Method for operating a brake device, and control unit

Abstract

A method is described for operating a hydraulic braking device of a vehicle, in particular a motor vehicle, including at least one brake booster, which sets a brake boost as a function of a driver's braking command. It is provided that the driver's braking command is ascertained via a vacuum sensor assigned to a pneumatic brake booster and a hydraulic brake boost is set.

Claims

1. A method for operating a hydraulic braking device of a vehicle that includes at least one brake booster which sets a hydraulic brake boost as a function of a braking command of a driver, comprising: ascertaining the braking command via a vacuum sensor assigned to a pneumatic brake booster; setting the hydraulic brake boost; detecting a pressure value by the vacuum sensor; comparing the detected pressure value to at least one first specifiable limiting value; setting a first brake boost when the first limiting value is exceeded; comparing the detected pressure value to a second specifiable limiting value; if the second limiting value is exceeded, setting a second hydraulic brake boost, wherein: the second limiting value is higher than the first limiting value, and the second brake boost is higher than the first brake boost; comparing the detected pressure value to a third specifiable limiting value; and if the third limiting value is exceeded, setting a third hydraulic brake boost, wherein: the third limiting value is higher than the second limiting value, and the third brake boost is higher than the second brake boost.

2. The method as recited in claim 1, wherein the vehicle is a motor vehicle.

3. The method as recited in claim 1, wherein the hydraulic brake boost is set as a function of at least one of a wear, a service life, and an operating temperature of the braking device.

4. The method as recited in claim 1, wherein the hydraulic brake boost is set as a function of a current road gradient.

5. A control unit operating a hydraulic braking device of a vehicle that includes at least one brake booster which sets a hydraulic brake boost as a function of a braking command of a driver, comprising: an arrangement which ascertains the braking command via a vacuum sensor assigned to a pneumatic brake booster; an arrangement which sets the hydraulic brake boost; and an arrangement which: detects a pressure value by the vacuum sensor, compares the detected pressure value to at least one first specifiable limiting value, a first brake boost being set when the first limiting value is exceeded; compares the detected pressure value to a second specifiable limiting value, a second hydraulic brake boost being set if the second limiting value is exceeded, the second limiting value being higher than the first limiting value, and the second brake boost being higher than the first brake boost; and compares the detected pressure value to a third specifiable limiting value, a third brake boost being set if the third limiting value is exceeded, the third limiting value being higher than the second limiting value, and the third brake boost being higher than the second brake boost.

6. The control unit as recited in claim 5, wherein the control unit is a brake system control unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an advantageous method for operating a braking device of a motor vehicle.

(2) FIG. 2 shows a relationship between a brake booster prechamber pressure detected by a vacuum sensor and a brake master cylinder pressure.

DETAILED DESCRIPTION

(3) FIG. 1 shows a diagram on the basis of which an advantageous method for operating a braking device of a vehicle is to be explained. For this purpose, FIG. 1 shows, plotted in a diagram over time t, a vehicle deceleration a.sub.x while a braking operation is being carried out, pressure p.sub.mc in a brake master cylinder of the braking device and braking pressure p.sub.h additionally set by a hydraulic brake boost.

(4) It is assumed that at instant t.sub.0 a brake pedal of the braking device is being operated by the driver of the vehicle. As a result, pressure p.sub.mc in the brake master cylinder increases in accordance with the brake pedal actuation up to an instant t.sub.2 at which the brake pedal has reached its end position as set by the driver. The presently considered braking device includes a pneumatic brake booster that automatically increases the braking pressure beyond the braking pressure specified by the driver. This produces a vehicle deceleration ax.sub.1.

(5) Additional pressure p.sub.h that is set by the hydraulic brake booster is displayed in the lower section of FIG. 1. The vehicle deceleration is normally used as the trigger criterion. For this purpose the current vehicle deceleration is compared, for example, to a limiting value and, when the limiting value is reached, the hydraulic brake booster additionally sets the additional braking pressure, as shown in FIG. 1, so that vehicle deceleration ax.sub.2 (solid line) results. The additional brake boost by the hydraulic brake booster occurs only starting at instant t.sub.2.

(6) In the present case, a vacuum sensor is used to monitor a vacuum acting on the pneumatic brake booster. The vacuum here is provided, for example, by the intake port of an internal combustion engine of the motor vehicle or by a separate vacuum pump that acts correspondingly on the pneumatic brake booster.

(7) FIG. 2 shows the relationship between vacuum p.sub.v of the pneumatic brake booster and pressure p.sub.mc of the brake master cylinder. To this end, the two pressures p.sub.v and p.sub.m are plotted over time t. At an instant t.sub.a, if the brake pedal is being operated, pressure p.sub.v detected by the vacuum sensor increases. Pressure p.sub.mc in the brake master cylinder likewise increases. If the brake pedal is located in its end position, vacuum pressure p.sub.v detected by the vacuum sensor slowly decreases again within a time period t.sub.b, while the pressure in the brake master cylinder stays the same. If the driver terminates the braking operation by, for example, removing his foot from the brake pedal at an instant t.sub.c, the detected vacuum pressure increases to a maximum value, while the pressure in the brake master cylinder decreases again.

(8) In the present case, it is now provided that the hydraulic brake boost is set or triggered based on pressure p.sub.v detected by the vacuum sensor. As is evident from FIG. 2, a change in the pressure detected by the vacuum sensor takes place nearly simultaneously with the brake pedal actuation. Accordingly, an especially early signal is available for triggering the hydraulic brake boost. Because the provision of vacuum sensors, especially for engine regulation, is already known in many motor vehicles, the provision of an additional sensor, it is possible in particular to eliminate the provision of a pressure sensor in the hydraulic circuit of the braking device. Pressure p.sub.v detected by the vacuum sensor is compared here to three different limiting values. When pressure value p.sub.v increases and exceeds the particular limiting value, a higher brake boost is set by the hydraulic brake booster.

(9) The described approach now makes it possible to set the hydraulic brake boost much earlier than previously, namely already at instant t.sub.21. Thus, FIG. 1 shows as a dashed line additional braking pressure p.sub.h* provided by the hydraulic brake boost and vehicle deceleration ax.sub.2* for the previously described case in which the hydraulic brake boost is set as a function of the pressure detected by the vacuum sensor. Thus, it is obvious that the brake boost is set or settable as a function of the pressure value not only earlier, namely at time t.sub.1, but also with greater assistance, that is, with a higher additional braking pressure p.sub.h*. This results in earlier and sharper deceleration ax.sub.2* of the motor vehicle.

(10) The brake boost is preferably terminated in a ramp-like manner as shown in FIG. 1, the boost and the negative slope of the ramp preferably decreasing incrementally over time. The ramp-like reduction of the brake boost contributes in particular to driving comfort and is also a function of the residual vehicle speed. Preferably, a current road gradient and/or wear of the braking device are/is taken into account in the setting of brake boost p.sub.h* and compensated by the setting of hydraulic brake boost p.sub.h*. As an alternative to the provision of three fixed specifiable limiting values for the setting of the hydraulic brake boost, it is also conceivable to set a continuous brake boost as a function of the current pressure value of vacuum sensor p.sub.v.