Abstract:
The invention relates to an automatic parking brake for motor vehicles having a plurality of brake actuators ( 4   a   , 4   b ) and an operating element ( 6 ) for actuating the parking brake, the braking actuators ( 4   a   , 4   b ) being connected to a controller ( 1, 11 ) performing individual wheel slip control upon occurrence of a critical driving situation. Simultaneous locking and thus the loss of lateral force can be prevented if the controller ( 1, 11 ) is designed such that the wheel slippage (A 2 ) of a second wheel ( 13   b ) is considered when controlling the slippage of a first wheel ( 13   a ).

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to an automatic parking brake for motor vehicles, a method for controlling slip, and a control unit having a slip controller. 
       BACKGROUND INFORMATION 
       [0002]    Automatic parking brakes (APB) typically include an operating element such as, e.g., a push-button switch by which the parking brake may be locked or released. When the operating element is actuated, a control unit connected to it recognizes the parking brake command and accordingly controls an actuator such as, e.g., a hydraulic pump or an electric motor in order to build up braking force on the wheels of the vehicle or to release the brake. 
         [0003]    The following considers systems in which electric motors (having gear units) are situated directly on the wheel brakes (so-called “motor on caliper”). Such parking brake systems are typically designed so that the driver may operate them when the vehicle is at a standstill and while driving. The possibility of an operation while driving is provided particularly for the purpose of being able to brake the vehicle in the case of a defective service brake. In this case, the vehicle is braked with a predefined deceleration, e.g., 3 m/s 2 . For this purpose, the deceleration of the vehicle is usually controlled. 
         [0004]    A difficulty of such parking brake systems using electric motors is that the response of the mechanical components is substantially slower than in a hydraulic parking brake system. This results in a relatively poor controllability of the electromechanical systems. This may cause the braked wheels to lock too long while being controlled. In a critical driving situation, for example, when oversteering or understeering while cornering, a vehicle may lose its cornering force. 
       SUMMARY 
       [0005]    Example embodiments of the present invention provide for the creation of an electromechanical parking brake that is more controllable and yet achieves a predefined deceleration of the vehicle than earlier systems. 
         [0006]    Example embodiments of the present invention provide for performing a slip control on the braked wheels and designing the corresponding controller in such a way that when controlling the slip on a first wheel, the wheel slip of a second wheel is taken into account. For example, depending on the magnitude of the wheel slip of the other wheel, a decision is made whether or not the braking force on the controlled (first) wheel may be increased. Thus, the possibility that both wheels lock at the same time or have too much slip is eliminated. The cornering force is thus maintained at least on one wheel. 
         [0007]    In example embodiments, the other wheel taken into account according to the present invention is the wheel of the same axle situated on the opposite side of the vehicle. 
         [0008]    In example embodiments, the wheel slip of another wheel is ascertained and an increase of the braking force on the controlled (first) wheel is allowed only if the wheel slip of the other wheel is lower than a predefined threshold value. The threshold value may be, for example, between 3% and 5% wheel slip. The threshold value may be, for example, approximately 4% wheel slip. This makes it possible to prevent the two wheels from locking at the same time and to allow them to continue to take up cornering force. 
         [0009]    In example embodiments, the slip controller is designed in such a way that the braking force on the controlled wheel is increased only if the wheel slip on this wheel is lower than a predefined threshold value. This threshold value may be approximately 4%. In an example embodiment, if the wheel slip is greater than the threshold value, then the braking force is reduced. 
         [0010]    In example embodiments, the slip control additionally takes the deceleration of the vehicle into account. In this case, an increase of the braking force on the controlled wheel is allowed only if the deceleration of the vehicle is lower in terms of its absolute value than a predefined threshold value. Otherwise, in an embodiment, the braking force is left as it is. 
         [0011]    In example embodiments, after a braking operation is triggered, the control operation on the left and right wheel is started asynchronously. That is, the electric motors are triggered in a time-delayed fashion. This has positive effects on the control response since it reduces the danger of an oscillating system in which both wheel brakes are synchronously opened or closed. 
         [0012]    In example embodiments, the slip controller is integrated as a software algorithm in a control unit, which is connected to wheel speed sensors and/or an acceleration sensor and/or triggers the electric motors of the parking brake as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    In the following, the present invention is explained in greater detail by way of example with reference to the attached drawings. 
           [0014]      FIG. 1  shows a schematic representation of a motor vehicle braking system having an automatic parking brake. 
           [0015]      FIG. 2  shows a flow chart for illustrating the control strategy of the slip control. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  shows a block diagram of a hydraulic braking system having an automatic parking brake according to an embodiment of the present invention. In an available manner, the braking system includes a foot brake pedal  5 , which acts together with a main brake cylinder via a brake booster (combined in block  7 ). The brake pressure produced by the driver and amplified is conducted via a hydraulic unit  2 , which is designed to perform a slip control, and brake lines  10   a ,  10   b  to wheel brakes  3 ,  8 . Wheel brakes  3 ,  8  are implemented in this example embodiment as disk brakes, which respectively include a brake caliper  3  and a brake disk  8 . 
         [0017]    The automatic parking brake includes an operating element  6  (e.g. a push-button switch) for activating and deactivating the parking brake, a control unit  1 , connected to push-button switch  6 , in which a parking brake algorithm having a slip controller  11  is stored, and multiple electric motors  4   a ,  4   b , which are respectively mounted on a brake caliper  3   a  and  3   b , respectively (motor on caliper). The system is supplied with voltage from a battery  9 . 
         [0018]    In example embodiments, to secure the vehicle in the parked state, the driver briefly operates push-button switch  6 . Control unit  1  recognizes this as a parking brake command and thereupon activates the electric motors disposed on brake calipers  3  in order to build up braking force and to lock the brake pads. When electric motors  4  are in the locked position, then the brake pistons are prevented from returning into the start position such that the brakes stay clamped. 
         [0019]    When push-button switch  6  is activated while driving, control unit  1  interprets this as a braking command. Control unit  1  thereupon triggers actuators  4  in such a way that the vehicle is braked at a predefined deceleration, e.g., 3 m/s 2 . For example, if the wheels lock in such an emergency braking operation, e.g., because of a slippery roadway, a wheel-specific slip control is performed on wheels  13   a  and  13   b , as shown in  FIG. 2 . 
         [0020]      FIG. 2  shows a flow chart concerning an example control strategy for the slip control of wheels  13   a ,  13   b  in an emergency braking operation using the parking brake embodiment. Wheels  13   a ,  13   b  are rear axle wheels. In this example, the example control strategy method used is identical for both wheels  13   a ,  13   b ; accordingly, in the following, only the left part of the flow chart is explained. 
         [0021]    In this embodiment, as soon as the driver has activated push-button switch  6  in step  20 , actuators  4   a ,  4   b  are triggered to brake the vehicle. In steps  21  and  22 , the slip of wheel  13   a  is then checked. If slip λ 1  in step  21  is greater than e.g. 4%, then the clamping force of brake  3   a ,  8   a  is reduced in step  27 . If λ 1  is lower than 4% and higher than 2% (N in steps  21  and  22 )—corresponding to the ideal range between 2% and 4%—then there is no change in the clamping force (step  25 ). If slip λ 2  by contrast is lower than 2% (J in step  22 ) and thus theoretically the clamping force could be increased further on this wheel, slip λ 2  of the other wheel  13   b  is first taken into account in step  23 . This is shown by an arrow illustrating the interaction between the two wheels. In an embodiment, if slip λ 2  of the other wheel  13   b  is greater than 4% (N in step  23 ), then there is no change in the clamping force (step  25 ) because the other wheel  13   b  already has a high slip. In an embodiment, if the slip of the other wheel  13   b  by contrast is lower than 4% (J in step  23 ), then the clamping force on wheel  13   a  is increased without losing the cornering stability. In an embodiment, as shown, for example, in step  24 , additionally the deceleration of the vehicle is checked. In an embodiment, if the acceleration a x  is greater than −2.5 m/s 2  (that is, for example, the acceleration is −1 m/s 2 ), then the clamping force may be increased further in step  26 . In an embodiment, if the acceleration a x  is lower than −2.5 m/s 2  (N in step  25 ), then no clamping force change occurs on brake  3   a ,  8   a.    
         [0022]    Embodiments of the present invention are advantageous, in part, because at least one of the wheels has a low slip and is thus able to take up sufficient cornering force. In an embodiment, this is carried out analogously for the second wheel  13   b  by taking into account the wheel slip of first wheel  13   a.    
         [0023]    In an embodiment, following the triggering of the braking operation by activating push-button switch  6 , the control operation is started on wheels  13   a  and  13   b , respectively, e.g., in an asynchronous manner. In an embodiment, the triggering of the corresponding actuators  4   a ,  4   b  occurs left/right time-delayed by 150 ms, for example. This has positive effects on the control response, for example, since the time-delayed triggering reduces the danger of an oscillating system in which both wheel brakes are synchronously opened or closed.