Method for controlling a hydraulic braking system to avoid the hydraulic fallback level

Abstract

A method for controlling a hydraulic brake system of a motor vehicle comprising a linear actuator (5) as a driver-independent pressure supply device, and a hydraulic valve arrangement (26) between the linear actuator (5) and the wheel brakes (8) of the hydraulic brake system, wherein, in the event of reduced availability of the linear actuator (5), a holding mode is performed in which the hydraulic pressure in the wheel brakes (8) is locked in by means of the valve arrangement (26) and the power of the linear actuator (5) is reduced, wherein the system pressure profile (70) is measured in the holding mode and, based on the system pressure profile (70), a transition from the holding mode to a substitute mode is performed, characterized in that a substitute mode is a cooperative mode in which an additional hydraulic pressure supply device (57) is activated to increase the locked-in pressure and subsequently the system switches back to the holding mode.

Description

[0001] The invention relates to a method for controlling a hydraulic braking system of a motor vehicle, comprising a linear actuator as a driver-independent pressure supply device, and a hydraulic valve arrangement between the linear actuator and the wheel brakes of the hydraulic braking system. The invention also relates to a corresponding hydraulic braking system.

[0002] The driver-independent pressure build-up via a linear actuator makes it possible to implement a multitude of complex control systems that improve the comfort and safety of a braking system. However, the motor of the linear actuator has mechanical and, in particular, thermal limits that preclude continuous operation. In the event of an overload and the associated reduced availability, the linear actuator must therefore be degraded, up to and including complete shutdown, to prevent irreversible damage. For this purpose, the braking system is switched to a hydraulic fallback mode, in which the driver must apply braking force solely through muscle power.

[0003] From DE 10 2016 220 752 A1 a braking system is known in which the pressure is maintained even during a pressure holding phase by means of the pressure supply device, by regulating the speed to 0.

[0004] German patent DE 10 2018 221 757 A1 discloses a braking system whose master cylinder has an integrated pedal feel simulator, which is used in a normal mode while the pressure is provided by an actuator. In an emergency mode, the master cylinder directly actuates the wheel brakes.

[0005] It is therefore an object of the present invention to avoid premature switching to the hydraulic fallback level.

[0006] The problem is solved by a method for controlling a motor vehicle's hydraulic braking system, comprising a linear actuator as a driver-independent pressure supply device and a hydraulic valve arrangement between the linear actuator and the wheel brakes of the hydraulic braking system. In the event of reduced availability of the linear actuator, a holding mode is implemented in which the hydraulic pressure in the wheel brakes is held in place by means of the valve arrangement, and the linear actuator's output is reduced, in particular to zero. During holding mode, the system pressure profile—that is, the hydraulic pressure in a region connected to the wheel brakes and therefore providing information about the braking force—is measured, and a transition from holding mode to a substitute mode is performed based on this system pressure profile.

[0007] As a starting condition for the hold mode, it could be, for example, that the linear actuator indicates reduced availability, such as a thermal overload, the vehicle is stationary, and a detected driver braking request corresponds to a pressure request greater than 5 to 10 bar. As a termination condition for the hold mode, it could be that the vehicle is rolling, i.e., has a speed greater than zero, the driver request decreases by a predetermined absolute or percentage amount, or the driver applies more than 500 N to the brake pedal.

[0008] The transition to the backup mode avoids the need to directly degrade the braking system to a full hydraulic fallback mode.

[0009] In a preferred embodiment of the invention, the extent to which the system pressure has dropped in holding mode is determined, with the system switching to the substitute mode when the system pressure has dropped below a threshold value. The threshold value can, in particular, be selected as a percentage of the initial value at the start of holding mode. Alternatively, the system pressure profile can also be analyzed in other ways; for example, the rate at which the system pressure drops can be checked.

[0010] In a particularly preferred embodiment of the invention, the threshold value is determined based on the road gradient. On a steep slope, only smaller deviations from the original value are permitted than on level ground. For example, two threshold values ​​can be provided. If the vehicle gradient is less than a gradient limit a and the measured system pressure during the holding mode decreases by a first percentage threshold, then the standstill protection is switched to the substitute mode. If, on the other hand, the vehicle gradient is greater than or equal to the gradient limit a, the standstill protection is switched to the substitute mode as soon as the measured system pressure during the holding mode decreases by a second percentage threshold, which is smaller than the first threshold.

[0011] In a further preferred embodiment of the invention, the reduced availability of the linear actuator is detected when a temperature associated with the linear actuator exceeds a threshold value. The temperature can be determined at the linear actuator using a temperature sensor or calculated from quantities such as the electrical resistance of motor coils. Alternatively or additionally, the temperature can be determined using temperature models. These temperature models assume a starting temperature of the components. This starting temperature can be determined, for example, by a sensed ambient temperature. Subsequently, the electrical energy input of the motor, i.e., the current level and duration, is added. This energy input is added to the starting temperature of the components, thus determining the new actual temperature.

[0012] In a further preferred embodiment of the invention, a re-pumping mode is performed based on the system pressure profile and the availability of the linear actuator. In this mode, the locked pressure is increased by the linear actuator, and the system then switches back to holding mode. The re-pumping mode can be performed, in particular, as soon as the system pressure falls below a second threshold value that is above the first threshold value. Simultaneously, the linear actuator must allow at least brief re-pumping. For this purpose, a second temperature threshold can be provided, for example. By re-pumping, the total time for which brake pressure can be maintained can be significantly increased, since the linear actuator is only briefly electrically actuated to build up pressure and can cool down again during the pressure maintenance phase.

[0013] In a further preferred embodiment of the invention, the holding mode is performed when the motor vehicle is stationary. When stationary, the safety requirements are lower, so maintaining pressure is sufficient for the operation of the braking system.

[0014] In another preferred embodiment of the invention, the substitute mode is not the hydraulic fallback level in which only the driver can exert a braking force.

[0015] In a further preferred embodiment of the invention, the substitute mode is a parking brake mode in which an electromechanical parking brake of the motor vehicle is applied and the hydraulic braking system is deactivated, thereby holding the vehicle by the parking brake. In this way, in addition to the linear actuator, the electrohydraulic valves can also be put into a de-energized state, allowing them to cool down. The hydraulic braking system can also be partially deactivated as needed.

[0016] In a further preferred embodiment of the invention, the substitute mode is a cooperative mode in which an additional hydraulic pressure supply device is activated to increase the locked pressure and subsequently switches back to the holding mode. The use of the additional pressure supply device thus relieves the load on the linear actuator.

[0017] In a further preferred embodiment of the invention, the substitute mode is a half-mode in which the valve arrangement is controlled to connect the wheel brakes of a first axle in a flow-open manner to a master brake cylinder that can be actuated by the driver, and to lock in the hydraulic pressure of the wheel brakes of a second axle. Thus, the two circuits are separated by a circuit isolator valve.

[0018] In a particularly preferred embodiment of the invention, when switching to half-mode, the pressure of the wheel brakes is equalized to the pressure of the master brake cylinder before the valve arrangement establishes a flow-open connection. For this purpose, the outlet valves can be briefly opened, for example. This prevents pressure pulses from being transmitted to the brake pedal and thus to the driver.

[0019] In a further preferred embodiment of the invention, a linear actuator builds up a pressure at the beginning of the holding mode that is greater than the requested pressure, and this higher pressure is then held. This already anticipates a certain pressure drop over time, which can be determined, for example, from known leakage values ​​of the individual valves. Due to the elevated initial pressure at the beginning of the holding mode, the threshold value can be chosen to be proportionally lower, and a minimum pressure sufficient to hold the vehicle stationary can be maintained for a longer period.

[0020] The problem is further solved by a hydraulic braking system for a motor vehicle comprising a linear actuator as a driver-independent pressure supply device, a hydraulic valve arrangement between the linear actuator and wheel brakes of the hydraulic braking system, and a control device which is configured to carry out the aforementioned procedure.

[0021] Further features, advantages, and applications of the invention will also become apparent from the following description of exemplary embodiments and the drawings. All features described and / or illustrated, both individually and in any combination, are part of the subject matter of the invention, even independently of their compilation in the claims or their cross-references. Fig. Figure 1 schematically shows a braking system according to the invention of a first embodiment, Fig. Figure 2 schematically shows a braking system according to the invention of a second embodiment, Fig. Figure 3 shows a time sequence of the transfer to hold mode;

[0022] The in Fig. The braking system for a motor vehicle shown in Figure 1 comprises four hydraulically actuated wheel brakes 8a-8d. The braking system includes a master brake cylinder 2 actuated by means of an actuating or brake pedal 1, a travel simulator or simulation device 3 interacting with the master brake cylinder 2, a pressure medium reservoir 4 at atmospheric pressure, an electrically controlled pressure supply device 5, and a valve arrangement comprising wheel-specific brake pressure modulation valves, which are designed, for example, as inlet valves 6a-6d and outlet valves 7a-7d. Furthermore, the braking system includes at least one electronic control unit 12 for controlling the electrically actuated components of the braking system.

[0023] For example, wheel brake 8a is assigned to the left front wheel (FL), wheel brake 8b to the right front wheel (FR), wheel brake 8c to the left rear wheel (RL) and wheel brake 8d to the right rear wheel (RR).

[0024] The master brake cylinder 2 comprises a master brake cylinder piston 15 within a housing 16, which defines a hydraulic pressure chamber 17, and is a single-circuit master brake cylinder 2. The pressure chamber 17 accommodates a return spring 9, which positions the piston 15 in its initial position when the master brake cylinder 2 is not actuated. The pressure chamber 17 is connected to the hydraulic fluid reservoir 4 via radial bores formed in the piston 15 and a corresponding pressure equalization line 41, which can be closed off by a relative movement of the piston 15 within the housing 16. The pressure chamber 17 is also connected to a brake supply line 13 via a hydraulic line section 22 (also referred to as the first supply line), to which the inlet ports of the inlet valves 6a-6d are connected. Thus, the pressure chamber 17 of the master brake cylinder 2 is connected to all inlet valves 6a-6d.

[0025] For example, no hydraulic valve, in particular no electrically or hydraulically actuated valve and no check valve, is arranged in the pressure equalization line 41 or in the connection between the pressure chamber 17 and the pressure medium reservoir 4.

[0026] Alternatively, a diagnostic valve, preferably a normally open (NC) diagnostic valve, or preferably a parallel connection of a normally open (NC) diagnostic valve with a check valve closing towards the NC reservoir 4, can be included in the pressure equalization line 41 or between the master brake cylinder 2 and the pressure medium reservoir 4.

[0027] The valve arrangement can also include further hydraulic valves. A shut-off valve 23 is arranged between the supply line 22 connected to the pressure chamber 17 and the brake supply line 13. Alternatively, the pressure chamber 17 is connected to the brake supply line 13 via the first supply line 22 and a shut-off valve 23. The shut-off valve 23 is designed as an electrically actuated, preferably normally open (SO), 2 / 2-way valve. The hydraulic connection between the pressure chamber 17 and the brake supply line 13 can be shut off by the shut-off valve 23.

[0028] A piston rod 24 couples the pivoting movement of the brake pedal 1 resulting from pedal actuation with the translational movement of the master brake cylinder piston 15, whose actuation travel is detected by a preferably redundant displacement sensor 25. The corresponding piston travel signal is thus a measure of the brake pedal actuation angle. It represents a driver's braking request.

[0029] A pressure sensor 20 connected to the first supply line 22 detects the pressure built up in the pressure chamber 17 by the movement of the piston 15. This pressure value can also be evaluated to characterize or determine the driver's braking request. Alternatively, a force sensor 20 can also be used to determine the driver's braking request.

[0030] The simulation device 3 is, for example, hydraulically designed and hydraulically coupled to the master brake cylinder 2. The simulation device 3 essentially comprises, for example, a simulator chamber 29, a simulator return chamber 30, and a simulator piston 31 separating the two chambers 29 and 30. The simulator piston 31 is supported against a housing by an elastic element 33 (e.g., a simulator spring) arranged in the (for example, dry) simulator return chamber 30. The hydraulic simulator chamber 29 is, for example, connected to the pressure chamber 17 of the master brake cylinder 2 by means of a preferably electrically actuated, preferably normally closed simulator release valve 32.

[0031] The braking system comprises one inlet valve 6a-6d and one outlet valve 7a-7d for each hydraulically actuated wheel brake 8a-8d. These valves are hydraulically connected in pairs via center connections and are attached to the wheel brake 8a-8d. Each inlet valve 6a-6d is connected in parallel to a non-return valve (not further specified) that opens towards the brake supply line 13. The outlet ports of the outlet valves 7a-7d are connected to the hydraulic fluid reservoir 4 via a common return line 14.

[0032] The electrically controlled pressure supply device 5 is designed as a hydraulic cylinder-piston arrangement (or a single-circuit electrohydraulic actuator) or linear actuator, the piston 36 of which can be actuated by a schematically indicated electric motor 35 via an interposed rotary-translational transmission 39, also schematically depicted. The piston 36 defines the single pressure chamber 37 of the pressure supply device 5. A rotor position sensor, only schematically indicated, which serves to detect the rotor position of the electric motor 35, is designated by reference numeral 44.

[0033] A line section (also referred to as the second supply line) 38 is connected to the pressure chamber 37 of the electrically controlled pressure supply device 5. The supply line 38 is connected to the brake supply line 13 via an electrically actuated, preferably normally closed, switching valve 26 as part of the valve arrangement. The switching valve 26 allows the hydraulic connection between the pressure chamber 37 of the electrically controlled pressure supply device 5 and the brake supply line 13 (and thus the inlet ports of the inlet valves 6a-6d) to be opened and closed in a controlled manner.

[0034] The actuator pressure generated by the force exerted by the piston 36 on the pressure medium enclosed in the pressure chamber 37 is fed into the second supply line 38. In a "brake-by-wire" operating mode, particularly when the brake system is functioning correctly, the supply line 38 is connected to the brake supply line 13 via the switching valve 26. During normal braking, this results in the build-up and release of wheel brake pressure for all wheel brakes 8a-8d by the forward and retraction of the piston 36.

[0035] When the pressure is reduced by retracting the piston 36, the pressure medium previously moved from the pressure chamber 37 of the pressure supply device 5 into the wheel brakes 8a-8d flows back into the pressure chamber 37 via the same route.

[0036] Alternatively, different wheel brake pressures can be easily set using the inlet and outlet valves 6a-6d and 7a-7d. When the pressure drops accordingly, the hydraulic fluid released via the outlet valves 7a-7d flows through the return line 14 into the hydraulic fluid reservoir 4.

[0037] The intake of pressure medium into pressure chamber 37 is possible by retracting the piston 36 with the switching valve 26 closed, allowing pressure medium to flow from the reservoir 4 through line 42 and a check valve 53, which opens in the flow direction towards the actuator 5, into the actuator pressure chamber or pressure chamber 37. For example, pressure chamber 37 is also connected to the pressure medium reservoir 4 via one or more vent holes when the piston 36 is not actuated. This connection between pressure chamber 37 and pressure medium reservoir 4 is severed when the piston 36 is (sufficiently) actuated in the actuation direction 27.

[0038] An electrically actuated, normally open circuit isolating valve 40 is arranged in the brake supply line 13, through which the brake system is divided into two hydraulic sub-circuits. The brake supply line 13 is divided into a first line section 13a, which is connected (via the isolating valve 23) to the master brake cylinder 2, and a second line section 13b in the second hydraulic sub-circuit, which is connected (via the switching valve 26) to the pressure supply device 5. The first line section 13a is connected to the inlet valves 6a, 6b of the wheel brakes 8a, 8b, and the second line section 13b is connected to the inlet valves 6c, 6d of the wheel brakes 8c, 8d.

[0039] With the circuit isolating valve 40 open, the brake system is configured as a single circuit. By closing the circuit isolating valve 40, the brake system can be separated or divided into two hydraulic sub-circuits, brake circuits I and II, particularly when controlled according to the situation. In the first brake circuit I, the master brake cylinder 2 (via the isolating valve 23) is connected only to the inlet valves 6a, 6b of the wheel brakes 8a, 8b of the front axle VA, and in the second brake circuit II, the pressure supply device 5 (with the switching valve 26 open) is connected only to the wheel brakes 8c and 8d of the rear axle HA.

[0040] With the circuit separator valve 40 open, the inlet ports of all inlet valves 6a-6d can be supplied with pressure via the brake supply line 13. In a first operating mode (e.g., "brake-by-wire" mode), this pressure corresponds to the brake pressure provided by the pressure supply device 5. In a second operating mode (e.g., a de-energized fallback mode), the brake supply line 13 can be pressurized with the pressure from the pressure chamber 17 of the master brake cylinder 2.

[0041] Advantageously, the brake system includes a level measuring device 50 for determining a pressure medium level in the pressure medium reservoir 4.

[0042] For example, the hydraulic components, namely the master brake cylinder 2, the simulation device 3, the pressure supply device 5, the valve assembly with the hydraulic valves 6a-6d, 7a-7d, 23, 26, 40 and 32, and the hydraulic connections including the brake supply line 13, are arranged together in a hydraulic control unit 60 (HCU). The electronic control unit (ECU) 12 is assigned to the hydraulic control unit 60. Preferably, the hydraulic and electronic control units 60 and 12 are designed as a single unit (HECU).

[0043] The brake system includes a pressure sensor 19, or system pressure sensor, for detecting the pressure supplied by the pressure supply device 5. The pressure sensor 19 is located downstream of the switching valve 26, as viewed from the pressure chamber 37 of the pressure supply device 5.

[0044] In addition to the hydraulic actuation, the two rear wheel brakes 8c, 8d are each equipped with an integrated parking brake 48c, 48d, which are designed as electromechanical parking brakes.

[0045] In normal operating mode, the isolating valve 23 is closed and the switching valve 26 and the circuit isolating valve 40 are open, so that the hydraulic pressure in all wheel brakes 8a to 8d is controlled by the linear actuator 5. If brake pressure is maintained for an extended period while the vehicle is stationary, the linear actuator must continuously maintain a counterforce to the hydraulic pressure, for which it is operated with a non-zero electrical power. This can lead to overheating of the linear actuator 5. If a temperature threshold is detected being exceeded, the actuator switches to holding mode according to the invention.

[0046] In the Fig. Figure 2 shows a further embodiment of a brake system. The connection to the front wheel brakes 8a, 8b is made via a further brake unit 54, each with a changeover valve 59a, 59b, which is open during normal operation. For redundancy reasons, the further brake unit includes a further pressurization device 57, which is designed as two hydraulic pumps 57a, 57b with a common motor. The hydraulic pumps 57a, 57b are connected on the suction side via normally closed pump isolation valves 58a, 58b to an associated low-pressure accumulator 55a, 55b, which in turn has a connection to the brake fluid reservoir 4. The low-pressure accumulators 55a, 55b are also each connected to the associated wheel brake 8a, 8b via a further valve 56a, 56b.

[0047] The hydraulic pump 57 and the linear actuator 5 can be controlled by two separate control units.

[0048] Fig. Figure 3 shows a corresponding sequence with a system pressure of 70 and a valve flow rate of 60 of the switching valve 26. The valve flow rate 60 of the switching valve 26 is briefly increased to the opening flow rate 61, causing the switching valve 26 to open. Afterwards, the valve flow rate 60 is reduced to a holding flow rate 62, which keeps the switching valve 26 open. While the switching valve 26 is open, hydraulic pressure is built up by the linear actuator 5, and the system pressure 70 increases accordingly.

[0049] To switch to holding mode, the switching valve 26 is closed, thereby locking in the hydraulic pressure in the wheel brakes 8a to 8d. For this purpose, the valve flow 60 of the switching valve 26 is reduced from the holding flow 62 to a closing flow 63. The electrical current of the linear actuator 5 can then be switched off, allowing the linear actuator to cool down. As in Fig. As shown in Figure 3, the trapped hydraulic pressure does not remain unchanged at the initial level, since all hydraulic units, especially the hydraulic valves of the valve arrangement, exhibit a certain leakage flow.

[0050] According to the invention, the system pressure 70 is therefore monitored. At a time 71, the system pressure 70 drops below a threshold value 73, which is set at 90% of the original value 74 when transitioning to the holding mode. Accordingly, a switch to a substitute mode is made according to the invention.

[0051] In a first embodiment, a parking brake mode is activated as a backup mode. For this purpose, the electromechanical parking brakes 48c and 48d are applied, thus holding the vehicle stationary. The hydraulic braking system can therefore be deactivated or at least switched in such a way that all components can cool down and be fully operational again at a later time.

[0052] In a second embodiment, a cooperative mode is used as a substitute mode. This is possible with the braking system of the Fig.2, which, in addition to the linear actuator 5, also has a further pressure supply device 57. Here, the electric pumps 57a and 57b are controlled to increase the brake pressure in the wheel brakes 8a and 8b. Once a target pressure is reached, the pressure can be locked again. The cooperative mode can also be combined with the parking brake mode by simultaneously activating the electromechanical parking brakes 48c and 48d on the rear axle.

[0053] In a third embodiment, the system switches to half-mode as a substitute mode. In this mode, a circuit isolating valve 40 is closed to divide the brake system into two partial circuits. The rear axle wheel brakes 8c and 8d then remain in a state where the hydraulic pressure is locked in. At the front axle, the wheel pressure of the wheel brakes 8a and 8b is adjusted to the hydraulic pressure in the master brake cylinder 2 by briefly opening the outlet valves. As soon as the pressure is essentially equal, the isolating valve 23 is opened, thereby connecting the master brake cylinder 2 to the wheel brakes 8a and 8b. This gives the driver direct control over the front wheel brakes. Should the braking force be insufficient to hold the vehicle stationary, the driver can apply further pressure to the brake pedal to increase the braking force.

[0054] The use of the substitute modes according to the invention thus prevents the braking system from having to switch directly to a hydraulic fallback level, in which only the driver has to build up the required brake pressure via the master brake cylinder. Reference symbol list: 1 Brake pedal 2 master brake cylinders 3 Simulation equipment 4 pressure medium reservoirs 5. Print supply unit 6 a to d Inlet valves 7 a to d exhaust valves 8 a to d wheel brake 9 Return spring 12 Tax system 13 Brake supply line 14 Return line 16 cases 17 Pressure chamber 19 System pressure sensor 20 Master cylinder pressure sensor 22 first supply line 23 Isolation valve 24 piston rod 25 Displacement sensor 26 Switching valve 29 Simulator Chamber 30 Simulator back chamber 31 simulator pistons 32 Simulator release valve 33 Elastic element 35 pistons 36 Electric motor 37 Printing chamber 38 Supply line 39 rotation-translation gears 40 Circular separation valve 41 Pressure equalization line 42 Management 44 Rotor position sensor 50 level sensor 53 Check valve 54 Diverter valve 55 Low-pressure storage tanks 56 Suction valve 57 Hydraulic pump 58 Pump separation valve 60 Valve flow 61 Opening current 62 Holding current 63 Closing current 70 system pressure 71 Switchover point 72 Threshold 73 Original value

Claims

[1] A method for controlling a hydraulic braking system of a motor vehicle comprising a linear actuator (5) as a driver-independent pressure supply device, and a hydraulic valve arrangement (26) between the linear actuator (5) and the wheel brakes (8) of the hydraulic braking system, wherein, in the event of reduced availability of the linear actuator (5), a holding mode is performed in which the hydraulic pressure in the wheel brakes (8) is locked in by means of the valve arrangement (26) and the power of the linear actuator (5) is reduced, wherein the system pressure profile (70) is measured in the holding mode and, based on the system pressure profile (70), a transition from the holding mode to a substitute mode is performed. characterized by , that a backup mode is a cooperative mode in which an additional hydraulic pressure supply device (57) is activated to increase the locked pressure and subsequently switches back to the holding mode. [2] Method according to claim 1 characterized by , that determines how far the system pressure (70) has dropped in hold mode, switching to substitute mode when the system pressure has dropped below a threshold (72). [3] Method according to claim 2 characterized by , that the threshold (72) is determined based on the road gradient. [4] Method according to any of the preceding claims characterized by , that the reduced availability of the linear actuator (5) is detected when a temperature associated with the linear actuator (5) exceeds a threshold value. [5] Method according to any of the preceding claims characterized by , that based on the system pressure profile (70) and the availability of the linear actuator (5) a re-pumping mode is performed in which the locked pressure is increased by the linear actuator (5) and subsequently switched back to the holding mode. [6] Method according to any of the preceding claims characterized by , that the hold mode is performed when the vehicle is stationary. [7] Method according to any of the preceding claims characterized by that the backup mode is not the hydraulic fallback level. [8] Method according to any of the preceding claims characterized by , that a substitute mode is a parking brake mode in which an electromechanical parking brake (48) of the motor vehicle is applied and the hydraulic braking system is deactivated, thereby holding the vehicle by the parking brake (48). [9] Method according to any of the preceding claims characterized by, that a substitute mode is a half-mode in which the valve arrangement (23, 26) is controlled to connect the wheel brakes (8a, 8b) of a first axle in a flow-open manner to a master brake cylinder (2) that can be operated by the driver and to lock in the hydraulic pressure of the wheel brakes (8c, 8d) of a second axle. [10] Method according to claim 9 characterized by , that when switching to half-mode the pressure of the wheel brakes (8a, 8b) is equalized to the pressure of the master brake cylinder (2) before the valve arrangement (23) establishes a flow-open connection. [11] Method according to any of the preceding claims characterized by , that at the beginning of the holding mode a pressure is built up by a linear actuator (5) which is greater than the requested pressure, and the greater pressure is locked in. [12] Hydraulic brake system for a motor vehicle comprising a linear actuator (5) as a driver-independent pressure supply device, a hydraulic valve arrangement (26) between the linear actuator (5) and wheel brakes (8) of the hydraulic brake system and a control device (12), characterized by , that the control device (12) is configured to carry out a method according to one of claims 1 to 11.

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