Motor vehicle braking system with protection mode

Abstract

The invention relates to a motor vehicle braking system comprising a first assembly with a first pressure supply device and a second assembly with a second pressure supply device, wherein an isolation valve is provided for separating the first and the second assembly and The second unit comprises a circuit isolating valve, which divides the second unit into a first brake circuit comprising the first wheel brakes and the second pressure supply unit, and a second brake circuit comprising the second wheel brakes. To prevent problems with the isolating valve, a control device is provided which, in the event of a fault in the first unit, switches to a fallback mode in which only the second unit builds up and regulates the pressure in the wheel brakes. The control device then switches to a protective mode in which, when pressure is requested, the isolating valve is open and the circuit isolating valve is closed.

Description

[0001] The invention relates to a motor vehicle brake system comprising a first structural unit with a first pressure supply device and a second structural unit with a second pressure supply device, wherein an isolation valve is provided for separating the first and second structural units, and the second structural unit comprises a circuit separation valve which separates the second structural unit into a first brake circuit comprising first wheel brakes and the second pressure supply device, and a second brake circuit comprising second wheel brakes. The invention also relates to a method for controlling the motor vehicle brake system. The structural units can be designed as separate housings or formed in a common housing and / or block.

[0002] From DE 10 2020 214 634 A1, another type of redundant motor vehicle brake system is known in which two wheel brakes are connected to the first structural unit and two wheel brakes to the second structural unit, which accordingly does not include a circuit isolation valve.

[0003] A motor vehicle brake system with only one component is known, for example, from DE 10 2020 212 838 A1.

[0004] Hydraulic valves are typically asymmetrically constructed; a hydraulic pressure difference closes the valve in a first direction and opens it in a second direction. Normally open valves are also designed so that they remain open without an electrical current through a switching coil and without a pressure difference. Such valves therefore also have a pressure-dependent holding current. This means that an electrical current that holds the valve closed has a smaller value when a pressure difference is present in the first direction and a larger value when a pressure difference is present in the second direction.

[0005] It can therefore happen that in various operating situations a valve has to be controlled with a high valve current for a long time, so that its coil, and thus also other electronic components located in the vicinity, gradually heats up. Since sufficient cooling of these elements cannot always be guaranteed, this can cause damage to the braking system, particularly its electronic units. Furthermore, an appropriate power supply cannot always be guaranteed with certainty. This is particularly problematic in fallback levels in which the valves are operated outside of normal operating situations. This issue is particularly relevant for redundant braking systems, which comprise two components to provide the aforementioned redundancy.However, it is crucial that a failure in one component does not negatively impact the other, which is why the components must be separated. If a valve designated for this purpose cannot be closed properly, the redundancy fails.

[0006] It is therefore an object of the present invention to provide a motor vehicle brake system which avoids the above disadvantages.

[0007] The problem is solved by a control device which is configured to switch to a fallback mode in the event of a fault in the first structural unit, in which only the second structural unit builds up and regulates the pressure in the wheel brakes, wherein the control device switches to a protective mode in which, in the event of a pressure request, the isolation valve is open and the circuit isolating valve is closed. The isolation valve and circuit isolating valve can be designed to be normally open, so that the isolation valve experiences no or only a lower thermal load when it is open. The first and second structural units can in particular have separate housings which can be arranged at a distance from one another. Required connections can be made between the structural units via electrical and / or hydraulic lines.

[0008] In a preferred embodiment of the invention, the control device is configured to open and / or leave the isolation valve open and to close and / or leave the circuit isolation valve closed in the protection mode only when the pressure demand is less than a pressure threshold. However, when the pressure demand is greater than the pressure threshold, the isolation valve is closed and the circuit isolation valve is open. The pressure threshold can preferably be between 0 and 50 bar, particularly preferably between 10 and 30 bar. For example, a pressure threshold of 20 bar can be selected.

[0009] The isolation valve can be aligned such that pressure on the side of the first pressure supply device pushes it open and / or holds it open, and pressure on the side of the second pressure supply device closes or holds it shut. A higher pressure on the side of the second pressure supply device leads in particular to a lower holding current, which is required to keep the isolation valve securely closed. This means that the isolation valve can be used at higher pressures with low valve currents without, for example, thermal problems occurring or the current control capability being exceeded. At lower pressures, however, when the required holding current would be higher, the circuit isolation valve can be used instead, for example to prevent overheating of the isolation valve or its surrounding electronics.

[0010] In a further preferred embodiment of the invention, when the isolation valve is open and the circuit isolation valve is closed, the second pressure supply device is only openly connected to the first wheel brakes, and when the isolation valve is closed and the circuit isolation valve is open, the second pressure supply device is only openly connected to the first wheel brakes and the second wheel brakes. The first wheel brakes can preferably be arranged on a rear axle and the second wheel brakes on a front axle. It is therefore preferable to brake with the circuit isolation valve open, since in this case more wheel brakes are available. This is used by the invention whenever high deceleration is desired and at the same time does not cause any problems, thus ensuring reliability.

[0011] In a further preferred embodiment of the invention, the first pressure supply device is a linear actuator whose pressure output is connected to the isolation valve and, in particular, to a brake fluid reservoir via a normally open isolating valve. Linear actuators enable pressure control in which both pressure buildup and pressure reduction can be adjusted directly with the pressure source, since this can both release and absorb the brake fluid in a controlled manner.

[0012] In a further preferred embodiment of the invention, the control device is configured to directly activate the protection mode in the fallback mode. This means that in the event of a fault that causes the control device to switch to fallback mode, the protection mode is always active, regardless of other variables. This is particularly useful when damage would otherwise be expected quickly due to sensitive electronics in the vicinity of the isolation valve and / or in the case of isolation valves with high electrical power to maintain the closed state. This is thus preventatively prevented.

[0013] In another preferred embodiment of the invention, the control device is configured to activate the protection mode in the fallback mode when the availability of the isolation valve is at least limited. This is the case when the isolation valve cannot at least completely ensure the separation between the first unit and the second unit.

[0014] In another preferred embodiment of the invention, the control device is configured to activate the protection mode if thermal problems are detected in the fallback mode. If this is not the case, the isolation valve remains closed and the circuit isolation valve remains open in the fallback mode, so that pressure is built up at more wheel brakes to decelerate the vehicle. Thermal problems can be detected if the temperature of the isolation valve exceeds a temperature threshold, whereby the temperature can be derived from a corresponding sensor or a model calculation. It can be a time value, a current integral, and / or an ohmic resistance value.

[0015] In a further preferred embodiment of the invention, the control device is configured to activate the protection mode when problems with the current control capability are detected in the fallback mode. This can be caused, for example, by undervoltage in the vehicle electrical system or a fault / overload in the power supply. Thus, the variability of the protection mode is also utilized in this case, so that at high pressures, when only smaller valve currents are required, braking can be performed with the first and second wheel brakes.

[0016] In a further preferred embodiment of the invention, the circuit isolation valve is designed and aligned such that a pressure on the side facing the second pressure supply device keeps the circuit isolation valve closed.

[0017] In a further preferred embodiment of the invention, in a pressure range lower than the pressure threshold, the holding current and / or the electrical power required to keep the circuit isolation valve closed is lower than the holding current and / or the electrical power required to keep the isolation valve closed. Thus, by using the circuit isolation valve appropriately, the electrical power can be reduced, thereby reducing the load on the valve, electronics, and vehicle electrical system.

[0018] In a further preferred embodiment of the invention, the pressure threshold is designed in two parts to implement hysteresis behavior. Thus, there is a first, lower pressure threshold and a second, higher pressure threshold. The isolation valve is opened and the circuit separation valve is closed when the pressure demand falls below the first pressure threshold, and the isolation valve is closed and the circuit separation valve is opened when the pressure demand rises above the second pressure threshold. This avoids excessive switching between these two operating modes, thereby preventing unnecessary noise from valve switching. Preferably, a first pressure threshold of 10 bar and a second pressure threshold of 30 bar can be selected.

[0019] In a further preferred embodiment of the invention, the control device is configured to adapt the pressure provided by the second unit to the number of wheel brakes supplied. A total braking torque should always be provided that is independent of the current switching state of the valves. Depending on the switching state, the total braking torque can be provided by the first wheel brakes or by the first and second wheel brakes. In the latter case, a lower braking torque per wheel brake, and thus a lower pressure, is required than if the same total braking torque had to be provided with fewer wheel brakes.

[0020] In a further preferred embodiment of the invention, the control device is configured to close the isolation valve and open the circuit isolation valve when the pressure demand gradient exceeds a gradient threshold. This allows for immediate braking with the first and second wheel brakes, even at low absolute pressure demand values. This is possible because, with a high gradient, a value is expected that exceeds the pressure demand threshold after a short time. The thermal input during this short period of time is assumed to be low.

[0021] In a further preferred embodiment of the invention, the control device is configured to close the inlet valves of the second wheel brakes before opening the circuit isolation valve and subsequently to gradually build up the hydraulic pressure in the second wheel brakes, in particular with a linear ramp. Thus, when the circuit isolation valve is opened, there is no sudden increase in the braking force on the second wheel brakes, allowing the driver to safely control the vehicle.

[0022] In a further preferred embodiment of the invention, the isolation valve is part of the second unit. This allows it to be controlled even if the first unit fails.

[0023] In a further preferred embodiment of the invention, the control device is constructed in two parts, comprising a first control unit and a second control unit. The first control unit is assigned to the first structural unit and, in particular, exclusively controls the hydraulic actuators of the first structural unit, and the second control unit is assigned to the second structural unit and, in particular, exclusively controls the hydraulic actuators of the second structural unit. This also ensures electronic redundancy.

[0024] The object is further achieved by a method for controlling a motor vehicle brake system, wherein, in the event of a fault in a first structural unit having a first pressure supply device, a fallback mode is switched on in which only a second structural unit having a second pressure supply device builds up and regulates pressure in the wheel brakes, wherein a protection mode is switched on in which, in the event of a pressure request, an isolation valve is opened to separate the first and second structural units, and a circuit separation valve is closed, which separates the second structural unit into a first brake circuit comprising first wheel brakes and the second pressure supply device and a second brake circuit comprising second wheel brakes. This also means keeping the respective valves open or closed if they are already in the state to be set.

[0025] Further features, advantages, and possible applications of the invention will become apparent from the following description of exemplary embodiments and the drawings. All described and / or illustrated features, both individually and in any combination, are part of the subject matter of the invention, regardless of their summary in the claims or their references. Fig. 1 shows schematically a braking system according to the invention; Fig. 2 shows a characteristic curve of the isolation valve;

[0026] In Fig. Figure 1 shows a redundant hydraulic braking system for motor vehicles. For example, the braking system is designed to actuate four hydraulically actuated wheel brakes 8; expansion to more wheel brakes is easily possible. For example, the two upper wheel brakes (HL, HR) in the figure, referred to here as the first wheel brakes, are assigned to the rear axle, and the wheel brakes (VL, VR), referred to here as the second wheel brakes, are assigned to the front axle of the vehicle.

[0027] The brake system comprises a first structural unit, which is embodied, for example, as a first electrohydraulic brake control unit with a valve block and a first electronic control unit ECU1, and a second structural unit, which is embodied, for example, as a second electrohydraulic brake control unit with a valve block and a second electronic control unit ECU2. The valve blocks each form a separate housing.

[0028] A pressure fluid reservoir 4 with two chambers is arranged on the first structural unit, with a first reservoir connection being assigned to the first chamber and a second reservoir connection being assigned to the second chamber. Reservoirs with three or more chambers are also possible.

[0029] A first electrically actuated pressure source 5 is arranged in the first structural unit.

[0030] In the second structural unit, a second electrically actuated pressure source 2 and wheel-individual brake pressure modulation valves are arranged, which are designed as an electrically actuated inlet valve 6 and an electrically actuated outlet valve 7 for each wheel brake 8.

[0031] The first pressure source 5 and the second pressure source 2 are connected on the pressure side to a brake supply line to which the four inlet valves 6 are connected. Thus, all four wheel brakes 8 can be actuated by means of the first pressure source 5 and / or the second pressure source 2.

[0032] An electrically actuated circuit isolation valve 40 is arranged in the brake supply line and thus in the second structural unit, so that when the circuit isolation valve 40 is closed, the brake supply line is separated into a first line section, to which the inlet valves 6 or the first wheel brakes 8 of the rear axle are connected, and a second line section, to which the inlet valves 6 or the second wheel brakes 8 of the front axle are connected. The second pressure source 2 is hydraulically connected to the first line section, and the first pressure source 5 is hydraulically connected to the second line section. When the circuit isolation valve 40 is closed, the brake system is thus separated or divided into two hydraulic brake circuits I and II, or a first partial circuit and a second partial circuit.In the first brake circuit I, pressure source 2 is connected (via the first line section) only to the wheel brakes 8 of the rear axle, and in the second brake circuit II, the first pressure source 5 is connected (via the second line section) only to the wheel brakes 8 of the front axle. The circuit separation valve 40 is advantageously designed to be open when de-energized. Such an operating mode can be called circuit separation mode or ACS for "Active Circuit Separation."

[0033] As already mentioned, the braking system comprises an inlet valve 6 and an outlet valve 7 for each hydraulically actuated wheel brake 8, which are hydraulically interconnected in pairs via central connections and each connected to a hydraulic wheel connection of the second structural unit to which the corresponding wheel brake 8 is connected. A check valve opening towards the brake supply line is connected in parallel to each of the inlet valves 6. The output connections of the outlet valves 7 are connected to the pressure fluid reservoir 4 or its second chamber via a common return line. The input connections of all inlet valves 6 can be supplied with a pressure provided by the first pressure source 5 or, for example, if the first pressure source 5 fails, by the second pressure source 2.

[0034] The first electrically controllable pressure source 5 of the valve block is designed as a hydraulic cylinder-piston arrangement (or a single-circuit electro-hydraulic actuator (linear actuator)), the piston of which can be actuated by a schematically indicated electric motor with the interposition of a rotation-translation gear, also shown schematically, in particular can be moved forwards and backwards in order to build up and reduce pressure in a pressure chamber. The piston delimits the pressure chamber of the pressure source 5. To control the electric motor, a rotor position sensor (only schematically indicated) is provided that detects the rotor position of the electric motor. By means of this sensor, the piston position and speed, and thus the released or absorbed volume of brake fluid, can be determined.

[0035] A system pressure line section is connected to the pressure chamber of the first electrically controllable pressure source 5. By means of this line section, the pressure source 5 or its pressure chamber is connected to a hydraulic connection of the first structural unit, which is connected to a hydraulic connection of the second structural unit via a hydraulic connecting element. This connection represents the only hydraulic pressure connection between the first and second structural units. It is a hydraulic connection for transmitting brake pressure for actuating the wheel brakes 8.

[0036] The pressure chamber is connected to the pressure fluid reservoir 4 via a (suction) line, regardless of the piston's actuation state. A check valve 53, closing toward the pressure fluid reservoir 4, is arranged in the line and connected to the second chamber. An electrically switchable valve 23 forms a further connection to this line, which is connected to the output port of the linear actuator 5. This isolating valve 23 is designed to be open when de-energized, so that when de-energized, the wheel brakes 8 are connected to the brake fluid reservoir 4. The cylinder-piston arrangement 5, for example, has no sniffing holes.

[0037] The second electrically controllable pressure source 2 of the second assembly is designed, for example, as a two-piston pump whose two pressure sides are interconnected. The suction sides are connected to the return line and thus to the pressure fluid reservoir 4. The pressure sides are connected to the first line section of the brake supply line.

[0038] In addition to the pressure source 2 and the brake pressure modulation valves 6, 7, an electrically actuated, preferably normally open, isolation valve 26 is arranged in the second structural unit, for example. Isolation valve 26 is hydraulically arranged between the connection and the second line section of the brake supply line. Thus, the first pressure source 5, and thus the entire first structural unit, is separably connected to the second line section or the brake supply line via the isolation valve 26.

[0039] For example, the braking system includes a pressure sensor in brake circuit I, which is thus assigned to the second pressure source 2. This is advantageous for burst protection during active circuit separation, i.e., when the circuit separation valve 40 is closed. However, the pressure sensor can also be located in brake circuit II.

[0040] For example, the brake system for leakage monitoring comprises a level measuring device for determining a pressure medium level in the pressure medium reservoir 4.

[0041] Each valve block is assigned an electronic control unit (ECU1 and ECU2). Each electronic control unit comprises electrical and / or electronic elements (e.g., microcontrollers, power units, valve drivers, other electronic components, etc.) for controlling the electrically actuated components of the associated valve block and, if applicable, the associated sensors—thus, the entire respective assembly. The valve block and electronic control unit are advantageously designed as an electrohydraulic unit, as is known.

[0042] The first electronic control device controls the first pressure source 5. For example, the first pressure source 5 is supplied with energy (from a first electrical energy source) via the first electronic control device.

[0043] The second electronic control device controls the second pressure source 2. For example, the second pressure source 2 is supplied with energy (from a second electrical energy source) via the second electronic control device.

[0044] For example, the first pressure source or primary pressure source 5 can be or is controlled exclusively by the first electronic control device and the second pressure source or secondary pressure source 2 can be or is controlled exclusively by the second electronic control device.

[0045] During normal operation, the pressure in the wheel brakes is built up by primary pressure source 5. The pressure in primary pressure source 5 is released by retracting its piston. The pressure is modulated for each wheel as needed by the inlet and outlet valves. If necessary, isolation valve 26 is closed so that primary pressure source 5 can draw in additional volume.

[0046] If a particularly high flow rate is required, both pressure sources 5 and 2 operate in parallel. If a particularly high pressure is required, the isolation valve 26 is closed, and the secondary pressure source 2 increases the pressure above the pressure of the primary pressure source 5. Outside of braking situations, atmospheric pressure equalization can be permanently ensured via the isolation valve 23 and the isolation valve 26.

[0047] The control units ECU1 and ECU2 are configured to implement a pressure buildup using the linear actuator 5 and / or the piston pump 2 based on pressure requests. These requests can originate from automatic functions of the assistance program and, in particular, from the driver, who transmits the driver's request via the brake pedal. This is transmitted to the control units as a brake pedal actuation variable. The brake pedal actuation variable can be a brake pedal travel, a brake pedal force, and / or similar variables.

[0048] If a leak occurs in the brake system, the circuit separation valve 40 is closed, dividing the system into two independent brake circuits, I and II. The leak can be monitored using various monitors. A Volume Deviation Monitor (VDM) is a leak monitoring device that can detect a hydraulic leak based on the pressure-to-volume ratio (PV ratio). The volume is determined by sensing the linear actuator or pedal position. The expected PV behavior can be stored using functions or stored characteristic curves. A low brake fluid sensor level (BFLS) in the reservoir or excessively low pressure can also lead to leak detection.

[0049] Isolation valve 26 is preferably controlled by the secondary ECU. The following description of operation in the event of a fault refers to this valve assignment.

[0050] If the secondary system fails electrically, specifically the secondary ECU or its voltage source, the pressure is increased and decreased via the primary pressure source 5, as in normal operation. Individual wheel pressure control is not required, but joint modulation of the wheel pressures remains possible to prevent the vehicle from being destabilized by locking wheels.

[0051] If the primary system fails electrically, particularly the primary system or its power supply, the secondary ECU switches to a fallback mode in which the second component is the sole pressure source for the vehicle's wheel brakes 8. To do so, it closes the isolation valve 26 to allow pressure to build up via the secondary pressure source 2. Without the isolation valve 26 closed, the brake fluid would continue to flow into the reservoir via the normally open isolating valve 23, thus preventing pressure from building up. Pressure is released via the outlet valves 7. Preferably, the inlet and outlet valves are controlled by the secondary ECU so that the pressure can be modulated for each wheel.

[0052] In this fallback mode, the temperature of the isolation valve 26 is monitored. This can be done using sensors, which calculate the ohmic resistance of the isolation valve 26, which is temperature-dependent, from the applied voltage and current. By determining the resistance, the temperature increase can be determined using the temperature coefficient of copper. Copper changes its resistance by a factor of 0.00393 (1 / K). A temperature increase of 100K leads to a resistance increase of 39.3%. Alternatively, a temperature model can be used, from which the temperature is determined based on the electrical power curve of the isolation valve coil.

[0053] If the temperature exceeds a threshold value stored in the ECU2, it switches to protection mode in addition to the fallback mode. In this protection mode, the isolation valve 26 and the circuit isolation valve 40 are used depending on the pressure requirement to shut off the first component and thus the pressure-free area. For example, if the brake pedal is only lightly depressed by the driver, resulting in a pressure requirement of 5 bar, the isolation valve 26 would have to be supplied with a very high holding current to keep it closed, as can be seen from the characteristic curve 61 of the Fig. 2 can be seen.

[0054] According to the invention, in the protection mode, the circuit isolation valve 40 is closed instead, and the isolation valve 26 is not supplied with current. The circuit isolation valve 40 must be supplied with a lower holding current at the same differential pressure, as can be seen from the characteristic curve 62 of the Fig. 2. The second pressure supply device 2 is then only connected to the first wheel brakes 8 of the rear axle, while the second wheel brakes on the front axle are not pressurized at all. The isolation valve 26 can therefore cool down, while such a small driver braking request can be easily implemented by just two wheel brakes 8. To ensure this, when only the first wheel brakes are actuated, a characteristic curve is used that indicates a relationship between the actuation of the brake pedal and the pressure to be built up, which requires a higher pressure for the same brake pedal actuation in order to account for the smaller number of wheel brakes. For example, the pressure requirement can be increased from 5 bar to 10 bar at two wheel brakes.

[0055] If the pressure requirement continues to rise, for example because the driver presses the brake pedal harder, the pressure requirement will eventually exceed a second, higher threshold of 30 bar. With a differential pressure of 30 bar across the isolation valve 26, a significantly lower holding current would be required, as can be seen from the Fig. 2. This is considered unproblematic with regard to thermal input. Accordingly, when this second pressure threshold is exceeded, the isolation valve 26 is closed and the circuit separation valve 40 is opened to connect both the first and second wheel brakes 8 of both axles to the pressure supply device 2.

[0056] To prevent a sudden increase in the total braking torque, which would occur due to the sudden connection of the additional wheel brakes, their inlet valves 6 are closed before the circuit isolation valve 40 opens. Subsequently, the pressure in these wheel brakes is regulated to the desired pressure by appropriately controlling the inlet valves. In particular, the pressure can be slowly increased to the desired target pressure using a pressure ramp.

[0057] If in the future the pressure requirement falls below a first lower pressure threshold, the circuit isolation valve 40 is closed again and the isolation valve 26 is opened.

[0058] The invention ensures that the braking system remains operational even during long braking phases in the low pressure range.

Claims

[1] Motor vehicle brake system comprising a first structural unit with a first pressure supply device (5) and a second structural unit with a second pressure supply device (2), wherein an isolation valve (26) is provided for separating the first and the second structural unit and the second structural unit comprises a circuit separation valve (40) which separates the second structural unit into a first brake circuit comprising first wheel brakes (8) and the second pressure supply device (2) and a second brake circuit comprising second wheel brakes (8), characterized by that a control device is provided which is designed to switch to a fallback mode in the event of a fault in the first structural unit, in that only the second structural unit builds up and regulates the pressure in the wheel brakes, wherein the control device switches to a protection mode in which the isolation valve is opened and the circuit isolation valve is closed in the event of a pressure request. [2] Motor vehicle brake system according to claim 1 characterized by in that the control device is designed to open and / or leave open the isolation valve (26) and to close and / or leave closed the circuit separation valve (40) in the protection mode only when the pressure requirement is less than a pressure threshold, but wherein the isolation valve (26) is closed and the circuit separation valve (40) is open when the pressure requirement is greater than the pressure threshold. [3] Motor vehicle brake system according to claim 1 or 2, characterized by that when the isolation valve (26) is open and the circuit separation valve (40) is closed, the second pressure supply device (2) is only connected to the first wheel brakes (8) in a flow-open manner, and when the isolation valve (26) is closed and the circuit separation valve (40) is open, the second pressure supply device (2) is only connected to the first wheel brakes (8) and the second wheel brakes (8) in a flow-open manner. [4] Motor vehicle brake system according to one of the preceding claims, characterized by that the first pressure supply device (5) is a linear actuator whose pressure output is connected to the isolation valve (26) and to a brake fluid reservoir (4). [5] Motor vehicle brake system according to one of the preceding claims, characterized by that the control device is designed to always activate the protection mode in the fallback mode. [6] Motor vehicle brake system according to one of the preceding claims, characterized by that the control device is designed to activate the protection mode in the fallback mode when the availability of the isolation valve (26) is at least limited. [7] Motor vehicle brake system according to one of the preceding claims, characterized bythat the control device is arranged to activate the protection mode if thermal problems of the isolation valve (26) are detected in the fallback mode. [8] Motor vehicle brake system according to one of the preceding claims, characterized by that the control device is arranged to activate the protection mode if problems with the current control capability are detected in the fallback mode. [9] Motor vehicle brake system according to one of the preceding claims, characterized by that the circuit isolation valve (40) is designed and aligned such that a pressure on the side facing the second pressure supply device (2) keeps the circuit isolation valve (40) closed. [10] Motor vehicle brake system according to one of the preceding claims, characterized bythat in a pressure range smaller than the pressure threshold, the holding current (61) and / or the electrical power required to keep the circuit isolation valve (40) closed is smaller than the holding current (61) and / or the electrical power required to keep the isolation valve (26) closed. [11] Motor vehicle brake system according to one of the preceding claims, characterized by that the pressure threshold is designed in two parts in order to implement hysteresis behavior. [12] Motor vehicle brake system according to one of the preceding claims, characterized by that the control device is designed to adapt the pressure provided by the second structural unit to the number of wheel brakes (8) supplied. [13] Motor vehicle brake system according to one of the preceding claims, characterized bythat the control device is designed to close the isolation valve (26) and open the circuit isolation valve (40) when the pressure demand gradient is greater than a gradient threshold value. [14] Motor vehicle brake system according to one of the preceding claims, characterized by that the control device is designed to close the inlet valves (6) of the second wheel brakes (8) before opening the circuit isolation valve (40) and subsequently to control the hydraulic pressure in the second wheel brakes (8) to gradually build up. [15] Motor vehicle brake system according to one of the preceding claims, characterized by that the isolation valve (26) is part of the second structural unit. [16] Motor vehicle brake system according to one of the preceding claims, characterized bythat the control device is constructed in two parts with a first control unit and a second control unit, wherein the first control unit is assigned to the first structural unit and the second control unit is assigned to the second structural unit. [17] Method for controlling a motor vehicle brake system according to one of the preceding claims, characterized byin that, in the event of a fault in a first structural unit of the motor vehicle brake system, having a first pressure supply device (5), a fallback mode is switched on in which only a second structural unit of the motor vehicle brake system, having a second pressure supply device (2), builds up and regulates a pressure in wheel brakes (8), wherein in the fallback mode a protective mode is additionally switched on in which, in the event of a pressure request, an isolation valve (26) for separating the first and second structural units is opened or remains open and a circuit separation valve (40), which separates the second structural unit into a first brake circuit comprising first wheel brakes (8) and the second pressure supply device (2) and a second brake circuit comprising second wheel brakes (8), is closed or remains closed.

Images