Abstract:
A method is described for automatically keeping a motor vehicle at a standstill. The motor vehicle includes a braking system having a valve for limiting the brake pressure to a predefinable pressure threshold value. Gradient information and a pressure threshold value dependent on the gradient information are ascertained. The pressure threshold value is set at the valve as soon as the motor vehicle is at a standstill and the service brake is applied.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for automatically keeping a motor vehicle at a standstill as well as a corresponding device for carrying out the method according to the present invention. 
       BACKGROUND INFORMATION 
       [0002]    Modern motor vehicles frequently include a hill hold control (HHC) which automatically keeps the vehicle on a gradient without the driver having to apply the brake for this purpose. Hill hold systems generally function as follows: During a braking operation in which the driver decelerates the vehicle to a stop, the brake pressure prevailing on the wheel brakes is blocked with the aid of shift valves—normally the so-called switching valves. For this purpose, the valves are activated by a control unit, in which the HHC function is stored, using an appropriate current. The valve current set at the valve determines the level of the brake pressure which the relevant valve is capable of maintaining. If the differential pressure dropping at the valve is greater than the pressure threshold value, medium flows into the valve. It is thus possible to maintain a predefined pressure in the wheel brakes, even if the driver&#39;s foot is removed from the brake pedal. To release the brakes, the driver must normally depress the accelerator pedal, or the HHC function is automatically deactivated when the brake pedal has no longer been depressed for, for example, approximately 2 seconds. 
         [0003]    In the case of the hill hold systems known from the related art, the valves are normally not closed until the driver has already partially released the brake pedal and the brake pressure prevailing at the master brake cylinder has dropped to a predefined target pressure. To be able to close the valves at the above-named point in time, the brake pressure prevailing at the master brake cylinder is monitored with the aid of a pressure sensor, the so-called pre-pressure sensor. The target pressure is normally a pressure which is at least high enough to hold the vehicle securely at a standstill. 
         [0004]    Known hill hold systems therefore require at least two sensors, namely, a sensor for determining the road gradient and a pressure sensor for monitoring the brake pressure in a brake circuit. Accordingly, common hill hold systems are relatively complex and are in particular too expensive as optional equipment in vehicles in the lower price category. 
       SUMMARY 
       [0005]    An object of the present invention is therefore to provide a more cost-effective device for automatically holding a motor vehicle at a standstill, as well as a corresponding method. 
         [0006]    According to the present invention, it is proposed to ascertain gradient information and a pressure threshold value which is dependent on it in a motor vehicle equipped with a hill hold control (HHC), and to set the pressure threshold value at a valve with the aid of which it is possible to maintain the brake pressure prevailing on one wheel brake, independently of the level of the brake pressure prevailing in the brake circuit. Since the pressure threshold value is set at the valve temporally independently of the brake pressure prevailing in the brake circuit, it is possible to dispense with the pre-pressure sensor required in the related art, making the hill hold system significantly more cost-effective. 
         [0007]    The valve according to the present invention is preferably a control valve, at which a certain pressure threshold value may be set by specifying a valve current. The pressure threshold value determines the pressure which the valve is capable of maintaining As long as the differential pressure dropping at the valve is greater than the set pressure threshold value, medium flows into the valve. 
         [0008]    According to a preferred specific embodiment of the present invention, the pressure threshold value is set at the valve before the driver has begun to release the brake. Preferably, the pressure threshold value is set as soon as the service brake is applied and the vehicle has come to a stop. It is possible to detect that the service brake is applied via, for example, the stop light switch (SLS). SLS closed denotes “brake applied” and SLS open denotes “brake not applied.” It is possible to detect the standstill of the vehicle in a known manner with the aid of the wheel speed sensors. 
         [0009]    For smaller gradients, less brake pressure is required in principle than for larger gradients to keep the vehicle at a standstill. The pressure threshold value set at the valve is therefore preferably lower for smaller gradients than for larger gradients. On the level, it may amount to, for example, 15 bar, and at a gradient of 20%, it may amount to, for example, 50bar. 
         [0010]    According to one specific embodiment of the present invention, as a function of the gradient, a minimum brake pressure is ascertained which is required to keep the vehicle at a standstill, and the pressure threshold value is then set at least as high as the minimum brake pressure. In this way it is ensured that the vehicle will not be unintentionally set in motion. The minimum brake pressure may be read out from, for example, a characteristic curve. 
         [0011]    For safety reasons, a higher value than the minimum brake pressure may be set at the valve. According to one specific embodiment of the present invention, a pressure increase dependent on the gradient may, for example, be ascertained, and the pressure threshold value may be set higher than the minimum brake pressure by the amount of the pressure increase. 
         [0012]    The gradient may be estimated, for example, with the aid of an acceleration sensor or measured directly using a gradient sensor. 
         [0013]    The pressure threshold value preferably remains set at the valve until a startup intent of the driver has been detected. The valve may be opened, for example, when a startup intent has been detected and the engine torque is greater than the torque required for starting on the gradient. 
         [0014]    In a vehicle having multiple brake circuits and/or multiple valves, identical or different pressure threshold values may be set in principle. 
         [0015]    For carrying out the method described above, a control device having a corresponding algorithm is preferably provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1   a  shows the time curve of the brake pressure at one wheel and at the master brake cylinder in a brake circuit of a vehicle which is equipped with a known hill hold system. 
           [0017]      FIG. 1   b  shows the time curve of a valve current of a valve for automatically maintaining a brake pressure at a wheel brake. 
           [0018]      FIG. 2   a  shows the time curve of the brake pressure in a brake circuit of a vehicle which is equipped with a hill hold system according to the present invention. 
           [0019]      FIG. 2   b  shows the associated time curve of a valve current of a valve for automatically maintaining a brake pressure at a wheel brake. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  shows an exemplary curve of brake pressure p at a wheel (p wheel ) and at the master brake cylinder (p vor ) in a brake circuit of a vehicle, which is equipped with a hill hold control (p vor ) known from the related art (hill hold system). It is assumed that the vehicle is on a gradient at a standstill at point in time t 0 . The driver keeps the brake pedal depressed. A brake pressure Po prevails in the vehicle&#39;s brake circuit. 
         [0021]    Starting at point in time t 1 , the driver takes his foot off the brake pedal, and brake pressure p vor  begins to fall. The lowering of brake pressure p vor  is monitored with the aid of a pressure sensor (pre-pressure sensor). At point in time t 2 , brake pressure p vor  reaches a minimum brake pressure p min , which is the minimum required for keeping the vehicle at a standstill. This minimum brake pressure p min  is dependent on the gradient and other influencing factors, such as the weight of the vehicle, and is calculated in a control unit of the hill hold system. 
         [0022]    When pressure p vor  prevailing in the brake circuit at point in time t 2  reaches minimum braking pressure p min , a value—normally a so-called switching valve—is closed in the brake circuit, which locks instantaneous brake pressure p vor  in the wheel brake. For this purpose, the valve is activated using a certain current, which determines the level of the brake pressure, which the valve is capable of maintaining without medium flowing into it.  FIG. 1   b  shows the corresponding curve of valve current l v . In the present case, a pressure threshold value p s  is set at the valve at a level equal to minimum brake pressure p min ; p s =p min  applies. Thereafter, wheel brake pressure p wheel  remains at the level of minimum brake pressure p min  while master brake cylinder pressure p vor  continues to fall. Due to the pressure locked in the wheel brakes, the vehicle is automatically kept at a standstill. 
         [0023]    Subsequently, the driver begins to depress the accelerator pedal in order to start driving. As soon as the engine torque has reached a predefined threshold value at point in time t 3  the switching valve is opened. For this purpose, valve current lv is slowly lowered (ramp-shaped) to a value of zero. At point in time t 4 , the valve current is equal to zero. The wheel brakes are then completely free of pressure, and the vehicle is able to start driving. 
         [0024]      FIG. 2   a  shows the curve of the brake pressure in the same braking situation, but in a vehicle that is equipped with a hill hold control (hill hold system) according to the present invention. In contrast to the system described above, this hill hold control requires no pre-pressure sensor. It is thus unnecessary to install a pre-pressure sensor in the vehicle. 
         [0025]    At point in time t 0 , the vehicle is again on a gradient and the driver activates the brake pedal. The state “vehicle standstill” is detected, for example, with the aid of the wheel speed sensors. The absence of a pre-pressure sensor makes it impossible to measure the brake pressure prevailing in the braking system. An activation of the service brake is only ascertained qualitatively from the switching state of the stop light switch (SLS). SLS closed denotes “brake applied” and SLS open denotes “brake not applied.” 
         [0026]    In the example shown, the valve is immediately closed at point in time t 0  after it has been detected that the vehicle is at a standstill and the brake is applied simultaneously. A pressure threshold value is set at the valve which is sufficiently high to keep the vehicle at a standstill.  FIG. 2   b  shows in turn the corresponding curve of valve current l v . 
         [0027]    Pressure threshold value p s  is for reasons of safety preferably set to a value that is larger by one pressure increase Δp than a minimum brake pressure p min , which is the minimum required to keep the vehicle at a standstill. Pressure threshold value p s  is calculated from an algorithm while taking into consideration the gradient, or it is read out from a characteristic curve. Depending on the gradient, pressure threshold value p s  may amount to, for example, between 10 bar (on the level) and 50 bar (at, for example, a 20% gradient). 
         [0028]    Starting at point in time t 1 , the driver begins to release the brake, and brake pressure p vor  begins to fall. As long as the differential pressure prevailing at the valve is greater than pressure threshold value p s , medium flows through the switching valve in the direction of the master brake cylinder. At point in time t 2 , brake pressure p vor  reaches pressure threshold value p s . While brake pressure p vor  subsequently continues to fall, wheel brake pressure p wheel  remains at the level of pressure threshold value p. The vehicle is thus automatically kept at a standstill. 
         [0029]    Starting at point in time t 3 , a startup intent of the driver is again detected; the switching valve is opened slowly and brake pressure p wheel  begins to fall. This causes the wheel brakes to release and the vehicle is able to start.