Brake system and method for operating a brake system

An electrohydraulic brake system, comprising a brake master cylinder. A pressure generating device for actuating wheel brakes and an additional brake actuator may both be activated electronically. In a normal mode of operation, an open- and closed-loop control unit detects a braking demand based on the actuation of a brake pedal by the driver and activates the pressure generating device to build up braking torque at the wheel brakes. If the pressure generating device is not activated, the driver gains direct access to the wheel brakes and the control unit activates the additional brake actuator to build up braking torque. In the fallback mode, when a predetermined pedal travel threshold value is reached, the control loop activates the additional brake actuator to build up braking torque. In the event of a succession of brake pedal actuations by the driver, the pedal travel threshold value is increased at least once.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of PCT Application PCT/EP2016/072403, filed Sep. 21, 2016, which claims priority to German Application DE 10 2015 219 001.3, filed Oct. 10, 2015. The disclosures of the above applications are incorporated herein by reference.

TECHNICAL FIELD

The invention relates to an electrohydraulic brake system.

BACKGROUND

In motor vehicles “brake-by-wire” brake installations are being used ever more widely. Brake installations of this kind often have not only a brake master cylinder that can be actuated by the vehicle driver but also an electrically activatable pressure generating device (activatable “by-wire”), by means of which actuation of the wheel brakes takes place in the “brake-by-wire” operating mode.

In these brake systems, especially electrohydraulic brake systems with the “brake-by-wire” operating mode, the driver does not have direct access to the brakes. When the pedal is actuated, a pedal decoupling unit and a simulator are usually actuated, and the braking demand of the driver is detected by a sensor system. The pedal simulator is used to give the driver a brake pedal feel which is as familiar and comfortable as possible. The detected braking demand leads to the determination of a setpoint braking torque, from which the setpoint brake pressure for the brakes is then determined. The brake pressure is then built up actively in the brakes by a pressure generating device.

In the “by-wire” operating mode, the actual braking is achieved by active pressure buildup in the brake circuits with the aid of a pressure generating device, which is activated by an open- and closed-loop control unit. By virtue of the hydraulic decoupling of the brake pedal actuation from the pressure buildup, a large number of functionalities, such as ABS, ESC, TCS, slope starting assistance etc., may be implemented in a convenient manner for the driver in brake systems of this kind.

In brake systems of this kind, a hydraulic fallback mode is usually provided, by means of which the driver may brake or halt the vehicle by muscle power by actuating the brake pedal if the “by-wire” operating mode fails or is disrupted. Whereas, in the normal mode, there is the above-described hydraulic decoupling between brake pedal actuation and brake pressure buildup, by means of a pedal decoupling unit for example, this decoupling is canceled in the fallback mode, thus enabling the driver to displace brake fluid directly into the brake circuits. A transition is made to the fallback mode when, for example, it is no longer possible to build up pressure with the aid of the pressure generating device. This is the case inter alia if the relevant sensor system for the activation of the pressure generating unit fails or if the brake pressure or the piston travel, for example, has no longer been reliably detected by the sensor system.

The pressure generating device in the brake systems described above is also referred to as an actuator or hydraulic actuator. In particular, actuators are designed as linear actuators or linear units, in which a piston is moved axially into a hydraulic pressure chamber to build up pressure.

Conventional brake systems predominantly comprise an actuating unit with a vacuum brake booster, a hydraulic tandem brake master cylinder and a downstream electronically controlled modulator unit for control functions connected with driving dynamics. A central electronic open- and closed-loop control unit (ECU) processes various sensor signals and commands, which may come from sensors installed in the vehicle or from other ECUs.

At a raised functionality level, the actuating unit may also initiate braking without the driver, e.g. in the case of an active booster. The brake booster may be of electrohydraulic or electromechanical design. Current brake systems essentially consist of separate modules for pressure generation and control functions. These may be accommodated in one or more housings in the vehicle.

DE 600 30 271 T2 discloses the practice of activating the parking brake to build up additional braking torque during a braking operation in the hydraulic fallback mode of a by-wire brake installation if the electric braking mode fails. However, the parking brake is not designed for such uses and can thus be overloaded and damaged.

SUMMARY

A brake system is assisted in getting to the fallback mode but, at the same time, the parking brake is protected.

In respect of the brake system, in the fallback mode, the open- and closed-loop control unit monitors the brake pedal travel and, when a predetermined pedal travel threshold value is reached, activates the additional brake actuator to build up braking torque, and wherein, in the event of a succession of brake pedal actuations by the driver, the pedal travel threshold value is increased at least once.

The use of an additional brake actuator, for example, the parking brake, is expedient in a hydraulic fallback mode. In the fallback mode, the driver must build up the brake pressure by muscle power and accept longer brake pedal travels. Through the activation of the additional brake actuator, the parking brake, additional braking torque is built up for the motor vehicle, and therefore the driver does not get the false impression that it is no longer possible to brake the vehicle at all or that the brake system has failed completely because of the altered braking characteristic. In addition, the vehicle is also braked more powerfully by means of the additional braking torque component.

However, the additional brake actuator should be protected, especially if it is designed as a parking brake, since it is not designed for service braking operations and may be damaged if used too intensively and may completely or partially cease to operate.

As has now been recognized, it is possible to achieve assistance for the driver while also sparing the additional brake actuator if, in a sequence of brake pedal actuations, the activation of the additional brake actuator is shifted successively toward longer brake pedal travels. As a result, the driver initially finds out that it is still possible to brake the vehicle. Owing to the ever-increasing length of the brake pedal travels, the driver furthermore learns that it is necessary to cover ever longer brake pedal travels and therefore becomes accustomed to the new situation of the fallback mode.

The wheel brakes, which may be actuated by the brake master cylinder or the pressure generating device, which are probably part of the service brake. If the pressure generating device is not activated or cannot be activated, the driver gains direct access to the wheel brakes via the brake master cylinder in the hydraulic fallback mode.

In the event of a succession of brake pedal actuations, the pedal travel threshold value is shifted toward ever longer brake pedal travels. The driver must, therefore, depress the brake pedal ever further before the additional braking torque is applied by the additional brake actuator. From this, the driver learns that the additional braking effect occurs later and later and, at some point, will presumably not occur at all.

The sequence of increased brake pedal travels may have substantially equal pedal travel intervals. This regularity and hence foreseeability gives the driver reliability and certainty in learning the new braking behavior.

After a predetermined applicable maximum number of successive activations, the additional brake actuator may be no longer activated. In the meantime, the driver has learned that they are receiving assistance only transitionally and they have become accustomed to the long brake pedal travels and the powerful actuation of the brake pedal. The additional brake actuator is then spared and protected from overloading or damage. The number of successive activations may be between 30 and 50.

The intensity of the braking torque generated by the additional actuator is substantially equal. That is to say that, the braking torque built up by the additional brake actuator is substantially the same in each activation.

The additional brake actuator may be designed as a parking brake. In this case, the parking brake performs a dual function. Therefore, it may be activated when the vehicle is parked in order to hold or secure the vehicle, or it may serve as a braking aid in the fallback mode and indicates to the driver that the vehicle may still be braked, despite an altered braking characteristic.

In the normal mode of operation, the open- and closed-loop control unit detects a braking demand on the basis of the actuation of the brake pedal by the driver and activates the pressure generating device to build up braking torque at the wheel brakes of the service brake, wherein, if the pressure generating device is not activated or cannot be activated, the driver gains direct access to the wheel brakes and the open- and closed-loop control unit activates the additional brake actuator of the parking brake to build up braking torque in the hydraulic fallback mode.

If the parking brake is of electromechanical design. In this case, different designs of the electromechanical parking brake may be used, e.g. a brake cable assembly, duo-servo actuators, integrated brake caliper actuators, pure parking brake calipers. Here, the type of brake, e.g. disk or drum brake, or the principal of action, e.g. simplex, duo-servo or combination brake caliper, is not relevant.

Many other systems may be used as an electronically activatable additional brake actuator, provided they are suitable for assisting the braking demand of the driver, e.g. eddy current brakes, hydrodynamic brakes, retarders, the action of an electric motor.

The electronically activatable parking brake may be arranged on the rear axle and comprises at least one actuator. By way of example, this may be embodied with a single-cable brake cable assembly, which acts on both parking brake calipers in a manner similar to the handbrake lever.

In respect of the method, in the fallback mode, the brake pedal travel is monitored and, when a predetermined pedal travel threshold value is reached, the additional brake actuator is activated to build up braking torque, and wherein, in the event of a succession of brake pedal actuations by the driver, the pedal travel threshold value is increased at least once.

The alteration of the activation point of the additional actuator depending on the brake pedal travel on the one hand indicates to the driver that the vehicle may be still be braked in the fallback mode and, on the other hand, also indicates that the necessary brake pedal travels have changed and that longer brake pedal travels must be covered. Particularly if the activation of the additional brake actuator is shifted toward ever longer brake pedal travels in a sequence of brake pedal actuations, the driver learns that ever-increasing brake pedal travels are becoming necessary for a braking torque of the same magnitude. A uniform increase in the necessary brake pedal travel conveys to the driver a sense that the brake installation is predictable and functional, allowing said driver to adjust to the new situation in the fallback mode. If a parking brake is used as the additional actuator, there is furthermore no need to employ another actuator.

Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the disclosure, are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.

DETAILED DESCRIPTION

In all of the figures, identical parts are denoted by the same reference designations.

FIG. 1shows aN exemplary embodiment of an electrohydraulic brake system1or brake installation. The brake system1comprises a brake master cylinder2, which may be actuated by means of an actuating or brake pedal1a, a simulation device3interacting with the brake master cylinder2, a pressure medium reservoir4under atmospheric pressure associated with the brake master cylinder2, an electrically controllable pressure generating device5, which is formed by a cylinder-piston assembly having a hydraulic pressure chamber37, the piston36of which may be moved by an electromechanical actuator comprising a motor35and a rotation/translation mechanism, an electrically controllable pressure modulation device for setting wheel-specific brake pressures and an electronic open- and closed-loop control unit12.

By way of example, the pressure modulation device (not designated specifically) comprises, for each hydraulically actuable wheel brake8,9,10,11of a motor vehicle (not shown), an inlet valve6a-6dand an outlet valve7a-7d, which are hydraulically interconnected in pairs by central ports and are connected to the wheel brakes8,9,10,11. The inlet ports of the inlet valves6a-6dare supplied by means of brake circuit supply lines13a,13bwith pressures which, in a “brake-by-wire” operating mode, are derived from a system pressure present in a system pressure line38connected to the pressure chamber37of the pressure generating device5.

A check valve50a-50dopening in the direction of the brake circuit supply lines13a,13bis connected in parallel with each of the inlet valves6a-6d. In a fallback operating mode, the brake circuit supply lines13a,13bare supplied via hydraulic lines22a,22bwith the pressures of pressure chambers17,18of the brake master cylinder2. The outlet ports of the outlet valves7a-7dare connected to the pressure medium reservoir4by a return line14b.

The brake master cylinder2has two pistons15,16arranged in series, which delimit the hydraulic pressure chambers17,18. On the one hand, the pressure chambers17,18are connected to the pressure medium reservoir4by radial holes formed in the pistons15,16and corresponding pressure compensation lines41a,41b, wherein the connections may be shut off by a relative movement of the pistons17,18. On the other hand, the pressure chambers17,18are connected to the abovementioned brake circuit supply lines13a,13bby means of the hydraulic lines22a,22b.

A normally open valve28is contained in pressure compensation line41a. The pressure chambers17,18contain return springs (not designated specifically), which position the pistons15,16in an initial position when the brake master cylinder2is not actuated. A piston rod24couples the pivoting movement of the brake pedal1aresulting from a pedal actuation with the translational movement of the first brake master cylinder piston15, the actuation path of which is detected by a displacement sensor25, which may be of redundant design. As a result, the corresponding piston displacement signal is a measure of the brake pedal actuation angle. It represents a braking demand of the vehicle driver.

Arranged in each of the line sections22a,22bconnected to the pressure chambers17,18is a respective block valve23a,23b, which is designed as an electrically actuable, preferably normally open, 2/2-way valve. By means of the block valves23a,23b, the hydraulic connection between the pressure chambers17,18of the brake master cylinder and the brake circuit supply lines13a,13bmay be shut off. A pressure sensor20of redundant design connected to line section22bdetects the pressure built up in pressure chamber18by a movement of the second piston16.

The simulation device3may be coupled hydraulically to the brake master cylinder2and, by way of example, essentially comprises a simulator chamber29, a simulator spring chamber30and a simulator piston31separating the two chambers29,30. The simulator piston31is supported on the housing21by an elastic element (e.g. a spring), which is arranged in the simulator spring chamber30and may be preloaded. The simulator chamber29may be connected to the first pressure chamber17of the brake master cylinder2by means of an electrically actuable simulator valve32. When a pedal force is input and simulator valve32is open, pressure medium flows from brake master cylinder pressure chamber17into the simulator chamber29. A check valve34arranged hydraulically antiparallel to the simulator valve32allows the pressure medium to flow back from the simulator chamber29to brake master cylinder pressure chamber17largely unhindered, irrespective of the switching state of the simulator valve32. Other embodiments and connections of the simulation device to the brake master cylinder2are conceivable.

The electrically controllable pressure generating device5is designed as a hydraulic cylinder-piston assembly or a single-circuit electrohydraulic actuator, the pressure piston36of which, which delimits the pressure chamber37, may be actuated by a schematically indicated electric motor35via a likewise schematically illustrated rotation/translational mechanism. A rotor position sensor, indicated only schematically, which serves to detect the rotor position of the electric motor35, is denoted by reference sign44. In addition, a temperature sensor for sensing the temperature of the motor winding may also be used.

The actuator pressure generated by the effect of the force of the piston36on the pressure medium enclosed in the pressure chamber37is fed into the system pressure line38and detected by means of a pressure sensor19, which is preferably of redundant design. When the connecting valves26a,26bare open, the pressure medium enters the wheel brakes8,9,10,11for the actuation thereof. Thus, when the connecting valves26a,26bare open, a wheel brake pressure buildup and reduction for all the wheel brakes8,9,10,11takes place during a normal braking operation in the “brake-by-wire” operating mode by means of the forward and backward displacement of the piston36.

During the pressure reduction, the pressure medium that has previously been displaced from the pressure chamber37into the wheel brakes8,9,10,11flows back into the pressure chamber37over the same route. During a braking operation with wheel brake pressures that differ between individual wheels and are controlled with the aid of the inlet and outlet valves6a-6d,7a-7d(e.g. during an antilock control operation (ABS control)), in contrast, the pressure medium component discharged via the outlet valves7a-7dflows into the pressure medium reservoir4and is thus initially no longer available to the pressure generating device5for the actuation of the wheel brakes8,9,10,11. Additional pressure medium may be drawn into the pressure chamber37by retraction of the piston36while the connecting valves26a,26bare closed.

By way of example, the brake installation1has a parking brake50of electromechanical design, which is an additional brake actuator. The parking brake50is designed as an electromechanical brake and is mounted on the rear axle of the vehicle. When the vehicle is parked, it is activated by the open- and closed-loop control unit12to build up braking torque to hold the vehicle.

In the normal “brake-by-wire” operating mode of the brake system1, the open- and closed-loop control unit12detects the braking demand of the driver by means of the pedal travel sensor25, which detects the brake pedal travel covered. From this, a setpoint braking torque or associated brake pressures to be set in the wheel brakes8,9,10,11are determined. The open- and closed-loop control unit12activates the pressure generating device5to build up pressure.

In a hydraulic fallback mode, to which a switch or transition is made when the pressure generating device5may no longer build up sufficient brake pressure in the wheel brakes8,9,10,11, the pressure generating device5is separated hydraulically from the wheel brakes8,9,10,11by closing the connecting valves26a,26b. To enable this to take place even when there is a failure of the onboard electrical system, the connecting valves26a,26bmay be of normally closed design. The block valves23a,23b, which may be of normally open design, are open or are opened, enabling the driver to displace brake fluid into the wheel brakes8,9,10,11by actuating the brake master cylinder2by muscle power.

During the transition from the normal “by-wire” operating mode to the hydraulic fallback mode, the braking properties of the brake system1change significantly. During this process, the brake pedal characteristic changes from “short and firm” to “long and soft”. It is now only possible for braking torque to be built up directly by muscle power by the driver. When the brake pedal is actuated, brake fluid is displaced into at least one wheel brake8,9,10,11by the movement of the pistons15,16of the brake master cylinder. For this purpose, the driver must depress the brake pedal1amore strongly than in the by-wire mode and also cover longer brake pedal travels to achieve the same braking torque as in the by-wire mode.

To ensure that, in the fallback mode, the driver does not mistakenly think that the brake system may no longer generate any braking force or that there is a total failure owing to the longer brake pedal travel and weaker braking force, the open- and closed-loop control unit12activates the parking brake50to build up braking torque in the example. The vehicle therefore brakes more sharply than it would if there were only the braking torque generated by the muscle power of the driver.

The brake system1is thus rendered capable of assisting the driver in this way in the fallback mode. At the same time, however, the parking brake is protected from overloading since it is not designed for continuous braking.

For this purpose, the open- and closed-loop control unit12varies the pedal travel threshold value at which the parking brake is activated in the event of successive brake pedal actuations. In the one embodiment, this variation takes place in such a way that the pedal travel threshold value selected is longer in successive brake pedal actuations. Thus, the braking torque of the parking brake50is built up with ever longer brake pedal travels. In this case, the maximum braking torque built up by the parking brake50is not varied. The respective braking torque applied by the parking brake50is between 0.25 and 0.4 g on a high friction coefficient, while the specific value depends on the instantaneous or dynamic axle load distribution and the characteristics of the roadway.

The way in which the parking brake50is activated according to the example and what effects the activation thereof has on the braking process of the vehicle are explained below with reference to diagrams shown inFIGS. 2, 3 and 4.

According to the example,FIG. 2shows a diagram in which the pedal force is indicated in newtons on an x axis60and the vehicle deceleration is indicated in m/s2on a y axis64. Here, a first curve68shows the deceleration of the vehicle in the normal “by-wire” operating mode. A second curve70shows the vehicle deceleration in the normal hydraulic fallback mode, in which the driver builds up braking torque by muscle power (only). Curve70is accordingly significantly shallower than curve68. It crosses the zero line after a pedal force threshold value66and then rises in a substantially linear manner.

A third curve72shows the vehicle deceleration in the hydraulic fallback mode, wherein the parking brake is additionally activated. Here, curve72rises steeply in a linear manner after a pedal force threshold value78, after which it bends and continues to rise in a linear manner. It may be seen here that, when the parking brake is activated, the vehicle deceleration rises very quickly and then flattens off with a slope which is shallower than in the case of the normal fallback mode. The rapid rise encourages the driver to continue braking since the vehicle responds quickly and clearly to the driver's braking demand. Reference signs74and76describe the legal minimum requirement, according to which a minimum deceleration of the vehicle of 2.44 m/s2must be achieved with a pedal force of 500 N.

InFIG. 3, the brake pedal travel is plotted in millimeters on the x axis60; the vehicle deceleration is once again plotted in m/s2on the y axis64. Curve80shows the deceleration behaviour in the normal (“by-wire”) operating mode, while curve84illustrates the deceleration behaviour in the fallback mode. Depending on the brake pedal travel, deceleration starts later in the fallback mode, i.e. with a longer brake pedal travel, thereby possibly disconcerting the driver. Curve88shows the vehicle deceleration with an additionally activated parking brake. In comparison with the normal fallback mode, vehicle deceleration starts even with relatively short brake pedal travels and initially rises more steeply, enabling the driver to recognize that the vehicle may still be braked—even if the pedal feel has changed.

If the parking brake is activated uniformly with each pedal actuation in the hydraulic fallback mode, this may lead to damage or functional impairment of the parking brake. According to the example, therefore, the activation point thereof is varied in the event of a sequence of brake pedal actuations, as illustrated inFIG. 4. Here, the x axis60once again shows the brake pedal travel, and the y axis64shows the vehicle deceleration.

Curve90shows the vehicle deceleration in the normal operating mode of the brake system, in which the braking demand of the driver is detected and a setpoint braking torque is determined therefrom. The pressure generating device5is activated accordingly by the open- and closed-loop control unit12in order to build up in the wheel brakes8,9,10,11the brake pressure required for the braking torque.

Curve94shows the vehicle deceleration in the hydraulic fallback mode (without activation of the additional brake actuator). Curve94rises significantly less steeply than curve90, which means that the driver has to depress the pedal significantly further to achieve the same deceleration as in the normal operating mode.

Curves100,102,104,106,108,110each show the vehicle decelerations in the event of a sequence of brake pedal actuations by the driver. During the first actuation of the brake pedal in this sequence, this being symbolized by curve100, the parking brake is activated or selected when a first pedal travel threshold value120is reached. During a second pedal actuation, which, may have been preceded by a prior release of the brake pedal, the parking brake is activated or selected at a second, higher pedal travel threshold value124.

The assistance by the additional brake actuator is preferably activated by an applicable travel threshold value “driver braking” of the brake pedal (variable pedal travel threshold value). If this is undershot, assistance is withdrawn.

In addition to activation via the pedal travel threshold value, a switch-off is preferably superimposed, switching the activation of the additional brake actuator on or off depending on the vehicle deceleration. Additionally, assistance may be reduced when an upper limit value of the vehicle deceleration is exceeded, and assistance is reapplied when a lower limit value is undershot, as long as the travel threshold value is above “driver braking”.

The additional brake actuator or parking brake is preferably activated when the brake pedal travel is greater than the (current) pedal travel threshold value and the vehicle deceleration is less than the maximum value (e.g. lower limit value).

Subsequent pedal actuations lead to activation of the parking brake at respectively higher pedal travel threshold values124,126,128,130etc. The driver therefore has to depress the brake pedal further and further before the additional braking torque is built up by the parking brake. In this way, the driver finds out or learns that the increasing braking effect was only being used as initial assistance and that they must adjust to the fact that this assistance will disappear in the event of further braking operations. The driver is also led to ever longer brake pedal travels by the ever later onset of the reinforcement and, in this way, they become accustomed to the now longer brake pedal travels. Thus, they are as it were conditioned to the new braking situation.