to as a daylight harvesting system. Basically, the light output is dimmable within a range extending from light output to the maximum light output that the light source 2 is able to deliver. However, additionally, the range can be limited. In this embodiment the upper limit is adjustable by means of the external limiting signal received by the driver unit 3, and more particularly by the logic circuit 8. Thus, a user of the lighting device 1 , or a lighting system comprising several similar lighting devices, and having a central controller, is able to remotely set a maximum light output level, i.e. the upper limit of the range. While the adjustment of the upper limit is done occasionally, the instantaneous light output level is continuously controlled by the controller 6 in dependence on input from the light sensor circuit 5. The logic circuit 8 thus receives a dimming signal from the controller 6, which adjusts the light output level within the range. If the maximum light output level has been decreased by means of the limiting signal, the controller 6 may still apply a dimming signal asking which would generate a higher level, but the logic circuit 8 will then provide the drive circuit 7 with a drive current level signal which corresponds with the limited maximum light output level. Thus, the logic circuit 8 outputs a drive current level signal corresponding with the lowest of the limiting signal and the dimming signal.

In this embodiment the minimum light output level is zero, i.e. no light output at all, while in other embodiments there may be a non-zero minimum light output in order to provide at least a minimum illumination of a room as long as the lighting device 1 is turned on.

The lighting device 1 is arranged to perform an automatic calibration sequence to calibrate the light sensor unit 4. Referring to the flow chart of Fig. 2, according to a first embodiment of a method of performing the automatic calibration sequence it is controlled by the controller 6 as follows. When the controller 6 receives a trigger input, which in this embodiment is when the mains input transits from an off state to an on state, i.e. when the lighting device 1 is turned on, in box 20, the controller 6 is arranged to start the calibration sequence, in box 21 . The calibration sequence begins with the controller 6 adjusting the dimming signal to the minimum level, typically zero, in box 22. Then the controller 6 performs a first measurement M1 of the total light level, i.e. it samples the sensed total light signal from the light sensor circuit 5 and stores the measurement as a first sensed value, and also stores the associated first dimming signal value, in box 23. Next, the controller 6 gradually increases the dimming signal while monitoring the sensed total light signal, in box 24. As long as the sensed total light signal increases the controller 6 keeps increasing the dimming signal, but when the sensed total light signal stops to increase, in box 25, the controller 6 performs a second measurement M2 and stores a second sensed value in conjunction with a second dimming signal value associated with the thus sensed maximum total light level, in box 26. For example, the controller 6 can be arranged to compute a slope of the total light signal, for instance based on two successive measurements, and to determine that the sensed total light signal stops to increase when the slope is null of below a given threshold value. This typically corresponds with that the light output reaches its maximum, as illustrated in Fig. 3. The stored values are then used to adjust the light sensor unit 4 such that the interval between the first dimming signal value and the second dimming signal value is defined as the full control range for controlling the light output of the light source 2 in dependence of the sensed total light level during the subsequent operation of the lighting device 1 . Thus, the outcome of the automatic calibration sequence is used in the daylight harvesting control loop. If the limiting signal has been set to reduce the maximum light output, this results in that the maximum sensed total light level is reached before the dimming signal has reached its maximum, since the light output does not increase beyond that level. By means of the calibration the thereby reduced maximum dimming signal level is redefined as the maximum level that the controller 6 is able, or allowed, to output during the following operation of the lighting device 1 . This is illustrated in Fig. 4. Thus, in Fig. 3 the maximum sensed total light level is reached when the dimming signal has been ramped-up to its absolute maximum. In Fig. 4, on the other hand, the sensed total light level stops increasing when the dimming signal reaches half its absolute maximum, because the limiting signal is set to half of its

maximum value. Different techniques can be used for controlling the driver unit 3 with the limiting signal. In this embodiment a phase-cut technique is used for the limiting signal. Consequently, for instance, the halved limiting signal is obtained by a 90 degree phase-cut. According to an alternative embodiment the limiting signal is based on an analog voltage between between 1 -10V according to IEC-60929.

The measurements are only used for calibration if a sufficient accuracy can be ensured, in box 27. For instance the dimming signal range between the first and second dimming signal values has to be large enough.

Therefore, if the accuracy is too low the measurements are rejected, in box 28, and the previous setting is used instead, otherwise the light sensor unit is calibrated on basis of the new values, in box 29. Moreover it is possible to use a weighted average of a previous calibration and the newly determined one, in order to have a gradual transition from a previous situation to a changed situation. This would help to limit the effect of an accidental error in the calibration, should this occur, e.g. in case a person should be accidentally walking through the area of detection during the execution of the calibration procedure.

As described above, the controller 6 is arranged in the light sensor unit 4. This is done in order to enable the use of a simple driver unit 3, which can be a standard type of driver unit, which reduces the cost of the lighting device. However, alternative constructions are possible, where the controller 6 is a separate element or is integrated in the driver unit 3, which however is less advantageous.

In a second embodiment of the method of performing the automatic calibration sequence, delays are introduced before and after each

measurement. More particularly, the increase, such as the ramp-up, of the dimming signal is paused during a time period, and the values of the total light level and of the dimming signal level are measured and stored during that time period, such as approximately in the middle of the time period, as shown in Fig. 5. In other words, at start the dimming signal is held at its minimum level during a pre-delay time before the very measurement, and during a post-delay time after the very measurement. Then the dimming signal is ramped-up a predetermined fraction of the maximum dimming signal to a second level, where it is held again for the same pre-delay time before a second measurement is performed followed by the same post-delay time. Next a further ramp-up step is taken and the measurement is repeated again. This is continued until the maximum dimming level has been reached or until the total light level is no higher than at the preceding measurement. Then the calibration is finished in the same way as in the first embodiment of the method described above. By the introduction of the delays, the method becomes less sensitive to possible jitter in the starting time and/or the slope of the ramp-up of the dimming signal when there are more than one lighting device which are turned on at the same time. It is ensured that the

measurements are performed in a situation where all lighting devices are at the same stage of the calibration process. In Fig. 5 an example with two lighting devices is shown, where their automatic calibration sequences L1 , L2 are begun at slightly different points of time, but at each measurement both lighting devices are at the same stage of holding the dimming signal. This is advantageous if the lighting devices are placed so close that the total light detected by the light sensor circuit of a lighting device contains light originating from another lighting device.

According to an aspect of the present invention the method is performed by means of a computer program downloaded to the lighting device. The computer program can be provided as a downloadable computer program product, comprising executable program portions for performing an automatic calibration sequence, comprising the following operations:

- performing a plurality of measurements of total light level and a control signal, comprising:

- receiving a trigger input trigging the sequence;

- setting a dimming signal to a driver unit to a minimum level; - sampling an output of a light sensor circuit and storing the sample together with an associated value of the dimming signal as first values;

- increasing the level of the dimming signal while monitoring the output of the light sensor circuit;

- detecting that the output of the light sensor circuit stops to increase; and

- sampling the output of the light sensor circuit and storing the sample together with an associated value of the dimming signal as second values; and

- using the plurality of measurements to calibrate a light sensor unit comprising the light sensor circuit.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.