Abstract:
The invention relates to a method for actuating an external-force parking brake system of a hydraulic vehicle brake system that has a hydraulic pump. For actuating the parking brake system by external-force, the invention proposes pumping brake fluid with the hydraulic pump out of a wheel brake, by opening an outlet valve, into a wheel brake, which has a locking device for locking in the actuated state. A switchover valve is preferably closed, to avoid feedback effects on a master cylinder.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to an improved method for actuating an external-force parking brake system in a hydraulic vehicle brake system.  
         [0003]     2. Description of the Prior Art  
         [0004]     It is standard now for passenger cars to be equipped with a hydraulic service brake system that is manually actuated by means of a pedal-actuated master cylinder. Typically, a negative-pressure brake booster is present, and electrohydraulic brake boosting with an electric-motor-driven hydraulic pump is also known. Often, the hydraulic service brake systems have an electric-motor-driven hydraulic pump, also called a return pump, for purposes of anti-lock braking, traction control, and/or electronic stability control (ABS, ATC, ESP), with which brake fluid can be pumped into or out of the wheel brakes in order to increase or decrease a wheel brake pressure in the wheel brakes. Below, for the sake of brevity, only anti-lock braking will be discussed. For that purpose, each wheel brake, or a group of wheel brakes, is preceded and followed by a brake pressure buildup and a brake pressure lowering valve, respectively. Such vehicle brake systems are known to one skilled in the art in various embodiments and will therefore not be described in further detail here.  
         [0005]     Electrohydraulic service brake systems, that is, external-force service brake systems, are also known, in which a brake pressure is built up with an electric-motor-drivable hydraulic pump as the source of external energy. A brake pedal serves as the brake force set-point transducer, and a master cylinder is required only for auxiliary braking (emergency braking) if the external energy source fails. One such electrohydraulic vehicle brake system is disclosed in International Patent Disclosure WO 98/31576. The known vehicle brake system has an ABS device, which in principle is embodied identically to the ABS device described above in a muscle-or auxiliary-force brake system, with the specification that the hydraulic pump forming the external energy source is also used for anti-lock purposes.  
         [0006]     European Patent Disclosure EP 0 995 659 A1 discloses a vehicle brake system with a hydraulic service brake system and a likewise hydraulic external-force parking brake system (parking brake). For embodiment as a parking brake system, wheel brakes of the vehicle brake system have hydraulically actuatable, releasable locking devices, with which the wheel brakes can be mechanically locked in the actuated state. A brake force that has been built up is as a result maintained even if pressure fails. The locking can be activated hydraulically, electromechanically, or in some other way. For unlocking the locking device and releasing the parking brake system, it may be necessary to actuate the service brake system in order to release a mechanical tightening of the locking device and to make it possible to release the locking device, or to release it easily.  
         [0007]     A wheel brake with a hydraulically switchable mechanical locking device is disclosed in German Patent Disclosure DE 195 02 927 A1. The method of the invention can also be used to release the parking brake system, and therefore actuating the parking brake system should also be understood in the sense of releasing the parking brake system.  
         [0008]     In the known vehicle brake systems, a feedback effect on the brake pedal or in general on an actuating device of the service brake system can occur when the external-force parking brake system is actuated while the service brake system is also actuated. Because brake fluid is pumped in pulsating fashion with the hydraulic pump, typically operated as a piston pump, vibration can be sensed in the depressed brake pedal. The brake pedal is also pulled away from under the driver&#39;s foot when the hydraulic pump aspirates brake fluid from a master cylinder. These effects, and especially the vibration of the brake pedal, can occur not only upon tightening but also upon release of the external-force parking brake system.  
       OBJECT AND SUMMARY OF THE INVENTION  
       [0009]     For actuating the parking brake system, the present invention provides that with the hydraulic pump, brake fluid from at least one wheel brake, which has no parking brake system, be pumped into at least one wheel brake that does have a parking brake system, and as a result a wheel brake pressure is built up in the at least one wheel brake that has a parking brake system. The wheel brake that has the parking brake system is actuated (pulled tight) as a result and is then locked with the locking device in its actuated position. In the ideal case, the brake fluid from the at least one further wheel brake suffices to actuate the wheel brake that has the locking device. No brake fluid for external-force actuation of the parking brake system is aspirated from the master cylinder, which has the advantage that when the master cylinder is actuated, a foot brake pedal or other actuating device is not moved in the actuation direction, or in other words is not “pulled away” from under the driver&#39;s foot. If the brake fluid from the at least one further wheel brake does not suffice to actuate the wheel brake that has the locking device, then brake fluid can additionally be aspirated directly, bypassing the master cylinder, from a brake fluid supply tank.  
         [0010]     The invention is especially suitable for vehicle brake systems with so-called X brake circuit distribution, or for vehicle brake systems in which all the wheel brakes are connected hydraulically parallel, as is typically the case in electrohydraulic vehicle brake systems. X brake circuit distribution means a vehicle brake system with two brake circuits hydraulically disconnected from one another, in which the wheel brakes of one front wheel and of the diagonally opposite rear wheel are connected to one brake circuit. In a II brake circuit distribution, in which the wheel brakes are connected axle by axle to a brake circuit, one front and one rear wheel must be equipped with a locking device in order to perform the method of the invention, or else some possibility must be created so that with the hydraulic pump of one brake circuit, brake fluid can be pumped into the other brake circuit. A further alternative would be to actuate only one wheel brake.  
         [0011]     The invention also provides for disconnecting a muscle-power-actuatable master cylinder of the service brake system, for external-force actuation of the parking brake system, hydraulically from the vehicle brake system. This is preferably done by closing one typically present switchover valve for each brake circuit, which valve connects the master cylinder to the brake circuit or disconnects them. As a result, a feedback effect on the master cylinder and its actuating device is avoided; for instance, no pulsations, which the hydraulic pump typically embodied as a piston pump generates because of its discontinuous mode of pumping, are perceptible in the foot brake pedal.  
       BRIEF DESCRIPTION OF THE DRAWING  
       [0012]     The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of a preferred embodiment taken in conjunction with the sole drawing figure which shows a hydraulic circuit diagram of a vehicle brake system for performing the method of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     The drawing shows a hydraulic vehicle brake system  10  with a dual-circuit master cylinder  12 , to which two brake circuits I, II independent of one another are connected; of them, only brake circuit I is shown in the drawing.  
         [0014]     A branching master brake line  14  leads from the master cylinder  12  to two wheel brakes  15 ,  16 , connected to this brake circuit I, and of these one wheel brake  16  has a locking device  17 , with which the wheel brake  16  can be fixed, that is, mechanically locked, in the actuated position. Thus the vehicle brake system  10  has a hydraulically actuatable external-force parking brake system. The wheel brakes  15 ,  16  connected to one brake circuit I are assigned to one front wheel and one diagonally opposite rear wheel of a vehicle; the wheel brakes  15 ,  16  of the other brake circuit II are assigned to the other two vehicle wheels. The vehicle brake system shown and described accordingly has what is known as an X brake circuit distribution.  
         [0015]     A switchover valve  18  that is open in its basic position is disposed in a common portion of the master brake line  14 . Two inlet valves  20 ,  21  that are open in their basic position are also disposed in the branching parts of the master brake line  14  and are each upstream of one wheel brake  15 ,  16 . From the wheel brakes  15 ,  16 , a uniting return line  22 , in which one outlet valve  24 ,  25  that is closed in its basic position is disposed for each of the two wheel brakes  15 ,  16 , leads to the intake side of a hydraulic pump  26 , which is also called a return pump. A hydraulic reservoir  28  is connected to the return line  22 .  
         [0016]     A pressure side of the hydraulic pump  26  is connected to the master brake line  14  via a damper chamber  30  and a throttle  32  between the switchover valve  18  and the inlet valves  20 ,  21 .  
         [0017]     Via an intake line  34 , in which there is an intake valve  36  that is closed in its basic position, the intake side of the hydraulic pump  26  is connected to a brake fluid supply tank  37 , which is seated on the master cylinder  12 .  
         [0018]     The hydraulic pumps  26  of the brake circuits I, II both shown and not shown can be driven by a common electric pump motor  38 . All the valves listed up to now, namely the switchover valve  18 , the inlet valves  20 ,  21 , the outlet valves  24 ,  25 , and the intake valves  36 , are magnet valves, which for purposes of anti-lock and traction control are controllable with an electronic control unit  40 , which also switches the pump motor  38  on and off. The electronic control unit  40  receives signals from wheel rotation sensors  42 , which are evaluated in order to ascertain an inclination of a vehicle wheel to lock upon braking or to ascertain slip upon starting up. The control unit  40  also receives a signal from a brake pedal sensor (brake light switch)  44 , with which an actuation of the master cylinder  12  can be ascertained.  
         [0019]     With the vehicle brake system described, service braking is possible in a manner known per se. Anti-lock, traction control and/or electronic stability control (ABS, ATC, ESP) are also possible in a manner known per se. Below, for the sake of brevity, only the anti-lock mode will be discussed. To that end, wheel brake pressures in the wheel brakes  15 ,  16  are regulated (modulated) for individual wheels by means of the electronic control unit  40 , by clocked control of the inlet valves  20 ,  21  and outlet valves  24 ,  25 , in the course of which the hydraulic pump  26  is put into operation. The switchover valve  18  is closed for the anti-lock mode, and as a result the master cylinder  12  is disconnected hydraulically from the rest of the vehicle brake system  10 . For a rapid pressure buildup in a traction control and/or electronic stability control event, the intake valve  36  is opened, so that the hydraulic pump  26  aspirates brake fluid directly from the brake fluid supply tank  37 .  
         [0020]     For the external-force actuation of a parking brake system in the vehicle brake system  10 , a button  46  or other operator control element is actuated. The electronic control unit  40  closes the switchover valves  18 , so that the master cylinder  12  is disconnected from the rest of the vehicle brake system  10 . The pump motor  38  of the hydraulic pump  26  is switched on, and the inlet valve  21  of the wheel brake  16  that has the locking device  17  is open or remains open, while conversely the inlet valve  20  of the wheel brake  15  that has no locking device is closed. The outlet valve  24  of the wheel brake  15  that has no locking device is opened, and the outlet valve  25  of the wheel brake  16  that has the locking device  17  is or remains closed. As a result, the hydraulic pump  26  aspirates brake fluid from the wheel brake  15  that has no locking device, and it pumps the brake fluid into the wheel brake  16  that does have the locking device  17 . The latter wheel brake  16  is actuated (pulled tight) as a result, and after that in the actuated state is fixed or locked with the locking device  17 . The brake force built up continues to be maintained as a result of the locking with the locking device  17 , even when there is no pressure, when the hydraulic pump  26  is at a stop and currentless, or in other words when the magnet valves  18 ,  20 ,  21 ,  24 ,  25 ,  26  have dropped back into their basic positions. If the brake fluid aspirated from the wheel brake  15  that has no locking device does not suffice for an adequate pressure buildup in the wheel brake  16  that has the locking device  17 , then by opening the intake valve  36  brake fluid can additionally be aspirated from the brake fluid supply tank  37 .  
         [0021]     The method of the invention can be performed in principle in the same way in other hydraulic vehicle brake systems that have a parking brake system, for instance including electrohydraulic vehicle brake systems. What is necessary is that from at least one wheel brake  15 , brake fluid can be pumped with the hydraulic pump  26  into at least one other wheel brake  16  that does have a locking device  17 . Therefore vehicle brake systems  10  in particular with an X brake circuit distribution or electrohydraulic vehicle brake systems, in which the wheel brakes are connected hydraulically parallel, are suitable for performing the method of the invention. In a II brake circuit distribution, in which the two front wheels are assigned to one brake circuit and the two rear wheels to another brake circuit, it would for instance be necessary for one front and one rear wheel, assigned to different brake circuits, each to have a locking device (not shown). Another possibility would be a hydraulic communication between the two brake circuits, or between wheel brakes both brake circuits, via a magnet valve (not shown) or via fixing (locking) only one vehicle wheel.  
         [0022]     For releasing the locking device  17 , a wheel brake pressure in the wheel brake  16  that has the locking device  17  can be built up with the hydraulic pump  26 , in order to mechanically untense the locking device  17 , making it easily releasable or unlockable. The pressure buildup in the wheel brake  16  is effected as in the actuation (pulling tight) of the wheel brake  16  as described above.  
         [0023]     The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.