Method for adjusting brake pressure.

Abstract

Method for adjusting a brake pressure in a brake system with four wheel brakes (18a, 18b, 18c, 18d) using a piston-cylinder system (14, HZ, THZ), wherein each of the wheel brakes (18a, 18b, 18c, 18d) is assigned a solenoid valve (17a, 17b, 17c, 17d), the method comprising the following steps: - Performing a pressure build-up in at least one selected wheel brake (18a, 18b, 18c, 18d) when a pressure change in the selected wheel brake (18a, 18b, 18c, 18d) is required, wherein the pressure build-up in the selected wheel brake (18a, 18b, 18c, 18d) comprises the following steps: - Adjustment of a piston of the piston-cylinder system (14, HZ, THZ) by the electric motor, so that in a pressure build-up phase a pressure (p R1 , p R2 ) at least in the selected wheel brake (18a, 18b, 18c, 18d) is built up in such a way that in a first time interval of the pressure build-up phase a difference between a pressure (p HZ) in the piston-cylinder system (14, HZ, THZ) and the pressure (p R1 , p R2 ) in the selected wheel brake (18a, 18b, 18c, 18d) increases and in a second time interval of the pressure build-up phase the difference between the pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) and the pressure (p R1 , p R2 ) decreases in the selected wheel brake (18a, 18b, 18c, 18d).

Description

[0001] The present invention relates to methods for adjusting a brake pressure. State of the art

[0002] With ABS / ESP, the accuracy and dynamics of the pressure curve determine the control quality and thus the braking distance and stability of the vehicle. Fast and precise pressure control is crucial for good control. With the exception of the electromechanical brake EMB, all hydraulic systems use 2 / 2-way solenoid valves. The Brake Manual, 2nd edition of 2004, pages 114-119, with literature references, provides detailed basic information on this. Without special measures, these valves have a purely digital switching behavior, i.e. they are either open or closed. The rapid closing creates pressure oscillations with a large amplitude that depend on the pressure gradient. These oscillations affect wheel behavior and, above all, cause noise. The pressure gradient depends on the differential pressure, which fluctuates greatly in the control range between µ = 0.05 (ice) and µ = 1.0 (dry asphalt) and is also dependent on the greatly fluctuating THZ pressure of the brake booster.The control of the often pulsed pressure build-up amplitude in the range of 1-10 bar (setpoint) is relatively imprecise. Improvement can be achieved through sophisticated PWM control of the 2 / 2-way solenoid valves. This allows the transition from pressure build-up to pressure maintenance to be influenced, reducing pressure oscillations and noise. This PWM control is difficult and relatively imprecise because it must take the pressure gradient, pressure amplitude, and temperature into account. This PWM control is not used for pressure reduction.

[0003] WO 2006 / 111393 A1 describes a method for pressure control using an electric motor and piston control. The HZ piston movement of the brake booster determines the pressure control and thus offers significant advantages in terms of precise pressure control and variable gradients. WO 2006 / 111393 A1 also describes the pressure control of multiple wheel brakes using the so-called multiplex method (MUX method). Among other things, it is described that the 2 / 2-way solenoid valves should have a large flow cross-section with negligible throttling effect, and the lines from the piston-cylinder system to the brake cylinder should have negligible flow resistance. Furthermore, it is stated that pressure reduction at two wheel brakes can occur simultaneously if approximately the same pressure level prevails initially.

[0004] Despite the measures described in WO 2006 / 111393 A1, the multiplexing process has the disadvantage that simultaneous pressure reduction is not possible if the pressure level in two wheel brakes is unequal. This is because, with the dimensioning described in WO 2006 / 111393 A1, pressure equalization can occur between two to four wheel brakes during pressure reduction if the flow resistance from the HZ or THZ to the wheel cylinder is too low. In addition, two or more pressure reduction requests, which occur slightly offset in time, cannot be carried out simultaneously or partially simultaneously due to the aforementioned problem of possible pressure equalization between the wheel cylinders. This is particularly problematic because the temporal offset of pressure requests with the same sign can occur frequently.

[0005] As mentioned above, pressure reduction and pressure build-up can occur simultaneously or semi-simultaneously. Simultaneous occurs when two or more solenoid valves open and close simultaneously. Partially simultaneous pressure occurs when two or more solenoid valves open or close with a time delay.

[0006] Furthermore, WO 2006 / 111393 A1 does not provide for simultaneous pressure buildup. This means that a potential pressure increase cannot be implemented in the short term, potentially resulting in a longer braking distance. Object of the invention

[0007] The object of the invention is to further improve the multiplexing method known from WO 2006 / 111393 A1 so that a simultaneous or partially simultaneous pressure reduction and pressure build-up of two or more wheel cylinders from different pressure levels can take place. Solution to the task

[0008] The object is achieved according to the invention with a method according to claims 1, 2, 8, 9, 35 and 36. Further advantageous embodiments of the braking systems and the methods result from the features of the subclaims.

[0009] The invention is advantageously characterized by the fact that simultaneous or partially simultaneous pressure reduction and pressure build-up are possible even at different pressure levels of all wheel brakes. This is achieved by appropriately high piston speeds, the dimensioning of the flow resistances RL of the line from the 2 / 2-way solenoid valve to the working chamber of the piston-cylinder system (HZ or THZ), and the flow resistance RV of the 2 / 2-way solenoid valve and the hydraulic lines to the wheel cylinder. The condition is that the flow resistance RL must be smaller than the flow resistance RV. It is particularly advantageous if the flow resistance RL is a factor of 1.5 to 3 smaller than the flow resistance RL.

[0010] In an improved embodiment of the invention, it can be taken into account that the total flow resistance (RL + RV) is designed in such a way that at maximum HZ piston dynamics, which corresponds to the maximum motor dynamics of the brake booster drive, and with two or more open solenoid valves, no pressure equalization can take place in the short term (ie within the valve opening times) due to the simultaneous volume intake or volume release of the wheel cylinder brakes.

[0011] When designing the switching valves, it is therefore important to ensure a very low flow resistance that does not fall below the minimum described above. It is important to ensure that there is sufficient pressure difference between the HZ or THZ and the wheel cylinder during simultaneous pressure reduction so that pressure equalization cannot occur between the individual wheel cylinders of the wheel brakes during simultaneous pressure reduction.

[0012] Another way to prevent pressure equalization during simultaneous pressure reduction or pressure buildup is to reduce the flow cross-section of the valves using PWM control, thereby increasing the flow resistance. The advantage of this is that even during simultaneous or partially simultaneous pressure buildup and pressure reduction, the pressure gradients can be selected depending on the situation, and there is no constraint on the pressure curves specified by the design of the RL and RV. This also makes simultaneous or partially simultaneous pressure reduction or pressure buildup with extremely different pressure levels in two or more wheels manageable.

[0013] Since the maximum possible flow velocity drops to low pressures during pressure reduction and the pressure-volume characteristics of the individual wheels represent a non-linear function, a variable or different piston speed is absolutely necessary for simultaneous or partially simultaneous pressure reduction and pressure build-up.

[0014] In the case of simultaneous or partially simultaneous pressure reduction, the piston of the HZ or THZ must be adjusted by means of an appropriate control or regulation due to the volume flow from the wheel cylinder into the HZ or THZ in order to maintain the pressure difference. The volume that flows out of the HZ or THZ into the wheel cylinder would lead to a pressure increase and static pressure equalization without adjustment of the HZ piston. This piston adjustment is primarily carried out via the controller, which calculates the necessary pressure difference, determines the volume absorption in the HZ accordingly, and uses the HZ pressure and, ideally, a pressure model for this purpose. When adjusting the HZ or THZ piston, care must be taken to ensure that the HZ or THZ pressure is always below the minimum pressure level of all wheel cylinders that are currently connected to the HZ or THZ via an open solenoid valve or switching valve. The same applies to simultaneous or partially simultaneous pressure build-up.Here, the controller again specifies the pressure level for the pressure increase. The HZ or THZ pressure is adjusted accordingly via the piston travel and piston speed to account for the volume of the wheel brake cylinders for pressure buildup. When adjusting the HZ piston, it is important to ensure that the HZ or THZ pressure is always above the maximum pressure level of all wheel cylinders currently connected to the HZ or THZ via an open solenoid valve.

[0015] Knowledge of the pressure-volume characteristic curve of the individual wheels is crucial for both the simultaneous or semi-simultaneous pressure buildup and the simultaneous or semi-simultaneous pressure reduction. This is recorded at intervals for each wheel while the vehicle is stationary. The volume is measured via the corresponding piston travel, taking into account the known pressure in the central or semi-simultaneous cylinders (HCC) or the central or semi-simultaneous cylinders (THC). This process occurs with relatively low dynamics, so that the wheel cylinder pressure corresponds to the pressure in the central or semi-simultaneous cylinders (HCC).

[0016] It is well known that in highly dynamic pressure control processes, there is a large pressure difference both during pressure build-up and pressure reduction due to flow resistance in the switching valve, which is usually a solenoid valve, and in the hydraulic lines to the wheel cylinder. The controller determines the pressure change at the wheel brake, which is proportional to the braking torque. Therefore, conventional ABS / ESP systems, even with a pressure sensor at the solenoid valve output, can only measure the wheel pressure statically. A pressure model with limited accuracy is used for dynamic measurement. Furthermore, it is complex to install a pressure sensor for each wheel. However, in the inventive system with piston control, if the pressure-volume characteristic curve is known, the wheel cylinder pressure can be precisely adjusted even under varying dynamic conditions.

[0017] With simultaneous or semi-simultaneous pressure build-up and pressure reduction, two or more wheel cylinders are operated simultaneously. The pressure difference predetermined by the controller is converted into a corresponding piston travel using the pressure-volume characteristics of the wheels. Using an additional pressure model, the wheel cylinder pressure is continuously calculated. As soon as the target pressure for a wheel is reached, the corresponding solenoid valve is closed. The piston of the HZ or THZ then moves on to operate the remaining wheel cylinders. For the last wheel cylinder to be controlled, pressure control is carried out via the piston travel previously calculated from the pressure-volume characteristic. The solenoid valve of the last wheel brake can then also be closed.

[0018] The pressure model for piston control is very important for the braking system in connection with the simultaneous pressure reduction and pressure build-up, as it is used to calculate or estimate the wheel cylinder pressures. The wheel cylinder pressures calculated with this model are used both to calculate the closing and opening times of the 2 / 2 solenoid valves (switching valves) and as the actual value of the pressure controller's controlled variable in the multiplex process. In addition, the wheel cylinder pressures from the pressure model are used in higher-level control structures (e.g., ABS / ESP, driver assistance functions such as ACC, etc.).

[0019] Since it is advantageous for the HZ or THZ pressure to initially be brought close to the output pressure of the wheel cylinder to be controlled before the pressure change in the wheel cylinder occurs, it is necessary to continuously calculate and store the wheel cylinder pressures. This task is also performed by the pressure model.

[0020] The pressure model is therefore extremely important for the control dynamics, the resulting noise and the control accuracy, especially in connection with the simultaneous or partially simultaneous pressure reduction and pressure build-up.

[0021] The pressure model uses the HZ or THZ pressure as its input signal. The pressure model then calculates the various wheel cylinder pressures. The model parameters, such as equivalent flow resistance, equivalent line inductance, and pressure-volume characteristic curve, can be adapted via temperature (e.g., ambient temperature or a separate temperature sensor on a solenoid valve). Should changes in the transient behavior occur, the model parameters can also be adjusted via adaptation.

[0022] The process of simultaneous or partially simultaneous pressure changes is relatively rare during normal ABS / ESP braking and occurs more frequently in borderline cases such as asymmetric or inhomogeneous road surfaces. It is therefore crucial that the multiplexer can switch from one wheel cylinder to the next as quickly as possible. This is possible because the piston speed, and thus the rate of pressure change, is variably adjustable, allowing the piston to be controlled with maximum dynamics in extreme cases. This variability normally makes it possible to reduce the piston speed and only resort to maximum dynamics in extreme cases. Furthermore, the switching time between the start of the piston movement and the opening or closing of the solenoid valve depends on the pressure difference to be controlled and the absolute pressure in the wheel cylinder.

[0023] When designing the HZ or THZ, it is important to ensure that the HZ or THZ is as rigid as possible when the solenoid valves or switching valves are closed, as the elasticity or rigidity of the HZ or THZ has a significant influence on the switching time. A HZ or THZ that is as rigid as possible thus enables very short switching times.

[0024] To check and, if necessary, correct the wheel cylinder pressures calculated by the pressure model during a longer control intervention, a comparison of the wheel cylinder pressure with the HZ or THZ pressure is carried out at longer intervals. With the piston stationary and the solenoid valve open, a static adjustment is therefore carried out after a certain pressure settling time. Due to the structure of the pressure model, this adjustment runs automatically without additional adaptation rules or extensions to the pressure model. The check can also be performed if the slip or wheel acceleration specified by the controller is not achieved. It is also possible to operate without simultaneous or partially simultaneous pressure change, based solely on the pressure-volume characteristic curve and corresponding piston adjustment proportional to the controller requirement.

[0025] WO 2006 / 111393 A1 describes a braking system that uses a travel simulator. The braking system according to the invention can include a travel simulator. However, for cost reasons, a travel simulator can be omitted. In this case, a feedback effect on the brake pedal can be achieved via the electric drive and a mechanical connection between the brake pedal and the brake booster.

[0026] The invention is explained in more detail below with reference to drawings.

[0027] They show: Fig. 1: Basic structure of the actuators for pressure control; Fig. 2: Control cycle for pressure control of a wheel cylinder; Fig. 3: Control cycle for partially simultaneous pressure control of two wheel cylinders; Fig. 4: Block diagram of a pressure model.

[0028] The Fig. 1 shows the basic structure of the braking system according to the invention consisting of HZ or THZ 14, EC motor 10, spindle 11 for driving the push rod piston, spindle return 12 and rotary angle sensor 13 for determining the position of the piston and detecting the rotor position or the piston travel.

[0029] If the piston receives the command to build up a specific pressure, the corresponding piston movement is initiated via position sensor 13 and pressure sensor 19 in the push rod circuit based on the previously recorded pressure-volume characteristic curve stored in a characteristic map. During a brief, constant pressure period, which usually occurs during braking, a correlation comparison is made based on new measured data with the stored characteristic map data. If a deviation is detected, the pressure-volume characteristic curve is recorded again individually for each wheel brake when the vehicle is later stationary, and the characteristic map is corrected. If the deviation is significant, e.g., in a wheel cylinder, the driver is advised to visit the workshop.

[0030] The pressure generated in the HZ or THZ is transferred via lines 15 and 16 from the push rod piston and floating piston via the 2 / 2-way solenoid valves 17a-d to the wheel cylinders 18a and 18d. The dimensioning of the flow resistances RL from the HZ to the solenoid valve 17i (where i=a,b,c,d) in lines 15 and 16, and subsequently the flow resistances RV in the solenoid valve and the hydraulic connection to the wheel cylinder, is of great importance. Both resistances RL and RV should be low, whereby RL should be much smaller than RV, preferably RL smaller than RV divided by a factor, whereby the factor should be 1.5 to 3. The 2 / 2-way solenoid valves 17a-d with lines 15 and 16 and pressure sensor 19 are preferably integrated into a single block; for this purpose, the HZ or THZ can also be included.

[0031] If the pressure reduction command is given, the pressure is again adjusted via the piston travel and then compared with the pressure measurement. Pressure build-up and reduction correspond to the usual brake control function. This requires the addition of components such as a pedal, pedal travel sensor, travel simulator, and others, as described in the aforementioned WO 2006 / 111393 A1. However, the braking system of WO 2006 / 111393 A1 focuses on pressure control and modulation and does not require all of the above-mentioned components.

[0032] If pressure modulation is now required, e.g., for the ABS / ESP function, the MUX function is activated. If, for example, the pressure at wheel 18a is to be reduced after the HZ or THZ 14 has previously generated a certain pressure in lines 15 and 16 and wheel cylinders 18b and 18d via a motor 10, the solenoid valves 17b to 17d are closed.

[0033] If the pressure reduction p specified by the regulator is achieved via the corresponding piston travel ab is reached, the solenoid valve 17a is closed, and the piston of the HZ or THZ moves to the desired position specified by the controller. If, for example, a pressure build-up p is to be achieved in the wheel cylinder 18d, auf the solenoid valve 17d opens and the piston is moved to the new target position for the target value p auf If a simultaneous or partially simultaneous pressure reduction p ab in the wheel cylinders 18a and 18d, the solenoid valves 17a and 17d are de-energized and thus switched to the open position, and the solenoid valves 17b and 17c are closed. Here, too, the piston moves to the new target position. These processes for pressure modulation occur extremely quickly with special switching conditions for the motor and solenoid valves. These are defined in the Fig. 2 and the Fig. 3 described.

[0034] In Fig. Figure 2 shows a control cycle using the MUX method for a wheel brake. In the upper xy diagram, p R the wheel cylinder pressure and with p HZ The HZ and THZ pressures are shown schematically. Below, the y-axis label s K the position of the push rod piston is plotted over time. The control signals U MV for the solenoid valves are shown in the diagram below.

[0035] In the time course shown, the pressure is first reduced p ab . At time 6, a pressure build-up p auf for a wheel brake.

[0036] Before time 1, all switching valves 17a-d are closed and the HZ piston is stationary. At 1, a pressure reduction command is issued for the wheel shown. Between times 1 and 2, the so-called switching time T Umshown in which, using the pressure-volume characteristic curve of the master cylinder, an attempt is made to adjust the pressure in the HZ to the wheel pressure known from the pressure model by means of a piston displacement, so that when the valve opens at the end of phase 2, a pressure in the HZ is already approximately equal to the pressure in the wheel cylinder, so that pressure equalization between the HZ and the wheel cylinder is almost achieved. Using the pressure-volume characteristic curve of the wheel cylinder, the necessary volume that must be taken from the wheel cylinder in order to bring the pressure to a certain or required level is now calculated at 2. The HZ piston is now driven and adjusted accordingly, and the respective switching valve 17i is simultaneously activated and opened. The time range between times 2 and 3 represents the entire dead time until an effect of the pressure reduction is also noticeable in the wheel pressure.The HZ piston then moves between phases 3 and 4 to the calculated target position, which it reached at the end of phase 4. Should the target pressure increase again in the meantime, e.g., due to a command from higher-level controllers, the motor would abort the process earlier. Once the target position is reached, a settling time T is used to calm the flow conditions in the hydraulic system. ein phases 4 to 5, in which the HZ piston is stationary before the switching valve 17i closes. Here, pre-control of the switching valves is used, whereby the signal is advanced by the closing time of the valve. The settling time 4-5 contributes to a more accurate estimation of the wheel pressures and reduces the noise when the solenoid valve closes at time a. In phases 5 to 6, all switching valves 17a-d are closed again. The actuator would now have time to operate other wheels. If it does not need to change the pressure in the HZ in the meantime, the switching time T UM = 0. The following pressure build-up occurs analogously to the pressure reduction described above.

[0037] The Fig. 3 differs from the Fig. 2 by the fact that in the top diagram a second wheel cylinder pressure p R2shown and at the very bottom another diagram for the control signal of the second switching valve is added.

[0038] Basically, the figure describes the control sequence for a partially simultaneous pressure modulation p ab and p auf in the wheel cylinders. Until time 1, the HZ piston is stationary and all solenoid valves are closed. Then, a pressure reduction request for the first wheel cylinder occurs. Simultaneously or at a later time, a second pressure reduction request for a second wheel cylinder occurs. This now allows simultaneous or partially simultaneous pressure reduction at two wheels. The same applies, of course, to three or four wheels. From time 1 to time 2, the switching time T Umshown, in which, as described above, an attempt is made to approximate the pressure of the first wheel cylinder known from the pressure model, so that when the switching valve belonging to the first wheel brake is opened, pressure equalization between the HZ and the first wheel cylinder is almost achieved. The pressure-volume characteristic curve is then used to calculate the volume required to be taken from the first wheel cylinder in order to bring the pressure to a certain level during or at the end of phases 1-2. If the desired reduction for the second wheel cylinder is also already known, the required HZ piston travel is also calculated here using the stored pressure-volume characteristic curve. In principle, however, this calculation step can also take place at time 3. The HZ piston now moves at time 2 and the solenoid valve belonging to the first wheel brake is controlled and opened at the same time.Time range 2-3 represents the entire dead time until the effect of pressure reduction can also be seen in the wheel cylinder pressure. As soon as it is foreseeable from the pressure model and the resulting calculated volume flow (time 3) that the wheel cylinder pressure p. HZ within the known valve opening time the wheel pressure p R2 falls below, the second solenoid valve MV2 is switched on via U MV2controlled and opened. Shortly before time 4, it is foreseeable from the pressure model and the volume flow or pressure gradient calculated with it that the first wheel cylinder will reach the target pressure within the known valve closing time. Therefore, the solenoid valve MV1 is now closed. At time 4, the valve MV1 is then also closed and the volume flow in the valve MV1 is stopped. This results in pressure oscillations in the wheel cylinder pressure at b. By PWM control of the solenoid valves, the pressure oscillations in the wheel pressure can be reduced. Coincidentally, the HZ piston also reaches its target position at time 4, which was previously calculated as described above. The volume flow in the solenoid valve MV2 can now adjust itself within the settling time T ecalm down before this valve MV2 is also closed at time 5. Due to the settling time, hardly any pressure oscillations occur at the wheel cylinder at a during this process. PWM control of the solenoid valves would not bring any advantages here. Following phases 5 to 6, the previously described process for the simultaneous pressure build-up is repeated. It is important for the simultaneous pressure build-up that when the solenoid valve MV is open i The HZ pressure is always above the lowest wheel cylinder pressure to maintain a positive pressure gradient and avoid pressure equalization between two or more wheels. In general, during simultaneous or semi-simultaneous pressure reduction, PWM control of the solenoid valves would offer the advantage of allowing the pressure gradients to be influenced online even during simultaneous or semi-simultaneous pressure reductions or pressure build-ups.

[0039] The Fig.Figure 4 shows a possible pressure model for calculating the individual wheel cylinder pressures. The pressure model uses the HZ pressure p as the input signal. HZ (t) . Using a model, the various wheel cylinder pressures are calculated as a function of the pressure curve p HZ (t) in the HZ or THZ, where p R (t) = f (p HZ). Model parameters such as equivalent flow resistance, equivalent line inductance and pressure-volume characteristic of the wheel cylinder and the HZ or THZ can be adapted via the temperature, e.g. the ambient temperature or by means of the temperature measured by a temperature sensor on a solenoid valve. For this purpose, the model parameters are either stored in a table dependent on the temperature or the model parameters are calculated using a physical calculation rule and passed on to the model. The adaptation of the model parameters can also be influenced by other events and / or input signals, which are shown in dashed lines. If, for example, changes in the transient behavior occur, it is also possible to adjust the model parameters via adaptation.The pressure model is constantly taken into account and is extremely important for the accuracy of the pressure setting, especially in connection with the pressure modulation in ESP / ABS or other higher-level controllers.

[0040] In addition, the following embodiments are disclosed: 1. Brake system with a brake booster, the piston-cylinder system (14, HZ, THZ) of which is mechanically or hydraulically driven by an electric motor through a transmission means, wherein at least one working chamber of the piston-cylinder system (14, HZ, THZ) is connected to at least two wheel brakes via hydraulic lines, wherein a 2 / 2-way switching valve (17a, 17b, 17c, 17d) is assigned to each wheel brake and the hydraulic connecting lines between the wheel brakes (18a, 18b, 18c, 18d) and the piston-cylinder system (14, HZ) can be closed either separately or jointly by means of the 2 / 2-way switching valves (17a, 17b, 17c, 17d), so that in the wheel brakes (18a, 18b, 18c, 18d) a pressure can be regulated successively in the sense of a multiplexing process and / or simultaneously, wherein the electric motor and the switching valves (17a, 17b, 17c, 17d) are controlled by a control device,and that the hydraulic connecting line from the working chamber of the piston-cylinder system (14, HZ, THZ) to the respective solenoid valve has a flow resistance RLi, and each switching valve together with the hydraulic line to the wheel cylinder (17a, 17b, 17c, 17d) has a flow resistance RVi, characterized in that the flow resistances RLi and RVi are small, such that the HZ piston speed determines the pressure reduction gradient and pressure build-up gradient in each wheel brake (18a, 18b, 18c, 18d), wherein the flow resistance RLi is smaller than the flow resistance RVi, and that the control device regulates or controls the piston movement and piston speed during pressure build-up and pressure reduction depending on the pressure-volume characteristic curve of the wheel brakes (18a, 18b, 18c, 18d). 2. Braking system according to embodiment 1, characterized in that the flow resistance RVi is greater than the flow resistance RLi, in particular RVi is at least 1.3 to 2.5 times greater, preferably 1.5 to 2 times greater, than the flow resistance RLi. 3. Brake system according to embodiment 1 or 2, characterized in that the maximum achievable pressure gradient of the piston-cylinder system (14, HZ) with closed switching valves (17a, 17b, 17c, 17d) is greater by at least a factor of 2 to 4, in particular a factor of 3, than the maximum achievable pressure gradient in the wheel brakes (18a, 18b, 18c, 18d) with at least one open switching valve (17a, 17b, 17c, 17d). 4. Braking system according to one of embodiments 1 to 3, characterized in that the flow resistances RLi and RVi, in particular the sum of the flow resistances RLi and RVi of each wheel brake (18a, 18b, 18c, 18d), are designed such that at maximum dynamics of the piston-cylinder system (HZ) and its drive and at least one, in particular two, open switching valves (17a, 17b, 17c, 17d) due to the simultaneous volume absorption or volume release of the wheel brakes (18a, 18b, 18c, 18d) during the time in which the switching valves (17a, 17b, 17c, 17d) are open, no pressure equalization takes place between the wheel brakes (18a, 18b, 18c, 18d). 5. Braking system according to one of embodiments 1 to 4, characterized in that in order to reduce the flow cross-section of a switching valve, the control device controls this switching valve by means of pulse width modulation PWM. 6. Brake system according to one of the preceding embodiments, characterized in that the hydraulic connecting line from the working chamber of the piston-cylinder system (14, HZ) to the respective solenoid valve (17a, 17b, 17c, 17d) is shorter than 30 cm, in particular shorter than 20 cm, particularly preferably shorter than 5 cm. 7. Braking system according to one of the preceding embodiments, characterized in that the adjustment of the pressure in each wheel brake is carried out on the basis of the pressure-volume characteristic curve of the respective wheel via a travel control of the piston of the piston-cylinder system (14, HZ). 8. Braking system according to one of the preceding embodiments, characterized in that a higher-level controller, in particular the ABS and ESP controller, specifies the target pressure for the piston-cylinder system (14, HZ). 9. Brake system according to one of the preceding embodiments, characterized in that the necessary and to be calculated pressure gradient in the piston-cylinder system is a function of the amount of the required pressure change in a wheel brake. 10. Braking system according to one of the preceding embodiments, characterized in that the control device, in particular during the control process, continuously calculates the pressure levels of the wheel brakes based on a pressure model. 11. Braking system according to embodiment 10, characterized in that an input variable of the pressure model is the actual pressure in the piston-cylinder system (14, HZ). 12. Braking system according to embodiment 10 or 11, characterized in that the pressure model uses as model parameter an equivalent flow resistance which corresponds to the hydraulic resistance of the path from the piston-cylinder system (14, HZ) via the switching valve (17a, 17b, 17c, 17d) to the wheel cylinder of the wheel brake. 13. Braking system according to embodiment 12, characterized in that the pressure model in the total pressure loss within the hydraulic path from the piston-cylinder system (14, HZ) via the switching valve (17a, 17b, 17c, 17d) to the wheel cylinder of the wheel brake takes into account a weighting of the flow conditions laminar and turbulent. 14. Braking system according to one of embodiments 10 to 13, characterized in that the pressure model calculates the closing time of each switching valve (17a, 17b, 17c, 17d) as a function of the calculated actual pressure and the current pressure gradient. 15. Braking system according to one of embodiments 10 to 14, characterized in that the pressure model takes into account a hydraulic equivalent inductance as a model parameter, which represents the mass and / or inertia of the brake fluid. 16. Braking system according to one of embodiments 10 to 15, characterized in that the pressure model includes as model parameters the pressure-volume characteristic curve of each individual wheel brake, which represents the capacity or volume absorption of the wheel brake. 17. Braking system according to one of embodiments 10 to 16, characterized in that the model parameters of the pressure model are a function of the temperature, in particular the ambient temperature of the switching valves (17a, 17b, 17c, 17d), and / or are adapted or adjusted via the temperature. 18. Braking system according to one of embodiments 10 to 17, characterized in that the control device corrects or adapts the pressure model if the slip and / or the wheel acceleration specified by the controller is / are not achieved. 19. Braking system according to one of embodiments 10 to 18, characterized in that the control device checks the pressure model and / or adjusts its values ​​and parameters by determining the actual pressure in the piston-cylinder system (14, HZ) with the switching valve (17a, 17b, 17c, 17d) open after waiting for a short pressure settling time. 20. Braking system according to embodiment 19, characterized in that the control device carries out the checking and adjustment of the pressure model several times in succession or at shorter time intervals if the pressure model deviates from the actually measured values. 21. Braking system according to one of embodiments 10 to 20, characterized in that the time difference (TUm) between the start of the piston movement and the switching of a switching valve is variable such that it can be controlled via the piston speed. 22. Brake system according to one of the preceding embodiments, characterized in that the piston of the piston-cylinder system (14, HZ, THZ) and the lines from the piston-cylinder system (14, HZ, THZ) to the switching valves (17a, 17b, 17c, 17d) are designed to be rigid. 23. Method for adjusting a brake pressure in at least one wheel brake using a brake system according to one of the preceding embodiments, characterized in that the pressure in at least two wheel brakes (18a, 18b, 18c, 18d) is built up or reduced successively, simultaneously (simultaneously) or in a temporally overlapping manner (partially simultaneously). 24. Method for adjusting a brake pressure in at least one wheel brake using a brake system according to one of embodiments 1 to 22 or the method according to embodiment 23, characterized in that during simultaneous pressure build-up or pressure reduction in at least two wheel brakes (18a, 18b, 18c, 18d), the pressure difference between the working chamber of the piston-cylinder system (14, THZ) and the respective wheel brakes (18a, 18b, 18c, 18d) is selected to be so large that no pressure equalization takes place between the respective wheel brakes (18a, 18b, 18c, 18d). 25. Method according to embodiment 24, characterized in that the piston of the piston-cylinder system (14, THZ) is adjusted by the control device to maintain the pressure difference when the switching valves (17a, 17b, 17c, 17d) are open. 26. Method according to embodiment 25, characterized in that the control device calculates the necessary pressure difference and thus the required volume change of the working chamber of the piston-cylinder system (14, THZ), in which no volume compensation takes place between the wheel brakes (18a, 18b, 18c, 18d) whose associated switching valve (17a, 17b, 17c, 17d) is open or whose opening is imminent, and that in order to set the required differential pressure, the control device adjusts or controls the piston of the piston-cylinder system (14, THZ) accordingly for pressure regulation. 27. A method for adjusting a brake pressure in at least one wheel brake using a brake system according to one of embodiments 1 to 22 or a method according to one of embodiments 23 to 26, characterized in that the pressure build-up and / or pressure reduction in at least two wheel brakes takes place simultaneously and / or with a temporal overlap (partially simultaneous), wherein the initial levels of the respective wheel brakes are different from one another. 28. Method for adjusting a brake pressure in at least one wheel brake using a brake system according to one of embodiments 1 to 22 or a method according to one of embodiments 23 to 27, characterized in that before the start of the journey the pressure-volume characteristic curve of each wheel brake is adjusted by statically comparing the piston travel with the pressure in the working chamber of the piston-cylinder system (14, HZ). 29. Method for adjusting a brake pressure in at least one wheel brake using a brake system according to one of embodiments 1 to 22 or a method according to one of embodiments 23 to 28, characterized in that the pressure in the working chamber of the piston-cylinder system (14, HZ) is set or regulated to the pressure in the respective wheel brake (18a, 18b, 18c, 18d) or a lower pressure before the start of the pressure reduction, and thereafter the associated switching valve (17a, 17b, 17c, 17d) is opened by the control device. 30. Method for adjusting a brake pressure in at least one wheel brake using a brake system according to one of embodiments 1 to 22 or a method according to one of embodiments 23 to 29, characterized in that the pressure in the working chamber of the piston-cylinder system (14, HZ) is adjusted or regulated to the pressure in the respective wheel brake (18a, 18b, 18c, 18d) or a higher pressure before the start of the pressure build-up, and thereafter the associated switching valve (17a, 17b, 17c, 17d) is opened by the control device. List of reference symbols: 1-9 phases in the control cycle p HZ Master cylinder pressure p R Wheel cylinder pressure p auf Pressure build-up p ab Pressure reduction p* ab Pressure change rate during pressure reduction p* an Pressure change rate during pressure build-up s kHZ piston travel s* k HZ piston speed T E Settling time before valve closing T Um Switching time from the start of piston movement to opening the valve T MUX Total time to set the desired pressure on one or more wheels a Transition curve in the pressure-time behavior with settling time before valve closing b Transition curve in the pressure-time behavior during hard valve closing without settling time MV i Solenoid valve / switching valve U MV Voltage curve 2 / 2 solenoid valve RL Flow resistance in the line from the HZ or THZ to the solenoid valve / switching valve RV flow resistance in the solenoid valve 10 EC motor 11 spindle 12 Spindle reset 13 Angle encoder (position encoder) 14 Hz or THz 15 Pressure line from the push rod piston 16 Pressure line from the floating piston 17a-17d 2 / 2-way solenoid valves as switching valves 18a-18d wheel cylinder 19 pressure sensors

Claims

[1] Method for adjusting a brake pressure in a brake system with four wheel brakes (18a, 18b, 18c, 18d) using a piston-cylinder system (14, HZ, THZ), wherein each of the wheel brakes (18a, 18b, 18c, 18d) is assigned a solenoid valve (17a, 17b, 17c, 17d), the method comprising the following steps: - Carrying out a pressure build-up in at least one selected wheel brake (18a, 18b, 18c, 18d) when a pressure change in the selected wheel brake (18a, 18b, 18c, 18d) is required, wherein the pressure build-up in the selected wheel brake (18a, 18b, 18c, 18d) comprises the following steps: - Adjusting a piston of the piston-cylinder system (14, HZ, THZ) by the electric motor, so that in a pressure build-up phase a pressure (p R1 , p R2 ) at least in the selected wheel brake (18a, 18b, 18c, 18d) in such a way that in a first time interval of the pressure build-up phase a difference between a pressure (p HZ) in the piston-cylinder system (14, HZ, THZ) and the pressure (p R1 , p R2 ) in the selected wheel brake (18a, 18b, 18c, 18d) increases and in a second time interval of the pressure build-up phase the difference between the pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) and the pressure (p R1 , p R2 ) in the selected wheel brake (18a, 18b, 18c, 18d). [2] Method for adjusting a brake pressure in a brake system with four wheel brakes (18a, 18b, 18c, 18d) using a piston-cylinder system (14, HZ, THZ), wherein each of the wheel brakes (18a, 18b, 18c, 18d) is assigned a solenoid valve (17a, 17b, 17c, 17d), the method comprising the following steps: - Carrying out a pressure build-up in at least one selected wheel brake (18a, 18b, 18c, 18d) when a pressure change in the selected wheel brake (18a, 18b, 18c, 18d) is required, wherein the pressure build-up in the selected wheel brake (18a, 18b, 18c, 18d) comprises the following steps: - Adjusting a piston of the piston-cylinder system (14, HZ, THZ) by the electric motor, so that in a pressure build-up phase a pressure (p R1 , p R2 ) at least in the selected wheel brake (18a, 18b, 18c, 18d) in such a way that in a first time interval of the pressure build-up phase a difference between a pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) and the pressure (p R1 , p R2 ) in the selected wheel brake (18a, 18b, 18c, 18d) increases and in a second time interval of the pressure build-up phase the difference between the pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) and the pressure (p R1 , pR2 ) in the selected wheel brake (18a, 18b, 18c, 18d), wherein during the adjustment of the piston the solenoid valve (17a, 17b, 17c, 17d) of at least one other wheel brake (18a, 18b, 18c, 18d) is closed in at least one time interval. [3] Method according to claim 1 or 2, characterized by that the second time interval follows the first time interval. [4] Method according to one of the preceding claims, characterized by that in the first time interval a first pressure gradient is set in the piston-cylinder system (14, HZ, THZ) and in the second time interval a second pressure gradient is set in the piston-cylinder system (14, HZ, THZ), wherein the first pressure gradient is higher than the second pressure gradient. [5] Method according to claim 4, characterized bythat in a third time interval a third pressure gradient is set in the piston-cylinder system (14, HZ, THZ), wherein the third pressure gradient is lower than the first pressure gradient and / or higher than the second pressure gradient. [6] Method according to one of claims 2 to 5, characterized by that during the adjustment of the piston, the solenoid valve (17a, 17b, 17c, 17d) of the other wheel brake (18a, 18b, 18c, 18d) is opened in such a way that the difference between the pressure (p R1 , p R2 ) in the selected wheel brake (18a, 18b, 18c, 18d) and the pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) higher or lower than the difference between the pressure (p R1 , p R2 ) in the other wheel brake (18a, 18b, 18c, 18d) and the pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) to maintain the pressure (p R1 , p R2 ) in the selected wheel brake (18a, 18b, 18c, 18d) and the pressure (p R1 , p R2) in the other wheel brake (18a, 18b, 18c, 18d) simultaneously or in an overlapping manner. [7] Method according to one of the preceding claims, characterized by that the pressure build-up is carried out in at least two selected wheel brakes (18a, 18b, 18c, 18d) whenever a pressure change is required, wherein the adjustment of the piston is carried out in such a way that the pressures (p R1 , p R2 ) in the selected wheel brakes (18a, 18b, 18c, 18d) simultaneously or in an overlapping manner. [8] Method for adjusting a brake pressure in a brake system with four wheel brakes (18a, 18b, 18c, 18d) using a piston-cylinder system (14, HZ, THZ), wherein each of the wheel brakes (18a, 18b, 18c, 18d) is assigned a solenoid valve (17a, 17b, 17c, 17d), the method comprising the following steps: - carrying out a pressure reduction in at least one selected wheel brake (18a, 18b, 18c, 18d) when a pressure change in the selected wheel brake (18a, 18b, 18c, 18d) is required, wherein the pressure reduction in the selected wheel brake (18a, 18b, 18c, 18d) comprises the following steps: - Adjusting a piston of the piston-cylinder system (14, HZ, THZ) by the electric motor, so that in a pressure reduction phase a pressure (p R1 , p R2 ) at least in the selected wheel brake (18a, 18b, 18c, 18d) in such a way that in a first time interval of the pressure reduction phase a difference between a pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) and the pressure (p R1 , p R2 ) in the selected wheel brake (18a, 18b, 18c, 18d) increases and in a second time interval of the pressure reduction phase the difference between the pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) and the pressure (p R1 , p R2) in the selected wheel brake (18a, 18b, 18c, 18d). [9] Method for adjusting a brake pressure in a brake system with four wheel brakes (18a, 18b, 18c, 18d) using a piston-cylinder system (14, HZ, THZ), wherein each of the wheel brakes (18a, 18b, 18c, 18d) is assigned a solenoid valve (17a, 17b, 17c, 17d), the method comprising the following steps: - carrying out a pressure reduction in at least one selected wheel brake (18a, 18b, 18c, 18d) when a pressure change in the selected wheel brake (18a, 18b, 18c, 18d) is required, wherein the pressure reduction in the selected wheel brake (18a, 18b, 18c, 18d) comprises the following steps: - Adjusting a piston of the piston-cylinder system (14, HZ, THZ) by the electric motor, so that in a pressure reduction phase a pressure (p R1 , p R2) at least in the selected wheel brake (18a, 18b, 18c, 18d) is reduced in such a way that in a first time interval of the pressure reduction phase a difference between a pressure (pHZ) in the piston-cylinder system (14, HZ, THZ) and the pressure (p R1 , p R2 ) in the selected wheel brake (18a, 18b, 18c, 18d) increases and in a second time interval of the pressure reduction phase the difference between the pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) and the pressure (p R1 , p R2 ) in the selected wheel brake (18a, 18b, 18c, 18d), wherein during the adjustment of the piston the solenoid valve (17a, 17b, 17c, 17d) of at least one other wheel brake (18a, 18b, 18c, 18d) is closed in at least one time interval. [10] Method according to claim 8 or 9, characterized by that the second time interval follows the first time interval. [11] Method according to one of claims 8 to 10, characterized bythat in the first time interval a first pressure gradient is set in the piston-cylinder system (14, HZ, THZ) and in the second time interval a second pressure gradient is set in the piston-cylinder system (14, HZ, THZ), wherein the first pressure gradient is lower than the second pressure gradient. [12] Method according to claim 11, characterized by that in a third time interval a third pressure gradient is set in the piston-cylinder system (14, HZ, THZ), wherein the third pressure gradient is higher than the first pressure gradient and / or lower than the second pressure gradient. [13] Method according to one of claims 9 to 12, characterized by that during the adjustment of the piston, the solenoid valve (17a, 17b, 17c, 17d) of the other wheel brake (18a, 18b, 18c, 18d) is opened in such a way that the difference between the pressure (p R1 , p R2 ) in the selected wheel brake (18a, 18b, 18c, 18d) and the pressure (p HZ) in the piston-cylinder system (14, HZ, THZ) higher or lower than the difference between the pressure (p R1 , p R2 ) in the other wheel brake (18a, 18b, 18c, 18d) and the pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) to maintain the pressure (p R1 , p R2 ) in the selected wheel brake (18a, 18b, 18c, 18d) and the pressure (p R1 , p R2 ) in the other wheel brake (18a, 18b, 18c, 18d) simultaneously or in an overlapping manner. [14] Method according to one of claims 8 to 13, characterized by that the pressure reduction is carried out in at least two selected wheel brakes (18a, 18b, 18c, 18d) whenever a pressure change is required, wherein the adjustment of the piston is carried out in such a way that the pressures (p R1 , p R2 ) in the selected wheel brakes (18a, 18b, 18c, 18d) are reduced simultaneously or in an overlapping manner. [15] Method according to one of the preceding claims, characterized by that during the adjustment of the piston, the solenoid valve (17a, 17b, 17c, 17d) of at least one further wheel brake (18a, 18b, 18c, 18d) is controlled by means of pulse width modulation such that the flow resistance of the solenoid valve (17a, 17b, 17c, 17d) is increased. [16] Method according to one of the preceding claims, characterized by that a higher-level controller specifies a target pressure for the piston-cylinder system (14, HZ, THZ) as well as a switching state (OPEN, CLOSED, pulse width modulation PWM) of the solenoid valves (17a, 17b, 17c, 17d). [17] Method according to one of the preceding claims, characterized by that during simultaneous pressure build-up or pressure reduction to adjust the pressure gradient, the flow cross-section of at least one solenoid valve (17a, 17b, 17c, 17d) is reduced via a pulse width modulation control of the solenoid valves (17a, 17b, 17c, 17d). [18] Method according to one of the preceding claims, characterized bythat the necessary and to be calculated pressure gradient in the piston-cylinder system (14, HZ, THZ) is a function of the amount of the required pressure change in a wheel brake (18a, 18b, 18c, 18d). [19] Method according to one of the preceding claims, characterized by that in the pressure build-up phase / pressure reduction phase at least once an actual pressure (p R1 , p R2 ) in at least the selected wheel brake (18a, 18b, 18c, 18d) is calculated using a pressure model. [20] Method according to claim 16, characterized by that the adjustment of the piston in the direction of a desired position takes place at a desired speed, so that a (wheel-individual) predetermined pressure gradient is set in the at least one selected wheel brake (18a, 18b, 18c, 18d), wherein the desired position and the desired speed are determined based on the at least one pressure change using the pressure model. [21] Method according to claims 19 and 20, characterized by that the pressure build-up / pressure reduction in the selected wheel brake (18a, 18b, 18c, 18d) comprises the following step: - Closing the solenoid valve (17a, 17b, 17c, 17d) of the selected wheel brake (18a, 18b, 18c, 18d) when the actual pressure (p R1 , p R2 ) in the selected wheel brake (18a, 18b, 18c, 18d) corresponds to a target pressure corresponding to the pressure change and / or the piston has reached the target position in order to adjust the pressure change in the selected wheel brake (18a, 18b, 18c, 18d). [22] Method according to claim 21, characterized by that a settling time (T e ) is provided to calm the flow conditions between the piston reaching the desired position and the closing of the solenoid valve (17a, 17b, 17c, 17d). [23] Method according to one of the preceding claims, characterized by that the time difference (T Um) between the start of the piston movement and the switching of a solenoid valve (17a, 17b, 17c, 17d) is variable, such that it can be controlled via the piston speed. [24] Method according to one of claims 19 to 23, characterized by that an input variable of the pressure model is an actual pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ). [25] Method according to one of claims 19 to 24, characterized by that the pressure model includes as model parameter a wheel-individual pressure-volume characteristic curve of each of the wheel brakes (18a, 18b, 18c, 18d), which represents the capacity or volume absorption of the wheel brake (18a, 18b, 18c, 18d). [26] Method according to one of claims 19 to 25, characterized by that the pressure model determines the closing time of each solenoid valve (17a, 17b, 17c, 17d) depending on the calculated actual pressure (p R1 , p R2) and the current pressure gradient in the respective wheel brake (18a, 18b, 18c, 18d). [27] Method according to one of claims 19 to 26, characterized by that the pressure model takes into account a hydraulic equivalent inductance as a model parameter, which represents the mass and / or inertia of a brake fluid. [28] Method according to one of claims 19 to 27, characterized by that the model parameters of the pressure model are a function of temperature and / or are adapted or adjusted via temperature. [29] Method according to one of claims 19 to 28, characterized by that a control device corrects or adapts the pressure model if the slip and / or wheel acceleration specified by the controller is / are not achieved. [30] Method according to one of claims 24 to 29, characterized bythat a control device checks the pressure model and / or adjusts its values ​​and parameters by adjusting the actual pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) with the solenoid valve (17a, 17b, 17c, 17d) open after waiting for a short pressure settling time. [31] Method according to claim 30, characterized by that the control device checks and adjusts the pressure model several times in succession or at shorter time intervals if the pressure model deviates from the actually measured values. [32] Method according to one of claims 25 to 31, characterized by that before starting the journey, the pressure-volume characteristic curve of each wheel brake (18a, 18b, 18c, 18d) is determined by static comparison of the piston travel with the pressure (p HZ ) in the working chamber of the piston-cylinder system (14, HZ, THZ). [33] Method according to one of claims 25 to 32, characterized by following step: - Recording the wheel-individual pressure-volume characteristic curve for each of the wheel brakes (18a, 18b, 18c, 18d) when the vehicle is stationary. [34] Method according to claim 33, characterized by that the step of recording the wheel-individual pressure-volume characteristic curve is repeated if a deviation of the actual pressure (p R1 , p R2 ) in at least one wheel brake (18a, 18b, 18c, 18d) from the actual pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) with the piston at a standstill and the solenoid valve (17a, 17b, 17c, 17d) of the wheel brake (18a, 18b, 18c, 18d) open is detected after a pressure settling time in order to correct or adapt the pressure model. [35] Method for adjusting a brake pressure in at least one wheel brake (18a, 18b, 18c, 18d) using a piston-cylinder system (14, HZ, THZ), wherein a solenoid valve (17a, 17b, 17c, 17d) is assigned to the wheel brake (18a, 18b, 18c, 18d), the method comprising the following steps: - Carrying out a pressure build-up in the wheel brake (18a, 18b, 18c, 18d) when a pressure change in the wheel brake (18a, 18b, 18c, 18d) is required, wherein the pressure build-up is carried out in such a way that a volume flow from the piston-cylinder system (14, HZ, THZ) through the solenoid valve (17a, 17b, 17c, 17d) into the wheel brake (18a, 18b, 18c, 18d) causes the pressure change (completely); and - carrying out a pressure reduction in the wheel brake (18a, 18b, 18c, 18d) when a pressure change in the wheel brake (18a, 18b, 18c, 18d) is required, wherein the pressure reduction is carried out in such a way that a volume flow from the wheel brake (18a, 18b, 18c, 18d) through the solenoid valve (17a, 17b, 17c, 17d) into the piston-cylinder system (14, HZ, THZ) causes the pressure change (completely). [36] Method for adjusting a brake pressure in at least one wheel brake (18a, 18b, 18c, 18d) using a piston-cylinder system (14, HZ, THZ), the method comprising the following steps: - Recording a wheel-specific pressure-volume characteristic curve for the wheel brake (18a, 18b, 18c, 18d) when the vehicle is stationary, whereby the following steps are carried out: - Adjusting a piston of the piston-cylinder system (14, HZ, THZ) in such a way that along a piston path a pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) a pressure (p R1 , p R2 ) in the wheel brake (18a, 18b, 18c, 18d); and - Detecting the pressure (p HZ ) in the piston-cylinder system (14, HZ, THZ) as a function of a volume in the wheel brake (18a, 18b, 18c, 18d) along the piston path in order to record the pressure-volume characteristic curve for the wheel brake (18a, 18b, 18c, 18d); - Carrying out a pressure build-up and / or pressure reduction in the wheel brake (18a, 18b, 18c, 18d) when a pressure change in the wheel brake (18a, 18b, 18c, 18d) is required.

Images