Method and apparatus for operating a hydraulic braking system, and braking system

Abstract

The invention relates to a method for operating a hydraulic braking system (1) for a motor vehicle, wherein the braking system (1) has at least one controllable pressure generator (11) to which at least one brake circuit (9, 10) is connected, which has a plurality of hydraulically actuated wheel brakes (2-5), wherein each wheel brake (2-5) is associated with a controllable inlet valve (16-19) and a controllable outlet valve (20-23), wherein the pressure generator (11) is controlled as a function of a braking request by specifying a setpoint hydraulic pressure (psoll), and wherein an actual hydraulic pressure (pist) is monitored in the brake circuit (9, 10). According to the invention, a braking dynamic is determined as a function of the monitored actual pressure (pist) and the setpoint pressure (psoll) is changed as a function of the determined braking dynamic.

Description

[0001] Description

[0002] title

[0003] Method and device for operating a hydraulic brake system, brake system

[0004] The present invention relates to a method for operating a hydraulic braking system for a motor vehicle, wherein the braking system has at least one controllable pressure generator to which at least one brake circuit is connected, which has a plurality of hydraulically actuated wheel brakes, wherein each wheel brake is assigned a controllable inlet valve and a controllable outlet valve, wherein the pressure generator is controlled as a function of a braking request by specifying a hydraulic target pressure, and wherein an actual hydraulic pressure in the brake circuit is monitored.

[0005] Furthermore, the invention relates to a device for operating the hydraulic brake system described above, which has a control unit.

[0006] Furthermore, the invention relates to a braking system as described above, with such a device.

[0007] State of the art

[0008] Methods of the type mentioned above are already known from the state of the art. With the increasing electrification of motor vehicles, concepts are also being developed that eliminate the need for a mechanical connection between input devices that can be operated by the driver and actuators that are intended to implement the driver's input request.

[0009] Corresponding developments exist both in steering systems as so-called steer-by-wire solutions, as well as in braking systems as so-called brake-by-wire solutions.

[0010] In braking systems that have an electromechanical brake booster, which is present in particular in addition to or as the only pressure generator in the braking system, the pressure applied by the driver using a brake pedal to a master brake cylinder is decoupled during operation from the actual braking force acting on the wheel brake, which is provided by the electrically controlled and operated pressure generator. Such a pressure generator is often implemented with a piston pump driven by an electric motor in order to ensure a high hydraulic pressure. Only through this decoupling can a boost function, i.e. an increase in the brake pressure above the brake pressure specified by the driver, be implemented. Furthermore, this decoupling can also be used to reduce the pre-pressure acting in the braking system if it becomes unnecessarily high, for example if the driver presses the brake pedal too hard.This can also be advantageous from an energy perspective. Furthermore, excessive hydraulic pressure can cause annoying noises and impair the durability of the braking system. Limiting the effective pressure to a maximum permissible pressure is therefore advantageous. This can be achieved advantageously through decoupling. In the case of ABS control, decoupling results in a further advantage: the so-called hydraulic preload, which describes the excess pressure maintained in the hydraulic system compared to the target brake pressure, can thus be adjusted independently of brake pedal actuation. Due to the preload, the pressure provided by the pressure generator is therefore higher than the pressure actually required at the wheel brakes, which allows for rapid pressure control processes. Typically, the preload is selected such that it exceeds each of the wheel brake locking pressures.

[0011] Disclosure of the invention

[0012] The method according to the invention with the features of claim 1 has the advantage that the preload pressure is reduced to a favorable level and increased only when necessary, thus increasing the durability of the braking system and simultaneously enabling safe and dynamic brake application and control. According to the invention, braking dynamics are determined based on the monitored actual pressure, and the target pressure is changed based on the determined braking dynamics. The braking dynamics are thus taken into account as the basis for determining the target pressure and thus also the preload. This target pressure differs from the target braking pressure to be exerted on one of the wheel brakes. In particular, the target pressure is always higher than the target braking pressure to ensure preload.The fact that the target pressure and thus the preload is adjusted depending on the braking dynamics results in the advantage that, on the one hand, a permanent load on the braking system caused by a high preload is avoided and, on the other hand, rapid brake pressure control is ensured during dynamic braking processes.

[0013] For this purpose, it is particularly preferred that the braking dynamics be determined based on the pressure pulses detected by the monitoring system. Thus, the actual pressure in the brake circuit or braking system detected by the pressure sensor is monitored for the occurrence of pressure pulses. An increasing number of pressure pulses in a given period of time indicates that the braking dynamics are increasing. A decreasing number of pressure pulses in the same period of time indicates that the braking dynamics are decreasing. Monitoring the pressure pulses thus ensures simple detection of the braking dynamics.

[0014] Preferably, the target pressure is increased with increasing or enhanced braking dynamics. This ensures that during dynamic braking, there is increased preload in the braking system, which ensures that the desired braking force can be generated when controlling the inlet and outlet valves of the wheel brakes.

[0015] Furthermore, it is preferably provided that the target pressure is reduced when braking dynamics decrease or are reduced. This ensures that the preload is reduced when dynamics are reduced, thus hydraulically relaxing the braking system or reducing its load, resulting in increased durability of the braking system.

[0016] According to a preferred embodiment of the invention, only pressure pulses that occur consecutively within a time period shorter than the maximum control time of a pressure regulator of the pressure generator are taken into account to determine the braking dynamics. This ensures that only pressure changes that cannot be compensated for by the pressure generator itself are taken into account.

[0017] In particular, to detect the actual pressure, the sensor signals of a pressure sensor assigned to the pressure generator are monitored, and the sensor signals are subjected to a low-pass filter with a time constant of 40 to 60 milliseconds, particularly 50 milliseconds. This ensures that irregularities that the pressure generator itself can compensate for are ignored when considering the braking dynamics.

[0018] To detect the braking dynamics, preferably only the magnitude of the deviation of the sensor signals from the filtered sensor signal is taken into account. In particular, the deviation of a current pressure signal from the filtered mean value of the pressure signal is determined, which corresponds to the amplitude of the pressure generator pressure. Because only the magnitude of the deviation is of interest, preferably only the absolute value of the deviation is calculated.

[0019] Furthermore, it is preferably provided that detected deviations are stored in a shift register and optionally weighted depending on their age. The shift register thus evaluates the braking dynamics for a specified period of time using weighted deviations. The preferential weighting depending on the age of the respective deviation results in advantageous braking dynamics monitoring. In particular, a shift register with, for example, 20 values ​​from the last 100 milliseconds is used. 100 milliseconds normally corresponds to the control speed of an ABS activation and deactivation control cycle. The weighting is carried out in such a way that the most recent value has the highest weighting and the oldest value the lowest weighting.

[0020] Preferably, the target pressure is specified as a function of a weighted average of the deviations stored in the shift register. Preferably, the weighted average is divided by the sum of all values, each multiplied by their weighting, so that the final result is a standardized value that has the same pressure unit as the pressure sensor signal.

[0021] Preferably, the target pressure is specified as a function of the hydraulic preload to a target brake pressure. To ensure that the absolute value of the preload does not reach a critical value, a minimum and a maximum value are preferably specified for the preload, by which the preload is limited to a permissible range. A range of 5 bar to 60 bar is preferably selected. Optionally, the raw value of the preload is also filtered to prevent it from containing unnecessarily high frequency components. For example, a slave pointer filter with an applicable filter width is used for this purpose. In addition, a change gradient of the target value is preferably limited to prevent an excessively rapid increase and / or decrease. In this way, a change gradient of the pressure target value is limited to a maximum gradient and a minimum gradient.

[0022] The device according to the invention with the features of claim 11 is characterized in that the control unit is specifically designed to carry out the method according to the invention. This results in the advantages already mentioned above.

[0023] The braking system according to the invention with the features of claim 11 is characterized by the device according to the invention. This results in the advantages already mentioned above. The method is preferably used only with active pressure control by an ABS system or with active ABS control. Further advantages and preferred features and combinations of features emerge in particular from the above description and from the claims. The invention will be explained in more detail below with reference to the drawings.

[0024] To show

[0025] Figure 1 shows a braking system for a motor vehicle,

[0026] Figure 2 is a diagram explaining a hydraulic preload of the brake system, and

[0027] Figure 3 is a flow chart explaining an advantageous method for operating the braking system.

[0028] Figure 1 shows a simplified representation of an advantageous braking system 1 for a motor vehicle (not shown in detail here), which has several wheels FL, RR, RL, and FR. Each wheel is assigned a wheel brake 2, 3, 4, 5 of the braking system, each of which is hydraulically actuated.

[0029] For this purpose, the braking system 1 has a master brake cylinder 6, which is designed as a tandem cylinder in this case and can be actuated by a driver of the motor vehicle by depressing the brake pedal 7. The master brake cylinder 6 is connected, on the one hand, to a hydraulic reservoir 8 and, on the other hand, to two brake circuits 9 and 10. Brake circuit 10 is assigned to or connected to wheel brakes 2 and 3, and brake circuit 9 is assigned to or connected to wheel brakes 4 and 5.

[0030] The braking system 1 further comprises a controllable pressure generator 11, which has an electric motor 12 operatively connected to a piston pump 13 for driving the latter. The piston pump 13 is connected to the reservoir 8 on the suction side and to the two brake circuits 9, 10 on the pressure side, optionally with the interposition of shut-off valves 14, 15.

[0031] Each of the wheel brakes 2 to 5 is further assigned an inlet valve 16, 17, 18, 19 and a controllable outlet valve 20, 21, 22, 23 in the respective brake circuit. The valves 16 to 23, like the pressure generator 11, can be controlled by a control unit 24, which, for example, in an ABS situation when one of the wheels is about to lock, controls one of the outlet valves 20 to 23 to reduce the pressure at the respective wheel brake 2 to 5 in order to prevent or interrupt locking. In addition, the control unit 24 detects a braking request from the driver by means of a pressure sensor 25 and / or travel sensor 26, which is assigned to the brake pedal 7, in dependence on which the control unit 24 controls the pressure generator 11 to generate a target pressure in the brake circuit 9, 10, which is then available to the wheel brakes 2 to 5 for carrying out a braking operation.By controlling the inlet and outlet valves 16 to 23, the control unit 24 sets a target brake pressure for a respective wheel brake 2 to 5 depending on the braking request.

[0032] The master brake cylinder 6 is further connected to a brake feeling simulator 27, so that the hydraulic volume displaced by the driver by actuating the brake pedal 7 is pushed into a hydraulic accumulator 28, but not into one of the brake circuits 9, 10. As a result, the brake circuits 9, 10 are hydraulically separated or separable from the master brake cylinder 6, in particular by means of additional separating valves 29, 30, so that the actual pressure pi St in the hydraulic system or in the brake circuits 9, 10 can be provided solely by the pressure generator 11.

[0033] Figure 2 shows a simplified diagram in which the brake target pressure PRB_S OII is plotted over time t. Furthermore, the target pressure Psoii specified to the pressure generator as a function of the brake target pressure and the actual pressure pi actually provided in the brake circuits 9, 10 by the pressure generator 11 st plotted over time t. The distance between the target pressure p S0 n and the target brake pressure defines the hydraulic preload Apv.

[0034] With reference to Figure 3, an advantageous method is described below that determines a particularly situation-optimal minimum value for the preload Apv. This optimally sets the preload in every situation, both to maximize the continuous load capacity of the braking system 1 and to ensure dynamic brake pressure control.

[0035] Various ABS control scenarios are known in which increased preload pressure is advantageous, for example, in very unstable ABS control on poor road surfaces with varying friction coefficients. A high preload helps to implement large pressure strokes—i.e., a large build-up and release of pressure in the respective wheel brake—in a short time. The preload serves as a buffer for the potential wheel pressure buildup. Other ABS operating scenarios, such as on consistently smooth ice, in which a significantly lower preload pressure is advantageous, are also known.

[0036] Basically, two disturbances must be taken into account, which are determined by the boundary conditions of the braking system and should be taken into account:

[0037] The first boundary condition to be mentioned is pressure peaks. Highly dynamic pressure peaks from the pressure generator or piston pump 13 disrupt the control of intake valves 16-19 because they respond hydraulically or mechanically very quickly to the pressure peaks, and in turn, the electronic control system cannot react as quickly as desired to compensate for the respective pressure peak in the force balance of the respective valve. The resulting error is a so-called crosstalk of the pressure increase from piston pump 13 to the respective wheel. This crosstalk leads to unstable ABS control because the possible locking of the wheel is triggered by the disturbance in the pressure supply and not by the road surface itself.To improve robustness against pressure peaks, a hydromechanically slower valve would be advantageous because the electronic control can then react sufficiently quickly before the pressure peaks hydraulically override the wheel.

[0038] The second boundary condition is volumetric tolerances.

[0039] Volumetric inaccuracies in brake system 1 lead to deviations in the pressure control accuracy. The hydraulic models of the piston pump and control system, as well as the wheel pressure control with inlet valves 16 to 19, are based on the assumption of hydraulic elasticity between volume and pressure in the respective wheel brake (the so-called Pv characteristic curve). This component property is subject to tolerances and is dependent on wear, condition, and temperature. Therefore, model-based pressure controls are often inaccurate and, in the case of larger deviations, inconvenient because the resulting differences must first be subsequently corrected by, for example, closed-loop controllers. A more effective means of counteracting volumetric tolerances, however, is a fast valve that hydromechanically assumes its pressure operating point very quickly, even if the volumetric pilot control is not accurate within larger tolerances.

[0040] Thus, the systematic decision regarding the selection of dynamic properties, particularly for intake valves 16 to 19, is subject to a conflict of objectives. The compromise between robustness against pressure peaks and robustness against volumetric tolerances can only be determined once through a hardware design. The method described below takes advantage of the fact that the intake valves have the property that their time constant, which describes the dynamic adjustment of a static operating point (differential pressure, volume flow at a given electrical current), depends on the preload Apv. Thus, the respective intake valve 16 to 19 becomes slower with increasing preload and faster with decreasing preload. Thus, by selecting an appropriate preload, the compromise between fast and slow valve behavior can be resolved.Therefore, the advantageous method provides that during more dynamic braking operations, the preload is increased in order to slow down the valve behavior and thus make it more robust against occurring pressure peaks, and during less dynamic braking operations or control operations, the preload is reduced in order to accelerate the valve behavior and allow pressure control errors due to volumetric tolerances to subside more quickly. Furthermore, the noise development, the load on the vehicle electrical system, and the component load of the braking system 1 as a whole benefit. For this purpose, the advantageous method provides that the braking dynamics are continuously monitored, in particular during ABS control. For this purpose, in a first step S1, a current hydraulic pressure pi is first measured. st monitored

[0041] In a subsequent step S2, the pressure signal from pressure sensor 31 is filtered. In particular, a 50-millisecond pT1 low-pass filter is applied to smooth the sensor signal from pressure sensor 31. The time constant of 50 milliseconds corresponds in particular to the time response of a piston pump controller. Of particular interest are pressure deviations that cannot be compensated for by the piston pump controller. A deviation Aö of the current pressure signal pi st of this filtered pressure signal corresponds to the amplitude of the piston pump pressure, with a frequency f higher than the piston pump controller can regulate. In this case, f = 1 / T1 = 1 / 50 ms = 20 Hz.

[0042] However, only the amount of deviation Aö is relevant in the following, which is why only the absolute value Ap a bs is calculated in step S3.

[0043] In a subsequent step S4, the deviation of the last 100 milliseconds from these determined deviations, which corresponds to, for example, 20 values, is recorded in a shift register. The duration of 100 milliseconds normally corresponds to the control speed of an ABS setup and deactivation control cycle. Using a weighted evaluation algorithm, the deviations in step S4 are weighted preferentially according to their age and summed in step S5. The most recent value has the highest weighting (specifically, a factor of 19), and the oldest value has the lowest weighting (specifically, a factor of 0).

[0044] Subsequently, the weighted mean value of all shift register elements is divided by the sum of all elements, each multiplied by its weight, so that the result is a normalization in step S6, which has the same pressure unit as the pressure signal of the sensor 31. The sum of the 20 elements in this embodiment is thus 2i[i=0..19] = 190. This sum obtained in step S6 is the raw value of the hydraulic preload Apv. In order to ensure that the resulting preload Apv does not have unnecessarily high frequency components, various filters are applied in the subsequent steps. Thus, in a step S7, a slave pointer filter with an applicable filter width is first applied. Optionally, the filter width, or a filter width of, in particular, 2 bar, is set.

[0045] Subsequently, in step S8, the signal's change gradient is preferably additionally limited to prevent an excessively rapid increase or decrease. The signal is thus limited to a maximum gradient and a minimum gradient. In particular, a decrease limit of -75 bar per second and a decrease limit of 25 bar per second are set.

[0046] In order to ensure that the absolute value of the preload Apv also remains in an advantageous range, the final result is preferably limited in a step S9 to a smallest reasonable minimum value, in this case for example 5 bar, and a largest reasonable maximum value, in this case for example 60 bar.

[0047] At the beginning of the process, the filter values ​​are advantageously initialized with a default or initial value of 30 bar to start the process. Finally, in step S10, the minimum preload is provided, and the setpoint for the pressure generator 11 is calculated based on this minimum preload.

Claims

Claims 1 . Method for operating a hydraulic braking system (1) for a motor vehicle, wherein the braking system (1) has at least one controllable pressure generator (11), to which at least one brake circuit (9, 10) is connected, which has a plurality of hydraulically actuated wheel brakes (2-5), wherein each wheel brake (2-5) is assigned a controllable inlet valve (16-19) and a controllable outlet valve (20-23), wherein the pressure generator (11) is controlled as a function of a braking request by specifying a hydraulic target pressure (p S0 n) is controlled, and an actual hydraulic pressure (pi St ) in the brake circuit (9, 10), characterized in that a braking dynamic is determined as a function of the monitored actual pressure (pist) and the target pressure (p S0 n) is changed.

2. Method according to claim 1, characterized in that braking dynamics are determined as a function of pressure pulses detected by the monitoring system.

3. Method according to one of the preceding claims, characterized in that the target pressure (p S0 n) is increased with increasing or increased braking dynamics.

4. Method according to one of the preceding claims, characterized in that the target pressure (p S0 n) is reduced when braking dynamics decrease or decrease.

5. Method according to claim 2, characterized in that, for determining the braking dynamics, only pressure pulses are taken into account which occur one after the other in a time which is shorter than a maximum control time of a pressure regulator of the pressure generator (11).

6. Method according to one of the preceding claims, characterized in that for detecting the actual pressure (pi St) the sensor signals of a pressure sensor (31) associated with the pressure generator (11) are monitored, and that the sensor signals are subjected to a low-pass filter which in particular has a time constant of 40-60 ms, in particular 50 ms.

7. Method according to one of the preceding claims, characterized in that for detecting the braking dynamics only the amount of deviation of the sensor signal from the filtered sensor signal is taken into account.

8. Method according to one of the preceding claims, characterized in that detected deviations are stored in a shift register and optionally weighted depending on their age.

9. Method according to one of the preceding claims, characterized in that the target pressure is specified as a function of a weighted mean value of the deviations in the shift register.

10. Method according to one of the preceding claims, characterized in that the setpoint value is specified as a function of a hydraulic preload (Apv) to a wheel brake device setpoint pressure.

11. Device for operating a braking system (1) for a motor vehicle, wherein the braking system (1) has at least one controllable pressure generator (11) to which at least one braking circuit (9, 10) is connected, which has a plurality of hydraulically actuated wheel brakes (2-5), wherein each wheel brake (2-5) is assigned a controllable inlet valve (16, 19) and a controllable outlet valve (20-23), characterized by a control unit (24) which is specially designed to carry out a method according to one of claims 1 to 10 when used as intended.

12. A braking system for a motor vehicle, wherein the braking system (1) comprises at least one controllable pressure generator (11) to which at least one braking circuit (9, 10) is connected, which has a plurality of hydraulically actuated wheel brakes (2-5), wherein each wheel brake (2-5) is assigned a controllable inlet valve (16-19) and a controllable outlet valve (20-23), characterized by a device according to claim 11.

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