Differential pressure control of wheel brakes

Abstract

The invention relates to a method for controlling the hydraulic pressure in at least one wheel brake of a hydraulic motor vehicle braking system, in which the system pressure is generated by an electric pressure supply device and in which a required hydraulic pressure lower than the system pressure is set in the at least one wheel brake by controlling an inlet valve having an opening current characteristic and opening in particular in a current-free state. In order to improve the pressure control, according to the invention the inlet valve is provided to open in particular by application of an opening current below the opening current characteristic and to be switched by an opening current to an intermediate current on the opening current characteristic.

Description

Technical Field

[0001] The present invention relates to a method for controlling the hydraulic pressure in at least one wheel brake of a hydraulic motor vehicle braking system, wherein the system pressure is generated by an electric pressure supply device, and a required hydraulic pressure lower than the system pressure is set in at least one wheel brake by controlling an inlet valve having an opening current characteristic curve.

Background Art

[0002] Today's modern braking systems often operate according to the brake-by-wire principle, in which the driver does not have a direct hydraulic connection to the individual wheel brakes. Instead, a pressure supply device is provided that supplies the brake pressure for the individual wheel brakes. However, this type of braking system generally has more individual wheel brakes than pressure supply devices. Usually, only one pressure supply device is provided for such a braking system. When it is intended to set different wheel pressures in the individual wheel brakes, said wheel pressures are realized by using wheel valves. Also in conventional braking systems, for the implementation of assist functions, an electric pressure supply device has to generate the brake pressure that has to be distributed by the wheel valves.

[0003] For this purpose, in the prior art, the change is made between a current below the opening current characteristic curve and a current above the opening current characteristic curve, and thus the inlet valve is opened in a pulsed manner. The opening time is determined based on the volume requirement.

[0004] From German Patent Application Publication No. 10 2012 222 897 A1, it is known to individually control the inlet valve of a wheel brake in the same way in order to generate the brake pressure specific to the wheel. This means that the pressure medium volume flow provided by the pressure and volume control unit is assigned to each wheel brake having pressure change requirements. The operating current of the inlet valve of the selected wheel brake is periodically reduced in relation to each other such that the duration of the current reduction defines the proportion of the selected wheel brake in the pressure increasing volume.

[0005] Due to inaccuracies in the parameters and models, such control necessarily leads to pressure control errors, which can be a problem especially for open-loop control functions. Furthermore, such operation of the inlet valve leads to a high noise load.

SUMMARY OF THE INVENTION

PROBLEM TO BE SOLVED BY THE INVENTION

[0006] Accordingly, an object of the present invention is to enable improved pressure setting in individual wheel brakes.

MEANS FOR SOLVING THE PROBLEM

[0007] The object is achieved according to the present invention by first applying to the inlet valve, especially in the case of a normally open inlet valve, an opening current below the opening current characteristic curve, as a result of which the inlet valve opens. The inlet valve is here arranged especially between the pressure supply device and the wheel brake. The opening current characteristic curve is defined for various differential pressures above the inlet valve and indicates from which current the inlet valve no longer opens. Thus, the valve opens below the characteristic curve and closes above the characteristic curve.

[0008] From the turn-on current, a transition is then made to an intermediate current on the turn-on current characteristic curve. After a short turn-on time, the inlet valve does not then close completely, but rather transitions to an intermediate state. In the intermediate state, a volume that allows the wheel pressure to follow the pressure requirement flows through the inlet valve, as long as the pressure requirement does not involve too rapid a change. The wheel pressure is thus set very precisely and noise emission is significantly reduced.

[0009] In a preferred embodiment of the invention, the intermediate current is determined from the turn-on current characteristic curve based on the pressure difference between the system pressure and the required fluid pressure. Thus, generally, the current actual wheel pressure, which cannot be directly measured due to the lack of a suitable sensor and is instead calculated from a pressure model, is not required.

[0010] In another preferred embodiment of the invention, the turn-on current is set based on the current pressure difference across the inlet valve and / or based on the required volumetric flow rate through the inlet valve. The turn-on current is the current at which the inlet valve opens clearly. Thus, since the current to open the valve is not simply reduced to zero, the volumetric flow rate is adjustable and the noise load is minimized.

[0011] In a particularly preferred embodiment of the invention, the pressure difference across the inlet valve is determined from the system pressure and the actual wheel pressure, in particular, the actual wheel pressure is determined from a model calculation and not from a wheel-specific pressure sensor. The system pressure may generally be understood to mean the wheel inlet pressure.

[0012] In another advantageous embodiment of the invention, as soon as the actual wheel pressure reaches the setpoint wheel pressure within an allowable deviation, a transition is made from the turn-on current to the intermediate current. The full opening of the inlet valve thus first allows a large volumetric flow rate in order to overcome the difference between the setpoint pressure and the actual pressure as quickly as possible, and then a transition to an intermediate flow rate is made to enable accurate and quiet differential pressure control.

[0013] In another preferred embodiment of the present invention, as soon as the difference between the actual wheel pressure and the setpoint wheel pressure becomes greater than a threshold value, an open current is applied to the inlet valve. For example, a value between 0.5 and 3 bar, particularly about 1 bar, can be selected as the threshold value. Thus, in the case of a rapid change in the setpoint value, when a large discrepancy occurs again between the setpoint value of the wheel pressure and the actual value, the inlet valve is fully opened by reapplying the open current in order to enable a rapid adjustment to the setpoint pressure.

[0014] In another preferred embodiment of the present invention, the inlet valve is controlled without being pulsed, and thus the pressure equalization of the actual wheel pressure to the setpoint wheel pressure is carried out continuously. Thereby, the noise radiation due to the control of the wheel pressure is significantly reduced.

[0015] In a further preferred embodiment of the present invention, the follow-up phase is started at a constant setpoint wheel pressure. The electric valve current is maintained at an intermediate current during the follow-up period. A value between 100 ms and 500 ms can be selected as the follow-up period. The actual wheel pressure is thereby accurately set to the setpoint wheel pressure even if there was an error previously.

[0016] In a further preferred embodiment of the present invention, stabilization pulses are periodically applied to the inlet valve at a pressure gradient smaller than the threshold value, i.e., the time derivative of the setpoint wheel pressure. As already explained, when an intermediate current is applied, the inlet valve is not in a stable state. Thus, due to the change in the flow rate, the valve may be pushed fully open. To prevent this, switching is alternately carried out between the calculated intermediate current and a larger stabilization current. However, the stabilization pulses with the stabilization current are applied only for a short time such that the valve plunger does not move perceptibly. In particular, pulses having a duration of 1 ms can be applied every 10 ms to 20 ms. A current 50 to 500 mA, particularly 100 mA, higher than the intermediate current can be selected as the stabilization current.

[0017] In another preferred embodiment of the present invention, the valve current is calculated based on pulse control, the valve current is compared with an intermediate current, and the smaller of the two currents is applied to the inlet valve. This ensures that the pressure setting is not carried out more slowly by differential pressure control than by known pulse control or volume control.

[0018] The object is also an electrohydraulic braking system for a motor vehicle, having an electric pressure supply device, at least one wheel brake, a normally open inlet valve assigned to the wheel brake, and a control unit designed to generate a system pressure by means of the electric pressure supply device and to set a hydraulic pressure lower than the system pressure in at least one wheel brake by controlling the normally open inlet valve having an open current characteristic curve, wherein the inlet valve opens by application of an open current below the open current characteristic curve and is switched from the open current to an intermediate current on the open current characteristic curve, which is also achieved by an electrohydraulic braking system.

[0019] Further features, advantages and possible uses of the present invention can also be obtained from the description of the following exemplary embodiments and the drawings. All features shown and / or described, individually as well as in any combination, belong to the subject matter of the present invention independently of the claims or their summary in their citation.

Brief Description of the Drawings

[0020]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Best Mode for Carrying Out the Invention

[0021] The automotive braking system shown in FIG. 1 includes four hydraulically actuable wheel brakes 8a to 8d. The braking system includes a master brake cylinder 2 actuated by an actuating pedal, i.e., a brake pedal 1, a moving simulator, i.e., a simulation device 3 interacting with the master brake cylinder 2, a pressure medium reservoir 4 under atmospheric pressure, an electrically controllable pressure supply device 5, and a valve arrangement including wheel-specific brake pressure regulating valves configured as inlet valves 6a to 6d and outlet valves 7a to 7d according to the present embodiment. Further, the braking system includes at least one electronic open-loop and closed-loop control unit 12 for controlling the electrically actuable components of the braking system.

[0022] According to the embodiment, the wheel brake 8a is assigned to the left front wheel (FL), the wheel brake 8b is assigned to the right front wheel (FR), the wheel brake 8c is assigned to the left rear wheel (RL), and the wheel brake 8d is assigned to the right rear wheel (RR).

[0023] The master brake cylinder 2 has a master brake cylinder piston 15 defining a hydraulic chamber 17 within a housing 16, constituting a single-circuit master brake cylinder 2. The pressure chamber 17 houses a return spring 9 that positions the piston 15 at the starting position when the master brake cylinder 2 is not actuated. The pressure chamber 17 is connected at one end to the pressure medium reservoir 4 via a radial bore formed in the piston 15 and a corresponding pressure equalization line 41, and the bore and the line can be blocked by the relative movement of the piston 15 within the housing 16.

[0024] The pressure chamber 17 is connected at its other end by means of a hydraulic line section (also referred to as the first feed line) 22 to a brake supply line 13 to which the input connections of the inlet valves 6a to 6d are connected. The pressure chamber 17 of the master brake cylinder 2 is thus connected to all the inlet valves 6a to 6d.

[0025] According to the embodiment, no electrically or hydraulically actuable valve is arranged in the pressure equalization line 41 or at the connection between the pressure chamber 17 and the pressure medium reservoir 4.

[0026] As an alternative, in particular the parallel connection between a normally open diagnostic valve, preferably between a normally open diagnostic valve and a non-return valve closing in the direction of the pressure medium reservoir 4, can be accommodated in the pressure equalization line 41 or between the master brake cylinder 2 and the pressure medium reservoir 4.

[0027] The valve arrangement may also include other hydraulic valves. A shut-off valve 23 is arranged between the feed line 22 connected to the pressure chamber 17 and the brake supply line 13, or the pressure chamber 17 is connected to the brake supply line 13 via a first feed line 22 having a shut-off valve 23. The shut-off valve 23 is designed as an electrically actuable, preferably normally open (NO) 2 / 2-way valve. The hydraulic connection between the pressure chamber 17 and the brake supply line 13 can be shut off by the shut-off valve 23.

[0028] The piston rod 24 couples the pivotal movement of the brake pedal 1 as a result of pedal actuation to the translational movement of the master brake cylinder piston 15, and its actuating movement is preferably detected by a movement sensor 25 of redundant design. Thus, a corresponding piston movement signal is an indication of the brake pedal actuation angle. This represents the braking requirement of the vehicle driver.

[0029] The pressure sensor 20 connected to the first feed line 22 detects the increased pressure in the pressure chamber 17 as a result of the displacement of the piston 15. This pressure value can also be evaluated to characterize or identify the braking requirement of the vehicle driver. As an alternative to the pressure sensor 20, a force sensor 20 can also be used to identify the braking requirement of the vehicle driver.

[0030] According to this embodiment, the simulation device 3 is of a hydraulic configuration and is hydraulically coupled to the master brake cylinder 2. The simulation device 3 substantially has, for example, a simulator chamber 29, a simulator rear chamber 30, and a simulator piston 31 that separates the two chambers 29, 30 from each other.

[0031] The simulator piston 31 is supported on the housing by an elastic element 33 (for example, a simulator spring) disposed in the simulator rear chamber 30 (dry according to this example). According to this embodiment, the hydraulic simulator chamber 29 is connected to the pressure chamber 17 of the master brake cylinder 2 by a preferably electrically operable, preferably normally closed simulator enable valve 32.

[0032] The braking system comprises inlet valves 6a - 6d and outlet valves 7a - 7d for each hydraulically actuable wheel brake 8a - 8d. The inlet valves and the outlet valves are hydraulically interconnected in pairs via a central connection part and are connected to the wheel brakes 8a - 8d. A check valve (specifically not shown) that opens in the direction of the brake supply line 13 is connected in parallel to each of the inlet valves 6a - 6d. The output connections of the outlet valves 7a - 7d are connected to the pressure medium reservoir 4 via a common return line 14. The valves, especially the inlet valves, may in particular be seat valves. Such seat valves have only two stable states, fully open or fully closed, when there is no flow. When flow passes through the seat valve, in addition to spring force, magnetic force, and compression force, there is also a flow force resulting from the pressure change as a result of the flow. By appropriate selection of the spring and furthermore electromagnetic characteristics (residual air gap, coil), the valve can be designed such that multiple stable positions are generated by the flow force. However, this is not comparable to the quality of a proportional valve, and usually, in such valves, there is a problem that the plunger tends to vibrate in the intermediate position, which in turn leads to noise and vibration (NVH).

[0033] The electrically controllable pressure supply device 5 is in the form of a hydraulic cylinder piston arrangement (or a single - circuit electro - hydraulic actuator) or a linear actuator, and its piston 36 is operable by an electric motor 35 shown schematically, which has an intermediate connection to a rotary - translational transmission 39 also shown schematically. The piston 36 delimits a single pressure chamber 37 of the pressure supply device 5. A rotor position sensor, which serves to detect the rotor position of the electric motor 35 shown only schematically, is indicated by reference numeral 44.

[0034] The line section (also referred to as the second feed line) 38 is connected to the pressure chamber 37 of the electrically controllable pressure supply device 5. The supply line 38 is connected to the brake supply line 13 via an electrically actuatable, preferably normally closed, sequence valve 26 as part of the valve arrangement. The sequence valve 26 enables the hydraulic connection between the pressure chamber 37 of the electrically controllable pressure supply device 5 and the brake supply line 13 (and thus the input connections of the inlet valves 6a - 6d) to be opened and closed in a controlled manner.

[0035] The actuator pressure generated by the action of the force of the piston 36 on the pressure medium accommodated in the pressure chamber 37 is supplied to the second feed line 38. In the "brake - by - wire" operating mode, in particular, when there is no fault in the braking system, the feed line 38 is connected to the brake supply line 13 via the sequence valve 26. In this way, during normal braking, the movement of the piston 36 forward and backward causes an increase and decrease in the wheel brake pressure of all the wheel brakes 8a - 8d.

[0036] In the case of pressure reduction due to the backward movement of the piston 36, the pressure medium that has previously moved from the pressure chamber 37 of the pressure supply device 5 to the wheel brakes 8a - 8d also flows back into the pressure chamber 37.

[0037] Alternatively, different wheel brake pressures for each wheel can be easily adjusted by the inlet valves 6a - 6d and the outlet valves 7a - 7d. In the case of a corresponding pressure reduction, a part of the pressure medium discharged via the outlet valves 7a - 7d flows into the pressure medium reservoir 4 via the return line 14.

[0038] Additional pressure medium can be drawn into the pressure chamber 37 in that, with the sequence valve 26 closed, upon rearward movement of the piston 36, the pressure medium can flow from the reservoir 4 via a line 42 having a check valve 53 which opens in the flow direction to the actuator 5 into the actuator pressure chamber, that is to say the pressure chamber 37. According to the embodiment, the pressure chamber 37 is additionally connected to the pressure medium reservoir 4 via one or more breather holes in the non-operating state of the piston 36. This connection between the pressure chamber 37 and the pressure medium reservoir 4 is disconnected when the piston 36 (sufficiently) operates in the operating direction 27.

[0039] In the brake supply line 13, an electrically actuatable normally open circuit cut-off valve 40 is arranged, via which the brake system is divided into two hydraulic sub-circuits. The brake supply line 13 is divided into a first line section 13a which is connected to the brake master cylinder 2 (via the shut-off valve 23) and a second line section 13b in a second hydraulic sub-circuit which is connected to the pressure supply device 5 (via the sequence valve 26). The first line section 13a is connected to the inlet valves 6a, 6b of the wheel brakes 8a, 8b, and the second line section 13b is connected to the inlet valves 6c, 6d of the wheel brakes 8c, 8d.

[0040] With the circuit cut-off valve 40 open, the brake system has a single-circuit design. By closing the circuit cut-off valve 40, the brake system, which is controlled in particular depending on the situation, can be separated or divided into two hydraulic sub-circuits, brake circuits I and II. Here, in the first brake circuit I, the master brake cylinder 2 is connected only to the inlet valves 6a, 6b of the wheel brakes 8a, 8b of the front axle VA (via the shut-off valve 23), and in the second brake circuit II, the pressure supply device 5 is connected only to the wheel brakes 8c and 8d of the rear axle HA (with the sequence valve 26 open).

[0041] With the circuit breaker 40 open, a pressure corresponding to the brake pressure provided by the pressure supply device 5 in the first operating mode (e.g., the "brake-by-wire" operating mode) can be supplied to the input connection parts of all the inlet valves 6a to 6d by the brake supply line 13. In the second operating mode (e.g., the non-energized fallback operating mode), the pressure in the pressure chamber 17 of the master brake cylinder 2 can be applied to the brake supply line 13. This pressure is also applied to all of the inlet valves 6a to 6d when the circuit breaker 40 is open, and is thus also referred to as the system pressure.

[0042] The braking system advantageously comprises a level measuring device 50 for determining the pressure medium level / filling level in the pressure medium reservoir 4.

[0043] According to this embodiment, the hydraulic components, namely the master brake cylinder 2, the simulation device 3, the pressure supply device 5, the hydraulic valves 6a to 6d, 7a to 7d, 23, 26, 40, and 32, and also the valve arrangement having the brake supply line 13, and furthermore the hydraulic connections including the brake supply line 13, are arranged together within a hydraulic open-loop and closed-loop control unit 60 (HCU). The electronic open-loop and closed-loop control unit (ECU) 12 is assigned to the hydraulic open-loop and closed-loop control unit 60. The hydraulic and electronic open-loop and closed-loop control units 60, 12 are preferably configured as one unit (HECU).

[0044] The braking system comprises a pressure sensor 19 or a system pressure sensor for detecting the pressure provided by the pressure supply device 5. Here, the pressure sensor 19 is arranged downstream of the sequence valve 26 when viewed from the pressure chamber 37 of the pressure supply device 5.

[0045] In addition to hydraulic actuation, the two rear-wheel brakes 8c, 8d each comprise an integrated parking brake 48c, 48d designed as an electromechanical parking brake.

[0046] In the normal operating mode, the shut-off valve 23 is closed, the sequence valve 26 and the circuit breaker valve 40 are open, and thus the hydraulic pressure in all wheel brakes 8a to 8d is set by the linear actuator 5. To control the different brake pressures in the individual wheel brakes 8a to 8d, the respective inlet valves 6a to 6d must be controlled according to the situation.

[0047] Such control of the inlet valves, as known from the prior art, is shown in FIG. 2. The setpoint pressure 51 of the front axle is higher than the setpoint pressure 52 of the rear axle. Thus, the setpoint pressure 51 of the front axle can be set directly by the linear actuator 5 when the inlet valves 6a, 6b of the wheel brakes 8a, 8b of the front axle are fully open. In contrast, the setpoint pressure 52 of the rear axle is controlled by pulse control of the inlet valves 6c, 6d. As shown in FIG. 2, initially only the setpoint pressure 51 of the front axle increases, and the setpoint pressure of the rear axle remains zero. Correspondingly, a closing current 53a that surely closes the inlet valves is supplied to the inlet valves 6c, 6d of the rear axle. After a short time, this closing current is reduced to a holding current 53b sufficient to surely keep the inlet valves in the closed state.

[0048] When the setpoint pressure (p req ) 52 of the rear axle rises, immediately in the said "volume control", in the first step, from the pressure demand p req and the actually estimated wheel pressure p mod , a differential volume dV is specified. The pressure-volume characteristic curve (pV characteristic curve) stored in the braking system is used for this purpose. dV = pV(p req ) - pV(p mod )

[0049] Furthermore, the desired volume flow rate q is specified from the setpoint pressure gradient p grad and the derivative of the pV characteristic curve.

Equation

[0050] In a second step, a current for the inlet valve is then determined, which enables a volume flow rate q for the currently prevailing differential pressure via the valve. To convey a differential volume dV through the valve by the volume flow rate q, the valve is left open for a valve actuation time Tau = dV / q. After the valve actuation time Tau, a closing current is applied to the inlet valve, whereby the inlet valve is closed again completely. As in the embodiment shown in FIG. 2, when the setpoint pressure increases further, now, a difference occurs between the new setpoint pressure 52(p req ) and the current actual wheel pressure. Thus, the above steps are repeated and another opening pulse is applied to the inlet valve. If the setpoint 52 of the rear axle remains constant, a holding current is set after the last closing pulse to keep the inlet valve in the closed state.

[0051] The pressure curve obtained from the volume control is shown in FIG. 3. Initially, the actual wheel pressure 54 of the front axle follows the setpoint wheel pressure 51 very accurately because it is set directly by the linear actuator 5. The setpoint pressure 52 of the rear axle also increases, and as soon as the volume control opens the inlet valve in pulses, both the pressure 55 at the rear axle at the wheel and the pressure 54 at the front axle at the wheel result in a number of small pressure peaks.

[0052] FIG. 4 shows the open current characteristic curve of a typical inlet valve 6. The open current characteristic curve 56 shows the tendency of various differential pressures DP through the inlet valve, the current range in which the inlet valve is closed (above the open current characteristic curve), and the current at which the inlet valve is closed (below the open current characteristic curve).

[0053] As equivalent to FIG. 2, FIG. 5 shows the differential pressure control according to the present invention. The set pressure curves 51 and 52 of the front axle and the rear axle are the same as those in FIG. 2. Therefore, the current curve 53 again has pulses 53a, and then, while the holding current 53b that keeps the inlet valves 6c and 6b of the rear axle fully closed follows, the pressure requirement of the set pressure 52 of the rear axle still remains zero. As soon as the set pressure 52 of the rear axle increases, the first opening pulse 53c is connected. For this purpose, the valve current and the valve operation time Tau can be calculated as described above. However, the switching from this opening current to the closing current is not performed, and the valve current on the opening current characteristic curve 56 is selected. Correspondingly, the inlet valve 6 is neither in a predetermined closed state nor in a predetermined open state. Rather, the inlet valve 6 is in an intermediate state. The valve current is selected from the opening current characteristic curve 56 for the differential pressure calculated between the system pressure and the set value 52. Therefore, this directly involves the set value of the pressure difference, rather than the actual value of the pressure difference. However, since the set value and the actual value are close to each other, the difference is small. When the change in the set value 52 is extremely slow, exactly the same amount of volume flows through the inlet valve 6 so that the actual wheel pressure 60 can accurately follow the set wheel pressure 52. The differential pressure between the set value 52 of the rear axle and the set value 51 of the front axle gradually decreases. As shown in FIG. 4, the valve current 57 therefore moves to the left on the opening current characteristic curve 56.

[0054] As can be inferred from FIG. 6, the actual wheel pressures 54 and 55 of the front axle and the rear axle follow the regulations by the set values 51 and 52 much more accurately. In particular, it should be noted that the control of the valve current on the opening current characteristic curve depends only on the pressure difference between the system pressure and the set value 52p for each wheel brake. req In particular, the actual wheel pressure, which generally cannot be directly measured, is not included in the pressure control. Instead, it is derived from model calculations. The pV characteristic curve, which may show a large inaccuracy, is also not included in the pressure control in this region. This greatly improves the accuracy and robustness of the pressure control.

[0055] FIG. 7 additionally shows the noise emission 61 during pressure setting by volume control and the noise emission 62 during pressure setting by differential pressure control. The noise emissions are initially still equal, while in volume control, rapid opening and closing of the inlet valve causes a number of noise peaks, which have been found to result in high noise levels. In differential pressure control, these peaks do not exist, and a much quieter noise level is achieved. The present invention may also include the following aspects: 1. A method for controlling the hydraulic pressure in at least one wheel brake (8a, b, c, d) of a hydraulic automotive braking system, wherein the system pressure is generated by an electric pressure supply device (5), and by controlling a normally open inlet valve (6a, b, c, d) having an open current characteristic curve (56), a required hydraulic pressure lower than the system pressure is set in the at least one wheel brake (8a, b, c, d). In the method, the inlet valve (6a, b, c, d) opens, in particular, by applying an open current below the open current characteristic curve (56), and is switched from the open current to an intermediate current on the open current characteristic curve (56). 2. The method according to 1. above, characterized in that the intermediate current is determined from the open current characteristic curve (56) based on the pressure difference between the system pressure and the required hydraulic pressure. 3. The method according to 1. or 2. above, characterized in that the open current is set based on the current pressure difference through the inlet valve (6a, b, c, d) and / or based on the required volume flow rate through the inlet valve (6a, b, c, d). 4. The method according to 3. above, characterized in that the pressure difference through the inlet valve (6a, b, c, d) is determined from the system pressure and the actual wheel pressure, and in particular, the actual wheel pressure is determined from a model calculation. 5. The method according to any one of 1. to 4. above, characterized in that the switching from the open current to the intermediate current is performed immediately when the actual wheel pressure reaches the setpoint wheel pressure. 6. The method according to any one of 1. to 5. above, characterized in that the open current is applied to the inlet valve (6a, b, c, d) immediately when the difference between the actual wheel pressure and the setpoint wheel pressure becomes greater than a threshold value. 7. The method according to any one of 1. to 6. above, characterized in that the inlet valve (6a, b, c, d) is controlled without being pulsed, and thus the pressure equalization of the actual wheel pressure to the setpoint wheel pressure is performed continuously. 8. The method according to any one of 1. to 7. above, characterized in that in a follow-up stage where the setpoint wheel pressure remains constant, the electric valve current is maintained at the intermediate current during the follow-up period. 9. The method according to any one of 1. to 8. above, characterized in that a stabilizing pulse is periodically applied to the inlet valves (6a, b, c, d) at a pressure gradient smaller than the threshold value. 10. The method according to any one of 1. to 9. above, characterized in that the valve current is calculated based on pulse control, the valve current is compared with the intermediate current, and the smaller of the two currents is applied to the inlet valve. 11. In a hydraulic braking system for a motor vehicle, comprising an electric pressure supply device (5), at least one wheel brake (8a, b, c, d), normally open inlet valves (6a, b, c, d) assigned to the wheel brakes (8a, b, c, d), and a control unit (12) designed to generate a system pressure by the electric pressure supply device (5) and set a hydraulic pressure lower than the system pressure in the at least one wheel brake (8a, b, c, d) by controlling the normally open inlet valves (6a, b, c, d) having an opening current characteristic curve (56), the inlet valves (6a, b, c, d) open by application of an opening current below the opening current characteristic curve (56) and are switched from the opening current to an intermediate current on the opening current characteristic curve (56).

Explanation of Signs

[0056] 1 Brake pedal 2 Master brake cylinder 3 Simulation device 4 Pressure medium reservoir 5 Pressure supply device 6a~d Inlet valve 7a~d Outlet valve 8a~d Wheel brake 9 Return spring 12 Control system 13 Brake supply line 14 Return line 16 Housing 17 Pressure chamber 19 System pressure sensor 20 Master cylinder pressure sensor 22 First feed line 23 Shut-off valve 24 Piston rod 25 Mobile sensor 26 Sequence valve 29 Simulator chamber 30 Rear simulator chamber 31 Simulator piston 32 Simulator enable valve 33 Elastic element 35 Piston 36 Electric motor 37 Pressure chamber 38 Feed line 39 Rotary translation mechanism 40 Circuit breaker valve 41 Pressure equalization line 42 Line 44 Rotor position sensor 45 Check valve 50 Level sensor 51 Setpoint pressure of the front axle 52 Setpoint pressure of the rear axle 53 Valve current of the pulsed inlet valve 54 Pressure curve of the front axle 55 Pressure curve of the rear axle 56 Open current characteristic curve 57 Pressure rise on the current curve 58 Follow-up period of the current value 59 Follow-up period 60 Model pressure 61 Volume control of noise emission 62 Pressure difference control of noise emission

Claims

1. A method for controlling the hydraulic pressure in at least one wheel brake (8a, b, c, d) of a hydraulic motor vehicle braking system, wherein the system pressure is generated by an electric pressure supply device (5) and the required hydraulic pressure lower than the system pressure is set in the at least one wheel brake (8a, b, c, d) by controlling normally open inlet valves (6a, b, c, d) having an energization current characteristic curve (56). The method is characterized in that the inlet valves (6a, b, c, d) open by applying an energization current below the energization current characteristic curve (56), and are switched from the energization current to an intermediate current on the energization current characteristic curve (56).

2. The method according to claim 1, characterized in that the intermediate current is determined from the energization current characteristic curve (56) based on the pressure difference between the system pressure and the required hydraulic pressure.

3. The method according to claim 1 or 2, characterized in that the energization current is set based on the current pressure difference via the inlet valves (6a, b, c, d) and / or based on the required volume flow rate through the inlet valves (6a, b, c, d).

4. The method according to claim 3, characterized in that the pressure difference via the inlet valves (6a, b, c, d) is determined from the system pressure and the actual wheel pressure, and the actual wheel pressure is determined from a model calculation.

5. The method according to claim 4, characterized in that the switching from the energization current to the intermediate current is performed as soon as the actual wheel pressure reaches the setpoint wheel pressure.

6. The method according to claim 5, characterized in that the energization current is applied to the inlet valves (6a, b, c, d) as soon as the difference between the actual wheel pressure and the setpoint wheel pressure becomes greater than a threshold value.

7. The method according to claim 5, characterized in that the inlet valves (6a, b, c, d) are controlled without being pulsed, and thus the pressure equalization of the actual wheel pressure to the setpoint wheel pressure is performed continuously.

8. The method according to claim 5, characterized in that in a follow-up stage where the setpoint wheel pressure remains constant, the electric valve current is maintained at the intermediate current during the follow-up period.

9. The method according to claim 1 or 2, characterized in that stabilization pulses are periodically applied to the inlet valves (6a, b, c, d) at a pressure gradient smaller than the threshold value.

10. The pilot current is calculated based on pulse control, the pilot current is compared with the intermediate current, and the smaller of the pilot current and the intermediate current is applied to the inlet valve (6a, b, c, d). The method according to claim 1 or 2, characterized in that.

11. In a hydraulic braking system for a motor vehicle, comprising an electric pressure supply device (5), at least one wheel brake (8a, b, c, d), a normally open inlet valve (6a, b, c, d) assigned to the wheel brake (8a, b, c, d), and a control unit (12) designed to generate a system pressure by the electric pressure supply device (5) and to set a hydraulic pressure lower than the system pressure in the at least one wheel brake (8a, b, c, d) by controlling the inlet valve (6a, b, c, d) having an open current characteristic curve (56). The inlet valve (6a, b, c, d) opens by application of an open current below the open current characteristic curve (56), and is switched from the open current to an intermediate current on the open current characteristic curve (56). A hydraulic braking system, characterized in that.

12. The inlet valve (6a, b, c, d) is controlled without being pulsed, and thus the pressure equalization of the actual wheel pressure to the setpoint wheel pressure is carried out continuously. The method according to claim 6, characterized in that.

13. In the follow-up stage where the setpoint wheel pressure remains constant, the electric valve current is maintained at the intermediate current during the follow-up period. The method according to claim 6, characterized in that.

14. In the follow-up stage where the setpoint wheel pressure remains constant, the electric valve current is maintained at the intermediate current during the follow-up period. The method according to claim 7, characterized in that.

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