Abstract:
A device for triggering a lighting device of a motor vehicle includes a control arrangement which generates at least one pulse width modulated control signal for at least one switching arrangement via which at least one lighting device may be supplied with electric power. The control arrangement receives a measure of the power supply voltage supplied at least to the lighting device in order to influence the control signal as a function of the power supply voltage. The control arrangement determines the control signal as a function of at least one additional operating parameter, in addition to the power supply voltage, of the motor vehicle.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for triggering a lighting device in a motor vehicle. 
     BACKGROUND INFORMATION 
     European Patent Application No. 0 935 404 describes a circuit for stabilizing the voltage for a lamp, i.e., for headlights on a motor vehicle. The lamp voltage is stabilized at a reduced power loss by the fact that an additional time-delay switch controllable by a trigger circuit is included in the lamp circuit; when the power supply voltage is lower than the predefined nominal operating voltage of the lamp, the trigger circuit keeps the time-delay switch switched on continuously; and at a higher power supply voltage than the nominal operating voltage of the lamp, the trigger circuit periodically switches the time-delay switch on and off, the ratio of the on time to the duration of the period corresponding to the square of the ratio of the nominal operating voltage to the applied power supply voltage. 
     It is an object of the present invention to further stabilize the lighting intensity of a lighting device arranged in a motor vehicle, even for different operating states of the vehicle. 
     SUMMARY OF THE INVENTION 
     The device according to the present invention for triggering a lighting device of a motor vehicle includes a control arrangement which generates at least one pulse width modulated control signal for at least one switching arrangement via which at least one lighting device may be supplied with electric power. The control arrangement receives a measure of the power supply voltage which supplies power at least to the lighting device for influencing the control signal as a function of the power supply voltage. The control arrangement determines the control signal as a function of at least one additional operating parameter of the vehicle in addition to the power supply voltage. Due to the variable adaptation of the pulse width modulated control signal, different operating situations of the motor vehicle may be taken into account in which, as experience has shown, light intensities which fluctuate to different extents may occur. The driving speed is available as one possible operating parameter. The battery is usually under the greatest load when the vehicle is standing still, so that stabilization has priority. However, the brightness of the headlights need not be maximized when the vehicle is stationary. Driving speed as a possible operating parameter is taken into account in determining the control signal. Furthermore, flickering of the lights caused by other electric loads being turned on or off is hardly noticeable while driving. Thus, at high speeds, the stabilization function is not so strongly emphasized as adequate brightness of the lamp, i.e., the headlights. 
     In an example embodiment, the control arrangement specifies a maximum pulse width as a control signal when the power supply voltage drops below a lower limit. The lower limit is variable according to the present invention as a function of at least one operating parameter of the vehicle. The calculation procedure for determining the pulse width is stored in the control arrangement as a function of the lower limit value. Only this one value need be varied as a function of the operating parameter in order to take into account the prevailing operating situation of the vehicle in setting the control signal. The corresponding algorithm thus has a relatively simply structure. 
     In an example embodiment, the control signal may depend on the charge state of the automotive battery. The charge state is determined, for example, in conjunction with the power supply voltage before starting the vehicle and is used for setting the new lower limit value, for example. In the case of a high power supply voltage, indicating a fully charged automotive battery, the lower limit value may be set higher, because stabilization then plays a subordinate role. When the battery is relatively low, however, the tendency of the lighting device to flicker is much greater. Therefore, fluctuations in power supply voltage in a larger stabilization range are taken into account in setting the control signal. 
     In an example embodiment, the control arrangement specifies a minimum pulse width as the control signal when the power supply voltage exceeds an upper limit value. The minimum pulse width is variable as a function of the operating parameter. Due to this definition of the minimum pulse width, the stabilization range of the control arrangement may also be adapted to the prevailing operating situation. With an increase in driving speed, the minimum pulse width is increased for adaptation of the stabilization range. In addition, this provides overvoltage protection and lengthens the lifetime of the battery. 
     An example embodiment of the device according to the present invention is illustrated in the drawings and is described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram. 
     FIG. 2 shows the triggering for a lighting device as a function of operating parameters. 
    
    
     DETAILED DESCRIPTION 
     An output signal of a first operating element  12  and an output signal of a second operating element  14  are sent to a microcontroller  10 . Microcontroller  10  generates a trigger signal  16  according to a certain computation algorithm, this signal then triggering a switching arrangement  18 , opening or closing it. A lamp  20  is supplied with power supply voltage UB provided by a battery  22  via switching arrangement  18 . Power supply voltage UB is also sent to microcontroller  10 , which also exchanges data over a bus system  24 , over which driving speed v detected by a speed sensor  26  is sent to microcontroller  10 . 
     Trigger signal  16  is a pulse width modulated signal which varies between a maximum pulse width PWM max  of 100% and a minimum min PWM min  of minimum pulse width PWM min . FIG. 2 shows a linear relationship between pulse width PWM and power supply voltage UB. Three characteristic curves are shown as a function of driving speed v as an example. Lower limit value U min  decreases from max U min  to min U min  with a decrease in driving speed. When power supply voltage UB reaches or falls below lower limit value U min , microcontroller  10  generates a control signal  16  having a maximum pulse width PWM max  of 100%, for example, i.e., switching arrangement  18  assumes a continuous on state. In the range between lower limit value U min  and upper limit value U max , pulse width PWM decreases linearly with an increase in operating voltage. If operating voltage UB reaches or exceeds upper limit value U max , pulse width PWM remains constantly at the level of minimum pulse width PWM min . Upper limit value U max  retains its value independently of a change in driving speed v. However, minimum pulse width PWM min  increases with an increase in driving speed v. It thus fluctuates between minimum min PWM min  of minimum pulse width PWM min  and maximum max PWM min  of minimum pulse width PWM min . With an increase in driving speed v, there is a decrease in the linear, directly proportional range within which the varying operating voltage UB is stabilized by adjusting pulse width PWM of control signal  16  as a function of operating voltage UB. 
     Microcontroller  10  generates pulse width modulated trigger signal  16  with which switching arrangement  18  is switched on or off. Pulse width is understood to refer to the duration of the on time based on a fixed cycle time. The brightness of lamp  20  may vary due to variations in pulse width PWM. Microcontroller  10  should then ensure that a uniform light intensity is always achieved, even when there is a fluctuation in power supply voltage UB. Flickering of lamp  20  is prevented, i.e., when there are great load changes which may directly affect power supply voltage UB. To this end, microcontroller  10  detectors power supply voltage UB and determines the instantaneous pulse width according to the following algorithm: 
     
       
           PWM =((( U   max   −UB )[÷]( U   max   −U   min ))·( PWM   max   −PWM   min ))+ PWM   min   
       
     
     where 
     U max =upper limit value 
     UB=power supply voltage 
     [U min ]=lower limit value 
     [PWM max ]=maximum pulse width 
     [PWM min ]=minimum pulse width 
     This yields the linear relationship for the average operating voltage range (U min &lt;UB&lt;U max ) as illustrated in FIG.  2 . If the absolute value of power supply voltage UB detected is below lower limit value U min , then switching arrangement  18  is triggered by trigger signal  16  having maximum pulse width PWM max . This may amount to 100%, for example, so that in this case switching arrangement  18  would be closed continuously. Maximum possible power supply voltage UB is thus available to lamp  20  to an unlimited extent. Lamp  20  is thus triggered at the maximum possible power and therefore at the maximum possible intensity, i.e., in cases of heavy load (UB&lt;U min ). 
     If power supply voltage UB is between lower limit value U min  and upper limit value U max  (U min &lt;U max ), then pulse width PWM is directly proportional to UB. When power supply voltage UB reaches upper limit value U max  (UB≧U max ), lamp  20  is triggered at a maximum pulse width PWM min  to ensure adequate lighting in any case. 
     FIG. 2 shows as an example a linear relationship between power supply voltage UB and pulse width PWM. However, to keep the light intensity constant as a function of the power supply voltage, a parabolic or other mathematical relationship between pulse width PWM and power supply voltage UB could also be used. The characteristic curve according to FIG. 2 is adapted to the type of lighting device accordingly. 
     In agreement with FIG. 2, the characteristic line is now shifted as a function of the operating parameter to adapt the stabilization of the light intensity to different operating states. In the example embodiment according to FIG. 2, driving speed v is used as the operating parameter. This speed is detected by a speed sensor  26  and is conveyed to microcontroller  10  via bus system  24 , for example. Lower limit value U min  varies between a minimum lower limit value min U min  and a maximum lower limit value max U min  as a function of the driving speed. With an increase in driving speed v, the absolute value of lower limit value U min  increases. The characteristic curves illustrated in FIG. 2 ensure that a great measure of stabilization is achieved when the vehicle is at a standstill (v=0). Therefore, the stabilization function intervenes earlier in the form of the linear characteristic curve (in the range of U min &lt;UB&lt;U max ) than when the vehicle is moving. If the vehicle is moving (v&gt;0), then the maximum brightness of lamp  20  is of interest for the user. Therefore, pulse width PWM remains at maximum value PWM max  for a greater operating voltage range than at low driving speeds. The stabilization effect is thus reduced with an increase in speed in favor of maximum brightness. 
     With an increase in driving speed v, minimum pulse width PWM min  is increased while upper limit value U max  remains constant. 
     The speed dependence of lower limit value U min  and/or minimum pulse width PWM min  is stored in microprocessor  10  in the form of tables or as a direct mathematical relationship. For calculation of instantaneous pulse width PWM, lower limit value U min , adapted according to driving speed v, and minimum pulse width PWM min  are used as the basis for the computation algorithm. 
     In another example embodiment (not shown here), upper limit value U max  is configured to be dependent on the operating parameter and to decrease in absolute value with an increase in driving speed v. 
     In another example embodiment, the charge state of battery  22  is used as an operating parameter. Before starting the vehicle, microcontroller  10  detects power supply voltage UB as a measure of the charge state of battery  22 . The degree of stabilization may be increased at a lower charge state of battery  22  by increasing the operating voltage range by defining the pulse width as a function of operating voltage (U min &lt;UB&lt;U max ) . The situation of a low charge state of battery  22  thus corresponds to that of low driving speeds v. In agreement with FIG. 2, there is similarly an adaptation of lower limit value U min  and minimum pulse width PWM min . 
     In addition, the output signals of operating elements  12 ,  14  are sent to the microcontroller. Lamp  20  is activated or deactivated as a function of these signals, with pulse width PWM thus determined. 
     Lamp  20  may be the headlight of a motor vehicle or the interior lighting. However, the present invention is not limited to these cases.