Method and braking system for determining an operating current characteristic map

Abstract

Method for determining a working current characteristic of a hydraulic inlet valve (6) of a motor vehicle brake system characterized by the steps: a) Opening the inlet valve (6), b) Setting a hydraulic wheel pressure (62) in the wheel brake (5) associated with the inlet valve (6), c) Closing the inlet valve (6), d) Setting a hydraulic system pressure (63), e) Setting an estimated working flow (64) at the inlet valve (6) based on the difference between the set wheel pressure (62) and the set system pressure (63) and a volume flow rate (65), f) Controlling a pressure supply device (2) to deliver the volume flow (65), g) Recording the course of the actual pressure difference for a given time period and determining a pressure difference reference point, h) Determining a support point of the operating current characteristic map for the set operating current (64), the delivered volume (65) and the pressure differential support point;

Description

[0001] The invention relates to a method for determining a working current characteristic of a hydraulic inlet valve of a motor vehicle braking system and to a corresponding hydraulic braking system.

[0002] In modern brake-by-wire systems, hydraulic brake pressure is not directly supplied by the driver. Instead, an electrical pressure supply unit provides hydraulic pressure based on a command from the driver or an assistance system. For cost reasons, individual pressure supply units are typically not provided for each wheel to be braked; rather, a single pressure supply unit serves all or at least several wheels. To accommodate varying brake pressure requirements, analog inlet valves are used at the wheel brakes to distribute the system pressure supplied by the pressure supply unit to the wheel brakes as needed. The operating current characteristics of the inlet valves are required for this purpose. Inaccuracies in these operating current characteristics directly lead to inaccurate brake pressures.However, the inlet valves and thus their operating current characteristics can vary to a considerable extent due to manufacturing tolerances and / or wear.

[0003] From DE 10 2013 203 599 A1, a method for determining the actuation characteristic of a hydraulic valve is known. In this method, a brake pressure is generated by a pressure supply device and measured with a pressure sensor. The determined brake pressure is stored together with a valve actuation parameter as a calibration point.

[0004] Furthermore, DE 10 2005 014 097 A1 teaches that, for a calibration process of an inlet valve, an associated isolating valve is closed and a pressure differential is built up across the inlet valve. The inlet valve is then controlled with a valve opening signal, and the volume that consequently flows into the wheel brake is determined by pumping it back using a hydraulic pump.

[0005] It is therefore an object of the present invention to enable a precise specification of the brake pressures.

[0006] The task is solved by determining the operating current characteristic curves, through the following steps: a) Opening the inlet valve whose operating current characteristic is to be measured, b) Setting a hydraulic wheel pressure in the wheel brake associated with the inlet valve, in particular by means of a pressure supply device, by providing a corresponding system pressure on the side of the inlet valve facing away from the wheel brake. The pressure in the wheel brake is provided by the open inlet valve. c) Closing the inlet valve so that the hydraulic pressure is locked into the wheel brake. A corresponding outlet valve of this wheel brake is closed. d) Setting a hydraulic system pressure by again using the pressure supply device to provide a corresponding system pressure on the side of the inlet valve facing away from the wheel brake. The closed valve thus establishes a pressure differential across the inlet valve. e) Setting an estimated working flow at the inlet valve, based on the difference between the set wheel pressure and the set system pressure, as well as a volume flow rate, f) Controlling a pressure supply device to deliver the volumetric flow rate. If the estimated operating flow rate corresponds to the correct operating flow rate for the delivered volumetric flow rate and the set pressure differential, the pressure differential remains constant at the set value. However, if the value is not exactly correct, the pressure differential will change. g) To determine this, the course of the actual pressure difference is recorded for a given period of time and a pressure difference reference point is determined from it. h) Determining a reference point of the operating current characteristic curve for the set operating current, the delivered volume and the pressure differential reference point.

[0007] Using the resulting triplet of values ​​for operating current, volumetric flow rate, and pressure difference, a portion of the operating current characteristic curve is determined. Preferably, the operating current characteristic curve can be adjusted directly by shifting a stored curve accordingly. Alternatively, however, the preceding steps are repeated to determine measured values ​​for various operating currents, pressure differences, and volumetric flow rates, thereby adjusting and / or directly determining the entire operating current characteristic curve.

[0008] In a preferred embodiment of the invention, the operating current characteristic curve for a specific volume flow through the inlet valve and a specific pressure difference across the inlet valve indicates the required electrical operating current of the inlet valve coil.

[0009] In a further preferred embodiment of the invention, a mean value, a final value or an extrapolated value of the course of the actual pressure difference is used to determine a pressure difference support point.

[0010] In a further preferred embodiment of the invention, the system pressure is measured by means of a pressure sensor, in particular at the output of the pressure supply device, and / or the wheel pressure is determined from a wheel pressure model, in particular by means of a pV curve, in order to record the course of the actual pressure difference. Thus, individual pressure sensors for each wheel brake can be dispensed with, thereby significantly reducing the costs of the braking system.

[0011] In a further preferred embodiment of the invention, the estimated operating current is determined from a previous operating current characteristic curve. This can, for example, be a design characteristic curve, i.e., an operating current characteristic curve defined for the valve type, which is stored in a control unit of the brake system during manufacturing. Alternatively, it can be an operating current characteristic curve of the inlet valve measured during or after manufacturing.

[0012] In a further preferred embodiment of the invention, the remaining inlet valves are closed and / or kept closed during the measurement. This ensures that only the inlet valve currently being measured influences the measured values.

[0013] In a further preferred embodiment of the invention, the hydraulic wheel pressure is set to 0 bar in step b), and in particular, a corresponding outlet valve remains open during the determination. This eliminates the wheel pressure model as a source of error, since the wheel pressure always remains at 0 bar due to the open outlet valve, and this measured value is always precisely known. Thus, the pressure difference can be determined with particular accuracy.

[0014] In a further preferred embodiment of the invention, the pV characteristic of the affected wheel brake is determined before the operating current characteristic is determined. This improves the accuracy of the wheel pressure determination using the pressure model.

[0015] In a further preferred embodiment of the invention, if the deviation of the actual pressure difference from the set pressure difference exceeds a threshold value, the measurement is repeated with an adjusted estimated operating current. Thus, the accuracy of the operating current characteristic can be improved iteratively.

[0016] In a further preferred embodiment of the invention, the inlet valve is designed as an analog inlet valve without a check valve and / or the outlet valve is digital. The analog inlet valve allows for particularly precise control of the wheel pressure and, in particular due to the absence of a check valve, also enables the containment of a wheel pressure greater than the system pressure.

[0017] In a further preferred embodiment of the invention, to implement vehicle deceleration, particularly in ABS operation, a target value for wheel pressure is determined based on the actuation of an e-Pedal or the input of a virtual driver or assistance system. To adjust the wheel pressure, the associated inlet valve is set with an operating current based on the determined operating current characteristic map. In normal braking operation, the inlet valves are typically fully open. Utilizing the controllability of the valves with their operating current characteristics is particularly useful in ABS operation, where the aim is to regulate an analogous pressure build-up gradient in situations where the wheel pressure is lower than the system pressure.

[0018] The problem is also solved by a hydraulic brake system comprising an electric pressure supply device, at least one wheel brake with an associated inlet valve and a control device for regulating the pressure supply device and the inlet valve, wherein the control device is configured to perform one of the above.

[0019] Further features, advantages, and applications of the invention will also become apparent from the following description of exemplary embodiments and the drawings. All features described and / or illustrated, both individually and in any combination, are part of the subject matter of the invention, even independently of their compilation in the claims or their cross-references. Fig. Figure 1 shows an embodiment of a brake system according to the invention, Fig. Figure 2 shows a section of an operating current characteristic map, Fig. Figure 3 shows sizes during the inventive process, Fig. Figure 4 shows the volume flow rate during the inventive method;

[0020] In Fig. Figure 1 is a first embodiment of a braking system 1 according to the invention for a motor vehicle with four hydraulically actuated wheel brakes 5a-5d shown schematically.

[0021] The braking system 1 advantageously comprises a brake control unit (HECU) with a hydraulic block 20 (hydraulic control and regulating unit HCU, valve block) with an output port 4a-4d for each of the wheel brakes 5a-5d. A pressure medium reservoir 3 at atmospheric pressure is arranged on the valve block 20. By way of example, the output ports 4a, 4b are assigned to the wheel brakes 5a, 5b of the front axle (front), e.g., output port 4a to the left front wheel FL (wheel brake 5a) and output port 4b to the right front wheel FR (wheel brake 5b), and the output ports 4c, 4d are assigned to the wheel brakes 5c, 5d of the rear axle (rear), e.g., output port 4c to the left rear wheel RL (wheel brake 5c) and output port 4d to the right rear wheel RR (wheel brake 5d).

[0022] The fill level of the pressure medium reservoir 3 is measured by means of a fill level sensor 44.

[0023] The pressure medium storage tank 3 comprises at least two tank chambers 84, 85, which are at least partially separated from each other by a bulkhead 86. Furthermore, the pressure medium storage tank 3 comprises a first connection 71 and a second connection 72. The first connection 71 is connected to the first tank chamber 84, and the second connection 72 is connected to the second tank chamber 85. In this sense, the first tank chamber 84 is assigned a first connection 71 of the pressure medium storage tank 3, and the second tank chamber 85 is assigned the second connection 72 of the pressure medium storage tank 3.

[0024] Each output port 4a-4d is assigned an inlet valve 6a-6d and an outlet valve 7a-7d, wherein the inlet valves 6a-6d are advantageously normally open and the outlet valves 7a-7d are advantageously normally closed. None of the inlet valves 6a-6d is connected in parallel to a check valve 8a-8d closing towards the output ports 4a-4d or wheel brakes 5a-5d, so that pressure can be trapped in the wheel brakes.

[0025] The respective outlet ports 4a-4d are connected to the pressure medium reservoir 3 via the outlet valve 7a-7d. The outlet valves 7a-7d are connected to the first port 71 of the pressure medium reservoir 3 via a return line 161b, 161 (which is at least partially shared).

[0026] An electrically controlled hydraulic pressure source 2 is provided, which is formed by a cylinder-piston arrangement with a pressure chamber 30, the piston 31 of which can be actuated by an electromechanical actuator with a schematically indicated electric motor 32 and a schematically depicted rotary-translational transmission 33. The pressure source 2 is, for example, designed as a single-circuit electrohydraulic linear actuator (LAC) with only one pressure chamber 30. The piston 31 can be advanced by means of the electromechanical actuator to build up pressure (brake actuation direction) and retracted to release pressure.

[0027] For example, the electric motor is designed as a double-wound electric motor 32 with a first motor winding 34a and a second motor winding 34b. If both motor windings 34a, 34b are energized, the electric motor 32 delivers its full power. If only one of the two motor windings 34a, 34b is energized, the power of the electric motor 32 is reduced, but pressure can still be generated by the pressure source 2, albeit at a reduced level and with reduced dynamics.

[0028] Pressure chamber 30 is hydraulically connected via a pressure port 83 to a brake line section 160, which is hydraulically connected to the inlet valves 6a-6d (more precisely to the inlet ports of the inlet valves 6a-6d).

[0029] For drawing pressure medium from the pressure medium reservoir 3 into the pressure source 2, the pressure source 2 has a suction port 81. Pressure chamber 30 is connected via the suction port 81 and a suction valve 14, which is designed, for example, as a check valve opening towards the pressure chamber 30, to the first port 71 of the pressure medium reservoir 3 and thus to the first chamber 84 of the pressure medium reservoir 3.

[0030] Furthermore, pressure source 2 has a compensating port 82 for the hydraulic connection of the pressure chamber 30 with the pressure medium reservoir 3 in a predetermined position of the piston 31. This allows the wheel brakes 5a-5d to be depressurized, i.e., connected to the pressure medium reservoir 3 at atmospheric pressure, in the predetermined position of the piston 31. The predetermined position of the piston 31 is advantageously the unactuated state of the piston 31. The compensating port 82 is connected to the other (second) reservoir chamber 85, i.e., the second port 72, of the pressure medium reservoir 3.

[0031] For example, pressure source 2 has a vent hole 80 as a compensating port 82. The vent hole 80 is connected via a follow-up line section 162 to the second port 72 of the pressure medium reservoir 3. In the unactuated state of the piston 31, pressure chamber 30 is connected via the vent hole 80 and the follow-up line section 162 to the second reservoir chamber 85 of the pressure medium reservoir 3, whereby the vent hole 80 is closed when the piston 31 is actuated, thus disconnecting the connection to the pressure medium reservoir 3. For example, the piston 31 is provided with at least one bore through which, in the unactuated state of the piston 31, the hydraulic connection between pressure chamber 30 and after-run section 162 is established, and which, when the piston 31 is actuated, passes over a seal, so that the hydraulic connection between pressure chamber 30 and after-run section 162 is separated.

[0032] For example, the return line 161b, 161 for the outlet valves 7a-7d and a suction line 161a, 161 of the pressure source 2 are combined in sections. For this purpose, the suction port 81 is connected to the return line 161b, 161 via the suction valve 14 and a suction line section 161a. The outlet valves 7a-7d and the suction valve 14 are thus connected to the first port 71 of the pressure medium reservoir 3 via a common line section 161. In other words, the intake line section 161a and a return line section 161b lead into the line section 161 to the first connection 71. In this sense, the pressure chamber 30 is connected via the suction valve 14 and the intake line section 161a to the return line 161b, 161 (from return line section 161b and line section 161).

[0033] The after-line section 162 and the suction line section 161a (or the return line 161b, 161 or the return line section 161b or the line section 161) are not directly connected to each other. There is only a certain indirect hydraulic connection via the pressure medium reservoir 3 if the fill level of the pressure medium reservoir 3 is above the bulkhead 86.

[0034] The suction line 161a, 161 of the pressure source 2 is not connected to the second connection 72 of the pressure medium reservoir 3.

[0035] A (system) pressure sensor 40 is connected to the brake line section 160, by means of which the pressure generated by the pressure source 2 can be determined. Preferably, the pressure sensor 40 is the only pressure sensor of the brake system 1 or of the brake control unit.

[0036] To control the pressure source 2, brake system 1 includes redundant sensor elements for detecting a rotational speed or angle of rotation of the electric motor 32. For example, a first motor angle sensor 43 and a second motor angle sensor 42 are provided.

[0037] Braking system 1 advantageously comprises only the one hydraulic pressure source 2. Braking system 1 does not include a second electrically controlled hydraulic pressure source or a pressure source that can be actuated by the driver, e.g. a brake pedal-actuated master brake cylinder, to realize a hydraulic, driver-actuated fallback level.

[0038] Braking system 1 further comprises a first electronic control unit (ECU) A and a second electronic control unit (ECU) B for controlling the electrically actuated components of braking system 1. The first electronic control unit A and the second electronic control unit B are advantageously electrically independent of each other in the sense that a failure of the first electronic control unit does not cause a failure of the second electronic control unit, and vice versa. For this purpose, the electronic control units A and B can be implemented as separate units, but they can also be implemented as independent subunits within the same electronic control unit.

[0039] The arrows A or B on the electrical or electrically actuated components, such as the valves 6a-6d, 7a-7d and the sensors 40, 42, 43, 44, indicate the assignment to the electronic control unit A or B.

[0040] The electric motor 32 of the pressure source 2 can be controlled by the first and the second electronic control and regulation units A, B, i.e., each of the electronic control and regulation units A, B is capable of independently building up brake pressure via the pressure source 2 to perform a service brake.The pressure source 2 and the electronic control units A, B are designed such that, in the event of a failure of the first electronic control unit A, the pressure source 2 can be controlled by means of the second electronic control unit B in order to build up a brake pressure for actuating the wheel brakes 5a-5d during a service or control braking operation, and that, in the event of a failure of the second electronic control unit B, the pressure source 2 can be controlled by means of the first electronic control unit A in order to build up a pressure for actuating the wheel brakes 5a-5d during a service or control braking operation.

[0041] As an example, the electric motor is designed as a double-wound electric motor 32 with the first motor winding 34a and the second motor winding 34b. The electric motor 32 of the pressure source 2 is controlled by the first and the second electronic control unit in the sense that the first motor winding 34a (preferably only) is controlled by the first electronic control unit A (labeled with an arrow) and the second motor winding 34b (preferably only) is controlled by the second electronic control unit B (labeled with an arrow), in particular being supplied with electrical energy by the electronic control unit. For this purpose, motor winding 34a is connected to the first control unit A and the other motor winding 34b to the second control unit B.To control the pressure source 2, each of the two control and regulation units A, B includes a motor processor for processing the motor control functions, a power stage with transistors for providing the phase voltages to the electric motor 32 (e.g. B6 bridge) and a driver stage (Gate Drive Unit) for controlling the transistors of the power stage.

[0042] In the event of a failure of one of the electronic control units A or B, the pressure source 2 is controlled by the other electronic control unit B or A, and pressure is built up to actuate the wheel brakes 5a-5d in brake-by-wire mode for service braking. With one functioning electronic control unit B of A, the pressure source 2 or the electric motor 34 is operated with at least part of its power to build up pressure for actuating the wheel brakes.

[0043] The electrically actuated inlet and outlet valves 6a-6d, 7a-7d and the sensors 40, 42, 43, 44 of brake system 1 are each assigned to only one of the electronic control units. This means that each electrically actuated valve 6a-6d, 7a-7d is controlled exclusively by electronic control unit A or exclusively by electronic control unit B. This avoids the need for complex, dual-actuated valves / valve coils. Similarly, the signals from each sensor 40, 42, 43, or 44 are fed exclusively to electronic control unit A or exclusively to electronic control unit B.

[0044] All electrically actuated valves, i.e., for example, the electrically actuated inlet and outlet valves 6a-6d, 7a-7d, are advantageously assigned to the same electronic control unit, for example, the electronic control unit A, and are controlled exclusively by the electronic control unit A.

[0045] The signals from the (first) motor angle sensor 43 are fed to the second electronic control unit B and evaluated by it, whereas the signals from the (second) motor angle sensor 42 are fed to the first electronic control unit A and evaluated by it.

[0046] The signals from the pressure sensor 40 are advantageously supplied to the same electronic control unit A which also controls the inlet and outlet valves 6a-6d, 7a-7d, i.e. the signals from the pressure sensor 40 are supplied to the first electronic control unit A and evaluated by it.

[0047] If one of the electronic control units A or B fails, the pressure source 2 can still be controlled by means of one of the motor windings 34a or 34b and one of the motor angle sensors 42 or 43 (or possibly the pressure sensor 40), thus building up a suitable pressure, although possibly at a reduced level and / or with reduced dynamics. This (central) pressure can be used to actuate all wheel brakes 5a-5d. The (central) pressure can also be modulated by moving the piston 31 back and forth.

[0048] Advantageously, the braking system 1 is powered by a redundant electrical system with two independent power sources (a first electrical power supply and a second electrical power supply), so that both control units A and B are not powered by the same electrical power supply. For example, control unit A is powered by the first electrical power supply and control unit B is powered by the second electrical power supply.

[0049] Braking system 1 includes, for example, electric parking brakes 50a, 50b on the wheels of one of the axles, for example, on the rear wheels RL, RR. The electric parking brakes 50a, 50b are controlled or actuated by the electro-hydraulic brake control unit.

[0050] Advantageously, the rear axle wheel brakes are designed as combination brake calipers with a hydraulic wheel brake 5c, 5d and an integrated, electrically operated parking brake (IPB).

[0051] For example, one of the electric parking brakes, e.g., parking brake 50a, is actuated / controlled by the first electronic control unit A (indicated by the arrow with A), while the other electric parking brake, e.g., parking brake 50b, is actuated / controlled by the second electronic control unit B (indicated by the arrow with B). If one of the control units A or B fails, the vehicle can still be secured using at least one of the parking brakes, which is actuated by the functioning control unit B or A. This eliminates the need for a transmission parking lock.

[0052] Preferably, the braking system also includes an operating unit for a driver (not shown in Fig. 1) The actuation unit is connected to the brake control unit (HECU) on the signal side for transmitting a driver request signal, however there is no mechanical-hydraulic connection from the actuation unit to the brake control unit or the wheel brakes 55a-5d.

[0053] In the event of a failure of the second electronic control unit B, the hydraulic braking function of the braking system can be carried out by means of the pressure source 2, including the wheel pressure control by means of the inlet and outlet valves 6a-6d, 7a-7d, using the first electronic control unit A, the only limitation being that the power of the pressure source 2 may be reduced.

[0054] Since the outlet valves 7a-7d are normally closed and the inlet valves 6a-6d are normally open, all wheels can be hydraulically braked in the event of a failure of the first electronic control unit A. For this purpose, the second electronic control unit B regulates the delivered hydraulic fluid volume when no pressure signal is available. Common pressure modulation at all wheel brakes 5a-5d remains possible.

[0055] When the brakes are not applied, the wheel brakes 5a-5d should be at atmospheric pressure. For this purpose, the pressure source 2 is controlled and returned to its unactuated state in such a way that a hydraulic connection to the pressure medium reservoir 3, which is at atmospheric pressure, is created via the vent hole 80.

[0056] When using the outlet valves 7a-7d for individual wheel pressure control, hydraulic fluid volume is consumed in the sense that hydraulic fluid is released from the pressure chamber 30 via the wheel brakes 5a-5d into the hydraulic fluid reservoir 3. Accordingly, hydraulic fluid volume must be drawn into the pressure chamber 30 of the pressure source 2 via the suction valve 14 at the latest when the hydraulic fluid volume in the pressure chamber 30 reaches a lower limit.

[0057] In the valve block 20 of the brake system, there is no hydraulic connection between the suction port 81 and the second port 72 of the pressure medium reservoir 3, and no hydraulic connection between the equalization port 82 and the first port 71 of the pressure medium reservoir 3.

[0058] In summary, the brake system has no isolating valve and no check valves. Pressure chamber 30 of pressure source 2 is hydraulically connected directly to brake line section 160 and the inlet valves 6a-6d; that is, for example, no electrically actuated valve is provided. Similarly, no valve, including a check valve, is located in the hydraulic connection between pressure chamber 30 and each of the inlet valves 6a-6d. This offers the advantage of low throttling losses in the main flow path from pressure source 2 to the inlet valves 6a-6d and the wheel brakes 5a-5d.

[0059] Advantageously, the exemplary braking system 1 comprises Fig. 1. In addition to the electrically actuated inlet and outlet valves 6a-6d, 7a-7d, there are no other electrically actuated valves.

[0060] In Fig. Figure 2 shows a section of the operating current characteristic curve as a family of curves. Curve 60 shows the opening current characteristic, which indicates the opening current for a volume flow rate of 0. Curve 61 shows the relationship between the pressure difference across the inlet valve and the operating current of the valve coil for a specific volume flow rate. Corresponding curves exist for all possible volume flow rates, but are not shown here.

[0061] The method according to the invention can be illustrated using the diagram of the Fig. 3 and Fig. Step 4. Initially, the valve current 64 through the inlet valve 6 is zero amperes, so the inlet valve 6 is fully open, as it is a normally open valve. In step 1, the volume flow 65 provided by the pressure supply device 2 increases, flowing through the open inlet valve 6 into the associated wheel brake 5. Accordingly, the wheel pressure 62 of this wheel brake 5 increases until it reaches its target value in step 2. In step 3, the inlet valve 6 is now supplied with a valve current 64 of 1200 mA, ensuring it is securely closed. From now until step 4, the pressure supply device 2 continues to be activated, which, due to the now closed inlet valve 6, increases the system pressure 63, i.e., the pressure on the side of the inlet valve 6 facing away from the wheel brake 5. This results in the desired pressure differential across the inlet valve 6.

[0062] From step 4 onwards, the actual measurement begins, in which the valve flow 64 is set to an estimated working flow for the set pressure differential at the inlet valve 6 and a specific volume flow rate q. The pressure supply device 2 is controlled to deliver the specified volume flow rate q. For this purpose, it is operated at a velocity v = q / A, where A is the effective cross-sectional area of ​​the piston of the pressure supply device. In the example shown, the Fig.In step 3, the operating flow rate at the inlet valve 6 was set slightly too high. This means that, given the differential pressure across the inlet valve, less than the set volume flow rate passes through the valve, causing the system pressure 63 to rise faster than the wheel pressure 62. Consequently, the differential pressure increases. A new reference value for the differential pressure can now be determined from the differential pressure curve. Preferably, one waits until step 5, when the differential pressure has stabilized. The differential pressure in the range between steps 5 and 6 is then averaged, and the average value is used as the reference value.

[0063] Using the resulting triplet of values ​​for operating current, volumetric flow rate, and pressure difference, the operating current characteristic curve can be adjusted directly, for example, by shifting curve 61 accordingly. However, it is preferable to repeat the preceding steps to determine measured values ​​for various operating currents, pressure differences, and volumetric flow rates, thereby adjusting and / or directly determining the entire operating current characteristic curve.

Claims

[1] Method for determining a working current characteristic of a hydraulic inlet valve (6) of a motor vehicle brake system characterized by the steps: a) Opening the inlet valve (6), b) Setting a hydraulic wheel pressure (62) in the wheel brake (5) associated with the inlet valve (6), c) Closing the inlet valve (6), d) Setting a hydraulic system pressure (63), e) Setting an estimated working flow (64) at the inlet valve (6) based on the difference between the set wheel pressure (62) and the set system pressure (63) and a volume flow rate (65), f) Controlling a pressure supply device (2) to deliver the volume flow (65), g) Recording the course of the actual pressure difference for a given time period and determining a pressure difference reference point, h) Determining a support point of the operating current characteristic map for the set operating current (64), the delivered volume (65) and the pressure differential support point; [2] Method according to claim 1, characterized by , that the operating current characteristic for a certain volume flow (65) through the inlet valve (6) and a certain pressure difference across the inlet valve (6) specifies the required electrical operating current (64) of the inlet valve coil. [3] Method according to claim 1 or 2, characterized by , that to determine a pressure difference support point, a mean value, a final value or an extrapolated value of the course of the actual pressure difference is used. [4] Method according to any one of the preceding claims, characterized by , that to record the course of the actual pressure difference the system pressure (63) is measured by means of a pressure sensor and / or the wheel pressure (62) is determined from a wheel pressure model. [5] Method according to any one of the preceding claims, characterized by , that the estimated operating current (64) is determined from a previous operating current characteristic map. [6] Method according to any one of the preceding claims, characterized by , that the remaining inlet valves (6) are closed and / or remain closed during the investigation. [7] Method according to any one of the preceding claims, characterized by , that the hydraulic wheel pressure (62) is selected to 0 bar in step b). [8] Method according to any one of the preceding claims, characterized by , that the pV characteristic of the affected wheel brake (5) is determined before the determination of the operating current characteristic. [9] Method according to any one of the preceding claims, characterized by , that if the deviation of the actual pressure difference from the set pressure difference exceeds a threshold, the measurement is repeated with an adjusted estimated operating current. [10] Method according to any one of the preceding claims, characterized by , that the inlet valve (6) is designed as an analog inlet valve without a check valve and / or the outlet valve (7) is designed digitally. [11] Method according to any one of the preceding claims, characterized by , that to implement a vehicle deceleration, a target value for a wheel pressure (62) is determined based on the actuation of an e-pedal or the specification of a virtual driver or assistance system, and to adjust the wheel pressure (63), the associated inlet valve (6) is set with an operating current based on the determined operating current map. [12] Hydraulic brake system comprising an electrical pressure supply device (2), at least one wheel brake (5) with an associated inlet valve (6) and a control device for controlling the pressure supply device (2) and the inlet valve (6), characterized by, that the control device is configured to execute one of the methods according to claims 1 to 11.

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