Braking system for a motor vehicle and method for operating a braking system

Abstract

Brake system (1) for a motor vehicle, with at least one wheel brake, an operating element for actuating the wheel brake and a vacuum brake booster (2) which is operatively connected between the operating element and the wheel brake and which can be pressurized with vacuum from a vacuum source (4), wherein the vacuum source (4) is connected to the vacuum brake booster (2) via a vacuum line (3), wherein the vacuum line (3) can be connected to an external ambient flow by means of a valve (5) which is fluidically arranged between the vacuum source (4) and the vacuum brake booster (2) in the vacuum line (3), characterized in that an electric vacuum pump (4) serves exclusively as the vacuum source (4) and the valve (5) is designed as a solenoid valve.

Description

The invention relates to a brake system for a motor vehicle, having at least one wheel brake, an operating element for actuating the wheel brake, and a vacuum brake force booster which is present in an operative connection between the operating element and the wheel brake and can be acted upon by a vacuum source with a vacuum, wherein the vacuum source is connected to the vacuum brake force booster via a vacuum line, wherein the vacuum line can be connected to an external ambient flow by means of a valve which is present in the vacuum line in terms of flow between the vacuum source and the vacuum brake force booster. The invention further relates to a method for operating a brake system for a motor vehicle.The brake system serves for decelerating the motor vehicle from driving to standstill and / or for holding the motor vehicle at standstill. The brake system is present as a service brake system which can be actuated during normal driving operation of the motor vehicle. By means of the brake system, a braking force which can be adjusted with the aid of the operating element can be imposed via the wheel brake on a wheel of the motor vehicle. Preferably, a respective wheel brake of this type is assigned to a plurality of wheels of the motor vehicle, in particular to all wheels of the motor vehicle. A plurality of these wheel brakes, preferably all of the wheel brakes, can now be actuated jointly and / or simultaneously by means of the operating element.The operating element is present, for example, in the form of a brake pedal. The operating element must be acted upon by an operating force in order to achieve a specific braking force at the wheel brake. The operative connection between the operating element and the wheel brake is preferably at least partially or exclusively provided via a hydraulic connection. In addition to the hydraulic connection, a mechanical connection can be provided, in particular for safety reasons. However, the connection can also be purely mechanical.In order to be able to provide or generate a high braking force even at a low operating force, the vacuum brake booster is present in the operative connection, in particular in the hydraulic connection. This is supplied with negative pressure from the negative pressure source for its operation. This operates using a pressure difference between the negative pressure and an ambient pressure, which is present in an external environment of the brake system or of the motor vehicle. Using the vacuum brake booster, the operating force acting on the operating element is converted into a braking force applied to the wheel brake or exerted by the latter on the wheel.The prior art discloses, for example, the publication DE 10 2008 051 851 A1. This relates to an operating method for a vacuum brake booster of a vehicle, which has a vacuum chamber connected to a vacuum source, which is in particular designed as a vacuum pump, and a working chamber separated from the vacuum chamber by a movable partition wall, which working chamber is connected to the vacuum chamber in the absence of a braking request, while the working chamber is ventilated in the presence of a braking request, wherein a nonreturn valve, which is assigned to the vacuum chamber and which closes off the vacuum chamber, is provided in a connecting line between the vacuum chamber of the brake booster and the vacuum source if the vacuum prevailing in the vacuum chamber is greater in absolute value than that of the vacuum source. In this case, in operating states in which the vacuum source cannot provide sufficient vacuum, the section of the connecting line between the vacuum source and the aforementioned nonreturn valve is automatically ventilated, that is to say is subjected to ambient pressure. Preferably, a ventilation valve unit is provided for this purpose, with a diaphragm valve which is prestressed in the opening direction by a spring element and in the process ventilates the connecting line and which is transferred into the closed state or held in the closed state when sufficient negative pressure is provided by the negative pressure source.Furthermore, the publication DE 100 16 737 A1 is known from the prior art.It is now the object of the invention to propose a brake system for a motor vehicle which has advantages over known brake systems, in particular enables extensive test operation of the brake system.This is achieved according to the invention with a brake system having the features of claim 1. It is provided that only an electric vacuum pump serves as the vacuum source and the valve is designed as a solenoid valve.The brake system described here is intended to be used in particular during a test operation of the brake system or of the motor vehicle. It enables a test operation with a high range during which numerous fault cases can be simulated in order to observe or establish the behavior of the brake system in response to these fault cases. The brake system according to the invention provides the vacuum necessary for operating the vacuum brake booster exclusively with the aid of the electric vacuum pump. The vacuum source in the form of the vacuum pump is also the only vacuum source connected fluidically to the vacuum brake booster.Compared to otherwise customary vacuum pumps which are driven by an internal combustion engine of the motor vehicle, this has the advantage that the vacuum can be set completely independently of the operating mode and / or the operating point of the internal combustion engine. Of course, the vacuum pump operatively connected to the internal combustion engine can also be installed in the motor vehicle. However, this is completely fluidically separated from the vacuum brake booster at least during the test operation. This is therefore supplied with negative pressure exclusively by the electric vacuum pump. For this purpose, the vacuum pump is connected to the vacuum brake booster via the vacuum line.The vacuum line can be evacuated with the aid of the vacuum pump, so that the vacuum is present therein. The valve is also assigned to the vacuum line; for example, the valve is arranged in the vacuum line. The valve serves to selectively put the vacuum line in fluid communication with the external environment or to separate it from the external environment in terms of fluid flow. In a first switching state of the valve, the flow connection is present between the vacuum line and the external environment, so that a pressure equalization can take place between them. In a second switching position, on the other hand, the flow connection between the vacuum line and the external environment is interrupted, so that the pressure equalization does not take place.If the flow connection is present between the vacuum line and the external environment, ambient air can flow into the vacuum line and correspondingly reduce the vacuum present there. The negative pressure is understood to mean a pressure which is less than the ambient pressure. The negative pressure can be specified either as absolute negative pressure or as relative negative pressure with respect to the ambient pressure. The valve is designed as a solenoid valve. For example, it is a discretely switching valve, which is thus either completely open or completely closed. Of course, the valve can also be designed as a continuous valve.The invention provides that the valve is fluidically present between the vacuum pump and the vacuum brake booster in the vacuum line. The valve thus makes it possible to reduce the negative pressure present at the vacuum brake booster, in particular also during operation of the negative pressure pump for evacuating the negative pressure line. The flow cross section of the valve or the flow cross section of the flow connection between the vacuum line and the external environment that can be produced by means of the valve is thus selected such that even during operation of the vacuum pump, in particular at nominal power or at maximum power, a reduction of the vacuum or at least a keeping constant of the vacuum in the vacuum line can take place.Within the scope of a further preferred embodiment of the invention, it is provided that a differential pressure sensor for measuring a pressure difference between the vacuum in the vacuum line and the ambient pressure is arranged in the vacuum line. It has already been explained above that the negative pressure can be specified as the relative negative pressure. This takes place in the form of the pressure difference between the negative pressure and the ambient pressure in the external environment. The differential pressure sensor is preferably fluidically present between the valve and the vacuum brake booster. Alternatively, it can of course be fluidically present between the vacuum pump and the valve.A further development of the invention provides that the vacuum pump, the valve and the differential pressure sensor are connected to a common control unit, in particular to a pressure regulator of the control unit. The control device is preferably different from a brake system control device of the brake system and is present in particular in addition to the latter. The brake system control device is used, for example, to implement driver assistance devices, in particular to implement an electronic stability program (ESP) and / or a brake assist. The control device preferably serves exclusively for operating the vacuum pump, the valve and the differential pressure sensor, for example, thus for receiving measurement signals of the differential pressure sensor and for controlling the vacuum pump and the valve.The control device preferably has the pressure regulator, which serves for the regulating setting of a specific negative pressure in the negative pressure line. An input variable of the pressure regulator is, for example, a difference between the pressure difference and a setpoint pressure difference or a difference between the negative pressure present in the negative pressure line and a setpoint negative pressure. On the basis of this controlled variable, the control unit generates actuating variables for the vacuum pump and the valve, i.e. adjusts these to reduce the difference mentioned.Finally, within the scope of a further embodiment, the brake system may be characterized by a further operating element for activating the vacuum pump and / or the valve, in particular for activating and / or deactivating the vacuum pump and / or for presetting a specific vacuum in the vacuum line and / or by a display for displaying the vacuum in the vacuum line and / or the pressure difference. The further operating element serves for the targeted activation of the vacuum pump or of the valve by a user, for example by an occupant of the motor vehicle.With the aid of the further operating element, the vacuum pump can be activated and / or deactivated, for example. Alternatively, the further operating element can be provided for presetting the determined negative pressure in the negative pressure line, i.e. for presetting the setpoint negative pressure or the setpoint pressure difference. The further operating element is different from the aforementioned operating element. The further operating element can be present, for example, in the form of a button, a switch or the like.Additionally or alternatively, the indication may be present, which serves to indicate the negative pressure in the negative pressure line and / or the pressure difference. The display is arranged, for example, in the field of vision of the driver or of occupants of the motor vehicle. The display is preferably in the form of a screen display or the like.The invention further relates to a method for operating a brake system for a motor vehicle, in particular a brake system according to the preceding explanations, wherein the brake system has at least one wheel brake, an operating element for actuating the wheel brake, and a vacuum brake booster which is provided in an operative connection between the operating element and the wheel brake and can be acted upon by a vacuum source with a vacuum, wherein the vacuum source is connected to the vacuum brake booster via a vacuum line, wherein the vacuum line can be connected to an external ambient flow by means of a valve which is provided in the vacuum line in terms of flow between the vacuum source and the vacuum brake booster. It is provided that exclusively an electric vacuum pump is used as the vacuum source and a solenoid valve is used as the valve.The advantages of such a procedure or such a configuration of the brake system have already been discussed. Both the method and the brake system can be further developed according to the above explanations, so that reference is made to these in this respect. It has already been explained above that the operation of the brake system is preferably carried out within the scope of a test operation. The method is thus used for operating the brake system for testing or, in other words, for carrying out a test operation of the brake system.A further embodiment of the invention provides that a specific negative pressure is set in the negative pressure line by means of the negative pressure pump. First, the vacuum pump is thus operated in such a way that the determined vacuum is established. For example, for this purpose, the control device can actuate the vacuum pump accordingly, in particular using the differential pressure sensor.A preferred embodiment of the invention provides that the vacuum pump is deactivated after the setting until actuation of an operating element or is operated in a controlled and / or regulated manner in order to keep the vacuum constant, and / or that the vacuum is reduced by means of the valve after the setting. As soon as the determined vacuum in the vacuum line is reached, the vacuum pump can be operated in different operating modes. One possibility here is to completely deactivate the vacuum pump, so that the vacuum line is not evacuated any further. Ideally, the negative pressure in the negative pressure line remains constant or at least almost constant even when the negative pressure pump is deactivated, at least when the valve is closed and / or the operating element is not actuated at the same time.However, a reduction in the negative pressure, i.e. an increase in the pressure in the negative pressure line, can occur if the operating element is actuated, i.e. a braking force is to be generated by means of the wheel brake. If the vacuum pump remains deactivated despite such a reduction in the vacuum, a defect in the vacuum brake booster can be simulated, for example. Alternatively, it can of course be provided that after the negative pressure has been set, the negative pressure pump is operated in order to keep the negative pressure constant. This can be done using the differential pressure sensor. For example, the vacuum pump remains deactivated until the actuation of the further operating element or is operated to keep the vacuum constant.Additionally or alternatively, it can be provided that after the setting of the determined negative pressure, the negative pressure in the negative pressure line is reduced by opening the valve for establishing a flow connection between the negative pressure line and the external environment. In this way, for example, a leakage of the vacuum line can be simulated.A further development of the invention provides that the vacuum pump is deactivated after the setting and is operated again for setting the specific vacuum only after a reduction of the vacuum in the vacuum line by a specific vacuum difference. The vacuum pump is therefore not operated directly to keep the vacuum constant after the setting of the determined vacuum. Rather, it is initially deactivated. If the vacuum subsequently falls by the determined vacuum difference, i.e. to a vacuum which is below the determined vacuum, the vacuum pump can be put into operation again. In this case, the vacuum pump is operated in such a way that the determined vacuum is set again. In this respect, an interval-like operation of the vacuum pump and a hysteresis-like behavior of the vacuum in the vacuum line are realized.Finally, in a further embodiment of the invention, it can be provided that a predefined vacuum profile is set in the vacuum line over time by means of the vacuum pump and / or the valve. The vacuum profile is stored in the control device, for example. The negative pressure profile has a predefined negative pressure for a plurality of points in time. When the respective point in time is reached, the negative pressure present in the negative pressure line is set to this predefined negative pressure using the negative pressure pump and / or the valve. The vacuum profile is preferably continuous. However, a discrete vacuum profile, in particular with abrupt or step-like changes of the preset vacuum, can also be provided. In this way, for example, a sudden leak in the vacuum line can be simulated.The invention is explained in more detail below with reference to the exemplary embodiments shown in the drawing, without any restriction of the invention being effected. The only one is shown hereFIG. is a schematic illustration of a region of a brake system for a motor vehicle.The FIGURE shows a schematic illustration of a part of a brake system 1 for a motor vehicle. The brake system 1 has, in addition to at least one wheel brake and an operating element for actuating the wheel brake, both of which are not shown, a vacuum brake force booster 2. This is present in the operative connection between the operating element and the wheel brake in order to convert an operating force acting on the operating element into a greater braking force present at the wheel brake. For this purpose, the vacuum brake booster 2 is supplied with vacuum via a vacuum line 3 from a vacuum source 4, which is designed as an electric vacuum pump. The vacuum brake booster 2 is only supplied with negative pressure from this negative pressure source 4.The vacuum line 3 is also assigned a valve 5, by means of which the vacuum line 3 can be connected in terms of flow to an external environment. The valve 5 is designed as a solenoid valve. Both the vacuum source 4, i.e. the vacuum pump, and the valve 5 are connected to a control unit 6. Both can be controlled by the control device 6. Furthermore, a differential pressure sensor 7 is connected to the control unit 6. The differential pressure sensor 7 is used to measure a pressure difference between the negative pressure in the negative pressure line 3 and the ambient pressure in the external environment.With the aid of the construction of the brake system 1 described here, comprehensive test operation of the brake system 1 can be carried out. In particular, a predefined vacuum profile over time can be set in the vacuum line 3 with the aid of the vacuum source 4 and the valve 5, and any scenarios for testing the brake system 1 can thus be realized.

Claims

Brake system (1) for a motor vehicle, having at least one wheel brake, an operating element for actuating the wheel brake, and a vacuum brake force booster (2) which is present in an operative connection between the operating element and the wheel brake and can be acted upon by a vacuum source (4) with a vacuum, wherein the vacuum source (4) is connected to the vacuum brake force booster (2) via a vacuum line (3), wherein the vacuum line (3) can be connected to an external ambient flow by means of a valve (5) which is present in the vacuum line (3) in terms of flow between the vacuum source (4) and the vacuum brake force booster (2), characterized in that exclusively an electric vacuum pump (4) serves as the vacuum source (4), and the valve (5) is designed as a solenoid valve.Brake system according to Claim 1, characterized in that a differential pressure sensor (7) for measuring a pressure difference between the ambient pressure in the vacuum line (3) and the ambient pressure is arranged in the vacuum line (3).Brake system according to Claim 2, characterized in that the vacuum pump (4), the valve (5) and the differential pressure sensor (7) are connected to a common control unit (6).Brake system according to Claim 2 or 3, characterized bya further operator control element for actuating the vacuum pump (4) and / or the valve (5) for activating and / or deactivating the vacuum pump (4) and / or presetting a specific vacuum in the vacuum line (3) and / or by a display for displaying the vacuum in the vacuum line (3) and / or the pressure difference.Method for operating a brake system (1) for a motor vehicle according to one or more of the preceding claims, wherein the brake system (1) has at least one wheel brake, an operating element for actuating the wheel brake and a vacuum brake force booster (2) which is present in an operative connection between the operating element and the wheel brake and can be acted upon by a vacuum source (4) with a vacuum, wherein the vacuum source (4) is connected to the vacuum brake force booster (2) via a vacuum line (3), wherein the vacuum line (3) can be connected to an external ambient flow by means of a valve (5) which is present in terms of flow between the vacuum source (4) and the vacuum brake force booster (2) in the vacuum line (3), characterized in that exclusively an electric vacuum pump (4) is used as the vacuum source (4) and a throttle valve is used as the valve (5).Method according to claim 5, characterised in that a specific vacuum is set in the vacuum line (3) by means of the vacuum pump (4).Method according to Claim 6, characterized in that the vacuum pump (4) is deactivated after the setting up until the actuation of a further operating element or is operated in a controlled and / or regulated manner in order to keep the vacuum constant, and / or in that the vacuum is reduced by means of the valve (5) after the setting.Method according to Claim 6 or 7, characterized in that the vacuum pump (4) is deactivated after the setting and is operated again for setting the specific vacuum only after a reduction of the vacuum in the vacuum line (3) by a specific vacuum difference.Method according to one of Claims 6 to 8, characterized in that a predefined vacuum profile over time is set in the vacuum line (3) by means of the vacuum pump (4) and / or the valve (5).

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