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.
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.
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.
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.
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.
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.
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.
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.