Abstract:
A brake system with a master cylinder, a first piston which is coupled to a brake pedal via a push rod transmitting actuating forces, a second piston which can be actuated by the first piston and which can be brought into force-transmitting connection to a third piston, via which the master cylinder is actuated, at least one elastic element which forms a pedal travel simulator which, in the “brake-by-wire” operating mode, gives the vehicle driver a pleasant pedal sensation, an interspace capable of being acted upon with hydraulic pressure, between the second and the third piston, the action of pressure upon the interspace loading the second and the third piston in opposite directions, a pressure supply device which has a high-pressure source and which allows both a filling of the interspace with pressure medium and emptying thereof, and a valve device for varying pressure fed in the interspace.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national phase application of PCT International Application No. PCT/EP2008/053334, filed Mar. 19, 2008, which claims priority of German Patent Application No. 10 2007 018 895.3, filed Apr. 19, 2007 and German Patent Application No. 10 2007 049 620.8, filed Oct. 17, 2007, the contents of such applications being incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a brake system for motor vehicles, with a master cylinder to which wheel brake cylinders are connectable, with a first piston which is coupled to a brake pedal via a push rod transmitting actuating forces, with a second piston which actuates the master cylinder, with a third piston which can be actuated by the first piston and which can be brought into force-transmitting connection to the second piston, with at least one elastic element which forms a pedal travel simulator which, in the “brake-by-wire” operating mode, gives the vehicle driver a pleasant pedal sensation, with means for coupling the movements of the first and of the third piston, with an interspace, capable of being acted upon with hydraulic pressure, between the second and the third piston, the action of pressure upon the interspace loading the second and the third piston in opposite directions, with an electrically controllable pressure supply module which allows both a filling of the interspace with pressure medium and the emptying thereof, and with a valve device, by means of which the pressure fed in the interspace can be varied. 
     2. Description of the Related Art 
     In motor vehicle technology, “brake-by-wire” brake systems are in increasingly widespread use. In these brake systems, the brake can be “extraneously” actuated, even without the active assistance of the driver, by means of electronic signals. These electronic signals may be output, for example, by an electronic stability program ESP or a headway control system ACC. If there is an overlap of an extraneous actuation of this type with a driver actuation, the driver of the motor vehicle detects a reaction in the brake pedal. This reaction effect on the brake pedal may be surprising and unpleasant to the driver, and therefore, in a critical road traffic situation, the driver does not actuate the brake pedal to an extent matched to this situation, since he is irritated by the reaction on the brake pedal caused by the extraneous actuation of the brake. 
     A brake system of the generic type initially mentioned is known from international patent application WO 2006/042823 A1. In order to make it possible to assist the actuating force even in the event of a failure of the control electronics or of the electrical energy supply, the known brake system has arranged between the first and the third piston a fourth piston which, in interaction with the third piston, delimits a hydraulic chamber which has a hydraulic connection, capable of being shut off as a result of brake pedal actuation, to a pressure medium reservoir, and in which pressure can be built up by means of the actuating forces, a valve arrangement being activatable by the hydraulic pressure fed in the chamber, and the pressure fed in the interspace being variable by means of said valve arrangement. It is found to be a disadvantage of the known brake system that the pedal travel simulator is switched off as a function of the movement travel of the third piston which constitutes a loss of travel during actuation. 
     SUMMARY OF THE INVENTION 
     An object of the present invention, therefore, is to propose a brake system of the generic type initially mentioned, in which the pedal travel simulator is switched off and switched on, without detectable losses of travel occurring during actuation on the brake pedal. 
     This object is achieved, according to aspects of the invention, in that the means for coupling the movements of the first and of the second piston are automatically active in the event of an absent pressure supply and can be deactivated by the hydraulic pressure provided by the pressure supply device. 
     To put the idea of the invention into more concrete terms, in an advantageous development of the subject of the invention at least one further (fourth) piston is provided, which can be acted upon with the hydraulic pressure provided by the pressure supply device and which allows a movement of the first piston with the effect of the actuation of the pedal travel simulator or prevents such a movement. 
     The present invention is explained in more detail below by means of two exemplary embodiments, with reference to the accompanying diagrammatic drawing in which identical components are given the same reference symbols. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the set-up of a first version of the brake system according to an exemplary embodiment of the invention; 
         FIG. 2  shows the actuation device according to  FIG. 1  on an enlarged scale; and 
         FIG. 3  shows the set-up of a second version of the brake system according to another exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The brake system according to aspects of the invention, illustrated in the drawing, consists essentially of an actuation device  1 , of a pressure supply device  2 , the actuation unit and the pressure supply device forming a brake booster, and of a brake master cylinder or a tandem master cylinder  3  which operatively follows the brake booster and of which the pressure spaces, not illustrated, can be connected to the chambers of a first pressure medium reservoir  18  which are under atmospheric pressure. On the other hand, the pressure spaces have connected to them wheel brake circuits I, II which, with a known ABS or ESP hydraulic assembly or an electrically controllable wheel brake pressure modulation module  4  being interposed, supply the wheel brakes  5 - 8  of a motor vehicle with hydraulic pressure medium. The wheel brakes  7 ,  8  are preferably equipped with electrical actuation elements  38 ,  39  which can be activated by means of a parking brake switch  40  and by means of which a parking brake function can be implemented. A sensor cluster  34 , indicated merely diagrammatically, which contains a transverse acceleration sensor, not shown, and a yaw rate sensor, serves, in combination with the abovementioned wheel brake pressure modulation module  4 , for implementing an electronic stability function (ESP). The wheel brake pressure modulation module  4  is assigned an electronic control and regulation unit  41 . The actuation device  1 , which is arranged in a housing  20  to which the tandem master cylinder  3  is connected, can be activated via a brake pedal  9  which is coupled mechanically via an actuating rod  10  to a first piston  11  of the actuation device  1 . The actuating travel of the brake pedal  9  is detected by means of a travel sensor  19  which senses the travel of the first piston  11 . However, a rotary angle sensor which detects the rotary angle of the brake pedal  9  may also be used for the same purpose. 
     The abovementioned pressure supply device  2  comprises a hydraulic high-pressure source  30  designed as a motor/pump assembly, optionally a high-pressure accumulator  29 , pressure-regulating valves  35 ,  36 ,  37  and a second pressure medium reservoir  21  which may be arranged, for example, on the housing  20  of the actuation unit  1  and which preferably forms with the first pressure medium reservoir  18  a structural unit. A pressure sensor  24 , which may likewise be integrated in the pressure supply device  2 , serves for monitoring the pressure discharged by the high-pressure source  30 . The high-pressure accumulator  29  delivers the energy for brake actuation when the motor/pump assembly of the high-pressure source  30 , because of the mass inertia of its rotor, cannot immediately provide the pumping power necessary, for example, for sudden full braking, or when there is no electrical energy available for driving the motor/pump assembly. The pressure supply device  2  is assigned an electronic control unit  33  which is fed, inter alia, with output signals from the travel sensor  19  and the pressure sensor  24  and which serves for activating the motor/pump assembly  30  and the pressure-regulating valves  35 - 37 . The high-pressure source  30  is connected to the actuation unit  1  by means of a hydraulic line which is given the reference symbol  31 , while the line portion between the pressure-regulating valves  35 ,  36  is likewise connected to the actuation unit  1  by means of a further hydraulic line  32 . The function of the two lines  31 ,  32  is explained in more detail in the following description. 
     As may be gathered particularly from  FIG. 2 , the first piston  11  is guided displaceably in a guide  13  of a second piston  12  which cooperates with a third piston  14  which may form the primary piston of the tandem master cylinder  3 , in the example illustrated a pressure intensification piston  16  being arranged between the second  12  and the third  14  piston. Between the second piston  12  and the pressure intensification piston  16 , an interspace  22  is delimited, as a result of the action upon which with a hydraulic pressure the second piston  12  is held against a stop  23  formed in the housing  20 , while the pressure intensification piston  16  and consequently the primary piston  14  of the tandem master cylinder are acted upon with the effect of a pressure build-up in the tandem master cylinder  3 . A movement of the pressure intensification piston  16  which results from this load is detected by means of a second travel sensor  25 . The pressure fed in the interspace  22  is detected by means of a second pressure sensor  26 . An elastic element or a compression spring  42  is arranged operatively between the first piston  11  and a force transmission element  45  and, together with an elastomeric spring  43 , forms a pedal travel simulator which, when the brake system is actuated, gives the vehicle driver the accustomed pedal sensation which corresponds to a usual brake pedal characteristic. This means that the resistance rises slowly in the case of a small brake pedal travel and increases superproportionally in the case of a greater brake pedal travel. 
     For mechanical guidance of the elastic element  42 , that end of the force transmission element  45  which faces the pedal is designed as a hollow cylinder  44 . The force transmission element  45  is displaceably guided, sealed off, in the second piston  12  and delimits therein a pilot control chamber  46 , to which the pressure-regulating valves  35 - 37  are connected via the abovementioned hydraulic line  32 , so that said pilot control chamber can be fed with a pilot control pressure which is set by the suitable activation of the pressure-regulating valves  35 - 37 . On the other hand, the pilot control chamber  46  is delimited by a valve body  47  of a pressure-regulating valve device  50  which is designed as a slide valve and with the aid of which the pressure fed in the interspace  22  can be varied. A first control edge  51  of the slide valve body  47  in this case cooperates with the issue of a pressure medium duct  52  which is formed in the second piston  12  and which allows the inner space of the valve body  47  to be connected to the pressure medium reservoir  21 . By contrast, a second control edge  53  of the valve body  47  cooperates with the issue of a further pressure medium duct  54  which is formed in the second piston  12  and which is connected to an annular space  55  which is delimited by the second piston  12  in the housing  20  and into which the abovementioned hydraulic line  31  issues. A further pressure medium duct  56  connects the inner space of the valve body  47  to the interspace  22 . In the position of rest of the valve body  47 , as shown in the drawing, the latter bears under the prestress of a valve spring  57  against the force transmission element  45  which is supported on a stop  58  provided in the second piston  12 . 
     Moreover, it can be gathered from  FIG. 2  that, in the second piston  12 , a hydraulic space  60  is provided which is connected to the abovementioned annular space  55  and which receives a compression spring  59  of relatively high rating which is supported on a fourth piston  61 , arranged displaceably coaxially to the guide  13 , and which holds said piston in bearing contact against the end of the guide  13 . A radial collar  62 , on which the first piston  11  is supported axially, serves for the bearing of the fourth piston  61 , so that, as a result of the action of the strong spring  59 , a movement of the first piston  11  with the effect of the actuation of the pedal travel simulator  42 ,  43  is prevented. 
     In a first (normal) operating mode, that is to say in a hydraulic booster operating mode, if the brake system is functioning perfectly the annular surface, designated by “A”, of the fourth piston  61  is acted upon with the high pressure provided by the high-pressure source  30 , so that the fourth piston  61  moves to the left in the drawing and releases the first piston  11 , so that the pedal travel simulator  42 ,  43  is actuated and the driver is given the usual pedal sensation. By means of a corresponding activation of the pressure-regulating valves  35 - 37 , the pilot control chamber  46  is acted upon with a pilot control pressure, so that, as a result of a subsequent displacement of the valve body  47 , the pressure medium duct  52  leading to the pressure medium reservoir  21  is shut off and the pressure medium duct  54  is released, so that the interspace  22  is acted upon with a regulated high pressure. As a result of the action of the high pressure, the second piston  12  is held against the abovementioned stop  23  and the pressure intensification piston  16  is displaced in the direction of actuation of the brake master cylinder  3 . The pressure in the interspace  22  is otherwise freely selectable, so that any desired dependence of the brake pressure on the brake pedal actuation can be programmed. Undesirable pedal reactions are in this case ruled out. This electronic actuation pressure regulation has the advantage that its transmission behavior can be selected freely within the framework of the dynamics afforded by the technical data of the pressure accumulator and pressure-regulating valves. Consequently, a jumper function, as it may be referred to, that is to say the jump to a predetermined brake pressure value when the brake pedal  9  is touched, a brake assistance function, deceleration control and autonomous braking, such as is required, for example, for ASR (traction control), ESP (electronic stability program) and ACC (adaptive cruise control), can be implemented by means of software measures. For this purpose, the driver&#39;s instruction in the form of a brake pedal actuation, detected by travel, force or other sensors, is converted by a computing unit, not explicitly illustrated, using suitable algorithms, into wheel brake pressures which are implemented with the aid of the electronically switchable valves in the pressure supply device  2  and the following ABS/ESP control module  4 . 
     In a second operating mode, which corresponds to an occurrence of a mechanical and/or electrical fault, causing a malfunction of the pressure-regulating valves  35 - 37 , and therefore to a first relapse level, no pilot control pressure can be fed in the pilot control chamber  46 . In this case, the pressure-regulating valve device  50  is actuated by means of a transmission of the actuating force introduced at the brake pedal  9  to the force transmission element  44 , so that the interspace  22  is acted upon with a regulated high pressure, as in the case described above. 
     In a third operating mode which is characterized by the absence of a pressure generated by the hydraulic pressure supply device  2  or by a failure of the high-pressure source  29  or  30 , and in which no high pressure is made available and which corresponds to a second relapse level, a pressure build-up takes place in the brake master cylinder  3  as a result of purely mechanical force transmission between the first ( 11 ) or the second  12  piston and the pressure intensification piston  16  or the third piston  14 , during which the abovementioned strong spring  59  is compressed by the actuating force. 
     The set-up of the second embodiment, illustrated in  FIG. 3 , of the brake system according to aspects of the invention corresponds largely to the first version shown in  FIG. 1 . A motor/pump assembly  30   a  is used as high-pressure source. A further difference can be seen in the design of the means for coupling the movements of the first ( 11 ) and of the second ( 12 ) piston, which means are formed by a plurality of pistons  63 ,  64  arranged at regular angular intervals around the abovementioned guide  13  of the second piston  12 , and by an annular force transmission disk  65  cooperating with the pistons  63 ,  64 . The force transmission disk  65  is prestressed counter to the direction of actuation of the first piston  11  via a compression spring  66  and under the prestress of this compression spring  66  bears axially against the pistons  63 ,  64  and also the guide  13  and the first piston  11 . In the exemplary embodiment illustrated, the compression spring  66  is arranged in the dry inner space of the second piston  12 . The release of the first piston  11  takes place, in a similar way to the first version, by the pistons  63 ,  64  being acted upon with the pressure provided by the high-pressure source  30   a , thus resulting in a movement of the pistons  63 ,  64  and therefore of the force transmission disk  65  to the left in the drawing. 
     By virtue of the present invention, a brake system of simple set-up is achieved in which the brake pedal characteristic is not dependent on the state of actuation of the remaining brake system, with the result that, in the event of braking by the driver, the pedal sensation cannot be disturbed either by the simultaneous presence of extraneous braking or by other control activities of the brake system, such as antilock control, traction control or driving-stability control. 
     The brake system has the advantage, furthermore, that it has a simpler set-up than conventional brake systems. Vehicles with an electronic stability control function (ESP) require, for example, special ESP hydraulics which are more complicated than normal ABS hydraulics, since, in contrast to ABS hydraulics, they must also possess the capability for building up wheel brake pressures above the master cylinder pressure. 
     In vehicles with the brake system according to aspects of the invention, special ESP hydraulics are superfluous—the extraneous brake hydraulics according to aspects of the invention, in conjunction with a conventional ABS system, afford far better functioning. Fewer electromagnetically actuable valves are required than for conventional ESP hydraulics. Moreover, the brake system according to aspects of the invention has a better energy balance and a lower generation of noise than conventional ESP hydraulics, because the circulatory pumping, required there in the ESP operating mode, of brake fluid for the generation of dynamic pressure at a pressure-limiting valve is unnecessary. An improvement in the noise behavior, as compared with the prior art indicated, can also be achieved, since no noise-generating components, such as valves, motors or pumps, are arranged inside the module which is surrounded by the housing and which is connected in the vehicle to the splashboard which is critical in terms of structure-borne sound. 
     The set object of maintaining brake boosting in the event of a brief failure of the electrical energy supply is achieved in that only the (normal) operating mode presupposes the presence of electrical energy, whereas the relapse levels require only hydraulic or mechanical energy.