Method for actuating an automatic parking brake

A method for releasing a brake device for an automatic parking brake, a corresponding device, and a control device enable safely releasing the parking brake without stress on components. A method for actuating a parking brake with a brake device having a brake caliper housing with a chamber configured to receive a fluid, a brake piston, a spindle and a spindle nut, includes determining a first hydraulic driver-applied pre-pressure in the chamber during a mechanical application process of the parking brake. The method further includes comparing the first driver-applied pre-pressure to a predetermined pressure threshold value via the control device; and building up a hydraulic support pressure in the chamber in order to unload the spindle when the first driver-applied pre-pressure is above the pressure threshold value.

This application claims priority under 35 U.S.C. §119 to patent application no. DE 10 2014 202 178.2, filed on Feb. 6, 2014 in Germany, the disclosure of which is incorporated herein by reference in its entirety.

The present disclosure relates to a method for actuating an automatic parking brake, to a corresponding automatic parking brake and to a control device.

BACKGROUND

A method for releasing a parking brake, whereby a hydraulic fluid is fed into the hydraulic chamber in principle in the first step, is known from DE 10 2004 046 871 A1. The electric motor is then energized in such a way that it attempts to move the spindle nut of the brake device away from the brake disk. However, as a large spring force acts on the spindle via a spring element and a corresponding auxiliary piston, the electric motor does not at first succeed in moving the spindle. Only when the pressure in the hydraulic chamber is greater than the spring force of the spring element acting via the auxiliary piston is the spindle unloaded and begins to rotate. To prevent the spindle from jamming, a ball is provided on the head of the spindle in order to keep friction as low as possible. In addition, a stop comprising a first and a second element may also be provided to prevent jamming of the spindle, the first element being arranged on the head of the spindle and the second element on a housing part of the parking brake. Such design measures for preventing jamming of the spindle are, however, complex and costly as well as liable to failure.

A release process for a parking brake is further described in DE 10 2011 004 763 A1, whereby the electric brake motors of the two electromechanical brake devices, which together form the parking brake and are each installed on a respective wheel of a common axle, are first activated in the release device, so that the pressure of the brake pistons on the brake disk is reduced and the clamping force is decreased. As soon as a fault is detected during the release, both electric brake motors of the parking brake are switched off. After the electric brake motors have been switched off a maximum braking pressure is then built up automatically via the hydraulic vehicle brake and acts on the brake disks via the same brake pistons which are also subjected to pressure by the electric brake motors. A sufficient clamping force for securely locking the vehicle is thereby to be ensured. A recalibration is then carried out.

Automatic parking brake systems, or automatic “motor on caliper” parking brake systems, which are combined with electronic stability program (ESP) systems, are also known. These systems have at least one electromechanical actuator on the rear wheel brake, a spindle driven by a motor-transmission unit being located in the piston of the service brake. By rotation of the spindle a spindle nut guided in the brake piston is displaced, whereby the brake piston is also displaced. The clamping force on the rear axle can therefore be exerted either mechanically, hydraulically or in a combined manner. In commercially available systems a permanently energized DC motor is generally used to drive the electromechanical actuator.

However, in the event of a superposition of pressures through actuation of the service brake by the driver during actuation of the parking brake, it can happen that the spindle provided in the brake piston, especially when the hydraulic pressure applied by the driver is reduced again with the parking brake engaged, is exposed, on subsequent release without hydraulic pre-pressure, to high longitudinal forces acting via the spindle nut. These forces can cause the spindle400to flex elastically, as represented in exaggerated form inFIG. 1, since the spindle acts against the spring force F of the brake caliper (see arrow inFIG. 1: Fcaliper=ccaliper*spiston, where ccaliperis the spring constant of the brake caliper and spistonthe distance travelled by the brake piston). Such flexing of the spindle can have the result that the frictional forces between the threads of the spindle and of the spindle nut increase during opening or release of the automatic parking brake (binding), whereby opening of the automatic parking brake without a corresponding pre-pressure applied by the driver may not be ensured.

It is therefore the object of the present disclosure to provide a method for releasing a brake device for an automatic parking brake, and a corresponding device or a control device, by means of which the parking brake can be released reliably and without stress on components.

SUMMARY

This object is achieved by the features of the claims of the present disclosure. Developments are specified in the claims, the drawings, and the present disclosure.

The method for actuating a parking brake with a brake device including a brake caliper housing with a chamber for receiving a fluid, a brake piston, a spindle and a spindle nut advantageously comprises the following steps: determining a first driver-applied hydraulic pre-pressure in the chamber during a mechanical application process of the parking brake; comparing the first driver-applied pre-pressure to a predetermined pressure threshold value (limit value—force/area) by means of a control device; and building up a hydraulic support pressure in the chamber (during release of the parking brake) in order to unload the spindle, if the first driver-applied pre-pressure is above the pressure threshold value. According to the present disclosure, therefore, if a certain hydraulic pre-pressure has been exerted by the driver via the service brake during actuation of the parking brake, the release process of the parking brake is supported hydraulically, for example by means of an ESP modulator. In this way the mechanical subsystem consisting of the spindle and the spindle nut is unloaded, since the brake piston is again displaced hydraulically in the direction of the brake disk. In this way an elastic deformation or flexing of the spindle can be counteracted during the release process of the parking brake. If the driver-applied pre-pressure is below the pressure threshold value, no such support is needed, reducing stress on components, in particular with regard to the hydraulic device (ESP modulator).

The method advantageously includes the following further step: determining a second hydraulic driver-applied pre-pressure in the chamber at the start of a release process of the parking brake, the hydraulic support pressure in the chamber (23) for unloading the spindle (4) being built up only if the difference between the first driver-applied pre-pressure and the second driver-applied pre-pressure is greater than the pressure threshold value. It is thereby achieved that if, for example, a very high first driver-applied pre-pressure was present and the second driver-applied pre-pressure is above the pressure threshold value, a hydraulic pressure support nevertheless takes place if the difference between the first driver-applied pre-pressure and the second driver-applied pre-pressure is greater than the pressure threshold value, in order in this way to effect an unloading of the spindle.

The hydraulic support pressure in the chamber advantageously takes place adaptively during the release as a function of the difference determined between the first driver-applied pre-pressure and the second driver-applied pre-pressure. In this way the spindle can be unloaded optimally in that, for example, the hydraulic support pressure corresponds exactly to the difference between the first and the second driver-applied pre-pressures.

The hydraulic support pressure in the chamber during the release advantageously takes place adaptively as a function of the difference between the difference determined between the first driver-applied pre-pressure and the second driver-applied pre-pressure, and the pressure threshold value.

The hydraulic pressure support in the chamber is advantageously enabled for the following release process if the first driver-applied pre-pressure is above the pressure threshold value. In this way a two-stage system, whereby a hydraulic pressure support takes place only when required, is implemented.

The first driver-applied pre-pressure is advantageously measured during the mechanical application process of the parking brake and is stored by means of a storage device; only thereby is corresponding data-processing made possible.

The predetermined pressure threshold value is advantageously calculated from characteristic values, such as the brake piston diameter, the stiffness of the brake caliper housing and of the spindle of the parking brake, together with an additional safety margin, whereby trouble-free operation of the brake system is made possible.

The first driver-applied pre-pressure and the second driver-applied pre-pressure are advantageously determined in an analog manner by at least one pressure sensor. Through the use of the same measuring method and the same pressure sensor the measured values are suitably robust.

A suitable control and regulation device and a corresponding brake device are provided for implementing the method.

DETAILED DESCRIPTION

A first embodiment of the present disclosure is described below with reference toFIG. 2. A brake device1for an automatic (automated) parking brake (or locking brake) comprises a brake caliper housing2or a brake caliper, a brake piston3, a spindle4, in particular a threaded spindle, and a spindle nut5. The parking brake further has a motor-transmission unit (MGU)100, at least one brake pad6on the brake piston side, and a brake disk7.

The brake caliper housing2comprises a first end21with a first opening21a for receiving the brake piston3, a second end22oriented in the direction of the motor-transmission unit100, and a chamber23for receiving a fluid or a brake fluid. A hydraulic line24for conducting the brake fluid into/out of the chamber21is further provided in the brake caliper housing2.

The brake piston3is substantially cylindrical and has a blind hole oriented in the direction of the second end22, in which blind hole the spindle4and the spindle nut5mounted thereon are provided. In this case the brake device1is configured such that the brake piston3either is moved hydraulically in the direction of the brake disk7, or this process is performed mechanically via the spindle4and the spindle nut5, or a combination thereof is used.

In operation the brake device1for the parking brake according to the disclosure is operated as follows. Vehicles, in particular motor vehicles equipped with ESP, offer the possibility of modulating the braking pressure in a wheel-individual manner. The present brake device1, or the parking brake, is equipped with at least one pressure sensor (not shown), so that, upon actuation of the parking brake, any driver-applied pre-pressure present, that is, the pre-pressure exerted by the driver via the service brake (not shown) of the vehicle, can be measured. In the application phase of the automatic parking brake the driver-applied pre-pressure can therefore be determined and stored by means of a suitable storage device (not shown). As soon as it is detected by means of a suitable control device (not shown) that the driver-applied pre-pressure is above a predetermined pressure threshold value, a hydraulic support may be effected preventively during a subsequent release process of the automatic parking brake in order to unload the spindle4mechanically. The hydraulic support may be implemented by means of a suitable hydraulic unit (ESP modulator).

With reference toFIG. 3, The functional sequence of operating the parking brake can be divided into the following steps:First, in the application phase, that is, during actuation of the parking brake, a first driver-applied pre-pressure in the chamber23of the brake caliper housing2, exerted via the service brake and via the hydraulic line24, is measured (S1).A pressure threshold value is or has been defined (S2) and is selected such that, when it is exceeded, there is at least a danger that the spindle4jams during release on account of excessive elastic deformation (flexing), or at least a certain binding of the system occurs on account of the increased frictional forces arising as a result of the flexing of the spindle5, in particular with regard to the threaded connection between the spindle4and the spindle nut5, which in the worst case leads to an unreleasable parking brake or to damage to components. The pressure threshold value is dependent on the characteristic curve of the mechanical components involved. This includes, for example, parameters such as the brake piston diameter, the stiffness of the brake caliper, of the spindle4, of the brake piston3, etc., which stiffness depends on the respective geometry and the material used, and the properties of the threaded connection between the spindle4and the spindle nut5. An additional safety margin may also be included.It is determined by means of a suitable control unit whether the first measured driver-applied pre-pressure is above the defined pressure threshold value (S3). If the first driver-applied pre-pressure is above the defined pressure threshold value, a hydraulic pressure support for the following release process is enabled (S5′). That is, a pressure support during the following release process is possible but not mandatory. If the first driver-applied pre-pressure is below the defined pressure threshold value, the parking brake is opened without hydraulic support (S5), since it must be assumed that the longitudinal forces arising, which act on the spindle4, are small and therefore do not adversely influence the operation or the release process of the parking brake.The first driver-applied pre-pressure is stored by the storage device (S4).At the start of the release process a second driver-applied pre-pressure is measured in order to check via the control unit whether a hydraulic support (which may be enabled) is required or not, and with which pressure it should be carried out. What is relevant here is the difference between the first driver-applied pre-pressure measured during application of the parking brake and the second driver-applied pre-pressure measured again at the start of the release process, if the second driver-applied pre-pressure at the start of the release process is lower than the first driver-applied pre-pressure during closing of the parking brake. If this difference at the start of the release process of the automatic parking brake is below the pressure threshold value, or the driver-applied pre-pressure at the start of the release process is higher than during the closing of the parking brake—if, therefore, the differential value is, so to speak, negative—the parking brake is opened without hydraulic support (S5).

If the difference at the start of the release process of the automatic parking brake is above the pressure threshold value, a hydraulic support takes place via a corresponding return pump (not shown) of the hydraulic unit (not shown) in order to unload the spindle4mechanically.The hydraulic support during the release can be set adaptively, in particular as a function of the difference determined between the two driver-applied pre-pressures measured. For example, a hydraulic pressure support equal to the pressure difference determined may be carried out. However, the hydraulic support pressure in the chamber during the release may also take place adaptively as a function of the difference between the difference determined between the first driver-applied pre-pressure and the second driver-applied pre-pressure, and the pressure threshold value. It is ensured by means of the additional safety margin that the amount of the support pressure is not selected merely such that a pressure difference finally remains at the level of the pressure threshold value, but that a smaller pressure difference is produced. Jamming of the spindle4is thereby reliably prevented.

In the extreme case that a very high first driver-applied pre-pressure has been exerted during actuation of the parking brake, the second driver-applied pre-pressure then having been reduced to zero prior to the release process of the parking brake, so that the chamber23of the brake caliper housing2is substantially unpressurized, large longitudinal forces (indicated inFIG. 1by the arrow from right to left) act via the brake piston3and the spindle nut5on the spindle4, whereby the spindle4is deformed elastically, as shown inFIG. 1.

By means of the method according to the disclosure a hydraulic support pressure is built up if required in the chamber23via the hydraulic line24and the ESP modulator, which support pressure presses the brake piston3in the direction of the brake disk7, thus unloading the spindle nut5and the spindle4. The hydraulic forces generated in this way (indicated inFIG. 2by the three arrows from left to right) counteract or cancel out the above-mentioned longitudinal forces. In this way the parking brake can be released with a minimized elastic deformation of the spindle5.