Abstract:
To achieve a specifiable, and thus reproducible final pressure in at least one part of the brake circuit, the pump is first actuated for pressure buildup or pressure reduction when the valve is closed. When a first specified setpoint pressure is reached, or when a pressure threshold value is exceeded, the valve is put into an open position using a first actuation. After the pump is switched off, the actuation of the valve is modified in such a way that, during a specifiable time, a continuous change of the first actuation is undertaken all the way up to a second actuation, in which the valve assumes a holding position, particularly while taking into consideration the pressure difference which prevails at the valve.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and a device for controlling a hydraulic or pneumatic braking system. 
       BACKGROUND INFORMATION 
       [0002]    Hydraulic braking systems for motor vehicles are known in many different variations. Such a braking system has at least one brake circuit in which at least one arrangement that conveys the pressure medium, a pump, in particular, is situated which is also designated as a return pump. In addition, at least one further arrangement for conveying the pressure medium may optionally be provided, especially a self-priming charge pump, which is connected via a suction line to a storage container for the pressure medium. The inlet and outlet of the pressure medium into the brake circuit is controlled, just as is the inlet and outlet of the pressure medium into the respective wheel brake cylinder, via cutoff devices for the inlet and/or the outlet and/or the passage of the pressure medium, particularly via valves. This is discussed in DE 195 46 682 A1, for example. In an electrohydraulic braking system, such as the one in DE 195 48 248 A1, one also finds this arrangement in principle, in which the pressure medium is supplied to valves or comes from valves by a pump via an interposed pressure reservoir. In this context, the pressure medium is introduced into the wheel brake cylinder or let out from it by opening and closing the inlet and outlet valves according to the braking command of the driver and/or the activating signals of a mediating logic, such as an antilock system, a traction control system or an electronic stability program. 
         [0003]    With that, the described arrangement in principle applies in the same way, for instance, in a hydraulic braking system by a return pump having inlet and outlet valves or a charge pump having charge valves or switchover valves, and also as, for instance, in an electrohydraulic braking system by a turbine pump having inlet and outlet valves. 
         [0004]    In the case of an unregulated pump actuation, in response to a pressure buildup or a pressure drop, in an emergency, there may be a noise development and/or vibrations which may have a disturbing effect in the passenger compartment. For this purpose, it may be provided that the pump is shut off after a pressure buildup or a pressure drop has taken place. However, in such a design approach it is a disadvantage that the final pressure created in the brake circuit, in this context, is not reproducible after the pump is shut off. The reason for this, among other things, is that the slowing down of the pump varies, based on different boundary conditions, such as the load, the friction, the rotary speed, the wear or the temperature that occur, and consequently supplies differently sized contributions to the final pressure. In addition, however, the switching performance of the valves present in the brake circuit is normally not clearly specifiable, since each valve has tolerances within certain limits based on the pressure present at it. 
       SUMMARY OF THE INVENTION 
       [0005]    Consequently, an object of the exemplary embodiments and/or exemplary methods of the present invention is to achieve a reproducible final pressure by controlling at least one switchover valve. 
         [0006]    The exemplary embodiments and/or exemplary methods of the present invention describes a method and a device for controlling an hydraulic or pneumatic braking system. It is provided, in the process, that the braking system has at least one brake circuit, whose pressure is able to be increased or decreased using at least one electrically controllable pump motor. Furthermore, at least one electrically controllable valve is provided, such as in the form of a switchover valve, which may be used for pressure regulation in the brake circuit, and is moved to an open state when it is switched without current. 
         [0007]    In order to achieve a specifiable, and thus reproducible final pressure in at least one part of the brake circuit, the pump is first controlled for pressure buildup or pressure drop with the valve closed. When a first predefined setpoint pressure is reached, or when a pressure threshold value is exceeded, the valve is put into an open position using a first actuation. According to the exemplary embodiments and/or exemplary methods of the present invention, after shutting off the pump, the control of the valve is modified in such a way that, during a specifiable time, a continuous change of the first actuation is undertaken all the way up to a second actuation, in which the valve takes on a holding position, particularly while taking into consideration the pressure difference present at the valve. 
         [0008]    Because of such an actuation of the valve, a specifiable, and thus reproducible pressure is able to be set in the brake circuit of the braking system, independent of the slowing down of the pump after shutoff. 
         [0009]    In one embodiment of the present invention, it is provided that the first actuation should take place using a starting current, and the second control using a hold current. The starting current may expediently be greater, in this instance, than the hold current, the reverse actuation being also possible. 
         [0010]    In response to a (high) present pressure difference, the valve advantageously requires a lower current consumption than when no pressure difference, or only a small one, is present at the valve. 
         [0011]    In order to determine the starting current and/or the specifiable time, in which, for instance, the starting current is brought continuously to the hold current, the slowing down of the pump is recorded after its shutoff. Thus one may conclude, for instance, what the required starting current should be and the required specifiable time, from one or more past pump cycles. In this context, typical variables which represent the slowing down of the pump are the rotary speed, the regenerative voltage and the time up to which the pump comes to rest without actuation. 
         [0012]    In one refinement of the exemplary embodiments and/or exemplary methods of the present invention, the continuous change of the first actuation of the valve is undertaken in such a way on the second actuation that a second setpoint pressure is set in the brake circuit separated by the valve. The time in which the continuous change is undertaken is advantageously specified as a function of the second setpoint pressure, in this context. 
         [0013]    In order to achieve the pressure in the brake circuit, it is provided in one embodiment of the present invention that one should close the valve by the second actuation or the hold current. 
         [0014]    For the take-up of the pressure in the brake circuit, a pressure sensor or a model for estimating the pressure may optionally be used. Typical models use pump parameters, such as the rotary speed, the actuation and/or the slowing down voltage during the actuation, in order to estimate the volume conveyed. In addition, however, it is also possible to estimate the volume flow through the valve to obtain the pressure in the brake circuit. 
         [0015]    Further advantages result from the following description of exemplary embodiments, and from the dependent patent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  schematically shows a brake circuit of a braking system in a block diagram. 
           [0017]      FIG. 2  shows a device according to the present invention. 
           [0018]      FIG. 3   a  shows a known valve actuation as a function of a corresponding pump actuation. 
           [0019]      FIG. 3   b  shows a known valve actuation as a function of a corresponding pump actuation. 
           [0020]      FIG. 3   c  shows a known valve actuation as a function of a corresponding pump actuation. 
           [0021]      FIG. 3   d  shows a known valve actuation as a function of a corresponding pump actuation. 
           [0022]      FIG. 3   e  shows a known valve actuation as a function of a corresponding pump actuation. 
           [0023]      FIG. 4   a  shows the valve actuation according to the present invention, also as a function of the corresponding pump actuation. 
           [0024]      FIG. 4   b  shows the valve actuation according to the present invention, also as a function of the corresponding pump actuation. 
           [0025]      FIG. 4   c  shows the valve actuation according to the present invention, also as a function of the corresponding pump actuation. 
           [0026]      FIG. 4   d  shows the valve actuation according to the present invention, also as a function of the corresponding pump actuation. 
           [0027]      FIG. 4   e  shows the valve actuation according to the present invention, also as a function of the corresponding pump actuation. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]      FIG. 1  shows schematically a hydraulic vehicle braking system  10  having a dual-circuit brake master cylinder  12 , to which two mutually independent brake circuits I and II are connected. To simplify things, however, Drawing  1  shows only brake circuit I, the other brake circuit II being correspondingly implemented. It should, however, be explicitly mentioned that correspondingly outfitted pneumatic vehicle brake systems are able to be operated using the method according to the present invention. 
         [0029]    A branching main brake line  14  runs from brake master cylinder  12  to two wheel brakes  15  and  16  that are connected to brake circuit  1 . In the present exemplary embodiment, the two wheel brakes  15  and  16  are assigned to a front wheel and a diagonally opposite rear wheel of the appertaining vehicle, brake circuit II being assigned to the other two wheels. However, besides this so-called X-braking apportioning, any other apportioning of the brake circuits is also conceivable. 
         [0030]    In a common part of main brake line  14 , a switchover valve  18  is situated between brake master cylinder  12  and pump  26 . In addition, in the branched parts of main brake line  14  there are situated two inlet valves  20  and  21 , that are open in their basic setting, which are respectively connected upstream of wheel brakes  15  and  16 . A uniting return line  22 , in which an outlet valve  24  and  25 , that are closed in their basic setting, is provided for each of the two wheel brakes  15  and  16 , leads from wheel brakes  15  and  16  to the suction side of a hydraulic pump  26 , which may also be designated as a return pump. In addition, a hydraulic reservoir  28  is connected to return line  22 , which is able to accommodate brake fluid from wheel brakes  15  and  16  in case the brake pressure is to be reduced, when pump  26  is switched off and outlet valves  24  and  25  are open. The pressure side of hydraulic pump  26  is optionally connected to main brake line  14  via a damping chamber  30  and a throttle  32  between switchover valve  18  and inlet valves  20  and  21 . The suction side of hydraulic pump  26  is connected to a brake master cylinder  12  via a suction line  34 , in which a suction valve  36  is situated that is closed in its basic setting. The pressure in brake master cylinder  12 , which corresponds to the braking command of the driver by his operation of the brake pedal, is able to be picked up with the aid of an appropriate sensor  49 . 
         [0031]    Hydraulic pumps  26  of shown, and not shown brake circuits I and II, for instance, a 6-piston pump, are able to be driven using separate pump motors. However, in a further exemplary embodiment it is also possible to drive the pumps using a common electric pump motor  38 . Switchover valve  18 , inlet valves  20  and  21 , outlet valves  24  and  25  and high-pressure switching valve  36  are provided in the present exemplary embodiment as solenoid valves which, among other things, are able to be controlled for lockup protection control and for traction control using an electronic control unit  40 . This control unit  40  may also assume the control of pump motor  38 , and it receives signals from wheel sensors  42  and possibly at least wheel pressure sensor  48 , the sensors being able to be evaluated so as to establish the lockup tendency of a vehicle wheel during braking or the slip when driving off. In addition, control unit  40  receives a signal from an accelerator sensor (brake light switch)  44 , using which the operation of brake master cylinder  12  is able to be established. 
         [0032]      FIG. 2  schematically shows a control device  100 , having a control unit  160 , for instance, a microprocessor, which supplies actuating signals to switchover valve  130  as a function of the operation of pump  110 . Control device  100  is typically identical with control unit  40  in  FIG. 1 , but it may also be provided that control device  100  is developed separately, for instance, in a decentralized manner. Alternatively or in addition, control device  100  may also evaluate data on pressure response p(t) and volume flow Q(t) in the brake circuit, and take these into account in the actuation of switchover valve  130 . Arrangement  120  may be provided for recording the pressure response and the volume flow, which supply the parameters required for this. For recording the pressure, a pressure sensor may be provided in the brake circuit, for instance, in this context, while using subtraction of various pressures in the braking system may be provided for ascertaining the volume flow. It should further be mentioned that the pressure or the pressure response in the brake circuit may also be concluded from the operating parameters of pump  110  and  26 . The (pump-specific and valve-specific) parameters t* as well as I 0 , I 1  and I 2  required for the actuation of switchover valve  130  may be stored in a memory  140 . These parameters are able to be modified via an external interface  150 . 
         [0033]      FIGS. 3   a ,  3   b ,  3   c ,  3   d  and  3   e  show the various actuation parameters or operating parameters of the braking system as a function of time, in response to a typical known actuation of a switchover valve  130 . In  FIG. 3   a , actuating current I P  and rotary speed n of pump  110  are shown, and in this instance, at time t 0  the pump is switched on, and at time t 1  it is switched off again. Because of a delay on account of inertia, speed n of the pump does not increase in a stepwise manner after it is switched on, but in a parallel shape, at first having a certain offset in time, up to the maximum speed. Correspondingly, the kinetic energy stored in the pump keeps the pump in motion after the switching off of the actuating current I P  after time t 1 , until the damping caused by the hydraulic medium damps the pump down to a standstill. During this slowing down of the pump a regenerative voltage is generated in the pump which may be acquired.  FIG. 3   b  shows the pressure curve p(t) in the brake circuit, beginning with an initial pressure or starting pressure p S . The pressure in the brake circuit rises parabolically, in this case, after the pump is switched on at time t 0 , until it has reached the maximum pressure at time t 1 . Actuation I USV  of the switchover valve is shown in  FIG. 3   c . In this instance, the assumption is that the switchover valve is open in the current-free state. 
         [0034]    In order to achieve a more rapid closing of the switchover valve, in one exemplary embodiment, the switchover valve has a slight current I 0  preapplied to it, while in another exemplary embodiment this preapplication of current may be omitted. At point t 1 , the closed-loop control of the switchover valve is started, in which the valve is actuated using a current I 1 . Because of this current I 1  (&gt;I 0 ), the valve is put into a partially open state, so that the opening or the lift of the valve (see  FIG. 3   e ) is a function of volume flow Q USV  (see  FIG. 3   d ) through the valve. As may be recognized in taking an overall view of  FIGS. 3   a  to  3   e , volume flow Q USV  rises parabolically after the pump is switched on. By using a switchover valve which has an opening response that is a function of the pressure difference, a large volume flow Q USV  holds the switchover valve open. Consequently, the lift of the switchover valve also increases parabolically up to point t 1 , at which the pump is switched off. After the pump is switched off, the final achievable pressure or holding pressure p H  depends, among other things, on the slowing down of the pump and on the opening response of switchover valve  18 . Since neither time t 3  up to which the pump arrives at a standstill (pump slowing-down time), nor the transition of the valve from the partially open state (closed-loop control) to the closed state is reproducible because of the many boundary conditions (load, friction, rotary speed of the pump, temperature, volume flow via the switchover valve), a specified holding pressure p H  cannot be set in the brake circuit. 
         [0035]    Corresponding to the curves with respect to time in  FIGS. 3   a  to  3   e , in  FIGS. 4   a ,  4   b ,  4   c ,  4   d  and  4   e , the curves are shown that are in response to the method according to the present invention. The switching response of the pump in  FIG. 4   a , in this instance, corresponds to the switching response as shown in  FIG. 3   a . However, whereas the holding pressure is not definably specifiable in the current actuation of the switchover valve,  FIG. 4   b  shows that, on account of the method according to the present invention, a reproducible final pressure p H  is able to be achieved. Along with this, as is described in  FIG. 4   c , the switchover valve, upon the pump being switched on at time t 1 , is actuated at first using a specifiable current I 2 , which is greater than holding current I 1 . After the pump is switched off, current I 2  is continuously brought down to holding current I 1 , at which the switchover valve remains closed. It should be noted, in this context, that time t*, in which the current is brought down, has to be greater than the slowing down of the pump, that is, 
         [0000]        t *=Δ( t   3   −t   1 )&gt;Δ( t   2   −t   1 ).
 
         [0036]    In  FIG. 4   d  it may thus be recognized that the slowing down response of the pump, first of all up to time t 2 , determines volume flow Q USV  and also lift USV  of the switchover valve. Holding current I 1  is selected in such a way, in this instance, that the valve constantly remains open at least partially in response to a specified volume flow Q*. The switchover valve closes only after an equalization of the pressure difference over the valve is achieved at time t 3 , and thus the volume flow dies down. 
         [0037]    By specifying the time t* at which current I 2  is brought down to holding current I 1 , a specified holding current p H  may thus be achieved in the brake circuit, or vice versa. On the other hand, the time of slowing down of the pump has no further influence on the setting of the holding pressure. 
         [0038]    Time t* may be specified specifically for the pump, based on values from experience for the slowing down phase of the pump used, and stored in a memory  140 . It is advantageous, in this context, if this value is able to be implemented or overwritten when pump  110  is exchanged by a service technologist  150 . A further possibility in the ascertainment and the specification of time t* is that, during the operation of the braking system, continuous data on the slowing down phase of the pump are collected and stored in memory  140 . From these data, for example, using weighted average formation, one may conclude what the currently required time Δ(t 2 −t 1 ) is for the slowing down phase of the pump. By the addition of a buffer time for this required time, time t* is then able to be determined. 
         [0039]    In one additional specific embodiment it is provided that the switchover valve be opened and thus overflowed, after the switching on of the pump, and when a certain setpoint pressure (p S +x) is achieved. Because of the volume flow required for the pressure equalization, the switchover valve subsequently remains open until the pump is shut down. Because of the pressure equalization in the brake circuit, and because of the specified lowering of actuating current I USV , at the switchover valve from I 2  to I 1 , the switchover valve is slowly closed. Actuating current I 1  secures the closed state of the switchover valve.