Method for controlling a redundant hydraulic motor vehicle braking system

Abstract

The invention relates to a method for controlling a hydraulic motor vehicle braking system comprising a first pressure supply device (5), which is designed as a linear actuator, the pressure output of which is connected via an isolation valve (26) to inlet valves (6) of wheel brakes (8) and to a pressure output of a second pressure supply device (2), wherein in an operating mode in which the second pressure supply device (2) provides the pressure supply for the wheel brakes (8) and supplies a first pressure, the isolation valve (26) is closed. For robust control of the braking system, the linear actuator (5) provides a second pressure lower than the first pressure to minimize the differential pressure across the isolation valve (26), whereby the isolation valve (26) is controlled to open at least partially, so that a volume flow passes through the isolation valve (26) into the linear actuator (5).

Description

[0001] The invention relates to a method for controlling a hydraulic motor vehicle braking system comprising a first pressure supply device, which is designed as a linear actuator, the pressure output of which is connected via an isolation valve to inlet valves of wheel brakes and to a second pressure supply device, wherein in an operating mode in which the second pressure supply device provides the pressure supply for the wheel brakes and establishes a first pressure, the isolation valve is closed. The invention also relates to a corresponding hydraulic motor vehicle braking system.

[0002] Such a braking system is known, for example, from DE 10 2023 200 932, which shows a division into two modules, each with its own electrical pressure supply device. The first module also includes a hydraulically coupled master cylinder. However, braking systems with a purely electrically coupled brake pedal are also conceivable.

[0003] From the subsequently published DE 10 2023 205 735 A1, a brake system is known which has a linear actuator and a piston pump. A switching valve PFV can be closed to supply pressure to the wheel brakes via the piston pump.

[0004] From DE 10 2021 204 854 A1, a braking system is known which includes a brake pedal-operated master brake cylinder that, in normal operating mode, delivers pressure to a simulator. The pressure in the wheel brakes is provided by a linear actuator. Due to the redundancy required for autonomous driving functions, such braking systems have duplicate components, which is particularly evident in the two pressure supply devices, resulting in high costs.

[0005] It is therefore an object of the present invention to provide a method for such a braking system and such a braking system which minimizes costs.

[0006] The problem is solved according to the invention by a method for controlling a hydraulic motor vehicle brake system according to claim 1, comprising a first pressure supply device, which is designed as a linear actuator, the pressure output of which is connected via an isolation valve to inlet valves of wheel brakes and to a second pressure supply device, wherein in an operating mode in which the second pressure supply device provides the pressure supply for the wheel brakes and establishes a first pressure, the isolation valve is closed, whereby the linear actuator and the pressure side of the second pressure supply device are separated at least to the extent that a pressure differential exists across the isolation valve, which allows a target pressure to be set by means of the second pressure supply device.Simultaneously, the linear actuator generates a second pressure lower than the first pressure to minimize this differential pressure across the isolation valve. This is performed by the piston pump, particularly throughout the entire duration of the pressure application. Due to the hydraulically rigid space, this pressure must be maintained with the isolation valve closed, without any significant volume flow. Consequently, the linear actuator, and thus its motor, remains in the same position at high power, resulting in high thermal stress. To counteract this, the isolation valve is controlled, according to the invention, to open at least partially, allowing a volume flow to pass through the isolation valve into the linear actuator. This volume flow is compensated for by retracting the linear actuator, or rather its piston, thereby changing its motor position.This leads to a better distribution of the input heat, allowing the linear actuator to maintain the second pressure for a longer period without exceeding thermal limits that would cause it to degrade.

[0007] This allows braking via the second pressure supply device to be maintained at high pressures for extended periods.

[0008] The second pressure supply device can in particular be a hydraulic piston pump whose maximum pressure is greater than the maximum pressure of the linear actuator.

[0009] In a preferred embodiment of the invention, the isolation valve is designed to be normally open (de-energized), with one installation orientation such that pressure on the side of the second pressure supply device has an opening effect. An opposing pressure thus keeps the valve closed. A control system can then adjust the holding current, which keeps the isolation valve in a closed state, to the prevailing pressure differential. The greater the pressure generated by the second pressure supply device, the greater the holding current is set to keep the isolation valve sealed. Such hydraulic valves accordingly have a maximum current with which they can be supplied and thus also a maximum pressure differential that can be maintained.

[0010] In a preferred embodiment of the invention, the isolation valve is pulsed. This periodically directs brake fluid volume through the isolation valve to the linear actuator, which, via the control system, causes the linear actuator's motor to rotate. The linear actuator can be controlled, in particular, by pressure or torque. The pulse duration can be adjusted to the required volume. The isolation valve thus alternates between a closed and an open state. Preferably, the isolation valve is closed for more than 95% of the time during the pressure phase by the piston pump. This minimizes the discharged volume, ensuring it is only as large as necessary to address temperature issues.

[0011] In a further preferred embodiment of the invention, the isolation valve comprises an electrically controlled valve and a parallel-connected check valve, which opens towards the wheel brakes. This ensures that a flow of fluid can always be directed towards the wheel brakes when pressure builds up in the linear actuator. This is particularly important in fallback systems, for example, if the control of the isolation valve malfunctions.

[0012] In a particularly preferred embodiment of the invention, the isolation valve can maintain a maximum pressure differential of less than 200 bar, and in particular less than 150 bar. As explained above, this is particularly because otherwise such a high holding current would be necessary, which would overload either the valve coil or an available voltage / current supply. This limitation allows the use of less expensive components, thereby reducing the overall cost of the system.

[0013] In a particularly preferred embodiment of the invention, the opening of the isolation valve is achieved by a brief reduction of the holding current. The isolation valve is thus initially held in the fully closed state by a holding current. The holding current is then reduced, for example, to 50%. "Briefly" here means on the order of 10 to 500 ms. For example, the holding current, which might be 1.2 A, can also be reduced to 0 A for 50 ms. Afterwards, a switching pulse of 3.5 A can be applied for 80 ms to ensure the valve closes reliably, followed by the normal holding current of approximately 1.2 A.

[0014] In a particularly preferred embodiment of the invention, the isolation valve remains closed until the linear actuator reaches a thermal limit. The isolation valve opening step is therefore not performed immediately, but only when needed. This allows the linear actuator to utilize its maximum potential without unnecessarily moving it to a position that prevents further volume absorption. The thermal limit can be determined directly by sensors, in particular temperature sensors or electrical components used as such. Alternatively, temperature models or the duration of current flow to individual phases can be used. In particular, the temperature of a motor winding of the linear actuator can be considered.

[0015] In a further preferred embodiment of the invention, the opening of the isolation valve is aborted when the linear actuator reaches a position smaller than a minimum position. As described, the opening of the compensating valve draws brake fluid into the linear actuator. However, the actuator can only hold a finite amount of brake fluid before reaching its rear end stop. By providing a limit value, it is ensured that the actuator still has sufficient reserves to allow for pressure relief.

[0016] In a further preferred embodiment of the invention, the volume flow rate is selected such that a motor phase changes before it exceeds a temperature threshold. This can occur continuously, quasi-continuously, or intermittently. The volume required for changing a motor phase can, for example, be between 0.2 and 1 cm³ in typical systems. Depending on the design, the change of a motor phase can occur every 1 to 100 seconds. This results in a volume flow rate of 0.002 to 1 cm³ / s.

[0017] In a further preferred embodiment of the invention, the volume flow rate is selected based on the second pressure. The higher the pressure to be supplied by the linear actuator, the more power the motor must provide and the greater the heat to be dissipated. Therefore, a pressure-dependent parameter can be stored in the control unit to determine the volume flow rate. For example, at higher pressures, an opening pulse can be applied to the isolation valve more frequently, so that the motor phase opens at shorter intervals.

[0018] In a further preferred embodiment of the invention, the temperature of the active motor phase is determined, and if a limit value is exceeded, the motor is advanced by one phase by activating the isolation valve to discharge a volume corresponding to the advancement of one phase. This avoids a predetermined pattern, which, due to numerous influencing factors such as ambient temperature and aging processes, requires a large safety margin, but rather operates precisely according to the demand to maintain the temperature within an acceptable range. The volume required for the advancement of one phase can be stored in the system. Alternatively, the advancement of one phase can be detected via motor monitoring.

[0019] In a further preferred embodiment of the invention, the linear actuator position is monitored during the opening of the isolation valve, and the isolation valve is closed again as soon as the linear actuator position has changed by a predetermined amount. Suitable sensors, in particular a rotor position sensor or a displacement sensor, can be used to monitor the linear actuator position.

[0020] In a further preferred embodiment of the invention, the linear actuator position is monitored when the isolation valve is closed, and the opening of the isolation valve is suspended if the linear actuator position indicates a leakage current greater than a threshold value. Particularly with older valves, a leakage current can occur through the valve even when it is supposedly closed, sufficient to trigger a timely motor phase change.

[0021] In a further preferred embodiment of the invention, the isolation valve is opened while the second pressure supply device is pumping volume. During the pressure setting by the second pressure supply device, it does not operate continuously but rather has active and passive phases. Thus, during the passive phase, the opening of the isolation valve is suppressed, and it is only permitted to open during the active phase when the second pressure supply device is pumping volume. In this way, the volume flowing out through the isolation valve is immediately replenished, which allows the pressure to be maintained particularly well and consistently.

[0022] The problem is also solved by a hydraulic motor vehicle brake system comprising a first pressure supply device, which is designed as a linear actuator, the pressure output of which is connected via an isolation valve to inlet valves of wheel brakes and to a second pressure supply device, wherein a control unit is provided which is configured to carry out one of the above methods.

[0023] 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, Fig. Figure 2 shows an alternative design of the first component of the brake system according to Fig. 1, Fig. Figure 3 shows another alternative design of the first component of the brake system according to Fig. 1,

[0024] In Fig. Figure 1 shows a redundant hydraulic braking system for motor vehicles. As an 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 wheel brakes (HL, HR) are assigned to the rear axle and the wheel brakes (VL, VR) to the front axle of the vehicle.

[0025] The braking system comprises a first assembly, which is designed, for example, as a first electro-hydraulic brake control unit with a valve block and a first electronic control unit ECU1, and a second assembly, which is designed, for example, as a second electro-hydraulic brake control unit with a valve block and a second electronic control unit ECU2.

[0026] The first assembly unit has a pressure medium reservoir 4 with two chambers, wherein the first chamber is assigned a first reservoir connection and the second chamber is assigned a second reservoir connection. Reservoirs with three or more chambers are also possible.

[0027] In the first assembly unit, a first electrically actuated pressure source 5 is arranged.

[0028] In the second assembly unit, a second electrically actuated pressure source 2 and wheel-specific 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.

[0029] 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. This allows all four wheel brakes 8 to be actuated by means of the first pressure source 5 and / or by means of the second pressure source 2.

[0030] An electrically actuated circuit isolator valve 40 is arranged in the brake supply line, so that when the circuit isolator valve 40 is closed, the brake supply line is divided into a first line section, to which the inlet valves 6 and the wheel brakes 8 of the rear axle are connected, and a second line section, to which the inlet valves 6 and the 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 isolator valve 40 is closed, the brake system is thus divided into two hydraulic brake circuits I and II, or a first sub-circuit and a second sub-circuit.In the first brake circuit I, pressure source 2 (via the first line section) is connected only to the rear axle wheel brakes 8, and in the second brake circuit II, pressure source 5 (via the second line section) is connected only to the front axle wheel brakes 8. The circuit isolating valve 40 is advantageously designed to be normally open (de-energized). Such an operating mode can be called circuit isolating mode or ACS for "Active Circuit Separation".

[0031] As already mentioned, the brake system comprises, for each hydraulically actuated wheel brake 8, an inlet valve 6 and an outlet valve 7, which are hydraulically connected in pairs via center connections and each pair is connected to a hydraulic wheel connection of the second assembly, to which the corresponding wheel brake 8 is connected. A check valve opening towards the brake supply line is connected in parallel to each inlet valve 6. The outlet connections of the outlet valves 7 are connected to the pressure medium reservoir 4 or its second chamber via a common return line. The inlet connections of all inlet valves 6 can be supplied with pressure via the brake supply line (i.e., with the circuit separator valve 40 open), which is provided by the first pressure source 5 or, for example, in the event of a failure of the first pressure source 5, by the second pressure source 2.

[0032] The first electrically controlled pressure source 5 of the valve block is designed as a hydraulic cylinder-piston arrangement (or a single-circuit electrohydraulic actuator (linear actuator)), whose piston can be actuated by a schematically indicated electric motor via a similarly schematically depicted rotary-translational transmission, in particular by moving it back and forth to build up and release pressure in a pressure chamber. The piston defines the pressure chamber of pressure source 5. A rotor position sensor, also only schematically indicated, is provided to control the electric motor. This sensor allows the piston position and speed, and thus the volume of brake fluid delivered or received, to be determined.

[0033] A section of system pressure line is connected to the pressure chamber of the first electrically controlled pressure source 5. This line section connects the pressure source 5, or rather its pressure chamber, to a hydraulic connection of the first component, which in turn is connected via a hydraulic coupling element to a hydraulic connection of the second component. This connection constitutes the only hydraulic pressure connection, and in particular the only hydraulic connection, between the first and second components, since the return line leads directly to the reservoir. This hydraulic connection is used to transmit brake pressure for actuating the wheel brakes 8.

[0034] The pressure chamber is connected to the pressure medium reservoir 4 via a (suction) line, regardless of the piston's actuation state. A check valve 53, closing towards the pressure medium reservoir 4, is located in the line and connected to the second chamber. An electrically switchable compensating valve 23 forms a further connection to the brake fluid reservoir 4, with the output port of the linear actuator 5. This compensating valve 23 is normally open, so that in the de-energized state, the wheel brakes 8 are connected to the brake fluid reservoir 4. The cylinder-piston assembly 5, for example, has no vent holes. Even outside of braking operations, atmospheric pressure equalization can be permanently ensured via the compensating valve 23 and the isolation valve 26.

[0035] The second electrically controlled pressure source 2 of the second assembly is, for example, designed as a two-piston pump whose two pressure sides are connected together. The suction sides are connected to the return line and thus to the pressure medium reservoir 4. The pressure sides are connected to the first section of the brake supply line.

[0036] In addition to the pressure source 2 and the brake pressure modulation valves 6, 7, the second assembly unit includes, for example, an electrically actuated isolation valve 26, which is advantageously normally open. The isolation valve 26 is hydraulically connected between the connection to the first assembly unit and the second section of the brake supply line. Thus, the first pressure source 5 is disconnected from the second section of the brake supply line via the isolation valve 26. The isolation valve is designed and oriented such that pressure on the side of the linear actuator 5 keeps the isolation valve 26 closed, and pressure on the side of the wheel brakes 8 and the piston pump 2 opens it.Furthermore, a check valve is connected in parallel to the isolation valve 26, so that when pressure builds up, the linear actuator 5 does not keep the isolation valve 26 in the closed state, but can pump pressure into the wheel brakes 8 via the check valve.

[0037] The brake system includes, for example, a pressure sensor 19 in brake circuit I, which is thus assigned to the second pressure source 2. This is advantageous for burst protection when the circuit isolating valve 40 is closed. The pressure sensor can also be located in brake circuit II. Furthermore, another pressure sensor is also located next to the isolating valve 26 on the side of the linear actuator.

[0038] For example, the brake system for leakage monitoring includes a level measuring device for determining a pressure medium level in the pressure medium reservoir 4.

[0039] 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 components, valve drivers, other electronic components, etc.) for controlling the electrically actuated components of the associated valve block and, if applicable, the associated sensors. Advantageously, the valve block and electronic control unit are designed as a single electrohydraulic unit, as is known.

[0040] 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.

[0041] 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.

[0042] For example, the first pressure source 5 can be controlled exclusively by the first electronic control device and the second pressure source 2 can be controlled exclusively by the second electronic control device.

[0043] Under normal operating conditions, the pressure in the wheel brakes is built up by the primary pressure source 5. The pressure in the primary pressure source 5 is reduced by retracting the piston. The pressure is modulated individually for each wheel as needed by the inlet and outlet valves. If necessary, the isolation valve 26 is closed to allow the primary pressure source 5 to draw in additional volume.

[0044] When a particularly high flow rate is required, both pressure sources 5 and 2 operate simultaneously in parallel. When a particularly high pressure is required, the isolation valve 26 is closed, and the secondary pressure source 2 increases the pressure above the maximum pressure of the primary pressure source 5. The pressure setting is then taken over by the second pressure supply device. Preferably, the isolation valve 26 is controlled by the secondary ECU. Simultaneously, the primary pressure source 5 is also controlled to provide a second pressure that is lower than the pressure of the secondary pressure source 2. Due to the closed equalization valve 23 and the closed isolation valve 26, no volume is delivered to the wheel brakes from the primary pressure generator 5, i.e., the linear actuator 5. Instead, the pressure build-up serves to minimize the pressure differential across the isolation valve 26.This allows the valve to remain closed, preventing any significant volume of brake fluid delivered by pump 2 from flowing back towards the reservoir via the isolation valve 26. The second pressure, set by the linear actuator 5, is selected to create a pressure differential at which the isolation valve remains closed. Depending on the design of the isolation valve, this differential can be kept below 150 bar, for example.

[0045] The linear actuator 5 must provide quite high pressures for this purpose, which can even be on the order of its maximum capacity. Since the linear actuator 5 provides the pressure, especially without a significant flow rate, it essentially remains stationary.

[0046] In particular, if the linear actuator is designed as a brushless motor, only a single phase, and thus the coil winding of the linear actuator 5, is continuously supplied with a high current, causing it to heat up. Due to the lack of phase rotation and the absence of cooling through motor movement, this heating can quickly lead to overheating and thus to degradation or even damage to the motor. According to the invention, the temperature of the linear actuator is therefore monitored. For this purpose, a model is used into which the motor current of the linear actuator is fed. If a temperature exceeding a threshold value is determined, the linear actuator should switch the active motor phase. To achieve this, the holding current of the isolation valve 26 is briefly reduced, so that a volume flow from the side of the piston pump 2 flows through the isolation valve 26 to the linear actuator 5.The piston of the linear actuator 5 moves backwards to maintain the required pressure. This movement changes the motor phase, thus preventing overheating.

[0047] If the primary system fails electrically, in particular the primary control unit or its power supply, the secondary ECU closes the isolation valve 26 to build up pressure via the secondary pressure source 2. 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 individually for each wheel.

[0048] If the secondary system fails electrically, particularly the secondary ECU or its power supply, the pressure is built up and released via the primary pressure source 5 as in normal operation. Individual wheel pressure control is not possible, but collective modulation of the wheel pressures remains possible to prevent the vehicle from being destabilized by wheel lock-up.

[0049] The control units ECU1 and ECU2 are configured to implement pressure build-up using the linear actuator 5 and / or the piston pump 2 based on pressure requirements. These requirements can originate from automatic functions, assistance programs, and, in particular, from the driver, who transmits the driver's braking request via the brake pedal. In this embodiment, the brake pedal is a dry design and therefore has no hydraulic connection, nor any switchable hydraulic connection. The driver's request is transmitted to the control units as a brake pedal actuation signal. This signal can be a brake pedal travel, a brake pedal force, and / or similar parameters.

[0050] As soon as a pressure request is received, the compensating valve 23 closes to isolate the pressure side of the linear actuator 5 from the brake fluid reservoir 4, allowing brake pressure to build up. After the pressure request is completed and the linear actuator 5 releases the pressure, the compensating valve 23 reopens to ensure the system is depressurized.

[0051] Fig. Figure 2 shows an alternative design of the first component of the brake system. Fig. 1. The connection between linear actuator 5 and container 4 is configured such that the isolation valve 23 is arranged between the pressure outlet and a first chamber of the container 4. The check valve 53 is arranged between the pressure outlet and a second chamber of the container 4. A third chamber of the container is connected to the suction side of the pump 2. This connection is configured as shown in Fig. 1. It is formed directly between the container 4 and the second assembly. Alternatively, it can be routed through the housing block of the first assembly.

[0052] Fig. Figure 3 shows another alternative design of the first component of the brake system. Fig.1, which includes a brake pedal-operated master brake cylinder 1. In normal operation, such a brake system also operates according to the brake-by-wire principle, meaning that the isolating valve 23 is closed and the master brake cylinder 1 delivers fluid to a simulator 3 when actuated. The actual pressure build-up in the wheel brakes then also occurs as described above via the linear actuator 5. Only in the event of a brake system failure is the master brake cylinder 1 connected directly to the wheel brakes 8 via an open isolating valve 23. A switching valve 27 is provided directly at the output of the linear actuator 5 and can disconnect the linear actuator 5 so that the volume of the master cylinder 1 flows not into it but to the wheel brakes 8.

Claims

[1] Method for controlling a hydraulic motor vehicle brake system comprising a first pressure supply device (5) designed as a linear actuator, the pressure output of which is connected via an isolation valve (26) to inlet valves (6) of wheel brakes (8) and to a pressure output of a second pressure supply device (2), wherein in an operating mode in which the second pressure supply device (2) provides the pressure supply for the wheel brakes (8) and provides a first pressure, the isolation valve (26) is closed characterized by , that the linear actuator (5) sets a second pressure lower than the first pressure to minimize the differential pressure across the isolation valve (26), whereby the isolation valve (26) is controlled to open at least partially, so that a volume flow passes through the isolation valve (26) into the linear actuator (5). [2] Method according to claim 1, characterized by, that the isolation valve (26) is designed to be open when de-energized, wherein one installation direction is such that pressure on the side of the second pressure supply device (2) has an opening effect. [3] Method according to any one of the preceding claims, characterized by , that the isolation valve (26) is opened in a pulsed manner. [4] Method according to any one of the preceding claims, characterized by , that the isolation valve (26) comprises an electrically controlled valve and a parallel-connected check valve which opens in the direction of the wheel brakes (8). [5] Method according to any one of the preceding claims, characterized by , that the isolation valve (26) can withstand a maximum pressure differential that is less than 200 bar, in particular less than 150 bar. [6] Method according to any one of the preceding claims, characterized by , that the opening of the isolation valve (26) is implemented by a brief reduction of the holding current. [7] Method according to any one of the preceding claims, characterized by , that the isolation valve (26) remains completely closed until the linear actuator (5) reaches a thermal limit. [8] Method according to any one of the preceding claims, characterized by , that the opening of the isolation valve (26) is aborted when the linear actuator (5) reaches a position smaller than a minimum position. [9] Method according to any one of the preceding claims, characterized by , that the volume flow is selected such that a motor phase changes before it exceeds a temperature threshold. [10] Method according to any one of the preceding claims, characterized by , that the volume flow rate is chosen based on the second pressure. [11] Method according to any one of the preceding claims, characterized by, that a temperature of the active motor phase is determined and if a limit value is exceeded the motor is rotated further by one phase by controlling the isolation valve (26) to discharge a volume which corresponds to the further rotation of one phase. [12] Method according to any one of the preceding claims, characterized by , that the linear actuator position is observed during the opening of the isolation valve (26), with the isolation valve being closed again as soon as the linear actuator position has changed by a predetermined amount. [13] Method according to any one of the preceding claims, characterized by , that with the isolation valve (26) closed, the linear actuator position is observed, and the opening of the isolation valve is suspended if the linear actuator position indicates a leakage current greater than a threshold. [14] Method according to any one of the preceding claims, characterized by, that the isolation valve is opened while the second pressure supply device is pumping volume. [15] Hydraulic motor vehicle brake system comprising a first pressure supply device (5) designed as a linear actuator, the pressure output of which is connected via an isolation valve (26) to inlet valves (6) of wheel brakes and to a second pressure supply device (2), characterized by that a control unit is provided which is configured to carry out a method according to any one of claims 1 to 14.

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