Abstract:
A vehicle brake system, comprising a hydraulically controllable disk brake device, which has an electromechanical actuating device for activating a parking brake function, wherein the vehicle brake system has a hydraulic circuit having a hydraulic pressure source and controllable hydraulic functional elements in order to hydraulically control the disk brake device according to a service brake action of a driver or according to an automatic activation of a driving assistance system, wherein the vehicle brake system further comprises a control device in order to control the electromechanical actuating device according to a parking brake action of the driver or according to an automatic activation of the parking brake function. In order to avoid overload states the control device is designed to control at least one of the hydraulic functional elements in such a way that, before the parking brake function is activated by controlling the electromechanical actuating device, a hydraulic volume currently acting on the disk brake device in order to produce a hydraulic pressure that causes a service brake action can be hydraulically isolated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national stage of International Application No. PCT/EP2011/003334 filed Jul. 5, 2011, the disclosures of which are incorporated herein by reference in entirety, and which claimed priority to German Patent Application No. DE 10 2010 033 253.4 filed Aug. 3, 2010, the disclosures of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a vehicle brake system comprising a hydraulically controllable disk brake device, which has an electromechanical actuating device for activating a parking brake function, wherein the vehicle brake system has a hydraulic circuit having a hydraulic pressure source and controllable functional elements in order to hydraulically control the disk brake device according to a service brake action of a driver or according to an automatic activation of a driving assistance system, wherein the vehicle brake system further comprises a control device in order to control the electromechanical actuating device according to a parking brake action of the driver or according to an automatic activation of the parking brake function. The invention further relates to a method for controlling a vehicle brake system of the previously described type. 
         [0003]    A variety of vehicle brake systems are known from the background art. Specifically the document DE 10 2005 051 082 A1, and corresponding U.S. Pat. No. 8,322,495 B2, both of which are incorporated by reference herein in entirety, describes a vehicle brake system having a disk brake that is designed as a floating caliper disk brake. In this case, a friction lining arrangement comprising two opposing friction linings on either side of a brake disk is provided. The one brake lining is attached to a floating caliper, while the other brake lining may be hydraulically displaced relative to the floating caliper by means of an actuating piston. As a result of a hydraulic loading of an actuating piston the movable brake lining is displaced towards the brake disk. The brake lining fixed to the floating caliper is in an, as such, known manner pulled towards the brake disk from the other side by means of the floating caliper function, so that a braking force acts from both sides upon the brake disk. 
         [0004]    In addition to this floating caliper function, which has been known for a long time, this background art moreover also provides an electromechanical actuating device. This is used to realize a parking brake function. Given corresponding control the disk brake, besides the hydraulic displacement, may in addition be actuated electromechanically. To avoid having to dimension the electromechanical components, in particular the electric motor, far too large and powerful, it may be provided that the activation of the parking brake is assisted hydraulically. In other words the brake is hydraulically preloaded, the hydraulic preloading pressure is maintained and the parking brake is then blocked by means of the electromechanical actuating device. In a correspondingly elaborate manner hydraulic assistance also has to be provided for releasing the parking brake function. 
         [0005]    From the prior art WO 2005/073043 A1, and corresponding U.S. Pat. No. 7,744,166 B2, both of which are incorporated by reference herein in entirety, a corresponding disk brake device and a method of controlling this disk brake device are further known, whereby using various sensors and employing predetermined mathematical models in dependence upon the actual operating situation, such as for example a loaded state of the vehicle or an inclination of the road, it is possible also to generate braking forces that lie substantially above the braking forces that may be generated solely by the electromechanical actuating device. 
         [0006]    It has however emerged that, given such a combination of hydraulic and electromechanical brake, because of the high collective load of hydraulically and electromechanically generated brake application forces upon activation of the parking brake function the electromechanical components of the electromechanical actuating device have to be designed with a corresponding load rating in order to avoid failure. Particularly during the subsequent release of the parking brake it is then namely necessary to overcome the brake application force, which is generated hydraulically and mechanically in order to activate the parking brake function and is, as it were, conserved during the braking state, by means of a correspondingly powerful mechanism or using hydraulic assistance. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    A feature of the present invention is to provide a disk brake device of the type described in the introduction, in which with simultaneous utilization of a service-braking-related applied hydraulic pressure the collective load upon activation of the parking brake function may be reduced and the releasing of the parking brake function may be effected in a less elaborate manner. 
         [0008]    This feature is achieved by a disk brake of the type described in the introduction, in which it is provided that the control device is designed to control at least one of the hydraulic functional elements in such a way that, before the parking brake function is activated by controlling the electromechanical actuating device, a hydraulic volume currently acting on the disk brake device in order to produce a hydraulic pressure that causes a service brake action can be hydraulically isolated. 
         [0009]    In other words, according to the invention it is provided that the hydraulic volume acting in any case upon the disk brake device as a result of an already activated service brake function is initially hydraulically locked in, i.e. isolated from the pressure supply, so that the hydraulic pressure prevailing in the disk brake device is maintained at the start of activation of the parking brake function. However, as soon as the electromechanical actuating device is triggered in order to activate the parking brake function, i.e. the disk brake device is applied electromechanically, the hydraulic volume increases as a result of displacement of the actuating piston of the disk brake device. The hydraulic pressure in the disk brake device accordingly decreases rapidly, so that the hydraulically built-up braking forces decrease during the electromechanical brake application upon activation of the parking brake function. Consequently, during the activation of the parking brake function starting from the hydraulic pressure applied as a result of an (automatic or driver-initiated) service braking operation substantially electromechanically generated brake application forces come into effect, so that in the course of the purely electromechanical release an overloading of the brake system or of individual components of the disk brake devices that results from the collective hydraulically and electromechanically generated brake application forces may be ruled out. 
         [0010]    According to the invention it may be provided that the hydraulic circuit comprises a plurality of controllable valves in order selectively to supply hydraulic pressure to, or hydraulically isolate, individual disk brake devices of the vehicle brake system. Usually the valves are triggered in the course of a service braking operation in accordance with the default settings of a slip regulating system. In this connection it may be provided according to the invention that the valves are controllable by means of the control device in such a way that only the disk brake devices of the vehicle brake system that are configured with a mechanical actuating device can be hydraulically isolated. These are, as a rule, the disk brake devices at the rear wheels. However the invention also makes it possible for parking brake devices to be disposed even on the conventionally larger-sized disk brake devices of the front wheels and to be controlled in a suitable manner, without the parking brake device having to be in each case of a particularly stable or powerful design. 
         [0011]    As already indicated above, according to the invention it is possible for the control device to be designed with a slip regulating function. In this case it may be provided that the hydraulic volume can be isolated by means of at least one valve associated with the slip regulating function. The valve may be for example a brake-circuit shut-off valve or an ABS inlet valve of a slip regulating system. 
         [0012]    According to the invention it may further be provided that the control device prior to an activation of the parking brake function detects whether a hydraulic pressure is applied to the disk brake device as a result of a service brake action initiated by the driver or automatically. This is possible with a low technical outlay for example in that the control device detects the presence of a service brake action by the driver or of a braking operation that is initiated automatically by a driving assistance system (for example ACC or Hill-hold with hill ascent—and hill descent assistance) or a safety system (for example ESP) on the basis of available parameters, in particular on the basis of the current switching state of a brake light switch, the hydraulic pressure in a master cylinder, or the pedal travel of a brake pedal. 
         [0013]    The invention further relates to a method of controlling a vehicle brake system for activating a parking brake function of the previously described type, comprising the following steps:
       detect an actual state of a disk brake device,   isolate a hydraulic volume exerting a hydraulic pressure in the disk brake device, and   activate the electromechanical actuating device to apply the parking brake function.       
 
         [0017]    In this method it may be provided according to the invention that the actual state is detected on the basis of available parameters, in particular on the basis of the current switching state of a brake light switch, the hydraulic pressure in a master cylinder, or the pedal travel of a brake pedal. 
         [0018]    Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a block diagram of a vehicle brake system according to the invention, a disk brake device for a front wheel and a disk brake device for a rear wheel being shown diagrammatically; 
           [0020]      FIG. 2  is a part-sectional detail view of the disk brake device for the rear wheel according to  FIG. 1  with corresponding parking brake device; 
           [0021]      FIG. 3  is the schematic diagram according to  FIG. 1  in the situation of a triggering to release the parking brake function; and 
           [0022]      FIG. 4  is a diagram to explain the mode of operation of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    In  FIG. 1  a block diagram for a vehicle brake system  100  according to the invention is schematically shown. This system comprises a pressure source  102 , which is configured with a brake booster  104 , a reservoir  106 , a master cylinder  108  and a brake pedal  110  actuable by the driver. The pressure source  102  is coupled hydraulically to a hydraulic brake circuit  112 . This comprises a motor-controllable pump  114 , as well as a pressure accumulator  116 . Further evident are two disk brake devices, namely a disk brake device  10  for a rear wheel and a disk brake device  11  for a front wheel. 
         [0024]    Associated with the disk brake devices  10 ,  11  there are in each case valves  118 ,  120 ,  122 ,  124 , which are controllable by means of a non-illustrated control device. Additionally evident are inlet valves  126  and  128 , which enable the hydraulic brake circuit  112  to be hydraulically uncoupled from the pressure source  102  actuable by the driver. 
         [0025]    With regard to the service brake function the schematically illustrated vehicle brake system  100  operates in an, as such, known manner. The non-illustrated control device further offers a slip regulating function in order to be able to carry out anti-skid control operations (ABS), anti-spin control operations (ASR), vehicle dynamics control operations (ESP) as well as automatic braking operations, for example for adaptive cruise control (ACC). 
         [0026]    For a clearer understanding the structure of the disk brake device  10  on the rear wheel is described in detail below. 
         [0027]    In  FIG. 2  the disk brake device  10  according to the invention is represented in a part-sectional view. It comprises a caliper  12 , which is float-mounted in a conventional manner by means of a non-illustrated brake anchor plate and spans a brake disk  14 , which is coupled in a rotationally fixed manner to a vehicle wheel. Provided in the caliper  12  is a brake lining arrangement, which comprises a brake lining  16  lying adjacent to the caliper  12  as well as a brake lining  18  lying adjacent to an actuating piston  22 . The two brake linings  16  and  18  face one another and in the released position shown in  FIG. 2  are disposed with slight clearance on either side of the brake disk  14  so that no significant residual drag torques arise. The movable brake lining  18  is disposed via a brake lining carrier  20  on an actuating piston  22  for joint movement. The actuating piston  22  is mounted displaceably in a cylindrical hollow space  24  in the caliper  12 . In the region of this hollow space  24  lying near the brake disk a circumferential receiving groove  26  is provided, in which an annular sealing element  28  is accommodated. 
         [0028]    It is further evident that the actuating piston  22  at its, in  FIG. 2  left, end facing the brake disk  14  is provided with an annular gaiter  30 , which prevents dirt from being able to penetrate into the region between the actuating piston  22  and the peripheral wall defining the receiving hollow space  24  of the caliper  12 . 
         [0029]    In  FIG. 2  it is further evident that the actuating piston  22  is of a hollow construction. Accommodated in it is a thrust piece  32  of an electromechanical actuating arrangement  34 . The electromechanical actuating arrangement  34  comprises a drive assembly  36  comprising an electric motor and a gear arrangement. An output shaft  38  of this drive assembly  36  drives a drive spindle  42 , which is supported via a thrust bearing  40  and the thread of which is accommodated in a thread receiver  44  of the thrust piece  32 . 
         [0030]    The thrust piece  32  in its, in  FIG. 2  left, region facing the brake disk  14  has a conical portion  46 , which may be brought into abutment with an inner surface  48  of the actuating piston  22  that is of a complementary conical configuration. In the release position shown in  FIG. 2  there is a clearance between the two conical surfaces  46  and  48 . 
         [0031]    If the brake is hydraulically actuated by the driver via the brake pedal or automatically by a driving assistance system (for example ACC or Hill-hold with hill ascent—and/or hill descent assistance) or by a safety system (for example ESP), then in an, as such, known manner a hydraulic pressure is built up in the hollow space  24  so that the actuating piston  22  is displaced in  FIG. 1  to the left along the longitudinal axis A. Consequently, with the floating caliper arrangement in an, as such, known manner the brake lining  18  is pressed onto the brake disk  14  and at the same time the brake lining  16  at the other side of the brake disk  14  is pulled against the brake disk  14  as a result of corresponding displacement of the caliper  12 . 
         [0032]    As a result of loading of the inner space  24  with pressurized hydraulic fluid the actuating piston  22  is displaced in  FIG. 2  to the left along the longitudinal axis A, resulting in a braking effect. The radially outer sealing element  28  under the applied pressure and as a result of the movement of the actuating piston  22  deforms elastically into the region of a chamfer-shaped recess  29 . 
         [0033]    If then, for example as a result of releasing the brake pedal or deactivating the driving assistance—and/or safety system, the hydraulic pressure in the inner space  24  of the caliper  12  is reduced, the sealing element  28  is able to elastically expand and adopts its original shape in accordance with  FIG. 2 . This means that in the course of its elastic expansion it pulls the actuating piston  22  back out of the actuating position because of the mutual friction contact and hence moves the disk brake device into the initial state thereof. This corresponds to the, as such, known rollback function, which is achieved in conventional brakes by means of the outer seal  28  at the outer circumference of the actuating piston  22 . 
         [0034]    With regard to an activation of the parking brake function a distinction is to be made between two situations: if there is no hydraulic pressure being applied in the interior  24  of the disk brake device  10  and preloading the actuating piston  22 , then according to the invention in order to activate the parking brake function only the electromechanical actuating device  34  is triggered. If however a hydraulic pressure is being applied at the disk brake device  10  and preloading the actuating piston  22 , for example because a driver prior to activating the parking brake function has pressed down the brake pedal or because an automatic braking operation has been carried out, then the hydraulic volume situated in the inner space  24  is isolated, as represented in  FIG. 3 , from the rest of the braking system  100  by closing the inlet valve  120  (ABS-Iso), said hydraulic volume being so to speak shut in or locked in. The pressure state prevailing because of the service braking operation is accordingly conserved in the inner space  24  at the start of activation of the parking brake function. 
         [0035]    The electromechanical actuating device  34  is then triggered. Under the action of the electromechanical actuating device  24  the thrust piece  32  moves forward until the clearance is used up and the conical surface  46  is positioned against the corresponding conical surface  48  in the interior of the actuating piston  22 . The result achieved is that the actuating piston  22  is supported axially via the thrust piece  32  and the thrust bearing  40  against the housing of the caliper  12 . The thrust piece  32  then moves the actuating piston  22  further forward. In this case the volume of the inner space  24  increases. However, as a result of the increase in volume of the inner space  24  the hydraulic pressure in the inner space  24  decreases. This is because the hydraulic volume—as previously described—has been shut in. Thus, unlike conventional solutions where the hydraulic pressure is kept at a constant level also upon displacement of the actuating piston  22  in the inner space  24  during activation of the parking brake function, leading to a superimposing of hydraulic and electromechanical preloading, the present invention provides that such a superimposing of hydraulic and electromechanical preloading is prevented. The result is therefore substantially no collective load during the activation of the parking brake function, this instead being effected solely on the basis of the electromechanical preloading. 
         [0036]    The parking brake state is maintained because of the position of the thrust piece  32  and the self-locking (for example by means of a self-locking gear between spindle  42  and receiver  44 ). The brake linings  16 ,  18  pressing against the brake disk  14  are supported via the thrust piece  32 . 
         [0037]    If the parking brake state is to be released, the electromechanical actuating device  34  is then triggered again. According to the invention for reasons of a simple design no hydraulic assistance occurs. As the activation of the parking brake function and the application forces then arising were summoned up solely by means of the electromechanical actuating device  34 , the performance capability of said device is then also sufficient to release the parking brake state. There is therefore no need for the actuating device  34  to be of such a powerful and stable design that for the purpose of releasing the parking brake state it summons up release forces that correspond to the collective load of hydraulically summoned-up application force and electromechanically summoned-up application force. 
         [0038]    The advantages of the invention are explained with reference to the diagram according to  FIG. 4 . Featuring in this diagram are various curves that show for the activating (applying) and deactivating (releasing) of the parking brake:
       the characteristic of the rotational speed of the electromechanical actuating device  34  in dash-dot-dot line K 1 ;   the characteristic of the application force of the disk brake device  10  in dotted line K 2 ;   the characteristic of the voltage across the electromechanical actuating device  34  in continuous line K 3 ; and   the characteristic of the current intensity electromechanical of the actuating device  34  in continuous line K 4 .       
 
         [0043]    To apply the parking brake, at the time T 1  a voltage U (K 3 ) is applied to the electric motor of the electromechanical actuating device  34 . Initially the thrust piece  32  because of the prevailing hydraulic pressure may be moved substantially free of resistance until it comes into abutment with the actuating piston  92 . At this time the current intensity (K 4 ) rises strongly, and from the time T 2  there occurs a further application of the disk brake device  10 . At the time T 3  the desired application force (K 2 ) is reached. The voltage supply (K 3 ) is interrupted and the parking brake is applied. 
         [0044]    If the parking brake is to be released, then at the time T 4  a voltage (K 3 ) is again applied across the electromechanical actuating device  34 . The current intensity (K 4 ) immediately rises strongly because of the high resistance of the self-locking of the electromechanical actuating device  34 , but after overcoming of the blocked state rapidly decreases. At the time T 5  the parking brake is fully released and the voltage supply (K 3 ) is disconnected again. 
         [0045]    Owing to the fact that no constant hydraulic pressure was applied up to the end (time T 3 ) of the brake application operation, the maximum amount of the current intensity arising in the electromechanical actuating device  34  during release of the parking brake is smaller by the value ΔI than the maximum amount of the current intensity during the brake application. This means that as a whole the electromechanical actuating device  34  need be of a less powerful rating than would be the case if the activation of the parking brake were to be effected with permanent utilization of the collective load of hydraulic and electromechanical application force. 
         [0046]    In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.