Method for operating a brake-by-wire braking system and brake-by-wire braking system for a motor vehicle

Abstract

The invention relates to a method for operating a brake-by-wire braking system (1) for a motor vehicle and a brake-by-wire braking system (1) for a motor vehicle, wherein a brake torque is verified by a pressure generating device (4) on the basis of a measured motor current, and / or wherein a brake pressure measured by a pressure sensor (7) in the brake circuit (2a, 2b) is validated on the basis of the measured motor current.

Description

The invention relates to a method for operating a brake-by-wire brake system for a motor vehicle and to a brake-by-wire brake system for a motor vehicle.Brake systems for motor vehicles, which have at least one brake circuit which is designed as an electro-hydraulic brake circuit, are sufficiently known from the prior art. Such brake systems having an electrohydraulic brake circuit are also referred to as brake-by-wire brake systems. In such a brake system, the actual brake triggering device, for example a brake pedal, is generally decoupled from the electro-hydraulic brake circuit, at least when the brake system is operated in a brake-by-wire operating mode. In the brake-by-wire operating mode, for example, the actuation of the brake pedal is detected via a sensor arrangement, the desired braking torque or the desired braking effect is determined on the basis of the output signals of the sensor data, and a pressure supply device of the electro-hydraulic brake circuit is then actuated in such a way that the corresponding brake pressure is generated in the brake circuit.Frequently, in connection with a brake-by-wire brake system, the term "one-box" or the term "one-box" is also used, in particular when an ESP hydraulic module and the brake force generation are located in a device.A brake-by-wire brake system is known from DE 10 2013 216 477 A1. DE 20 2015 008 975 U1 likewise discloses a brake system of this type. DE 10 2018 133 218 A1 discloses a method for operating a brake-by-wire brake system having the features of the preamble of claim 1.In order to operate a brake system in an optimum manner, the knowledge of the dead volume in the brake circuit is of particular importance or the knowledge of when the wheel brake actually brings about a braking torque insofar as, for example, the brake lining bears against a brake disc or drum is of particular importance. Until the point in time at which the brake linings bear against the brake disc, no braking torque or only a very low braking torque is namely generated by the wheel brake. The distance of the brake pads from the brake disk in the unactuated state is also referred to as the clearance and results from the fact that a gap is present between the brake pad or the brake shoe and the brake disk. A particular problem with regard to the clearance results from the fact that during operation of the vehicle, disk tilting can occur, as a result of which the brake pads are pressed away from the brake disk. This tip-out is not necessarily constant during operation of the vehicle and may depend on the specific driving situation. For example, when cornering, there may be a different clearance than when traveling straight ahead. Accordingly, the instantaneous air clearance may depend on the driving situation. In addition, the clearance can also change during operation of the motor vehicle due to other influences, for example a transverse acceleration, wear of the brake linings and / or wear of the brake disks. In addition, return springs are frequently used for reducing consumption in conventional and electric vehicles in order to keep the residual friction torque as low as possible, since this has a positive effect on the consumption of the vehicle.A consequence of the clearance is a so-called dead volume, which must first be displaced in the brake circuit before the brake pads bear against the brake disc and a braking action is initiated. The so-called dead volume can also be influenced by other factors known from practice. Knowing the currently present dead volume in the brake circuit is, however, decisive for the actually applied braking action, since as a rule the determination or calculation of the applied braking torque is determined by means of a pressure-volume characteristic curve, also referred to as pV characteristic curve, via the displaced volume. As a rule, the brake pressure is calculated via the pV characteristic curve on the basis of the known displaced volume and this brake pressure is multiplied by a specific brake coefficient of friction in order to calculate the brake torque. The brake pressure is thus ascertained via the displaced volume in the brake circuit. It is problematic here that the dead volume can change or the currently present dead volume is generally not exactly known. As a rule, a pressure sensor is therefore provided in the brake circuit, this pressure sensor serving for measuring the brake pressure in the brake circuit.As a rule, the dead volume, caused by the air clearance, is determined for different operating conditions of the brake system in series of tests. These operating conditions include, for example, the operating temperature and the lateral acceleration. The tables, characteristic curves etc. determined are stored in a control device of the brake system. A disadvantage of such a method which accesses such characteristic curves or tables is that a currently present dead volume is not actually taken into account, as a result of which the braking action only approximately corresponds to the desired braking action. Although, as already explained, one or more pressure sensors are generally arranged in the brake circuit in order to determine the actually present brake pressure, these pressure sensors have the disadvantage that they have a certain inertia and are in particular also sensitive to back-up pressures which can occur in the brake circuit during the process of braking or pressure build-up. As a result, the measured value of the brake pressure is distorted. Moreover, malfunctions or inaccuracies can occur in certain working ranges in the pressure sensors, as a result of which there is the risk that the actually desired braking action is not achieved.It is an object of the present invention to specify a method for operating a brake-by-wire brake system and to specify a brake-by-wire brake system which overcomes the aforementioned disadvantages.These objects are achieved by a method having the features of claim 1 and a brake system having the features of claim 8.In the method according to the invention for operating a brake-by-wire brake system, it is provided that the brake system has at least one brake circuit, the brake circuit being designed as an electro-hydraulic brake circuit. This brake circuit is coupled to at least one wheel brake for generating a braking torque, wherein a brake element of the wheel brake is hydraulically coupled to the brake circuit. The brake circuit has an electrically driven pressure supply device for building up pressure in the brake circuit, for the purpose of actuating the wheel brake. The wheel brake can be, for example, a disc brake, wherein the brake element is formed by one or more brake pads which are pressed against a brake disc for the purpose of initiating a braking action. The pressure supply device has an electromotive drive and has a current sensor for measuring a motor current flowing through the drive.It is provided that it is verified on the basis of the measured motor current whether a braking torque is generated by the wheel brakes.The verification as to whether a braking torque is generated by the wheel brakes is based on the basic idea that, if only dead volume is displaced in the brake circuit, thus no braking torque is yet generated by the wheel brake, the force required for the application of the wheel brake is relatively small and therefore the motor current absorbed by the drive is also relatively small. It is assumed that, in the case of a stable on-board power supply system, the motor current absorbed by the drive is proportional to the force to be applied by the drive. As soon as a braking torque is generated by the wheel brake, for example when the brake pads are in contact with the brake disc, the motor current absorbed by the drive will increase abruptly at this torque. It can thus be verified on the basis of the measured motor current whether a braking torque is generated by the wheel brake. This can be done, for example, using software stored in an evaluation device, which evaluates the output signals of the current sensor. As a result, it can be verified via the measured motor current that a braking force is actually applied or that the brake pads bear against the brake disc. As soon as it is verified that a braking force is applied, it can be assumed that any further displacement of volume in the brake circuit leads to an increase in the pressure in the brake circuit, since any dead volume has already been displaced. Accordingly, it can be concluded by the motor current recorded whether the dead volume has already been shifted or the dead volume has already been in a linear or approximately linear range of the pV characteristic curve and accordingly the braking force or the brake pressure increases linearly or approximately linearly with the additionally shifted volume with further volume shifting.It is preferably provided that a brake pressure in the brake circuit measured by a pressure sensor is checked for plausibility on the basis of the measured motor current, namely by the behavior of the current consumption of the drive of the pressure supply device, since with increasing brake pressure the current required for the provision of the corresponding brake pressure also increases, since the drive has to apply more force with increasing brake pressure. The signal of the pressure sensor can therefore be checked for plausibility via the behavior of the current consumption.It is considered to be particularly advantageous if the verification of the generation of a braking torque by the wheel brake takes place on the basis of a motor current characteristic curve. The motor current characteristic curve can be determined beforehand by series of experiments and stored in a control device or an evaluation device or a memory assigned to these components.It is also considered to be particularly advantageous if the plausibility check of the brake pressure measured by the pressure sensor takes place based on a motor current-brake pressure characteristic curve. Such a characteristic curve can also be determined beforehand in test series and stored in a control device or an evaluation device or an associated memory unit. The advantage when using motor current characteristic curves or motor current-brake pressure characteristic curves is that these characteristic curves are independent or substantially independent of the dead volume currently present in the brake circuit and thus also independent or substantially independent of a currently present air clearance.In a preferred embodiment, it is provided that a brake pressure is calculated using the measured motor current on the basis of the motor current-brake pressure characteristic curve, deviations of the brake pressure measured by the pressure sensor and of the calculated brake pressure being determined by forming differences and it being assessed on the basis of the magnitude of the deviation whether the measured brake pressure is plausible or not. If the deviations lie, for example, in the range of 10% of the calculated value, the pressure sensor can be considered valid.In a preferred embodiment, it is provided that the pressure supply device is designed as a piston-cylinder system, in particular as a piston pump.The brake-by-wire brake system preferably has a brake pedal that can be actuated by a driver of the motor vehicle, the brake system having one or more sensors for measuring a force acting on the brake pedal, a control device being provided, the control device being configured to actuate the pressure supply device of the electro-hydraulic brake circuit as a function of a force applied to the brake pedal.It is quite conceivable for the brake system to have a pedal force simulator, wherein the pedal force simulator interacts with the brake pedals that can be actuated by the driver of the motor vehicle. The pedal force simulator is used to generate a brake feel customary for the driver of the motor vehicle.It is quite conceivable for a hydraulic circuit to be provided, wherein the brake pedal is coupled to the hydraulic circuit. The hydraulic circuit can be coupled to the brake circuit if required in order to cooperate directly with the brake circuit, for example in the event of a failure of the pressure supply device. This can be effected, for example, by means of one or more valves which are arranged between the hydraulic circuit and the brake circuit.It is provided that, when a specific braking torque is requested by a driver of the vehicle, a setpoint displacement volume is determined based on a pressure-volume characteristic curve of the brake circuit, wherein the setpoint displacement volume corresponds to the volume which has to be displaced in addition to any dead volume in order to achieve the desired braking torque. The advantage is that it is not necessary to store a multiplicity of pressure-volume characteristic curves for different operating conditions of the vehicle in order to take account of different dead volumes in the respective operating condition of the vehicle, but only a few, ideally a single, pressure-volume characteristic curve must be stored, from which the desired displacement volume can be determined independently of the dead volume, thus virtually at a dead volume of 0 m 3.Therefore, it is provided that any dead volume is ignored in the pressure-volume characteristic curve. Due to the possible verification of the contact of the brake pads on the brake disc or the verification of the generation of a braking torque, no dead volume is present any longer in the brake circuit at this torque, so that a further displacement of volume leads to an increase in the braking pressure and thus to an increase in the braking torque. As soon as the verification has taken place, the pressure supply device is activated in such a way that it still shifts the desired displacement volume determined from the pressure-volume characteristic curve.In the brake-by-wire brake system according to the invention, it is provided that the brake system has at least one brake circuit, wherein the brake circuit is designed as an electro-hydraulic brake circuit, wherein the brake circuit is coupled to at least one wheel brake for generating a braking torque, wherein a brake element of the wheel brake is hydraulically coupled to the brake circuit, wherein the brake circuit has an electrically driven pressure supply device for building up pressure in the brake circuit for the purpose of actuating the wheel brake. The pressure supply device has an electromotive drive and also has a current sensor for measuring a motor current flowing through the drive. The brake system preferably also has a pressure sensor for measuring a brake pressure in the brake circuit. The brake system has a control unit, wherein the control unit is configured to carry out the method according to the invention or one of the aforementioned embodiments of the method. The control unit is thus configured to verify, on the basis of the detected motor current, whether a braking torque is generated by the wheel brake, and / or the control unit is configured to plausibilize the brake pressure measured by the pressure sensor on the basis of the measured motor current.The explanations relating to the method according to the invention and the embodiments of the method apply correspondingly to the brake system according to the invention and vice versa.In the following figures, the invention is explained in more detail on the basis of exemplary embodiments, without being restricted to these.The following are shown: FIG. 1 shows a schematic illustration of a brake-by-wire brake system as is known from practice, FIG. 2 shows a schematic illustration of a control unit for carrying out the method, FIG. 3 shows a pressure-volume characteristic curve as used in the prior art.FIG. 1 shows a brake system 1 which is designed as a brake-by-wire brake system, as is known from practice. Since the components of a brake-by-wire brake system illustrated in FIG. 1 are known, only the components of the brake system 1 relevant to the present invention will be discussed in more detail below.The brake system 1 has a first electro-hydraulic brake circuit 2 aand a second electro-hydraulic brake circuit 2 b, each of these brake circuits 2 a, 2 bbeing coupled to two wheel brakes 3. The brake circuit 2 btherefore supplies a wheel brake 3 which is assigned to the left front wheel and a wheel brake 3 which is assigned to the right rear wheel. The brake circuit 2 a supplies a wheel brake 3 which is associated with the left rear wheel and a further wheel brake 3 which is associated with the right front wheel. Accordingly, the respective brake circuit 2 a, 2 bfunctions to brake a front wheel and a diagonally opposite rear wheel. This is thus a so-called X brake circuit distribution. Such a division is not absolutely necessary. Other braking force distributions, in particular a wheel-specific distribution, are also conceivable.The brake circuits 2 a, 2 bhave a common electrically driven pressure supply device 4, which is designed in the present case in the form of a piston-cylinder system. Via switchable valves 19, brake circuits 2 a, 2 bmay be selectively coupled to pressure supply device 4. By displacing the piston by means of an electromotive spindle drive 5, a volume, namely brake fluid, is displaced in the brake circuits 2 a, 2 bto actuate the wheel brakes 3 during brake-by-wire operation of the brake system 1. the pressure supply device 4 in this respect serves for the pressure build-up and thus for the generation of a brake pressure in the respective brake circuit 2 a, 2 b, wherein the actual brake pressure is only built up when a dead volume has been displaced in the respective brake circuit 2 a, 2 b. Only when the dead volume has been moved is the clearance overcome and the brake pads of the wheel brake 3 bear against a brake disc of the wheel brake 3. Only at this point in time is an actual or significant braking torque generated by the respective wheel brake 3 with further displacement of brake fluid in the brake circuit 2 a, 2 b. Even if a spindle drive is used in the embodiment shown, other drives are also conceivable in order to displace the piston in order to build up brake pressure therewith.The pressure supply device 4 has a current sensor 6 for measuring a motor current flowing through the drive 5. The greater the force opposing the displacement of the volume by means of the pressure supply device 4, the higher the motor current absorbed by the electric motor drive 5. Accordingly, the motor current necessary for displacing the dead volume is smaller than the motor current necessary for displacing the volume or brake fluid in addition to the dead volume. Therefore, it can be verified on the basis of the measured motor current whether or not a braking torque is generated by the wheel brakes 3. It is assumed that the measured motor current increases more or less abruptly following the displacement of the dead volume. On the basis of the sudden increase, it is therefore possible to verify when the dead volume has been displaced and when further displacement of volume actually contributes to the increase in the brake pressure. The measured motor current can be, for example, a phase current.The brake system 1 also has a pressure sensor 7 for measuring a brake pressure in the brake circuit 2 a, 2 b. The pressure sensor 7 is arranged between the valves 19 and the pressure supply device 4. On the basis of the measured motor current, the brake pressure measured by the pressure sensor 7 can be checked for plausibility, since the motor current likewise increases with increasing brake pressure.The brake system 1 further comprises a hydraulic circuit 9, wherein the hydraulic circuit 9 interacts with a brake pedal 10 that can be actuated by a driver of the motor vehicle. Further, a pedal force simulator 11 is coupled to the hydraulic circuit 9. The hydraulic circuit 9 is coupled to the brake circuits 2 a, 2 bvia switchable valves 17, wherein the valves 17 are closed in a brake-by-wire operating mode and are transferred into an open position, for example, in the event of a failure of the pressure supply device 4, in order to enable purely hydraulic braking in this case.Sensors 13 are arranged in the region of the brake pedal 10, these sensors 13 serving for determining the force exerted on the brake pedal 10. The signals of the sensors 13 in turn serve as input signals for a control unit 12, wherein this control unit 12 in turn serves to actuate the drive 5 of the pressure supply device 4. The control unit 12 is configured to actuate the drive 5 of the electro-hydraulic brake circuit 2 a, 2 bdepending on the force applied to the brake pedal 10 in order to achieve the desired braking effect. The control unit 12 is furthermore configured to verify, on the basis of the measured motor current, whether a braking torque is generated by the wheel brake, and is furthermore configured to plausibilize the brake pressure detected by the pressure sensor 7 on the basis of the measured motor current. Furthermore, the control unit 12 is coupled to a sensor 8, wherein this sensor 8 serves to detect the rotational position of the spindle of the drive 5, whereby the position of the piston of the pressure supply device 4 can be determined and the displaced volume of the brake fluid located in the brake circuit 2 a, 2 bcan be calculated via the known geometry of the piston and the cylinder.It is quite conceivable that a motor current-brake pressure characteristic curve is stored in a memory unit of the control unit 12 in order to calculate the brake pressure on the basis of the measured motor current. In the control unit 12, a difference can be formed between the brake pressure measured by the pressure sensor 7 and the brake pressure calculated from the motor current, in order to evaluate, on the basis of the magnitude of the deviation, whether or not the brake pressure measured by the pressure sensor 7 is plausible.Furthermore, a pressure-volume characteristic curve of the brake circuit 2 a, 2 bmay be stored in the memory unit of the control unit 12, wherein, when a driver of the vehicle requests a specific braking torque, a setpoint displacement volume is determined based on this pressure-volume characteristic curve of the brake circuit 2 a, 2 b, wherein the setpoint displacement volume, which is determined on the basis of this pressure-volume characteristic curve, corresponds to the volume which, in addition to any dead volume, has to be displaced in order to achieve the desired braking torque. In the stored pressure-volume characteristic curve, therefore, any dead volume is not taken into account. However, this is also not necessary since the measured motor current can be used to determine the state in which there is no longer any dead volume in the brake circuit 2 a, 2 b, insofar as the brake pads bear against the brake disks of the wheel brake 3. In this state, only the determined desired displacement volume then needs to be displaced by the pressure supply device 4. As already described in the general part, this has various advantages over the methods known from practice, in which a multiplicity of pressure-volume characteristic curves for different operating states are stored, wherein the respective pressure-volume characteristic curve takes into account the dead volume present in the respective operating state. Such a pressure-volume characteristic curve as is used in the prior art is shown by way of example in FIG. 3. The graph in FIG. 3 shows three curves 14, 15, 16, curve 14 showing the pressure-volume characteristic curve for one of two front wheel brakes, curve 15 showing the pressure-volume characteristic curve for one of two rear wheel brakes, and curve 16 showing the pressure-volume characteristic curve for the entire system, curve 16 being produced by summing the corresponding values of curve 14 and curve 15 and multiplying this sum by the factor 2. The steep increase at the beginning of the pressure-volume characteristic curve shown in FIG. 3 is due to the dead volume present in brake circuit 2 a, 2 b, since this dead volume must first be displaced before a significant increase in the brake pressure takes place with a further displacement of volume. The region of the diagram shown in FIG. 3 that can be assigned to the displacement of dead volume in brake circuit 2 a, 2 bis identified by dashed line 18 and is the region that is situated to the left of dashed line 18 in the diagram. The dead volume in the case shown in FIG. 3 is approximately 5,000 mm 3. Starting from the moment at which the brake pads are applied, the dead volume has thus been displaced, the change in braking force takes place in connection with the pV characteristic curve or the curves 14, 15, 16 in the region to the right of the dashed line 18, since the stiffness or elasticity is defined or known. As can be seen from the curves 14, 15, 16, the change of the brake pressure to the right of the dashed line 18 takes place approximately linearly with the displaced volume.The braking torque provided by the respective wheel brake 3 can be calculated at least approximately by multiplying a brake characteristic value by the brake pressure. The braking characteristic value is also frequently referred to as the braking coefficient of friction.List of reference characters1 Brake system 2 a Circuit 2 b Bremskreis circuit 3 Wheel brake 4 Pressure supply device 5 Drive 6 Current sensor 7 Pressure sensor 8 Sensor 9 Hydraulic circuit 10 Brake pedal 11 Pedal force simulator 12 Control unit 13 Sensors 14 Curve 15 Curve 16 Curve 17 Valve 18 Line 19 Valve

Claims

Method for operating a brake-by-wire brake system (1) of a motor vehicle, wherein the brake system (1) has at least one brake circuit (2a, 2b), wherein the brake circuit (2a, 2b) is designed as an electro-hydraulic brake circuit, wherein the brake circuit (2a, 2b) is coupled to at least one wheel brake (3) for generating braking torque, wherein a brake element of the wheel brake (3) is hydraulically coupled to the brake circuit (2a, 2b), wherein the brake circuit (2a, 2b) has an electrically driven pressure supply device (4) for building up pressure in the brake circuit (2a, 2b) for the purpose of actuating the wheel brake (3), wherein the pressure supply device (4) has an electric motor drive (5) and has a current sensor (6) for measuring a motor current flowing through the drive (5), characterized in that it is verified on the basis of the measured motor current whether a braking torque is generated by the wheel brake (3), wherein, when a specific braking torque is requested by a driver of the vehicle, a setpoint displacement volume is determined on the basis of a pressure-volume characteristic curve of the brake circuit (2a, 2b), wherein the setpoint displacement volume corresponds to the volume which has to be displaced in addition to any dead volume in order to achieve the desired braking torque, wherein any dead volume is ignored in the pressure-volume characteristic curve, wherein, as soon as the verification has taken place, the pressure supply device (4) is actuated in such a way, that said sensor still displaces the desired displacement volume determined from the pressure-volume characteristic curve.Method according to Claim 1, wherein a brake pressure in the brake circuit (2a, 2b) measured by a pressure sensor (7) is checked for plausibility on the basis of the measured motor current.Method according to Claim 1 or 2, wherein the verification of the generation of the braking torque by the wheel brake (3) takes place on the basis of a motor current characteristic curve.Method according to one of Claims 1 to 3, wherein the plausibility check of the brake pressure measured by the pressure sensor (7) takes place on the basis of a motor current-brake pressure characteristic curve.Method according to Claim 4, wherein a brake pressure is calculated by means of the measured motor current on the basis of the motor current-brake pressure characteristic curve, wherein deviation of the brake pressure measured by the pressure sensor (7) and of the calculated brake pressure is determined by forming a difference and it is assessed on the basis of the magnitude of the deviation whether the measured brake pressure is plausible or not.Method according to one of Claims 1 to 5, the brake circuit (2a, 2b) being subject to a dead volume, the dead volume corresponding to the volume which has to be displaced by the wheel brake until a braking torque is generated, the brake system (1) having a sensor (8) for ascertaining a volume displaced by the pressure supply device (4), a current dead volume of the brake circuit (2a, 2b) being determined on the basis of the ascertained displaced volume and the measured motor current.Method according to one of Claims 1 to 6, wherein the pressure supply device (4) is designed as a piston-cylinder system, in particular as a piston pump.Brake-by-wire brake system (1) for a motor vehicle, wherein the brake system (1) has at least one brake circuit (2a, 2b), wherein the brake circuit (2a, 2b) is designed as an electro-hydraulic brake circuit, wherein the brake circuit (2a, 2b) is coupled to at least one wheel brake (3) for generating braking torque, wherein a brake element of the wheel brake (3) is hydraulically coupled to the brake circuit (2a, 2b), wherein the brake circuit (2a, 2b) has an electrically driven pressure supply device (4) for building up pressure in the brake circuit (2a, 2b) for the purpose of actuating the wheel brake (3), wherein the pressure supply device (4) has an electric motor drive (5) and has a current sensor (6) for measuring a motor current flowing through the drive (5), wherein the brake system (1) comprises a control unit (12), wherein the control unit (12) is configured to perform the method according to any one of claims 1 to 7.

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