Method for operating a hydraulic vehicle braking system with two pressure generators

Abstract

The invention relates to a method for operating a hydraulic vehicle braking system (1) with at least one first and one second electrically actuated pressure generator (2, 2'), several wheel brakes (3) and a separating valve (4) for hydraulically decoupling the first pressure generator (2) from the second pressure generator (2'), wherein the method comprises the following steps: - Receiving a print build-up command whose requirements profile cannot be fulfilled by the first print generator (2) alone (S10); - Simultaneous operation of the first and second pressure generators (2, 2') and increasing the pressure at the wheel brakes (3) until a first pressure threshold is reached (S11); - Closing of the separating valve (4), thereby hydraulically separating the first pressure generator (2) from the second pressure generator (2') and the wheel brakes (3) (S12); - Continue operating the second print generator (2') until the target pressure has been reached according to the print build-up command (S13).

Description

[0001] The invention relates to a method for operating a hydraulic vehicle braking system and a hydraulic braking system of a vehicle.

[0002] It is known to use electrically operated pressure generators in hydraulic vehicle braking systems. For example, a first pressure generator is used to actuate the wheel brakes during normal operation. A second pressure generator serves as a backup in case the first pressure generator fails or in the event of a leak in the line system.

[0003] In particular, emergency braking situations, where high pressure must be built up at the wheel brakes in a very short time, require the first pressure generator to be very large in order to meet the requirements regarding the pressure build-up time and the required pressure level at the wheel brakes (hereinafter also referred to as system pressure). This results in a relatively large and expensive first pressure generator.

[0004] WO 2024 / 012639 A1 discloses a braking system comprising two electrical pressure supply devices, each of which can be connected to the wheel brakes. When a particularly high flow rate is required, both pressure sources are operated in parallel. When a particularly high pressure is required, an isolation valve is closed, and the secondary pressure source increases the pressure above the pressure of the primary pressure source.

[0005] DE 10 2017 113 563 A1 discloses another braking system comprising two electrical pressure supply devices. Switching between them is performed based on predefined pressure limits.

[0006] DE 10 2020 214 972 A1 discloses another braking system comprising two electrical pressure supply devices. Here, the hydraulic pump switches from synchronous operation to individual operation when the linear actuator reaches its knee point, an ABS control is triggered, or a predetermined locking pressure is reached.

[0007] Furthermore, DE 10 2021 112 434 A1 discloses a braking system with an electrofluidic pressure generation unit and a pressure boosting unit. Here, the pressure is first built up with the pressure generation unit alone and then further increased with the pressure boosting unit.

[0008] A braking system with two pressure supply devices is also known from DE 20 2019 101 596 U1. Up to a pressure level P1, one pressure supply device is used, and above this pressure level, the other pressure supply device is used.

[0009] Based on this, it is the object of the invention to provide a method for operating a hydraulic braking system of a vehicle which enables a braking system that is optimized with regard to costs and the dimensioning of the first pressure generator.

[0010] This object is achieved by a method having the features of independent patent claim 1. Preferred embodiments are the subject of the dependent claims. A braking system is the subject of independent patent claim 13.

[0011] According to a first aspect, a method for operating a hydraulic vehicle braking system is disclosed. The vehicle braking system comprises at least a first and a second electrically actuated pressure generator, a plurality of wheel brakes, and a separating valve for decoupling the first pressure generator from the second pressure generator. The method comprises the following steps: First, a pressure build-up command is received. The pressure build-up command includes a requirement profile that cannot be fulfilled by the first pressure generator alone. The requirement profile can, for example, specify a time period within which the pressure at the wheel brakes must be increased to a predetermined pressure value. Alternatively or additionally, the requirement profile can include a pressure value to which the pressure at the wheel brakes must be increased. The time period and / or the pressure value can be selected such that these specifications cannot be met by the first pressure generator alone.

[0012] To meet the requirements profile, the first and second pressure generators are operated simultaneously, at least for an initial period of time. The pressure at the wheel brakes is increased until the system pressure reaches a first pressure threshold.

[0013] Once the first pressure threshold is reached, the isolation valve is closed, hydraulically separating the first pressure generator from the second pressure generator and the wheel brakes. This decouples the first pressure generator from the second pressure generator, so that the pressure that is to be further increased in the area of ​​the wheel brakes no longer affects the first pressure generator.

[0014] After the isolation valve is actuated, the second pressure generator continues to operate alone (i.e. without the involvement of the first pressure generator) until the target pressure has been reached according to the pressure build-up command.

[0015] The technical advantage of the proposed method is that the first pressure generator can be dimensioned smaller and, in the event of an emergency braking, the necessary requirements regarding volume flow and maximum pressure can still be met by the interaction of the first and second pressure generators.

[0016] According to one embodiment, the first pressure generator comprises an electrically operated piston-cylinder unit. The piston-cylinder unit comprises, for example, a piston that can be linearly displaced in a pressure chamber under sensor control. This allows the first pressure generator to generate a precisely controllable brake pressure, both in the positive direction (pressure increase) and in the negative direction (pressure reduction), resulting in comfortable braking behavior.

[0017] According to one embodiment, the second pressure generator comprises an electrically operated piston pump. The piston pump is preferably a two-piston pump. This allows the second pressure generator to generate high fluid pressures and thus provide high braking power to the wheel brakes.

[0018] According to the invention, the first pressure threshold is defined by the control point of the first pressure generator. The first and second pressure generators thus operate simultaneously until the first pressure generator has reached its control point. “Reaching the control point” means that the first pressure generator delivers the maximum pressure it can provide on the output side. The two pressure generators are thus operated simultaneously as long as the first pressure generator can contribute to the pressure increase at the wheel brakes. When the first pressure generator can no longer contribute, the closing of the isolation valve hydraulically decouples the first pressure generator from the second pressure generator, allowing the second pressure generator to continue increasing the pressure at the wheel brakes on its own, without this pressure having a feedback effect on the first pressure generator.

[0019] According to the invention, the reaching of the control point of the first pressure generator is determined by monitoring the speed of the motor of the first pressure generator and by detecting the drop in the speed of the motor by a predetermined speed amount. The speed of the motor of the first pressure generator depends on the pressure provided on the output side. When the first pressure generator reaches its maximum pressure, the motor must be operated at maximum torque, which leads, for example, to a drop in the rated motor speed, for example a drop by 5% to 15%, in particular by 10% or substantially 10%. The motor speed can thus be used as an indication of the reaching of the maximum pressure of the control point of the first pressure generator.

[0020] According to one embodiment, after the isolation valve closes, the pressure provided by the first pressure generator is reduced. This allows the first pressure generator to be transferred to a relief mode, in which the load on the motor and / or the thermal load on the electronics is reduced.

[0021] According to one embodiment, the pressure reduction of the first pressure generator is achieved by moving the piston back a partial stroke. The partial stroke is preferably selected to be small, for example, 1 mm or essentially 1 mm. This allows a significant pressure reduction to be achieved, but the pressure in the first pressure generator can also be increased very quickly if it is to be reintegrated into the hydraulic circuit of the wheel brakes by opening the isolation valve.

[0022] According to one embodiment, the first pressure generator is switched from pressure-controlled operation to position-controlled operation after the isolation valve closes or simultaneously with the closure of the isolation valve. Preferably, a pressure sensor is provided in the area of ​​the wheel brakes, by means of which the pressure in the area of ​​the wheel brakes can be measured. When the isolation valve is closed, the measured value of this pressure sensor is no longer meaningful for the pressure applied to the first pressure generator. Position control eliminates the need for an additional pressure sensor in the area of ​​the first pressure generator, since the position of the piston of the first pressure generator is proportional to the applied pressure.

[0023] According to one embodiment, after the target pressure is reached, the pressure at the wheel brakes is reduced in a first period immediately following the target pressure being reached by actuating the outlet valves assigned to the respective wheel brakes. If the braking power at the wheel brakes is to be reduced with the isolation valve closed, this is preferably done by opening the outlet valves, since the second pressure generator itself cannot reduce the pressure (unlike the first pressure generator). The outlet valves are preferably opened and closed in a cyclical manner to achieve a controlled pressure reduction.

[0024] According to one embodiment, after the pressure at the wheel brakes drops to a pressure threshold, the isolation valve is opened, and with the outlet valves closed, further pressure reduction at the wheel brakes occurs via the first pressure generator. This is advantageous because the pressure reduction by the first pressure generator can be carried out more comfortably and without acoustic disturbance.

[0025] According to one embodiment, when the isolation valve is opened, the pressure provided by the first pressure generator essentially corresponds to the pressure applied to the wheel brakes. This allows the first pressure generator to be reintegrated into the hydraulic circuit of the wheel brakes with a steady pressure curve, i.e., without, or essentially without, a jump or drop in the system pressure.

[0026] According to one embodiment, the pressure provided by the first pressure generator is increased before the isolating valve opens. This occurs in particular if the outlet pressure of the first pressure generator has been reduced after the isolating valve closes. This allows the first pressure generator to be reconnected to the hydraulic circuit of the wheel brakes with an adjusted pressure, so that this reconnection occurs without, or essentially without, a jump or drop in the system pressure.

[0027] According to one embodiment, before the isolation valve is opened, the piston of the first pressure generator is moved in a position-controlled manner to the position it occupied when the isolation valve was previously closed. This allows the first pressure generator to be reintegrated into the hydraulic circuit of the wheel brakes with the correct pressure even without a pressure sensor in the area of ​​the first pressure generator.

[0028] In the alternative case that the pressure provided by the first pressure generator is measurable by a pressure sensor, the piston of the first pressure generator can be moved in a pressure-controlled manner to the position at which the pressure provided by the first pressure generator corresponds to the pressure applied to the wheel brakes before the isolating valve is opened.

[0029] According to one embodiment, the first pressure generator is switched from position-controlled operation to pressure-controlled operation simultaneously with the opening of the isolation valve. This allows the first pressure generator to control the system pressure according to the required target pressure after the first pressure generator is reconnected to the wheel brake hydraulic circuit.

[0030] According to a further aspect, a hydraulic braking system for a vehicle is disclosed. The hydraulic braking system has at least two electrically actuated pressure generators, a plurality of wheel brakes, and a separating valve for hydraulically decoupling the first pressure generator from the second pressure generator. The vehicle braking system comprises at least one control unit configured to perform the following steps: - Receiving a pressure build-up command whose requirement profile cannot be executed by the first pressure generator alone; - Simultaneous operation of the first and second pressure generators until a first pressure threshold is reached; - closing the isolation valve, whereby the first pressure generator is hydraulically separated from the second pressure generator and the wheel brakes; and - Continue to operate the second pressure generator until the target pressure has been reached according to the pressure build-up command.

[0031] The terms “approximately”, “essentially” or “about” mean, in the sense of the invention, deviations from the exact value by + / - 10%, preferably by + / - 5% and / or deviations in the form of changes that are insignificant for the function.

[0032] Further developments, advantages, and possible applications of the invention will become apparent from the following description of exemplary embodiments and from the figures. All described and / or illustrated features, individually or in any combination, are fundamentally part of the invention, regardless of their summary in the claims or their reference back to them. The content of the claims is also incorporated into the description.

[0033] The invention is explained in more detail below with reference to exemplary embodiments and the figures. They show: Fig. 1 shows, by way of example, a schematic representation of a hydraulic braking system of a vehicle; Fig. 2 shows, by way of example, the time courses of the piston position of the first pressure generator, the target pressure, the pressure provided by the first and second pressure generators and the control current of the isolating valve during pressure build-up at the wheel brakes Fig. 3 shows, by way of example, the time profiles of the piston position of the first pressure generator, the target pressure, the pressure provided by the first and second pressure generators, and the control current of the isolating valve during pressure reduction at the wheel brakes; and Fig. 4 shows an example block diagram illustrating the process steps of the operation of the hydraulic braking system with two cooperating pressure generators.

[0034] Fig. 1 shows schematically a hydraulic braking system 1 of a vehicle.

[0035] The braking system 1 comprises, for example, a first module 100, also called a pressure supply module, a second module 200, also called a wheel pressure supply module, and a pressure fluid reservoir 7 under atmospheric pressure. The first and second modules 100, 200 are structurally separate and can, for example, be arranged separately in the vehicle. The pressure fluid reservoir 7 is, for example, assigned to the first module 100 and arranged on it. A brake fluid is held in the pressure fluid reservoir 7 as the pressure medium.

[0036] The first module 100 comprises a first, electrically actuated pressure generator 2. The first pressure generator 2 is designed, for example, as a hydraulic piston-cylinder arrangement and has, for example, a pressure chamber 2.1 and a piston 2.2, which can be displaced within the pressure chamber 2.1 by an electric actuator 2.3 in order to vary the volume of the pressure chamber and thereby apply pressure to the brake fluid located in the pressure chamber 2.1. The actuator 2.3 can be an electric motor with a rotor position sensor 2.4.

[0037] The output pressure connection 100A of the first module 100 is directly coupled to the pressure chamber 2.1 by means of a connecting line.

[0038] The pressure chamber 2.1 is connected to the interior of the pressure fluid reservoir 7 via a check valve 8. The check valve 8 is arranged such that it allows a flow of brake fluid from the pressure fluid reservoir 7 into the pressure chamber 2.1, but prevents a backflow of the brake fluid from the pressure chamber 2.1 into the pressure fluid reservoir 7.

[0039] In addition, the first module 100 has, for example, a separating valve 9. This separating valve 9 is provided in a line that couples a line running between the pressure medium reservoir 7 and the check valve 8 to the connecting line between the pressure chamber 2.1 and the output pressure connection 100A.

[0040] The second module 200 has a first inlet port 200E1 connected to the interior of the pressure medium reservoir 7. Furthermore, the second module 200 has a second inlet port 200E2 coupled to the output pressure port 100A of the first module 100.

[0041] The second module 200 comprises a second pressure generator 2', which is designed, for example, as a brake fluid pump. The brake fluid pump can comprise a piston pump driven by an electric motor, in particular a two-piston pump. The suction side of the second pressure generator 2' is fluidly connected to the inlet port 200E1, and the pressure side of the second pressure generator 2' is fluidly connected to a connecting line 10, which establishes a connection to at least two brake circuits B1, B2 of the vehicle.

[0042] The brake circuits B1, B2 each apply braking force to a part of the wheel brakes 3 of the vehicle, for example the brake circuit B1 applies braking force to the wheel brakes of the vehicle's rear axle and the brake circuit B2 applies braking force to the wheel brakes of the front axle.

[0043] Each wheel brake 3 is assigned an inlet valve 5 and an outlet valve 5'. The inlet valves 5 are connectable on the inlet side to the first and second pressure generators 2, 2' via the connecting line 10. On the outlet side, the inlet valves 5 are each connected to a wheel brake 3. Pressurized brake fluid can thus be supplied to the wheel brakes 3 in a controlled manner via the inlet valves 5, thereby generating a braking force at the respective wheel.

[0044] The outlet valves 5' are coupled on the inlet side to the connecting line that runs between the respective wheel brake 3 and the inlet valve 5 assigned to that wheel brake 3. The outlets of the outlet valves 5' are connected to the pressure fluid reservoir 7 via a connecting line. The braking force in the wheel brakes 3 can thus be reduced by reducing the pressure via the outlet valves 5'.

[0045] The second module 200 also has a separating valve 4. The second module 200 can be fluidically decoupled from the first module 100 via the separating valve 4, for example, if a leak is detected there. The separating valve 4 can also be used—as described in more detail below—to separate the brake circuits B1, B2 from each other, for example, to generate the pressure required for emergency braking through the second pressure generator 2' without affecting the first pressure generator 2.

[0046] The second module 200 can also include a circuit isolation valve 11. By means of the circuit isolation valve 11, the connecting line 10 can be interrupted in a region that connects the two brake circuits B1, B2, so that when the circuit isolation valve 11 is closed, the brake circuits B1, B2 are fluidically separated from each other.

[0047] The following describes a method for the temporary parallel operation of the first and second pressure generators 2, 2' and a method for transferring from a parallel operation of the two pressure generators 2, 2' to a sole provision of the pressure to the wheel brakes 3 by the second pressure generator 2' and a subsequent change of the pressure provision from the second pressure generator 2' to the first pressure generator 2.

[0048] The previously described vehicle braking system 1 is normally pressurized, i.e., without leakage and during braking in normal operation (i.e., up to a medium braking force without emergency braking), preferably exclusively by the first pressure generator 2. The first pressure generator 2, with its linearly movable piston 2.2, can effect both a controlled pressure buildup and a controlled pressure reduction, thus achieving comfortable braking behavior of the vehicle.

[0049] For cost reasons, the first pressure generator 2 can be dimensioned such that it can only provide a limited pressure, for example, a pressure between 50 bar and 100 bar, in particular between 70 and 90 bar, particularly preferably 80 bar or substantially 80 bar. This is essentially due to the dimensioning of the actuator 2.3 of the first pressure generator 2, which only provides a limited torque, resulting in a limited maximum pressure. This allows cost advantages to be achieved.

[0050] Especially in the case of emergency braking, where pressure must be built up at the wheel brakes in a short time, for example a pressure greater than 120 bar, in particular greater than 150 bar, the pressure provided by the first pressure generator 2 is not sufficient.

[0051] In order to achieve maximum pressure at the wheel brakes as quickly as possible and / or to achieve maximum pressure, the first and second pressure generators 3, 3' are operated simultaneously, i.e., pressure buildup occurs in parallel by activating both pressure generators 2, 2'. This achieves the maximum possible volume flow.

[0052] Fig. 2 shows several time diagrams which illustrate the interaction of the first and second pressure generators during pressure build-up in the braking system, wherein the top diagram shows the time course of the position of the piston 2.2 of the first pressure generator 2, the middle diagram shows the pressure request (also called target pressure), the pressure provided by the first pressure generator 2 and the system pressure (i.e. the pressure in the area of ​​the wheel brakes 3) and the lower diagram shows the control current of the isolating valve 4.

[0053] As can be seen in the middle diagram, a pressure request is received at time t1. The pressure request includes a pressure build-up command, which in this case involves increasing the pressure as quickly as possible to a pressure value of 150 bar.

[0054] Once this pressure build-up command has been received, the first and second pressure generators 2, 2' are operated simultaneously and in parallel, i.e. the pressure build-up is initially effected simultaneously by the first and second pressure generators 2, 2'. As can be seen in the top diagram, the piston 2.2 of the first pressure generator 2 is advanced in the period between times t1 and t2, so that the first pressure generator 2 takes over a first part of the pressure build-up. At the same time, the motor of the second pressure generator 2' is controlled so that it takes over the second part of the pressure build-up. At time t2, the first pressure generator 2 reaches its control point, i.e. the actuator 2.3 is driven with maximum torque, so that the first pressure generator 2 reaches its maximum pressure. The control point of the first pressure generator 2 can be recognized in particular by the speed of the actuator 2.3 becomes smaller than the rated speed, for example 10% or substantially 10% smaller than the rated speed.

[0055] Once this control point has been reached, the isolation valve 4 is closed, so that the first pressure generator 2 is separated on the pressure side from the second pressure generator 2'. This allows the system pressure to be further increased by the second pressure generator 2' without this having a negative effect on the first pressure generator 2. The period between times t2 and t3, which is, for example, between 80 ms and 120 ms, in particular 100 ms or essentially 100 ms, serves as a buffer to ensure that the isolation valve 4 has actually reached the closed state. At time t3, for example, the closing current of the isolation valve 4 is reduced, since from this point on it is assumed that the valve is actually closed.

[0056] From time t3, the second pressure generator 2' alone takes over the further increase of the pressure at the wheel brakes 3 until the target pressure according to the pressure request has been reached.

[0057] After the isolation valve 4 closes, the first pressure generator 2 can no longer contribute to the further pressure buildup at the wheel brakes 3. The first pressure generator 2 can, for example, be switched from a pressure control to a position control. To protect the actuator 2.3 and its electronics, or to protect the actuator 2.3 from overheating, the pressure in the area of ​​the first pressure generator 2 can be reduced at time t4. This is achieved, for example, by moving the piston 2.2 back a partial distance, in particular a partial distance in the range of 0.5 - 2 mm, particularly preferably by 1 mm or essentially 1 mm. This relieves the load on the first pressure generator 2.

[0058] After such a rapid and / or strong increase in brake pressure, for example, during emergency braking, the pressure at the wheel brakes 3 must then be reduced to reduce the braking effect. For this purpose, the second pressure generator 2' is preferably deactivated or switched to idle.

[0059] Fig. 3 illustrates this case by means of several diagrams, wherein the top diagram shows the time course of the position of the piston 2.2 of the first pressure generator 2, the middle diagram shows the requested target pressure, the pressure provided by the first pressure generator 2 and the system pressure (ie the pressure in the area of ​​the wheel brakes 3) and the lower diagram shows the control current of the isolation valve 4.

[0060] In the period between times t5 and t6, the pressure at the wheel brakes 3 is to be reduced according to the pressure request. At time t5, this pressure is higher than the maximum pressure to be provided by the first pressure generator 2, so that the isolation valve 4 remains closed, i.e., the first pressure generator 2 is hydraulically decoupled from the second pressure generator 2'.

[0061] In this case, the pressure in the area of ​​the wheel brakes 3 is reduced by the outlet valves 5', which are assigned to the respective wheel brakes 3. These are opened and closed in a controlled manner, for example, in a clocked manner.

[0062] When this pressure has essentially reached the pressure value at which the first pressure generator 2 was deactivated, a hydraulic transfer can take place from the second pressure generator 2' to the first pressure generator 2, so that after this transfer the further pressure reduction can take place by the first pressure generator 2.

[0063] Preferably, the piston 2.2 of the first pressure generator 2 is first moved back to the position in which it was in the pressure build-up phase when the isolating valve 4 was closed. This is done in the illustration according to Fig. 3 from time t6. At time t6, the control current of the isolating valve 4 is also reduced, so that this isolating valve 4 opens and the first pressure generator 2 is again hydraulically coupled to the second pressure generator 2' and the wheel brakes 3.

[0064] After the piston 2.2 of the first pressure generator 2 has reached its original position (i.e. the position before the isolation valve 4 is closed), it is transferred from the position control back to the pressure control so that the piston 2.2 is moved based on the pressure request at the wheel brakes 3.

[0065] This is in Fig. 3, this is the case from approximately time t7. From this point onward, the pressure reduction at the wheel brakes 3 is no longer achieved by opening the outlet valves 5', but by the controlled retraction of the piston 2.2 of the first pressure generator 2.

[0066] The pressure in the area of ​​the wheel brakes 3 can now be controlled exclusively by the first pressure generator 2. In the event that a very rapid increase in the system pressure and / or a system pressure above the maximum pressure to be achieved by the first pressure generator 2 is requested again, the second pressure generator 2' can be activated again, and the previously described transition from parallel operation of the first and second pressure generators 2, 2' to sole operation of the second pressure generator 2' can be carried out to achieve the requested target pressure.

[0067] Fig.4 shows a flow chart illustrating the method steps of a method for operating a hydraulic vehicle brake system with at least two pressure generators.

[0068] First, a pressure build-up command is received, the requirement profile of which cannot be executed by the first pressure generator alone (S10).

[0069] Subsequently, the first and second pressure generators are operated simultaneously, thereby increasing the pressure at the wheel brakes until the first pressure generator has reached a pressure threshold (S11).

[0070] After the pressure threshold is reached, the isolation valve is closed, hydraulically separating the first pressure generator from the second pressure generator and the wheel brakes (S12).

[0071] After the isolation valve is closed, the second pressure generator continues to operate until the target pressure has been reached according to the pressure build-up command (S13).

[0072] The invention has been described above using exemplary embodiments. It is understood that numerous changes and modifications are possible without departing from the scope of protection defined by the patent claims. List of reference symbols 1 vehicle braking system 2 first pressure generator 2.1 Pressure chamber 2.2 Piston 2.3 Actuator 2.4 Rotor position sensor 2' second pressure generator 3 wheel brakes 4 Isolating valve 5 Inlet valve 5' exhaust valve 6 Control unit 7 pressure medium reservoir 8 Check valve 9 Isolating valve 10 connecting cable 11 Circuit isolation valve 100 first module 100A output pressure connection 200 second module 200E1 first inlet port 200E2 second inlet port B1 first brake circuit B2 second brake circuit

Claims

[1] Method for operating a hydraulic vehicle braking system (1) with at least a first and a second electrically operated pressure generator (2, 2'), a plurality of wheel brakes (3) and a separating valve (4) for hydraulically decoupling the first pressure generator (2) from the second pressure generator (2'), the method comprising the following steps: - receiving a pressure build-up command whose requirement profile cannot be executed by the first pressure generator (2) alone (S10); - Simultaneous operation of the first and second pressure generators (2, 2') and increasing the pressure at the wheel brakes (3) until a first pressure threshold value has been reached (S11), which is defined by the control point of the first pressure generator (2), wherein the reaching of the control point of the first pressure generator (2) is determined by monitoring the speed of the motor of the first pressure generator (2) and by detecting the decrease in the speed of the motor by a predetermined speed amount; - closing the isolation valve (4), whereby the first pressure generator (2) is hydraulically separated from the second pressure generator (2') and the wheel brakes (3) (S12); - Continue to operate the second pressure generator (2') until the target pressure has been reached according to the pressure build-up command (S13). [2] Method according to claim 1, characterized by that the first pressure generator (2) has an electrically operated piston-cylinder unit. [3] Method according to claim 1 or 2, characterized by that the second pressure generator (2') has an electrically operated piston pump. [4] Method according to one of the preceding claims, characterized by that after the isolating valve (4) is closed, the pressure provided by the first pressure generator (2) is reduced. [5] Method according to claim 4, characterized by that the pressure reduction is achieved by moving the piston back a partial stroke. [6] Method according to one of the preceding claims, characterized by that the first pressure generator (2) is switched from a pressure-controlled operation to a position-controlled operation after the isolating valve (4) is closed or simultaneously with the isolating valve (4) is closed. [7] Method according to one of the preceding claims, characterized bythat after reaching the target pressure, the pressure at the wheel brakes (3) is reduced in a first period directly following reaching the target pressure by outlet valves (5') assigned to the respective wheel brakes (3). [8] Method according to claim 7, characterized by that after the pressure at the wheel brakes (3) has dropped to a pressure threshold value, the isolating valve (4) is opened and, with the outlet valves (5') closed, the further pressure reduction at the wheel brakes (3) is carried out by the first pressure generator (2). [9] Method according to claim 8, characterized by that when the isolating valve (4) is opened, the pressure provided by the first pressure generator (2) essentially corresponds to the pressure applied to the wheel brakes (3). [10] Method according to claim 8 or 9, characterized by that before opening the isolating valve (4) the pressure provided by the first pressure generator (2) is increased. [11] Method according to one of claims 8 to 10, characterized by that the piston of the first pressure generator (2) is moved in a position-controlled manner to the position which it had when the isolating valve (4) was previously closed before the isolating valve (4) is opened. [12] Method according to one of claims 8 to 11, characterized by that the first pressure generator (2) is switched from a position-controlled operation to a pressure-controlled operation at the same time as the isolating valve (4) opens. [13] Hydraulic braking system for a vehicle with at least two electrically operated pressure generators (2, 2'), a plurality of wheel brakes (3) and a separating valve (4) for hydraulically decoupling the first pressure generator (2) from the second pressure generator (2'), wherein the vehicle braking system (1) comprises at least one control unit (6), wherein the control unit (6) is designed to carry out the following steps: - receiving a pressure build-up command whose requirement profile cannot be executed by the first pressure generator (2) alone; - Simultaneous operation of the first and second pressure generators (2, 2') until a first pressure threshold value has been reached, which is defined by the control point of the first pressure generator (2), wherein the reaching of the control point of the first pressure generator (2) is determined by monitoring the speed of the motor of the first pressure generator (2) and by detecting the decrease in the speed of the motor by a predetermined speed amount; - closing the isolation valve (4), whereby the first pressure generator (2) is hydraulically separated from the second pressure generator (2') and the wheel brakes (3); and - Continue to operate the second pressure generator (2') until the target pressure has been reached according to the pressure build-up command.

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