FIELD OF THE INVENTION 
     The present invention relates to a method for operating a multi-voltage electrical system for a motor vehicle, which includes a first subsystem operable at a first operating voltage and a second subsystem operable at a second operating voltage, a corresponding multi-voltage electrical system and means for implementing the method. 
     BACKGROUND 
     So-called multi-voltage electrical systems for motor vehicles are known in principle. Multi-voltage electrical systems are used, for example, when consumers in a motor vehicle have different performance requirements. Multi-voltage electrical systems, as the term is used in this application, include so-called subsystems which are configured to operate at the same or at different voltage levels, referred to in this case as “operating voltages.” In particular, multi-voltage electrical systems may be designed as dual voltage electrical systems, in which the operating voltages may total 48 V (in a so-called high voltage subsystem) and 12 V (in a so-called low voltage subsystem). 
     Two subsystems of a multi-voltage electrical system may be connected to each other via a DC/DC converter. At least one of the two subsystems includes a generator operable electric machine which feeds the respective subsystem. The respective other subsystem connected via the DC/DC converter may in turn be fed from the subsystem having the generator operable electric machine if the other subsystem itself does not include a generator operable electric machine. 
     In principle, the present invention may be used for all multi-voltage electrical systems for motor vehicles which have a first subsystem operable at a first operating voltage and a second subsystem operable at a second operating voltage. The operating voltages may be identical or different. Identical operating voltages in two subsystems are used, for example, when in one of the subsystems safety-related electrical consumers are grouped which are intended to be protected from potential voltage spikes or voltage drops in the respective other subsystem. Thus, the use of the present invention is not limited to dual-voltage electrical systems, i.e., electrical systems with exactly two subsystems. Multi-voltage electrical systems, however, include at least two subsystems referred to within the scope of this application as “first subsystem” and “second subsystem.” In conventional dual-voltage electrical systems, for example, the first subsystem has a higher operating voltage (high voltage subsystem) and the second subsystem has a lower operating voltage (low voltage subsystem). 
     However, the present invention relates in particular to the dual-voltage electrical system explained, in which a generator operable electric machine is provided in only one of the (two) subsystems. In this connection, the subsystem (of a dual- or multi-voltage electrical system), as the term is used in this application, which includes the generator operable electric machine, is referred to as the first subsystem. The second subsystem is then fed from the first subsystem via the DC/DC converter. Conventional DC/DC converters are typically installed as separate devices having a separate housing or as separate devices in a housing together with a pulse controlled inverter or a battery. A corresponding DC/DC converter, as mentioned, has the task of ensuring the exchange of energy between the subsystems. 
     Multi-voltage electrical systems are used, in particular, in so-called recuperation systems for recovering brake energy. For the purpose of recuperation, at least one generator operable electric machine is integrated into the first subsystem and designed to be able to provide sufficient braking power. The electrical system must therefore be designed as a multi-voltage electrical system. In this connection, it is known from JP 2007-259511 A1, U.S. Pat. No. 7,407,025, EP 1 219 493 B1, JP 2012-021687 A, and EP 1 138 539 B1 to use a DC/DC converter for stabilizing the voltage supply of the second subsystem. 
     An attenuator is typically provided in the first subsystem, for example, in the high voltage subsystem explained, which includes the generator operable electric machine and which is configured to supply the second or additional subsystems via the DC/DC converter. A so-called high voltage battery, for example, is installed in a high voltage subsystem. The attenuator may also be a capacitor (in particular a so-called super capacitor). 
     Typically, consumers situated in the first subsystem are coupled to the first subsystem and may be decoupled from it. The terms “couple” and “decouple” include all measures which in each case cause a current to flow into the respective consumers or a corresponding current to be suppressed, for example a switching on and off. The consumers in the first subsystem are naturally those which have correspondingly high performance requirements. If an attenuator is present in the first subsystem, sudden load variations caused by coupling or decoupling of corresponding consumers are then sufficiently compensated for by the attenuator. This means that coupling or decoupling of corresponding consumers causes no sudden load variation, which manifests itself in the form of a sudden rise in voltage or sudden drop in voltage. A corresponding voltage may fall or rise, but this occurs within a time window in which the generator operable electric machine has sufficient time to compensate for the load increase or load reduction by increasing or reducing its power output. 
     Problems arise, however, when a corresponding attenuator in the first subsystem fails and/or must be switched off. In such cases, it is possible to maintain the energy supply of the motor vehicle only with difficulty, because sudden load variations caused by the coupling or decoupling of the consumers may result in strong voltage fluctuations. If the generator in the first subsystem is not able to adjust corresponding sudden load variations rapidly enough, undervoltages or overvoltages may occur. 
     Hence, the first subsystem and, potentially, the second subsystem (through degradation of the interconnected DC/DC converter) may acquire an undervoltage when a consumer is coupled and an overvoltage when a consumer is decoupled. The result of this may be, for example, that a touch voltage limit cannot be adhered to or a provided overvoltage protective circuit is overloaded. Depending on the provided regulating strategy, voltage fluctuations may lead to consumer failures within the first subsystem or to failure of the entire first subsystem. 
     Moreover, switching the generator to the de-energized state (i.e., with no corresponding energy store in the first subsystem) is not readily possible, particularly if the control panel and/or the controller is/are supplied from the first subsystem. Therefore, a corresponding system in such cases is normally switched to a safe state which, however, includes switching off the first subsystem, and the vehicle malfunctions. 
     SUMMARY 
     Hence, there continues to be the need for improved options for operating corresponding multi-voltage electrical systems. 
     Against this background, example embodiments of the present invention provide a method for operating a multi-voltage electrical system for a motor vehicle, which includes a first subsystem operable at a first operating voltage and a second subsystem operable at a second operating voltage, and a corresponding multi-voltage electrical system. 
     The present invention may be used in multi-voltage electrical systems, the subsystems of which have identical or essentially identical operating voltages. The present invention may also be used in, and is described below with reference to, dual-voltage electrical systems in which a considerable difference exists between the operating voltages of the two subsystems, but is explicitly not limited to such dual-voltage electrical systems. As explained, consumers in such multi-voltage electrical systems may be coupled to the first subsystem and may be decoupled from the first subsystem. As also explained, in the event of a failure of an attenuator in the first subsystem, negative or positive sudden load variations may occur in the event of a corresponding coupling or decoupling. These are manifested as voltage drops or voltage spikes. 
     According to example embodiments of the present invention, a multi-voltage electrical system is designed such that a current may be selectively fed from the first subsystem into the second subsystem or from the second subsystem into the first subsystem. According to the present invention, a special operating mode is provided in this connection, in which a negative sudden load variation caused by coupling of at least one consumer to the first subsystem is counteracted by feeding current from the second subsystem into the first subsystem, and a positive sudden load variation caused by decoupling of at least one consumer from the first subsystem is counteracted by feeding current from the first subsystem into the second subsystem. Such “counteracting” occurs in each case over a limited “feed time period.” 
     The respective current may be fed in the form of an increase in a current normally fed via the DC/DC converter into the second subsystem and/or in the form of a reduction of a corresponding current, e.g., based on a normal value. 
     Such a special operating mode is carried out, in particular, if a failure of an attenuator provided in the first subsystem occurs. This may be detected, for example, by a measuring circuit, or is known if a corresponding attenuator, for example, a battery, is actively switched off. 
     The current is fed from the second subsystem into the first subsystem or vice versa advantageously over the respective feed time periods to allow a generator operable electric machine provided in the first subsystem sufficient time to counteract the respective (negative or positive) sudden load variations by a corresponding adjustment. Thus, during the feed time periods a power output of a generator operable electric machine provided in the first subsystem is gradually increased in the case of a negative sudden load variation and gradually reduced in the case of a positive sudden load variation. The duration of the respective time periods is determined by the capabilities of the generator operable electric machine. 
     Thus, positive and negative sudden load variations are essentially compensated for using the measures according to the present invention. In addition, a switching on of the generator operable electric machine in the first subsystem without the attenuator, for example, without a battery, is possible in the first subsystem because, for this purpose, a current may be fed from the second subsystem. 
     In summary, within the scope of the present invention, the respective current feed, as explained, may have an attenuating effect on the corresponding sudden load variations in the event of failure of an attenuator (for example, a battery) situated in the first subsystem, and therefore, in the event of loss of a corresponding attenuating behavior of this attenuator. To feed the current in such a case, a DC/DC converter in particular is used which is bi-directionally operable. Such DC/DC converters have significantly lower time constants than do electric machines due to the lack of masses to be moved. A corresponding intervention gives the generator operable electric machine sufficient time to adapt its power output. 
     The method according to example embodiment of the present invention is used in particular in dual-voltage electrical systems, in which the first operating voltage is higher than the second operating voltage. In this case, a DC/DC converter, selectively operable as a step-down converter or a step-up converter, is used to feed the current from the first subsystem into the second subsystem, or from the second subsystem into the first subsystem. Such DC/DC converters are also known as step-down, step-up converters or as buck-boost converters. The DC/DC converter used within the scope of the present invention may, for example, shift its operating mode according to a corresponding activation from a step-down converter operation (for example, 48 V to 12 V) to a step-up converter operation (for example, 12 V to 48 V). 
     Particular advantages result, for example, when in a corresponding special operating mode the operating voltage of the first subsystem is also reduced. This may take place, for example, in the explained dual voltage electrical systems which have different operating voltages. For example, the operating voltage of the first subsystem may be reduced from 48 V to 36 V. The level of the maximum allowable electrical system voltage is thus potentially increased so that also sudden load variations which are not completely adjustable do not result in inadmissible voltage values, for example, exceeding the maximum allowable touch voltage. 
     Feeding the current from the first subsystem into the second subsystem in this mode may result in a particularly strong charging of the present battery (for example, a lead-acid battery) or even in its outgassing. This may also be used as a measure for draining excess energy. It may also be provided to briefly convert corresponding energy into heat with the aid of the DC/DC converter. This enables an efficient “discharge” of excess energy. 
     According to an example embodiment, a processor unit, for example, a control unit of a motor vehicle, is programmed to carry out a method according to the present invention. 
     The implementation of the method in the form of software is also advantageous, since this involves particularly low costs, in particular if a performing control unit is also used for other tasks and is therefore present anyway. Suitable data media for providing the computer program are, for example, diskettes, hard disks, flash memories, EEPROMs, CD-ROMs, DVDs and the like. It is also possible to download a program from computer networks (e.g., the Internet, an intranet, etc.). 
     Additional advantages and embodiments of the present invention are understood from the description and the appended drawing. 
     It is understood that the features cited above and those to be explained below are applicable not only in each specified combination, but also in other combinations or alone, without departing from the scope of the present invention 
     The present invention is schematically shown in the drawings with reference to example embodiments and is explained in greater detail below with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic circuit diagram of a conventional dual-voltage electrical system. 
         FIG. 2  is a schematic circuit diagram of an operable dual-voltage electrical system according to an example embodiment of the present invention. 
         FIG. 3  shows voltage and current curves in conjunction with a method according to an example embodiment of the present invention. 
         FIG. 4  is a flow chart that illustrates a method according to an example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the figures, corresponding elements of the various drawings are denoted by identical reference numerals. For the sake of clarity, these will not be repeatedly explained. 
       FIG. 1  is a schematic circuit diagram of a conventional dual-voltage electrical system  110 . Dual-voltage electrical system  110  has a first subsystem  10  and a second subsystem  20 . In this embodiment, first subsystem  10  is designed, for example, for operation at 48 V as an operating voltage. Second subsystem  20  in this embodiment is designed, for example, for operation at 12 V as an operating voltage. Thus, first subsystem  10  is a so-called high voltage subsystem, and second subsystem  20  is a so-called low voltage subsystem. 
     An electric machine  11  having a converter  12  is provided in first subsystem  10 . Electric machine  11  may be at least generator operated and may feed a current into first subsystem  20  via converter  12 . In particular, electric machine  11  may also be motor operated. For example, braking power may be recovered in so-called recuperation systems with the aid of electric machine  11 , and/or the instantaneous power of an internal combustion engine may be boosted. 
     Provided in first subsystem  10  is a correspondingly designed energy store  13 , which is configured for operation with the first operating voltage, for example, a battery or a capacitor. A consumer  14  is schematically illustrated in subsystem  10 . 
     Provided in second subsystem  20  is a starter motor  21 , for example, which may be used for a start of a motor vehicle, in which dual-voltage electrical system  110  is formed. Also provided in second subsystem  20  is an energy store  23  also configured for a corresponding operating voltage, for example, a conventional vehicle battery. Here, a consumer  24  is also schematically illustrated. 
     First subsystem  10  and second subsystem  20  are connected to one another via a DC/DC converter  30 . In the example shown, a generator operable electric machine  11  is provided only in first subsystem  10 , so that ultimately second subsystem  20  is fed exclusively from first subsystem  10 . “Fed exclusively” also expressly includes a feed with the aid of energy store  23 , which is itself in turn charged from first subsystem  10 . 
     A control unit  50  (not shown in  FIG. 1 ) is configured to activate dual-voltage electrical system  110  via activation lines  51 . 
       FIG. 2  is a schematic circuit diagram of a dual-voltage electrical system  100 , which is operable according to an example embodiment of the present invention. Dual-voltage electrical system  100 , as shown, includes all the components of previously explained dual-voltage electrical system  110  (cf.  FIG. 1 ). Here, a (potential) interruption of a contact between battery  13  and first subsystem  10  is indicated by reference numeral  130 . As explained, instead of battery  13 , it is also possible to provide another suitable energy store, for example, a capacitor, which performs an attenuating function in first subsystem  10 . In the example shown, DC/DC converter  30  is designed as an active converter including a half bridge with appropriate activatable switch elements S and an inductance L. Switch elements S may be activated, for example, by a control unit  50  via corresponding control signals  51 . DC/DC converter  30  may be operated as a bi-directional converter, selectively as a step-down converter, converting a (higher) operating voltage of first subsystem  10  into a (lower) operating voltage of second subsystem  20 , or as a step-up converter, vice versa. 
     A voltage drop against ground in first subsystem  10  is illustrated in the form of an arrow  131 . A current fed by electric machine  11  is illustrated with an arrow  111 . A current flowing via consumer  14  is illustrated with an arrow  141 . Illustrated with an arrow  31  is a current which (in a step-up converter operation of DC/DC converter  30 ) is fed from second subsystem  20  into first subsystem  10 . 
     The effects of a coupling or decoupling of a consumer  14  are explained in greater detail with reference to  FIG. 3 .  FIG. 3  shows in diagrams  310  and  320  a voltage U in V, and a current I in A on the ordinate against a time t in ms on the abscissa. 
     A voltage curve  311  in a dual-voltage electric system  110  is illustrated in diagram  310  which occurs during a coupling or decoupling of a consumer  14 , when the attenuator  13  fails. In this case, consumer  14  is coupled at a point in time t 0  and decoupled at a point in time t 2 . 
     The coupling of consumer  14  at point in time t 0  results, due to lack of attenuation, in a voltage drop, which potentially falls below a minimum allowed voltage level denoted by UB min  of dual-voltage electrical system  110 . The current flowing through consumer  14  is illustrated by current curve  321  in diagram  320  (cf. arrow  141  in  FIG. 2 ). The current fed by electric machine  11  into dual-voltage electrical system  110  is represented there by  323  (cf. arrow  111  in  FIG. 2 ). As is apparent, electric machine  11  increases its power output and thereby feeds a greater amount of current into dual-voltage electrical system  110  to compensate for the current consumption of consumer  14 . Due to the slow time constant of electric machine  11 , electric machine  11  has not compensated for the increased current consumption until a point in time t 1 . At this point in time, the operating voltage of the first subsystem has again reached the setpoint voltage, denoted here by UB. 
     These explanations also apply conversely to a decoupling of consumer  14 . In this case, the maximum allowable operating voltage of the first subsystem, denoted here by UB max , is potentially exceeded after a point in time t 2 —see voltage curve  311 . The voltage potentially exceeds the maximum allowable touch voltage (typically 60 V) and therefore represents a safety risk. Here too, the current fed by electric machine  11  into dual-voltage electric system  110  may only be gradually reduced. 
     The measures according to example embodiments of the present invention essentially include feeding a current (cf. arrow  31  in  FIG. 2 ) from the second subsystem into the first subsystem between points in time t 0  and t 1  and feeding a current from the first subsystem into the second subsystem between points in time t 2  and t 3  (see current curves  322  in diagram  320 ). Sudden load variations are therefore counteracted by a corresponding feed. This results in a smoothing of the voltage curve in an operable dual-voltage electrical system  100  according to an example embodiment of the present invention, as illustrated by  312  in diagram  310 . 
       FIG. 4  is a flow chart of a method according to an example embodiment of the present invention. The method includes a submethod  411  on the generator side and a submethod  412  on the converter side, as a result of which the increase or decrease in the power output of electric machine  11  on the one hand and the feeding of the currents by DC/DC converter  30  on the other hand are implemented in each case essentially in the form of a control loop. 
     A corresponding special operating mode  410  is then initiated if, for example, a failure of an attenuator  13  is detected in the first subsystem with the aid of a corresponding sensor structure, or a corresponding attenuator  13  is switched off. 
     Starting from step  410  (cf. submethod  411  on the generator side) an adaptation of the generator power output is carried out (step  420 ) until the presence of a normal operation is detected in a step  430 , i.e., for example, a corresponding attenuator  13  is re-connected. In this case, submethod  411  on the generator side transitions to a rest state  440  or idle operation. 
     In a step  450  in the example shown a sudden load variation is detected, which may, for example, have a value of from 0 to 100%. In a step  460  (cf. submethod  412  on the converter side), it is checked whether this sudden load variation exceeds or falls below a potentially existing tolerance value. According to the result, either an emergency supply to the first subsystem is undertaken (step  470 ) by feeding current from the second subsystem into the first subsystem or as an alternative, current may be fed from the first subsystem into the second subsystem (step  475 ). In this case too, a corresponding submethod on the converter side is carried out (step  480 ) until a normal operation mode is present. In this case, submethod  412  on the converter side also transitions to a rest state  490  or idle operation.
A multi-voltage electrical system of a motor vehicle includes first and second subsystems operable at operating voltages, where a current is selectively and alternatively feedable from the first subsystem into the second subsystem and from the second subsystem into the first subsystem, and an electrical consumer is coupleable to and decoupleable from the first subsystem. A method for operating the system includes, responsive to a failure or lack of an attenuator, implementing a special operating mode in which negative and positive sudden load variations caused by coupling or decoupling of the electrical consumer is counteracted by, respectively, feeding current from the second subsystem into the first subsystem or vice versa, in each case over a respective feed time period.