Method for improving a pre-pressure estimation in a hydraulic circuit of a slip-controlled motor vehicle braking system and hydraulic circuit of a slip-controlled motor vehicle braking system

Abstract

Method for improving pressure estimation in a hydraulic circuit of a slip-controlled motor vehicle braking system, - in which an electrically clocked control of an electric motor of a motor-pump unit (43, 53) takes place, - wherein, according to the electrically clocked control, hydraulic volume is extracted from at least one low-pressure accumulator (46, 56) by the motor-pump unit (43, 53) and used to generate pressure on the outlet side of the motor-pump unit (43, 53), - wherein a pressure reduction in at least one wheel brake cylinder (49, 410, 59, 510) causes the filling of at least one low-pressure accumulator (46, 56) with hydraulic volume, - wherein, during an electrical cycle pause, a pressure-loaded coasting behavior of the motor-pump unit (43, 53) is detected in the form of a generator voltage applied to the electric motor, - where a pressure is estimated based on the generator voltage using a pressure estimation and - wherein a fill level of the at least one low-pressure storage tank (46, 56) is estimated by means of a low-pressure storage tank estimation, characterized in that - that by means of pressure reduction and / or by means of pumping hydraulic volume into the at least one low-pressure accumulator (46, 56) the fill level of the at least one low-pressure accumulator (46, 56) is regulated to a constant or variable value above a fill level threshold during slip control operation.

Description

[0001] The invention relates to a method for improving a pre-pressure estimation in a hydraulic line of a slip-controlled motor vehicle brake system according to the preamble of claim 1 and to a hydraulic line of a slip-controlled motor vehicle brake system according to the preamble of claim 10.

[0002] With increasing demands on comfort for electronic braking systems in motor vehicles, ever higher requirements are being placed on noise development, among other things. Valve control noises that are perceptible to the driver, for example in ABS control, pose a particular problem in this regard, as they are perceived as unpleasant and unacceptable by the driver. Furthermore, such noise-causing valve vibrations can even be associated with a longitudinal oscillation in the vehicle speed that is clearly perceptible to the driver. This can only be avoided by precisely controlling the analogue digital valves currently in use. However, in order to precisely control the analogue digital valves with a suitable current, it is necessary to know not only the valve's opening current characteristic but also the pressure difference across the valve as precisely as possible.However, in order to ensure cost-effective production of the vehicle brake system, pressure sensors are often not available, so that the knowledge of the current pre-pressure required for control must be determined using a pre-pressure model.

[0003] In this context, DE 10 2010 028 083 A1 discloses a method for the individual calibration of a pump motor in a pressure control system of a motor vehicle. After the pressure control system has been manufactured, the pump's output or speed is measured under defined pressure conditions as a function of the electrical control, and the measured data is stored in the pressure control system. Using, for example, a structure-borne sound sensor, the speed is determined during pump operation at a known motor current, and a conclusion is drawn about the pre-pressure based on the measured data stored during the individual calibration.

[0004] WO 2005 / 007475 A1 describes a method that allows the current pre-pressure to be estimated based on the deceleration behavior of a motor-pump unit. An electronic unit electrically controls the motor of the motor-pump unit as needed using pulse-width modulation, with a generator voltage generated by the motor being tapped during a shutdown phase. The generator voltage is fed to the electronic unit, which estimates the pre-pressure present in the braking system based on the generator voltage. A speed reduction, detected via the generator voltage and experienced by the motor-pump unit during a shutdown phase due to the pre-pressure load, is used as a benchmark for the pre-pressure.

[0005] DE 10 2007 032 588 A1 proposes a motor vehicle braking system in which the fill level of a low-pressure accumulator is calculated using a model. According to the model, the fill level of the low-pressure accumulator is determined from the volume discharged into the low-pressure accumulator by the outlet valves and the flow rate of the return pump that empties the low-pressure accumulator. The outlet volume of the outlet valves can be determined via the activation time and the opening width of the outlet valves, as well as the pressure difference. The activation time and the opening width of the outlet valves are known. The pressure difference can be calculated, for example, using a method such as that disclosed generically in DE 10 2010 028 083 A1 or WO 2005 / 007 475 A1. Thus, the fill level of the low-pressure accumulator can be determined using the described model.

[0006] DE 10 2005 041 556 A1 describes a method for determining a pre-pressure prevailing between a master brake cylinder and an inlet valve of a wheel brake cylinder of a motor vehicle brake system, which determines the pre-pressure taking into account the curve of a run-on voltage of a clocked motor of a pump which is used to return brake fluid from a low-pressure accumulator to the master brake cylinder, wherein several parameters of the voltage curve are measured and each used to determine a pre-pressure value and wherein an evaluation of the quality and / or reliability of the measured parameters, a filtering and / or processing of the parameters and / or the pre-pressure values ​​determined therefrom in the case of insufficient quality and / or reliability of the measured parameters, an averaging of the pre-pressure values ​​determined from the various parameters,where only pressure values ​​of similar magnitude are taken into account and a temporal averaging of the pre-pressure values ​​is carried out to dampen fluctuations.

[0007] Furthermore, DE 10 2009 025 344 A1 describes a braking system with means for controlling brake and drive slip and for controlling driving dynamics, the braking circuit of which has a chamber accommodating an additional volume from which brake fluid can be extracted for controlling driving dynamics.

[0008] DE 10 2006 004 745 A1 describes a hydraulic brake system with anti-lock control, with a variable-speed electric motor for driving a hydraulic pump for emptying one or more low-pressure accumulators, which is suitable for temporarily absorbing pressure medium released from wheel brakes during slip-controlled braking, comprising an electronic controller and electromagnetic valves for wheel-individual modulation of the brake pressure in the wheel brakes of the slip-controlled wheels as a function of the wheel rotational behavior, wherein the electronic controller has a module which is provided for changing the speed of the electric motor, wherein the target speed of the electric motor is set or regulated as a function of the filling level in the at least one low-pressure accumulator.

[0009] However, the pressure estimation methods known in the prior art have the disadvantage that an estimation of the inlet pressure based on the run-down behavior of the motor-pump unit is only possible if a low-pressure accumulator arranged on the suction side of the motor-pump unit provides sufficient fluid volume. However, if the low-pressure accumulator is empty, a pressure estimation based on the behavior of the motor-pump unit is not possible. For this reason, in slip control mode, the control of the motor-pump unit must always be selected and adjusted to prevent the low-pressure accumulator from running dry. Thus, the control timing of the motor-pump unit is continuously adjusted to the fill level of the low-pressure accumulator.However, since a change in the control timing makes it impossible to make a meaningful pre-pressure estimate over a certain period of time, no pre-pressure values ​​are available for this period. This results in inappropriate control of the analogue digital valves, which can lead to anything from noise to longitudinal oscillations in the vehicle speed.

[0010] The object of the invention is therefore to propose an improved pre-pressure estimation method which avoids emptying of a low-pressure accumulator during slip control operation of a motor vehicle brake system without changing a control timing of a motor-pump unit.

[0011] This object is achieved according to the invention by the method for improving a pre-pressure estimation in a hydraulic line of a slip-controlled motor vehicle brake system according to claim 1 and the hydraulic line of a slip-controlled motor vehicle brake system according to claim 10.

[0012] The invention relates to a method for improving pre-pressure estimation in a hydraulic line of a slip-controlled motor vehicle brake system, in which an electrically clocked control of an electric motor of a motor-pump unit takes place. Hydraulic volume is extracted from at least one low-pressure accumulator by the motor-pump unit in accordance with the electrically clocked control and pressurized on the outlet side of the motor-pump unit. By reducing the pressure in at least one wheel brake cylinder, the at least one low-pressure accumulator is filled with hydraulic volume, wherein, during an electrical clock pause, a pressure-loaded run-down behavior of the motor-pump unit is detected in the form of a generator voltage applied to the electric motor.Based on the generator voltage, a pressure is estimated by means of a pressure estimation, and a fill level of the at least one low-pressure accumulator is estimated by means of a low-pressure accumulator estimation. The method according to the invention is characterized in that, by means of pressure reduction and / or by means of pumping hydraulic volume into the at least one low-pressure accumulator, a fill level of the at least one low-pressure accumulator is regulated to a constant or variable value above a fill level threshold during slip control operation. According to the method of the invention, the control of the electric motor of the motor-pump unit is therefore not changed, but rather the fill level of the low-pressure accumulator is maintained above a fill level threshold, the reaching or falling below of which would necessitate an adjustment of the control of the electric motor of the motor-pump unit.The fill level of the low-pressure accumulator can be estimated, for example, based on opening times, opening widths, and by taking into account the orifice equations of an inlet valve and an outlet valve, as well as a differential pressure. This enables a significantly faster and more accurate estimation of the admission pressure by largely avoiding changes in the control of the electric motor. Such an improved estimation of the admission pressure also allows for an improved wheel pressure estimation using a wheel pressure model. This is particularly advantageous for braking systems that do not have a pressure sensor. However, the pressure estimation method improved according to the invention can also be used in braking systems with a pressure sensor (such as ESC systems). The invention can then be advantageously used to safeguard or check the pressure signal of a pressure sensor.The described pressure reduction or injection can be carried out by all wheel brake cylinders assigned to a low-pressure accumulator or only by one wheel brake cylinder assigned to a low-pressure accumulator.

[0013] It is preferably provided that, for the purpose of supplying pressure, both an inlet valve assigned to the at least one wheel brake cylinder and an outlet valve assigned to the at least one wheel brake cylinder are controlled and / or regulated into an open position and / or an intermediate position, in particular during simultaneous pressurisation by the motor-pump unit or a pressure transducer. In the context of the invention, the term pressure transducer refers to a master cylinder filled with hydraulic volume and a pressure control device, e.g. a brake pedal, which is operated by the driver and connected to the master cylinder. By simultaneously, at least partially opening the outlet valve and the inlet valve, a direct hydraulic short circuit can be established between the low-pressure accumulator and the pressure transducer, which allows hydraulic volume to be supplied directly from the pressure transducer into the low-pressure accumulator.Likewise, a direct hydraulic short circuit is created with the output side of the motor-pump unit, which alternatively or additionally allows the hydraulic volume delivered by the motor-pump unit to be directed into the low-pressure accumulator. The hydraulic volume delivered by the motor-pump unit into the low-pressure accumulator can be discharged from the pressure sensor, for example, when the switching valve is open. This allows the fill level of the low-pressure accumulator to be controlled or regulated.

[0014] It is particularly preferably provided that the inlet valve and the outlet valve are controlled and / or regulated in such a way that a pre-pressure in at least one wheel brake cylinder is kept constant during the supply, in particular by adapting the hydraulic resistances of the inlet valve and the outlet valve to the pre-pressure. Keeping the pre-pressure in the brake cylinder constant has the advantage that a currently implemented slip control is influenced as little as possible, although hydraulic volume is simultaneously supplied to the wheel brake cylinder through the inlet valve and discharged through the outlet valve. The control or regulation of the inlet valve and / or the outlet valve is adapted to the currently existing pre-pressure, since the pre-pressure during the supply and discharge is determined by the difference between the hydraulic resistances of the inlet valve and the outlet valve.According to the invention, the term "adjusting the hydraulic resistance" refers to both the adjustment of a valve's opening width and, additionally or alternatively, the adjustment of its opening time. For example, the hydraulic resistance of a digital valve can be adjusted by specifying an on-off modulation with a specific duty cycle.

[0015] It is expedient for the injection to take place at a time when the currently implemented slip control operation is essentially unaffected, in particular at a time when no pressure reduction and / or pressure buildup is taking place in at least one wheel brake cylinder. This results in the advantage that an adverse effect on the slip control operation can be particularly reliably avoided. Since the slip control operation is particularly easy to influence during a pressure reduction or pressure buildup, the injection can take place, for example, during a pressure maintenance phase.

[0016] In particular, it is expediently provided that the injection takes place at a time after a pre-pressure reduction phase, especially during a wheel monitoring phase. During the wheel monitoring phase, neither pressure buildup nor pressure reduction takes place. Instead, only the behavior of the vehicle wheel assigned to the wheel brake cylinder is monitored. This results in the advantage that the injection of hydraulic volume into the low-pressure accumulator during the wheel monitoring phase particularly reliably prevents any interference with the slip control operation.

[0017] Furthermore, it is advantageous that the fill level threshold is set or determined in such a way that no adjustment of the control of the motor-pump unit is necessary due to an insufficient fill level. This results in the advantage that, according to the concept of the invention, the low-pressure accumulator always stores sufficient hydraulic volume to make adjusting the control of the electric motor of the motor-pump unit unnecessary. Thus, a reliable pressure estimate can be made at any time, which in turn allows for precise control or regulation of the valves in the brake circuit at any time.

[0018] According to a further preferred embodiment of the invention, pressure reduction occurs by opening the outlet valve assigned to the at least one wheel brake cylinder, and pressure buildup occurs by opening the inlet valve assigned to the at least one wheel brake cylinder and simultaneously controlling pressure generated by the pressure sensor and / or by opening the inlet valve and simultaneously controlling pressure generated by the motor-pump unit. Opening the outlet valve is a simple and effective method for reducing pressure in the wheel brake cylinder. There is also the further advantage that opening the outlet valve also leads to filling the low-pressure accumulator with hydraulic fluid.Building up pressure by opening the inlet valve and applying pressure generated by the motor-pump unit or the pressure sensor is a simple and reliable way to increase pressure in the wheel brake cylinder. The outlet valve can be closed during the pressure buildup or at least controlled or regulated in such a way that it has a higher hydraulic resistance than the inlet valve.

[0019] Preferably, the low-pressure accumulator estimation takes into account the hydraulic resistances and the opening times of the inlet valve and the outlet valve, as well as the pressure, whereby the hydraulic resistances are determined by valve opening widths and / or valve opening times. This results in the advantage that all factors influencing the fill level of the low-pressure accumulator are taken into account for the low-pressure accumulator estimation. The low-pressure accumulator estimation thus provides a result that corresponds as closely as possible to the actual fill level.

[0020] It is expedient for the pre-pressure estimation and the low-pressure accumulator estimation to be carried out in an electronic control unit, wherein the electronic control unit controls and / or regulates the motor-pump unit, the exhaust valve and the inlet valve. The pre-pressure estimation and the low-pressure accumulator estimation are significantly influenced by the behavior of the motor-pump unit, the exhaust valve and the inlet valve. As the control and regulation of these devices is carried out in the same control unit which also carries out the pre-pressure estimation and the low-pressure accumulator estimation based on them, the output values ​​used for the estimates are directly available. The integration of the estimation methods and the control and regulation in a single electronic control unit also results in advantages compared to estimation and control or regulation.Control using separate control units results in cost savings due to reduced hardware requirements.

[0021] The invention further relates to a hydraulic line of a slip-controlled motor vehicle brake system, comprising at least one wheel brake cylinder which is hydraulically connected between an inlet valve and an outlet valve, at least one low-pressure accumulator which is hydraulically connected downstream of the outlet valve, a motor-pump unit which is hydraulically connected downstream of the at least one low-pressure accumulator and which, in accordance with an electrically clocked control, conveys hydraulic volume from the at least one low-pressure accumulator in the direction of the inlet valve and pressurizes the outlet side of the motor-pump unit, as well as an electronic control unit which controls and / or regulates the outlet valve and the inlet valve and electrically clocked controls an electric motor of the motor-pump unit. The electric motor runs down under pressure during an electrical clock pause and in the process generates a generator voltage.When the outlet valve opens, hydraulic volume flows into the at least one low-pressure accumulator and fills it. The electronic control unit reads the generator voltage and estimates a pre-pressure based on the generator voltage. It also estimates a fill level of the at least one low-pressure accumulator based on the hydraulic volume pumped from the at least one low-pressure accumulator and the hydraulic volume flowing into the at least one low-pressure accumulator. The hydraulic line according to the invention is characterized in that the electronic control unit controls and / or regulates the outlet valve and / or the inlet valve and / or the motor-pump unit such that the fill level of the at least one low-pressure accumulator remains at a constant or variable value above a fill level threshold during slip control operation.The hydraulic line according to the invention thus comprises all the necessary devices for implementing the method according to the invention and thus makes it possible to maintain the fill level of the low-pressure accumulator above a certain fill level threshold. This results in the advantages already described.

[0022] Preferably, the inlet valve and / or the outlet valve are analog digital valves. Analog digital valves allow not only setting an open position and a closed position, but also setting or adjusting intermediate positions. Thus, a specific opening width can be set using an analog digital valve, allowing comparatively precise control of the hydraulic resistance influenced by the opening width.

[0023] Further preferred embodiments emerge from the subclaims and the following description of an embodiment with reference to figures.

[0024] It shows Fig. 1 schematically shows four hydraulic lines according to the invention in two separate brake circuits of a motor vehicle and Fig. 2 a flow chart illustrating the individual steps of a possible embodiment of the method according to the invention.

[0025] Fig. Figure 1 shows a possible schematic layout of four hydraulic lines according to the invention in two separate brake circuits 4, 5 of a motor vehicle. Pressure sensor 1 is hydraulically coupled to brake circuits 4, 5 via hydraulic lines 2, 3. Each brake circuit 4, 5 comprises a switching valve 41, 51, a separating valve 42, 52, and two wheel brake cylinders 49, 410, 59, 510. Each of the wheel brake cylinders 49, 410, 59, 510 is assigned an inlet valve 45, 48, 55, 58 and an outlet valve 44, 47, 54, 57. Furthermore, each brake circuit 4, 5 comprises a low-pressure accumulator 46, 56 and a motor-pump unit 43, 53.A single hydraulic line according to the invention contained in brake circuits 4, 5 comprises pressure sensor 1, an inlet valve 45, 47, 55 or 58, a wheel brake cylinder 49, 410, 59 or 510, an outlet valve 44, 47, 54 or 57, a low-pressure accumulator 46 or 56, a motor-pump unit 43 or 53, and an electronic control unit 7. Some of the described devices are used, for example, for more than one of the hydraulic lines according to the invention. For example, pressure sensor 1 is used in all four hydraulic lines, while low-pressure accumulator 56 is used in two hydraulic lines. During slip control operation, the wheel rotation speed of the wheels assigned to wheel brake cylinders 49, 410, 59, and 510 is monitored and controlled by pressure reduction and pressure buildup in wheel brake cylinders 49, 410, 59, and 510. For the sake of clarity, only a single hydraulic brake line is considered below.However, the procedure can be applied without restriction to each of the four hydraulic brake lines shown.

[0026] The pre-pressure in wheel brake cylinder 59 is estimated by electronic control unit 7 from the run-down behavior of the electric motor of motor-pump unit 53. Electronic control unit 7 also estimates the fill level of low-pressure accumulator 56. The fill level decreases when motor-pump unit 53 discharges hydraulic volume from low-pressure accumulator 56, and it increases when hydraulic volume flows into low-pressure accumulator 56 by opening outlet valve 54. In order to be able to estimate the current pre-pressure from the behavior of motor-pump unit 53, it is important that motor-pump unit 53 is operated with a constant electrically timed control. The electrically timed control is specified by electronic control unit 7.Constantly adjusting the electrically timed control to a changing fill level of the low-pressure accumulator 56 would make estimating the pre-pressure using the motor-pump unit 53 unreliable or even impossible. However, knowing the prevailing pre-pressure as accurately as possible is essential to avoid squeaking noises from the valves and longitudinal vibrations in the vehicle speed. To estimate the pre-pressure, inlet valve 55 can be opened, creating a hydraulic short circuit between the wheel brake cylinder 59 and the motor-pump unit 53.

[0027] Electronic control unit 7 stores a fill level threshold for the hydraulic volume fill level in low-pressure accumulator 56, which must not be undercut during slip control operation in order to avoid an adjustment of the electrically clocked control. Since electronic control unit 7 specifies the electrically clocked control for motor-pump unit 53, electronic control unit 7 has information regarding the amount of hydraulic volume discharged from low-pressure accumulator 56 of motor-pump unit 53. To keep the fill level constant, the same amount of discharged hydraulic volume must be fed back in. This is done, for example, initially by reducing the pressure in wheel brake cylinder 59 by opening outlet valve 54, since it was detected during slip control operation that the corresponding pre-pressure is too high and must be reduced.Thus, a quantity of hydraulic volume calculable by electronic control unit 7 flows into low-pressure accumulator 56. Inlet valve 55 remains closed during this time. However, since the fill level of low-pressure accumulator 56 continues to decrease according to the low-pressure estimation performed in electronic control unit 7, both outlet valve 54 and inlet valve 55 are now opened and pressure generated by pressure sensor 1 is fed directly into low-pressure accumulator 56. Since, at the same time, a certain pre-pressure exists in wheel brake cylinder 59, which should not be changed by the introduction of hydraulic volume into low-pressure accumulator 56 in order not to influence the braking force acting on the corresponding vehicle wheel, the hydraulic resistance of outlet valve 54 and inlet valve 55 is adapted to the pressure present in wheel brake cylinder 59.Since a comparatively high pressure must be maintained in wheel brake cylinder 59, inlet valve 55, which can be controlled analogously, is opened almost completely. With this large opening width, the hydraulic resistance of inlet valve 55 is comparatively small. Outlet valve 54, on the other hand, is a purely digital valve and is regulated by a suitable selection of on-off modulation such that a higher hydraulic resistance is achieved compared to inlet valve 55. This allows the pre-pressure in wheel brake cylinder 59 to be kept constant, while simultaneously filling low-pressure accumulator 56 with hydraulic volume. Therefore, the electrically clocked control of motor-pump unit 53 does not need to be changed, and reliable estimates of the pre-pressure in wheel brake cylinder 59 are continuously available.

[0028] Fig.2 shows a flowchart illustrating the individual process steps of a possible embodiment of the method according to the invention. In step 21, slip control operation is started on the vehicle tires. In accordance with the wheel speed requirements, an electrically clocked control of a motor-pump unit takes place in process step 23 and a control of the inlet and outlet valves takes place in process step 22. From the control of the motor-pump unit and its run-down behavior, a pre-pressure is estimated in step 25 based on a pressure estimate. Furthermore, the fill level of the low-pressure accumulator is estimated from the control and run-down behavior of the motor-pump unit in step 24. The valve control selected in step 22, which influences the amount of hydraulic volume fed into the low-pressure accumulator, also flows into the estimation of the low-pressure accumulator fill level in step 24.Depending on the estimated pressure in step 25 and the estimated fill level of the low-pressure accumulator 24, the valve control in step 22 and also the electrically clocked control of the motor-pump unit in process step 23 are adjusted.

Claims

[1] Method for improving a pre-pressure estimation in a hydraulic line of a slip-controlled motor vehicle brake system, - in which an electrically clocked control of an electric motor of a motor-pump unit (43, 53) takes place, - wherein hydraulic volume is removed from at least one low-pressure accumulator (46, 56) by the motor-pump unit (43, 53) in accordance with the electrically timed control and is used to generate pressure on the outlet side of the motor-pump unit (43, 53), - wherein by means of a pressure reduction in at least one wheel brake cylinder (49, 410, 59, 510) a filling of the at least one low-pressure accumulator (46, 56) with hydraulic volume is effected, - wherein, in an electrical cycle pause, a pressure-loaded run-out behaviour of the motor-pump unit (43, 53) is detected in the form of a generator voltage applied to the electric motor, - where a pressure is estimated based on the generator voltage by means of a pressure estimation and - wherein a filling level of the at least one low-pressure accumulator (46, 56) is estimated by means of a low-pressure accumulator estimation, characterized by , - that by means of the pressure reduction and / or by means of a pumping of hydraulic volume into the at least one low-pressure accumulator (46, 56), a filling level of the at least one low-pressure accumulator (46, 56) is regulated to a constant or variable value above a filling level threshold during a slip control operation. [2] Method according to claim 1, characterized by in that, for the purpose of supplying the brake fluid, both an inlet valve (45, 48, 55, 58) associated with the at least one wheel brake cylinder (49, 410, 59, 510) and an outlet valve (44, 47, 54, 57) associated with the at least one wheel brake cylinder are controlled and / or regulated into an open position and / or an intermediate position. [3] Method according to claim 2, characterized by that the inlet valve (45, 48, 55, 58) and the outlet valve (44, 47, 54, 57) are controlled and / or regulated in such a way that a pre-pressure in at least one wheel brake cylinder (49, 410, 59, 510) is kept constant during the supply. [4] Method according to at least one of claims 1 to 3, characterized by that the injection is carried out at a time when the currently implemented slip control operation is essentially not affected. [5] Method according to claim 4, characterized by that the injection is carried out at a time after a pre-pressure reduction phase, in particular during a wheel observation phase. [6] Method according to at least one of claims 1 to 5, characterized by that the fill level threshold is set or determined in such a way that no adjustment of the control of the motor-pump unit (43, 53) is necessary due to the fill level being too low. [7] Method according to at least one of claims 1 to 6, characterized by that the pressure is reduced by opening the outlet valve (44, 47, 54, 57) assigned to the at least one wheel brake cylinder and that the pressure is built up by opening the inlet valve (45, 48, 55, 58) assigned to the at least one wheel brake cylinder and simultaneously controlling the pressure generated by the pressure sensor (1) and / or by opening the inlet valve (45, 48, 55, 58) and simultaneously controlling the pressure generated by the motor-pump unit (43, 53). [8] Method according to at least one of claims 1 to 7, characterized bythat the low-pressure accumulator estimation is carried out taking into account the hydraulic resistances and the opening times of the inlet valve (45, 48, 55, 58) and the outlet valve (44, 47, 54, 57) as well as taking into account the pressure, wherein the hydraulic resistances are characterized by valve opening widths and / or valve opening times. [9] Method according to at least one of claims 1 to 8, characterized by that the pre-pressure estimation and the low-pressure accumulator estimation are carried out in an electronic control unit (7), wherein the electronic control unit controls and / or regulates the motor-pump unit (43, 53), the outlet valve (44, 47, 54, 57) and the inlet valve (45, 48, 55, 58). [10] Hydraulic line of a slip-controlled motor vehicle braking system, comprising at least one wheel brake cylinder (49, 410, 59, 510) which is hydraulically connected between an inlet valve (45, 48, 55, 58) and an outlet valve (44, 47, 54, 57), at least one low-pressure accumulator (46, 56) which is hydraulically connected behind the outlet valve (44, 47, 54, 57), a motor-pump unit (43, 53) which is hydraulically connected behind the at least one low-pressure accumulator (46, 56) and, in accordance with an electrically timed control, delivers hydraulic volume from the at least one low-pressure accumulator (46, 56) in the direction of the inlet valve (45, 48, 55, 58) and pressurizes the outlet side of the motor-pump unit (43, 53), and an electronic control unit (7) which controls and / or regulates the outlet valve (44, 47, 54, 57) and the inlet valve (45, 48, 55, 58) and electrically timed controls an electric motor of the motor-pump unit (43, 53), where the electric motor runs out under pressure during an electrical cycle break and thereby generates a generator voltage, wherein, upon opening of the outlet valve (44, 47, 54, 57), hydraulic volume flows into the at least one low-pressure accumulator (43, 53) and fills it, and wherein the electronic control unit (7) reads the generator voltage and estimates a pressure based on the generator voltage and estimates a fill level of the at least one low-pressure accumulator (46, 56) based on the hydraulic volume delivered from the at least one low-pressure accumulator (46, 56) and the hydraulic volume flowing into the at least one low-pressure accumulator (46, 56), characterized by , that the electronic control unit (7) controls and / or regulates the outlet valve (44, 47, 54, 57) and / or the inlet valve (45, 48, 55, 58) and / or the motor-pump unit (43, 53) in such a way that the fill level of the at least one low-pressure accumulator (46, 56) remains at a constant or variable value above a fill level threshold during slip control operation. [11] Hydraulic line according to claim 10, characterized by that the inlet valve (45, 48, 55, 58) and / or the outlet valve (44, 47, 54, 57) is an analogue digital valve.

Images