Method for controlling an electronically slip-controllable power braking system

Abstract

A method for controlling an electronically slip-controllable power braking system of a motor vehicle. Power braking systems are equipped with a plunger unit, which includes a plunger piston accommodated in a plunger cylinder and delimiting a working chamber, for generating brake pressure in brake circuits. This plunger piston is actuatable by an electronically activatable drive in a pressure buildup direction or in the opposing spatial direction thereto in a pressure reducing direction. A time-limited drive of the plunger piston takes place in the pressure reducing direction as soon as an actual brake pressure generated by the plunger unit has reached a predefinable setpoint brake pressure and a decoupling of a wheel brake and the plunger unit from an associated brake circuit has taken place.

Claims

1. A method for controlling an electronically slip-controllable power braking system for a motor vehicle, the power braking system being equipped with a unit for at least indirectly specifying a setpoint brake pressure, a plunger unit, which, for generating an actual brake pressure in a brake circuit of the vehicle braking system, includes a displaceable plunger piston accommodated in a plunger cylinder which delimits a working chamber, the plunger piston being displaceable by an electronically controllable drive in a pressure buildup direction or in the opposite spatial direction thereto, in a pressure reducing direction, a wheel brake, which is controllably coupled to the brake circuit and to which the actual brake pressure may be applied, and an electronic control unit, to activate the drive of the plunger unit and to control the coupling of the wheel brake to the brake circuit to adapt the actual brake pressure to the setpoint brake pressure in consideration of the slip conditions prevailing at a wheel associated with the wheel brake, the method comprising: with the aid of a corresponding activation of the drive by the electronic control unit, actuating the plunger unit in the pressure reducing direction as soon as the actual brake pressure generated by the plunger unit has reached the predefined setpoint brake pressure and a decoupling of the wheel brake and the plunger unit from the brake circuit has been carried out.

2. The method as recited in claim 1, wherein the decoupling of the wheel brake from the brake circuit is carried out by the electronic control unit by electronic activation of a pressure buildup valve, which is associated with the wheel brake and by which a buildup of brake pressure in the wheel brake is controllable.

3. The method as recited in claim 1, wherein a decoupling of the plunger unit from the brake circuit is carried out by the electronic control unit by a withdrawal of an electronic activation of plunger control valves, using which a pressure medium connection of the plunger unit to the brake circuit is controllable.

4. The method as recited in claim 1, wherein the drive of the plunger unit in the pressure reducing direction is carried out when an unstable rolling behavior at a wheel associated with the wheel brake is established by the electronic control unit.

5. The method as recited in claim 4, wherein the unstable rolling behavior of the wheel may be established by the electronic control unit based on a comparison between a vehicle speed and a speed of the wheel at a circumference when a deviation between the vehicle speed and the speed of the wheel at the circumference is established which exceeds a limiting value which is established in the electronic control unit.

6. The method as recited in claim 1, wherein the plunger piston of the plunger unit is driven in the pressure reducing direction when wheel-specific control of the brake pressure is not carried out at the wheel brake.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Example embodiment of the present invention are shown in the figures and are explained in detail below.

(2) FIG. 1 shows the pressure-medium-conducting layout of the electronically slip-controllable power braking system, which was discussed above and on which the present invention is based.

(3) FIG. 2 shows a diagram including multiple pressure/volume characteristic curves of a braking system which result when the plunger piston of the plunger unit is driven by its drive in the pressure buildup direction.

(4) FIG. 3 shows the curve of the vehicle speed and the speed of the wheels at the circumference during a brake application with wheel-specific brake pressure control, for example, a brake application with antilock braking control is shown over time.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(5) In diagram 200 of FIG. 2, the volume of working chamber 70 of plunger unit 60 is plotted on X axis 202 and the pressure generated by this plunger unit 60 in one of brake circuits 14, 16 of the power braking system is plotted on Y axis 204. The characteristic curves shown thus indicate pressure/volume characteristic curve 206, 208 of a plunger unit 60 which results when plunger piston 62 is actuated by drive motor 66 in the pressure buildup direction, i.e., in the direction of a shrinking working chamber 70.

(6) The distance along X axis 202 from Y axis 204 to a vertical line 210 at the right end of diagram 200 illustrates the maximum volume (100%) of working chamber 70 of plunger unit 60, i.e., the volume of working chamber 70 at the beginning of a movement of plunger piston 62 by its drive motor 66.

(7) A dashed horizontal line 212 at the upper end of diagram 200 indicates a braking intention, or a setpoint brake pressure to be set by plunger unit 60, respectively. This setpoint brake pressure is specified either by the driver actuating brake pedal 12 of power braking system and/or by electronic control unit 108 in the case of an automatic brake application, which is to be carried out independently of the driver.

(8) Plunger piston 62 displaces pressure medium out of working chamber 70 of plunger unit 60 into brake circuits 14, 16 by electrical activation of drive motor 66 of plunger unit 60 in the pressure buildup direction and the volume of this working chamber 70 therefore decreases continuously. In return, the brake pressure continuously rises in brake circuits 14, 16 of the power braking system from the original atmospheric pressure. The setpoint brake pressure (horizontal line 212) is reached when plunger piston 62 in plunger cylinder 68 has moved into position 214. Parabolically curved pressure-volume characteristic curve 206 intersects horizontal line 212 here. The distance from position 214 on X axis 202 up to vertical line 210 at the right end of diagram 200 indicates the remaining residual volume of working chamber 70 of plunger unit 60. It may be used as needed to carry out a pressure control on individual wheels and is only still a fraction, in diagram 200, for example, approximately 50%, of the originally provided volume of working chamber 70.

(9) After the setpoint brake pressure has been reached, pressure buildup valves 38 through 44 are activated by electronic control unit 48 until further notice in such a way that they switch from their passage position into their blocking position and thus confine the brake pressure in wheel brakes 22 through 28. If plunger control valves 72, 74 are moved into their blocking position parallel thereto and plunger piston 62 is actuated by drive motor 66 in the pressure reducing direction, i.e., in the direction of working chamber 70 becoming larger, due to this movement of plunger piston 62 in working chamber 70, a pressure results which is less than the atmospheric pressure prevailing in reservoir 48. The pressure gradient at the check valve of supply line 76 opens this check valve and pressure medium from reservoir 48 flows into enlarging working chamber 70 of plunger unit 60. In plunger unit 60, the stored pressure medium volume increases from 50% up to this point, for example, back to 70%, for example, of its original volume and is available there for subsequent brake applications. Plunger piston 62 has moved from position 214 into position 216. The larger volume of working chamber 70 in position 216 is readable in diagram 200 on the basis of the distance from vertical line 210, which has become greater in relation to position 214. Characteristic curve 208 corresponds in its profile to characteristic curve 206, but is plotted once again at another point of diagram 200 due to the position shift of plunger piston 62.

(10) Due to the explained volume replenishment, a plunger unit 60 may control a larger number of successive pressure buildup procedures until reaching the minimal volume of its working chamber 70 before a required volume replenishment has to be carried out. Accordingly, plunger unit 60 is available for brake pressure control over a greater fraction of its operating time. Alternatively, an existing plunger unit 60 may be used to supply higher-performance wheel brakes, i.e., wheel brakes which have a greater demand for pressure medium, without a separately adapted, larger-dimensioned plunger unit 60 being used for this purpose.

(11) The volume replenishment of working chamber 70 of plunger unit 60 takes place, as explained, following a completed setting of a specified setpoint brake pressure by plunger unit 60. It is unimportant for carrying out the volume replenishment whether working chamber 70 of plunger unit 60 has already reached the fixed minimum value at this point in time or not. The volume replenishment may be triggered in brake applications without wheel-specific pressure control or also at the beginning of brake applications with wheel-specific brake pressure control. In the case of wheel-specific brake applications, however, a suitable triggering time is to be selected for the volume replenishment.

(12) This suitable triggering time during a brake application with wheel-specific pressure control is illustrated on the basis of the diagram according to FIG. 3. Diagram 300 shows X axis 302 as the time axis and Y axis 304 as a velocity axis. The plotted first characteristic curve 306 indicates the profile of the vehicle speed of a braked vehicle and second characteristic curve 308 indicates the speed of the wheels at the circumference of a braked wheel of this vehicle. At the beginning of the brake application shown, the vehicle speed and the speed of the wheels at the circumference are initially equal, thereafter the vehicle speed decreases continuously, so that first characteristic curve 306 is characterized by a continuous curved profile. The speed of the wheels at the circumference shown (second characteristic curve 308), in contrast, has a cyclic, wavy profile, since unstable rolling conditions, for example, because of the prevailing slip conditions, exist at the affected wheel. This wheel is thus initially decelerated significantly more strongly than the overall vehicle until the speed of the wheels at the circumference finally decreases enough that the wheel threatens to lock up. A risk of locking and/or a wheel which only still rotates very slowly is detected by wheel speed sensors 100 through 106 and transmitted as electronic information to electronic control unit 108. Electronic control unit 108 thereupon moves pressure buildup valve 38 through 44 of the affected wheel into its blocking position and opens pressure reducing valve 50 through 56 to discharge pressure medium from affected wheel brake 22 through 28 into reservoir 48 and thus reduce the applied brake pressure. A gradual acceleration of the rotational movement of the wheel accompanies the reduction of the brake pressure and thus a renewed increase of the speed of the wheels at the circumference, until the vehicle speed and the speed of the wheels at the circumference have approached one another again or are at least approximately equal. Subsequently thereto, the explained cycle begins due to a further application of the brake pressure still prevailing in brake circuits 14, 16 to wheel brakes 22 through 28. For this purpose, wheel brakes 22 through 28 are again connected to brake circuits 14, 16 to conduct pressure medium by opening pressure buildup valves 38 through 44.

(13) By way of a comparison of the speeds of vehicle and wheel during a brake application in progress with wheel-specific pressure control in electronic control unit 108, braking phases are detectable in which a relatively large deviation exists between vehicle speed (first characteristic curve 306) and speed of the wheels at the circumference (second characteristic curve 308) or, in other words, braking phases during which unstable rolling conditions exist at one or multiple wheels. Such a braking phase is apparent in diagram 300 between the two plotted vertical lines 310 and 312. During such braking phases, a brake pressure buildup does not take place in brake circuits 14, 16 or in one of wheel brakes 22 through 28 and therefore an actuation of plunger unit 60 does not occur. Inactive plunger unit 60 may therefore be decoupled from brake circuits 14, 16 by a withdrawal of the electronic activation of plunger control valves 72, 74 and simultaneously plunger piston 62 may be driven by electronic control unit 108 in the pressure reducing direction without these measures having an effect on the pressure conditions in brake circuits 14, 16 or in wheel brakes 22 through 28. Unstable rolling states at one or multiple wheels of the vehicle accordingly suggest that a volume replenishment of plunger unit 60 be performed.

(14) As soon as vehicle speed (first characteristic curve 306) and speed of the wheels at the circumference (second characteristic curve 308) have equalized with one another again in the relevant braking phase and therefore stable rolling conditions are established again at the affected wheel, the volume replenishment of plunger unit 60 is ended. For this purpose, plunger control valves 72, 74 and pressure buildup valves 38 through 44 are opened by appropriate activation of electronic control unit 108 and plunger piston 62 is possibly driven in the pressure buildup direction again for a subsequent brake pressure buildup in brake circuits 14, 16. As a function of the duration available between vertical lines 310 and 312, a partial or complete filling of working chamber 70 with pressure medium may take place due to the drive of plunger unit 60 in the pressure reducing direction. However, the filling only takes place when a minimum filling duration is available. In general, the time window available for the volume replenishment is not sufficient to fill up working chamber 70 with pressure medium back to 100%. Nonetheless, on the basis of the provided method, the operating time of plunger unit 60 up to which the minimum volume of working chamber 70 is reached and a pressure medium replenishment necessarily has to be carried out may be significantly extended in relation to the related art.

(15) Of course, modifications or additions to the described exemplary embodiment are possible without departing from the basic embodiments of the present invention.