Abstract:
A device for operating a drive unit generates setpoint values for controlling the drive unit based on, for example driver-generated input parameters. The device generates a first setpoint value for a front-axle drive and a second setpoint value for a rear-axle drive and includes a load impact damping unit which, in the event of a risk of load impacts, processes the setpoint values so as to damp the load impacts. The load impact damping unit is divided into a first load impact damping filter for processing the first setpoint value for the front axle drive and into a second load impact damping filter for processing the second setpoint value for the rear axle drive, and supplies corresponding filtered setpoint values to these drives.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2011/001917, filed Apr. 15, 2011, which designated the U.S. and has been published as International Publication No. WO 2011/131320 and which claims the priority of German Patent Application, Ser. No. 10 2010 015 425.3, filed Apr. 19, 2010, pursuant to 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The invention relates to a device for operating a drive unit of a motor vehicle and a method for operating a motor vehicle. 
     In vehicles with internal combustion engines as well as in vehicles having an exclusive or a partial electric drive, strong driver-generated torque jumps can cause vibrations in the drive train, also referred to as load reversals or load impacts, which can be perceived as an uncomfortable driving characteristic. This applies in particular to torque jumps between an operating phase without torque demand, i,e. a coasting mode, and an operational phase with torque demand, i.e., a traction mode. 
     DE 10 2007 013 253 A1 discloses a generic device for operating a drive unit of a motor vehicle. The device generates setpoints based on driver-generated input parameters, which are used for controlling the drive unit. The drive unit is in the present case an internal combustion engine and an electric machine. The device recognizes operating conditions having a risk of load impacts. The device hereby processes the setpoints so as to attenuate the load impacts. 
     Such load impact damping is problematic especially for vehicles with a front axle drive and a rear axle drive that is mechanically decoupled from the front axle drive. In such drive system, the front axle drive and the rear axle drive can respond to jumps in the setpoint torque with different dynamics. When the front axle and the rear axle drive respond differently, the load impact damping on the front axle and at the rear axle may not be matched, resulting in an uncomfortable driving sensation. 
     It is the object of the invention to provide a device and a method for operating a drive unit of a motor vehicle, wherein impact damping which is comfortable for the vehicle occupant is feasible, in particular in vehicles having front axle drive and rear axle drive. 
     SUMMARY OF THE INVENTION 
     The device of the invention is used in a vehicle having a front axle drive and a rear axle drive. The device of the invention generates, in response to the input parameters set by the driver, a first setpoint value and/or a first setpoint torque for the front axle drive and a second setpoint value and/or a second setpoint torque for the rear axle drive. 
     According to the invention, a central load impact damping unit is not provided; instead, a first load impact damping filter associated with the front axle drive, and a second load impact damping filter associated with the rear axle drive are provided. The first load impact damping filter can then modulate the first setpoint torque supplied to the front axle drive, whereas the second load impact damping can modulate the setpoint torque supplied to the rear axle drive. 
     The setpoint torque is intended to include all setpoints suitable for deriving therefrom wheel or axle torques. A drive power or a value derived from the drive power is mentioned here as an example. 
     Based on the first load impact damping filter and the second impact damping filter operating independent from the first load impact damping filter, mutually independent load impact damping interventions on the front axle or on the rear axle may be performed. Thus, for example, vehicle load impact damping may be performed on one of the two vehicle axes, whereas the other vehicle axle does not have load impact damping. Alternatively, the load impact damping on the front axle may have a time offset with respect to the load impact damping on the rear axle in order to attain certain driving characteristics. 
     The first and the second load impact damping filter have at least one detection member or separate, independently operating detection members which recognize whether a load impact risk exists for the front axle or for the rear axle. Such load impact risk results, for example, when a load on the front or rear axle drive changes between coasting and a traction mode. Alternatively or in addition, a load impact risk may occur for larger jumps in the setpoint torque, when the driver suddenly sets a high torque demand. 
     The first and second load impact damping filter may also include a respective modulation member configured to modulate the setpoint torque set by the driver with respect to damping of the load impact. In the case of the aforementioned load change between the traction mode and the coasting mode, the torque gradient can be reduced in the region of a zero crossing of the torque. This can be done, for example, by impressing a second degree parabola on the torque jump in the region of the zero-crossing of the torque. 
     In the event that the detection member does not detect load impact risk, the setpoint torque can bypass the modulation member and can be supplied without modulation to the front axle drive or to the rear axle drive. 
     The present invention can be employed in particular in vehicles with a front axle drive and rear axle drive which are mutually independent and mechanically decoupled. In this case, the front axle and the rear axle cannot be driven by a common drive train, but are instead driven independent of one another by the two aforementioned axle drives. 
     The term axle drive hereby refers to an internal combustion engine and/or an electric machine. The internal combustion engines and electric machines can be provided here as an axle drive either alternatively or in combination. For example, the front axle drive may be realized with an internal combustion engine and an electric machine. The rear axle drive may have an additional electric machine independent from the front axle drive. Alternatively, the axle drive may have a combination of an internal combustion engine and an electric machine or a combination of two electric machines. The front axle drive and the rear axle drive are hereby controlled not by a common drive train, but instead by separate drive trains. 
     Based on the aforementioned input parameters, the device may first determine a sum torque that can be supplied to a torque distribution unit. The torque distribution unit splits the sum torque in response to, for example, a driver assistance control, for example an all-wheel control, of a vehicle dynamics control or a vehicle cruise control and the like into the aforementioned first setpoint torque for the front axle drive and the second setpoint torque for the rear axle drive. 
     As already mentioned above, the front axle drive and the rear axle drive may have different engines/machines, i.e. internal combustion engines or electric machines, which may result in a different temporal response of the front axle drive and the rear axle drive for set setpoint torques. Against this background, it is important for a smooth driving characteristic that the separately performed load impact damping on the front axle and the rear axle are coordinated with each other. 
     To this end, at least one timing element may be associated with the first and second load impact damping filter. A time offset which takes into account the different temporal response of the front axle and the rear axle can be defined with the timing element. The time delay may be defined such that the load impact damping occurs simultaneously at the front axle and at the rear axle, Alternatively, the time offset may be defined such that the load impact damping occurs both on the front axle and on the rear axle at different times with a mutual time delay. In this way, a particular driving characteristic desired by the driver may optionally be set, for example during a sports mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       An exemplary embodiment of the invention will now be described with reference to the accompanying figures. 
       These show in: 
         FIG. 1  in a schematic diagram a drive system of a motor vehicle; 
         FIG. 2  a block circuit diagram showing, starting from a torque set by the driver, signal processing until a filtered setpoint torque for the front axle drive and a filtered setpoint torque for the rear axle drive are produced, and 
         FIG. 3  a torque-time diagram showing a jump in the setpoint torque set by the driver and profiles of the setpoint torque for the front axle drive and the rear axle drive modulated with respect to load impact damping. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows in a schematic diagram the drive system of a hybrid vehicle with an all-wheel drive unit  1 . An internal combustion engine  5  and an electric machine  7  are connected to a drive train on the front axle of the vehicle  3  and cooperate with a gear  9 . The gear  9  is drivingly connected with the front axle  3  via a transmission output shaft  11  and a schematically indicated axle differential  13 . A clutch  15 , which is disengaged or engaged depending on the driving situation, is connected between the internal combustion engine  5  and the electric machine  7 . 
     An additional electric machine  19 , which drives the two rear wheels of the vehicle via an axle differential  21 , is arranged on the rear axle  17  of the motor vehicle. 
     Insofar as it is necessary for an understanding of the invention, the front-axle drive composed of the internal combustion engine  5  and the electric machine  7  and the rear axle drive composed of the electric machine  19  are indicated in  FIG. 1 . Other drive components, for example the high voltage battery  2  for supplying power to the two electric machines  7 ,  19 , or the engine controller  4 , the transmission controller  6  or the power electronics  8  of the two electric machines  7 ,  19  are only roughly sketched without further description for sake of clarity. 
     According to  FIG. 1 , an accelerator pedal module  23  is thus in signal communication with a central controller  25 . The central controller  25  additionally detects a plurality of input parameters, which will not be described here in detail, wherein the front axle drive  5 ,  7  can be controlled with a first filtered setpoint torque M 1F  and the rear axle drive  19  can be controlled with a second setpoint torque M 2F  based on a torque desired by the driver transmitted from the pedal module  23  and based on these input parameters. 
     The signal path necessary for an understanding the invention between the pedal module and the front axle drive  5 ,  7  and the rear axle drive  19  is shown in simplified form in  FIG. 2 . Accordingly, a desired driver value is supplied from the pedal module  23  to a device  27  integrated in the controller  25  as a setpoint. This setpoint includes a torque, a power or a quantity derived from the torque or the power. In the present example, the setpoint corresponds to a sum torque M Sum  that is supplied to a distribution unit  29  of the device  27 . The torque distribution unit  29  splits the sum torque M Sum  as a function of generally known input parameters into a setpoint torque M 1  for the front axle drive  5 ,  7  and a setpoint torque M 2  for the rear axle drive  19 . The sum torque M Sum  is split according to  FIG. 2  by taking into consideration a driver assistance controller  31 . 
     According to  FIG. 2 , the signals from the setpoint torques M 1 , M 2  split by the torque distribution unit  29  are further processed in mutually independent parallel processing paths, wherein the setpoint torques M 1 , M 2  can be filtered in a low-pass filter unit  33  and in a load impact damping unit  37 , so that correspondingly filtered setpoint torques M 1F , M 2F  are forwarded to the front axle drive  5 ,  7  and to the rear axle drive  19 . 
     In the device  27  shown in  FIG. 2 , the processing paths for filtering the setpoint torques M 1 , M 2  are constructed identically, by way of example. The following discussion will therefore be limited to the signal processing of the setpoint torque M 1  supplied to the front axle  5 ,  7 . The setpoint torque M 1  is initially supplied to a first low-pass filter  34  of the low-pass filter unit  33 . The low-pass filter  34  may be, for example, a time-proportional element with a temporal characteristic such that, for example, a driver-generated input step function can be smoothed, as will be described further below. 
     In the further signal path, the setpoint torque M 1  is supplied to a timing element  36 , from which the setpoint torque M 1  is optionally forwarded only following a time delay. The load impact damping unit  37  is arranged downstream of the timing element  36  in the direction of the signal flow. 
     The load impact damping unit  37  includes first a detection unit  38 , from which a bypass signal line  40  with a switched load impact damping filter  41  and a main line  43  branch off. It is then determined in the detection unit  38  whether a load impact risk exists for the front axle  3 . Such load impact risk may occur, for example, during a load change of the front axle drive  5 ,  7  or the rear axle drive  19  when transitioning from a coasting mode to a traction mode. 
     The main line  43  downstream of the detection unit  38  bypasses the load impact damping filter  41 , with the bypass signal line  40  being returned to the main line  43  after the load impact damping filter  41 . The main line  43  is further in signal communication with the front axle drive  5 ,  7 , which is controlled by the setpoint torque M 1F  filtered in the low-pass filter unit  33  and in the load impact damping unit  37 . 
     In accordance with the  FIG. 2 , the signal path of the setpoint torque M 2  from the torque distribution unit  29  towards the rear axle drive  19  is implemented identical to the aforedescribed signal path of the setpoint torque M 1 . 
     An exemplary driving situation is shown in  FIGS. 2 and 3 , wherein a driver-generated step function is generated by actuation of the accelerator pedal  23 . In the illustrated step function, an input torque M E  switches abruptly from a coasting mode without torque demand to a traction mode with torque demand. The step function shown in  FIGS. 2 and 3  thus passes through a zero-crossing for the torque, i.e. through the abscissa of the torque-time diagram. 
     The torque jump specified by the driver is split in the torque distribution unit  29  into the first setpoint torque M 1  for the front axle drive  5 ,  7  and the second setpoint torque M 2  for the rear axle drive  19 . The two setpoint torques M 1 , M 2  are initially smoothed in the first and second low-pass filters  34 ,  35 . Thereafter, a time delay Δt between the torque buildup on the front axle drive  5 ,  7  and the torque buildup taking place on the rear axle drive  13  can be specified using the timing elements  36 . Such a time delay takes into account that the front axle drive  5 ,  7  has a more sluggish response due to the use of an internal combustion engine  5  compared to the rear axle drive  19 , which has exclusively an electric machine. 
     The time offset At may be designed, when taking into account these differences in the response times, so as to prevent an uncomfortable driving characteristic when the filtered setpoint torques M 1F , M 2F  are applied to the front axle drive  5 ,  7  and the rear axle drive  19 . 
     Furthermore, the detection unit  38 ,  39  detects whether or not a load impact risk exists. If the detection unit  38 ,  39  identifies a load impact risk, then a reduced zero-crossing torque gradient can be impressed at the critical instant on the already smoothed setpoint torque jump in load impact damping filter  41 ,  42 . The so-filtered torque jump is then supplied as a filtered setpoint torque M 1F , M 2F  to the front axle drive  5 ,  7  and the rear axle drive  19 , respectively. 
     In another exemplary driving situation, it is assumed that initially an all-wheel intervention does not take place, and that instead the entire sum torque M Sum  is supplied from the torque distribution unit  29  to the front axle drive  5 ,  7  as the first setpoint torque M 1 . The front axle drive  5 ,  7  is then in traction mode, whereas the rear axle drive  19  is carried along and thus operates in coasting mode, for example recuperation mode. 
     When the driver assistance controller  31  detects slip on the front axle  13 , the sum torque M Sum  is at least partially redistributed by the torque distribution unit  29  to the rear axle  17 . This torque redistribution causes a load change from coasting to traction on the rear axle  19 , prompting the detection member  39  associated with the rear axle  19  to detect a load impact risk. Accordingly, the setpoint torque M 2  redistributed to the rear axle  17  is subjected to load impact damping filtering in the load impact damping filter  42 . 
     In contrast to the rear axle  17 , the reduction of the setpoint torque M 1  on the front axle  3  may be so small that the detection member  38  associated with the front axle  3  does not detect a load impact risk. Therefore, there is no load impact damping on the front axle  3 . In analogy to the aforementioned torque redistribution, torque redistribution from the rear axle  17  to the front axle  3  may also arise in other driving situations. In this situation, load impact damping may only be performed on the front axle  3 , whereas the rear axle  17  is driven without load impact damping.