Abstract:
The recirculating pumps of brake systems that are equipped with a precharging device or other pressure sources, the pressure of which at least temporarily reaches the suction side of the recirculating pump, are highly stressed and consequently subjected to premature wear by this pressure. Over time, this results in large leaks which may cause damage to the pump motors. Consequently, the invention proposes to control the pressure medium supply from the precharging device or other pressure source to the suction side of the recirculating pump as a function of the pressure in this connecting line. The invention proposes to utilize a hydraulically actuated on-off

Description:
TECHNICAL FIELD  
         [0001]    The present invention [pertains to a] relates to hydraulic brake systems and more particularly relates to anti-lock brake systems [according to the preamble of claim  1 .] 
         BACKGROUND OF THE INVENTION  
         [0002]    A brake system of this type is known from German Patent No. 4,025,859 A1, for example. In this known brake system, the brake circuit is divided into a front axle brake circuit and a rear axle brake circuit, wherein one brake circuit is assigned to the nondriven axle and the other brake circuit is assigned to the driven axle. The brake circuit of the driven axle is equipped with a traction control device. A traction control represents an active braking maneuver because the braking process is initiated without vehicle operator actuation of [actuating] the brake pedal. Other types of active braking maneuvers are braking processes for controlling the yaw moment, for example, without participation of the driver or in addition to a pedal-actuated braking maneuver.  
           [0003]    During an anti-lock braking maneuver, the known brake system operates in accordance with the recirculation principle, wherein the brake system is equipped with a precharging pump for the recirculating pump. The suction side of the precharging pump is connected to the reservoir of the brake system, and the pressure side is connected to the suction side of the recirculating pump via a hydraulically actuated on-off valve. Once the delivery pressure of the recirculating pump reaches the maximum pressure of the brake system during a traction control maneuver, the conveyed pressure medium flows through a pressure limiting valve and into the control line for the hydraulically actuated on-off valve arranged between the precharging pump and the recirculating pump. During the subsequent traction control, no precharging volume is advanced to the recirculating pump until the pressure in the control line has decreased to such a level that the on-off valve opens again. The purpose of this measure is that one attempts to prevent the unnecessary conveyance of the pressure medium through the recirculating pump. The recirculating pump should only generate a pressure upon demand. Consequently, excess pressure medium is not discharged into the reservoir of the brake system via a pressure control valve, but the suction side of the recirculating pump is [rather] blocked until the delivery of the pressure medium is required again.  
           [0004]    A precharging pressure source represents one of the possible pressure sources for at least temporarily building up an increased pressure on the suction side of the recirculating pump. Other options for precharging a regenerative or nonregenerative recirculating pump are the utilization of an active vacuum brake booster, an additional pressure reservoir or, for example, a precharging device arranged between the reservoir and the main cylinder. In the latter instance, the volume flowing into the brake line through the main cylinder is used for precharging the recirculating pump when the brake pedal is not actuated.  
           [0005]    It is also possible for a pressure source other than a precharging device to build up the pressure on the suction side of the recirculating pump. Brake systems, the second suction line of which branches off the brake line above a block valve, are only able to isolate the main cylinder pressure from the suction side of the recirculating pump if the second suction line is blocked. However, this only takes place after a pedal-actuated braking maneuver has been detected in the active braking mode, i.e., once the main cylinder pressure is able to propagate to the suction side of the recirculating pump.  
           [0006]    In all aforementioned constructions, the recirculating pump is highly stressed and consequently subjected to premature wear by the precharging device in the active braking mode or by the pressure acting upon the recirculating pump from another pressure source. In extreme instances, large leaks can lead to the failure of the electric motors for the pumps. However, a submersible design of the electric motor would be associated with significant expenditures.  
           [0007]    The present invention is based on the objective of developing a hydraulic brake system [of the initially mentioned type] in which the recirculating pump is subjected to less wear. [This objective is attained with the characteristics disclosed in the characterizing portion of claim  1 .] 
           [0008]    The basic principle of the invention consists of limiting the pressure on the suction side of the recirculating pump to the required level. With respect to blocking the pressure medium supply, the delivery pressure of the recirculating pump is no longer decisive, but rather the pressure on the suction side of said recirculating pump.  
           [0009]    In order to limit the pressure on the suction side of the pump, a hydraulically actuated valve is preferably utilized because a valve of this type requires no separate control logic and no electric lines. Such a valve need not contain any effective surfaces for the pressure medium in the valve opening direction because the output pressure which acts in the closing direction is automatically built up due to the activity of the recirculating pump such that the valve is opened.  
           [0010]    [The object of the invention is described in greater detail below with reference to one embodiment of the invention which is illustrated in the figure.] 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The figure shows a brake system according to the invention with a hydraulically actuated on-off valve arranged in the connection between the precharging source and the recirculating pump. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]    The brake system shown contains a main cylinder  1  that is connected to a reservoir  2  and actuated by a brake pedal  3  via a vacuum brake booster  4 . The brake booster  4  is realized in the form of an active brake booster, i.e., it is able to displace the actuating piston of the main cylinder  1  and build up a certain pressure in the brake system by means of electric control, i.e., without an actuation of the brake pedal  3  of the brake booster  4 . Two brake lines  5  and  6  extend from the main cylinder  1  to the respective brake circuits. Only the brake circuit I is illustrated in detail, wherein the other brake circuit II is realized identically or, for example, merely provided with an anti-lock device if the brake system is equipped with a traction control and the brake circuit is divided into a front axle brake circuit and a rear axle brake circuit. This means that the brake circuit I extends from the main cylinder  1  to the wheel brakes  12  and  13  via the brake line  5 , a block valve  7  and two branch lines  8  and  9  which respectively contain one inlet valve  10  or  11 . One respective return branch line  14  or  15  extends from the wheel brakes  12  and  13  to a return line  18 , to which a low-pressure reservoir  19  is connected, via one respective outlet valve  16  or  17 . The low-pressure reservoir  19  is connected to the suction side of a regenerative recirculating pump  21  via a first suction line  20 . The recirculating pump contains a second suction line  22 , in which a hydraulically actuated on-off valve  23  as well as an electromagnetically actuated reversing valve  24  is inserted. The second suction line  22  is connected to the brake line  5  between the main cylinder  1  and the block valve  7 . A pressure line  25  connects the pressure side of the recirculating pump  21  to the brake line  5  between the block valve  7  and the inlet valves  10  and  11 .  
         [0013]    During a normal braking maneuver, i.e., when the brake system is actuated via the pedal and no critical slip values or other critical dynamic factors are present, all valves remain in the position shown except the hydraulically actuated on-off valve  23  which is shown in its switching position. In its normal position and during a pedal-actuated braking maneuver, the valve is open. The pressure in the wheel brakes  12  and  13  is built up via the brake line  5  and the branch lines  8  and  9 , wherein the pressure is also relieved via this path.  
         [0014]    During an anti-lock braking maneuver, the brake system operates in accordance with the known recirculation principle. The electromagnetic reversing valve  24  remains closed, and the block valve  7  remains open. Only the inlet valves  10  and  11  and the outlet valves  16  and  17  are controlled in conventional fashion so as to regulate the pressure in the wheel brakes  12  and  13 . The recirculating pump  21  displaces the pressure medium discharged into the low-pressure reservoir  19  back into the brake line  5 , i.e., between the block valve  7  and the inlet valves  10  and  11 , via the pressure line  25 . During an active braking maneuver, e.g., a traction control maneuver or an active braking maneuver in order to control the yaw moment, the block valve  7  is closed and the electromagnetically actuated reversing valve  24  is open. The brake booster  4  actuates the main cylinder  1  such that a pressure medium pressure propagates in the second suction line  22  up to the hydraulically actuated on-off valve  23  via the brake line  5  and the electromagnetically actuated reversing valve  24 .  
         [0015]    The hydraulically actuated on-off valve  23  is realized in such a way that the pressure medium inlet  30  ends in an inlet chamber  31  that is limited by a piston  32  on one side and opens into an outlet chamber  33  of larger cross section on the other side. A valve tappet  34  that carries a valve closing element  35  is mounted on the piston  32 . The valve closing element is arranged in the outlet chamber  33  and has a larger diameter than the piston  23 . The valve seat  36  that forms the transition between the inlet chamber  31  and the outlet chamber  33  has the same cross section as the piston  32 . A compression spring  37  acts on the side of the piston  32  which faces away from the closing element  35  in the valve opening direction. The piston  32  is subjected to the atmospheric pressure on this side.  
         [0016]    During a precharging by the active brake booster  4 , a certain pressure is built up in the inlet chamber  31  and in the outlet chamber  33  due to the open valve. Once this pressure multiplied by the cross-sectional area of the valve seat  36  exceeds the force of the compression spring  37 , the piston  32  moves toward the compression spring  37  such that the closing element  35  adjoins the valve seat  36 . This means that the on-off valve  23  is closed. During this process, the pressure in the outlet chamber  33  continues to act in the closing direction, wherein the inlet chamber  31  contains no effective surfaces that could displace the piston  32 .  
         [0017]    Since the recirculating pump  21  permanently generates suction, the pressure in the outlet chamber  33  is reduced again such that the hydraulic on-off valve  23  is opened again.  
         [0018]    However, the precharging pressure for the recirculating pump  21  can never exceed the closing pressure of the on-off valve  23 . Thus, the recirculating pump is protected because it is always subjected to a low precharging pressure. However, it is ensured that a sufficient precharging volume is always available because the valve automatically opens on demand.  
         [0019]    At this point, it should be noted that such a hydraulically actuated on-off valve  23  is only sensible for a brake system that is equipped with a precharging device. For example, it would be conceivable if the driver of the motor vehicle suddenly actuated the brake pedal  3  in the active braking mode without precharging and while the electrically actuated reversing valve  24  was open. In this case, a high braking pressure would also be able to propagate into the second suction line  22 . In such instances, the hydraulically actuated reversing valve  23  isolates pressure peaks from the suction side of the recirculating pump  21 . This applies to brake systems, the second suction line  22  of which is connected to the brake line  5 . However, there also exist other brake systems which are equipped with hydraulic pressure sources, the pressure of which at least temporarily propagates into the second suction line  22 . The hydraulically actuated on-off valve  23  should be used in all brake systems of this type.  
         [0020]    A hydraulically actuated reversing valve of the type described above is also able to prevent excess pressure from being built up during a precharging process if the precharging is not realized via the brake line, but rather a separate precharging pump which conveys the pressure medium directly from the reservoir  2  into the second suction line  22 . In this case, the second suction line naturally would not be connected to the brake line  5 .