Electronically slip-controlled braking system for a motor vehicle and method for controlling an electronically slip-controlled braking system

Abstract

The invention relates to an electronically slip-controlled braking system for a motor vehicle with a first brake pressure generator (18e) for generating a brake pressure in a brake circuit (10) with a connected wheel brake (16), depending on a brake request, and a brake pressure modulation device (18) for modulating the brake pressure depending on a wheel slip on one of the wheels assigned to the wheel brake (16). The brake pressure modulation device (18) comprises a second brake pressure generator (18e) driven by a drive motor (18f) and connected in parallel to the first brake pressure generator (12) to the brake circuit (10), an electrically controlled first directional control valve (18a) for regulating the flow of hydraulic fluid from the wheel brake (16), and a hydraulic fluid reservoir (18c) for receiving hydraulic fluid released from the wheel brake (16) during brake pressure regulation. An electronic control unit (28) provides demand-based control of the components. A first pressure medium connection (22) from the first brake pressure generator (12) to the second brake pressure generator (18e) is controlled by an electrically controlled second directional control valve (20), which according to the invention is designed as a normally open directional control valve.

Description

Technical background

[0001] The invention relates to an electronically slip-controlled braking system for a motor vehicle according to the features of the preamble of claim 1 and a method for controlling an electronically slip-controlled braking system according to the features of the preamble of dependent claim 6.

[0002] Electronically controlled braking systems for motor vehicles, in various technical configurations, represent the state of the art and are also known as vehicle braking systems with anti-lock braking (ABS) or vehicle braking systems with electronic stability control (ESP). The purpose of such vehicle braking systems is to prevent the wheels of a motor vehicle from locking up during braking and / or driving, thereby maintaining the vehicle's steerability and counteracting unstable driving conditions.

[0003] The prerequisite for this is to adapt the brake pressure prevailing at a wheel brake to the slip conditions currently prevailing at one of the wheels assigned to that wheel brake.

[0004] For this purpose, an electronically controlled braking system of a motor vehicle is equipped with a hydraulic unit to which the wheel brake is hydraulically connected and which has a device for modulating the brake pressure at the wheel brake. This brake pressure modulation device includes, among other things, an electrically driven (second) brake pressure generator, electrically controlled directional control valves, an accumulator, and hydraulic connections that link these components together. An electronic control unit of the hydraulic unit electrically controls the aforementioned components according to a braking request. This braking request can be made by a driver by actuating a brake request detection device in the form of a pedal or lever, which activates a first brake pressure generator or...The pressure on the master brake cylinder can be determined mechanically or electrically by vehicle electronics that monitor the driving conditions. The first brake pressure generator is hydraulically supplied from a reservoir containing hydraulic fluid at atmospheric pressure.

[0005] In known hydraulic power units, the pressure in a brake circuit is adjusted by means of a pressure regulating valve. The pressure regulating valve is located in a so-called secondary hydraulic connection, which connects the secondary brake pressure generator to the master cylinder or the primary brake pressure generator. In the prior art, this pressure regulating valve is designed as a normally closed directional control valve, which can be moved from its closed position to a fully open position by the electronic control unit via electrical actuation for pressure regulation. Furthermore, a bypass to this pressure regulating valve is provided, in which a check valve is located. This check valve releases the hydraulic connection from the master cylinder or the primary brake pressure generator.The flow from the first brake pressure generator to the second brake pressure generator is unobstructed, allowing a brake pressure provided by the first brake pressure generator, which is higher than the prevailing brake pressure in the brake circuit, to reach the wheel brake. If brake pressure builds up through the second brake pressure generator, which is connected to the brake circuit in parallel with the first brake pressure generator, the check valve closes, driven by the prevailing brake pressure in the brake circuit, and shuts off the aforementioned second hydraulic connection.

[0006] In a brake system known from the prior art, when it is in its de-energized initial or default position, the first brake pressure generator is hydraulically isolated from the brake circuit. For this purpose, a so-called second-way valve is provided in the first hydraulic connection of the brake circuit, which leads from the first brake pressure generator to the second brake pressure generator. In the prior art, this second-way valve is normally closed and is only opened by the electronic control unit via electrical actuation when brake pressure is to be built up by the second brake pressure generator and this second brake pressure generator needs to be supplied with hydraulic fluid from the reservoir of the first brake pressure generator.

[0007] A disadvantage of a normally closed second-way valve is that a pressure reservoir for a brake pressure modulation device, located downstream of the second-way valve and featuring a separating element pre-tensioned by an elastic return element, cannot automatically empty itself towards the first brake pressure generator. Emptying such a pressure reservoir therefore requires electrical actuation of the second-way valve into a free-flowing position, which, however, causes noticeable valve noise, increases the energy consumption of the brake system, and thus negatively impacts its energy efficiency. Furthermore, the increased number of actuation cycles increases the demands on the service life of the second-way valve.

[0008] From DE 10 2006 026 872 A1, an electronically controlled braking system for a motor vehicle according to the features of the preamble of claim 1 is known.

[0009] The second directional control valve or high-pressure switching valve for controlling the first pressure medium connection from the first to the second pressure generator is, in this state of the art, as explained above, designed as a normally closed directional control valve and thus prevents, in the de-energized state of the brake system, an automatic emptying of a pressure medium accumulator connected downstream of this second directional control valve to the first pressure medium connection or to the brake circuit.

[0010] Because the conventional braking system can enter an operating state in which the brake pressure modulated by the brake pressure modulation device must increase the brake pressure specified by the driver via the master cylinder or the first brake pressure generator, a second directional control valve must be used that is technically capable of opening against this brake pressure specified by the driver. The necessary opening force requires a correspondingly powerful and space-consuming valve actuator. To nevertheless be able to use the most compact actuator possible, the second directional control valve in the prior art is designed as a multi-stage valve with two control stages, each with its own valve closing element. However, such a valve design is complex and expensive.Furthermore, multiple control stages in a directional control valve result in a higher pressure loss coefficient of the directional control valve and consequently a deterioration of the pressure build-up dynamics in the associated brake circuit.

[0011] If an electrically assisted brake pressure generator is used as the first brake pressure generator, the energy efficiency of the braking system is further reduced by a multi-stage directional control valve due to the increased drive power of this brake pressure generator.

[0012] To enable the accumulator to empty automatically, the known brake system features a separate relief channel that directly connects the pressure accumulator to the first brake pressure generator or the master brake cylinder. This relief channel is controlled by a spring-loaded check valve that opens when the pressure in the pressure accumulator is higher than the brake pressure supplied by the first brake pressure generator. However, this solution is disadvantageous because of the space required on the hydraulic unit to accommodate the relief channel, its manufacturing costs, the additional check valve, and the assembly costs.

[0013] Another brake system according to the features of the preamble of claim 1 and with a normally closed second directional control valve or high-pressure switching valve for controlling the first hydraulic fluid connection is disclosed in DE 10 2008 002 244 A1. In this known solution, a hydraulic fluid connection is used to empty the hydraulic fluid reservoir, which leads via the directional control valves of the brake pressure modulation device. However, this solution also requires electrical control to open one of these directional control valves and thus also reduces the energy efficiency of the brake system and / or increases the requirements for the service life of the directional control valve being controlled. Advantages of the invention

[0014] According to the characterizing features of claim 1, an electronically controlled braking system for a motor vehicle has a second directional control valve or high-pressure switching valve for controlling the first pressure medium connection from the first brake pressure generator or master brake cylinder to the second brake pressure generator, which is designed as a normally open valve.

[0015] Due to this type of directional control valve, the pressure reservoir contacted by the first pressure medium connection is continuously connected to the first brake pressure generator in the valve's de-energized initial state and can empty itself automatically, i.e., driven by its mechanical return mechanism, towards this first brake pressure generator or the reservoir coupled to it. This emptying of the pressure reservoir does not require electrical actuation of the directional control valve or activation of the second brake pressure generator, which on the one hand reduces the operating time of these components or extends their service life and on the other hand reduces the brake system's external energy consumption.A calculation model stored in the electronic control unit for estimating the fluid level of the accumulator exhibits higher reliability in a brake system with a normally open second-way valve. This, in turn, reduces the operating time of the aforementioned electrically actuated components and consequently lowers the external power requirement. Furthermore, any perceptible switching or operating noises from the brake system are reduced, and the driver receives improved haptic feedback from the brake system when the actuator is engaged.

[0016] Further advantages or beneficial training opportunities arise from the sub-claims and / or from the following description.

[0017] If a third-way valve is used to control a second hydraulic connection from the second brake pressure generator to the first brake pressure generator or master cylinder, and this third-way valve is also normally open, similar to the second-way valve, then a bypass to this third-way valve, known from the prior art, and consequently a check valve for controlling this bypass, can be omitted without replacement. Ultimately, this saves further installation space, additional components, and assembly effort.

[0018] Eliminating a check valve also removes a potential internal leakage point in the brake system, which is particularly important during pressure-holding maneuvers, such as when holding a braked vehicle on an incline. This also saves operating time for the brake pressure generator to compensate for any leakage that may occur, consequently extending its service life, improving the brake system's external energy consumption, and reducing operating noise. drawing

[0019] An embodiment of the invention is shown in the drawing and is explained in detail in the following description. The drawing comprises a single figure illustrating the invention by means of a hydraulic circuit diagram. This hydraulic circuit diagram represents, by way of example, a brake circuit of a brake system according to the invention in a schematically simplified manner using hydraulic circuit symbols.

[0020] The inventive braking systems can of course also be equipped with several brake circuits, each of which is symmetrical to the brake circuit shown in the figure. Description of the exemplary embodiment

[0021] To signal a braking request from the driver, the brake circuit (10) of a brake system shown in the figure includes a master brake cylinder (12a), which can be actuated by muscle power via an actuating element (14) in the form of a pedal or a lever. Depending on the actuation, the master brake cylinder (12a) generates brake pressure in the connected brake circuit (10), which is why the master brake cylinder (12a) is also referred to below as the first brake pressure generator (12).

[0022] The brake circuit (10) is connected to a pressure-actuated wheel brake (16) via an upstream brake pressure modulation device (18). The latter includes, among other things, an electronically controlled first directional control valve (18a) or inlet valve, through which the flow of hydraulic fluid to the wheel brake (16) and thus the build-up of brake pressure in this wheel brake (16) can be regulated. Furthermore, an outlet valve (18b), also electronically controlled, is connected in parallel to the first directional control valve (18a), through which hydraulic fluid can flow from the wheel brake (16) towards a hydraulic fluid reservoir (18c) of the brake pressure modulation device (18) to reduce brake pressure.

[0023] In this embodiment, the pressure medium accumulator is designed as a piston accumulator with a storage piston (18c1) (dividing element) which delimits a storage chamber (18c2) whose volume can be varied. The storage piston (18c1) is slidably mounted inside a storage cylinder (18c4) against the force of an elastic return element (18c3), e.g., a coil spring.

[0024] In addition to an inlet (18c5) opening into the storage chamber (18c2), the pressure medium accumulator (18) has an outlet (18c6) opening from the storage chamber (18c2). This outlet is controllably connected to the brake circuit (10) by a return valve (18d) at a point located upstream of a second pressure generator (18e) of the brake pressure modulation device (18). This second pressure generator (18e) can be actuated by an external force from a drive motor (18f) and, when actuated, delivers pressure medium from the master brake cylinder (12) through the then opened first directional control valve (18a) or inlet valve to the wheel brake (16), thus creating a pressure build-up therein.

[0025] Upstream of the point where the outlet (18c6) of the pressure accumulator (18c) is coupled to the brake circuit (10), an electrically controlled second-way valve (20) or high-pressure switching valve (20a) is provided. This high-pressure switching valve (20a) is designed to decouple the master brake cylinder (12a) or the first pressure generator (12) from the brake circuit (10) when brake pressure is built up in the wheel brake (16) by the second brake pressure generator (18e). The second-way valve (20) accordingly controls a first pressure medium connection (22) that leads from the master brake cylinder (12a) or the first brake pressure generator (12) to the second brake pressure generator (18e).

[0026] To regulate the brake pressure generated by the second brake pressure generator (18e) in the brake circuit (10), a third directional control valve (24) or pressure regulating valve (24a) is provided. This is located in a second hydraulic connection (26) via which the second brake pressure generator (18e) is connected to the master brake cylinder (12a) or the first brake pressure generator (12).

[0027] Both the electrical control of the directional control valves (18a; 20, 24) and that of the electric motor (18f) of the second brake pressure generator (18e) are carried out by an electronic control unit (28) of the brake system as required, i.e. according to the prevailing slip conditions at one of the wheels assigned to the wheel brake (16).

[0028] According to the invention, a normally open directional control valve (20) is used as the second directional control valve, which is designed as a proportional control valve, comprises two hydraulic connections and can be moved from a flow position (basic position) to a closed position in a plurality of intermediate positions by electrical control.

[0029] In its normal position, i.e., in the non-electrically actuated state of this second directional control valve (20), there is therefore a continuous hydraulic connection between the first brake pressure generator (12) and the second brake pressure generator (18e), or between the outlet (18c6) of the pressure accumulator (18c) into the brake circuit (10) and the first brake pressure generator (12). Consequently, pressure fluid located in the storage chamber (18c2) of the pressure accumulator (18c) can flow towards the first brake pressure generator (12) when actuated by the resetting return mechanism (18c3) of the pressure accumulator (18c). The pressure accumulator (18c) is thus able to empty itself automatically when the brake system is de-energized, for example, after a vehicle equipped with the brake system has been parked and its drive engine has been switched off.

[0030] The normally open second-way valve (20) is designed as a single-stage directional control valve and accordingly has exactly one valve closing element and one valve cross-section controlled by this valve closing element (not shown). Such a cost-effective valve design is possible because this directional control valve (20) opens mechanically and therefore does not require electrical actuation by the electronic control unit (28). Furthermore, compared to a multi-stage directional control valve known from the prior art, a single-stage directional control valve exerts a lower throttling effect on the hydraulic fluid flowing through the directional control valve, thus improving the dynamics with which a pressure change can occur in the brake circuit (10).

[0031] Apart from that, in the invention the third-way valve (24) is also designed to be normally open. This third-way valve (24) is also designed as a proportional control valve, has two ports, and can be electrically actuated from a fully open position (normal position) to a fully closed position through any number of intermediate stages.

[0032] In the non-energized basic position, the third-way valve (24) is open, thus creating a continuous hydraulic connection from the second brake pressure generator (18e) to the first brake pressure generator (12).

[0033] The third-way valve (24) is hydraulically non-flow-through arranged in the second hydraulic connection (26), so that all the hydraulic fluid passing through this second hydraulic connection (26) flows through the third-way valve (24). The invention dispenses with a bypass around the third-way valve (24), as known from the prior art, as well as with a check valve for controlling flow through such a bypass. In addition to saving on parts and assembly costs, this also saves valuable installation space.

[0034] Vehicles that have both an internal combustion engine and an electric motor as their drive motor—so-called hybrid vehicles, or vehicles powered by an electric motor—can convert the vehicle's kinetic energy into electrical energy during braking by switching the drive motor from engine operation to generator operation, thereby decelerating the vehicle. In such vehicles, the total braking torque corresponding to the driver's braking request is thus composed of a braking torque generated by the generator and a braking torque supplied by the braking system. However, the generator's contribution to the total braking torque decreases as the vehicle speed drops and must be compensated for by a correspondingly increasing contribution from the braking system.Such a process is called blending of braking torques and, controlled by the electronic control unit (28) of the braking system, takes place largely unnoticed by the driver or the vehicle occupants.

[0035] If, in an electrically driven vehicle, the driver increases the initial braking request during a braking process by applying more pressure to the pedal or lever, i.e., the actuating element (14) of the first brake pressure generator (12), the increased brake pressure cannot reach the wheel brake (16) due to the lack of a bypass to the third-way valve (24) in a braking system according to the invention, because the third-way valve (24) normally used for this purpose is electrically controlled by the electronic control unit (28) during the braking process and accordingly assumes its closed position.

[0036] This disadvantage can be avoided by a brake system control system according to the invention, provided that this brake system is equipped with a sensor device (30) which detects the brake pressure at the first brake pressure generator (12) or at the master brake cylinder (12a) and transmits a corresponding electrical signal to the electronic control unit (28) of the brake system. During an evaluation of the signal, the control unit detects the pressure increase and subsequently terminates the electrical actuation of the second directional control valve (20) or high-pressure switching valve (20a) and / or the third directional control valve (24) or pressure regulating valve (24a). The directional control valves (20, 20a; 24, 24a) then open and thus transmit the pressure generated at the first brake pressure generator (12) (master brake cylinder, 12a), which is higher than the prevailing pressure in the brake circuit, to the wheel brake (16).In simplified terms, the invention implements the function of a check valve in a bypass to the third-way valve (24), which is known from the prior art, by means of a control method stored in the electronic control unit (28).

[0037] Of course, modifications or additions to the described embodiment beyond these explanations are conceivable without thereby departing from the scope of protection of the invention specified by the claims. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2006 026 872 A1

[0008] DE 10 2008 002 244 A1

[0013]

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