Abstract:
In a braking system and a method for braking a vehicle having a hybrid drive, at least one wheel, preferably at least the wheels of one axle, is brakable by a regenerative brake and by a service brake. The braking system includes a vehicle stability control system, which ascertains state variables describing a driving dynamics of the vehicle, for which there exist threshold values and/or threshold value combinations. The vehicle stability control system is arranged such that when reaching, exceeding and/or undershooting one of the threshold values or one of the threshold value combinations a wheel-specific braking force control is effected. A predefined maximum regenerative braking force is determined on the basis of the state variables ascertained by the vehicle stability control system and the regenerative brake is controlled such that the regenerative braking force exerted on the at least one wheel does not exceed the determined predefined maximum regenerative braking force.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a braking system for a vehicle having a hybrid drive, e.g., in which at least one wheel, preferably at least the wheels of one axle, is capable of being braked by a regenerative brake and by a service brake, a vehicle stability control system being provided, which ascertains state variables describing a driving dynamics of the vehicle, for which threshold values and/or threshold value combinations exist, the vehicle stability control system being designed to effect a wheel-specific braking force control when reaching, exceeding and/or undershooting one of the threshold values or one of the threshold value combinations. Furthermore, the present invention relates to a method for operating such a braking system. 
       BACKGROUND INFORMATION 
       [0002]    A hybrid drive of a vehicle is distinguished by the fact that the vehicle includes at least two different driving motors. Today, hybrid drives normally include one internal combustion engine and one electric machine. The advantage of a hybrid drive is that the vehicle may be driven in different driving situations using the different driving motors. This makes it possible to save energy. This is further augmented by the fact that the electric machine may be utilized for recuperating energy that is released when the vehicle is decelerated. This means that the electric machine is utilized as an electric generator for producing electric current. This electric current is used to charge an electrical energy store. The electrical energy stored in the electrical energy store may be used subsequently to drive the electric machine and thereby the vehicle. An electric machine operable as a generator thus represents a regenerative brake. Motor vehicles additionally have so-called service brakes, which are normally operated hydraulically, pneumatically, electromechanically etc. 
         [0003]    Fundamentally, it is desirable to utilize as great a regenerative braking force as possible in order to obtain as much energy as possible in a deceleration of the vehicle. The result of this, however, is that a braking force distribution between the individual axles of the vehicle normally deviates from an optimal braking force distribution that would have to be chosen for an optimal stability of the vehicle. This means that the at least one wheel or the wheels of the at least one axle that are braked regeneratively normally are overbraked in comparison to a braking force distribution that would be optimal for a vehicle stability. For example, if the regenerative brake is connected to the front wheels of a vehicle, then the vehicle tends to understeer due to an overbraking of the front wheels. On the other hand, if the wheels of a rear axle are braked regeneratively and thus frequently overbraked, then the vehicle tends to oversteer. 
         [0004]    In vehicles that are not equipped with a regenerative brake, a vehicle stability control system is known to be provided that ascertains and monitors state variables that describe a driving dynamics of the vehicle, for example a vehicle speed, a longitudinal acceleration, a transversal acceleration, etc. Threshold values exist for the individual state variables or a combination of state variables. If one of these threshold values is reached, then the vehicle stability control system uses the service brakes to effect a wheel-specific braking force control. This means that the braking force distribution between the individual wheels, i.e. a braking torque distribution, is changed so as to have a positive influence on the stability of the vehicle. 
         [0005]    German Published Patent Application No. 103 27 502 describes a regulation for regeneratively braking a motor vehicle, including a regulating system of yaw stability, an ability to provide a regenerative braking torque on wheels of at least one axle, non-regenerative brakes that are connected to the wheels of at least one axle, a regulator and a regulating system implemented in the regulator for controlling the regulator so as to regulate regenerative braking and non-regenerative braking when operating the regulating system of yaw stability. The system is developed such that it has the ability to receive an input signal from a yaw stability regulating system, to compare the current braking equilibrium with a desired braking equilibrium, to determine whether the front wheels or rear wheels are overbraked in comparison to the desired braking equilibrium and to set the regenerative braking variable and the non-regenerative braking variable. The yaw stability regulating system is used to determine whether a state of understeering or of oversteering exists. The regenerative braking force is thus adjusted as a function of the occurrence of a vehicle instability in the form of oversteering or understeering. 
         [0006]    U.S. Pat. No. 5,318,355 describes a vehicle having regenerative brakes in which it is possible to switch between a preferred regenerative braking mode and a normal braking mode such that a total braking force of all wheels remains constant. In the preferred regenerative operating mode, the wheels that are braked regeneratively are overbraked with respect to a theoretical braking force distribution. In the normal operating mode, the braking force distribution matches the theoretical braking force distribution. A switch between the operating modes occurs via a control system, for example if a steering angle and a vehicle speed are no longer lower than predefined values. With this device, regenerative braking is not possible at higher vehicle speeds. 
         [0007]    German Published Patent Application No. 198 42 472 describes a braking system for a motor vehicle having a plurality of wheels including at least one driven wheel having: a friction braking device for pressing a friction part on a rotor rotating with the wheels so as thereby to apply a friction braking torque onto each wheel; a regenerative braking device, which includes at least one electric motor, which is connected to the at least one driven wheel, for applying a regenerative braking torque onto the at least one driven wheel; and a total braking torque control device for controlling a total braking torque, which includes at least the regenerative braking torque or the friction braking torque that is applied to each of the wheels existing in the plural, the total braking torque control device working, when the total braking torque applied to at least one of the wheels existing in the plural exceeds an upper limit corresponding to a coefficient of friction of a road surface on which the motor vehicle is traveling, so as to bring to zero the regenerative braking torque of each of the wheels existing in the plural, of which at least one exists, and to control the friction braking torque of each of the wheels, of which at least one exists, while an influence of the process of bringing the regenerative braking torque to zero is reduced. Thus, if the braking torque applied on one of the wheels exceeds an upper limit that is a function of a coefficient of friction of the road, then the regenerative braking torque is regulated to zero. The vehicle described in German Published Patent Application No. 198 42 472 thus includes a regulating system that attempts to integrate the regenerative brakes into an anti-lock braking system. 
         [0008]    German Published Patent Application No. 199 12 425 describes a combined motor vehicle braking device including a friction brake and an electric machine brake, in particular in the form of a starter generator. To increase the ease of operation while at the same time preventing a vehicle battery, which is able to be powered by the electric machine brake in generator operation, from over loading, a brake activation element, in particular a foot brake pedal, is provided with a control system for automatically distributing the respective brake load between the friction brake and the electric machine brake. A brake load distribution between the electric machine brake and the friction brake may be selected as a function of a state of charge of the vehicle battery. 
         [0009]    The conventional systems do not ensure that the occurrence of an unstable driving state is prevented and that at the same time in any operating state there occurs the greatest possible regenerative braking operation and thus energy recuperation. 
       SUMMARY 
       [0010]    Example embodiments of the present invention provide a braking system for a vehicle having a hybrid drive and a method for braking such a vehicle, which will avoid critical driving states with respect to vehicle stability and at the same time obtain as much regenerative energy when braking as possible. 
         [0011]    Example embodiments of the present invention provide a braking system for a vehicle having a hybrid drive, in which at least one wheel, preferably at least the wheels of one axle, is capable of being braked by a regenerative brake and by a service brake, a vehicle stability control system being provided, which ascertains state variables describing a driving dynamics of the vehicle, for which threshold values and/or threshold value combinations exist, the vehicle stability control system being adapted to effect a wheel specific braking force control when reaching, exceeding and/or undershooting one of the threshold values or one of the threshold value combinations, a predefined maximum regenerative braking force being determinable on the basis of the state variables ascertained by the vehicle stability control system and the regenerative brake being controllable such that a regenerative braking force applied on the at least one wheel does not exceed the determined predefined maximum regenerative braking force. This means that a predefined maximum regenerative braking force is determined on the basis of the state variables ascertained by the vehicle stability control system. The regenerative brake is subsequently controlled such that the regenerative braking force exerted on the at least one wheel does not exceed the predefined maximum regenerative braking force. This ensures that the regenerative braking force is already limited beforehand such that a vehicle instability does not occur, rather than being reduced only in consequence of an unstable driving state that has already occurred. Since a regenerative brake normally reacts more slowly and is in particular difficult to control in a wheel-specific manner, it is necessary to regulate the regenerative braking force such that an unstable vehicle state is as much as possible avoided. While a conventional vehicle stability control system, which is referred to, for example, as an ESP (electronic stability program), is triggered when a transversal acceleration limiting value and a yaw rate limiting value are jointly reached/exceeded, both limiting values being possibly a function of speed, example embodiments of the present invention provide a regenerative braking force such that a triggering state of the vehicle stability control system is not reached because the latter is caused by at least one of the vehicle wheels being overbraked by the regenerative brake. 
         [0012]    Example embodiments therefore provide for the predefined maximum regenerative braking force to be determinable such that the individual ascertained state variables maintain a safety margin distance from the threshold values or threshold value combinations, the reaching, exceeding and/or undershooting of which result in the triggering of a wheel-specific braking force control. 
         [0013]    Example embodiments provide for one of the state variables to be a friction coefficient utilization. The friction coefficient utilization indicates how many percent of the possible forces transmittable onto the roadway via the wheels of the vehicle are already utilized. A variable of a roadway friction coefficient used for the calculation is already calculated in vehicle stability control systems such as are today part of the related art. 
         [0014]    Example embodiments of the present invention therefore provide for the friction coefficient utilization to be ascertained on the basis of a roadway friction coefficient and a transversal acceleration. 
         [0015]    Example embodiments, in which a longitudinal acceleration is included in ascertaining the friction coefficient utilization, allow for even more precise information about the friction coefficient utilization. In this case, the friction coefficient utilization is thus ascertained by taking into account the longitudinal and transversal acceleration. A friction coefficient utilization is calculated from the quotient of an acceleration and a product of the gravitational acceleration and a roadway friction coefficient. If the acceleration is composed of a transversal acceleration and a longitudinal acceleration, then a quadratic mean is determined. This means that use is made of the square root of the sum of the individual squares of acceleration. 
         [0016]    Example embodiments of the present invention provide for the state variables to include a transversal acceleration, a yaw rate and a vehicle speed. As already explained above, in vehicle stability control systems, the wheel-specific braking force control is often triggered if both the transversal acceleration as well as the yaw rate respectively exceed a predefined threshold value, i.e., exceed a combination of the threshold values in the sense of what is described here, the individual threshold values or the combination of the threshold values being a function of a vehicle speed. In such an example embodiment, an initial maximum regenerative braking force may be reduced, for example, when one of the two threshold values of the threshold value combination is exceeded. This ensures that a safety margin with respect to the individual, threshold values or the threshold value combination is maintained. 
         [0017]    In example embodiments, a setpoint value for the yaw rate is calculated within the vehicle stability control system from the vehicle speed and a steering wheel angle. For the setpoint value, there exist an upper threshold value that is greater than the setpoint value and a lower threshold value that is smaller than the setpoint value. The upper threshold value and the lower threshold value are a function of the setpoint value. If one plots the upper threshold value and the lower threshold value against the setpoint value of the yaw rate, then one obtains a tolerance band for the yaw rate. If a measured yaw rate lies outside of the tolerance band, i.e., if the measured yaw rate is greater than the upper threshold value for the ascertained setpoint value of the yaw rate or smaller than the associated lower threshold value, then a wheel-specific braking force control is initiated. A reduction of the predefined maximum regenerative braking force, however, is effected when “leaving” a narrower additional tolerance band. In this example embodiment, the ascertained state variables thus include a measured yaw rate, a vehicle speed and a steering wheel angle. The threshold values that form the additional tolerance band and which maintain a safety margin are a function of the setpoint value of the yaw rate just as the safety margin itself, i.e., of the vehicle speed and the steering wheel angle. Example embodiments of the present invention therefore provide for the state variables to include a vehicle speed, a steering wheel angle and a measured yaw rate. 
         [0018]    In addition to the determined predefined maximum regenerative braking force, for controlling the regenerative brake it is likewise necessary to know how great the braking force is that the regenerative brake is able to provide. This depends, for example, on how high a charge state of an electrical energy store is. If the latter is nearly or entirely charged, then the regenerative brake is only able to provide little or no regenerative braking energy. Example embodiments of the present invention therefore provide for a maximally applicable regenerative braking power to be ascertained and to be included in controlling the regenerative brake. 
         [0019]    In order to make a change in the braking force provided by the regenerative brake as transparent as possible for the driver, i.e. so that such a change in the braking force is not noticed by the driver, example embodiments of the present invention provide for the service brakes on the wheels of the vehicle to be controlled such that the braking force generated by the service brakes of the wheels corresponds to a difference between a braking force requested by a driver and the controlled regenerative braking force. 
         [0020]    The use of a braking system according to example embodiments of the present invention provides for the individual state variables to maintain a safety margin distance from their threshold values, which result in the triggering of a wheel-specific braking force control. If a driving state should nevertheless occur, in which a state variable or a combination of state variables respectively no longer maintain a second safety margin distance, which is lower than the safety margin distance, from the threshold values or the combination of threshold values, then example embodiments for the regenerative braking force be set to zero. This means that the regenerative braking force is entirely omitted if there is a danger of an imminent vehicle instability. In such a case, the regenerative braking force is preferably dispensed with in the continuing braking operation in order to exclude a vehicle instability entirely and to be able at any time to utilize to the full extent the higher reaction speed of the service brakes. Only in a new braking operation is the regenerative braking force again utilized for decelerating the vehicle. 
         [0021]    The features of the method according to example embodiments of the present invention have the same advantages as the corresponding features of the braking system according to example embodiments of the present invention. 
         [0022]    Example embodiments of the present invention are explained in greater detail below with reference to the appended Figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  a schematic representation of a motor vehicle having an example embodiment of a braking system; 
           [0024]      FIG. 2  a schematic block diagram for explaining the determination of the regenerative braking force and of the braking force applied by service brakes; and 
           [0025]      FIG. 3  a graphical representation of a measured yaw rate, a tolerance band for an intervention of a wheel-specific braking force control and another tolerance band, at the leaving of which a reduction of the predefined maximum regenerative braking force sets in, against time for a vehicle moved at constant speed, the steering wheel angle of which is changed. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  schematically shows a vehicle  1  that includes an example embodiment of a braking system. Vehicle  1  includes steerable, non-driven wheels  2  of a front axle  3 . Non-steerable wheels  4  of a rear axle  5  are drivable by a hybrid drive  6 . Hybrid drive  6  includes an internal combustion machine  7  and an electric machine  8 . Internal combustion machine  7  and electric machine  8  are operably connectible to rear axle  5  respectively via a clutch  9 ,  10 . One or more existing gear units are not shown. Electric machine  7  is coupled to an electrical energy store  11 . Electrical energy stored in electrical energy store  11  may be used to drive electric machine  8 . Electric machine  8 , however, may also be operated as a generator and feed electrical energy into electrical energy store  11 . A charging and monitoring unit  12  monitors the charge state of electrical energy store  11  and regulates an infeed and withdrawal of energy. Furthermore, charging and monitoring unit  12  provides information about a maximum energy absorbable by the battery to a control unit  13 , in which a braking control of the braking system is implemented. In the event of a braking request on the part of a driver via an actuator  14  in the form of a brake pedal, a desired braking force is supplied to control unit  13  as a function of a deflection of the brake pedal. 
         [0027]    A vehicle stability control system  15  is integrated into control unit  13 . Vehicle stability control system  15  evaluates sensor signals from wheel pulse sensors  16 , a longitudinal acceleration sensor  17 , a transversal acceleration sensor  18 , a yaw rate sensor  19  and possibly additional sensors that are not shown. In example embodiments, different and/or additional sensors may be evaluated in order to ascertain state variables that describe a driving dynamics of vehicle  1 . Vehicle stability control system  15  is coupled to service brakes  20  disposed on driven wheels  2  and non-driven wheels  4  such that a braking force may be controlled in a wheel-specific manner. Such a wheel-specific braking force control is exercised when vehicle stability control system  15  has established for individual state variables or a combination of state variables, which are ascertained on the basis of the received sensor signals, that a corresponding threshold value or a combination of threshold values has been exceeded, which indicates a vehicle instability. The wheel-specific braking force control is implemented such that the stability of the vehicle is maintained or reestablished. Service brakes  20  are furthermore coupled to a central braking force generator  21 , which normally takes the form of a pneumatic brake booster and which effects a balanced braking force distribution to individual service brakes  20 . Central braking force generator  21  is connected to control unit  13  and is controlled by the latter. 
         [0028]    Apart from being braked by service brakes  20 , driven wheels  4  may also be braked regeneratively by electric machine  8 , when the latter is operated as a generator. Control unit  13  of the braking system is arranged such that it determines and controls both the regenerative braking force applied by electric machine  8  in combination with electrical energy store  11  as well as the non-regenerative braking force  20  applied by the service brakes in cooperation with central braking force generator  15 . 
         [0029]    In  FIG. 2 , a block diagram shows in a rough schematic manner how the braking force applied by the service brakes and the braking force applied by the regenerative brake is defined and controlled. 
         [0030]    A braking force requested by the driver, which is proportional to a deflection of actuator  14  in the form of a brake pedal, is transmitted  31  to control unit  13 . An electric machine control unit  22  transmits  32  a maximum braking power of the electric motor to control unit  13 . Charging and monitoring unit  12  transmits  33  a maximum energy absorption of the battery. From the latter, the braking system determines by minimum formation a maximum braking force  34  suppliable by the regenerative brake. On the basis of the monitored state variables, vehicle stability control system  15  ascertains a predefined maximum regenerative braking force that the regenerative brake is able to apply without the danger of a vehicle instability  35 . The regenerative braking force is determined  36  via a new minimum value formation between the predefined regenerative braking force and the maximally suppliable regenerative braking force as well as the braking force requested by the driver. According to this determination, the regenerative brake is controlled  37  by electric machine control unit  22 . The braking force for the service brakes is ascertained  38  by a difference formation between the requested braking force and the regenerative braking force supplied by the regenerative brake. The service brakes are controlled  39  accordingly via central braking force generator  21 . 
         [0031]    A determination of the maximum braking power permitted from the perspective of driving dynamics, as is performed in functional block  35 , is preferably implemented on the basis of a friction coefficient utilization η. For this purpose, either the transversal friction coefficient utilization η q  or a longitudinal and transversal friction coefficient utilization η lq  may be used. The transversal friction coefficient utilization η q  is computed according to the following formula 
         [0000]    
       
         
           
             
               
                 η 
                 q 
               
               = 
               
                 
                   y 
                   ¨ 
                 
                 
                   μ 
                   · 
                   g 
                 
               
             
             , 
           
         
       
     
         [0000]    where ÿ indicates the transversal acceleration, μ a roadway friction coefficient and g the gravitational acceleration. In a vehicle stability control system, roadway friction coefficient μ is a known operand. The ascertainment will not be explained here in greater detail since it is known to one skilled in the art from the related art. 
         [0032]    The following formula describes the longitudinal and transversal friction coefficient utilization η lq : 
         [0000]    
       
         
           
             
               
                 η 
                 lq 
               
               = 
               
                 
                   
                     
                       
                         x 
                         ¨ 
                       
                       2 
                     
                     + 
                     
                       
                         y 
                         ¨ 
                       
                       2 
                     
                   
                 
                 
                   μ 
                   · 
                   g 
                 
               
             
             , 
           
         
       
     
         [0000]    where {umlaut over (x)} indicates the longitudinal acceleration. 
         [0033]    The predefined maximum regenerative braking force is preferably determined such that it is set to equal a maximum possible braking force of the vehicle when the friction coefficient utilization is below 50%. Above a friction coefficient utilization of 50% up to a friction coefficient utilization of 75%, the maximum predefined regenerative braking force is reduced steplessly or stepwise until it reaches a value of 0 at a friction coefficient utilization of 75%. At a friction coefficient utilization above 75%, the vehicle is thus braked exclusively by the service brake, while in the transitional range between a friction coefficient utilization of 50% and a friction coefficient utilization of 75%, the “missing” braking force not applied by the regenerative brake is applied by the service brakes and is thus quasi substituted on the basis of the described control by the braking system. The friction coefficient utilization limit values that were mentioned here may also be defined differently. Furthermore, the reduction of the predefined maximum regenerative braking force may occur linearly or along another monotonic, preferably continuous, function. For this purpose, additional state variables of such a function may be drawn upon, for example as parameters. In any event, the predefined maximum regenerative braking force is defined in such a way that each of the state variables, which is able to trigger a wheel-specific braking force control, maintains a safety margin distance from the corresponding threshold value or a threshold value combination. If this is undershot, then the predefined maximum regenerative braking force is reduced. As soon as a second safety margin distance situated closer to the threshold value or a threshold value combination is undershot, then the predefined maximum regenerative braking force is set to zero and is preferably not increased again in the same braking operation. 
         [0034]    Alternatively or additionally there is a provision in some vehicles for a setpoint value for the yaw rate of the vehicle to be calculated within the vehicle stability control system from a vehicle speed and a steering wheel angle. A tolerance band is around this setpoint value. The tolerance band is bounded by an upper threshold value and a lower threshold value, which respectively are a function of the calculated setpoint value of the yaw rate. Furthermore, for each setpoint value of the yaw rate a narrower additional tolerance band is defined. This additional tolerance band is bounded by another upper threshold value and another lower threshold value, which respectively are likewise a function of the setpoint value of the yaw rate. If the measured yaw rate exceeds the additional upper threshold value or if the measured yaw rate falls below the additional lower threshold value, then the predefined maximum regenerative braking force is reduced such that the upper threshold value is not exceeded and the lower threshold value is not undershot, which respectively would cause a triggering of a wheel-specific braking force control. 
         [0035]      FIG. 3  shows a graph  40  in which an upper threshold value  41  and a lower threshold value  42  are plotted against time. Upper threshold value  41  and lower threshold value  42  bound a tolerance band  43  for triggering a wheel-specific braking force control. An additional upper threshold value  44  and an additional lower threshold value  45 , which bound another tolerance band  46 , are likewise shown. In addition, a measured yaw rate  47  of a vehicle is plotted against time. The vehicle moves at a constant speed. At a time indicated by an arrow  48 , a steering motion sets in that brings with it a change of the steering wheel angle. This changes both the measured yaw rate as well as a setpoint value of the yaw rate, on which upper threshold value  41 , the lower threshold value, the additional upper threshold value  44  and the additional lower threshold value  45  depend. In the example shown, measured yaw rate  47  does not leave additional tolerance band  46  at any time such that during the entire steering operation the maximum regenerative braking force may be selected to equal a maximum possible regenerative braking force. If measured yaw rate  47  were to leave additional tolerance band  46 , then the predefined maximum regenerative braking force would be reduced, to zero if necessary, in order to avoid upper threshold value  41  from being exceeded and/or lower threshold value  42  from being undershot. 
       LIST OF REFERENCE CHARACTERS 
       [0000]    
       
           1  vehicle 
           2  non-driven wheels 
           2  front axle 
           4  driven wheels 
           5  rear axle 
           6  hybrid drive 
           7  internal combustion engine 
           8  electric machine 
           9 ,  10  clutches 
           11  electrical energy store 
           12  charging and monitoring unit 
           13  control unit 
           14  actuator 
           15  vehicle stability control system 
           16  wheel pulse sensors 
           17  longitudinal acceleration sensor 
           18  transversal acceleration sensor 
           19  yaw rate sensor 
           20  service brakes 
           21  central braking force generator 
           22  electric machine controller 
           31 - 39  functional blocks of a method for controlling the regenerative brake and the service brakes 
           40  graph 
           41  upper threshold value 
           42  lower threshold value 
           43  tolerance band 
           44  additional upper threshold value 
           45  additional lower threshold value 
           46  additional tolerance band 
           47  measured yaw rate 
           48  arrow