Abstract:
In order to impart to the vehicle operator the usual feeling at the brake pedal in an electrohydraulic brake system of the ‘brake-by-wire’ type or ensure a pre-selected flexibility of the brake pedal by means of an electrically controllable valve assembly, the invention discloses that the electrically controllable valve assembly is formed of the separating valve, a pressure regulating or control valve connected to the outlet side of the separating valve and having its outlet port connected to the pressure fluid reservoir, as well as further valves that allow applying the pressure introduced into at least one wheel brake and/or the pressure generated by the pressure source to the pressure chamber of the master brake cylinder.

Description:
TECHNICAL FIELD  
         [0001]    The present invention generally relates to electrohydraulic braking systems for motor vehicles and more particularly relates to an electrohydraulic braking system for motor vehicles, actuatable in a ‘brake-by-wire’ mode of operation both by the vehicle operator and independently of the vehicle operator.  
         BACKGROUND OF THE INVENTION  
         [0002]    A brake system of this type is generally disclosed in international patent application WO 98/31576. In the prior art brake system, the device for adjusting the flexibility of the brake pedal includes a first pedal travel valve and a second pedal travel valve, wherein one of the pedal travel valves is arranged between the master brake cylinder and the suction side of a pump associated with the pressure source or a low-pressure accumulator, and the other one of the pedal travel valves is arranged between the master brake cylinder and the pressure side of the pump or a high-pressure accumulator associated with the pressure source. The electronic control and regulating unit is designed for controlling the two pedal travel valves in dependence on an application of the brake pedal, the said pedal travel valves being configured as proportional travel or pressure valves.  
           [0003]    The comparatively great number of control valves entailing considerable expenditure and energy requirement is considered a disadvantage of the prior-art brake system.  
         BRIEF SUMMARY OF THE INVENTION  
         [0004]    In view of the above, an object of the invention is to provide a reliably operating brake system of the type initially mentioned with a reduced number of the necessary control valves or propose a brake system that offers the same comfort and can be realized with reduced expenditure and energy requirement.  
           [0005]    This object is achieved by the present invention in that the electrically controllable valve assembly is provided by the separating valve, a pressure regulating or control valve connected to the outlet side of the separating valve and having its outlet port connected to the pressure fluid reservoir, as well as further valves, which permit applying the pressure introduced into at least one wheel brake and/or the pressure generated by the pressure source to the pressure chamber of the master brake cylinder.  
           [0006]    To render the idea of the invention more precise, it is arranged for that the separating valve is configured as a normally open (NO) two-way/two-position directional control valve with analog control, while the pressure control valve is configured as a normally closed (NC) two-way/two-position directional control valve with analog control. Alternatively, the control valve connected to the outlet side of the separating valve can be designed as a normally closed (NC) on-off valve.  
           [0007]    In another favorable embodiment of the object of the invention, the valves allowing application of the pressure introduced into at least one wheel brake to the pressure chamber ( 4 ) of the master brake cylinder are the outlet valves connected to the outlet side of the first separating valve and associated with the individual wheel brakes.  
           [0008]    It is especially favorable when the valves that permit applying the pressure generated by the pressure source to the pressure chamber of the master brake cylinder are the inlet valves associated with the individual wheel brakes and the outlet valves.  
           [0009]    In another favorable variant of the object of the invention, a pressure control valve of analog control is interposed between the pressure source and the pressure chamber of the master brake cylinder.  
           [0010]    An advantageous improvement of the object of the invention provides that the inlet valves associated with the wheel brakes are configured as normally closed (NC) two-way/two-position directional control valves.  
           [0011]    In another favorable improvement of the object of the invention, the outlet valves associated with the wheel brakes are configured as normally open (NO) two-way/two-position directional control valves.  
           [0012]    In another aspect of the invention, where the master brake cylinder is designed as a tandem master cylinder, to the second pressure chamber of which a second brake circuit is connected by the intermediary of a second separating valve, the outlet ports of the outlet valves associated with the second brake circuit and the outlet port of the second separating valve are connectable to the pressure fluid reservoir by way of an electrically actuatable on-off valve. Further, it is especially expedient when a pressure control valve with a relief pressure function is connected between the outlet port of the pressure source and the first separating valve, said pressure control valve allowing a controlled application of the pressure generated by the pressure source to the pressure chamber of the master brake cylinder when the first separating valve is open.  
           [0013]    Another advantageous aspect of the object of the invention arranges for a pressure control valve of analog control to be interposed in the connection between the pressure source and the inlet side of the inlet valves.  
           [0014]    To achieve a favorable energy balance in the brake system of the invention, in particular in the bottom pressure range, a hydraulic-mechanical means is provided between the master brake cylinder and the separating valve, permitting a pre-selected flexibility of the brake pedal in the ‘brake-by-wire’ mode of operation.  
           [0015]    Further details, features, and advantages of the invention can be taken from the following description of three embodiments by making reference to the enclosed schematic drawings, wherein like components have been assigned like reference numerals. In the drawings, 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 shows the design of the brake system of the invention according to a first embodiment in the inactive or de-energized condition.  
         [0017]    [0017]FIG. 2 shows the design of a second embodiment of the brake system of the invention in the inactive or de-energized condition.  
         [0018]    [0018]FIG. 3 shows the design of a third embodiment of the brake system of the invention in a representation corresponding to FIG. 1 or  2 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    The brake system of the ‘brake-by-wire’ type only represented in the drawings includes an actuating unit  2  operable by means of an actuating pedal designated by reference numeral  1  that is essentially composed of a dual-circuit pressure generator or master brake cylinder  3  and a non-pressurized pressure fluid reservoir  6 . The master brake cylinder  3 , in turn, includes two pressure chambers  4 ,  5  separated from each other and being in communication with the pressure fluid reservoir  6 . A wheel brake  7  associated with the right front wheel of the motor vehicle and a wheel brake  8  associated with the left rear wheel of the motor vehicle are connected to the first pressure chamber (primary pressure chamber)  4  to which a pressure sensor  13  is connected by means of a closable first hydraulic line  11 . Line  11  is closed by means of a first separating valve  9 , which is preferably configured as an electromagnetically operable, normally open (NO) two-way/two-position directional control valve of analog control. The outlet side of the first separating valve  9  is connected to the pressure fluid reservoir  6  by means of a hydraulic connecting line  16  in which an electromagnetically operable, normally closed (NC) two-way/two-position directional control valve  17  of analog control is inserted. The first separating valve  9  and the two-way/two-position directional control valve  17  of analog control form an electrically controllable device which permits a pre-selected flexibility of the brake pedal  1  in the ‘brake-by-wire’ mode of operation and, thus, imparts the usual brake pedal feeling to the driver.  
         [0020]    The second pressure chamber  5  of the master brake cylinder  2  is connectable to the other pair of wheel brakes  14 ,  15  associated with the right rear wheel of the motor vehicle and the left front wheel of the motor vehicle by way of a second hydraulic line  12  closable by means of a second separating valve  10 . The second separating valve  10  is configured as an electromagnetically operable, normally open (NO) on-off valve, whose outlet side is connectable to the pressure fluid reservoir  6  by the intermediary of a likewise electromagnetically operable, normally open (NO) on-off valve  18 . In all other respects, the design of the hydraulic circuit connected to the second pressure chamber  5  of the master brake cylinder  2  is identical to the brake circuit  11  that has been explained in the preceding description so that it need not be discussed in the following text.  
         [0021]    As can be taken from the drawings in addition, a motor-and-pump assembly  20  with a high-pressure accumulator  21  is used as an auxiliary pressure source, said assembly, in turn, comprising a pump  23  driven by means of an electric motor  22 . The suction side of the pump  23  is connected to the inlet side of the above-mentioned pressure control valve  17  or the outlet side of the first separating valve  9  by way of a non-return valve  34 . The outlet of the high-pressure accumulator  21  is connected by way of a pressure-limiting valve  24  to a line portion  31  that connects the first separating valve  9  to the two-way/two-position directional control valve  17  of analog control. The fill condition of the high-pressure accumulator  21  is monitored by means of a travel sensor  25  (only represented).  
         [0022]    A third hydraulic line  26  connects the pressure side of the pump  23  or the outlet of the high-pressure accumulator  21  to the inlet side of two electromagnetically operable, preferably normally closed (NC) two-way/two-position directional control valves or inlet valves  27 ,  28  of analog control, said valves being connected upstream of the wheel brakes  7  and  8  serving to build up hydraulic pressure in these wheel brakes. Two electromagnetically operable, preferably normally open (NO) two-way/two-position directional control or outlet valves  29 ,  30  of analog control are used to decrease the hydraulic pressure introduced into the wheel brakes  7 , 9 , the outlet ports of said valves being connected to the above-mentioned line portion  31 . In addition, a pressure sensor  32  is associated with the wheel brake  7  and aids in determining the hydraulic pressure that prevails in the wheel brake  7 . An electronic control and regulating unit ECU  34  is used for the actuation of the motor-and-pump assembly  20 , the above-mentioned valves  9 ,  10 ,  17 ,  18 ,  27 ,  28 ,  29 ,  30  and the pressure control valves associated with the second brake circuit  12 . In particular the output signals of the pressure sensors  13 ,  32 , of the travel sensor  25  and a brake-request acquisition device  33  that is preferably designed redundantly and associated with the master brake cylinder  3  are sent to said ECU.  
         [0023]    In the second design of the electrohydraulic brake system of the invention as illustrated in FIG. 2, a two-way/two-position directional control valve  35  with pressure relief function of analog control is interposed between the outlet of the high-pressure accumulator  21  and the line portion  31  mentioned in connection with FIG. 1. Besides, a normally closed (NC) two-way/two-position directional control valve  36  of analog control is provided between the outlet of the high-pressure accumulator  21  and the hydraulic line  26 , said valve allowing a purposeful application of a defined pressure fluid volume to the wheel brakes  7 ,  9  when a pressure, equal zero is adjusted in line  26 . The pressure values determined by pressure sensors  32  may then be evaluated for detecting gases disposed in the system.  
         [0024]    Finally, both separating valves  9 ,  10  associated with the pressure chambers  4 ,  5  of the master brake cylinder  2  are designed as normally open (NO) two-way/two-position directional control valves with analog control in the third embodiment shown in FIG. 3. The outlets of the separating valves  9 ,  10  are connected to the outlet side of the outlet valves  29 ,  30 , on the one hand, and to the pressure fluid reservoir  6  by way of normally closed on-off valves  37 , on the other hand. Besides, the outlet of the high-pressure accumulator  21  is connected to the first pressure chamber  4  of the master brake cylinder  2  by way of a normally closed (NC) two-way/two-position directional control valve  38  of analog control.  
         [0025]    The mode of function of the brake system of the invention illustrated in FIG. 1 is explained in detail in the following text.  
         [0026]    In the non-depressed condition of brake pedal  1  all components assume their positions shown in the drawings, corresponding to the non-energized condition or standby condition. When in the preferred brake-by-wire mode of operation the brake pedal  1  is depressed or a pressure-increase phase initiated, the brake-request acquisition device  33  will produce control signals being sent to the above-mentioned electronic control unit  34 . Unit  34  produces actuating signals used to switch over the separating valves, pressure control valves and on-off valves described in the preceding text. These actuating signals serve for controlled activation of the first separating valve  9 , for closing the second separating valve  10  and opening the pressure control valve  17 . The on-off valve  18  remains closed which is arranged in the connection between the outlet side of the outlet valves associated with the second brake circuit  12  and the pressure fluid reservoir  6 . Simultaneously all outlet valves  29 ,  30  are closed and all inlet valves  27 ,  28  opened so that the pressure delivered by the high-pressure accumulator  21  is applied to the wheel brakes. Displacement of a pressure fluid volume through the first separating valve  9  activated in controlled manner and the open pressure control valve  18  into the pressure fluid reservoir  6  imparts a pedal feeling to the drive that corresponds to the pedal feeling of a conventional brake system.  
         [0027]    A phase of maintaining the pressure constant is initiated by switching the inlet valves  27 ,  28  over into their closed position.  
         [0028]    The inlet valves  27 ,  28  are closed and the corresponding outlet valves  29 ,  30  opened in a controlled way during pressure reduction. Besides, the valves  17 ,  18  arranged in the lines that lead to the pressure fluid reservoir  6  are also opened.  
         [0029]    In a release operation, initially the pressure fluid volume consumed in the wheel brakes  7 ,  8  is used to reset the master cylinder pistons. As this occurs, said pressure fluid volume flows through the now open outlet valves  29 ,  30  and the opened separating valve  9  into the first pressure chamber  4  of the master brake cylinder  2 . Because this volume is not sufficient in some cases to fully reset the master cylinder piston, the inlet valves  27 ,  28  will be opened as well so that pressure fluid volume can be supplied additionally from the high-pressure accumulator  21 . Using the volume out of the wheel brakes is favorable under energetic aspects.  
         [0030]    The release operation takes place similarly also in the embodiment according to FIG. 3. The above-mentioned on-off valves  37  are closed for this purpose, and the replenishment of the pressure fluid volume provided by the high-pressure accumulator  21  takes place through the pressure control valve  38  in a controlled manner, with inlet valves  27 ,  28  closed. The outlet valves  29 ,  30  are opened at the same time.  
         [0031]    In the event of power failure e.g. caused by a battery defect, a short-circuit or switch-off of the ignition, the brake system of the invention will automatically change over to a fallback mode of operation rendering braking operations by the driver possible. As this occurs, the separating valves  9 ,  10  and the outlet valves  29 ,  30  are switched to assume the inactive switch position shown in the drawing so that a hydraulic connection between the master brake cylinder  3  and the wheel brakes is opened, through which pressure increase can take place.