EP2088833B1 - Method for operating a motor vehicle Led-lighting device - Google Patents

Description

The invention relates to a method for operating a motor vehicle lighting device comprising at least one LED and a pulse width modulated driving the LED control device.

From the publication WO 2006/015476 A1 For example, a method of dimming an LED device having a plurality of LEDs is known in which all individual LEDs should be dimmable in a range of 0% to 100%. To operate such an LED device is further stated that this can be modulated with a pulse width modulation PWM, a pulse code modulation PCM or a frequency modulation FM. These types of modulation are generally known to the person skilled in the art. The variations described relate either to the amplitude or to the duty cycle in a pulse width modulation and not just to the frequency. With regard to the frequency, it is simply stated that it must be high enough to avoid the stroboscopic effect.

US 5,783,909 relates to a method for keeping the light intensity of an LED constant. Factors influencing light intensity are age and temperature. To measure the light intensity and the temperature, a light sensor and a temperature sensor are to be provided. When the light intensity changes, the average value of the current driving the LED is adjusted by changing either the pulse width, the frequency, or the baseline. The modulation frequency is changed depending on sensor data.

In motor vehicles, lighting devices with one or more LEDs are increasingly being used, be it as interior lighting, be it in the headlights or the taillights. To ensure the longest possible lifetime, it is known not to provide the LED with a constant, temporally constant current, but clocked by pulse width modulation clocked. That is, the current is switched on and off via a pulse width modulation interface via a high frequency pulse generator. The modulation frequency is constant, it is chosen in each case so high that the intermittent switching operation is not perceived by the human eye, so that it does not come to a stroboscopic effect. Usual frequencies are approx. 100 Hz. The brightness is adjusted via the pulse-pause ratio (the so-called duty cycle), which can be changed accordingly. An extension of the relative pulse duration, that is to say the switch-on duration, corresponds in the time average to a longer switch-on duration of the LED, thus a greater brightness, while a reduction of the pulse duration or pulse width corresponds to a lower brightness. The emitted luminous flux is approximately proportional to the mean value of the current, which is determined by the pulse-pause ratio.

Although a lifetime extension can be achieved by the pulse width modulation of the switching operation compared to a constant current operation, the operating frequency usually selected to avoid the stroboscopic effect is still so high that the use is correspondingly high high quality and therefore more expensive semiconductor devices than LED components is required, which are able to represent the modulation frequency.

The invention is therefore based on the problem to provide an operating method for a pulse-width modulated LED drive, which allows a reduction of the modulation frequency while avoiding a stroboscopic effect.

To solve this problem, it is provided according to the invention in a method of the type mentioned that the modulation frequency varies continuously.

Unlike in the past, in the method according to the invention, the LED is continuously driven with different frequencies over time. This means that there is no constant frequency relative to the set brightness, but the frequency changes continuously, while the pulse-pause ratio and thus the adjusted brightness remains constant. As a result of this constant frequency change, a stroboscopic effect is avoided with particular advantage, since there is no chronologically constant change between the switch-on and switch-off state, which could be perceived by the viewer. This means that there is no time-constant light pattern that can be detected. Rather, according to the invention, a continuously changing switching behavior is realized which counteracts or avoids the stroboscopic effect.

As a result, it is possible in the method according to the invention to lower the modulation frequency, which was chosen in terms of its height solely for avoiding the stroboscopic effect in previously known control methods. For in the method according to the invention, the stroboscopic effect is avoided by the frequency variation, but not by the frequency level alone. Of course, the frequency or the frequency band within which the modulation frequency varies, must be so high that also the varying frequency jumps just can not be perceived, but already frequencies are sufficient that are> 50 Hz. As a result of the frequency mean lower drive frequency over time also results in a lower stress on the LED semiconductor devices, which in turn means that less expensive semiconductor devices, ie LEDs can be used, however, when using the method according to the invention have similar lifetimes as previously used high-quality, expensive semiconductor LEDs, which are controlled with a constant, high frequency.

A further advantage is also to be seen in that the EMC behavior (EMC = electromagnetic compatibility) of the vehicle improves with regard to the frequency reduction. In principle, high control frequencies are disadvantageous with regard to possible interference or noise emissions with regard to the EMC behavior. This counteracts the invention according to the lower modulation frequency, as well as the reduction of any resonance probabilities is achieved via the continuous frequency variation.

With regard to the variation of the modulation frequency different possibilities are conceivable. According to a first embodiment of the invention, the modulation frequency may be repetitively increased or decreased in a frequency sweep, starting from a starting frequency, preferably linearly to a target frequency, and jumping back to the starting frequency upon reaching the target frequency. In this embodiment of the invention, therefore, the modulation frequency preferably increases or decreases linearly from a start frequency to a target frequency. After reaching the target frequency, the frequency is automatically changed in a frequency jump back to the starting frequency, after which the frequency increase or decrease takes place again. This pattern is going through continuously.

An alternative provides that the modulation frequency repetitively preferably linearly between a maximum and a minimum frequency changed. Here, therefore, no frequency jump takes place after reaching a target frequency to a start frequency, but rather there is a continuous frequency variation between a maximum and a minimum frequency, the frequency response here also preferably linearly changes, but the linearity is not a mandatory requirement.

Finally, a third alternative provides for the modulation frequency to be changed in a continuously changing manner within a defined frequency band via a random generator. In this case, any control frequencies within the frequency band that have been previously defined are selected via the random number generator, which are then controlled via the control unit. So it comes to continuous frequency jumps, the holding time of a frequency defined by the random generator is again as low as possible, and preferably below one second.

The control method according to the invention can be used both in lighting devices comprising only one LED, as well as in lighting devices with multiple LEDs combined in an array. In this case, all LEDs are then preferably driven together, that is, the entire array is operated via the frequency-varying method according to the invention.

If a plurality of separate arrays are used in an illumination device, as may be the case, for example, with LED-type headlights, the multiple arrays can all be driven together in the manner according to the invention if all the LEDs are to be operated.

In addition to the method, the invention further relates to a motor vehicle comprising a lighting device with at least one LED and a pulse width modulated the LED controlling control device, which is designed to carry out the method described.

Fig. 1

eine Prinzipdarstellung einer Beleuchtungseinrichtung mit zu- geordneter Steuerungseinrichtung zur Durchführung des erfin- dungsgemäßen Verfahrens,

Fig. 2

ein Prinzipdiagramm zur Darstellung der Veränderung der Steuerfrequenz in einem Frequenz-Sweep,

Fig. 3

ein Prinzipdiagramm zur Darstellung der Veränderung der Steuerfrequenz durch kontinuierlichen Wechsel zwischen einer Maximal- und Minimalfrequenz, und

Fig. 4

ein Prinzipdiagramm zur Darstellung der Veränderung der Steuerfrequenz durch zufällig ausgewählte Frequenzsprünge.

Further advantages, features and details of the invention will become apparent from the embodiments described below and with reference to the drawings. Showing:

Fig. 1

2 a schematic representation of a lighting device with associated control device for carrying out the method according to the invention,

Fig. 2

a schematic diagram showing the change of the control frequency in a frequency sweep,

Fig. 3

a schematic diagram showing the change in the control frequency by continuously switching between a maximum and minimum frequency, and

Fig. 4

a schematic diagram illustrating the change of the control frequency by randomly selected frequency jumps.

Fig. 1 shows a lighting device 1 according to the invention in the form of an array 2 consisting of several LEDs 3, of which four are shown in the example shown. Shown is also a control device 4, via which the operating current, which is applied to the array 2 or the LEDs 3 via the motor vehicle electrical system, is controlled pulse width modulated. The illumination device can be any illumination device of the motor vehicle, be it a signal or indicator light in the region of the rear of the vehicle, the vehicle side or the vehicle front, be it an LED headlight or a lighting device from the vehicle interior.

As described, the brightness is varied by adjusting the pulse-pause ratio, that is, the ratio between the turn-on and turn-off time of the individual LEDs 3. The frequency with which the switch-on and switch-off process is controlled, in the inventive method or the illumination device according to the invention shown However, unlike in the prior art, not constant, but changes over time continuously, as based on the Figures 2-4 is shown.

Fig. 2 shows a first embodiment of such a control method. Along the abscissa is the time t, along the ordinate, the control frequency f is plotted. Shown is a start frequency f _s and a target frequency f _z . In the example shown, it is assumed that the starting frequency f _{s is} lower than the target frequency f _z . Now, if the lighting device 1 is operated, the operating frequency continuously changes over time between the start frequency f _s and the target frequency f _z , wherein the frequency within each cycle, starting from the start frequency f _s in the example shown is linearly increased to the target frequency f _z after which the frequency is again reduced directly to the starting frequency f _s in a frequency jump, after which it in turn increases linearly to the target frequency f _z , etc. It forms over time t in Fig. 2 shown "sawtooth pattern" off. As a result of this continuous frequency variation, after a constant switching frequency does not occur, the stroboscopic effect is advantageously avoided since the viewer's eye is not given a time-constant switching pattern. The resulting average switching frequency is significantly lower than in conventional lighting devices that are operated at a constant high frequency of usually 100 Hz. In the example shown, the start frequency f _{s could be,} for example, 60 Hz and the target frequency f _z 90 Hz, this being merely an example, that is, other frequencies can also be selected. It would also be conceivable, starting from a higher starting frequency f _s, to lower the frequency to the target frequency f _z and then to increase it again to the starting frequency in a frequency jump. Again, a similar sawtooth pattern would result.

Fig. 3 shows a further possibility of a frequency response, whereby here again along the abscissa the time t and along the ordinate the frequency f is plotted. Here, the frequency varies continuously between a minimum frequency f _min and a maximum frequency f _max . Here, too, a linear frequency increase or frequency reduction is shown by way of example. Obviously, here too an average frequency results due to the avoidance of the stroboscopic effect due to the frequency variation is lower than previously usual constant control frequencies. For example, the minimum frequency f _min and the maximum frequency f _{max can be} 60 Hz and 90 Hz, and a minimum frequency of 50 Hz and a maximum frequency of 100 Hz or other frequency combinations would also be conceivable.

Finally shows Fig. 4 a third embodiment of a frequency response. Again, the time t is plotted along the abscissa and the frequency f is plotted along the ordinate. Here also a frequency band between the minimum frequency f _min and the maximum frequency f _{max is} defined. Via a random generator, which is part of the control device 4, a random frequency within the frequency band is then selected, which is then set by the control device 4. As Fig. 4 clearly shows arise over time a variety of frequency jumps, that is, the random number generator continuously selects new frequencies within the frequency band, so that the frequency changes continuously leaps and there is no continuous frequency control here. The individual frequency levels are kept very short, the hold time should be ≤ 1 second, which is also only an exemplary value. Also conceivable would be shorter or longer holding times, wherein the holding time can also depend on how high the actual control frequency chosen by the random generator is. Higher frequencies may be kept slightly longer than lower ones, or vice versa.

The in the Figures 2-4 shown control examples are merely exemplary nature. Of course, other frequency responses would be conceivable, be it non-linear frequency responses or less steep frequency edges when frequency jumps are made. The frequency response should always be selected so that a sufficient frequency change to avoid the stroboscopic effect is given and a load of the individual LEDs is as low as possible.

Claims (7)

1. Method for operating a motor vehicle lighting device comprising at least one LED and a control device which drives the LED using pulse width modulation,
   characterized in that
   the modulation frequency is continuously varied.
2. Method according to Claim 1,
   characterized in that
   the modulation frequency is repeatedly increased or reduced to a target frequency, preferably in a linear manner, starting from a starting frequency, in a frequency sweep and is returned to the starting frequency when the target frequency is reached.
3. Method according to Claim 1,
   characterized in that
   the modulation frequency is repeatedly changed between a maximum frequency and a minimum frequency, preferably in a linear manner.
4. Method according to Claim 1,
   characterized in that
   the modulation frequency is continuously alternately changed within a defined frequency band under the control of a random number generator.
5. Method according to one of the preceding claims,
   characterized in that,
   when a lighting device having a plurality of LEDs combined in an array is used, all LEDs are driven together.
6. Method according to Claim 5,
   characterized in that,
   when a lighting device having a plurality of arrays is used, all arrays are driven together.
7. Motor vehicle comprising at least one lighting device (1) having at least one LED (3) and a control device (4) which drives the LED (3) with pulse width modulation, designed to carry out the method according to one of the preceding claims.

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