Method and device for operating hydraulic brake system of motor vehicle, brake system

Abstract

The invention relates to a method for operating a hydraulic brake system of a motor vehicle, the brake system having a hydraulic main actuator with a brake master cylinder and an actuator that can be actuated to operate the brake master cylinder, the brake system also has at least one brake circuit, which is or can be hydraulically connected to the brake master cylinder and which has at least one hydraulically actuatable wheel brake, the at least one brake circuit has at least one secondary actuator and at least one actuatable inlet valve and at least one actuatable outlet valve associated with the respective wheel brake, the secondary actuator being actuatable to feed a hydraulic volume in the direction of the inlet valve, and the outlet valve being actuatable to discharge the hydraulic volume in the direction of the outlet valve. The at least one brake circuit has a hydraulic pressure accumulator connected downstream of the discharge valve.

Description

Technical Field

[0001] The present invention relates to a method for operating a hydraulic brake system of a motor vehicle, wherein the brake system has a hydraulic main actuator, which has a brake master cylinder and an actuator that can be controlled to operate the brake master cylinder, and the brake system also has at least one brake circuit that is hydraulically connected or can be hydraulically connected to the brake master cylinder, the brake circuit has at least one wheel brake that can be hydraulically operated, wherein the at least one brake circuit has at least one auxiliary actuator and at least one controllable inlet valve and at least one controllable outlet valve assigned to the wheel brake device, wherein the auxiliary actuator can be controlled to convey a hydraulic volume in the direction of the inlet valve, and wherein the at least one brake circuit has a hydraulic accumulator connected downstream of the outlet valve.

[0002] The invention also relates to a device for operating a hydraulic brake system of a motor vehicle, which device has a control unit which is specially designed to carry out the above-described method.

[0003] The invention also relates to a brake system for a motor vehicle, which is designed as described above and has an above-described control unit. Background Art

[0004] Methods and devices of the type mentioned at the outset are known in the prior art. Today's brake systems usually have a hydraulically operated brake master cylinder with a movably supported hydraulic single piston or hydraulic tandem piston, which the driver can move by actuating the brake pedal. Usually, the piston is mechanically coupled to the brake pedal so that the driver can generate hydraulic pressure in a hydraulic brake circuit connected to the brake master cylinder via the brake master cylinder in any driving situation. Usually, a brake pressure generating unit or brake pressure boosting unit is connected between the brake pedal and the brake master cylinder as a main actuator, which supports the driver in applying the required brake pressure or generates brake pressure or hydraulic pressure independently of the actuation of the pedal by actuating the brake master cylinder. In addition, modern motor vehicles usually have an ESP module (ESP=electronic stability program (elektronisches )) of a braking system, the ESP module ensures that the brake pressure or braking force of a single wheel is set in order to optimize the driving stability of the motor vehicle, for example during a braking process or an acceleration process. If a brake pressure generating unit or a brake pressure boosting unit fails, the ESP module can usually take over its function, but the performance will be greatly reduced. In this case, the ESP module detects the brake pressure generated by the driver in the master brake cylinder with the help of a pressure sensor and sets a brake wheel pressure by suitable actuation of valves and actuation of the secondary actuator of the ESP system, which exceeds the hydraulic pressure provided by the driver via the brake pedal. In this regard, the ESP module supports the driver during braking, thereby reducing the necessary pedal force in the event of a failure of the main actuator or ensuring the required braking even if the main actuator fails. Summary of the invention

[0005] The method according to the invention having the features of claim 1 has the advantage over known methods that even at low viscosities of the hydraulic medium The brake pressure can also be built up quickly enough at low temperatures which decrease and lead to an increased request to the slave actuator. According to the invention, this is achieved by carrying out the following steps in sequence each time the motor vehicle is started: First, in step a), the temperature of the hydraulic medium is determined or detected and compared with a predetermined limit value.

[0006] Only if the determined temperature is below the limit value is the subsequent step b) carried out. If the temperature is above the limit value, the brake system continues to operate normally without further steps. As an example, a temperature of -20°C, at least -10°C or, for example, 0°C is assumed as the limit value. In particular, the limit temperature is selected according to the temperature characteristics of the hydraulic medium so that the hydraulic medium reaches an undesirable viscosity at a temperature below the limit temperature. If the temperature is below the predetermined limit value, the main actuator is first controlled so that the hydraulic medium is delivered to the brake circuit in order to fill the accumulator with the hydraulic volume to a predetermined extent. By controlling the main actuator and the valves of the brake circuit, the hydraulic volume delivered by the master brake cylinder can be supplied to the accumulator without generating a brake pressure at one of the wheel brakes in the process. The accumulator can thus be filled efficiently and quickly.

[0007] Then in step c) it is monitored whether the master actuator and / or the master brake cylinder is functional during operation of the motor vehicle. Thus, filling the pressure accumulator serves only as a preparatory measure for a fault situation without the fault situation actually occurring or being anticipated.

[0008] Only when a failure of the main actuator, in particular the master brake cylinder and / or the actuator that can be controlled, is the slave actuator controlled in the subsequent step d) so that the hydraulic volume is delivered from the pressure accumulator to at least one brake circuit in the direction of the inlet valve of the brake circuit by the slave actuator. Now, the hydraulic volume is no longer sucked from the master brake cylinder as before, but the slave actuator delivers the hydraulic medium prefilled in the pressure accumulator to the brake circuit. Since the pressure accumulator is usually arranged closer to the wheel brake than the master brake cylinder in terms of hydraulic distance, when the corresponding inlet valve is opened, the brake circuit can be filled more quickly, that is, the desired brake pressure can be provided more quickly at the inlet valve or the wheel brake. By prefilling the pressure accumulator, it is possible to ensure that the pressure in the brake is quickly built up in the event of failure of the main actuator and, in particular, to bring the motor vehicle to a parking state quickly and safely.

[0009] According to a preferred embodiment of the invention, in step b), the main actuator is controlled so that a predetermined volume is delivered from the master brake cylinder to the pressure accumulator. The main actuator is thus controlled so that, knowing the piston cross section and the movement stroke of the hydraulic piston of the master brake cylinder, a desired amount of hydraulic medium is delivered to the pressure accumulator. The pressure accumulator is in particular designed as a passive pressure accumulator with an elastically deformable spring element, wherein the pressure accumulator in particular has a maximum filling capacity. Preferably, the pressure accumulator is not filled to the maximum extent by the main actuator, but only partially filled, so that during driving operation, the brake pressure can still be quickly reduced by opening the outlet valve and absorbing the hydraulic medium into the pressure accumulator.

[0010] According to an alternative embodiment of the invention, the pressure accumulator is preferably first completely filled in step b) by the main actuator and then emptied to a predetermined extent, in particular by actuating the slave actuator. This embodiment is particularly advantageous in cases where the brake lining can be easily actuated and ensures, by completely filling the pressure accumulator, that a distinction is made between whether the displaced hydraulic volume is delivered to the pressure accumulator or to the wheel brakes, so that a defined pressure is achieved in the brake circuit and thus ensures that the brake lining (when the inlet valve is open) is tight and that the pressure accumulator is completely filled. If the pressure accumulator is now completely filled, one or more outlet valves are closed and the main actuator is moved into the initial position or rest position. The excess hydraulic volume pushed into the pressure accumulator is then sucked out by means of the slave actuator and, in particular, pumped back into the hydraulic reservoir of the brake system.

[0011] According to a preferred improvement of the present invention, in step d), simple braking or emergency braking is performed according to the detected braking request. Specifically, simple braking and emergency braking are distinguished according to the braking force applied by the driver and / or the driver's operating speed. If the braking force is high and / or the operating speed is high, it is considered to be emergency braking or panic braking. If the braking force is low and / or the operating speed is low, it is considered to be a "normal" braking request and simple braking is initiated.

[0012] The brake system preferably has at least two wheel brakes for the front wheels of the motor vehicle and at least two wheel brakes for the rear wheels of the motor vehicle, wherein, for simple braking, a hydraulic volume is delivered to at least one wheel brake of the brake system via a secondary actuator. If the volume delivered by the secondary actuator is distributed to a plurality of wheel brakes, the hydraulic force acting on the individual wheel brakes, i.e. the respectively acting brake pressure, is reduced to a corresponding extent. Thus, in the case of simple braking, a reliable distribution to a plurality of wheel brakes is possible as long as the brake request is below a predetermined limit value given by the structural design of the brake system.

[0013] According to an alternative embodiment of the invention, the brake system has at least two wheel brakes for the front wheels of the motor vehicle and at least two wheel brakes for the rear wheels of the motor vehicle, wherein for simple braking, the hydraulic volume is delivered only to the wheel brakes for the front wheels by the slave actuator. This embodiment is advantageous in cases where the maximum deliverable hydraulic volume of the slave actuator is not sufficient, for example, to fill all wheel brakes of the brake system simultaneously with sufficient speed and sufficient pressure to satisfy the braking request. In this case, only the hydraulic volume to achieve the maximum braking effect is delivered to the wheel brakes for the front wheels.

[0014] Optionally, it is preferably provided that in step d) an electromechanical parking brake at least at the rear wheels of the motor vehicle is additionally activated. In particular, when the hydraulic volume is supplied only to the wheel brakes of the front wheels via the slave actuator, the electromechanical parking brake advantageously supports the overall braking behavior of the motor vehicle and ensures that a braking request is fulfilled that decelerates the motor vehicle to the desired degree.

[0015] Furthermore, it is preferably provided that in step d) at least one switching valve connected between at least one brake circuit and the master brake cylinder and associated with the slave actuator on the pressure side is closed. This prevents the hydraulic volume provided by the slave actuator from being conveyed in the direction of the master brake cylinder instead of in the direction of the corresponding inlet valve. This optimizes the operation of the brake system in step d).

[0016] Furthermore, it is preferably provided that in step d) at least one hydraulic valve connected between at least one brake circuit and the master brake cylinder and associated with the slave actuator on the intake side is opened. This allows the slave actuator to draw hydraulic volume on the intake side not only from the pressure accumulator but also from the master brake cylinder and / or a reservoir for hydraulic medium associated with the master brake cylinder. This increases the total hydraulic volume that can be drawn in step d).

[0017] Preferably, in this case the hydraulic valve is not opened until the hydraulic volume stored in the pressure accumulator has been delivered to the brake circuit by the slave actuator. This prevents the slave actuator from initially sucking in hydraulic medium from the reservoir or the master brake cylinder, which would be disadvantageous due to the determined low temperature of the hydraulic medium and the large hydraulic distances to the reservoir and the master brake cylinder.

[0018] The brake system preferably has at least two brake circuits, which are constructed similarly to the at least one brake circuit, wherein in step b), the at least two brake circuits fill a plurality of, in particular each or only one of the pressure accumulators to a predetermined extent. If the brake system has two brake circuits connected to the main actuator, which brake circuits in particular each have two wheel brakes, then preferably only one of the pressure accumulators or only the pressure accumulator of one of these brake circuits is filled to a predetermined extent. This reduces the preparation time of the brake system. In particular, the pressure accumulator of the brake circuit whose wheel brake is associated with the front wheel of the motor vehicle is filled. Optionally, the pressure accumulators of both brake circuits are filled to a predetermined extent. The secondary actuator preferably has a pump for each brake circuit, wherein each pump is provided with its own actuator, in particular an electric motor, or wherein the pump is provided with a common actuator, in particular an electric motor. In particular, in the latter case, the hydraulic volume is delivered to both or all brake circuits by actuating an actuator via the corresponding pump, thereby maximizing the total delivery volume.

[0019] Furthermore, it is preferably provided that, in the case where the motor vehicle is in a stationary state after executing step d), the current temperature is detected again, and when the temperature is below the limit value, step b) is executed again. Thus, after the motor vehicle is stationary, the brake system is again ready to provide brake pressure only via the slave actuator. This results in the advantages already mentioned above.

[0020] Preferably, after step d), a hydraulic volume is sucked from a reservoir associated with the master brake cylinder by means of a slave actuator and supplied to the wheel brakes for presetting the wheel brakes. In particular, the presetting is characterized in that a brake pressure is applied to the wheel brakes which is at least sufficient to overcome the air gap of the respective wheel brake. For this purpose, a brake pressure of, for example, 3 bar is generated by the presetting. In this case, corresponding valves, in particular switching valves, are also actuated for the presetting.

[0021] The device according to the invention having the features of claim 14 is characterized in that the control unit is specially designed to carry out the method according to the invention when used as intended. This results in the advantages already mentioned above.

[0022] The brake system according to the invention is characterized in that it has a controller according to the invention. This results in the advantages already mentioned above. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Further advantages and preferred features and combinations of features are particularly apparent from the above description and from the claims. The invention is explained in more detail below with reference to the accompanying drawings. In which:

[0024] Figure 1 The brake system is shown in a simplified diagram.

[0025] FIG. 2A to FIG. 2C A flow chart is shown which explains an advantageous method for operating the brake system. DETAILED DESCRIPTION

[0026] Figure 1 A simplified diagram shows a brake system 1 for a motor vehicle (not shown in detail here). The brake system 1 has a main actuator 2 which is designed to generate a hydraulic pressure in the brake system. For this purpose, the main actuator 2 has a brake master cylinder 3 and an actuator 5 which can be controlled and is associated with the brake master cylinder 3 and is coupled to a reservoir 4 in which a hydraulic medium is present. The actuator 5 is designed in particular as an electric or electromechanical actuator which serves to displace a simple hydraulic piston in the brake master cylinder 3 or as an actuator 5 which can be controlled and is associated with the brake master cylinder 3. Figure 1 The tandem pistons shown by way of example are arranged so that the hydraulic volume is pushed out of the master brake cylinder 3 by displacement.

[0027] Braking system 1 further comprises two brake circuits 6, 7 which are hydraulically connected to master brake cylinder 3, in particular to different chambers of tandem master brake cylinder 3. The two brake circuits 6, 7 are of substantially identical design, brake circuit 6 comprising wheel brakes 8, 9, brake 8 being associated with the left rear wheel HL of the motor vehicle and brake 9 being associated with the right front wheel VR of the motor vehicle, and brake circuit 7 comprising wheel brakes 10, 11, wheel brake 10 being associated with the left front wheel VL of the motor vehicle and wheel brake 11 being associated with the right rear wheel HR of the motor vehicle.

[0028] Since the designs of the brake circuits are basically the same, the structures of the brake circuits 6 and 7 are described below by taking the brake circuit 6 as an example. For the brake circuit 7, the same reference numerals are used for the same elements.

[0029] The brake circuit 6 is connected or can be connected to the master brake cylinder 3 via a switching valve 12 which is designed to open when the power is off and a hydraulic valve 13 which is designed to close when the power is off. The hydraulic valves 12 and 13 are connected in parallel with each other, wherein the switching valve 12 is also provided with a check valve in a bypass. Two inlet valves 15 and 16 are connected to the switching valve 12, wherein the inlet valve 15 is connected upstream of the wheel brake of one of the front wheels, in this case the wheel brake 9, and the inlet valve 16 is connected upstream of the wheel brake of the rear wheel, in this case the wheel brake 8. The inlet valves 15 and 16 are designed to open when the power is off and are each provided with a check valve in the bypass. If the switching valve 12 and the inlet valves 15 and 16 are open, the hydraulic volume can be pushed from the master brake cylinder 3 into the wheel brakes 8 and 9 by actuating the main actuator 2, so as to generate a brake pressure or a brake force acting on the front wheels LR and RF there.

[0030] Furthermore, the wheel brakes 8, 9 are each connected to an outlet valve 17, 18 which is designed to be closed when the power is off. The outlet valves 17, 18 are each connected on the outlet side to a common pressure accumulator 19 which is designed as a mechanical accumulator with a piston to which a spring force is applied. Furthermore, the outlet valves 17, 18 are connected to the hydraulic valve 13 and to the intake side of a pump 20. The pump 20 is operatively connected to a controllable actuator 21 which is designed in particular as an electric motor. The pump 20 and the electric motor 21 together form a secondary actuator 22 of the brake circuit 6.

[0031] On the pressure side, the pump 20 is connected to the brake circuit 6 between the switching valve 12 and the inlet valves 15, 16. If the slave actuator 22 is activated, the pump 20 delivers hydraulic medium in the brake circuit 6 in the direction of the inlet valves 16, 15, or more precisely in the direction of the wheel brakes 8, 9. Since the pump 20 is connected to the pressure accumulator 19 on the suction side, during delivery, hydraulic medium is also withdrawn from the pressure accumulator 19 and supplied to the brake circuit 6 on the pressure side. The excess hydraulic volume on the discharge side of the brake circuit 6 is also delivered, in particular, to the reservoir 4. The brake circuit 6 also has a pressure sensor 23, by means of which the hydraulic pressure between the master brake cylinder 3 and the switching valve 12 and the hydraulic pressure between the master brake cylinder 3 and the hydraulic valve 13 can be detected.

[0032] Inlet valves 15 , 16 , outlet valves 17 , 18 , pressure accumulator 19 and pump 20 preferably form part of an ESP system of motor vehicle 1 and for this purpose are arranged or designed, for example, in a common hydraulic block.

[0033] The brake system 1 also has a control unit 24 which is designed to control the main actuator 2 and the valves 12, 13, 15, 16, 17, 18 of the brake circuits 6, 7 as a function of the detected brake request in order to generate or set a desired brake pressure or a desired brake force at the wheel brakes 8, 9, 10, 11 individually, more precisely wheel by wheel. The brake request is specified or received, for example, by actuating a brake pedal 25 of the brake system 1. Figure 1 , the controller 24 is shown as being associated with the actuator 5. In principle, the controller 24 is arranged at any desired location in or on the brake system 1 and can also be realized, for example, by a central controller of the brake system 1 or an ESP controller.

[0034] In the present case, the brake system 1 is a so-called brake-by-wire system, in which the master actuator 2 is mechanically decoupled from the brake pedal 25. The brake request therefore reaches the controller 24 only as a signal and not as an actuating force to the master brake cylinder 3, as in conventional brake systems in which the brake pedal 25 is directly mechanically coupled to the master brake cylinder 3 and a brake booster is optionally connected therebetween. In this regard, the brake request can also be provided by an automatic driving system of the motor vehicle during autonomous driving operation.

[0035] If the main actuator 2 fails, for example due to a fault in the actuator 5 or the master brake cylinder 3, then in this case the user cannot mechanically generate any braking force on the wheel brakes 8, 9, 10, 11 by actuating the brake pedal 25. However, the control unit 24 is designed to control the slave actuator 22 of the corresponding brake circuit 6, 7 in this case to increase the hydraulic pressure in the corresponding brake circuit 6, 7. For this purpose, the pump 20 sucks in the hydraulic volume from the master brake cylinder 3 via the hydraulic valve 13. Due to the relatively long hydraulic path and at low temperatures and the resulting viscosity of the hydraulic medium, this may result in the brake pressure only being able to build up very slowly.

[0036] By means of the method described below, it is possible to provide sufficient brake pressure at wheel brakes 8 , 9 , 10 , 11 at a sufficient speed even at extremely low temperatures in the event of a failure of main actuator 2 .

[0037] In this regard, FIG. 2 shows a flow chart in several parts, according to which an advantageous method is explained. Currently, the flow chart is divided into Figure 2A , Figure 2B and Figure 2C The three parts are shown.

[0038] The method begins with Figure 2A In step S1 of FIG. 1 , the motor vehicle and thus the brake system 1 are put into operation. This is the case, for example, when the ignition of the motor vehicle 1 is started.

[0039] Then, in step S2, the current temperature T of the hydraulic medium is determined. For this purpose, for example, a temperature sensor 26 is used, which is, for example, associated with the reservoir 4. In step S2, the detected temperature is compared with a predetermined limit value. Here, a temperature of -20°C is preferably assumed as the limit value TG. Below this temperature, the hydraulic medium in the brake system, i.e. the brake fluid, generally loses viscosity and therefore requires greater force to convey it. If the comparison shows that the current temperature T is above the limit value TG, the brake system 1 continues to operate in normal operating mode according to step S3, provided that the main actuator 2 and the brake circuits 6, 7 are able to function.

[0040] However, if the comparison in step S2 shows that the current temperature T is below the predetermined limit value TG(j), then in step S4 all inlet valves 15, 16 and outlet valves 17, 18 are first opened. Then in step S5 the main actuator 2 is controlled to deliver a predetermined hydraulic volume VH into the pressure accumulator 19.

[0041] To this end, the actuator 5 moves the piston by a predetermined stroke, depending on the cross-sectional area of ​​the piston, so that the volume to be delivered to the pressure accumulator 19 is obtained from the volume of liquid moved. As an example, it is assumed that the hydraulic volume VH is VH=3.67 cm3 volume.

[0042] In the subsequent step S6, the inlet valves 15, 16 and the outlet valves 17, 18 are closed again and the actuator 5 is actuated so that the brake master cylinder 13 moves back to its initial position. Now, the predetermined hydraulic volume HV is retained in the pressure accumulator 19. In order to retain the hydraulic medium in the pressure accumulator 19, the spring force of the pressure accumulator 19 and the spring force of the non-return valve 27 located between the pump 20 and the hydraulic valve 13 are coordinated with each other.

[0043] In the case where the brake pads of wheel brakes 8 to 11 can be actuated so easily that it is not possible to clearly distinguish whether the pressure accumulator 19 is filled by the displaced hydraulic volume or the wheel brakes 8 to 11 are actuated, the pressure accumulator 19 can optionally be filled for a longer period of time until a defined pressure is present in the brake circuits 6, 7, thereby ensuring that the brake pads of the wheel brakes 8 to 11 are in contact and that the pressure accumulator 19 has been completely filled.

[0044] After the pressure accumulator 19 has been completely filled, the outlet valves 17, 18 are closed again and the main actuator 2 moves back into its initial position. Only subsequently in step S7 is the hydraulic volume delivered to the pressure accumulator 19 sucked out by means of the secondary actuator 20 and delivered, in particular, back into the reservoir 4. The desired hydraulic volume VH then remains in the pressure accumulator 19. Depending on the pressure required to completely fill the pressure accumulator 19, undesired actuation of the wheel brakes 8, 9, 10, 11 may occur, depending on the vehicle. In order to minimize or avoid this effect, the pressure accumulator 19 is preferably filled by each or only one of the brake circuits 6, 7 in this preparatory process.

[0045] In a subsequent query in step S8 it is checked whether the filling process of the pressure accumulator 19 was successful. If the brake system 1 is functional, the desired hydraulic volume is now permanently retained in the pressure accumulator 19 and serves as a dynamic volume reserve in the event that the main actuator 2 is unavailable due to a fault.

[0046] If a braking request is now detected, the slave actuator 22 draws hydraulic volume not from the master brake cylinder 3 but from the pressure accumulator 19. Since the hydraulic connection in this case is significantly shorter than the hydraulic connection to the master brake cylinder 3, the pressure builds up more quickly, thereby ensuring the desired braking force quickly and sufficiently even at particularly low temperatures.

[0047] In the event that the brake system 1 has an undesired leak and the desired hydraulic volume cannot be permanently retained in the pressure accumulator 19, the switching valve 12 and the inlet valves 16, 15 are preferably energized to prevent undesired emptying of the pressure accumulator 19. If an error is detected, a warning message is also issued to the driver of the motor vehicle in step S9.

[0048] If the filling is successful (j), then Figure 2B The operation of the motor vehicle is continued as shown. In step S10, the motor vehicle is first put into operation completely, for example, the engine is also started and it is determined that the motor vehicle can be operated. According to step S11, the motor vehicle 1 is accelerated actively by controlling the drive motor or passively on a slope by releasing the braking force or parking brake. If in the subsequent step S12, it is detected that the driver of the motor vehicle requests a braking torque, the following operation is performed:

[0049] First, in step S13, it is checked whether the main actuator 2 is functional. If it is functional (j), in step S14 the main actuator is activated in order to generate the desired brake pressure in the hydraulic system, more precisely in the brake system 1. In a subsequent query S15, it is checked whether the pressure has been generated. If this is confirmed (j), the process is repeated, i.e. the brake system 1 operates normally. However, if, for example, a query using the pressure sensor 23 shows that the desired brake pressure has not been generated or has not been fully generated (n), the slave actuator 22 is activated in order to increase the brake pressure, more precisely the hydraulic pressure, in the brake circuits 6, 7.

[0050] If a technical fault is detected in the main actuator 2, a warning message is preferably also displayed to the driver of the motor vehicle in step S16. If it is detected in step S17 that the braking request corresponds to a normal braking request, the secondary actuator 22 is preferably controlled in step S18 in the sense of a conventional ESP power request. In this case, the hydraulic valve 13 is opened and the switching valve is closed. The actuator 21 drives the secondary actuator with a power that causes the secondary actuator to rotate, for example, at 3000 revolutions per minute. This is sufficient to extract hydraulic medium from the pressure accumulator 19 at a sufficient speed and sufficient pressure and to deliver the hydraulic medium to the brake circuits 6, 7.

[0051] If it is detected in step S19 that the braking request exceeds the normal braking process, for example, because the driver actuates the brake pedal 25 with maximum force or maximum speed to indicate emergency braking, the switching valve 12 is closed, the hydraulic valve 13 is opened, and only the inlet valve 15 assigned to the front wheels VL, VR of the motor vehicle is opened. At the same time, the parking brake device 27 assigned to the rear wheels, in particular designed as an electromechanical parking brake, is actuated to generate a braking torque acting on the rear wheels. In this case, the parking brake is actuated in step S20 to generate the maximum possible clamping force.

[0052] The open hydraulic valve 13 brings the following advantages: As soon as the master brake cylinder 3 moves from the rest position, thereby opening at least one so-called through-hole to the reservoir, and as soon as the pressure accumulator 19 is empty, the slave actuator 22 automatically draws in additional hydraulic volume from the reservoir 4. Optionally, a non-return valve is connected between the master brake cylinder 3 and the reservoir 4, through which the hydraulic volume can be drawn in before the through-hole is released. Therefore, it is not necessary to switch the hydraulic valve 13 depending on the estimated filling volume of the pressure accumulator 19. However, if the filling level of the pressure accumulator 19 can be reliably monitored, the hydraulic valve 13 is preferably opened only when the pressure accumulator 19 has been emptied.

[0053] Due to efficiency requirements and the associated use of brake calipers of wheel brakes with a low residual friction moment and thus a large air gap at the wheels, the hydraulic volume HV stored in the pressure accumulator 19 may not be sufficient to generate the desired or requested braking pressure at the wheel brakes 8, 9, 10, 11. In order to still achieve the desired braking torque, it is therefore preferred that only the inlet valves of the wheel brakes 8, 9 assigned to the front wheels VL, VR are opened, and as already mentioned above, the parking brake 27 at the rear wheels HL, HR is activated, so that overall a high pressure builds up quickly at the wheel brakes. This allows the motor vehicle to be braked hydraulically and electromechanically at maximum deceleration, for example, depending on its current weight and possible brake pressure, even if the main actuator 2 has failed. In particular, if the wheel brakes 8 to 11 of the front or rear axle are not yet in the ABS control state, the still missing hydraulic volume can be sucked in from the reservoir 4 as soon as the through-holes of the master brake cylinder 3 are opened, so that ABS control is carried out on both axles of the motor vehicle.

[0054] If the motor vehicle is braked to a standstill by the above method in step S21, the current temperature T of the hydraulic medium is determined again in step S22 and compared with the limit value TG. If the limit value TG(j) is again or permanently lowered, and if the motor vehicle is accelerated again in the subsequent step S23 or the driver releases the accelerator pedal in step S24, the brake system 1 is pre-adjusted in step S25 by sucking hydraulic volume from the reservoir 4 by means of the slave actuator 22 and moving the hydraulic volume into the corresponding wheel brake 8-11, in particular to close the air gap of the corresponding wheel brake 8-11.

[0055] If a new braking request is detected in the subsequent step S26, it is determined in the subsequent step S27 whether the braking request is normal braking or panic braking or emergency braking, in particular based on the detected operating force or operating speed of the brake pedal 25. If this is normal braking (n), step S18 is executed again. However, if emergency braking (j) is detected, step S20 is executed again. Once it is determined in another step S28 that the vehicle is in a parking state again, the process of step S22 is repeated.

Claims

1. A method for operating a hydraulic brake system (1) of a motor vehicle, wherein: The brake system (1) comprises a hydraulic main actuator (2), the main actuator comprising a brake master cylinder (3) and an actuator (5) which can be controlled to operate the brake master cylinder (3), the brake system further comprising at least one brake circuit (6, 7) which is hydraulically connected or can be hydraulically connected to the brake master cylinder (3), the brake circuit comprising at least one wheel brake (8-11) which can be hydraulically actuated, wherein the at least one brake circuit (6, 7) comprises at least one slave actuator (22) and at least one controllable inlet valve (15, 16) and at least one controllable outlet valve (17, 18) associated with the respective wheel brake (8-11), wherein the slave actuator (22) can be controlled to convey a hydraulic volume in the direction of the inlet valve (15, 16), and wherein the at least one brake circuit (6, 7) comprises a hydraulic pressure accumulator (19) connected downstream of the outlet valve (17, 18), characterized in that the following steps are performed in sequence each time the motor vehicle is started: a) determining the current temperature (T) of the hydraulic medium and comparing the determined temperature (T) with a predetermined limit value (TG) and only if the determined temperature (T) is below said limit value (TG) b) controlling the main actuator (2) and the at least one brake circuit (6, 7) so that hydraulic medium is supplied to the at least one brake circuit (6, 7) until the hydraulic medium fills the pressure accumulator (19) to a predetermined extent, and then c) monitoring whether the main actuator (2) and / or the brake master cylinder (3) are functioning and only stopping the main actuator (2) when a failure of the main actuator (2) is detected; d) controlling the slave actuator (22) so that the slave actuator (22) delivers hydraulic medium from the pressure accumulator to the at least one brake circuit (6, 7) in the direction of the inlet valve (16, 17).

2. The method according to claim 1, characterized in that In step b), the master actuator (2) is controlled in such a way that a predetermined hydraulic volume is delivered from the master brake cylinder (3) into the pressure accumulator.

3. The method according to claim 1, characterized in that In step b), the pressure accumulator (19) is first completely filled and then emptied to a predetermined extent, in particular by actuating the slave actuator (22).

4. The method according to claim 1, characterized in that: In step d), simple braking or emergency braking is performed according to the detected braking request.

5. The method according to claim 4, characterized in that The brake system (1) has at least two wheel brakes (8, 9) for the front wheels (VL, VR) of the motor vehicle and at least two wheel brakes (10, 11) for the rear wheels (HL, HR) of the motor vehicle, and for simple braking, the hydraulic volume is delivered to all wheel brakes (8-11) of the brake system (1) via the slave actuator (22).

6. The method according to claim 4, characterized in that The brake system (1) has at least two wheel brakes (8, 9) for the front wheels (VL, VR) of the motor vehicle and at least two wheel brakes (10, 11) for the rear wheels (HL, HR) of the motor vehicle, and for simple braking, the hydraulic volume is delivered via the slave actuator (22) only to the wheel brakes (8, 9) for the front wheels (VL, VR).

7. The method according to any one of the preceding claims, characterized in that In step d), an electromechanical parking brake (27) located at least at the rear wheels (HL, HR) of the motor vehicle is additionally activated.

8. The method according to any one of the preceding claims, characterized in that In step d), at least one switching valve (12) connected between the at least one brake circuit (6, 7) and the master brake cylinder (3) and associated with the slave actuator (22) on the pressure side is closed.

9. The method according to any one of the preceding claims, characterized in that In step d), at least one hydraulic valve (13) connected between the at least one brake circuit (6, 7) and the master brake cylinder (3) and associated with the slave actuator (22) on the suction side is opened.

10. The method according to claim 9, characterized in that The hydraulic valve (13) is not opened until the hydraulic volume stored in the pressure accumulator (19) has been delivered to the at least one brake circuit (6, 7) via the slave actuator (22).

11. The method according to any one of the preceding claims, characterized in that The brake system (1) has at least two brake circuits (6, 7) which are constructed similarly to the at least one brake circuit (6, 7) and in step b) each of the two brake circuits (6, 7) fills one or only one of the pressure accumulators (19) to a predetermined level.

12. The method according to any one of the preceding claims, characterized in that If the motor vehicle is in a stopped state after executing step d), the current temperature (T) is detected again, and when the temperature (T) is lower than the limit value (TG), step b) is executed again.

13. The method according to any one of the preceding claims, characterized in that After step d) has been performed, hydraulic volume is sucked from a reservoir (4) associated with the master brake cylinder (3) by means of the slave actuator (22) and supplied to the wheel brakes (8-11) for pre-adjusting the wheel brakes.

14. A device for operating a hydraulic brake system (1) of a motor vehicle, the device comprising a control unit (24) which is specially designed to carry out the method according to claim 1 when used as intended.

15. A braking system (1) for a motor vehicle, wherein: The brake system (1) comprises a hydraulic main actuator (2) having a master brake cylinder (3) and an actuator (5) which can be controlled to operate the master brake cylinder (3), the brake system also comprising at least one brake circuit (6, 7) which is hydraulically connected or can be hydraulically connected to the master brake cylinder (3), the brake circuit comprising at least one wheel brake (8-11) which can be hydraulically actuated, wherein the brake circuit (6, 7) comprises at least one slave actuator (22) and at least one controllable inlet valve (15, 16) and at least one controllable outlet valve (17, 18) associated with the respective wheel brake (8-11), wherein the slave actuator (22) can be controlled to convey a hydraulic volume in the direction of the inlet valve (15, 16), and wherein the brake circuit (6, 7) comprises a hydraulic pressure accumulator (19) which is connected downstream of the outlet valve (17, 18), characterised in that the brake system comprises a device according to claim 14.

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