The present invention relates to a method for actuating an electrically controllable parking brake and an electrically controllable parking brake for motor vehicles comprising another electronic service brake system with anti-lock protection.
Up-to-date motor vehicle brake systems have to satisfy three functions mainly, i.e. service brake, parking brake and emergency brake functions, to meet legal provisions. To this end, prior-art brake systems generally include two brake systems independent of each other. Most frequently, the service brake is activated by application of a brake pedal and includes already in many cases electrohydraulic auxiliary means preventing the locking of the vehicle wheels, for example. Further, up-to-date motor vehicles are equipped with another, completely independent parking brake system which is actuated in many cases by means of a hand brake lever with a Bowden cable that acts mechanically on the brake actuators of the rear axle for brake application. In this arrangement, it is desired that the parking brake system be operable also during driving in order to enable the vehicle to come to a standstill even in the case of a defect of the service brake.
DE-A-195 16 639 discloses a service and parking brake system for motor vehicles, wherein independent actuating devices are provided to actuate the corresponding brake. The brake system comprises a pressure buildup means fed with external energy and allowing actuation of the friction brake of the parking brake system. Although the actuating device can be an element of a driving dynamics control device (ESP), no anti-lock function is provided for the parking brake system.
WO 99/38738 also discloses a motor vehicle brake system with a service brake and an electrically controllable parking brake, wherein the proportioning service brake system is equipped with ABS, TCS and ESP functions. The parking brake system is designed so that the service brake is driven upon actuation of the parking brake system at driving speeds of v≠0. The parking brake is applied only at driving speeds of v≈0. An anti-lock function for the parking brake is provided neither in this prior-art electronic parking brake.
DE-A-199 08 062 discloses a parking brake system for motor vehicles wherein the electronic parking brake is locked above a defined driving speed. Locking is necessary to prevent the motor vehicle from reaching an uncontrolled driving condition due to locking of the rear wheels when the actuators of the parking brake are enabled.
An object of the invention is to provide an electrically controllable parking brake system, which is safe to operate also at driving speeds of v≠0.
This object is achieved by the method according to claim 1 and the parking brake according to claim 10.
The basic idea of the invention involves finding an operating point on the μ-slip curve during anti-lock control that is favorable for a great braking effect, while using a method as simple as possible.
It is the purpose of the method to avoid a too high brake torque in order to prevent the wheels from locking in a dynamic braking operation.
To calculate a new nominal value, it is preferred to assess the maximum wheel slip behavior in a preceding unstable phase (release phase).
As this occurs, the brake torque request (ForceRequest) is reduced in the subsequent braking phase in particular depending on the maximum wheel slip in the preceding unstable phase.
In the method of the invention, it is preferred that the wheel encountering the greatest wheel slip in the respective unstable phases is taken into account to determine the magnitude of the brake torque request similarly to the per se known ‘select-low’ principle in ABS service brakes.
It is expedient to monitor in these cases whether the wheel slip (SlipPH2max) exceeds a defined slip threshold (SlipThr) on a wheel. This fact allows favorably reducing the demand in application force for the next braking phase. The slip threshold SlipThr to be applied in this regard amounts to roughly 30% to 50% of 1 in particular.
A new nominal value (ForceRequest) is calculated especially according to the following formula:
In this formula, the factors Factor1 and Factor2 are proportionality constants allowing an individual adaptation of the control to the conditions in the vehicle (e.g. rating of the parking brake) in an expert manner. The ratio of Factor1/Factor2 is preferably lower than 1. In a particularly preferred fashion, the ratio is lower than roughly 0.4, e.g. roughly 0.33.
The index n indicates the value for the ForceRequest to be presently calculated. The index n−1 designates the value determined in a previous calculation.
According to a preferred embodiment, the new nominal value ForceRequest(n) for the next braking phase is calculated in such a fashion that the initialization value of the application force request ForceRequest(n−1) is the current value of the actual brake torque at the time of detection of wheel instability. The time of instability is determined in particular by monitoring the time when a predetermined slip threshold is exceeded. If a value for the actual brake torque is not available directly, that means, measured by sensors, it is suitable to calculate a corresponding value by way of a physical model that can be derived in an expert manner.
To avoid underbraking conditions, that is to say, too low brake pressures in a ratio relative to the current coefficient of friction, it is preferred according to the method of the invention to increase the brake torque with a lower brake torque gradient, especially in steps, after a defined period of time when the wheels remain close to the vehicle reference speed. It is suitable to choose this period of time in such a way that the delay times of the entire system, electromotive actor, inertias of the wheels, etc. are taken into account. This means the brake torque gradient should be as high as possible, however, not so high that the brake system is no longer able to appropriately react to the changed application force request.
The brake torque is measured preferably by means of the application force or by measuring the travel of a moved element at a mechanical application device.
The method of the invention is advantageous because locking of the wheels or overload of the application force in a parking brake actuated during driving can be avoided.
Further preferred embodiments can be seen and the following description of an embodiment by way of Figures.
In the drawings,
In the diagrams of
The dotted lines 2 in
In range A in
In
At time tS (dotted line 4) an abrupt change in the coefficient of friction from a comparatively low coefficient of friction μL (e.g. on snow or ice) to a higher coefficient of friction μH (e.g. on dry roadways) is brought about to further explain the way the method is functioning. To avoid underbraking, the nominal value ForceRequest(n) is now gradually increased in range C until again a predetermined minimum slip occurs.
To avoid underbraking effects in a phase A′ where the brake torque is maintained constant, it can be monitored accordingly whether the wheel slip has not exceeded another predefined slip threshold for a defined time tÜ. If this is the case, the nominal value is stepwise increased similar to the method described in the preceding paragraph.
Number | Date | Country | Kind |
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10258647.0 | Dec 2002 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP03/13528 | 12/2/2003 | WO | 3/23/2006 |