Patent Publication Number: US-2012043806-A1

Title: Slip-Controlled Hydraulic Vehicle Brake System

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German Patent Application No. 10 2009 002 695.9, filed Apr. 28, 2009, German Patent Application No. 10 2009 045 714.3, filed Oct. 15, 2009, and PCT International Patent Application No. PCT/EP2010/055098, filed Apr. 19, 2010. 
     FIELD OF THE INVENTION 
     The present invention relates to a hydraulic vehicle brake system with slip regulation, and to a method for operating a brake system. 
     BACKGROUND OF THE INVENTION 
     A vehicle brake system of the above mentioned type is known for example from the international patent application WO 2008/017548. In the already known, preferably two-circuit brake system, a hydraulic line section which connects the outlet ports of outlet valves of a brake circuit is connected to a pressure medium storage reservoir and to the suction side of a hydraulic pump whose pressure side is connected via the inlet valves to vehicle wheel brakes. 
     It is considered to be a disadvantage of the known vehicle brake system that a leak of an outlet valve results in an elongation of the brake pedal actuating travel, wherein when the associated pressure chamber of the master brake cylinder is exhausted, the brake circuit connected thereto may fail. 
     It is therefore an object of the present invention, in a vehicle brake system of the generic type specified in the introduction, to propose measures which permit an increase in the operational reliability or a check of the leak-tightness of the outlet valves. 
     The above-mentioned object is achieved according to the invention, wherein features of a method for checking the leak-tightness of the outlet valves are also provided. 
     Advantageous further refinements of the vehicle brake system according to the invention are provided with further advantageous features of methods of operating a braking system. 
     Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a hydraulic braking system in accordance with a first embodiment of the invention; and 
         FIG. 2  is a schematic view of a hydraulic braking system in accordance with a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will be explained in more detail below on the basis of two exemplary embodiments illustrated in the appended drawing. Here,  FIGS. 1 and 2  each show a hydraulic circuit diagram of a first and a second embodiment of a vehicle brake system according to the invention. 
     The first embodiment of the vehicle brake system according to the invention, as shown in  FIGS. 1 , is provided for carrying out slip control processes, in particular anti-lock brake system (ABS), traction control (or anti-spin regulation (ASR)) and driving dynamics control (electronic stability program (ESP)). The brake system, which preferably has two brake circuits I and II consists substantially of an actuation unit  1 , a wheel brake pressure modulation device  2  which is connected to the actuation unit  1 , wheel brakes  3 ,  4 ,  5 ,  6  which are connected to the wheel brake pressure modulation device  2 , and an electronic control and regulating unit (ECU)  7  assigned to the brake pressure modulation device  2 . The reference numeral  8  denotes a sensor cluster (SC) which comprises the sensors required for carrying out the abovementioned regulating processes, the output signals of which sensors are supplied to the electronic control and regulating unit  7 . Here, the assignment of the wheel brakes  3 - 6  to the brake circuits I and II is such that the wheel brakes  3  and  4  connected to the first brake circuit I are assigned to the vehicle rear axle, while the wheel brakes  5  and  6  connected to the second brake circuit II serve for braking the vehicle front axle. Wheel rotational speed sensors  123 ,  124 ,  125 , and  126  serve for the direct measurement of the wheel rotational speeds and the indirect measurement of the pressure set in the wheel brakes  3 - 6  during a braking process. 
     As can also be seen from  FIG. 1 , the actuation unit  1  consists of a preferably pneumatic brake force booster  10  and of a two-circuit master brake cylinder  11  which in operational terms is connected downstream of the brake force booster  10 , to the pressure chambers (not illustrated) of which master brake cylinder the abovementioned brake circuits I and II are connected. A brake pedal denoted by the reference numeral  9  serves for the actuation of the described assembly. Furthermore, the master brake cylinder  11  is connected to a pressure medium (brake fluid) storage reservoir  12  whose chambers  12   a  and  12   b  are assigned to the master brake cylinder pressure chambers of the brake circuits I and II. The actuating travel of the brake pedal  9  is detected by means of a travel sensor  48  which senses the travel of the movable wall of the brake force booster  10  during an actuation. 
     The wheel brake pressure modulation device  2  comprises block valves  13   a  and  13   b  which are connected to the brake circuits I and II, which are designed as 2/2 directional valves which are open in the deenergized state, and to the outlets of which are connected inlet valves  14   a - d  assigned to the wheel brakes  3 - 6 , which inlet valves, in the open basic position in the deenergized state, permit a joint pressure build-up and dissipation and, upon electronic actuation, permit a different build-up of a hydraulic pressure in the wheel brakes  3 - 6  in a wheel-specific manner. A different dissipation of the pressure set in the wheel brakes  3 - 6  in a wheel-specific manner is realized by means of outlet valves  15   a - d , wherein the outlets of the outlet valves  15   a  and  15   b , and  15   c  and  15   d  assigned to the brake circuits I and II are connected to in each case one jointly used line sections  16   a  and  16   b . The line sections  16   a  and  16   b  lead to in each case one of safety outlet valves  17   a  and  17   b , the outlet of which is connected to in each case one of hydraulic low-pressure accumulators  18   a  and  18   b . Furthermore, each brake circuit I and II is assigned a circuit  19   a  and  19   b  of a two-circuit hydraulic pump  19  which is driven by means of an electric motor  20  and the suction side of which is connected to the low-pressure accumulators  18   a  and  18   b  via check valves  21   a  and  21   b  which close in the direction of the low-pressure accumulators  18   a  and  18   b . Further line sections  22   a  and  22   b  branch off from the section between the check valves  21   a  and  21   b  and the pumps  19   a  and  19   b , which further line sections  22   a  and  22   b  is connected, with the interposition of in each case one 2/2 directional valve  23   a  and  23   b  which is closed in the deenergized state, to the inlet port of the abovementioned block valves  13   a  and  13   b  or the brake circuits I and II. All of the valves described in this context are designed as electromagnetically actuable 2/2 directional valves, wherein in particular the individual pairs of inlet  14   a - d  and outlet valves  15   a - d  assigned to the individual wheel brakes  3 - 6  can be combined to form a 3/2 directional valve. Here, the inlet valves  14   a - d  are designed as 2/2 directional valves which are open in the deenergized state and which, in the energized state, perform the function of check valves which close in the direction of the wheel brake  3 - 6 , while the outlet valves  15   a - d  and the safety outlet valves  17   a  and  17   b  are designed as 2/2 directional valves which are closed in the deenergized state. 
     During a check for leak-tightness, which is carried out during a pressure holding phase for example at one of the outlet valves  15   a  or  15   b  assigned to the vehicle front axle, both the inlet valves  14   a  and  14   b  and also the outlet valves  15   a  and  15   b  are closed. For the purpose of the leak-tightness check, the corresponding safety outlet valve  17   a  is now opened. If one of the two outlet valves  15   a  or  15   b  has a leak, pressure medium flows out of the associated wheel brake  3  or  4  into the low-pressure accumulator  18   a  via the outlet valve  15   a  or  15   b  which has a leak. The resulting diminishing braking action is detected from the corresponding output signal of one of the abovementioned rotational speed sensors  123  or  124 . 
     During a check of the leak-tightness of the safety outlet valve  17   a , the inlet valve  14   b  is for example closed while the corresponding outlet valve  15   b  is opened. If the safety outlet valve  17   a  has a leak, pressure medium flows out into the low-pressure accumulator  18   a , which results in a loss of pressure medium and therefore a diminishing braking action of the wheel brake  4 . The diminishing braking action is detected again from the corresponding output signal of the rotational speed sensor  124 . The same approach self-evidently also applies to the other inlet and outlet valve pair  14   a  and  15   a.    
       FIG. 2  of the appended drawing shows an electrohydraulic brake system which can be operated in particular in a so-called “brake-by-wire” operating mode. Here, those parts of the brake system illustrated in  FIG. 2  which correspond to the components shown in  FIG. 1  are denoted by the same reference numerals. The actuation unit  1  has a first piston  25  which delimits a hydraulic pressure chamber  26  and which is coupled via a thrust rod  27 , which transmits actuating forces, to the brake pedal  9 . The actuation travel of the brake pedal  9  is detected by means of a travel sensor  33  which is preferably of redundant configuration and which senses the travel of the first piston  25 . The pressure chamber  26  can be connected to the pressure medium storage reservoir  12 , wherein said connection can be shut off by means of a relative movement of the first piston  25  with respect to a merely schematically indicated housing  40  in which the first piston  25  is guided. The actuation unit  1  is in operational terms connected downstream of a pedal decoupling unit  30  which is formed by a second piston  28 , which is likewise guided in the housing  40 , and by a hydraulic chamber  29  which is delimited by the second piston  28  in the housing  40 , the pressurization of which hydraulic chamber prevents a movement of the second piston  28  in the actuation direction. The second piston  28  constitutes a second delimitation of the above mentioned hydraulic chamber  26 , wherein a restoring spring is  34  arranged between the two pistons  25  and  28 . Furthermore, the second piston  28  can be connected in a force-transmitting manner to a first or primary piston  31  of the master brake cylinder  11 , wherein an intermediate chamber  32  which can be charged with hydraulic pressure is provided between the second piston  28  and the primary piston  31 , the pressurization of which intermediate chamber loads the second piston  28  and the primary piston  31  in opposite directions. 
     It can also be seen from  FIG. 2  that the abovementioned pressure chamber  26  is connected via a connecting line  35 , which can be shut off, to a hydraulically actuable travel simulation device  36 . The travel simulation device  36  has a simulator chamber  37  which is delimited by a simulator piston  38 . Here, the simulator piston  38  interacts with a simulator spring  39  and with an elastomer spring  41  which is connected in parallel with the simulator spring  39 . The shutting-off of the connecting line  35  takes place by means of a simulator shut-off valve  42  which is designed as an electromagnetically actuable 2/2 directional valve which is open in the deenergized state and which, in the energized state, performs the function of a check valve which closes in the direction of the simulator chamber  37 . A pressure sensor  43  serves for detecting the pressure prevailing in the pressure chamber  26 . 
     To realize the abovementioned “brake-by-wire” operating mode, an electrohydraulic pressure provision device  50  is provided which is formed from a hydraulic cylinder-piston arrangement  44  and from an electromechanical actuator  45 . Here, the electromechanical actuator  45  is designed as an electric motor with a step-down gearing, which ensures a translatory movement of a piston  46 , such that a hydraulic pressure is built up in a pressure chamber  47  of the hydraulic cylinder-piston arrangement  44 . The movement of the piston  46  is detected indirectly by a travel sensor  148  assigned to the actuator  45 . To charge the abovementioned intermediate chamber  32  with hydraulic pressure during a “brake-by-wire” braking operation, a first connecting line  49  is provided between the pressure chamber  47  of the hydraulic cylinder-piston arrangement  44  and the intermediate chamber  32 , to which first connecting line is connected a first pressure sensor  56 . A second connecting line  51 , in which is situated a second electromagnetically actuable 2/2 directional control valve  52  which is open in the deenergized state, forms a hydraulic connection, which can be shut off, between the hydraulic chamber  29  and the pressure chamber  47  of the hydraulic cylinder-piston arrangement  44 . Here, a check valve  57  which closes in the direction of the intermediate chamber  32  is connected between the first connecting line  49  and the second connecting line  51 . In the second connecting line  51 , there is connected upstream of the 2/2 directional valve  52  a third electromagnetically actuable 2/2 directional valve  53  which is open in the deenergized state and to the outlet port of which is connected a third connecting line  54  which is connected to the pressure medium storage reservoir  12 . 
     The assignment of the wheel brake pressure modulation valves or of the inlet and outlet valves and of the safety outlet valves to the individual brake circuits I and II corresponds to  FIG. 1  as explained above, with the exception of the fact that the reference numerals allocated to the said valves have been increased by 100 in  FIG. 2 . Here, the outlet ports of the safety outlet valves  117   a  and  117   b  are permanently connected via pressure dissipation lines  55  to the chambers  12   a  and  12   b  which are assigned to the individual brake circuits I and II of the pressure medium storage reservoir  12 . 
     During a check for leak-tightness carried out during a braking operation for example at one of the outlet valves  115   a  or  115   b  assigned to the vehicle front axle, the inlet valves  114   a  and  114   b  are opened while the outlet valves  115   a  and  115   b  are closed. For the purpose of the leak-tightness check, the corresponding safety outlet valve  117   a  is now opened. If one of the two outlet valves  115   a  or  115   b  has a leak, pressure medium flows out of the associated wheel brake  3  or  4  via the leaking outlet valve  115   a  or  115   b  into that chamber  12   a  of the pressure medium storage reservoir  12  which is assigned to the brake circuit I. By monitoring the volume consumption by means of the travel sensor  148 , if an outlet valve or safety outlet valve has a leak, an excessive volume consumption in relation to the master brake cylinder pressure is detected. Here, on the basis of the assignment of the occurrence of an additional volume consumption to the individual valve opening actions, it is detected whether one of the outlet valves or the safety outlet valve has the leak. After the discovery of a leak in one of the outlet valves, it is subsequently detected which of the two outlet valves of the brake circuit in question has the leak. For this purpose, the corresponding inlet valves  114   a  and  114   b  are closed, such that no pressure medium volume can be supplied to the wheel brakes  3  and  4  by the electrohydraulic pressure provision device  50 . As a result of a leak of one of the two outlet valves  115   a  or  115   b , the braking action at the associated wheel brake  3  or  4  diminishes, and this is detected from the corresponding output signal of one of the abovementioned rotational speed sensors  123  or  124 . 
     During a check of the leak-tightness of the safety outlet valve  117   a , the outlet valve  115   b  is for example opened while the corresponding inlet valve  114   b  remains open. If the safety outlet valve  117   a  has a leak, the pressure medium flows out into the associated chamber  12   a  of the pressure medium storage reservoir  12 , such that the loss of pressure medium caused in this way must be replenished by the electrohydraulic pressure provision device  50 . The replenishment of the pressure medium volume is determined indirectly at the hydraulic actuator  50  from the travel of the piston  46  detected by the travel sensor  148 . 
     The same approach is self-evidently also used for the other inlet and outlet valve pair  114   a  and  115   a.    
     While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation, and change without departing from the proper scope and fair meaning of the accompanying claims.