Abstract:
The invention proposes equipping a hydraulic vehicle brake system with a pedal travel modulator that has an electromechanically driven piston-cylinder unit. The piston-cylinder unit upon brake actuation “furnishes” additional brake fluid and thereby boosts a pedal travel of the vehicle brake system.

Description:
RELATED APPLICATIONS 
       [0001]    This application is based on German Patent Application 10 2008 014 462.2 filed Mar. 14, 2008, upon which priority is claimed. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a hydraulic vehicle brake system. 
         [0004]    2. Description of the Prior Art 
         [0005]    Such vehicle brake systems are conventional per se in motor vehicles. They have a master cylinder, which is actuated by muscle force by a vehicle driver by means of a brake pedal, or in the case of a motorcycle, by means of a manual brake lever. Wheel brake cylinders are connected hydraulically to the master cylinder. For antilock vehicle brake systems, brake pressure buildup valves and brake pressure reduction valves for modulating the brake pressure are disposed between the master cylinder and the wheel brake cylinders; for individual-wheel brake pressure modulation, one brake pressure buildup valve and one brake pressure reduction valve are disposed respectively upstream and downstream of each wheel brake cylinder. In simplified regulating systems, one brake pressure buildup valve and one brake pressure reduction valve can for instance be provided for the wheel brakes of one axle as well. Moreover, typically in each brake circuit is one hydraulic pump as a return pump and/or for pressure buildup, and a hydraulic reservoir as well as further hydraulic valves are provided. Such brake pressure regulating systems are known per se to one skilled in the art and will not be described further here, since they do not form the actual subject of the invention. 
         [0006]    In motor vehicles, underpressure brake boosters are typical, for enhancing the muscle force of the diver of the vehicle. Electrohydraulic brake boosters and external-force brake systems are also known, in both of which an external power supply to the vehicle brake system is accomplished with a hydraulic pump that is driven by an electric motor. Temporary storage of brake fluid under pressure in a hydraulic reservoir is typical. In the event of brake boosting, or in other words a so-called auxiliary force brake system, the brake pressure generated by the driver by muscle force by actuation of the master cylinder is enhanced with the external power supply; in the case of an external force brake system, the pressure buildup is effected solely with the external power supply; the master cylinder is disconnected hydraulically from the rest of the vehicle brake system. Only upon auxiliary braking, or in other words emergency braking in the event of failure of the external power supply is the actuation effected by muscle force using the master cylinder. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0007]    The hydraulic vehicle brake system of the invention takes a different course: Like conventional hydraulic vehicle brake systems, it has a master cylinder, to which the vehicle brake system is hydraulically connected and with which it is actuated by muscle force by the vehicle driver. The hydraulic vehicle brake system of the invention furthermore has an electrohydraulic pedal travel modulator that has a piston-cylinder unit which communicates with the master cylinder and which has an electromechanical drive. By actuation of the piston-cylinder unit of the pedal travel modulator, brake fluid is positively displaced from the piston-cylinder unit and reaches the master cylinder and the vehicle brake system. As a result, an actuation travel required for a brake actuation, that is, a displacement of a piston of the master cylinder, is shortened. Via the brake pedal, brake lever, or other user control element, the vehicle driver executes only a portion of the actuation travel and thus positively displaces a defined volume of brake fluid from the master cylinder into the vehicle brake system. A further volume of brake fluid is furnished by the pedal travel modulator. No force boosting takes place; the pressure in the master cylinder and in the wheel brakes is not increased by the pedal travel modulator. 
         [0008]    The displacement of the piston in the cylinder of the piston-cylinder unit of the pedal travel modulator, or in other words the volume of brake fluid that is positively displaced from the piston-cylinder unit and delivered to the vehicle brake system, is controlled or regulated as a function of a travel of the brake pedal, that is, as a function of a displacement of the piston of the master cylinder. The term “brake pedal” is understood in general also to mean a manual brake lever or other user control element for displacing the piston of the master cylinder. The pedal travel modulator can be said to effect a travel boost of the brake pedal, or in other words it increases the volume of brake fluid positively displaced, without enhancing the requisite muscle force. The dependency may, but need not, be linear, so that with the pedal travel modulator of the invention, as a further advantage, an intrinsically arbitrary pedal characteristic can be attained, the term “pedal characteristic” being understood to mean the dependency of the pedal travel on the pedal force. Force boosting can be attained by the hydraulic boosting of the master cylinder to the wheel brakes, in that the bore and piston diameter of the master cylinder is reduced and/or the piston diameter of the wheel brake cylinder is increased. An underpressure booster or other brake force booster can be dispensed with and according to the invention is not provided. In the event of a failure of the pedal travel modulator, auxiliary braking is possible solely by muscle force, and the pedal force is not increased by the failure of the pedal travel modulator; only the pedal travel increases. 
         [0009]    The drive of the piston-cylinder unit of the pedal travel modulator can be effected for instance with a linear motor, an electromagnet, or a piezoelectric element. An electromechanical drive with an electric motor and a mechanical gear is provided, which converts the rotary driving motion of the electric motor into a translational motion for displacing the piston in the cylinder of the piston-cylinder unit and which preferably includes a step-down gear. Since the invention makes a hydraulic force boosting from the pedal travel modulator to the master cylinder or the wheel brake cylinders possible, the invention makes do with a comparatively slight mechanical speed reduction, or even entirely without a step-down gear. 
         [0010]    At least one self-boosting wheel brake is provided in the vehicle brake system of the invention. Preferably, the brakes of one axle are structurally identical; that is, they are either self-boosting or non-self-boosting, or all the wheel brakes can be self-boosting. Both hydraulic and mechanical self-boosting devices are known, which need not be described further here. For mechanical self-boosting devices, wedge or ramp mechanisms are known, on which a friction brake lining of a disk brake is braced displaceably in the circumferential direction of the brake disk. The rotating brake disk urges the friction brake lining into an increasingly narrower wedge gap between the oblique wedge or ramp face and the brake disk, and by the principle of the wedge, brake boosting ensues. A travel-boosting self-boosting device is provided, in which the force is not boosted, but instead the travel, or in other words the positioning motion of the friction brake lining to the brake disk, is increased. For that purpose, the wedge or ramp mechanism is braced on the piston of the hydraulic wheel brake, so that the contact pressure of the friction brake lining against the brake disk is equal in magnitude to the tensing force generated by the brake piston. The bracing of the friction brake lining at an angle obliquely to the brake disk effects a positioning motion of the friction brake lining, or in other words a travel boost. This is known per se to one skilled in the art and therefore, and because the structural design of the self-boosting device of the wheel brake or brakes does not form the actual subject of the invention, will not be described in further detail here. The self-boosting devices are not limited to the structural types claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    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 drawings, in which: 
           [0012]      FIG. 1  is a schematic illustration of a vehicle brake system according to the invention; and 
           [0013]      FIG. 1   a  shows an alternative detail. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    The vehicle brake system according to the invention, shown in  FIG. 1  and identified overall by reference numeral  1 , has a dual-circuit master cylinder  2 , to which wheel brakes  4  are connected via a hydraulic anti-lock and traction control unit  3 . The traction control unit  3  is of a design known per se; for example, the ESP System 8.0 of the Applicant can be used, which makes individual-brake pressure regulation possible. Such anti-lock and traction control systems are known to one skilled in the art, so that they need not be explained here. 
         [0015]    The master cylinder  2  and with it the vehicle brake system  1  are actuated by muscle force, using a brake pedal  5 . In the case of a hand brake, a brake lever, instead of the brake pedal  5 , would be provided as a user control element. With a travel sensor  6 , a pedal travel of the brake pedal  5  or a piston travel of a rod piston  7  of the master cylinder  2 , which piston is connected mechanically to the brake pedal  5  via a piston rod  8 , is measured. A pressure sensor  9  is connected to one of the two brake circuits of the vehicle brake system  1 . 
         [0016]    An actuation of the vehicle brake system  1  is effected in the usual way by pressing down on the brake pedal  5 , or in other words by actuating the master cylinder  2 , from which brake fluid is positively displaced as a result; the brake fluid builds up a brake pressure in cylinders of the wheel brakes  4 , which communicate with the master cylinder  2 . Individual-wheel brake pressure regulation in the brake cylinders of the wheel brakes  4  is effected as needed automatically, with the hydraulic anti-lock and traction control unit  3 . 
         [0017]    One circuit of the master cylinder  2 , which in the exemplary embodiment is the rod circuit, communicates with a piston-cylinder unit  10  of a pedal travel modulator  11 . The piston-cylinder unit  10  of the pedal travel modulator  11  has a cylinder  12 , in which a modulator piston  13  is displaceable. The pedal travel modulator  11  has an electromechanical drive  14 , with an electric motor  15  that displaces the modulator piston  13  in the cylinder  12  via a gear wheel step-down gear  16  and a rack gear  17 . A piston rod  18  of the modulator piston  13  has a rack  19 , which is part of the rack gear  17 . 
         [0018]    Driving the modulator piston  13  causes brake fluid to be positively displaced out of the cylinder  12  of the piston-cylinder unit  10  of the pedal travel modulator  11  into the rod circuit of the master cylinder  2  or the vehicle brake system  1 . The hydraulic pressure is not increased as a result; it is determined solely by the muscle force exerted on the brake pedal  5 , which force acts on the rod piston  7  of the master cylinder  2 . However, the volume of brake fluid made available by the pedal travel modulator  11  shortens the pedal travel required for building up a defined brake force; that is, by means of the pedal travel modulator  11 , a kind of hydraulic travel boosting is effected. As a result, a greater hydraulic force boosting is possible by means of a smaller diameter of the pistons of the master cylinder  2  in proportion to the pistons in the cylinders of the wheel brakes  4 , and as a result, in turn, a brake booster, such as an underpressure brake booster, can be dispensed with. 
         [0019]    The pedal travel modulator  11  is controlled or regulated as a function of the pedal travel of the brake pedal  5 ; this travel is measured by the travel sensor  6 . The control or regulation of the pedal travel modulator  11  is effected with an electronic control unit, not shown, which preferably simultaneously controls the hydraulic anti-lock and traction control unit  3 , or in other words the ESP unit, as well. In other words, the control or regulation of the pedal travel modulator  11  can be integrated with the control unit, which is present anyway, of the hydraulic anti-lock and traction control unit  3 . The control or regulation of the pedal travel modulator  11  can be effected with a linear or other kind of fundamentally arbitrary dependency on the pedal travel. The pedal travel modulator  11  has a travel sensor  20  for measuring the displacement of the modulator piston  13 . As a result, an intrinsically arbitrary pedal characteristic can be achieved, the term pedal characteristic meaning the dependency of the pedal travel of the brake pedal  5  on the muscle force exerted on the brake pedal  5 . 
         [0020]    In the event of failure of the pedal travel modulator  11 , braking is effected solely by muscle force in the usual way, by pressing down on the brake pedal  5 . Since the pedal travel modulator  11  does not cause any force boosting but only travel boosting, the actuation force required for actuating the vehicle brake system  1 , that is, the muscle force to be exerted on the brake pedal  5 , is not increased; only the pedal travel is lengthened. The modulator piston  13  has a piston reverse-stroke limiter, which defines its basic position when the vehicle brake system  1  is not actuated but instead is released. In the exemplary embodiment, a spring ring  21 , as a piston reverse-stroke limiter  21 , is inserted into a groove of the cylinder  12  of the piston-cylinder unit of the pedal travel modulator  11 . The piston reverse-stroke limiter, in the event of failure of the pedal travel modulator  11 , prevents brake fluid upon brake actuation from being positively displaced out of the master cylinder  2  into the cylinder  12  of the piston-cylinder unit  10  of the pedal travel modulator  11 . In the event of failure of the pedal travel modulator  11 , the brake fluid positively displaced from the master cylinder  2  upon actuation is entirely available for the actuation of the wheel brakes  4 , and no brake fluid is “lost” into the cylinder  12  of the piston-cylinder unit  10  of the pedal travel modulator  11 . The piston reverse-stroke limiter  21  defines a basic position of the modulator piston  13  when the vehicle brake system is not actuated; displacement from the basic position and an alteration of the pedal characteristic are prevented. 
         [0021]    Since the pedal travel modulator  11  acts on only one of the two brake circuits of the vehicle brake system  1 , the hydraulic disconnection of the two brake circuits remains assured. 
         [0022]    A valve  22  is disposed between the piston-cylinder unit  10  of the pedal travel modulator  11  and the master cylinder  2 , or the vehicle brake system  1 . In the exemplary embodiment, this is a 2/2-way magnet valve, which in its currentless basic position connects the piston-cylinder unit  10  to the master cylinder  2  and the vehicle brake system  1  by means of a throttle restriction  23 . In a switching position when current is being supplied, the valve  22  is open; that is, it connects the piston-cylinder unit  10  of the pedal travel modulator  11  to the master cylinder  2  and to the vehicle brake system  1 . In normal operation, current is supplied to the valve  22 ; that is, it is open, and its opening is effected for instance with the activation of an ignition of a motor vehicle. In the event of an electrical failure or of the pedal travel modulator  11 , the valve  22  switches over to the currentless basic position. If the failure takes place during braking with the master cylinder  2  actuated, the throttle restriction  23  of the valve  22  throttles the flow of brake fluid from the master cylinder  2  into the cylinder  12  of the piston-cylinder unit  10  of the pedal travel modulator  11 , so that the brake pedal  5  does not “collapse” but instead yields slowly and spares the vehicle driver from the shock of sensing a total failure of the vehicle brake system  1 . The failure of the pedal travel modulator  11  can be ascertained from the lack of motion of the modulator piston  13  that is measured with the travel sensor  20 . Since the valve  22  is switched over quite infrequently, namely when the ignition is switched on and off, it has a very long service life. 
         [0023]    As an alternative, the valve  22 ′ shown in  FIG. 1   a  can be provided instead of the valve  22 . It is a 2/2-way magnet valve, which is open in its currentless basic position and closed in its switching position when current is being supplied. By closure of the valve  22 ′, the piston-cylinder unit  10  is disconnected hydraulically from the master cylinder  2  and the vehicle brake system  1 , so that a supply of current to the electric motor  15  can be switched off when the vehicle brake system is actuated during braking. In braking events that are long-lasting, the electric motor  15  can thereby be thermally relieved. 
         [0024]    In a preferred embodiment of the invention, some of the wheel brakes  4 , or all the wheel brakes  4 , are self-boosting. Preferably, the wheel brakes  4  of one or more or all the vehicle axles are self-boosting; because of the greater braking power, the wheel brakes  4  of a front axle are preferably self-boosting. In the drawing, the self-boosting of the two wheel brakes  4  shown on the left is represented by the symbol for a wedge mechanism  24 . Self-boosting wheel brakes are known to one skilled in the art, so that further explanation is unnecessary. Other known kinds of brakes include hydraulic and mechanical self-boosting devices, the latter in particular having wedge or ramp mechanisms that brace a friction brake lining, which is displaceable in the circumferential direction of a brake disk, obliquely to the brake disk. For the invention, travel-boosting wheel brakes  4  are preferred, in which some of the positioning travel of the friction brake linings toward the brake disk is effected by the displacement of the friction brake lining in the direction of rotation of the brake disk with the wheel brake  4  actuated, because of the oblique bracing in the brake caliper with the wedge or ramp mechanism. As a result, the volume of brake fluid, which must be positively displaced from the master cylinder  2 , that is required for brake actuation is reduced; the requisite pedal travel is shortened. The pedal travel shortening also results if even only some of the wheel brakes  4  are self-boosting (travel-boosting); it is not necessary for all the wheel brakes  4  to be self-boosting. Via the floating piston  25  of the master cylinder  2 , the travel boost also acts on the respective other brake circuit of the vehicle brake system  1 . 
         [0025]    A small diameter of the modulator piston  13  results in great travel reduction and great force boosting, as a result of which a slight travel reduction of the mechanical gear wheel gear  16  is possible, under some circumstances even an omission of the gear wheel gear  16  and a direct drive of the piston rod  18  via the gear rack drive  17  with the electric motor  15 . 
         [0026]    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.