Abstract:
A brake system for motor vehicles having a brake master cylinder that can be operated by a brake pedal, a fluid reservoir assigned to the brake master cylinder, a hydraulic booster stage which is operatively connected to the inlet side of the brake master cylinder, with a booster piston which is subjected to a hydraulic boost pressure supplied by a pressure source and which in all operating modes allows a direct mechanical action by the brake pedal on a hydraulic piston of the brake master cylinder, and having an electrically controllable pressure regulating valve arrangement for metering the boost pressure. In order to provide a brake system of simple design, which is cost-effective to produce and which functions reliably, the pressure regulating valve arrangement can be hydraulically controllable.

Description:
This application is the U.S. national phase application of PCT International Application No. PCT/EP2008/065889, filed Nov. 20, 2008, which claims priority to German Patent Application No. 10 2007 055 515.8, filed Nov. 21, 2007, and German Patent Application No. 10 2008 058 240.9, filed Nov. 19, 2008, the contents of such applications being incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates to a brake system for motor vehicles, having a brake master cylinder that can be operated by means of a brake pedal, a fluid reservoir assigned to the brake master cylinder, a hydraulic booster stage, which is operatively connected to the inlet side of the brake master cylinder, with a booster piston which is subjected to a hydraulic boost pressure supplied by a pressure source and which in all operating modes allows a direct mechanical action by the brake pedal on a hydraulic piston of the brake master cylinder, and having an electrically controllable pressure regulating valve arrangement for metering the boost pressure. 
     BACKGROUND OF THE INVENTION 
     Such an electro-hydraulic brake system is disclosed by the unexamined German application DE 27 25 941, which is incorporated by reference. In the known brake system the hydraulic booster stage is connected to the inlet side of a pedal-operatable brake booster. A 3/2-way directional control valve, which is electrically controlled, serves as pressure regulating valve arrangement for regulating the hydraulic boost pressure. A feature of the known brake system that has been found to be less advantageous are the facts that in addition to the arrangement mentioned for generating an electrically controlled brake servo assistance, the pneumatic brake booster has to be used in order to afford brake servo assistance in the absence of electrical control. Two brake boosters, one electrical and one pneumatic, are used connected in series. 
     On the basis of the aforementioned use of a pedal-operatable brake booster and an electrically operatable brake booster, an object of the invention is to produce such a brake system having only one brake booster, which is both pedal-operatable and electrically operatable. 
     SUMMARY OF THE INVENTION 
     According to aspects of the invention this object is achieved in that the pressure regulating valve arrangement can also be hydraulically controlled. The hydraulic control pressure corresponds to the pedal force, so that the pedal-operated brake booster connected on the input side is no longer needed. 
     The idea of the invention is embodied in that the pressure regulating valve arrangement is hydraulically controlled by means of the pressure generated by the brake master cylinder. 
     It is particularly advantageous here if the pressure regulating valve arrangement is of two-stage design. 
     In a development of the invention a substantial simplification of the construction of the boost pressure control is achieved in that the pressure regulating valve arrangement comprises an electrically controllable pilot control stage, a double-hydraulically controllable pilot control stage and a double-hydraulically controllable valve main stage. 
     In such an embodiment it is particularly advantageous if the pilot control stage is formed by the connection in series of an analog controllable 2/2-way directional control valve, closed when de-energized, and an analog controllable 2/2-way directional control valve, open when de-energized, the hydraulic center tap between the two valves delivering one of the control pressures for the valve main stage. The use of tried and tested electronic components ensures high operating reliability. 
     In a further embodiment of the subject matter of the invention a considerable increase both in the control performance and in the dynamics of the aforementioned boost pressure control is achieved in that the valve main stage comprises a 3/3-way directional control valve having a valve sleeve, a valve body axially displaceable in the valve sleeve, a first control piston and a second control piston, the first control piston defining a first hydraulic control chamber, the first and the second control pistons defining a second hydraulic control chamber, the second control piston and the valve body defining a hydraulic reservoir connecting chamber and the valve body and the valve sleeve defining a high-pressure connecting chamber and a working pressure connecting chamber. 
     An outstanding feature of an especially compact embodiment of the subject matter of the invention is that the valve body and the second control piston are integrally formed. 
     In another advantageous development of the invention the valve body on its end face remote from the second control piston projects into the working pressure connecting chamber and is subjected to the pressure of the high-pressure connecting chamber. 
     In a further embodiment of the subject matter of the invention a faultless hydraulic control of the aforementioned valve main stage is achieved in that the second control chamber is hydraulically connected to a center tap of the pilot control stage. 
     A further possibility for hydraulic control of the pressure regulating valve is to connect the first control chamber hydraulically to a hydraulic circuit of the brake master cylinder. 
     In an especially reliably functioning design variant of the subject matter of the invention the valve body, interacting with the valve sleeve, in a rest position establishes a hydraulic connection between the working pressure connecting chamber and a reservoir connecting chamber, the hydraulic passage cross section of which diminishes continuously under the application of a displacement travel of the valve body in the valve sleeve, and the valve body ( 14 ) in further displacement by a slight “overlap travel ” separates the three connecting chambers from one another, before under yet further displacement continuously opening a hydraulic connection between the high-pressure connecting chamber and the working pressure connecting chamber. 
     In a further development of the subject matter of the invention the first control piston is embodied as a stepped piston. 
     For faultless functioning of the pressure regulating valve it is particularly appropriate if an annular hydraulic chamber between the two sealing cross-sections of the stepped piston is hydraulically connected to the reservoir connecting chamber. 
     A further advantage of the invention is that the pedal-controlled component of the brake servo assistance can be deactivated by separating the hydraulic connection between the brake master cylinder and the pressure regulating valve. In another design variant such a separation between the brake master cylinder and the pressure regulating valve is ensured in that the connection between the brake master cylinder and the first control chamber can be shut off by means of an electromagnetically operatable 2/2-way directional control valve, open when de-energized, a non-return valve being provided, which is operatively connected in parallel to this valve and which allows a fluid volumetric flow leaving the first control chamber irrespective of the switching position of the 2/2-way directional control valve, open when de-energized. 
     For the aforementioned hydraulic control of the pressure regulating valve by the inlet pressure in the brake master cylinder it is particularly advantageous if the end face of the valve body facing the working pressure connecting chamber is larger than the face of the stepped piston defining the first control chamber. 
     In another preferred embodiment of the invention a hydraulic connection, in which a non-return valve closing towards the fluid reservoir is inserted, is provided between the working pressure connecting chamber and the fluid reservoir. 
     Finally, reliable registering of the driver&#39;s intended declaration is achieved in that a pressure sensor is provided for registering the inlet pressure in the brake master cylinder. 
     A redundant registration of the driver&#39;s intended deceleration is ensured in that a travel sensor is provided for registering the travel of the booster piston. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawings. Included in the drawings is the following Figures: 
         FIG. 1  shows the construction of one embodiment of the brake system according to aspects of the invention, and 
         FIG. 2  shows the construction of a pressure regulating valve that can be used in the embodiment according to  FIG. 1  on a larger scale. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The brake system according to aspects of the invention represented in  FIG. 1  substantially comprises an operating device  1 , a hydraulic control unit  2 , the operating device  1  and the control unit  2  forming a brake booster, and a brake master cylinder or tandem master cylinder  3 , which is operatively connected to the output side of the brake booster and the pressure chambers (not shown) of which are connected to wheel brake circuits I, II, which by way of a known ABS/ESP hydraulic unit or a controllable wheel brake pressure modulating module  4  supply the wheel brakes  5   a - d  of a motor vehicle with hydraulic fluid. An electronic brake system control unit  30  is assigned to the hydraulic control unit  2 , whilst an electronic control and feedback unit  31  is assigned to the wheel brake pressure modulation module  4 . The brake operating unit  1  is controlled through the provision of a brake pedal  33 , which is coupled to a piston rod  32 , which by way of a booster piston  5  has a force-transmitting connection to a first piston or primary piston  7  of the brake master cylinder  1 . The booster piston  5  is guided so that it is axially displaceable in a booster housing  36 , where it defines a hydraulic booster chamber, which is provided with the reference numeral  35 . 
     The aforementioned hydraulic control unit  2  substantially comprises a pressurizing device  6  and a pressure regulating valve arrangement  8 . The pressurizing device  6  is formed by a hydraulic high-pressure accumulator  38  and a motor-pump unit  39  for charging the high-pressure accumulator  38 , whilst the output of the pressure regulating valve arrangement  8  is connected on the one hand by a hydraulic connection  25  to the aforementioned fluid reservoir  37  and on the other via a line  40  attaching to the hydraulic connection  25  to the booster chamber  35 . The hydraulic control unit  2  furthermore has a pilot control stage  9 , the function of which will be explained in the following text, assigned to the pressure regulating valve arrangement  8 . A further line  41  connects the suction side of the motor-pump unit  39  to the aforementioned fluid reservoir  37 . The motor-pump unit  39  is preferably embodied as a module remote from the other components of the hydraulic control unit  2  and is equipped with fastenings and hydraulic connections that isolate any structure-borne noise. The hydraulic pressure held ready in the high-pressure accumulator  38  is registered by a pressure sensor, which is provided with the reference numeral  45 . 
     As can be seen in particular from  FIG. 2 , the pressure regulating valve arrangement  8  is of two-stage design and besides said electrically controllable pilot control stage  9  preferably comprises a double-hydraulically controllable valve main stage, which is provided with the reference numeral  10 , and a hydraulic control stage, the construction of which will be explained in the following description. 
     In a series connection the pilot control stage  9  comprises an analog controllable 2/2-way directional control  11  valve, closed when de-energized, and an analog controllable 2/2-way directional control valve  12 , open when de-energized, the hydraulic center tap  28  between the two valves  11 ,  12  delivering one of the control pressures for the valve main stage  10 . The hydraulic control stage is formed by a first control chamber  17 , a first control piston or stepped piston  15 , an annular chamber  22  connected to the fluid reservoir  37 , and a second control chamber  18 , which is defined by the stepped piston  15  and which among other things is connected to the aforementioned center tap  28  of the pilot control stage  9 . The second control chamber  18  is defined on the other side by a second control piston  16 , which together with a valve sleeve  13  defines a reservoir connecting chamber  19  and which in the embodiment shown is integrally formed with a valve body  14 . The valve sleeve  13  together with the valve body  14  forms the aforementioned main stage  10  of the pressure regulating valve arrangement  8 . 
     It can further be seen from  FIG. 2  that the first control chamber  17  is connected by way of an electromagnetically operatable, 2/2-way directional control valve  23 , open when de-energized, to the second brake circuit II. The inlet pressure in the second brake circuit is registered by a pressure sensor  27 , the 2/2-way directional control valve  23  in its energized switching position fulfilling the function of a non-return valve closing towards the control stage, which is indicated in  FIG. 2  by the corresponding, hydraulic symbol provided with the reference numeral  24 . 
     Meanwhile the valve body  13  together with the valve sleeve  13  forms a high-pressure connecting chamber  20 , which is connected to the outlet of the pressurizing device  6  or  38 . The high-pressure connecting chamber  20  can be connected by a displacement of the valve body  14  to a working pressure chamber  21 , which in the starting position or rest position of the valve body shown is connected to the reservoir connecting chamber  19  by means of fluid ducts  42 ,  43  formed in the valve body  14 . The inlet boost pressure in the working pressure chamber  21  is registered by a third pressure sensor  46 . It is advantageous here if the diameter of the valve body  14  guided in the valve sleeve  13  is greater than the diameter of the smaller step of the stepped piston  15 . It can be seen from  FIG. 2 , moreover, that the line  40  leading to the booster chamber  35  is connected to the working pressure chamber  21  (via the aforementioned connecting line  25 ) and that a further line  44  leading to the fluid reservoir  37  is connected to this former line. Also connected to the latter aforementioned line  44  is a non-return valve  26  closing towards the fluid reservoir  37 . 
     The operating principle of the brake system according to aspects of the invention will be explained in more detail below in conjunction with the drawing. 
     The first operating mode corresponds to a purely electrical, the so-called “brake-by-wire” operating mode, in which all components of the brake system are intact and functioning correctly. This corresponds to the nominal function of the brake system according to aspects of the invention. In this mode the valve  23  is closed and, in order to adjust the hydraulic pressure in the booster chamber  35  needed for instantaneous braking the pressure regulating valve arrangement  8  is controlled in such a way that the valve  12  is closed in order to boost the pressure, the valve  12  is opened so that fluid flows into the chamber  18 , thereby causing the valve body  16  to move, with the result that the pressure reduction control edge, that is to say the connection from the working pressure chamber  21  to the reservoir connecting chamber  18 , is closed and with further inflow the connection between the high-pressure connecting chamber  20  and the working pressure connecting chamber  21  is opened. To maintain pressure, metered opening of the valve  12  allows a partial outflow of fluid from the chamber  18 , so that the valve body  16  is displaced towards the right and the pressure build-up control edge, that is to say the connection from the high-pressure connecting chamber  20  to the working pressure connecting chamber  21  is shut off, whilst to reduce pressure through a further outflow of fluid from the chamber  18  the valve body  16  is displaced further towards the right, with the result that the pressure reduction control edge, that is to say the connection between the working pressure connecting chamber  21  and the reservoir connecting chamber  19  is opened. 
     The process of controlling the analog controllable 2/2-way directional control valves  11 ,  12  is coordinated by the electronic control unit  30 , in such a way that the pressure in the working pressure connecting chamber  21  is brought into line with a set-point value. This set-point value results on the one hand from a registered operating component of the brake pedal  33  and on the other from a remote operating component. The operating component of the brake pedal  33  is determined from the operating travel of the brake pedal  33  or the booster piston  5 , which is registered by means of the travel sensor  29  and which according to a preferably progressive characteristic curve is adjusted as a consequence of an operating force exerted on the brake pedal  33  in a relationship which by virtue of the progression is not proportional. 
     In a second operating mode, which is characterized by a voltage drop on the vehicle electrical system and which corresponds to a first return level, an electronically controlled pressure build-up in the working pressure connecting chamber  21  is not possible. For this purpose the pressure regulating valve arrangement  8  is hydraulically controlled by means of the inlet pressure  35  in the brake master cylinder  3  due to the operation of the brake pedal  33 , the control by hydraulic means ensuing via the open solenoid valve  23  and the pressure chamber  17 , and the fluid stored under pressure in the high-pressure accumulator  38  being used as energy source. Otherwise pressure is built up in the working pressure connecting chamber  21  in the same way as in the first operating mode. 
     In a third operating mode, which is characterized by the absence of a pressure generated by the hydraulic pressurizing device  6  or  38 , the brake system can be operated purely mechanically. Under the influence of brake pedal operation, the booster piston  5  moves from its limit stop towards the left and displaces the brake master cylinder piston  7  through mechanical contact. In the process the non-return valve  26  allows a further flow of fluid from the reservoir  37  into the booster chamber  35 . The brake master cylinder  3  is operated solely by the muscle power of the driver of the vehicle. 
     The present invention affords a brake system of simple construction, in which an electronic stability control function (ESP) can be more easily implemented than in conventional brake systems, because there is no need for special ESP hydraulics. In vehicles with the brake system according to aspects of the invention, special ESP hydraulics are superfluous—the remote brake hydraulics according to aspects of the invention in conjunction with a conventional ABS-System produces a far better function. Fewer electromagnetically operatable valves are needed than are required for conventional ESP hydraulics. Furthermore, the brake system according to aspects of the invention has a better energy balance and generates less noise than conventional ESP hydraulics, because there is no longer any need, in ESP mode to pump brake fluid around in order to generate dynamic pressure on a pressure-limiting valve.