Patent Publication Number: US-2005124462-A1

Title: Optimized wiping of vehicle brakes

Description:
FIELD OF THE INVENTION  
      The present invention relates to a method and a device for wiping a vehicle brake.  
     BACKGROUND INFORMATION  
      A water/dirt film, which impairs the braking effect of the brake, forms on the brake disks of a vehicle brake in wet conditions. The braking distance may thus be significantly lengthened under wet conditions.  
      To improve the braking effect, it is known to briefly apply the brake pads to the brake disks under low pressure, thus simply wiping away the water/dirt film. It is common in known braking systems to carry out a wiping operation either after a predefined time has elapsed, e.g., every three to five minutes, or after a predefined distance has been covered, e.g., every five kilometers.  
      The vehicle decelerates slightly due to the wiping operation, especially during slow driving at a constant speed. This is relatively easily noticeable to the driver and may irritate the driver.  
     SUMMARY OF THE INVENTION  
      Therefore, it is an object of the present invention to implement a wiping operation in such a way that the driver may hardly notice it or not notice it at all.  
      To select the point in time at which a wiping operation is triggered in such a way that it coincides with the driver pressing the accelerator pedal or with a velocity change of the vehicle (positive or negative acceleration) is an essential aspect of the present invention. During a velocity change, the vehicle acceleration is superposed by the vehicle deceleration caused by the wiping operation. A wiping operation is far less noticeable to the driver in such a driving situation than in a constant driving situation since the driver expects a velocity change anyway.  
      A wiping operation may alternatively also be triggered when the driver in fact presses the accelerator pedal, but no velocity change or only a minor velocity change takes place as in the event, for example, of the driver pressing the accelerator pedal while the vehicle continues moving uphill. A wiping operation is also far less noticeable to the driver in this situation than, for example, when driving on a flat stretch of road at a constant speed.  
      A wiping operation (in addition to other predefinable conditions) is preferably triggered under the condition that the change in the accelerator pedal position and/or the change in the vehicle velocity exceed(s) predefined threshold values.  
      Different variables, from which the mentioned variables are derivable, may be used as a measure for the change in the accelerator pedal position as well as for the change in the vehicle velocity. As a result, the acceleration may be determined, for example, from the signal of wheel speed sensors or from the signal of an acceleration sensor. An engine torque or the change in the engine torque, a drive torque or the change in the drive torque, or an engine speed or the change in the engine speed may optionally be used for checking whether the change in the vehicle velocity exceeds a predefined threshold value. The change in the accelerator pedal position may be determined, for example, via a position sensor or an angle sensor, situated at the accelerator pedal, or on the basis of an analog variable, such as the change in throttle position, for example.  
      According to a preferred embodiment of the present invention, at least two variables, from which the change in the accelerator pedal position or the change in the vehicle velocity is derivable, are monitored with respect to the threshold value, and a wiping operation is triggered when at least one of the variables exceeds a predefined threshold value. Two complementary strategies are thus implemented, and it is ensured that a wiping operation is triggered at the right moment.  
      The threshold values relevant for triggering the wiping operation are preferably variable and dependent on the driving situation. Triggering of a wiping operation may thus be adapted to different driving situations. The threshold value for the change in the accelerator pedal position is preferably dependent on the vehicle velocity or the threshold value for the change in the vehicle velocity is dependent on the accelerator pedal position. Different threshold values for the change in the vehicle velocity are also possible, depending on whether the change is positive or negative.  
      During a longer drive under relatively constant driving conditions, it is sensible, for purely preventive considerations, to trigger a wiping operation even when the monitored variables (accelerator pedal change/acceleration) do not exceed the particular threshold value, but a different condition is met which triggers a routine wiping operation. It is therefore proposed to carry out a wiping operation when a predefined time has elapsed since the last braking operation or when the vehicle has covered a predefined distance. The brakes are thereby wiped at least at regular intervals.  
      A regular wiping operation may be carried out, for example, using different parameters (pressure, length of time), than a wiping operation above the predefined threshold values for the accelerator pedal change/acceleration, i.e., the brakes may be wiped, for example, using a lower pressure over a longer period of time. This is far less noticeable to the driver under constant driving conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       FIG. 1  shows a flow chart of the essential method steps of an optimized wiping method;  
       FIG. 2  shows a system for wiping a vehicle brake, and  
       FIG. 3  shows a disk brake known from the related art.  
    
    
     DETAILED DESCRIPTION  
       FIG. 1  shows the essential method steps which are executed up to the triggering of a brake wiping operation. It is initially checked in step  1  whether basic preconditions are present to carry out a wiping operation (step  5 ) at all. One of the basic preconditions is the presence of wetness. Wetness may be detected, for example, using a moisture sensor or another variable representing the condition “wetness.” Activation of the windshield wiper may indicate the presence of wetness, for example.  
      A further basic precondition, for example, may be a minimum speed (e.g., 50 km/h) which the vehicle must have to trigger a wiping operation.  
      In addition, further criteria such as “the brakes are not operated at the moment or the accelerator pedal is pressed at the moment” may be predefined.  
      If necessary, the aforementioned criteria or other criteria may be jointly or alternatively linked.  
      If the basic preconditions in step  1  are not met (N), the method moves to step  4  and no wiping operation takes place. The routine then starts again.  
      If the predefined basic preconditions in step  1  are met (case J), further conditions are checked in step  2  which cause the triggering point of a wiping operation to fall into an advantageous driving situation. Such an advantageous driving situation exists in particular when the vehicle is subjected to a velocity change or when the driver presses the accelerator pedal. The wiping operation is far less noticeable to the driver in such a driving situation than in a constant driving situation, since the driver expects a velocity change anyway.  
      Therefore, it is checked in step  2  whether the change in the accelerator pedal position or the change in the vehicle velocity exceeds a particular predefined threshold value sw 1 , sw 2 .  
      If the answer is yes, a wiping operation is then triggered in step  5  which the driver may hardly notice as such or not notice it at all.  
      Any variables, from which the change in the accelerator pedal position or the change in the vehicle velocity is derivable, may be used as a measure for the change in the accelerator pedal position or for the change in the vehicle velocity. The accelerator pedal position may be determined, for example, using an accelerator pedal sensor or on the basis of the throttle position. The change in the vehicle velocity may be determined, for example, using wheel speed sensors, an acceleration sensor, via the engine torque, the drive torque or the engine speed.  
      In order to determine the triggering point more accurately and more reliably, preferably two or more of these variables are analyzed and monitored with respect to their threshold values. A wiping operation is triggered when at least one of the monitored variables exceeds a predefined threshold value.  
      If the additional conditions in step  2  are not met, it is checked in step  3  whether another condition is possibly met. This other condition is met, for example, when a length of time t since the last braking operation is greater than a threshold value sw 3  or a distance, which the vehicle has covered since the last braking operation, is greater than a threshold value sw 4 . It may be ensured in this way that, during constant driving, the brakes are at least wiped at regular intervals. If one of the other conditions in step  3  is met, a wiping operation is initiated in step  5 . Otherwise (N), the routine restarts at step  1 .  
      A wiping operation under the conditions of step  2  is preferably carried out using other parameters than a regular wiping operation according to step  3 . A wiping operation below predefined threshold values sw 1 , sw 2  is carried out, for example, using a lower pressure over a longer length of time than above threshold values sw 1 , sw 2 .  
       FIG. 2  shows a system for wiping brakes under wet conditions. The system includes a control unit  10  to which different sensor signals A, B, C from sensors  11 ,  12 ,  13  are supplied. Control unit  10  analyzes the signals and activates a brake  18  when the conditions of step  2  or step  3  are met.  
      Sensor  11  is a sensor for determining the accelerator pedal position, and sensor  12  is an acceleration sensor for determining a velocity change. Block  13  represents a number of sensors, additional variables, relevant for the above-described method, such as the condition “wetness,” the vehicle velocity, a brake activation, etc., being derivable from signals C of such sensors. Sensors  12  may thus include, for example, wheel speed sensors, a moisture sensor, a brake activation sensor, etc.  
      Control unit  10  processes sensor signals A, B, C, executes the threshold value comparisons described in  FIG. 1 , and controls the brake wiping operation, if needed.  
       FIG. 3  is merely used to illustrate a conventional disk brake. This disk brake essentially includes a brake caliper  15  in which a brake clip  17  together with a brake pad  16  are movably situated. Other brake pad  16  is situated on brake caliper  15  on the opposite side of brake disk  14 . By applying hydraulic pressure p, brake pads  16  are pressed against brake disk  14  in order to execute a wiping operation or braking operation. The brake pressure during a wiping operation amounts to a few bar, 5 bar, for example.  
     Reference Numerals  
     
         
           1 - 5  method steps  
           10  control unit  
           11  accelerator pedal sensor  
           12  acceleration sensor  
           14  brake disk  
           15  brake caliper  
           16  brake pads  
           17  brake clips  
           18  disk brake