Method for operating a braking system and braking system

A braking system for vehicles includes wheel brakes, a pressure-providing device for actuating the wheel brakes, a master brake cylinder having a primary pressure chamber, a secondary pressure chamber, and a secondary piston. The primary pressure chamber is separated from a first set of the wheel brakes by a first, normally open separating valve, and the secondary pressure chamber is separated from a second set of the wheel brakes by a second, normally open separating valve. A pressure medium reservoir is connected to the secondary pressure chamber in the unactuated state of the secondary piston, the braking system is activated in order to actively build up pressure in the brakes in a by-wire operating mode by the pressure-providing device. In the event of a pressure medium surplus in the wheel brakes, the second separating valve is opened if a release of the brake pedal by the driver is detected.

TECHNICAL FIELD

A method for operating a braking system for motor vehicles.

TECHNICAL BACKGROUND

In motor vehicle engineering, electrohydraulic “brake-by-wire” brake assemblies are being used ever more widely. Brake assemblies of this kind often have not only a master brake cylinder that can be actuated by the vehicle driver but also an electrically activatable pressure-providing device (activatable “by-wire”), by the wheel brakes are pressurized in the “brake-by-wire” operating mode.

In these electrohydraulic braking systems, the driver can be decoupled from direct access to the wheel brakes. This function is used in a “brake-by-wire” operating mode. When the pedal is actuated, the driver's braking request is detected and a pedal decoupling unit and simulator are activated. The hydraulic volume displaced from the master brake cylinder by the driver by pedal actuation flows into the simulator, which serves to give the driver as reliable and comfortable a brake pedal feel as possible. The braking request detected by sensors leads to the determination of a nominal braking effect, from which the nominal brake pressure for the brakes is then determined. The corresponding actual brake pressure is then actively provided by an electrically actuatable pressure-providing device.

The actual braking is thus achieved by active pressure build-up in the brake circuits by means of the pressure-providing device, which is activated by a control and regulation unit. In brake systems of this kind, because the brake pedal actuation is hydraulically decoupled from the pressure build-up in the wheel brakes, many functionalities such as ABS, ESC, TCS, slope launch assistance etc. can be implemented in a manner which is technically efficient and particularly comfortable for the driver because of the pedal decoupling.

In such brake systems, a hydraulic fall-back level is usually provided, by means of which the driver can 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, the above-described hydraulic decoupling between brake pedal actuation and brake pressure build-up at the wheel brakes is realized by means of a pedal decoupling unit, this decoupling is eliminated in the fall-back level, thus enabling the driver to directly displace pressure medium into the brake circuits or wheel brakes. A switch is made to the fall-back level if it is no longer possible to build up pressure with the aid of the pressure-providing device. This is the case inter alia if a check valve that connects the pressure-providing device to a reservoir no longer reliably shuts off, such that a pressure build-up is no longer reliably possible.

The pressure-providing device in the brake systems described above is also referred to as an actuator or electrohydraulic actuator. For example, an electrohydraulic actuator is formed by an electromechanical linear actuator which displaces a piston axially in a hydraulic pressure chamber in order to build up pressure. The electromechanical linear actuator is usually formed by the combination of an electric motor with a rotation-translation gear mechanism.

DE 10 2013 204 778 A1 relates to a “brake-by-wire” brake assembly for motor vehicles which comprises a tandem master brake cylinder which can be actuated by means of a brake pedal and the pressure chambers of which are in each case connected, separably by means of an electrically actuatable separating valve, to a brake circuit with two wheel brakes, an activatable and deactivatable simulation device which is hydraulically connected to the master brake cylinder, and an electrically controllable pressure-providing device which is formed by a piston-cylinder arrangement with a hydraulic pressure chamber, the piston of which is displaceable by an electromechanical linear actuator, wherein the pressure-providing device is connected to brake circuit supply lines via two electrically actuatable activation valves.

The braking system is activated, for example by actuation of the ignition, so that it switches to the by-wire operating mode. It may occur that the driver has already actuated the brake pedal before activation of the by-wire operating mode, and in this way has displaced brake fluid from the master brake cylinder in the direction of the wheel brakes. If the system is woken while the brake pedal is actuated and after actuation switches to by-wire mode, this causes a mismatch of the volume balance in the hydraulics, which means that the existing volume can no longer be adequately dissipated when the pedal is released since the linear actuator can only absorb again the volume which it has itself displaced, but not the volume already displaced by the driver. In order to dissipate this surplus volume, normally the outlet valves are actuated. This however causes perceptible noise which can lead to irritation and disturbance to the driver, and finally can lead to complaints. If the outlet valves are also opened when there is a very slight pressure difference, the problem of soiling exists and as a result leakage can occur.

SUMMARY

A braking system and a method for operating a braking system in that surplus volumes are dissipated with significantly less noise. It is furthermore sought to specify a corresponding braking system.

A braking system for motor vehicles includes wheel brakes, an electrically controllable pressure-providing device for hydraulically actuating the wheel brakes, a master brake cylinder having a primary pressure chamber, a secondary pressure chamber, and a floatingly mounted secondary piston. The primary pressure chamber can be hydraulically separated from a first set of the wheel brakes by a first, normally open separating valve, and the secondary pressure chamber can be hydraulically separated from a second set of the wheel brakes by a second, normally open separating valve. The system further includes a pressure medium reservoir connected to the secondary pressure chamber of the master brake cylinder in the unactuated state of the secondary piston. The system also includes a hydraulically designed simulator, wherein the braking system is activated in order to actively build up pressure in the wheel brakes in a by-wire operating mode by the pressure-providing device. In the event of a pressure medium surplus in the wheel brakes, the second separating valve is opened in the by-wire operating mode if a release of the brake pedal by the driver is detected, so that surplus pressure medium can flow into the secondary pressure chamber.

With reference to the method, in the event of a pressure medium surplus in the wheel brakes, the second separating valve is opened in the by-wire operating mode if a release of the brake pedal by the driver is detected, so that surplus pressure medium can flow into the secondary pressure chamber.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the apparatus may be practiced. These embodiments, which are also referred to herein as “examples” or “options,” are described in enough detail to enable those skilled in the art to practice the present embodiments. The embodiments may be combined, other embodiments may be utilized, or structural or logical changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense and the scope of the invention is defined by the appended claims and their legal equivalents.

Surplus brake volume is dissipated on activation of the by-wire braking system on a mismatch of the volume balance, irritates or disturbs the driver. The opening of the outlet valves is normally associated with loud noises which the driver does not expect and which gives them an uncomfortable feeling, or inaccurately suggests that the braking system is faulty. It would therefore be desirable to dissipate the surplus volume soundlessly but nonetheless reliably. Also, the outlet valves should be actuated rarely, in particular if only slight pressure differences exist, so as not to unnecessarily shorten their service life and so as to avoid soiling of the valve seat.

It has now been found that this can be achieved if the surplus volume is displaced into the secondary pressure chamber of the brake master cylinder. This process takes place substantially soundlessly and leads to no perceptible disturbance for the driver. If pressure medium flows into the secondary pressure chamber, the secondary piston is pushed back into its starting or release position so that the hydraulic connection is opened between the secondary pressure chamber and the pressure medium reservoir through the openings in the tandem master brake cylinder, known as blow holes. In this way, pressure medium can flow out of the secondary pressure chamber into the pressure medium reservoir.

A pressure medium surplus or brake fluid surplus here means a pressure medium volume which can no longer be absorbed in by-wire operation when the brake pedal is not actuated or the volume of the pressure chamber of the pressure-providing device is already filled to maximum.

In one or more embodiments, the braking system comprises at least one electrically actuatable wheel valve per wheel brake for setting wheel-individual brake pressures.

Advantageously, an elastic element which loads the secondary piston is arranged in the secondary pressure chamber. The secondary piston which is mounted in floating fashion is moved into its rest position by the elastic element, which is optionally formed as a spring, when the brake pedal is not actuated. In its rest position, the floating piston opens passage bores to the brake fluid reservoir, known as “blow holes”, through which the surplus brake fluid can flow soundlessly into the brake fluid reservoir.

The pedal position, i.e. the current position of the brake pedal, is observed or determined by means of a pedal travel sensor. The pedal travel sensor is advantageously configured in redundant fashion so that the reliability of its signal can be checked during operation. Release of the brake pedal is detected when the pedal travel is less than a predefined pedal travel threshold value, and/or when the pedal release speed lies below a predefined negative pedal release gradient and the pedal is moved in the direction of the release position.

The second separating valve remains open until a braking request by the driver is detected, and only then is it closed again. Then the first separating valve is also closed so that the tandem master brake cylinder is hydraulically decoupled from the wheel brakes, so that in by-wire operation, pressure can actively be built up by means of the pressure-providing device.

The second separating valve remains open until a braking request by the driver is detected. In particular, the second separating valve remains open until the predefined pedal travel threshold value has been exceeded. This threshold value is smaller than the travel necessary to close the openings (blow holes) in the secondary pressure chamber which connect this chamber hydraulically to the pressure medium reservoir when the secondary piston is not activated. In by-wire operation, the separating valves are thus closed when the pedal travel exceeds a predefined pedal travel threshold value. Thus pressure can be built up actively on the basis of a braking request from the driver in brake-by-wire operation.

The second separating valve is held closed until the pedal travel has fallen below a pedal travel threshold value. The driver has ended the braking by releasing the pedal, and the separating valves return to their normally open state.

If however actuation occurs before the system has been activated, the sensor does not sense the brake pedal travel and the separating valves are not activated.

The presence of a pressure medium surplus is detected if the pressure-providing device senses that it cannot build up the complete pressure alone.

With reference to the braking system, the above-mentioned method is achieved with a means for performing a method as described above. In particular, an electronic control and regulating unit is provided in which the method is implemented by software and/or hardware.

The advantages include that, with the proposed method, surplus brake volume can be dissipated with low noise so that the driver is neither irritated nor disturbed. Since the outlet valves need not be open for this, these are protected so that the service life of the braking system is extended.

Referring toFIG.1, the braking system2comprises a master brake cylinder10which can be actuated by means of an actuation pedal or brake pedal6, a simulation device14cooperating with the master brake cylinder10, a pressure medium reservoir18assigned to the master brake cylinder10and standing under atmospheric pressure, an electrically controllable pressure-providing device20which is formed by a cylinder-piston arrangement with a hydraulic pressure chamber26, the piston32of which can be displaced by an electromechanical actuator, an electrically controllable pressure-modulation device for setting wheel-individual brake pressures, and an electronic control and regulating unit40.

The pressure-modulation device (not designated in more detail) comprises for example hydraulically actuatable wheel brakes42,44,46,48, and for each actuatable wheel brake42to48a respective inlet valve50,52,54,56and an outlet valve60,62,64,66which are connected together hydraulically in pairs via central connections and connected to the wheel brakes42to48. The input connections of the inlet valves50to56are supplied with pressures by means of brake circuit supply lines70,72; in a “brake-by-wire” operating mode, these pressures are derived from a system pressure which is present in a system pressure line80connected to the pressure chamber26of the pressure-providing device20, and corresponds to the pressure provided by the pressure-providing device. Here, the brakes42,44are hydraulically connected to a first brake circuit84, and the brakes46,48are hydraulically connected to a second brake circuit88.

A respective check valve90,92,94,96which opens towards the brake circuit supply lines70,72is connected in parallel to each inlet valve50to56. In fall-back operating mode, the brake circuit supply lines70,72are loaded with the pressures of the brake medium from pressure chambers120,122of the master brake cylinder10via hydraulic lines100,102. The output connections of the outlet valves60to66are connected to the brake medium reservoir18via a return line130.

The master brake cylinder10has, in a housing136, two pistons140,142which are arranged in series and which delimit the hydraulic pressure chambers120,122. The pressure chambers120,122are connected on one side to the pressure medium reservoir18via radial bores formed in the pistons140,42and via corresponding pressure-balancing lines150,152, wherein the connections can be shut off by a relative movement of the pistons140,142in the housing136. On the other side, the pressure chambers120,122are connected to the above-mentioned brake circuit supply lines70,72by means of hydraulic lines100,102.

A normally open valve160is situated in the pressure-balancing line150. The pressure chambers120,122contain restoring springs (not designated specifically), which position the pistons140,142in a starting position when the master brake cylinder10is not actuated. A piston rod166couples the pivot movement of the brake pedal6due to pedal actuation to the translation movement of the first master brake cylinder piston140or primary piston, the actuation travel of which is detected by a travel sensor170, configured in redundant fashion. In this way, the corresponding piston travel signal is a measure of the brake pedal actuation angle. It represents a braking request by the vehicle driver.

A separating valve180,182which is configured as an electrically actuatable, normally open, 2/2-way directional control valve is arranged in each line portion100,102connected to the pressure chambers120,122. The separating valves180,182can shut off the hydraulic connection between the pressure chambers120,122of the master brake cylinder10and the brake circuit supply lines70,72. A pressure sensor188connected to the line portion102detects the pressure built up in the pressure chamber122by movement of the second piston142.

The simulation device14can be coupled hydraulically to the master brake cylinder10and substantially comprises for example a simulator chamber190, a simulator spring chamber194, and a simulator piston198separating the two chambers190,194from each other. This simulator piston198is supported on the housing136by an elastic element (e.g. a spring) arranged in the simulator spring chamber194and advantageously preloaded. The simulator chamber190is connectable to the first pressure chamber120of the master brake cylinder10by means of an electrically actuatable simulator valve200. When a pedal force is input and simulator valve200is open, pressure medium flows from the master brake cylinder pressure chamber120into the simulator chamber190. A check valve210arranged hydraulically antiparallel to the simulator valve200allows the pressure medium to flow back from the simulator chamber190to the master brake cylinder pressure chamber120largely unhindered, irrespective of the switching state of the simulator valve200. Other embodiments and connections of the simulation device to the master brake cylinder10are conceivable.

The electrically controllable pressure-providing device20is configured as a hydraulic cylinder-piston arrangement or a single circuit electrohydraulic actuator, in which the pressure piston32delimiting the pressure chamber26can be actuated by an electric motor220(indicated diagrammatically) with the interposition of a rotation-translation gear mechanism (also indicated diagrammatically) which is configured as a ball screw drive (KGT). A rotor position sensor (indicated merely diagrammatically) serving to detect the rotor position of the electric motor220carries reference sign226. In addition, a temperature sensor228may be used for sensing the temperature of the motor winding.

The actuator pressure generated by the effect of the force of the piston32on the pressure medium enclosed in the pressure chamber26is fed into the system pressure line80and detected by means of a pressure sensor230, which is of redundant design. When the pressure switching valves240,242are opened, the pressure medium reaches the wheel brakes42to48and actuates them. A wheel brake pressure is built up and dissipated for all wheel brakes42to48by the forward and return movement of the piston32, when the pressure actuation valves240,242are opened, in normal braking in brake-by-wire operating mode.

When the pressure dissipates, the pressure medium previously displaced from the pressure chamber26into the wheel brakes42to48returns to the pressure chamber26on the same route. In contrast, when braking with different wheel brake pressures for each individual wheel which are regulated using the inlet and outlet valves50to56,60to66(e.g. on ABS braking), the part of the pressure medium discharged via the outlet valves60to66flows into the pressure medium reservoir18and is therefore no longer available initially to the pressure-providing device20for actuating the wheel brakes42to48.

When the brake pedal6is actuated, pressure medium or brake fluid is displaced from the chambers120,122towards the wheel brakes42,44,46,48. It may also occur that the driver actuates the brake pedal6before the braking system2is activated, i.e. switched into brake-by-wire mode. If the braking system2is then activated, there is a mismatch in the volume balance in the hydraulics which means that the existing volume can no longer be adequately dissipated when the pedal is released.

In order to adjust the volume balance in the presence of a pressure medium surplus without opening the outlet valves60to66(which could lead to severe disturbance to the driver), the separating valve182is opened when it is detected that the driver releases the brake pedal6again after a braking process. The pedal travel is sensed using the pedal travel sensor170. The release of the brake pedal6is detected when the pedal travel falls below a predefined pedal travel threshold value. When the separating valve182opens, brake medium flows back to the second chamber122. This pushes the secondary piston142back, i.e. in the direction of its unactuated state. When this state is reached, the hydraulic access (“blow hole”) to the pressure medium reservoir18opens and the pressure medium can escape into the pressure medium reservoir18. Since the secondary piston142is working against a spring during this process, it is initially compressed before the process becomes evident at the primary piston140. Thus the driver feels no back-pressure or only a very little back-pressure at the brake pedal, so the adjustment to the hydraulic volume balance takes place largely comfortably and quietly.

After performing the method, the brake fluid volume balance in the braking system is balanced again, so that as long as the braking system remains in the by-wire mode, the method need not usually be repeated.

The above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Embodiments discussed in different portions of the description or referred to in different drawings can be combined to form additional embodiments of the present application. The scope should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.