WO2000010854A1

WO2000010854A1 - Method for operating a power-assist braking system

Info

Publication number: WO2000010854A1
Application number: PCT/EP1999/006245
Authority: WO
Inventors: Oliver Hecker; Steffen Ritz
Original assignee: Continental Teves Ag & Co. Ohg
Priority date: 1998-08-25
Filing date: 1999-08-25
Publication date: 2000-03-02
Also published as: JP2002523285A; DE19981606B4; DE19981606D2

Abstract

The invention relates to a method for operating a power-assist braking system. Said power-assist braking system has a first operating mode in which it is not activated, a second operating mode inwhich pressure is built up in the wheel brakes after an emergency braking situation has been detected and a third operating modewhich is provided for the transition from the second operating mode to the first. The pressure of the master cylinder (pTHZ) isdetermined before the third operating mode is initiated, resulting in improved operational safety and user comfort.

Description

Procedure for operating a brake assist system

The present invention relates to a method for operating a brake assistant system according to the preamble of patent claim 1.

In order to achieve the shortest possible braking distance of motor vehicles in emergency braking situations, it is necessary to increase the braking pressure compared to a pedal force caused by the driver. Since investigations have shown that normal drivers in emergency braking situations are often unable or unable to control the required pressure via the brake pedal, so-called brake assist systems have been developed which automatically increase the brake pressure above the level specified by the driver. The brake assist systems are drawn based on the direct or indirect observation speed of the brake pedal.

In principle, several different systems are known. In the case of a first one, the brake pressure is increased by external control of the pneumatic brake booster. In the second system, the active pressure increase takes place through suitable control of the ABS / ESP hydraulics by means of the electrical reclaim pump. Further brake assist systems are also known, all of which are further developed by the invention. Just as an example, reference is made to so-called mechanical or mechanical electrical brake assistants. A method for shortening the braking distance in critical driving situations is disclosed in German Patent DE 40 28 290 Cl. In the method disclosed in this document, the exceeding of a first threshold value by the driver's actuation speed of the brake pedal is the criterion for triggering an automatic braking process, such a braking pressure being built up immediately after the triggering of the automatic braking process that corresponds to the value of the Brake pressure with optimal deceleration of the vehicle corresponds. In order to ensure that the excessive brake pressure is released in time when the need for an automatic braking process has been eliminated, it is checked according to the teaching of this publication whether the brake pedal actuation speed is less than a predetermined threshold value, ie whether the driver of the vehicle is strong wants to reduce the braking process and thus only requires a braking process with a lower braking force

Brake assist systems are therefore known which have three operating modes. In the first operating mode, which can also be referred to as the state of passivity, the brake asstent system is not actuated or does not intervene. As soon as an emergency braking situation has been recognized, as already explained above, the second operating mode is changed, which contains the actual brake assist function. A full brake force build-up is generated by means of a recovery pump and a suitable valve control. As soon as a significant reduction in the pedal force applied by the driver is sensed, the

CONFIRMATION HEAD second into a third operating mode, which can also be referred to as dosing mode or modulating mode. In the third operating mode, the braking force is modulated depending on the pedal force. At the end of the maximum activation, the pressure increase is gradually reduced or increased again depending on the determined driver's request in order to achieve a more comfortable transition between the maximum support during the emergency braking situation and the conventional braking behavior in passive mode. In the dosing phase, the wheel brake pressure is increased compared to the pressure entered by the driver, in particular by means of a time-dependent multiplication function or the like. It should also be noted that the third operating mode is similar to a so-called "brake-by-wree" mode.

The prior art now has disadvantages of the technology in that, in the known method, in particular when the driver barely reaches the trigger threshold (s) when the brake assist system is drawn, the situation can arise that when the Pedal force a very low master cylinder pressure is opposed to a very high blocking pressure. This condition is questionable for safety reasons, since the required high gain factor leads to a behavior of the brake system that is difficult for the driver to understand. Here, particularly during a metering mode, slight changes in the pedal force adversely lead to strong fluctuations in the vehicle deceleration. The object of the present invention is therefore to avoid the disadvantages of the prior art, and in particular to provide a method for operating a brake assist system which is particularly safe and user-friendly.

The object according to the invention is achieved in a method of the type mentioned at the outset by the features specified in the characterizing part of patent claim 1.

According to the invention, the main cylinder pressure is sensed before the third operating mode is initiated, and a possible source of danger is recognized when an automatic brake assistant is ended.

An advantage of the invention is that, according to the invention, when the pressure difference between the master cylinder pressure and the wheel brake pressure is detected, the metering phase, which is safety-critical due to the inherent increase in such an operating state of the brake system, is not at all or, after carrying out a suitable system change, only is initiated with a delay. In particular, depending on the value of the main cylinder pressure, the third operating mode is initiated or not, since the initiation of the metering phase is particularly critical if the main cylinder pressure is too low.

The master cylinder pressure is preferably compared with a first predetermined threshold value and, if the master cylinder pressure is lower than the predetermined first threshold value, increased under external control. Through the active Increasing the master cylinder pressure or the pedal force is the otherwise very low master cylinder pressure, which in practice z. B. 12 bar, is adjusted to the high blocking pressure of the wheel brakes or approximates this. This results in a more natural response for the driver. Furthermore, the increase may be a suitable system change in the above sense.

The master cylinder pressure is advantageously increased by opening at least one isolating valve. In particular, clocked opening is preferred. The brake fluid flowing back from the wheel brakes into the master cylinder leads there, at least briefly, to the desired pressure increase.

The master cylinder pressure is preferably increased over a predetermined period of time. Such a predefined period of time thus represents a compromise between an immediate, but safety-critical, initiation of the dosing phase and the general foregoing of the dosing phase, which is comfortable per se.

According to an advantageous development of the invention, after the increase in the master cylinder pressure over the predetermined period of time, the master cylinder pressure is compared with a second predetermined threshold value. Now the success of the pedal force increase measure can be checked. Depending on the result of the comparison, the system switches to the third or the first operating mode. It is preferred that the third operating mode is initiated if the master cylinder pressure is greater than the second predetermined Threshold is. In this case, the master cylinder pressure was set to a level in which the metering phase can be operated without problems.

However, ending the increase in the master cylinder pressure is advantageous if the master cylinder pressure is less than the second predetermined threshold value. In this case, the dosing phase is not initiated at all and the transition from the active mode to the passive mode follows the phase according to the invention.

The invention, as well as further advantages and refinements thereof, will be explained in more detail below with reference to the attached drawings. All of the described and / or illustrated elements, alone or in any meaningful combination, form the subject matter of the present invention, irrespective of how they are summarized in the claims or their relationship. Throughout the drawings, the same reference numerals designate the same or corresponding elements or sizes. The drawings show:

Figure 1 is a schematic representation of the course of the master cylinder pressure and the valve position of the isolation valve.

2a shows a schematic illustration of the pedal travel, the master cylinder pressure and the wheel pressure in a conventional braking operation; 2b shows a schematic illustration of the pedal travel, the master cylinder pressure and the wheel pressure in a brake system with brake assist system; and

Fig. 3 is a schematic representation of the three operating modes of the brake assist system according to the invention and the transitions between the corresponding states.

The illustration in FIG. 2a shows the braking behavior of a conventional brake system. In Fig. 2a it can be seen that the master cylinder pressure p _{THZ follows} the pedal travel with a deceleration that is not constant over time. The wheel pressure P _RAD follows the master cylinder pressure with a deceleration that is also not constant in time and p _T Hz = PRAD applies from a certain point in time. In contrast to this, the active pressure build-up can be seen in FIG. 2b by a brake assistant, in particular a hydraulic one. Initially, the behavior of the system according to FIG. 2b corresponds to the behavior shown in FIG. 2a. As soon as it is sensed that the actuation speed of the brake pedal exceeds a critical limit, the brake assist function starts at time t _x . From this point, the force required to operate the wheel brakes wheel pressure P _AD by the master cylinder pressure P _ra z is uncoupled. It can be seen that the wheel pressure PRAD quickly reaches an increased value compared to the case in FIG. 2a. The master cylinder pressure p _TH z drops after reaching a maximum, but even from and between PRAD and p _TH z there is a large pressure difference, which leads to the difficulties mentioned at the beginning. In Fig. 1, the master cylinder pressure p _{THZ is shown schematically} as a function of time with an active pressure build-up by a hydraulic brake assistant (see. Fig. 2b). As soon as the master cylinder pressure p _{THZ reaches} a predetermined level (FIG. 2b), which is recognized in practice by comparing the master cylinder pressure p _TH z with a first predetermined threshold value p _S ι, a clocked opening of the isolating valve (duration of opening Δt; see the illustration in the lower half of FIG. 1), which leads to an increase in the pressure of the master cylinder pressure p _TH z due to the backflow of brake fluid. The isolating valve is clocked or periodically or pulsed over a predetermined time or test time T in each case by Δt '(Fig. 1: o = open; g = closed) and the resulting, oscillating pressure curve of the master cylinder pressure is in the upper one Part of Fig. 1 shown schematically. It should be noted that the operating state of the isolating valve is shown in dashed lines outside the phase according to the invention. Depending on the operating state of the brake system, a second threshold pressure p _s2 is now reached or not reached after the predetermined time period T. The course of the master cylinder pressure when the threshold value is reached is denoted by A in the figure and by B if the threshold value is not reached. If the master cylinder pressure p _TH z is greater than the predefined threshold value p _s2 after the time period T (case A), then a changeover to a dosing mode (FIG. 3) takes place to end the automatic braking force support. If the master cylinder pressure p _TH z does not exceed the threshold value p _s2 after the predefined time period T, that is to say is less than this, the brake assistant Function ended and the brake system returns to the "passive" mode (FIG. 3).

In Fig. 3 three operating modes or modes of the brake assist system according to the invention are shown schematically. 3 shows the function of the brake assist system as a state machine. The "passive" state means that the brake assist function is not active. As soon as an emergency braking situation is recognized, the system changes to the "full pressure build-up" state at time ti. In this state, a full pressure build-up of the wheel pressure (cf. FIG. 2b) is generated by means of a return pump and by suitable activation of isolating and switching valves. From the "full pressure build-up" state, a significant reduction in the pedal force, which, for. B. is detected via the pressure sensor installed in the master cylinder, both in the state of passivity and in the metering mode. The higher-ranking criteria for actuating the "metering" state from the "full pressure build-up" state are the recognition of a driver's request for a metered reduction in the braking force. This is sensed by a significant decrease in the master cylinder pressure p _TH z after the global blocking pressure level has been reached. A significant decrease in the master cylinder pressure p _TH z before the global blocking pressure level is reached leads to a direct transition from the “full pressure build-up” state to the “metering” state. In the "metering" state, the braking force is modulated as a function of the pedal force. After the maximum control has ended, the pressure increase is gradually reduced or, depending on the driver's request sensed by the measured master cylinder pressure also increased again in order to achieve a more comfortable transition between the maximum support during the emergency braking situation and the conventional braking behavior of the "passive" mode after the end of the emergency braking support. This state is similar to a brake-by-wire mode and can also be described as modulating. According to the invention, the state "full pressure build-up" is changed to the "passive" state if the master cylinder pressure does not rise above the predetermined value p _s within the predetermined time T (case B). If the master cylinder pressure p _TH z does not reach the threshold value p _s based on the predefined time period T, the system switches from the "full pressure build-up" state to the "passive" state.

The invention has been explained in more detail above on the basis of preferred embodiments thereof. However, it is obvious to a person skilled in the art that various modifications and modifications can be made without departing from the concept on which the invention is based. In particular, the core idea of the present invention is an active increase in pedal force when transitioning into a metering phase of a brake assist system. The brake system is therefore operated according to the invention in a fourth mode upstream of the metering mode, so to speak, in a reverse manner by feeding pressure from the wheel brakes into the master cylinder via the isolating valve.

CONFIRMATION HEAD Reference character list:

P _THZ master cylinder pressure

P _RAD wheel brake pressure

Psi first threshold or threshold _pressure p _s2 second threshold or threshold pressure

F _P pedal force

T time ti time

Δt pulse duration

Claims

claims

1. Method for operating a brake assist system, which has a first operating mode in which the brake assist system is not actuated, a second operating mode in which a pressure build-up of wheel brakes is generated after an emergency braking situation has been recognized, and a third operating mode which is used for Transition from the second to the first operating mode is provided, characterized in that the master cylinder pressure (P _THZ ) is sensed before the third operating mode is initiated.

2. The method according to claim 1, characterized in that depending on the value of the master cylinder pressure (P _THZ ), the third operating mode is initiated or not.

3. The method according to claim 1 or 2, characterized in that the master cylinder pressure (P _THZ ) is compared with a first predetermined threshold value (p _S ι) and, if the master cylinder pressure (p _TH z) less than the predetermined first Threshold (psi) is, is controlled externally.

4. The method according to any one of claims 1 to 3, characterized in that the increase in the master cylinder pressure (P _THZ ) is carried out by opening at least one isolation valve.

5. The method according to claim 4, characterized in that the opening of the isolation valve is carried out clocked.

6. The method according to any one of claims 3 to 5, characterized in that the master cylinder pressure (P _THZ ) is increased over a predetermined period (T).

7. The method according to claim 6, characterized in that after the increase in the master cylinder pressure (P _THZ ) over the predetermined period (T) of the master cylinder pressure (P _THZ ) is compared with a second predetermined threshold (p _s2 ).

8. The method according to claim 7, characterized in that the third operating mode is initiated if the master cylinder pressure (p _Tr ιz) greater than the second predetermined. Threshold (ps ₂ ).

9. The method according to any one of claims 7 or 8, characterized in that the increase in the master cylinder pressure (PTHZ) is ended if the master cylinder pressure (PTHZ) is less than the second predetermined threshold value (p _s2 ).