DE102009061892B4 - Method for adjusting a brake pressure. - Google Patents

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 respective solenoid valve (17a, 17b, 17c, 17d), wherein the method comprises 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 (pR1, pR2) is built up at least in the selected wheel brake (18a, 18b, 18c, 18d) such that in a first time interval of the pressure build-up phase a difference between a pressure (pHZ) in the piston-cylinder system (14, HZ, THZ) and the pressure (pR1, pR2) 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 (pHZ) in the piston-cylinder system (14, HZ, THZ) and the pressure (PR1, PR2) in the selected wheel brake (18a, 18b, 18c, 18d) decreases.

Description

The present invention relates to methods for adjusting a brake pressure.

State of the art

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 work with 2/2-way solenoid valves. The Brake Manual 2nd Edition of 2004 p.114-119 with literature references the detailed basic information. Without special measures these valves have a purely digital switching behavior, i.e. they are either open or closed (open/closed). The rapid closing results in pressure oscillations with a large amplitude depending on the pressure gradient, which affect the 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 also depends on the strongly fluctuating THZ pressure of the brake booster. The often clocked pressure build-up amplitude in the range of 1-10 bar (setpoint) can only be dosed relatively imprecisely. An improvement can be achieved by complex PWM control of the 2/2 solenoid valves. This can be used to influence the transition from pressure build-up to pressure maintenance in particular, so that the pressure oscillations and the noise are reduced. This PWM control is difficult and relatively inaccurate because it has to take into account the pressure gradient, the pressure amplitude and also the temperature. This PWM control is not used for pressure reduction.

In the WO 2006/111393 A1 A method for pressure control using an electric motor and piston control is described. The HZ piston movement of the brake booster determines the pressure control and thus has considerable advantages in terms of precise pressure control and variable gradients. The WO 2006/111393 A1 also describes the pressure control of several 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 that the lines from the piston-cylinder system to the brake cylinder should have negligible flow resistance. It is also stated that the pressure reduction can take place at two wheel brakes simultaneously if approximately the same pressure level prevails initially.

Despite this in the WO 2006/111393 A1 The multiplexing method has the disadvantage that if the pressure level in two wheel brakes is unequal, simultaneous pressure reduction is not possible, since the WO 2006/111393 A1 With the dimensioning described above, pressure equalization can occur between two to four wheel brakes during pressure reduction, provided that the flow resistance from the HZ or THZ to the wheel cylinder is too low. In addition, two or more pressure reduction requests that occur slightly offset in time cannot be carried out simultaneously or partially simultaneously due to the above-mentioned 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 more frequently.

As mentioned above, pressure reductions and pressure build-ups can occur simultaneously or partially simultaneously. Simultaneous means when two or more solenoid valves are opened and closed at the same time. Partially simultaneous means when two or more solenoid valves are either opened or closed at different times.

Furthermore, the WO 2006/111393 A1 no simultaneous pressure build-up is provided. This means that a possible pressure increase cannot be carried out in the short term, which may result in a longer braking distance.

task of the invention

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

solution to the task

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.

The invention is advantageously characterized by the fact that a simultaneous or partially simultaneous pressure reduction and pressure build-up is possible even with different pressure levels of all wheel brakes. This is achieved by correspondingly high piston speeds, the dimensioning of the flow resistance RL of the line from the 2/2-way solenoid valve to the working chamber of the piston-cylinder systems (HZ or THZ) and the flow resistance RV of the 2/2 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 smaller than the flow resistance RL by a factor of 1.5 to 3.

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 (i.e. within the valve opening times) due to the simultaneous volume intake or volume release of the wheel cylinder brakes.

When designing the switching valves, it is therefore important to ensure that a very low flow resistance is achieved 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 no pressure equalization can take place between the individual wheel cylinders of the wheel brakes during simultaneous pressure reduction.

Another way to prevent pressure equalization during simultaneous pressure reduction or pressure build-up is to reduce the flow cross-section of the valves using a PWM control and thus increase the flow resistance. The advantage of this is that the pressure gradients can be selected depending on the situation even during simultaneous or partially simultaneous pressure build-up and pressure reduction and there is no connection to the pressure curves specified by the design of RL and RV. Simultaneous or partially simultaneous pressure reduction or pressure build-up with extremely different pressure levels in two or more wheels can also be controlled in this way.

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.

In the case of simultaneous or partially simultaneous pressure reduction, the piston must be adjusted by means of an appropriate control or regulation as a result of 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 an increase in pressure without adjustment of the HZ piston and statically to a pressure equalization. This piston adjustment is carried out primarily via the controller, which calculates the necessary pressure difference, determines the volume absorption in the HZ accordingly and uses the HZ pressure and, preferably, a pressure model. 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 connected to the HZ or THZ at that moment via an open solenoid valve or switching valve. The same applies to simultaneous or partially simultaneous pressure build-up. Here, the controller again indicates the pressure level of the pressure increase. The HZ or THZ pressure is adjusted accordingly via the piston travel and the piston speed in order to take the volume of the wheel cylinders of the wheel brakes into account for the pressure build-up. 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 that are currently connected to the HZ or THZ via an open solenoid valve.

Knowledge of the pressure-volume characteristic curve of the individual wheels is of great importance for both the simultaneous or partially simultaneous pressure build-up and the simultaneous or partially simultaneous pressure reduction. This is recorded at intervals for each wheel when the vehicle is stationary by measuring the volume over the corresponding piston travel with knowledge of the HZ pressure or THZ pressure. The process takes place with relatively little dynamics, so that the wheel cylinder pressure corresponds to the pressure in the HZ or THZ.

It is known that in highly dynamic processes in pressure control, there is a large pressure difference both in the pressure build-up and in the pressure reduction due to the 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 can only measure the wheel pressure statically, even with a pressure sensor at the output of the solenoid valve. A pressure model is used for dynamic measurement, the accuracy of which is limited. In addition, it is complex to install a pressure sensor for each wheel. However, in the system according to the invention with piston control, if the pressure-volume characteristic curve is known, the wheel cylinder pressure can be set precisely even with different dynamics.

In the case of simultaneous or partially simultaneous pressure build-up and pressure reduction, two or several 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. The wheel cylinder pressure is constantly taken into account using an additional pressure model. 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, the pressure is controlled using the piston travel, which was previously calculated from the pressure-volume characteristics. The solenoid valve of the last wheel brake can then also be closed.

The pressure model for piston control is very important for the brake 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 it 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 controlled variable of the pressure regulator in the multiplex process. In addition, the wheel cylinder pressures from the pressure model are used in higher-level controller structures (e.g. ABS / ESP, driver assistance functions such as ACC, etc.).

Since it is advantageous that the HZ or THZ pressure is initially brought close to the output pressure of the wheel cylinder to be controlled before the pressure change in the wheel cylinder, it is necessary that the wheel cylinder pressures are continuously calculated and stored. This task is also carried out by the pressure model.

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.

The pressure model uses the HZ or THZ pressure as an input signal. The pressure model is then used to calculate 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 the temperature (e.g. ambient temperature or a separate temperature sensor on a solenoid valve). If changes in the transition behavior occur, it is also possible to adjust the model parameters via an adaptation.

The process of simultaneous or partially simultaneous pressure change is relatively rare in normal ABS/ESP braking and occurs more in borderline cases such as asymmetrical or inhomogeneous road surfaces. It is therefore very important 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 pressure change rate can be variably adjusted, meaning that in extreme cases the piston can be controlled with maximum dynamics. Due to the variability, it is normally possible to reduce the piston speed and only use 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 is in turn dependent on the pressure difference to be controlled and the absolute pressure in the wheel cylinder.

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 therefore enables very short switching times.

To check and, if necessary, correct the wheel cylinder pressures calculated by the pressure model during a longer control intervention, the wheel cylinder pressure is compared with the HZ or THZ pressure at longer intervals. With the piston at a standstill and the solenoid valve open, a static adjustment is therefore carried out after a certain pressure settling time, which runs automatically without additional adaptation rules or extensions in the pressure model due to the structure of the pressure model. The check can also be carried out if the slip or wheel acceleration specified by the controller is not achieved. It is also possible to work without simultaneous or partially simultaneous pressure change, only on the basis of the pressure-volume characteristic curve and corresponding piston adjustment proportional to the controller requirement.

In the WO 2006/111393 A1 A braking system is described in which a travel simulator is used. The braking system according to the invention can have a travel simulator. However, for cost reasons, a travel simulator can also be dispensed with. In this case, a reaction to the brake pedal can take place via the electric drive and a mechanical connection between the brake pedal and the brake booster.

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

• 1: Grundaufbau der Aktuatorik zur Drucksteuerung;
• 2: Regelzyklus zur Drucksteuerung von einem Radzylinder;
• 3: Regelzyklus zur teilsimultanen Drucksteuerung von zwei Radzylindern;
• 4: Blockschaltbild eines Druckmodells.

They show:

• 1 : Basic structure of the actuators for pressure control;
• 2 : Control cycle for pressure control of a wheel cylinder;
• 3 : Control cycle for partially simultaneous pressure control of two wheel cylinders;
• 4 : Block diagram of a pressure model.

The 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 reset 12 and rotary encoder 13 for determining the position of the piston and detecting the rotor position or the piston travel.

If the piston receives the command to build up a certain pressure, the corresponding piston movement is carried out via the previously recorded pressure-volume characteristic curve stored in a characteristic map via position sensor 13 and pressure sensor 19 in the push rod circuit. If the pressure is then briefly constant, which is usually the case when braking, the correlation is compared based on new measurement data with the stored characteristic map data. If there is a deviation, the pressure-volume characteristic curve is recorded again individually for each wheel brake when the vehicle is at a standstill 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.

The pressure generated in the HZ or THZ passes via lines 15, 16 from the push rod piston and floating piston via the 2/2 solenoid valves 17a-d to the wheel cylinders 18a and 18d. The dimensioning of the flow resistance RL from the HZ to the solenoid valve 17i (with i = a,b,c,d) in lines 15 and 16 and then the flow resistance RV in the solenoid valve and 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 solenoid valves 17a-d with lines 15 and 16 and pressure sensor 19 are preferably integrated in a block; for this purpose the HZ or THZ can also be included.

If the control command for pressure reduction is given, the pressure is again set via the piston travel and then compared with the pressure measurement. Pressure build-up and reduction correspond to the usual BKV function. For this, an addition of components such as pedal, pedal travel sensor, travel simulator, etc. is necessary, as described in the above-mentioned WO 2006/111393 A1 The braking system of the WO 2006/111393 A1 However, it focuses on pressure control and modulation and does not require all of the above components.

If a pressure modulation is now carried out, e.g. for the ABS/ESP function, the MUX function is switched on. 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 the lines 15 and 16 and wheel cylinders 18b and 18d via a motor 10, the solenoid valves 17b to 17d are closed.

If the pressure reduction p _ab specified by the controller is achieved via the corresponding piston travel, the solenoid valve 17a is closed and the piston of the HZ or THZ moves to the target position specified by the controller. If, for example, a pressure build-up p _auf is then to take place in the wheel cylinder 18d, 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 is to take place 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 the pressure modulation take place extremely quickly with special switching conditions for the motor and solenoid valves. These are in the 2 and the 3 described.

In 2 a control cycle in the MUX method for a wheel brake is shown. In the upper xy diagram, p _R represents the wheel cylinder pressure and p _HZ the HZ or THZ pressure is shown schematically. Below this, the position of the push rod piston is plotted over time with the y-axis label s _K. The control signals U _MV for the solenoid valves are shown in the lower diagram.

In the temporal progression shown, a pressure reduction p _down occurs first. At time 6, a pressure build-up p _up for a wheel brake follows.

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 switchover time T _Um is shown, 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 set that almost corresponds to the pressure in the wheel cylinder, so that a pressure balance between the HZ and the wheel cylinder is almost achieved. The pressure-volume characteristic curve of the wheel cylinder is now used at 2 to achieve the necessary Volume is calculated that must be taken from the wheel cylinder in order to bring the pressure to a certain or required level. 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 can also be seen in the wheel pressure. Then between 3 and 4 the HZ piston moves to the calculated target position that it reached at the end of phase 4. If the target pressure increases again in the meantime, e.g. due to a specification from higher-level controllers, the motor would abort the process earlier. Once the target position has been reached, a settling time T _e is observed in phases 4 to 5 to calm the flow conditions in the hydraulics; during this time the HZ piston stands still before the switching valve 17i closes. Here the switching valves are pre-controlled, with the signal being brought forward 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 now closed again. The actuator now has time to operate other wheels. If it does not have to change the pressure in the HZ in the meantime, the switching time T _UM = 0 for a subsequent pressure build-up request at time 6. The subsequent pressure build-up takes place in a similar way to the pressure reduction described above.

The 3 differs from the 2 by additionally showing a second wheel cylinder pressure P _R2 in the top diagram and adding another diagram for the control signal of the second switching valve at the very bottom.

Basically, the figure describes the control sequence for a partially simultaneous pressure modulation p _down and p _up in the wheel cylinders. Up to time 1, the HZ piston is stationary and the solenoid valves are all closed. Then a pressure reduction request comes for the first wheel cylinder. At the same time or at a different time, a second pressure reduction request for a second wheel cylinder occurs. This now makes simultaneous or partially simultaneous pressure reduction possible on two wheels. The same naturally applies to three or four wheels. From time 1 to time 2, the switchover time T _Um is shown, in which, as described above, an attempt is made to approach 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, a pressure equalization between the HZ and the first wheel cylinder is almost achieved. The pressure-volume characteristic curve is now used to calculate the volume required to be taken from the first wheel cylinder to bring the pressure to a certain level during or at the end of phases 1-2. If the reduction desired 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 only take place at time 3. The HZ piston now moves at time 2 and the solenoid valve associated with the first wheel brake is simultaneously controlled and opened. The time range 2-3 represents the entire dead time until an effect of the pressure reduction can also be seen in the wheel cylinder pressure. As soon as it can be foreseen from the pressure model and the volume flow calculated with it (time 3) that the HZ pressure p _HZ will fall below the wheel pressure P _R2 within the known valve opening time, the second solenoid valve MV ₂ is controlled and opened via U _Mv2 . Shortly before time 4, it can be foreseen 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. The solenoid valve MV ₁ is therefore now closed. At time 4, valve MV ₁ is then also closed and the volume flow in valve MV ₁ is stopped. This results in pressure oscillations in the wheel cylinder pressure at b. PWM control of the solenoid valves can reduce the pressure oscillations in the wheel pressure. By chance, the HZ piston also reaches its target position at time 4, which - as described above - was previously calculated. The volume flow in solenoid valve MV ₂ can now calm down in the settling time T _e before this valve MV ₂ is also closed at time 5. Due to the settling time, hardly any pressure oscillations occur in 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 simultaneous pressure build-up that when the MV _i solenoid valve is open, the HZ pressure is always above the lowest wheel cylinder pressure in order to maintain a positive pressure gradient and avoid pressure equalization between two or more wheels. In general, with simultaneous or partially simultaneous reduction, PWM control of the solenoid valves would have the advantage that the pressure gradients could then also be influenced online with simultaneous or partially simultaneous pressure reductions or pressure build-ups.

The 4 shows a possible pressure model for calculating the individual wheel cylinder pressures. The pressure model uses the HZ pressure p _HZ (t) as an input signal. A model is used to calculate the various wheel cylinder pressures as a function of the temporal 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 curve 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 depending 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 transition behavior occur, it is also possible to adjust the parameters of the model via adaptation. The pressure model is constantly taken into account and is extremely important for the accuracy of the pressure setting, particularly in connection with pressure modulation in ESP/ABS or other higher-level controllers.

1. 1. Bremssystem mit einem Bremskraftverstärker, dessen Kolben-Zylindersystem (14, HZ, THZ) von einem Elektromotor angetriebenen durch Übertragungsmittel mechanisch oder hydraulisch ist, wobei mindestens ein Arbeitsraum des Kolben-Zylindersystems (14, HZ, THZ) über hydraulische Leitungen mit mindestens zwei Radbremsen in Verbindung ist, wobei jeweils einer Radbremse ein 2/2-Wege-Schaltventil (17a, 17b, 17c, 17d) zugeordnet ist und die hydraulischen Verbindungsleitungen zwischen den Radbremsen (18a, 18b, 18c, 18d) und dem Kolben-Zylindersystem (14, HZ) wahlweise getrennt oder gemeinsam mittels der 2/2-Wege-Schaltventile (17a, 17b, 17c, 17d) verschließbar ist bzw. sind, so dass in den Radbremsen (18a, 18b, 18c, 18d) nacheinander im Sinne eines Multiplexverfahrens und/oder gleichzeitig ein Druck einregelbar ist, wobei der Elektromotor und die Schalt-ventile (17a, 17b, 17c, 17d) von einer Regeleinrichtung angesteuert werden, und dass die hydraulische Verbindungsleitung vom Arbeitsraum des Kolben-Zylindersystems (14, HZ, THZ) zum jeweiligen Magnetventil einen Strömungswiderstand RLi und jedes Schaltventil zusammen mit der hydraulischen Leitung zum Radzylinder (17a, 17b, 17c, 17d) einen Strömungswiderstand RVi aufweist, dadurch gekennzeichnet, dass die Strömungswiderstände RLi und RVi klein sind, derart, dass die HZ-Kolbengeschwindigkeit den Druckabbaugradienten und Druckaufbaugradienten in jeder Radbremse (18a, 18b, 18c, 18d) bestimmt, wobei der Strömungswiderstand RLi kleiner als der Strömungswiderstand RVi ist, und dass die Regelein-richtung die Kolbenbewegung und Kolbengeschwindigkeit beim Druckaufbau und Druckabbau in Abhängigkeit der Druck-Volumen-Kennlinie der Radbremsen (18a, 18b, 18c, 18d) einregelt bzw. einsteuert.
2. 2. Bremssystem nach Ausführungsform 1, dadurch gekennzeichnet, dass der Strömungswiderstand RVi größer ist als der Strömungswiderstand RLi, insbesondere RVi mindestens um den Faktor 1,3 bis 2,5, vorzugsweise, 1,5 bis 2 mal größer als der Strömungswiderstand RLi ist.
3. 3. Bremssystem nach Ausführungsform 1 oder 2, dadurch gekennzeichnet, dass der maximal erreichbare Druckgradient des Kolben-Zylindersystems (14, HZ) bei geschlossenen Schaltventilen (17a, 17b, 17c, 17d) um mindestens den Faktor 2 bis 4, insbesondere den Faktor 3, größer ist als der maximale erreichbaren Druckgradient in den Radbremsen (18a, 18b, 18c, 18d) bei mindestens einem geöffnetem Schaltventil (17a, 17b, 17c, 17d).
4. 4. Bremssystem nach einer der Ausführungsformen 1 bis 3, dadurch gekennzeichnet, dass die Strömungswiderstände RLi und RVi, insbesondere die Summe der Strömungswiderstände RLi und RVi jeder Radbremse (18a, 18b, 18c, 18d), so ausgelegt sind bzw. ist, dass bei maximaler Dynamik des Kolben-Zylindersystems (HZ) und dessen Antrieb sowie mindestens einem, insbesondere zwei, geöffneten Schaltventilen (17a, 17b, 17c, 17d) aufgrund der gleichzeitigen Volumenaufnahme oder Volumenabgabe der Radbremsen (18a, 18b, 18c, 18d) in der Zeit, in der die Schaltventile (17a, 17b, 17c, 17d) geöffnet sind, kein Druckausgleich zwischen den Radbremsen (18a, 18b, 18c, 18d) erfolgt.
5. 5. Bremssystem nach einer der Ausführungsformen 1 bis 4, dadurch gekennzeichnet, dass zur Verringerung des Strömungsquerschnitts eines Schaltventils die Regelein-richtung dieses Schaltventil mittels einer Pulsweitenmodulation PWM ansteuert.
6. 6. Bremssystem nach einer der vorhergehenden Ausführungsformen, dadurch gekennzeichnet, dass die hydraulische Verbindungsleitung vom Arbeitsraum des Kolbenzylindersystems (14, HZ) zum jeweiligen Magnetventil (17a, 17b, 17c, 17d) kürzer als 30 cm, insbesondere kürzer als 20cm, besonders bevorzugt kürzer als 5 cm, ist.
7. 7. Bremssystem nach einer der vorhergehenden Ausführungsformen, dadurch gekennzeichnet, dass die Einstellung des Drucks in jeder Radbremse auf der Basis der Druck-Volumen-Kennlinie des jeweiligen Rades über eine Wegsteuerung des Kolbens des Kolben-Zylindersystems (14, HZ) erfolgt.
8. 8. Bremssystem nach einer der vorhergehenden Ausführungsformen, dadurch gekennzeichnet, dass ein übergeordneter Regler, insbesondere der ABS- und ESP-Regler, den Solldruck für das Kolben-Zylindersystem (14, HZ) vorgibt.
9. 9. Bremssystem nach einer der vorhergehenden Ausführungsformen, dadurch gekennzeichnet, dass der notwendige und zu berechnende Druckgradient im Kolben-Zylindersystem eine Funktion des Betrags der geforderten Druckänderung in einer Radbremse ist.
10. 10. Bremssystem nach einer der vorhergehenden Ausführungsformen, dadurch gekennzeichnet, dass die Regeleinrichtung, insbesondere während des Regelvorgangs, laufend die Druckniveaus der Radbremsen anhand eines Druckmodells berechnet.
11. 11. Bremssystem nach Ausführungsform 10, dadurch gekennzeichnet, dass eine Eingangsvariable des Druckmodells der Ist-Druck im Kolben-Zylindersystem (14, HZ) ist.
12. 12. Bremssystem nach Ausführungsform 10 oder 11, dadurch gekennzeichnet, dass das Druckmodell als Modellparameter einen Ersatzströmungswiderstand verwendet, der dem hydraulischen Widerstand des Pfads vom Kolben-Zylindersystem (14, HZ) über das Schaltventil (17a, 17b, 17c, 17d) bis zum Radzylinder der Radbremse entspricht.
13. 13. Bremssystem nach Ausführungsform 12, dadurch gekennzeichnet, dass das Druckmodell im Gesamtdruckverlust innerhalb des hydraulischen Pfades vom Kolben-Zylindersystem (14, HZ) über das Schaltventil (17a, 17b, 17c, 17d) bis zum Radzylinder der Radbremse eine Gewichtung der Strömungsverhältnisse laminar und turbulent berücksichtigt.
14. 14. Bremssystem nach einer der Ausführungsformen 10 bis 13, dadurch gekennzeichnet, dass das Druckmodell den Schließzeitpunkt jedes Schaltventils (17a, 17b, 17c, 17d) in Abhängigkeit vom berechneten Ist-Druck sowie dem aktuellen Druckgradienten berechnet.
15. 15. Bremssystem nach einer der Ausführungsformen 10 bis 14, dadurch gekennzeichnet, dass das Druckmodell als Modellparameter eine hydraulische Ersatzinduktivität berücksichtigt, die für die Masse und/oder Trägheit der Bremsflüssigkeit steht.
16. 16. Bremssystem nach einer der Ausführungsformen 10 bis 15, dadurch gekennzeichnet, dass das Druckmodell als Modellparameter die Druck-Volumen-Kennlinie jeder einzelnen Radbremse beinhaltet, welche die Kapazität bzw. Volumenaufnahme der Radbremse darstellt.
17. 17. Bremssystem nach einer der Ausführungsformen 10 bis 16, dadurch gekennzeichnet, dass die Modellparameter des Druckmodells eine Funktion der Temperatur, insbesondere der Umgebungstemperatur der Schaltventile (17a, 17b, 17c, 17d), sind und/oder über die Temperatur adaptiert oder angepasst werden.
18. 18. Bremssystem nach einer der Ausführungsformen 10 bis 17, dadurch gekennzeichnet, dass die Regeleinrichtung das Druckmodell korrigiert bzw. anpasst, wenn der vom Regler vorgegebene Schlupf und/oder die Radbeschleunigung nicht erreicht wird bzw. werden.
19. 19. Bremssystem nach einer der Ausführungsformen 10 bis 18, dadurch gekennzeichnet, dass die Regeleinrichtung das Druckmodell überprüft und/oder dessen Werte und Parameter abgleicht, in dem die Regeleinrichtung den Ist-Druck im Kolben-Zylindersystem (14, HZ) bei geöffnetem Schaltventil (17a, 17b, 17c, 17d) nach Abwarten einer kurzen Druckeinschwingzeit ermittelt.
20. 20. Bremssystem nach Ausführungsform 19, dadurch gekennzeichnet, dass die Regeleinrichtung die Überprüfung und den Abgleich des Druckmodells mehrmals hintereinander oder in kürzeren Zeitintervallen vornimmt, wenn das Druckmodell von den tatsächlich gemessenen Werten abweicht.
21. 21. Bremssystem nach einer der Ausführungsformen 10 bis 20, dadurch gekennzeichnet, dass die Zeitdifferenz (TUm) zwischen Beginn der Kolbenbewegung und dem Umschalten eine Schaltventils variabel ist, derart dass sie über die Kolbengeschwindigkeit steuerbar ist.
22. 22. Bremssystem nach einer der vorhergehenden Ausführungsformen, dadurch gekennzeichnet, dass der Kolben des Kolben-Zylindersystems (14, HZ, THZ) und die Leitungen vom Kolben-Zylindersystems (14, HZ, THZ) zu den Schalt-ventilen (17a, 17b, 17c, 17d) steif ausgelegt sind.
23. 23. Verfahren zur Einstellung eines Bremsdrucks in mindestens einer Radbremse unter Verwendung eines Bremssystems nach einer der vorhergehenden Ausführungsformen, dadurch gekennzeichnet, dass der Druck in mindestens zwei Radbremsen (18a, 18b, 18c, 18d) nacheinander, gleichzeitig (simultan) oder zeitlich überlappend (teil-simultan) aufgebaut oder abgebaut wird.
24. 24. Verfahren zur Einstellung eines Bremsdrucks in mindestens einer Radbremse unter Verwendung eines Bremssystems nach einem der Ausführungsformen 1 bis 22 oder dem Verfahren nach Ausführungsform 23, dadurch gekennzeichnet, dass beim gleichzeitigen Druckaufbau oder Druckabbau in mindestens zwei Radbremsen (18a, 18b, 18c, 18d) die Druckdifferenz zwischen dem Arbeitsraum des Kolben-Zylindersystems (14, THZ) und den jeweiligen Radbremsen (18a, 18b, 18c, 18d) so groß gewählt wird, dass kein Druckausgleich zwischen den jeweiligen Radbremsen (18a, 18b, 18c, 18d) erfolgt.
25. 25. Verfahren nach Ausführungsform 24, dadurch gekennzeichnet, dass der Kolben des Kolben-Zylindersystems (14, THZ) zur Aufrechterhaltung der Druckdifferenz bei geöffneten Schaltventilen (17a, 17b, 17c, 17d) von der Regeleinrichtung nachgestellt wird.
26. 26. Verfahren nach Ausführungsform 25, dadurch gekennzeichnet, dass die Regeleinrichtung die notwendige Druckdifferenz und somit die benötigte Volumenänderung des Arbeitsraums des Kolbenzylindersystems (14, THZ) berechnet, bei der kein Volumenausgleich zwischen den Radbremsen (18a, 18b, 18c, 18d) erfolgt, deren zugeordnetes Schaltventil (17a, 17b, 17c, 17d) geöffnet ist bzw. deren Öffnung unmittelbar bevorsteht, und dass zur Einstellung des benötigten Differenzdrucks die Regeleinrichtung den Kolben des Kolben-Zylindersystems (14, THZ) zur Druckeinregelung entsprechend nachstellt bzw. ansteuert.
27. 27. Verfahren zur Einstellung eines Bremsdrucks in mindestens einer Radbremse unter Verwendung eines Bremssystems nach einer der Ausführungsformen 1 bis 22 oder einem Verfahren nach einer der Ausführungsformen 23 bis 26, dadurch gekennzeichnet, dass der Druckaufbau und/oder Druckabbau bei mindestens zwei Radbremsen simultan und/oder zeitlich überlappend erfolgt (teilsimultan), wobei die Ausgangs-niveaus der jeweiligen Radbremsen voneinander verschieden sind.
28. 28. Verfahren zur Einstellung eines Bremsdrucks in mindestens einer Radbremse unter Verwendung eines Bremssystems nach einer der Ausführungsformen 1 bis 22 oder einem Verfahren nach einer der Ausführungsformen 23 bis 27, dadurch gekennzeichnet, dass vor Fahrtbeginn die Druck-Volumen-Kennlinie jeder Radbremse durch statischen Vergleich des Kolbenweges mit dem Druck im Arbeitsraum des Kolben-Zylindersystems (14, HZ) abgeglichen wird.
29. 29. Verfahren zur Einstellung eines Bremsdrucks in mindestens einer Radbremse unter Verwendung eines Bremssystems nach einer der Ausführungsformen 1 bis 22 oder einem Verfahren nach einer der Ausführungsformen 23 bis 28, dadurch gekennzeichnet, dass der Druck im Arbeitsraum des Kolben-Zylindersystems (14, HZ) vor dem Beginn des Druckabbaus auf den in der jeweiligen Radbremse (18a, 18b, 18c, 18d) befindlichen Druck oder einen niedrigeren Druck eingestellt oder eingeregelt wird, und danach das zugehörige Schalt-ventil (17a, 17b, 17c, 17d) von der Regeleinrichtung geöffnet wird.
30. 30. Verfahren zur Einstellung eines Bremsdrucks in mindestens einer Radbremse unter Verwendung eines Bremssystems nach einer der Ausführungsformen 1 bis 22 oder einem Verfahren nach einer der Ausführungsformen 23 bis 29, dadurch gekennzeichnet, dass der Druck im Arbeitsraum des Kolben-Zylindersystems (14, HZ) vor dem Beginn des Druckaufbaus auf den in der jeweiligen Radbremse (18a, 18b, 18c, 18d) befindlichen Druck oder einen höheren Druck eingestellt oder eingeregelt wird, und danach das zugehörige Schaltventil (17a, 17b, 17c, 17d) von der Regeleinrichtung geöffnet wird.

In addition, the following embodiments are disclosed:

1. 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 together by means of the 2/2-way switching valves (17a, 17b, 17c, 17d), so that in the wheel brakes (18a, 18b, 18c, 18d) one after the other 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 controls the piston movement and piston speed during pressure build-up and pressure reduction in Depending on the pressure-volume characteristic curve of the wheel brakes (18a, 18b, 18c, 18d) it regulates or controls.
2. 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. 3. Braking 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 switching valve (17a, 17b, 17c, 17d) open.
4. 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), no pressure equalization takes place between the wheel brakes (18a, 18b, 18c, 18d) due to the simultaneous volume intake 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.
5. 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 a pulse width modulation PWM.
6. 6. Braking system according to one of the preceding embodiments, characterized in that the hydraulic connecting line from the working space 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. 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 path control of the piston of the piston-cylinder system (14, HZ).
8. 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. 9. Braking 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. 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 on the basis of a pressure model.
11. 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. 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. 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. 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. 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. 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. 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. 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. 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. 20. Braking system according to embodiment 19, characterized in 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.
21. 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. 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. 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 one after the other, simultaneously or in a temporally overlapping manner (partially simultaneous).
24. 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 occurs between the respective wheel brakes (18a, 18b, 18c, 18d).
25. 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. 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 space 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. 27. 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 in a temporally overlapping manner (partially simultaneously), wherein the initial levels of the respective wheel brakes are different from one another.
28. 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. 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 then the associated switching valve (17a, 17b, 17c, 17d) is opened by the control device.
30. 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 set 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 then 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

pHZ

master cylinder pressure

pR

wheel cylinder pressure

pauf

pressure build-up

pab

pressure reduction

p*ab

pressure change rate during pressure reduction

p*an

pressure change rate during pressure build-up

sk

HZ piston travel

s*k

HZ piston speed

TE

settling time before valve closing

TUm

Switching time from the beginning of piston movement to opening of the valve

TMUX

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

MVi

solenoid valve/switching valve

UMV

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

rotary 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 solenoid valves as switching valves

18a-18d

wheel cylinder

19

pressure sensor

Claims (36)

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 respective solenoid valve (17a, 17b, 17c, 17d), wherein the method comprises 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 ) is built up at least in the selected wheel brake (18a, 18b, 18c, 18d) such 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) decreases. 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 respective solenoid valve (17a, 17b, 17c, 17d), wherein the method comprises 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 ) is built up at least in the selected wheel brake (18a, 18b, 18c, 18d) such 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) decreases, 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. procedure according to claim 1 or 2 , characterized in that the second time interval follows the first time interval. Method according to one of the preceding claims, characterized in 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. procedure according to claim 4 , characterized in 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 lower than the first pressure gradient and/or higher than the second pressure gradient. Method according to one of the Claims 2 until 5 , characterized in 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) is 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) in order to build up 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 with an overlap in time. Method according to one of the preceding claims, characterized in 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 in each case, wherein the adjustment of the piston takes place in such a way that the pressures (p _R1 , p _R2 ) in the selected wheel brakes (18a, 18b, 18c, 18d) are built up simultaneously or in an overlapping manner. 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 respective solenoid valve (17a, 17b, 17c, 17d), wherein the method comprises 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 ) is reduced at least in the selected wheel brake (18a, 18b, 18c, 18d) such 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) decreases. 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 respective solenoid valve (17a, 17b, 17c, 17d), wherein the method comprises 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 ) is reduced at least in the selected wheel brake (18a, 18b, 18c, 18d) such 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) decreases, 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. procedure according to claim 8 or 9 , characterized in that the second time interval follows the first time interval. Method according to one of the Claims 8 until 10 , characterized in 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 lower than the second pressure gradient. procedure according to claim 11 , characterized in 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. Method according to one of the Claims 9 until 12 , characterized in 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) is 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) in order to adjust 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 with an overlap in time. Method according to one of the Claims 8 until 13 , characterized in that the pressure reduction is carried out in at least two selected wheel brakes (18a, 18b, 18c, 18d) when a pressure change is required, wherein the adjustment of the piston takes place 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. Method according to one of the preceding claims, characterized in 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. Method according to one of the preceding claims, characterized in that a higher-level controller specifies a target pressure for the piston-cylinder system (14, HZ, THZ) and a switching state (OPEN, CLOSED, pulse width modulation PWM) of the solenoid valves (17a, 17b, 17c, 17d). Method according to one of the preceding claims, characterized in 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). Method according to one of the preceding claims, characterized in that the necessary pressure gradient to be calculated 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). Method according to one of the preceding claims, characterized in that in the pressure build-up phase/pressure reduction phase an actual pressure (p _R1 , p _R2 ) in at least the selected wheel brake (18a, 18b, 18c, 18d) is calculated at least once using a pressure model. procedure according to claim 16 , characterized in 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. procedure according to claim 19 and 20 , characterized in 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). procedure according to claim 21 , characterized in that a settling time (T _e ) is provided for calming the flow conditions between the piston reaching the desired position and the closing of the solenoid valve (17a, 17b, 17c, 17d). Method according to one of the preceding claims, characterized in 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. Method according to one of the Claims 19 until 23 , characterized in that an input variable of the pressure model is an actual pressure (p _HZ ) in the piston-cylinder system (14, HZ, THZ). Method according to one of the Claims 19 until 24 , characterized in 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). Method according to one of the Claims 19 until 25 , characterized in that the pressure model calculates the closing time of each solenoid valve (17a, 17b, 17c, 17d) as a function of the calculated actual pressure (p _R1 , p _R2 ) and the current pressure gradient in the respective wheel brake (18a, 18b, 18c, 18d). Method according to one of the Claims 19 until 26 , characterized in that the pressure model takes into account as a model parameter a hydraulic equivalent inductance which represents the mass and/or inertia of a brake fluid. Method according to one of the Claims 19 until 27 , characterized in that the model parameters of the pressure model are a function of the temperature and/or are adapted or adjusted via the temperature. Method according to one of the Claims 19 until 28 , characterized in that a control device corrects or adjusts the pressure model, if the slip and/or wheel acceleration specified by the controller is not achieved. Method according to one of the Claims 24 until 29 , characterized in that a control device checks the pressure model and/or adjusts its values and parameters by determining 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. procedure according to claim 30 , characterized in 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. Method according to one of the Claims 25 until 31 , characterized in that before the start of the journey the pressure-volume characteristic curve of each wheel brake (18a, 18b, 18c, 18d) is adjusted by static comparison of the piston travel with the pressure (p _HZ ) in the working chamber of the piston-cylinder system (14, HZ, THZ). Method according to one of the Claims 25 until 32 , characterized by the following step: - recording the wheel-individual pressure-volume characteristic curve for each of the wheel brakes (18a, 18b, 18c, 18d) with the vehicle at a standstill. procedure according to claim 33 , characterized in 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) is detected with the piston at rest and the solenoid valve (17a, 17b, 17c, 17d) of the wheel brake (18a, 18b, 18c, 18d) open after a pressure settling time in order to correct or adapt the pressure model. Method for setting a brake pressure in at least one wheel brake (18a, 18b, 18c, 18d) using a piston-cylinder system (14, HZ, THZ), wherein the wheel brake (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 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). 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 at a standstill, the following steps being carried out: - adjusting a piston of the piston-cylinder system (14, HZ, THZ) such that along a piston path a pressure (p _HZ ) in the piston-cylinder system (14, HZ, THZ) corresponds to 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.

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