Abstract:
A method for operating a brake system of a motor vehicle, prefilling taking place to at least partially overcome an air gap of one wheel brake or a plurality of wheel brakes if the air gap of at least one wheel brake is increased, in particular as a result of dynamic influences of the vehicle motion.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method, a device and a computer program for operating a brake system of a motor vehicle.  
       BACKGROUND INFORMATION  
       [0002]     The function of electronic prefilling of a service brake (electronic brake prefill EBP) overcomes the air gap of the wheel brake by applying the brake linings. Overcoming the air gap reduces the response time of the brake system in a subsequent actuation of the brake pedal. This leads to a shorter pressure-generation time, and thereby shortens the stopping distance.  
         [0003]     From German Patent Application No. DE 10 2004 030464, for instance, a brake system having ELB (electronically controlled braking) and prefill function is known as well as a method for electronic brake control. To activate the ELB function in the related art, the accelerator pedal must be released very quickly, from which a possible emergency situation and subsequent panic braking are inferred. The air gap is overcome by an hydraulic control unit, which sets a slight wheel pressure for the application of the brake linings.  
         [0004]     The response of the service brake in the related art is improved only if the driver releases the accelerator pedal rapidly. In contrast, in driving situations with an especially large air gap, for instance due to so-called knock-back, the response of the brake can be very poor. Knock-back means an increase in the air gap, for example due to the transverse acceleration of a motor vehicle. In such a case, the brake pistons are enlarged by a transverse acceleration to which they are subjected, or by an increase in the imbalance of the brake disk due to mechanical effects acting on the wheel axle during cornering, for example. The response of the brake may be very poor in such a situation, so that a fairly large air gap must be overcome and thus a long pedal travel before there is a sufficient response of the service brake.  
       SUMMARY OF THE INVENTION  
       [0005]     Therefore, an objective of the present invention is to improve the response of the service brake.  
         [0006]     This problem is solved by a method for operating a brake system of a motor vehicle, in which prefilling takes place to at least partially overcome an air gap of one wheel brake or a plurality of wheel brakes if the air gap of at least one wheel brake is increased, in particular as a result of dynamic influences of the vehicle motion. The dynamic influences are, in particular, translatory or rotatory accelerations of the vehicle, but may also be, for example, deformations of the vehicle body, the axle geometries or individual components of the axles, such as the brake disks. By applying the brake linings through prefilling of the brake system, the present invention makes it possible to improve the response of the service brake and thereby shorten the stopping distance in the case of a larger air gap, especially as a consequence of so-called knock-back. The improved response also results in better pedal feel during braking.  
         [0007]     According to the present invention, the prefilling activation takes place independently of the gradient of the accelerator pedal, and thus independently of a potential emergency situation. Knock-back increases the air gap of the brake. If the driver brakes in a vehicle without the method of the present invention, he must first overcome the air gap. This has a detrimental effect on the response of the brake and leads to a longer stopping distance. Furthermore, the larger air gap results in an unusually soft pedal feel during the initial braking. In the case of a very large brake and very large air gap, the driver may also notice excessive travel of the brake pedal. In contrast, if the air gap is overcome or reduced once a larger air gap has come about, then the response time of the brake is briefer, the stopping distance shortened, and the pedal feel improved.  
         [0008]     Prefilling preferably takes place when a limit value of a transverse acceleration and/or a limit value of a steering angle and/or a limit value of a yaw rate have/has been exceeded. The prefilling may occur additionally or alternatively also when a measured limit value of the air gap was exceeded. Thus, an increased air gap may either be detected with the aid of measuring technology, or an increased air gap may be inferred on the basis of other measured physical operating parameters of the motor vehicle. An increased air gap may exist when the vehicle is cornering, for example. Cornering is able to be detected with the aid of a steering angle or the transverse acceleration, for instance. If the absolute value of the yaw rate exceeds a threshold value, then it is likewise possible to infer a larger air gap. The air gap may be removed as soon as an increased air gap has come about or as soon as the assumption has been made that such an increased air gap has come about, or else only later on, for instance when the driver releases the accelerator and it is likely that he intends to brake afterwards. Prefilling may occur at one axle or at both axles. Simultaneous or time-staggered prefilling of the axles is possible. The prefilling may be carried out after a time delay; the time delay may be a constant delay or depend on additional operating parameters of the motor vehicle.  
         [0009]     This problem mentioned in the introduction is also solved by a device, in particular a control device, for operating a brake system of a motor vehicle, in which prefilling takes place to at least partially overcome an air gap of one wheel brake or a plurality of wheel brakes if the air gap of at least one wheel brake has become larger, in particular as a result of dynamic influences of the vehicle motion.  
         [0010]     The objective indicated at the outset is also achieved by a computer program having program code for carrying out all steps of a method according to the present invention when the program is executed on a computer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows a sketch of an hydraulic brake system.  
         [0012]      FIG. 2  shows a flow chart of an exemplary embodiment of a method according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]     A power-assisted, hydraulic brake system (wheel brake system)  10  for passenger cars shown in  FIG. 1  has a brake circuit I assigned to wheel brakes  11 ,  12  of front axle FA of the vehicle, and a brake circuit  11  assigned to wheel brakes  13 ,  14  of rear axle RA. A servo-pressure source  15 , which supplies the energy required for the generation of braking force, is connected to both brake circuits I and II. Brake system  10  thus has a service brake, which is activated by external forces. Brake system  10  also includes a secondary brake, which is actuated by muscular energy. It has a main brake cylinder  17 , which is actuable by a brake pedal  16  and includes a pressure-medium reservoir  18 . Main brake cylinder  17  has a single-circuit design, i.e., it is connected to brake circuit I by a line  19  and a first valve  20  disposed therein. In the illustrated position of valve  20 , the secondary brake therefore acts only on wheel brakes  11  and  12  of front axle FA. With an active service brake, valve  20  blocks the connection between the main brake cylinder and wheel brakes  11  and  12 . First valve  20  assigned to main brake cylinder  17  is therefore denoted as shutoff valve in the following text.  
         [0014]     Servo-pressure source  15  aspirates pressure medium from reservoir  18  of main brake cylinder  17  and pressurizes it to high pressure for the functionality of the service brake. Pressure medium withdrawn from wheel brakes  11  through  14  while the service brake is in action is returned to reservoir  18 . To block brake circuit I with respect to pressure medium reservoir  18  when the secondary brake is active, a shutoff valve  24  is disposed in a line  23  leading to the pressure medium reservoir. In addition, two valves  25  and  26  for the modulation of the brake pressure in an active service brake are assigned to each wheel brake  11  through  14 .  
         [0015]     Brake system  10  is equipped with an electronic control device  29  to which, in addition to valves  20 ,  24 ,  25  and  26 , a displacement sensor  30  is connected, which detects the displacement of brake pedal  16 , as well as six pressure sensors  31  through  36  by which the pressure generated by main brake cylinder  17 , the pressure supplied by servo-pressure source  15 , and also the pressures applied into wheel brakes  11  through  14  are able to be detected. While the secondary brake operates hydraulically in the conventional manner, without involvement of control device  29 , the service brake operates electro-hydraulically, i.e., if the driver of the passenger car actuates brake pedal  16 , the electrical displacement signal detected by displacement sensor  30 , and possibly additional electrical signals are evaluated by control device  29  for the control of control valves  20 ,  24 ,  25  and  26  in order to generate braking pressure inside wheel brakes  11  through  14  according to the requested brake torque, the brake pressure being monitored by the electronic control device with the aid of the electrical signals from pressure sensors  31 ,  33  through  36 . In addition, the vehicle includes at least one acceleration sensor by which transverse acceleration A Q  is able to be determined, and, indirectly, yaw rate G as well. Furthermore, the steering angle of the vehicle is ascertainable via a sensor.  
         [0016]      FIG. 2  shows a flow chart of an exemplary embodiment of a method according to the present invention. The method begins in step  101 . In step  102 , transverse acceleration A Q  of the vehicle is determined. This may be done with the aid of an acceleration sensor, for example, which emits an electrical signal that is a measure for the magnitude of the transverse acceleration. In step  103  it is checked whether transverse acceleration A Q  is greater than a maximum value A QMax  of the transverse acceleration. If this is the case—indicated by the Y option—then prefilling so as to at least partially overcome the air gap of one wheel brake or a plurality of wheel brakes takes place in step  104 . Following successful prefilling in step  104 , rebranching in the sense of a loop takes place to the starting point of the method in step  101 . If the check in step  103  showed that transverse acceleration A Q  was smaller than the maximum value of transverse acceleration A QMax —denoted by the N option—, then steering angle υ L  of the vehicle will be determined in step  105 . The steering angle is the wheel angle of the front wheels relative to the straight-line rolling direction. In step  106 , it is checked whether steering angle υ L  is greater than a maximally permissible steering angle υ LMax . If this is the case—indicated by the Y option—, branching to step  104  takes place, and prefilling of the wheel brakes occurs as a result. If this is not the case—denoted by the N option—, then yaw rate G of the vehicle will be ascertained in step  107 . The yaw rate in a vehicle denotes the rotation about the vertical axis. This may be determined, for instance, in the form of a maximum value of the rotational acceleration or a maximum value of the angular velocity of the rotation. In step  108 , it will then be checked whether yaw rate G is greater than a maximally permissible yaw rate G max . If this is the case—indicated by the Y option—, branching to step  104  will take place and prefilling of the brake system occurs. If this is not the case—denoted by the N option—, immediate branching to the starting point of the method, i.e., step  101 , is implemented. A delay may be incorporated prior to step  104 , which is marked by step  109  including a time delay At, so that the prefilling is activated at a delay Δt. As an alternative to the time delay, a delay tied to other conditions may be provided.