Method for operating a hydraulic brake system of a motor vehicle and hydraulic brake system

Abstract

In a method according to the invention for operating a hydraulic brake system of a motor vehicle, when a brake pedal of the motor vehicle is actuated to generate a brake pressure p TMC in a master brake cylinder, a specified minimum remaining actuation travel s is undershot trigger of the brake pedal is detected and a pressure-generating device is activated to build up additional brake pressure in or in addition to the master brake cylinder. The pressure-generating device is only activated if an ABS device of the motor vehicle does not intervene. The invention also relates to a hydraulic braking system of a motor vehicle.

Description

[0001] The invention relates to a method for operating a hydraulic braking system of a motor vehicle and a hydraulic braking system of a motor vehicle.

[0002] Today's motor vehicle braking systems are generally designed as hydraulic brake systems, mostly as dual-circuit brake systems. A motor vehicle's hydraulic brake system features a master brake cylinder in which, by displacing a piston connected to a brake pedal, pressure is generated in a pressure chamber filled with brake fluid. This pressure is transmitted via hydraulic lines to at least one wheel brake cylinder. Due to the pressure of the brake fluid, the wheel brake cylinder brings brake pads resting on brake shoes, for example, into contact with a brake drum or brake disc. In this way, the braking force acting on the wheel connected to the brake drum or brake disc can be metered by actuating the brake pedal.The master brake cylinder is typically designed as a tandem master cylinder (TMC), which has two pressure chambers arranged one behind the other and separated by a secondary piston for pressurizing two independent brake circuits. To increase the braking force, a brake booster is usually connected to the piston rod. Depending on the brake pedal's actuation, this booster generates an additional force acting in the direction of actuation of the brake pedal, thereby moving the piston with increased force in the direction of actuation.

[0003] When the brake pedal is pressed and pressure is generated in the pressure chamber(s) of the master brake cylinder, brake fluid is displaced from the pressure chamber(s) in order to actuate the wheel brake cylinder(s).

[0004] To generate high braking force and for sensitive application of the braking force, it would therefore be desirable to have a large actuation travel for the brake pedal. On the other hand, the total available actuation travel of the brake pedal is limited by the space available for the brake pedal and the master brake cylinder. Furthermore, the operability of the brake pedal also requires a limitation of the pedal travel. The driver of the vehicle must be able to easily switch their foot from the accelerator pedal to the brake pedal in order to react quickly to an unexpected braking situation. Ultimately, the necessary actuation travel must be able to be covered sufficiently quickly, and the driver must also be able to exert a sufficiently high amount of force.

[0005] It cannot therefore always be guaranteed that a sufficiently high braking force can be generated under all conditions given the total available actuation travel of the brake pedal. For example, a situation may arise where the driver exerts a high force on the pedal to generate a high braking force, but the brake pedal is depressed so far that a stop is reached and the brake pedal approximately touches the floor panel. A further buildup of pressure in the braking system and a corresponding further increase in braking force, as would correspond to the force generated by the driver, is then no longer possible, resulting in an undesirably long braking distance.Such a situation can occur in particular when there are gas bubbles in the brake system, for example due to overheating of the brakes, or when a piston of a wheel brake cylinder has been displaced back into the cylinder, so that a considerable part of the total available pedal travel is required in order to bring the brake pads into contact with a brake disc or a brake drum.

[0006] DE 40 00 324 A1 discloses a braking system for motor vehicles in which the output volume of the master brake cylinder is designed to be sufficient for normal braking, but not for extreme braking loads. The additional brake fluid required for extreme braking loads is supplied to the two brake circuits typically provided by a feed pump, which is actuated either by a differential pressure switch or a differential pressure sensor, or by a pedal travel switch.

[0007] DE 36 19 487 A1 discloses a slip-controlled hydraulic brake system, in which an auxiliary pressure supply is activated by a travel-dependent switch actuated by a piston of the master brake cylinder to ensure a reserve volume or reserve stroke in the pressure chambers of the master brake cylinder. However, the interaction of the feed pump or the auxiliary pressure supply with an ABS device of the brake system can be problematic in terms of function and / or operability.

[0008] Further hydraulic vehicle braking systems with selective braking assistance by switchable hydraulic fluid pumps are known from DE 690 20 771 T2, DE 100 03 123 A1 and US 6 158 824 A.

[0009] An object of the present invention is to provide a method for operating a hydraulic brake system of a motor vehicle and a hydraulic brake system of a motor vehicle, wherein the aforementioned disadvantages are avoided or at least reduced.

[0010] This object is achieved by a method and by a hydraulic braking system as specified in the independent claims.

[0011] Advantageous embodiments of the invention are explained in the dependent claims.

[0012] In a method according to the invention for operating a hydraulic braking system of a motor vehicle, when a brake pedal of the motor vehicle is actuated to generate braking pressure in a master brake cylinder, it is detected whether a predetermined minimum remaining actuation travel of the brake pedal is undershot. To generate braking pressure, the brake pedal is operatively connected to a piston of the master brake cylinder, such that a force exerted on the brake pedal generates braking pressure in a pressure chamber of the master brake cylinder. The master brake cylinder is fluidly connected, in particular via a brake circuit, to at least one wheel braking device, such that a braking pressure generated in the master brake cylinder acts on the wheel braking device and triggers, for example, a pressing of brake pads against a brake disc of a disc brake corresponding to the braking pressure. This generates a braking force.The master brake cylinder can, for example, be designed as a tandem master brake cylinder (TMC) of a dual-circuit braking system. The master brake cylinder can be actuated from the brake pedal via a brake booster.

[0013] The remaining actuation travel of the brake pedal is the further actuation travel of the brake pedal that it can still travel from its current position until it reaches a stop. The stop can be caused, for example, by the brake pedal touching a floor plate or by a stop within the master brake cylinder. A minimum remaining actuation travel is determined in such a way that the corresponding remaining actuation travel indicates that the brake pedal is approaching the stop. The minimum remaining actuation travel can be fixed or specifiable. If the minimum remaining actuation travel of the brake pedal is undershot, i.e. a position of the brake pedal in which the only travel available for further actuation of the brake pedal to further build up braking pressure is less than the specified minimum remaining actuation travel, this can be detected by suitable sensors.For example, a sensor can be positioned in such a way that a signal is emitted when the minimum remaining actuation travel is exceeded, or a corresponding sensor can directly detect the distance from a stop, such as the distance from the vehicle floor. Such sensors are often already present in motor vehicles, for example, pedal angle sensors. The current remaining actuation travel can also be continuously determined and compared with the minimum remaining actuation travel. The actuation travel of the brake pedal can be measured both linearly and as an angle.

[0014] If the predetermined minimum remaining actuation travel of the brake pedal is not reached, a pressure generating device is activated according to the invention to build up additional brake pressure in or in addition to the master brake cylinder. The pressure generating device can in particular be designed as a pump or as a pressure reservoir which is connected to the master brake cylinder or to the brake circuit or is connected for activation. The additional brake pressure in or in addition to the master brake cylinder is transmitted to the at least one wheel brake device and therefore generates additional braking force. At the same time, the additional braking pressure causes an increased counterforce, which is exerted on the brake pedal via the piston of the master brake cylinder and is noticeable as an increased counterforce when the brake pedal is actuated and which counteracts further depression of the brake pedal.

[0015] According to the invention, the pressure-generating device is only activated when an ABS device of the motor vehicle does not intervene in the braking system. Since a number of conditions apply for the intervention of an anti-lock braking system (ABS), for example by controlling corresponding valves within the braking system, such as wheel slippage, this means that at least one condition for the ABS device to intervene is not met. If the ABS device intervenes in the braking system, a further increase in braking deceleration by increasing the brake pressure generated by the master brake cylinder is generally no longer possible.

[0016] By activating a pressure generating device to build up additional brake pressure in the master brake cylinder when the minimum remaining actuation travel of the brake pedal is undershot, this can prevent the brake pedal from reaching its stop and enable further pressure buildup. This can significantly reduce the braking distance and / or ensure the functionality of the braking system even in the event of gas bubbles in a brake circuit or in the event of a wheel brake cylinder piston being displaced backwards. Furthermore, by only activating the pressure generating device when an ABS device of the vehicle is not intervening in the braking system, ineffective activation of the pressure generating device can be avoided. This can also improve the operability of the braking system.

[0017] The remaining actuation travel of the brake pedal is preferably determined by calculating the difference between a predetermined maximum actuation travel, which represents the entire possible pedal travel or the maximum possible actuation travel, and the actuation travel currently traveled from a zero position of the brake pedal. For this purpose, the distance traveled from a zero position, which the brake pedal assumes without any actuation by the driver, can be recorded as a linear distance or as an angle using a travel sensor, for example. The actuation traveled can also be recorded indirectly, for example by recording the displacement of a piston in the master brake cylinder. The actuation travel covered from the zero position can be compared with the maximum actuation travel, and from this the currently remaining actuation travel until the stop is reached can be determined.As a rule, the minimum remaining actuation travel is a small part, in particular less than half, of the total possible pedal travel, so that the pressure generating device is only activated when the brake pedal comes close to the stop and there is no change in the response behavior of the braking system during braking operations in which the brake pedal is actuated less far.

[0018] According to a preferred embodiment of the invention, the motor vehicle has a driving dynamics control system, and the pressure generating device is a brake fluid pump of the driving dynamics control system. Such a driving dynamics control system generally comprises functions of an anti-lock braking system (ABS) and an electronic stability control (ESC). A driving dynamics control system generally comprises such a brake fluid pump, which serves to generate the pressure required for the driving dynamics control system to intervene in the braking system. Because the brake fluid pump of the driving dynamics control system serves as the pressure generating device, the method according to the invention can be carried out in a particularly simple manner. In particular, the method according to the invention can be embodied as a further function of the driving dynamics control system.By activating the pressure generating device only when an ABS device of the motor vehicle does not intervene in the braking system, any impairment of the function of the ABS device and, for example, an overload of the pressure generating device can be avoided.

[0019] According to the invention, it is further provided that the pressure-generating device is only activated when a current deceleration of the motor vehicle exceeds a predetermined minimum deceleration. The deceleration of the motor vehicle is the magnitude of a negative acceleration of the motor vehicle, in particular a longitudinal acceleration, whereby a forward-directed component of gravitational acceleration due to a gradient is equivalent to a deceleration and is also understood as such here. The deceleration can be determined, for example, by appropriate sensors; such sensors are often already present in motor vehicles as part of other systems, such as passive safety systems or vehicle dynamics control, such as wheel speed sensors.Because the pressure generating device is only activated when a predetermined threshold for the current deceleration is exceeded, it can be achieved that the response behavior of the braking system remains unchanged during slight decelerations, such as those that occur during normal operation of a motor vehicle; only in exceptional cases, such as emergency braking, does the additional increase in braking pressure according to the invention occur.

[0020] Accordingly, it is preferred that the pressure-generating device be activated only when the current brake pressure exceeds a predetermined minimum brake pressure. Since the risk of reaching the stop only exists when the driver exerts a high force on the brake pedal, this also further ensures that no change in the brake system's response behavior to which the driver is accustomed occurs during normal operation of the motor vehicle.

[0021] The minimum deceleration and / or the minimum brake pressure can be fixed or specifiable.

[0022] According to a preferred embodiment of the invention, the brake pressure is increased at most to a pressure that depends on the current actuation travel of the brake pedal. For this purpose, for example, the further actuation travel after activation of the pressure generating device can be detected and an additional pressure that increases in accordance with the further actuation travel, for example linearly or progressively increasing, can be generated in or in addition to the master brake cylinder and in the brake circuit. Appropriate sensor means can be provided to detect the further actuation travel. In particular, a travel sensor can be arranged to detect the travel traveled by the brake pedal from a zero position, which the brake pedal assumes without actuation by the driver, as a linear travel or as an angle, wherein the travel still traveled after the minimum remaining actuation travel is undershot is also detected.The remaining actuation travel can also be measured indirectly, for example, by measuring the displacement of a piston in the master brake cylinder. The generated additional brake pressure is then only increased until a brake pressure value dependent on the current actuation travel of the brake pedal is reached. This improves the operability of the braking system and enables the generation of braking force that can be controlled by the driver by pressing the brake pedal, even within the remaining actuation travel.

[0023] Preferably, the pressure generating device is controlled such that a counterforce, generated in particular by the master brake cylinder and acting on the brake pedal counter to the actuation, continuously increases with further actuation of the brake pedal. For this purpose, for example, the further actuation travel after activation of the pressure generating device can be detected, and an additional pressure increasing in accordance with the further actuation travel, for example, increasing linearly or progressively, can be generated in or in addition to the master brake cylinder and the brake circuit. Particularly preferably, the additional brake pressure generated and, likewise, the counterforce continuously increase with further actuation of the brake pedal. This further improves the operability of the braking system.

[0024] A hydraulic braking system of a motor vehicle according to the invention comprises a master brake cylinder which can be actuated by a brake pedal to generate a braking pressure, at least one wheel braking device which is in fluid communication with the master brake cylinder, sensor means for detecting when a predetermined minimum remaining actuation travel of the brake pedal is undershot, and a pressure generating device for building up an additional braking pressure in or in addition to the master brake cylinder, wherein the braking system is designed to activate the pressure generating device when the minimum remaining actuation travel is undershot and wherein the pressure generating device is only activated when an ABS device of the motor vehicle does not intervene.The braking system may comprise or be connected to a control device which is configured to detect a signal from the sensor means for detecting a shortfall in the minimum remaining actuation travel and to actuate the pressure generating device accordingly.

[0025] Furthermore, a hydraulic brake system according to the invention comprises an acceleration sensor for determining a current deceleration, wherein the brake system is designed to activate the pressure generating device only when the current deceleration d of the motor vehicle determined by the acceleration sensor exceeds a predetermined minimum deceleration d thres exceeds.

[0026] In particular, a hydraulic braking system according to the invention is designed to carry out the method described above.

[0027] The invention is explained in more detail below with reference to the drawings. They show: Fig. 1 a simplified flow diagram of a first embodiment of a method according to the invention, and Fig. 2 a simplified flow diagram of a second embodiment of a method according to the invention.

[0028] As in Fig. 1, according to a first embodiment of the invention, when the brake pedal of a motor vehicle is actuated, a residual actuation travel is determined, for example by detecting a position of the brake pedal or an actuation travel s traveled from a zero position using a position or travel sensor. By comparison with a predetermined total actuation travel s max , which is available for the operation of the brake pedal starting from a zero position, a currently remaining residual actuation travel s restof the brake pedal until a stop is reached, approximately until the brake pedal touches the floor panel: srest=smax−s.

[0029] If the current remaining actuation travel s rest is smaller than a predetermined or specifiable minimum remaining actuation travel s trigger , srest <strigger, the further steps of the procedure are triggered; otherwise the current remaining actuation path s is cyclically repeated rest and with the minimum remaining actuation travel s trigger If the brake pedal is released and returns to its zero position, which can also be determined based on the signal from the position or distance sensor, the process is terminated and only restarted when the brake pedal is pressed again (in Fig. 1 not shown).

[0030] If the current remaining actuation path s determined in the manner describedrest smaller than the minimum remaining actuation travel s trigger , it is checked whether the brake fluid pressure p TMC in the master brake cylinder, which can be detected by a pressure sensor provided as part of a vehicle dynamics control system, a predetermined threshold value p thres of pressure is reached or exceeded. If this is not the case, the procedure for determining the remaining actuation travel s rest back.

[0031] If the specified threshold value p thres reached or exceeded: pTMC≥pthres, a current deceleration value d, which is determined by an acceleration sensor and can be available in the context of an information system of the motor vehicle, is compared with a predetermined threshold value d thresthe deceleration. The deceleration d is measured in the longitudinal direction of the vehicle, so that, for example, a deceleration is recorded even when driving at a constant speed on a downhill stretch. If the deceleration d is below the threshold value d thres no intervention to increase the brake pressure is necessary, so that the procedure for determining the remaining actuation travel s rest returns.

[0032] If the threshold d thres currently reached or exceeded: d≥dthres, This determines whether an ABS system or an ABS function of a vehicle dynamics control system is currently intervening in the braking system. This can be determined, for example, using a data connection between a control device and a control device of the vehicle dynamics control system. Preferably, the control device of the vehicle dynamics control system itself is designed to implement the method according to the invention. If an ABS system is already intervening, the maximum possible braking deceleration under the existing driving conditions and the brake pressure required for this have generally already been reached, so that a further increase in the brake pressure would not be advisable.

[0033] If ABS intervention has not yet occurred, a brake fluid pump is activated to build up additional brake pressure in or in addition to the master brake cylinder or in the brake circuit(s). This both increases the braking effect and exerts increased counterpressure on the brake pedal. Further depressing of the brake pedal is thus made more difficult, preventing the brake pedal from reaching its stop.

[0034] If the brake pressure has been increased to the maximum possible braking pressure without locking the wheels, i.e., until the ABS system intervenes, the procedure is terminated. This ensures maximum braking deceleration and a minimum braking distance under the given driving conditions when the aforementioned conditions are met. It can also be provided that, when the aforementioned conditions are met, the brake pressure is not always increased until the ABS system intervention conditions are reached, but only to a lower value, which depends, for example, on the current position of the brake pedal and thus on the currently available remaining actuation travel s rest can depend (in Fig. 1 not shown). This allows the achievable brake pressure to remain controllable by further actuation of the brake pedal, thereby improving the operability of the braking system.

[0035] According to the Fig. In the embodiment of the invention schematically illustrated in Figure 2, no check is carried out to determine whether the brake pressure exceeds a threshold pressure; otherwise, the method runs as in Fig. 1. Therefore, if the other conditions are met, additional brake pressure is built up even at a lower pressure p TMC in the master brake cylinder. This ensures the buildup of sufficient brake pressure and thus proper functioning of the braking system even if gas bubbles have formed in a brake circuit, for example due to brake overheating. The same applies if a piston in a wheel brake cylinder has been pushed back into the cylinder and must travel a greater distance before braking force can be generated again.

Claims

[1] Method for operating a hydraulic brake system of a motor vehicle, wherein upon actuation of a brake pedal of the motor vehicle to generate a brake pressure p TMC in a master brake cylinder, a specified minimum remaining actuation travel s is undershot trigger of the brake pedal is detected and a pressure generating device is activated to build up additional brake pressure in or in addition to the master brake cylinder, and the pressure generating device is only activated if an ABS device of the motor vehicle does not intervene, characterized by , that the pressure generating device is only activated if a current deceleration d of the motor vehicle determined by an acceleration sensor exceeds a predetermined minimum deceleration d thres exceeds. [2] Method according to claim 1, characterized bythat the motor vehicle has a driving dynamics control system and the pressure generating device is a brake fluid pump of the driving dynamics control system. [3] Method according to one of the preceding claims, characterized by that the pressure generating device is only activated when a current brake pressure p TMC a specified minimum brake pressure p thres exceeds. [4] Method according to one of the preceding claims, characterized by that the additional brake pressure is increased up to a value which depends on the actuation travel s currently travelled from a zero position of the brake pedal. [5] Method according to one of the preceding claims, characterized by that a further actuation path of the brake pedal is detected after activation of the pressure generating device and an additional brake pressure is generated which increases in accordance with the further actuation path. [6] Method according to one of the preceding claims, characterized by that the pressure generating device is controlled in such a way that a counterforce acting on the brake pedal against the actuation increases continuously with further actuation of the brake pedal. [7] Hydraulic braking system of a motor vehicle with a master brake cylinder actuated by a brake pedal to generate a braking pressure p TMC can be actuated, at least one wheel brake device in fluid communication with the master brake cylinder, sensor means for detecting a shortfall of a predetermined minimum remaining actuation travel s trigger of the brake pedal and a pressure generating device for building up additional brake pressure in or in addition to the master brake cylinder, wherein the brake system is designed to activate the pressure generating device when the minimum remaining actuation travel s triggerand the braking system is designed such that the pressure generating device is only activated if an ABS device of the motor vehicle does not intervene, characterized by that an acceleration sensor is provided for determining a current deceleration and the braking system is designed to activate the pressure generating device only when the current deceleration d of the motor vehicle determined by the acceleration sensor exceeds a predetermined minimum deceleration d thres exceeds.

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