Method and device for activating an electric vacuum pump for a brake booster of a vehicle

Abstract

Described are a method and a device for activating an electric vacuum pump for a brake booster of a vehicle. The brake booster is supplied with a vacuum from a vacuum reservoir. The vacuum pump is designed to increase the vacuum in the vacuum reservoir as soon as the vacuum drops below a predefinable switch-on threshold. In order to avoid unnecessarily frequent switching on of the vacuum pump, which may irritate the driver, an expected decrease of the vacuum in the vacuum reservoir as a result of an instantaneous braking operation is estimated and the switch-on threshold of the vacuum pump is temporarily reduced if the vacuum instantaneously prevailing in the vacuum reservoir minus the estimated vacuum decrease is greater than a predetermined adjustment point vacuum, below which the brake booster can no longer act in a sufficiently boosting manner.

Claims

1. A method for activating an electric vacuum pump for a brake booster of a vehicle, the brake booster being supplied with a vacuum from a vacuum reservoir and the vacuum pump being adapted to increase the vacuum in the vacuum reservoir once the vacuum drops below a predefinable switch-on threshold, the method comprising: estimating an expected decrease of the vacuum in the vacuum reservoir as a result of an instantaneous braking operation; and temporarily reducing the switch-on threshold in the event a value for the vacuum instantaneously prevailing in the vacuum reservoir minus the estimated vacuum decrease is greater than a predetermined adjustment point vacuum, wherein the switch-on threshold is a threshold for switching on the vacuum pump.

2. The method as recited in claim 1, further comprising: ascertaining a pressure difference between a target braking pressure and an instantaneously prevailing pre-braking pressure in the brake booster; and estimating the decrease of the vacuum in the vacuum reservoir based on the ascertained pressure difference and the instantaneous vacuum in the vacuum reservoir.

3. The method as recited in claim 2, wherein the decrease of the vacuum in the vacuum reservoir is ascertained with the aid of a characteristic map indicating the vacuum decrease as a function of the ascertained pressure difference and the instantaneous pre-braking pressure in the brake booster.

4. The method as recited in claim 2, wherein the decrease of the vacuum in the vacuum reservoir is ascertained as proportional to the ascertained pressure difference.

5. The method as recited in claim 2, wherein the decrease of the vacuum in the vacuum reservoir is ascertained by extrapolation of a previously measured vacuum decrease.

6. The method as recited in claim 1, wherein an extent of the temporary reduction of the switch-on threshold is selected depending on the estimated expected decrease of the vacuum in the vacuum reservoir as the result of the instantaneous braking operation, and depending on the instantaneous vacuum in the vacuum reservoir.

7. The method as recited in claim 1, wherein the adjustment point vacuum is defined as a minimum vacuum in the vacuum reservoir, above which the vacuum in the vacuum reservoir is sufficient in order to assist a brake pedal activation with the aid of the brake booster.

8. The method as recited in claim 1, wherein the switch-on threshold is temporarily reduced for a duration of the instantaneous braking operation, and wherein the method further comprises one of the following: increasing, after the instantaneous braking operation as stopped, the switch-on threshold from a value to which it was reduced; and maintaining, after the instantaneous braking operation as stopped, the switch-on threshold at a value to which it was reduced.

9. The method as recited in claim 1, further comprising activating the electric vacuum pump after the instantaneous breaking operation has stopped.

10. A non-transitory computer-readable medium on which a computer program product is stored, the computer program product having machine-readable code, wherein the machine-readable code activates a machine for the purpose of carrying out a method for activating an electric vacuum pump for a brake booster of a vehicle, the brake booster being supplied with a vacuum from a vacuum reservoir and the vacuum pump being adapted to increase the vacuum in the vacuum reservoir once the vacuum drops below a predefinable switch-on threshold, the method comprising: estimating an expected decrease of the vacuum in the vacuum reservoir as a result of an instantaneous braking operation; and temporarily reducing the switch-on threshold in the event a value for the vacuum instantaneously prevailing in the vacuum reservoir minus the estimated vacuum decrease is greater than a predetermined adjustment point vacuum.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a braking system for a motor vehicle having a brake booster and an associated electric vacuum pump, as well as a device for activating the vacuum pump according to one specific embodiment of the present invention.

(2) FIG. 2 shows a flow chart for illustrating a method according to one specific embodiment of the present invention.

(3) FIG. 3 shows a characteristic map of a braking system having a brake booster for illustrating the method according to one specific embodiment of the present invention.

(4) The figures are purely schematic and not true to scale.

DETAILED DESCRIPTION

(5) FIG. 1 shows parts of a braking system 1 for a motor vehicle. A brake pedal 7 activates a brake booster 3 which transmits a force applied by a driver to brake pedal 7 reinforced to the brakes of a vehicle (not shown). Brake booster 3 has a vacuum reservoir 9 in which a vacuum of up to 1000 hPa, for example, may be generated with the aid of an electric vacuum pump 5. With the aid of this vacuum from vacuum reservoir 9 it is possible to load a diaphragm provided in brake booster 3 and in this way boost the force applied by brake pedal 7.

(6) A pressure sensor 13 measures the instantaneous vacuum inside vacuum reservoir 9. A control unit 11 is used to activate electric vacuum pump 5. For this purpose, control unit 11 receives the measurement data from pressure sensor 13 and, based on these data about the vacuum instantaneously prevailing in vacuum reservoir 9 and based on further information about the instantaneous braking operation and, in particular, on a maximum braking pressure to be generated by braking system 1, may decide whether or not electric vacuum pump 5 should be activated, or how a switch-on threshold for electric vacuum pump 5 should be instantaneously selected.

(7) FIG. 2 shows a flow chart provided to illustrate how control unit 11 may influence a switch-on threshold for activating electric vacuum pump 5.

(8) In a first step S1 a target braking pressure P.sub.B.sub._.sub.target and an instantaneously prevailing pre-braking pressure P.sub.B.sub._.sub.instantaneous are initially determined and a difference value P.sub.B of these two values is calculated.

(9) In a second step S2 an expected decrease of the vacuum P.sub.V in the vacuum reservoir during the instantaneous braking operation is determined. This value P.sub.V is ascertained as a function f (P.sub.B, P.sub.V.sub._.sub.instantaneous) of the previously ascertained pressure difference P.sub.B between the target braking pressure and the instantaneous pre-braking pressure as well as the vacuum P.sub.V.sub._.sub.instantaneous instantaneously prevailing in the vacuum reservoir. The functional relation may be derived, for example, from a previously stored characteristic map, or ascertained as proportional to a pressure difference ascertained between the target braking pressure and the instantaneous pre-braking pressure. Alternatively, the vacuum decrease may be ascertained by extrapolation of a previously measured vacuum decrease.

(10) In a subsequent third step S3 a vacuum level P.sub.V.sub._.sub.target is ascertained which is presumably set upon reaching the target braking pressure by subtracting the previously estimated vacuum decrease P.sub.V from the instantaneous vacuum P.sub.V.sub._.sub.instantaneous.

(11) In subsequent step S4 the target vacuum pressure P.sub.V.sub._.sub.target is compared with an adjustment point vacuum P.sub.A.

(12) In the event that the target vacuum P.sub.V.sub._.sub.target is greater than or equal to the adjustment point vacuum P.sub.A, a switch-on threshold P.sub.S.sub._.sub.temp to be temporarily adjusted is set in a step S5 to a value which is smaller than a standard switch-on value P.sub.S.sub._.sub.standard by a difference value P.sub.S. In other words, the switch-on threshold is temporarily reduced. In the process, the switch-on threshold indicates the pressure value below which the vacuum in the vacuum reservoir is meant to drop before the electric vacuum pump is to be activated.

(13) In the event the target vacuum pressure P.sub.V.sub._.sub.target is smaller than the adjustment point vacuum P.sub.A a switch-on threshold P.sub.S.sub._.sub.temp to be temporarily adjusted remains unchanged or is set to the P.sub.S.sub._.sub.standard in a step S6.

(14) The described method may be repeatedly carried out by a control unit 11. For example, the method may be carried out at intervals of 0.02 sec. so that the switch-on threshold may be adapted at appropriately short intervals to instantaneously prevailing requirements with respect to the vacuum to be generated by the vacuum pump.

(15) The described method for activating an electric vacuum pump is further clarified with reference to FIG. 3. FIG. 3 displays a typical characteristic curve of a brake booster system. Here, the X-axis indicates the pedal force F.sub.pedal. The braking pressure P.sub.B is indicated on the Y-axis.

(16) Starting with a minimal pedal force F.sub.0, braking pressure P.sub.B, after an initial sharp rise, ascends linearly to an adjustment point 15 within a range 17. The slope of this rise indicates a measure for the boosting effect of the brake booster.

(17) Above adjustment point 15 the brake booster is no longer able to contribute to an increase in the braking pressure, so that with a further increase in brake pedal force F.sub.Pedal, the increase in braking pressure P.sub.B is less than in boosted range 17 up to adjustment point 15.

(18) In terms of practical application, the braking system is aimed to always move within boosted range 17 of the characteristic curve, i.e., below adjustment point 15. However, the position of adjustment point 15 strongly depends on the vacuum prevailing within the vacuum reservoir. The greater this vacuum, the longer or up to higher braking pressures the brake booster is able to support the activation of a brake pedal. For example, dashed lines are drawn in FIG. 3 which represent from top to bottom a change in the characteristic curve of the brake booster with increasingly lowered vacuum in the vacuum reservoir. It is apparent, for example, that adjustment point 15 for case (c), the lowest vacuum in the vacuum reservoir, is much lower than for case (a) with a substantially higher vacuum.

(19) Area I indicates those cases in which the vacuum instantaneously prevailing in the vacuum reservoir minus the estimated vacuum decrease during the braking operation is great enough so that the minimum target braking pressure to be achieved remains below adjustment point 15. In such cases I, a switch-on threshold for the electric vacuum pump may be temporarily reduced so that the vacuum pump is not suddenly activated during the instantaneous braking operation and, therefore, to the driver, the feel of the brake pedal changes. It is continually ensured that a sufficient vacuum prevails in the vacuum reservoir, and that an assist from the brake booster lasts during the entire braking operation up to the target braking pressure, and the adjustment point is therefore not reached.

(20) For cases II in which the vacuum instantaneously prevailing in the vacuum reservoir is not sufficient to compensate for the vacuum decrease occurring during the braking operation without dropping below the adjustment point vacuum, the switch-on threshold for the vacuum pump is not reduced.

(21) Finally, it should be noted that the effectiveness of the described method may be enhanced with the use of a so-called overboost function in which an additional pressure buildup may be created by an ESP system (electronic stability program), in order to more or less virtually shift the adjustment point. As a result, the minimum deceleration or the desired target braking pressure may be reduced.