BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for reducing a ripple, caused by rotational nonuniformity of an internal combustion engine, in the output voltage of a generator driven by the internal combustion engine. 
         [0003]    2. Description of the Related Art 
         [0004]    Claw pole generators having electrical excitation are usually used as generators in motor vehicles. The current through the rotor winding is used as a manipulated variable for regulating the desired output voltage or a requested torque, and is predefined by an associated field controller. This type of regulation is necessary, since otherwise, due to the highly variable engine speeds, greatly fluctuating voltage values would be supplied by the generator which could possibly damage the downstream electrical system. 
         [0005]    Since the rotor winding typically has a time constant &gt;150 ms, rapid control operations are not possible. However, in particular at low rotational speeds, in particular in the range of the idle speed, this results in voltage ripples, since in this case the power output of the generator is highly dependent on the rotational speed. If the rotational speed fluctuates very rapidly due to rotational nonuniformity in this range, the field controller is not able to respond quickly enough to the fluctuations on account of the rotor time constant of the rotor field, resulting in an undesirable ripple in the generator output voltage. This effect is further intensified in generators, which in addition to the rotor winding have permanent magnets for leakage flux compensation, since in this case the power characteristic curve has an even steeper slope. 
         [0006]    Published European patent document EP 0 847 485 B1 discloses a method for damping torque fluctuations due to rotational nonuniformity of the internal combustion engine, a generator being used in motor mode to compensate for torque fluctuations. However, this further intensifies the voltage ripple. 
         [0007]    It is therefore desirable to provide an option for reducing a ripple in the output voltage of a generator driven by the internal combustion engine. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    According to the present invention, a method is proposed for reducing a ripple, caused by rotational nonuniformity of an internal combustion engine, in the output voltage of a generator driven by the internal combustion engine. 
         [0009]    The present invention may be used particularly advantageously in generators whose output voltage is rectified by a power converter having controllable switching elements. Such a power converter may be operated as a synchronous rectifier, a step-up converter, a step-down converter, a pulse-controlled inverter, etc. In the present case, a particularly simple option is provided for stator-side regulation by appropriate control of the switching elements, as a result of which the commutation angle and thus also the output direct current voltage are influenced. 
         [0010]    Power converters having controllable switching elements are being increasingly used in generators in motor vehicles. On the one hand, these power converters offer advantages with regard to efficiency, and on the other hand, they also allow operation of the generator as a motor, for example to assist the internal combustion engine in starting or during propulsion. A generator having such a power converter is disclosed in published German patent application document DE 100 27 859 A1, for example. 
         [0011]    In generators having a power converter (for example, starter generators having a pulse-controlled inverter), the current level on the stator side may be regulated via targeted control of the switching elements or rectifier elements, usually transistors. Since the stator winding has a significantly smaller time constant than the rotor winding, this type of regulation is particularly suitable for damping the described voltage fluctuations. Control voltage fluctuations in the vehicle electrical system are minimized. 
         [0012]    The present invention is based essentially on the measure of using a so-called electrical freewheeling system, in which in generator mode of the electric machine the commutation angle in the near-idling range is controlled in such a way that the generator current is decreased at the rotational speed maximum and increased at the rotational speed minimum. This results in a lower current ripple, and thus also a lower voltage ripple. Suitable input variables for the regulation are the generator rotational speed and the generator voltage, as well as the position of the exciter field. 
         [0013]    The regulation is preferably used when the voltage ripple exceeds a predefinable threshold value, for example 100 mV, the frequency of the generator alternating voltage is in a predefinable range, for example 25 Hz to 200 Hz, and the generator rotational speed is in a predefinable range, for example 1,000 rpm to 1,800 rpm. The regulation is advantageously terminated when the generator rotational speed exceeds a predefinable threshold value, for example 2,000 rpm. Regulation is subsequently carried out once again in a customary manner on the rotor side via the field controller, the power converter then normally being operated as a synchronous rectifier. When the regulation according to the present invention is used, the commutation angle is changed by appropriately controlling the switching elements until the voltage ripple is minimal or is below an acceptable threshold. As the result of changing the commutation angle, a portion of the current within the power converter is short-circuited, so that the ripple decreases at the output. 
         [0014]    An arithmetic unit according to the present invention, for example a control unit of a motor vehicle, is set up, in particular by programming, to carry out a method according to the present invention. 
         [0015]    Further advantages and embodiments of the present invention result from the description and the appended drawings. 
         [0016]    It is understood that the features stated above and to be explained below may be used not only in the particular stated combination, but also in other combinations or alone without departing from the scope of the present invention. 
         [0017]    The present invention is schematically illustrated with reference to exemplary embodiments in the drawings, and described in greater detail below with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  shows one specific embodiment of a generator having a power converter with controllable switching elements, which may be the basis for the present invention. 
           [0019]      FIG. 2  shows one specific embodiment of a control device, designed according to the present invention, for carrying out the present invention. 
           [0020]      FIG. 3   a  shows current, voltage, and rotational speed curves for a regulation according to the related art. 
           [0021]      FIG. 3   b  shows current, voltage, and rotational speed curves for a regulation according to one specific embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]      FIG. 1  illustrates an electric machine, which may be the basis for the present invention and which is denoted overall by reference numeral  100 , as a circuit diagram. The electric machine has a generator component  10  and a power converter component  20 . The power converter component is operated in generator mode of the machine, usually as a rectifier. 
         [0023]    Generator component  10  is illustrated strictly schematically in the form of stator windings  11 , wired in a star-shaped configuration, and an excitation winding or rotor winding  12  connected in parallel with a diode. The rotor winding is switched in a clocked manner by a power switch  13  which is connected to a connector  24  of power converter component  20 . 
         [0024]    Power switch  13  is controlled according to a field controller  15 , whereby power switch  13 , as well as the diode connected in parallel to rotor winding  12 , is generally integrated into an application-specific integrated circuit (ASIC) of the field controller. 
         [0025]    A three-phase generator is illustrated within the scope of the present patent application. In principle, however, the present invention is also usable with generators having fewer or more phases, for example five-phase generators. 
         [0026]    In the present case, power converter component  20  is designed as a B6 circuit, and has switching elements  21  which may be designed as MOSFETs  21 , for example. MOSFETs  21  are connected to the particular stator windings  11  of the generator via busbars, for example. In addition, the MOSFETs are connected to connectors  24 ,  24 ′, and when appropriately controlled, supply direct current to a vehicle electrical system  30  of a motor vehicle. Switching elements  21  are controlled by a control device  25  via control channels  26 , not all of which are provided with reference numerals for the sake of clarity. Control device  25  receives in each case the phase voltage of the individual stator windings via phase channels  27 . Further devices, not illustrated for the sake of clarity, may be provided for supplying these phase voltages. 
         [0027]    In (synchronous) rectifier mode, control device  25  carries out an evaluation of the phase voltages provided via phase channels  27 , and on this basis determines a respective switch-on and switch-off time of an individual MOSFET  21 . The control via control channels  26  acts on the gate connectors of MOSFETs  21 . 
         [0028]    Known field controllers, such as field controller  15  provided within the scope of the present specific embodiment, have a so-called V-terminal connector  19  which is connected to a phase of the stator winding of the generator. The frequency of the V-terminal signal or of the phase input signal is evaluated in controller  15 , and as a function of the parameters this signal is used for activating or deactivating the controller operation, and ultimately, for controlling power switch  13  via a control line  14 . The phase signal for phase signal input  19  may, as illustrated, also be conducted through control device  25 . 
         [0029]    When the engine is started, the generator rotor together with field winding  12  starts to turn, and a voltage is induced in stator windings  11 . These phase voltages are detected, for example based on frequency, by control device  25  and used for deducing or determining the control signals for individual MOSFETs  21 . 
         [0030]    One particularly preferred specific embodiment of the present invention is now described in greater detail with reference to  FIGS. 1 and 2 , in  FIG. 2  a control device  25  being shown which is configured to carry out one particularly preferred specific embodiment of the present invention. The variation over time of generator voltage U G , instantaneous rotational speed n G  of generator component  10 , and position angle φ of the generator component are supplied as input signals to control device  25 . The position sensor determines the phase position of the exciter field. If the electric machine were operated in generator mode with diodes, the diodes would commutate at the natural commutation time (zero crossing of the diode current). However, when switching elements, for example power semiconductors, are used, this commutation time may be shifted; i.e., the power semiconductors remain switched through for a longer or shorter period of time. 
         [0031]    Based on the present invention, control device  25  now determines appropriate control signals in order to control switching elements  21  for reducing an output voltage ripple. For this purpose, the commutation angle (and thus the phase position between the synchronous generated voltage and the stator voltage) is changed in such a way that the generator current is decreased at the rotational speed maximum and increased at the rotational speed minimum. 
         [0032]    According to one preferred specific embodiment of the present invention, the commutation angle is changed at low rotational speeds by appropriately controlling switching elements  21  until the voltage ripple is minimal or is below an acceptable threshold. 
         [0033]    The stator-side regulation of the generator voltage is preferably carried out only in a low rotational speed range around the idle speed, for example 1,000 rpm to 1,800 rpm. If generator rotational speed n G  exceeds a predefinable threshold value, for example 2,000 rpm, the generator voltage is once again controlled in a customary manner via field controller  15  by specifying the current through rotor winding  12 . 
         [0034]      FIG. 3   a  shows current, voltage, and rotational speed curves over time, which result during control of an electric machine  100  according to  FIG. 1  according to the related art. The variation over time of generator rotational speed n G  is illustrated in a diagram  301 , the variation over time of rectified output voltage U Batt  which results at connectors  24  and  24 ′ is illustrated in a diagram  302 , and the variation over time of associated output current I Batt  is illustrated in a diagram  303 . It is apparent that a distinct ripple in output voltage U Batt  as well as in output current I Batt  results due to the rotational nonuniformity. 
         [0035]    In comparison,  FIG. 3   b  illustrates the variation over time of output voltage U Batt  and of output current I Batt  which result during control of electric machine  100  according to  FIG. 1  according to the present invention. The variation over time of output current I Batt  is illustrated in a diagram  403 , and the variation over time of output voltage U Batt  is illustrated in a diagram  402 . The variation over time of generator rotational speed n G  according to  FIG. 3   b  corresponds to variation over time  301  according to  FIG. 3   a.  It is apparent that a distinct reduction in the ripple in output voltage U Batt  as well as in output current I Batt  is achievable due to the preferred control of the switching elements.
A method for reducing a ripple, caused by rotational nonuniformity of an internal combustion engine, in the output voltage of a generator which is driven by the internal combustion engine is provided. The generator has a stator winding, a rotor winding, a field controller, associated with the rotor winding, for regulating the output voltage of the generator, and a downstream power converter having controllable switching elements. To reduce the voltage ripple, the output voltage of the generator is regulated on the stator side by appropriately controlling the switching elements of the power converter.