BRAKING DEVICE FOR A HYDRAULIC MOTOR VEHICLE BRAKE SYSTEM

Abstract

A braking device for a hydraulic motor vehicle brake system, with an energy-store-free, electronically controlled electric-motor-driven on-demand braking force booster and with a master brake cylinder, in which, for the purpose of reducing the foot force on the brake pedal in the fall-back level, the braking pressure in the pressure chamber is built up stepwise and at least one active surface involved in generating brake pressure is switchable so as to be hydraulically ineffective.

Claims

1. A brake device for a hydraulic motor vehicle brake system, comprising: an energy-store-free, electronically controlled brake booster, an electric-motor-powered drive unit which is assigned to the brake booster, wherein when the drive unit is activated the brake booster generates a boosted braking force, and when the drive unit is deactivated the brake booster does not generate any boosted braking force, a master brake cylinder with at least one piston which can be displaced linearly therein and which bounds at least one pressure chamber filled with a pressure medium, in order to generate hydraulic brake pressure, wherein in the case of a usual service braking operation the brake booster acts on the piston, and in the case of an unusual braking operation the muscle force of a driver acts thereon, wherein at least one active area for generating brake pressure in the pressure chamber can be switched to a hydraulically ineffective setting under pressure control in the master brake cylinder, as a result of which the brake pressure in the pressure chamber is built up incrementally, and wherein the active area is switched off by a valve arrangement which is controlled by the pressure in the pressure chamber.

2. The brake device as claimed in claim 1, wherein the active area is formed on the piston.

3. The brake device as claimed in claim 1, wherein the piston is embodied as a stepped piston with the active area and at least one further active area.

4. The brake device as claimed in claim 3, wherein the active area is made larger than the further active area.

5. The brake device as claimed in claim 1, wherein during a braking process, before it is switched off the active area expels the pressure medium from a filling stage chamber into the pressure chamber and expels said pressure medium into a pressureless pressure medium container after switching off has occurred.

6. The brake device as claimed in claim 1, wherein the valve arrangement comprises a spring-loaded valve piston which inevitably opens when there is a defined pressure in the pressure chamber.

7. The brake device as claimed in claim 5, wherein the valve arrangement blocks a hydraulic connection between the filling stage chamber and the pressure medium container.

8. The brake device as claimed in claim 1, wherein the valve arrangement is arranged integrated into the piston or into a housing of the master brake cylinder.

9. The brake device as claimed in claim 1, wherein the boosted braking force is generated hydraulically in the brake booster.

10. The brake device as claimed in claim 1, wherein the boosted braking force is generated mechanically in the brake booster.

11. The brake device as claimed in claim 1, wherein the boosted braking force is generated pneumatically in the brake booster.

12. The brake device as claimed in claim 1, wherein the master brake cylinder is embodied as a separate component and is mounted on a housing of the brake booster.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Further details, features, advantages and application possibilities of the invention can be found in the subclaims together with the description with reference to the drawings. Corresponding components and structural elements are provided with the same reference signs where possible. In the drawings:

[0024] FIG. 1 shows a simplified illustration of an embodiment of the brake device according to the invention with a master brake cylinder in a tandem design and a single-stage piston.

[0025] FIG. 2 shows a simplified illustration of a further inventive embodiment of the brake device with another master brake cylinder with a two-stage piston.

[0026] FIG. 3 shows a diagram showing the ratio between the foot force and the braking deceleration in the usual operating mode and in the fall-back level with active areas of different sizes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1:

[0027] The brake device 1 comprises a master brake cylinder 4 which is filled with pressure medium and is mounted on the housing 12 of a brake booster which is illustrated only schematically. The brake booster 2 is driven by an electric-motor-powered drive unit 3 according to the on-demand principle. Here, the brake booster 2 draws the energy which is required to generate a boosted braking force exclusively from the drive unit 3 during its operation and does not have a separate energy store which would ensure the supply of energy when a drive unit is deactivated.

[0028] The control of the brake booster 2 is carried out by an electronic control unit 17.

[0029] During the usual operation of the brakes, the brake booster 2 provides the brake force via a coupling device 15 to the piston 5, connected downstream, of the master brake cylinder 4.

[0030] When the energy supply fails or there are other faults during which the brake booster 2 cannot function satisfactorilyin the so-called fallback levelthe piston 5 is activated directly by the brake pedal 13 via the coupling device 14 solely by the muscle force of the driver.

[0031] The two coupling devices 14, 15 can also be combined within the invention designed as a common coupling device.

[0032] Likewise, a simulator unit (not shown here), which generates a haptic feedback to the brake pedal 13 during the use of the brake booster 2 can be provided within an aspect of the invention, in the effect chain between the brake pedal 13 and the piston 5.

[0033] A sensor device 16 registers the braking actions of the driver in the normal braking mode and communicates it to the electronic control unit 17 for the purpose of corresponding actuation of the drive unit 3 or of the brake booster 2.

[0034] The piston 5 is displaced to be mounted in the housing 11 of the master brake cylinder 4 and bounds a pressure chamber 6 which is connected to a brake circuit (not shown). In the embodiment shown, the piston 5 is embodied as a stepped piston with two steps. When the piston 5 is displaced in the direction of the pressure chamber 6, the first step with the relatively large active area A1 and the second step with the relatively small active area A2 are involved in a hydraulically effective fashion in the buildup of the brake pressure in the pressure chamber 6. While the active area A2 is acting exclusively in the pressure chamber 6, the active area A1 expels the pressure medium from a separate filling stage chamber 8 passed the direction-dependent ceiling element 24 and into the pressure chamber 6.

[0035] A valve arrangement 7 with a valve piston 10 which is spring-loaded at the rear is arranged in the piston 5. At the start of the braking operation, the valve arrangement 7 is closed. However, as soon as the brake pressure in the pressure chamber 6 has reached a defined value which exceeds the spring load, the valve piston 10 moves counter to the spring load, with the result that the valve arrangement 7 opens and clears a hydraulic connection between the filling stage chamber 8 and the pressureless pressure medium container 9 via a chain of connecting ducts 20, 21, 22, 23. As a result, the pressure medium from the filling stage chamber 8 is expelled into the pressure medium container 9, and no longer into the pressure chamber 6, and the active area A1 is therefore switched to a hydraulically ineffective setting with respect to the buildup of brake pressure, or switched off, and the force-travel ratio at the piston 5 changes suddenly. From then on, the buildup of brake pressure in the pressure chamber 6 only takes place as result of the relatively small active area A2, and therefore overall in an incremental fashion.

[0036] In the inventive embodiment shown, the master brake cylinder 4 is embodied in a tandem design. In this context, a further front pressure chamber 19 which acts on a separate brake circuit (not shown) is arranged in the housing 11. A floating piston 18 is moveably guided between the pressure chambers 6 and 19, which floating piston 18 is usually actuated by the brake pressure in the pressure chamber 6, and is directly mechanically actuated by the piston 5 when there is a drop in pressure in the pressure chamber 6.

FIG. 2:

[0037] FIG. 2 is a simplified illustration of another embodiment of an aspect of the inventive brake device. The master brake cylinder 4 is not embodied in a tandem design here, and has a single three-stage piston 5 which is equipped with 3 active areas A1, A2 and A3 which are of different sizes.

[0038] The valve arrangement 7 is integrated into the housing 11 of the master brake cylinder 4. The active area A1 expels the pressure medium from the filling stage chamber 8 into an intermediate chamber 29, where it is expelled further into the pressure chamber 6 by the active area A3. In this context, there is a flow over each of the seal elements 24 and 25 in the direction of the pressure chamber 6. The smallest active area A2 compresses the pressure medium in the pressure chamber 6 and expels it into a brake circuit connected thereto.

[0039] The brake pressure in the pressure chamber 6 is passed on through the drilled hole 30 to the spring-loaded valve piston 10, which, when a defined pressure value is reached, clears a hydraulic connection through the connecting ducts 20, 21 between the intermediate chamber 29 and the pressureless pressure medium container 9. As a result, the pressure medium from the filling stage chamber 8 and the intermediate chamber 29 is expelled into the pressure medium container 9, and the active areas A1 and A3 are switched to a hydraulically ineffective setting.

FIG. 3:

[0040] FIG. 3 shows a diagram which basically illustrates the effectiveness of the brake device according to an aspect of the invention. The X axis stands for the necessary foot force on the brake pedal 13, and the Y axis for the braking deceleration of the vehicle. The perpendicular line F marks an upper limiting value for the foot force, for example a legally prescribed upper limit of 500N. The horizontal line D marks a defined target value for the braking deceleration of the vehicle, in particular a minimum value which is necessary for an emergency braking or full braking operation. The latter can also be legally prescribed and be, for example, of an order of magnitude of 0.643 g.

[0041] Graph 26 shows a profile of the foot force and the vehicle deceleration associated therewith given a satisfactory functioning brake booster 2 on a defined vehicle application. The foot force on the brake pedal 13 can be manipulated here in a targeted fashion by the regulation means of the brake booster 2 or by a separate stimulator unit, depending on the design.

[0042] In the case of failure of the brake booster 2 in the fall-back level, the brake deceleration depends, however, directly on the foot force. In the first phase of the braking operation the relatively large active area A1 is active. Although this permits a relatively short pedal travel, because a larger pressure medium volume is expelled, the foot force increases so quickly that a brake pressure which is necessary for the necessary braking deceleration D would not be reached within the upper limit of the foot force F, this is indicated by the graph 28.

[0043] When a defined brake pressure is reached, the active area A1 is switched off at the switching point S, with the result that only the relatively small active area A2 contributes to building up the brake pressure. As a result, the foot force increases only moderately and the target value for the brake deceleration D can be reached within the limiting value F. This is shown by graph 27.

LIST OF REFERENCE SIGNS:

[0044] 1 Brake device [0045] 2 Brake booster [0046] 3 Drive unit [0047] 4 Master brake cylinder [0048] 5 Piston [0049] 6 Pressure chamber [0050] 7 Valve arrangement [0051] 8 Filling stage chamber [0052] 9 Pressure medium container [0053] 10 Valve piston [0054] 11 Housing [0055] 12 Housing [0056] 13 Brake pedal [0057] 14 Coupling device [0058] 15 Coupling device [0059] 16 Sensor device [0060] 17 Electronic control unit [0061] 18 Floating piston [0062] 19 Pressure chamber [0063] 20 Connecting duct [0064] 21 Connecting duct [0065] 22 Connecting duct [0066] 23 Connecting duct [0067] 24 Seal element [0068] 25 Seal element [0069] 26 Graph of braking operation [0070] 27 Graph of fallback level with A2 [0071] 28 Graph of fallback level with A1 [0072] 29 Intermediate chamber [0073] 30 Drilled hole [0074] A1, A2, A3 Active area [0075] F Upper limit of foot force [0076] D Target value of braking deceleration [0077] S Switching point