Abstract:
The invention relates to a method for controlling/regulating the boosting of a brake force of a brake system ( 1 ), in particular of a power-assisted brake system ( 1 ) of a motor vehicle, wherein the brake system ( 1 ) comprises a brake force booster ( 100 ) having an actuator ( 130 ) by means of which a variable additional force (F 100 ) can be imparted to a master brake cylinder ( 200 ) of the brake system ( 1 ), wherein if a relatively high additional force (F 100 ) is demanded, a partial volume of a brake fluid is discharged from a brake circuit ( 300 ) of the brake system ( 1 ) in such a way that the actuator ( 130 ) of the brake force booster ( 100 ) can be placed relatively quickly into a position in which it can impart a greater additional force (F 100 ) to the master brake cylinder ( 200 ). The invention also relates to a brake force booster for boosting a brake force of a brake system ( 1 ), in particular of a power-assisted brake system ( 1 ) of a motor vehicle, having a gearing ( 120 ) which can be driven by an electric motor ( 110 ), wherein the gearing ( 120 ) has an actuator ( 130 ) by means of which a piston ( 210 ) of a master brake cylinder ( 200 ) can be actuated, and wherein the gearing ( 120 ) is a cam mechanism ( 120 ), in particular an asymmetrical cam mechanism ( 120 ), having a control surface ( 125 ) or a control groove.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a method for performing open-loop/closed-loop control of the boosting of a braking force of a brake system, in particular of a power-assistance brake system of a motor vehicle. In addition, the invention relates to a brake booster for boosting a braking force of a brake system, in particular of a power-assisted brake system of a motor vehicle. Furthermore, the invention relates to a control unit, in particular an ABS or ESP control unit or a control unit of a brake booster by means of which a method according to the invention can be carried out, and/or a brake booster according to the invention can be open-loop/closed-loop controlled. 
         [0002]    Activation of a brake system, for example that of a motor vehicle, is usually facilitated by means of a brake booster which increases a braking force applied by a driver of the motor vehicle by a specific amount or a specific factor, that is to say applies an additional force. A resulting total force or braking force is conveyed to the brakes of the motor vehicle, which subsequently brake one wheel or a plurality of wheels of the motor vehicle. A widespread type of brake booster operates on the basis of a partial vacuum accumulator which is evacuated by a suction port of an internal combustion engine of the motor vehicle, with the result that the partial vacuum accumulator makes available energy for boosting the braking force. Not every motor vehicle has an internal combustion engine which can be used to evacuate the pressure accumulator. For example, this is used in diesel vehicles or electric vehicles by way of example instead of using an electrically operated partial vacuum pump. However, partial vacuum accumulators and brake boosters which are based thereon are bulky and the arrangement of the brake booster with respect to the brake system therefore has little flexibility. 
         [0003]    DE 10 2007 016 863 A1 discloses an electromechanical brake booster for a motor vehicle, having a rotationally symmetrical cam mechanism roller which is driven by an electric motor. The cam mechanism roller has a pitch groove by means of which a pickup can be moved linearly to and fro on a longitudinal end of a lever during operation of the electric motor. At a longitudinal end of the lever opposite the latter, said lever is connected to a piston of a master brake cylinder and can thereby transmit a force, resulting from the electric motor to the piston. In a center region, the lever is mounted in an articulated fashion, wherein a length ratio of the pickup to the center joint and of the center joint to the mechanical connection to the piston sets a lever ratio of the brake booster. In the case of a method for operating the brake booster, the lever, the cam mechanism roller and therefore the pitch groove and a shaft of the electric motor always have the same position with respect to a specific position of the piston, with the result that an available transmission ratio cannot always be used in an optimum way. 
       SUMMARY OF THE INVENTION 
       [0004]    An object of the invention is to specify an improved method for operating, that is to say for performing open-loop or closed-loop control of, a brake booster, and to specify an improved brake booster. In addition, a control unit is to be specified for this purpose. In this context the intention is, in particular, that it should be possible to apply, at substantially any time, a maximum additional force, resulting from the brake booster, to a piston of a master brake cylinder, wherein the dynamics of the brake system are to be preferably retained. In addition, the brake booster is to be as maintenance-friendly as possible, is to take up little space and is to be capable of being installed in a flexible way. 
         [0005]    The object of the invention is achieved by means of a method for performing open-loop/closed-loop control of the boosting of a braking force of a brake system, in particular of a power-assisted brake system of a motor vehicle; a brake booster for boosting a braking force of a brake system, in particular of a power-assisted brake system of a motor vehicle, and a control unit, in particular an ABS or ESP control unit or a control unit of a brake booster. 
         [0006]    The brake booster according to the invention has a transmission which can be driven by a drive, in particular an electric motor, wherein the transmission has an actuator or a lever by means of which a piston of a master brake cylinder can be activated. In this context, the transmission is a cam mechanism, in particular a nonuniform cam mechanism which has a control face or a control groove. That is to say, the brake booster is preferably embodied as an electromechanical brake booster. In embodiments of the invention, the transmission can have a cam plate on which a pickup of the actuator can slide or roll, wherein the piston of the master brake cylinder can be activated by means of an activation section of the actuator. In this context, sensing occurs on one side, i.e. the pickup runs on the control face onto which it is preferably pressed by a spring force. Of course, positive guidance, such as for example a control groove in a guide disk, can also be applied according to the invention. 
         [0007]    In particular the drive, in particular the electromotor, of the brake booster, can be made smaller and therefore more cost-effective by means of a nonuniform transmission, it being possible to dispense with a comparatively large vacuum brake booster. This reduction in size can be of such an extent that the drive only just reaches the required maximum force only after, for example, approximately 25% up to approximately 33% of the travel of the piston of the master brake cylinder, and that said drive can constitute required braking dynamics on only the first approximately 30% of this travel. The resulting drive with a transmission is highly reduced and the electromechanical configuration of the brake booster is simple and maintenance-friendly, and therefore provides an advantage in terms of installation space and costs. 
         [0008]    The brake booster according to the invention can be operated here, i.e. open-loop or closed-loop controlled, with a method according to the invention. In this context, the brake booster is configured in such a way that said brake booster itself, or that, by means of the brake booster, such a method is implementable and can be correspondingly implemented. That is to say, a control unit which actuates or adjusts a brake booster or the brake booster according to the invention is either part of the brake booster, in particular the electric motor therefor, or the brake booster can be actuated or adjusted externally. This then preferably occurs by means of an ABS or ESP control unit. That is to say also that the method according to the invention can be carried out by these known control units or by other control units in any suitable brake booster, and can be correspondingly carried out. 
         [0009]    In the method according to the invention, when a comparatively large additional force is requested by a brake booster, a comparatively small partial volume of a brake fluid is discharged or removed from a brake circuit of a brake system in such a way that an actuator of the brake booster can be moved comparatively quickly into a position in which it can impart a relatively large additional force onto the master brake cylinder. The discharging or removal of the partial volume of the brake fluid from the brake circuit preferably takes place in such a way that a pressure of the brake fluid in the brake circuit initially remains constant on average and subsequently preferably rises. In this context, the actuator is adjusted in accordance with a piston of the master brake cylinder, wherein the adjustment occurs, in particular, with power assistance by means of the electric motor. Any instant pressure drop and re-increase in the brake circuit owing to opening of a valve in order to discharge or remove the partial volume of the brake fluid is not intended to be taken into account here. 
         [0010]    By means of the method according to the invention, it is possible, given a sudden request for a large braking force, in particular for a required, substantially maximum brake boosting, to make available said boosting in an electromechanical brake booster. That is to say, the cam plate or the guide disk of the nonuniform transmission, and as a result thereof a position of the actuator and of the piston of the master brake cylinder, does not firstly have to be implemented under considerable application of force but instead can, according to the invention, be established substantially more quickly; by virtue of the fact that a braking point of the piston of the master brake cylinder is moved into a position in which the brake booster can apply its entire boosting, which can preferably occur even without a loss of pressure and, if appropriate, even with a rise in pressure in the brake circuit. That is to say, it is possible to apply a maximum additional force to the piston of the master brake cylinder substantially at any time, wherein dynamics of the brake system are retained. 
         [0011]    The discharging or removal of the partial volume of the brake fluid from the brake circuit preferably takes place by means of an open-loop/closed-loop controllable valve which discharges the partial volume into an equalizing circuit or a low-pressure accumulator. The discharging or removing of the partial vacuum preferably occurs by means of an ABS or an ESP system of the motor vehicle, wherein the ABS or the ESP system preferably discharges the partial vacuum to a rear wheel. The discharging of brake fluid at a rear axle has a tangentially stabilizing effect here, which is therefore advantageous. According to the invention, a control unit of the brake booster or for the brake booster can output an electrical control signal for discharging the partial volume of the brake fluid. In this context, the control unit preferably outputs the control signal to the ABS or the ESP system, which consequently reacts correspondingly. If large to extremely large brake boosting forces are requested, use of the ABS system is very probable, providing the possibility of influencing brake pedal travel up to the regions which give rise to an increase in the boosting conditions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention will be described in more detail below by means of exemplary embodiments and with reference to the appended, schematic drawing, in which: 
           [0013]      FIG. 1  is a basic illustration of a brake booster according to the invention on a brake system, by means of which a method according to the invention is explained; and 
           [0014]      FIG. 2  is a force/travel diagram of the brake booster from  FIG. 1 , which represents an additional force which can be reached by the brake booster, plotted against a travel of a piston of a master brake cylinder. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  shows a detail of a brake system  1 , preferably of a vehicle brake system  1 , in particular of a power-assisted brake system  1 , where just one brake booster  100 , one master brake cylinder  200 , a brake circuit  300  or brake fluid circuit  300  and an equalizing circuit  400  which lead through the master brake cylinder  200 , and a control unit  500 , in particular an ABS or ESP control unit  500 , are illustrated. The control unit  500  performs open-loop or closed-loop control of the brake booster  100  or the drive  110  thereof, preferably in cooperation with a control unit  114 , which is, in particular, a component of the brake booster  100 , said drive  110  being preferably embodied as an electric motor  110 . Two brake circuits  300  (connections  230  on the master brake cylinder  200 ), which are separate from one another, can preferably be supplied by means of the master brake cylinder  200 , with the result that each brake circuit  300  can activate at least one brake (not illustrated). The equalizing circuit  400  (connections  220  on the master brake cylinder  200 ) in which an equalizing container and/or a low-pressure accumulator (both not illustrated), preferably of an ABS or ESP assembly, are/is provided, takes up excess brake fluid or leakages in the brake circuit  300 . 
         [0016]    Depending on an operating state, the brake booster  100  can activate a piston  210  of the master brake cylinder  200 , as a result of which in addition to a pedal force, preferably resulting from a brake pedal (not illustrated), an additional force F 100 , resulting from the brake booster  100 , can be applied to the piston  210 . That is to say, a resulting total force F total  acting on the piston  210  or a braking force F total  results from the addition of the additional force F 100  to the pedal force. In this context, the piston  210  travels through a specific adjustment travel x; see in this respect also  FIG. 2 . The brake booster  100  itself is preferably embodied as an electromechanical brake booster  100  whose drive unit is composed of the drive  110  and a transmission  120 . The brake booster  100  must be capable of meeting both a request for dynamics and a request to reach a maximum additional force F 100 . By means of a nonuniform transmission  120  it is possible to ensure that at a start of the adjustment travel x (position in  FIG. 1 ) the rotations of the drive  110  are converted into more adjustment travel, and after the start or at an end they are converted into more additional force F 100 . As a result, at the start more dynamics are available, as are usually required, and later, that is to say, for example, from a third or from half of the adjustment travel x onwards, the maximum additional F 100  is available. 
         [0017]    In one embodiment according to the invention, the transmission  120  is embodied as a cam mechanism  120  with a variable transmission ratio. That is to say, for example, the transmission  120  has a drive disk  122  with preferably an outer toothing  123 , preferably a worm gear  122 , which can be driven by an outer toothing  113  of a motor shaft  112 , for example a worm  113 , of the drive  110  which is preferably embodied as an electric motor  110 . It is, of course, possible to apply a different configuration in order to cause the transmission  120  to move rotationally in a uniform or else nonuniform fashion. Adjacent to the drive disk  122 , and connected thereto in a rotationally fixed fashion, there is a cam plate  124  or a guide plate (not illustrated in the drawing), which activates an actuator  130  of the transmission  120 , which in turn activates the piston  210  of the master brake cylinder  200 . In this context, the actuator  130  can be embodied, for example, as a lever  130  which is spring-biased in the direction of the cam plate  124 . Biasing is not necessary in the case of a cam plate, since in such a case the actuator  130  is positively guided in a control groove and does not slide or roll on a control face  125  as in the case of a cam plate  124 . 
         [0018]    During operation of the electric motor  110  by means of a pickup  132  which is embodied or provided thereon, the actuator  130  senses the control face  125  or the control groove. Lying opposite with respect to a center of gravity  133  of the actuator  130 , the latter has an activation section  134  by means of which the actuator  130  activates the piston  210 . In this context, the actuator  130  is mounted in a rotatable or pivotable fashion at the center of gravity  133 , and the pickup  132  and the activation section  134  are preferably provided or embodied on longitudinal end sections of the actuator  130  lying opposite one another. The corresponding dimensions are selected here in such a way that in its maximum movement path the activation section  134  mainly carries out a translational movement, or a contact geometry or joint geometry between the actuator  130  and the piston  210  is selected such that a translational transmission of movement from the activation section  134  of the actuator  130  to the piston  210  is possible. In the present case, a longitudinal end of the piston  210  is of convex design here or is provided with a rotatable roller on which the activation section  134  bears with a radial face. 
         [0019]    The brake booster  100  with the cam plate  124  or the guide plate is configured here in such a way that a transmission ratio over an adjustment travel x is nonuniform, as illustrated by way of example in  FIG. 1  and clarified in more detail in  FIG. 2 . That is to say, from a zero position of the piston  210  of the master brake cylinder  200  or of the cam plate  124  or of the guide disk, which zero position is illustrated in  FIG. 1 , the guide face  125  or guide groove is configured in such a way that the additional force F 100  acting on the piston  210  initially increases as a function of the adjustment travel x of the piston  210 , preferably increases substantially linearly, before then remaining at a specific maximum additional force F 100  (see  FIG. 2 ). That is to say, starting from the zero position, a radius to the guide disk  124  whose outer edge forms the guide face  125  initially increases strongly, preferably with an exponent, before then growing only linearly. This can be implemented with any desired curved shape of the control face  125 . For example sections of parabolas, evolvents and/or evolutes are suitable for this. The behavior is similar with the guide groove of the guide plate. 
         [0020]    According to the invention, starting from the zero position, that is to say at the start of the adjustment travel x of the piston  210  of the master brake cylinder  200 , the brake booster  100  does not have the potential to apply the full boosting force F 100  to the master brake cylinder  200 . This is generally not a problem either since the brake system  1  in its entirety constitutes a spring which only builds up a full opposing force over the adjustment travel x. However, it may be the case that the brake system  1  already has a high counterpressure rather early, that is to say at low adjustment travel values x of the piston  210 , for example when a driver assistance system such as, for example, ACC (Adaptive Cruise Control) has already built up pressure in the brake circuit  300 . In this case, the controller  114  or  500  of the brake booster  100  can state that after a short adjustment travel x, for example 20%, the drive  110  already applies a large part of its maximum torque, for example 90%. This can then be conveyed by the brake booster  100  to, for example, a suitable system, for example an ABS or an ESP system of the motor vehicle, by means of a suitable signal duct  115 . The ABS or ESP system can then, according to the invention, increase the adjustment travel x of the piston  210  by discharging a small partial volume of the brake fluid, preferably from the rear wheel brake cylinders, which leads to relatively high stress forces of the brake booster  100 . 
         [0021]    That is to say if a comparatively large or maximum additional force F 100  is already required at the start of the adjustment travel x of the piston  210  of the master brake cylinder  200  and if the brake booster  100  must make this force available, according to the invention, a partial volume of the brake fluid is removed from the brake circuit  300  and preferably discharges into the equalizing circuit  400 . This can take place at any desired location on the brake circuit  300 , as long as a corresponding device or apparatus, for example a valve, in particular an open-loop/closed-loop controllable valve, is present; in particular the ABS or ESP system is suitable for this. If the control unit  114  or  500  or some other controller therefore detects that more braking force is required than can be reached at a position of the actuator  130 , said control unit  114  transmits a corresponding signal  115  to a corresponding device such as, for example, to the ABS or ESP system. This signal  115  then causes the device or the ABS or ESP system to discharge some brake fluid from the brake circuit  300 . As a result, the piston  210  migrates into a position, a new “braking point” at which the drive unit can generate, with its nonuniform transmission  120 , a relatively large force in the brake circuit  300 . 
         [0022]    In the case of a currently conventional embodiment of the ABS or ESP, a volume reduction occurs in the brake circuits there usually in a low-pressure accumulator or the low-pressure accumulators of the ABS or ESP system or assembly. According to the invention, ABS and ESP systems discharge an excess partial volume of the brake fluid into the low-pressure accumulator or a low-pressure accumulator within the ABS or ESP assembly by using the discharge valves there. In an application case of the ABS or ESP, the partial volume of the brake fluid is then fed back again behind a brake pedal into the master brake cylinder  200  by means of a feedback pump, driven by a motor, in order to prevent the brake pedal from dropping. Consequently, the motor cannot be actuated again either, so that the transmission  120  of the non-linear electrical brake booster  100  drops further, in order to allow the additional force F 100  to rise. The low-pressure accumulator or accumulators of the ABS or ESP assembly must, if appropriate, be adapted in terms of a volume in order to ensure that the necessary partial volume of the brake fluid is taken up. 
         [0023]    It is preferred that the pressure of the brake fluid in the brake circuit  300  does not drop but instead remains at least the same, in particular rises; apart from a possible pressure drop peak when the valve is opened in order to discharge the partial volume of the brake fluid. This is achieved in that the drive  110  is correspondingly actuated, as a result of which the actuator element  130  moves the piston  210  of the master brake cylinder  200  under an at least constant pressure in the brake circuit  300  in the case of an open valve (see above) in the brake circuit  300 . As a result, only the “working window” of the piston  210  is shifted, albeit into a position in which the brake booster  100  can make available its maximum additional force F 100 . In this context, the actuator  130  is preferably firstly moved into a position in which the brake booster  100  can for the first time make available approximately 100% of the additional force F 100 . This is the case in  FIG. 2  at approximately 33% of the adjustment travel x of the piston  210 . 
         [0024]    In this context, the brake booster  100  is configured or actuated in such a way that the method is not started until a torque or an electric current of the drive  110 , in particular of the electric motor  110 , reaches approximately 70% to approximately 95%, preferably approximately 75% to approximately 90% and, in particular, approximately 80% to approximately 85% of a torque or electric current which is possible at the respective operating point. When this torque or this electric current is reached, the corresponding electrical control signal  115  is preferably output by the control unit  114  of the brake booster  100 , in particular to the ABS or ESP control unit  500 . However, this can also take place by means of another control unit, wherein an ECU (Engine Control Unit) can also perform this task. In addition, in particular the transmission  120 , embodied as a cam mechanism  120 , of the brake booster  100  is configured in such a way that, from the zero position of the piston  210  of the master brake cylinder  200 , the maximum additional force F 100  is not reached until after approximately 20% to 25%, preferably only after approximately 25% to 30%, in particular only after approximately 30% to 35% (see  FIG. 2 ), particularly preferably only after approximately 35% to 40% and, in particular, particularly preferably only after approximately 40% to approximately 45% of an adjustment travel x of the piston  210 .