Patent Publication Number: US-2023136153-A1

Title: Method for operating a brake system of a motor vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Priority Application No. 102021128764.2, filed Nov. 4, 2021, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The disclosure relates to a method for operating a brake system of multiple-track motor vehicle. 
     BACKGROUND 
     The brake systems of modern motor vehicles cooperate with a multiplicity of open-loop and closed-loop control systems in the vehicle. The systems intervene in the normal operation of the vehicle in a manner dependent on the driving situation, and cause an actuation of the brakes of individual wheels of the motor vehicle automatically. In other words the systems act independently of the actions of the vehicle driver and therefore perform automated brake interventions. For example, the anti-lock braking system (ABS) prevents blocking of the wheels during braking and a traction control system (ASR) avoids spinning of the wheels. 
     It is further customary to provide an electronic driver assistance system to counteract swerving of the motor vehicle by braking individual wheels. This system is often called ESP. This system prevents oversteer of the motor vehicle, for example by braking the respective front wheel on the outside of the corner, whereas understeer is corrected by braking the respective rear wheel on the inside of the corner. 
     A rollover protection system is known as a further assistance system which detects indications for imminent lateral tilting of the motor vehicle and proactively prevents a rollover of the motor vehicle by throttling and braking individual wheels. This system is as rule activated when a possible imminent rollover situation is detected and brakes the wheel with the greatest loading, as a rule the outer front wheel. In addition, the outer rear wheel can also be braked. 
     Within the context of this application, open-loop and closed-loop control systems that can provide active safety are considered to be parts of a general driving dynamics system, wherein these parts communicate among one another. 
     The data, on the basis of which systems of this type make their decisions, is supplied by sensors in the motor vehicle which detect the respective variables. 
     SUMMARY 
     The disclosure describes improved automated brake operations by way of rollover protection system. 
     The disclosure describes a method for operating a brake system of double-track, or multiple-track, motor vehicle which comprises two brakable wheels, wherein the wheels are arranged at opposite ends of an axle, and wherein the vehicle comprises a rollover protection system that can cause braking of the wheels in order to prevent a rollover situation. To this end, the following steps are carried out:
     causing of automatic braking of that wheel of the axle loaded more greatly when cornering by way of the rollover protection system, subsequently   detecting a counter-steering movement when a predefined steering angle change is exceeded in a predefined time period in the direction counter to the cornering direction, and, thereupon,   causing a brake force to be built up at the opposite wheel which is loaded less greatly by way of the rollover protection system.   

     This procedure corresponds for example to the situation of hard counter-steering in a corner or a swerving movement, in the case of which the counter-steering movement causes an impending rollover situation to be expected in the case of the directly imminent loading of the wheel, which has previously been loaded to a lesser extent on the current inner side of the corner. Therefore, the rollover protection system already proactively causes the build-up of a brake force at the brake of this wheel at this time, although the wheel load at this wheel at this time is still relatively low. In this way, this wheel is already braked before it becomes the wheel which is loaded more greatly. The brake effect therefore starts at an earlier time, which leads to improved stabilization of the vehicle. 
     Since the gradient of the brake force build-up is limited by the capacity of the brake system, in particular the power output of a pump and a hydraulic system or a mechanical drive, a higher maximum brake force can thus be generated by way of a longer lead time than if the build-up of the brake force begins only when the rollover protection system requires breaking of this wheel as standard at the time when it becomes the wheel which is loaded more greatly. 
     The rollover protection system can be designed, as is known, in such a way that the wheel of the axle which is loaded more greatly is always braked in the case of an impending rollover situation. 
     The brake system is generally designed in such a way that the brake force can be specified for each wheel individually and differently with regard to time, duration and magnitude. 
     The braking of the wheel, which is loaded to a lesser extent and is opposite on the axle can take place in the case of a counter-steering movement only if at the same time a vehicle lateral acceleration, a vehicle speed, a roll angle and/or a yaw rate of the motor vehicle exceed/exceeds a predefined value. The predefined value is dimensioned in such a way that exceeding for the specific vehicle in the specific driving situation might lead to a rollover event. The current values are advantageously provided by sensors which are present in the vehicle. 
     In order to avoid abrupt braking manoeuvres, a brake force request which is specified by the rollover protection system for the wheel which is loaded to a lesser extent is initially increased steadily and, as soon as this wheel becomes the wheel which is loaded more greatly, is increased rapidly. Since the brake force increase for the new wheel which is loaded more greatly does not begin at zero, but rather at the brake force value which has already been reached up to the time of the load change, a higher maximum brake force can be achieved, in particular, than if the build-up of the brake force began only at the load change. 
     Suitable data for determining the presence of a counter-steering movement can be obtained for example by detecting a steering wheel angle, a wheel lock angle, a roll angle, a yaw rate of the motor vehicle, a vehicle speed and/or a lateral acceleration of the motor vehicle. 
     The wheels which are braked by the rollover protection system can be the front wheels, and the axle is a front axle of the motor vehicle. It is optionally also possible for the braking of wheels on a rear axle of the motor vehicle to be caused by way of the rollover protection system in addition or as an alternative. 
     In addition, it is possible, during a direction change of a steering movement, for the two brakeable wheels on the axle to be braked if the rollover protection system detects a phase with low lateral acceleration. This situation can occur, for example, in the case of a lane change or generally in the case of two corners with an opposite curvature which follow one another, where a brief straightahead driving phase necessarily occurs. An aim here, in a driving situation in which braking does not have a negative influence on the stability of the motor vehicle, is to dissipate kinetic energy. Here, an ABS system can of course act for example in an assisting manner in order to reduce the vehicle speed as greatly as possible. 
     It is also conceivable, during a direction change of a steering movement, if the rollover protection system detects a phase with low lateral acceleration, for all the wheels on all the axles to be braked if a particularly large amount of kinetic energy is to be dissipated. 
     An algebraic sign change of the lateral acceleration, the detection of a counter-steering movement and/or the detection of a change of the wheel which is loaded more greatly of an axle can be incorporated into the determination of the time to brake the two brakeable wheels on an axle. All of these values are suitable for detecting a brief phase of straightahead driving, in which the two wheels can be braked without problems. 
     The double-sided braking of the wheels is ended, for example, when the rise in the lateral acceleration after a change in the algebraic sign or a rise in the yaw rate above a predefined value is detected. At this time, for example, the rollover protection system can change back again to single-sided braking of the wheel which is loaded more greatly, or can end the entire brake operation. 
     All the described method steps can always be carried out exclusively in driving situations, in which the rollover protection system is already active in its standard function, that is to say when an impending rollover situation has been detected. 
     It is fundamentally possible for the described rollover protection system to be used together with one or more autonomously operating assistance systems, for example an emergency steering assistance system, by which the current driving situation is handled without the influence of a human vehicle driver. 
     In general, a model of the driving dynamics of the vehicle can be stored in the driving dynamics system, with the result that the driving dynamics system knows the vehicle behaviour in certain driving situations and also fundamental vehicle data such as the centre of gravity and the wheelbase. These data are supplemented, for example, by current values which are supplied by the various sensors in the vehicle and are generally also available to the assistance systems. 
     An electronic open-loop and/or closed-loop control unit can be used as rollover protection system which can be realised, in particular, as an independent electronic unit or in an integrated manner in other vehicle systems, in particular the driving dynamics system. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the following, the disclosure provides greater detail with reference to the appended figures, in which: 
         FIG.  1    shows a diagrammatic illustration of a motor vehicle with a brake system for carrying out a method according to the disclosure, 
         FIG.  2    shows a diagrammatic illustration of driving situations, in which the method according to the disclosure can be applied, 
         FIG.  3    shows a diagrammatic illustration of a brake force profile in accordance with a situation from  FIG.  2   , 
         FIG.  4    shows a fundamental sequence of method steps of a method according to the disclosure, 
         FIG.  5    shows a diagrammatic illustration of a brake force profile of a situation from  FIG.  2   , 
         FIG.  6    shows a fundamental sequence of method steps of a method according to the disclosure, and 
         FIG.  7    shows a further diagrammatic illustration of a brake profile in accordance with a method according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a double-track motor vehicle  10  (here, a passenger car) with a total of four wheels  12  which are arranged on two axles  14  in such a way that in each case two wheels  12  are arranged at opposite ends of an axle  14 . The axles  14  extend along the transverse direction Q of the motor vehicle  10 , which transverse direction Q lies perpendicularly with respect to a longitudinal direction L. 
     One of the axles  14  is a front axle  14   v , the two wheels  12  on this axle accordingly also being called a left-hand and right-hand front wheel  12   L ,  12   R . The other axle  14  is a rear axle  14   H  which supports the other two wheels  12 . 
     In this example, all the wheels  12  can be braked independently and individually by way of a brake system  16  which is merely indicated in  FIG.  1   , it being possible for the time, duration and brake force F to be selected in a situation-dependent manner and individually for each wheel  12 . 
     The brake system  16  can be of hydraulic design, electromechanical design, or can be designed in a combination consisting of hydraulic and electromechanical components. 
     Here, the driving direction of the motor vehicle  10  can be influenced via a steering wheel  18  which can be actuated by a vehicle driver. 
     As an alternative or in addition, it is also conceivable that the steering movement is taken over by an assistance system (not shown), or the motor vehicle  10  is generally designed for autonomous driving. 
     Various known sensors  20  and the motor vehicle  10  detect current values for suitable variables such as, for example, a steering wheel angle, a wheel lock angle, a vehicle speed, a yaw rate and a roll angle about a vertical axis V the of the motor vehicle  10 , a transverse acceleration and similar variables, and possibly also data relating to the surrounding area of the motor vehicle  10 . 
     The sensors  20  are connected to a general driving dynamics system  22 , in which various active safety systems of the motor vehicle  10  are combined here, and in which a vehicle model is stored which allows predictions about how the motor vehicle  10  will behave in certain driving situations. 
     Moreover, there is a rollover protection system  24  which is designed here as part of the driving dynamics system  22  and which is designed to detect potential rollover situations and to take measures to prevent them. To this end, the rollover protection system  24  can fundamentally carry out the standardized procedure that, in the case of detection of a critical driving situation, it causes the wheel which is loaded more greatly of the two wheels  12  on the front axle  14   v  and possibly also the wheel which is loaded more greatly of the two wheels  12  on the rear axle  14   H  to be braked in an automated manner with a relatively high brake force F, in order to stabilize the motor vehicle  10  again. The respective brake requests are fulfilled by the brake system  16 . 
     Both the driving dynamics system  22  and the rollover protection system  24  are realised here as purely electronic systems. They can be combined in a single electronics unit, or can be installed on a plurality of separate units in the motor vehicle  10 . 
       FIG.  2    shows a driving situation in steps I to V, in which, coming from the right, the motor vehicle  10  turns into a left-hand corner and describes a right-hand corner after a brief straightahead driving phase. 
       FIG.  3    shows the profile of the brake force F for the left-hand front wheel  12   L  in  FIG.  2    during the left-hand corner. 
       FIG.  4    shows the fundamental procedure of a method to brake the motor vehicle  10  while driving around the left-hand corner. 
     The speed of the motor vehicle  10  and the curvature radius of the corner are such, in this example, that the rollover protection system  24  responds at the entry into the corner and already causes the front wheel  12   R  which is on the outside of the corner and is loaded to a more pronounced extent on the front axle  14   v  and, in this example, also the rear wheel  12  which is on the outside of the bend and is loaded more greatly on the rear axle to be braked (sections I and II in  FIG.  2   ). 
     The braked wheels are in each case shown in red in  FIG.  2   . 
     In this situation, a violent counter-steering movement is detected, in the case of which a predefined steering angle change is exceeded in a predefined time period in the direction counter to the current corner direction. The predefined steering angle change in the predefined time period are selected in such a way, for example in general by the driving dynamics system  22 , that a continuation of the risk of a rollover situation is to be expected if they are exceeded in the given conditions. 
     In order to assess the situation, current sensor values for the vehicle lateral acceleration, the vehicle speed, the roll angle and/or the yaw rate of the motor vehicle  10  are also used here, for which respective predefined values are likewise defined. 
     The predefined values can in general vary, for example, in a manner which is dependent on the driving situation, possibly also on the ambient conditions or loading of the vehicle. Values of this type are stored, for example, in the driving dynamics model of the driving dynamics system  22 . 
     As a consequence of the detected counter-steering movement, in addition to the right-hand front wheel  12   R  on the outside of the bend which is loaded more greatly and has already been braked up to now, the rollover protection system  24  causes the opposite front wheel  12   L  which is on the inside of the corner and is currently loaded to a lesser extent to be braked (see section III in  FIG.  2    and time t 0  in  FIG.  3   ). 
     As  FIG.  3    illustrates, the brake force F is built up constantly here, starting from zero, with a relatively low gradient (see curve section  26 ). 
     As a result, the left-hand front wheel  12   L  is already braked from this time t 0  with a small but rising brake force F. 
     At time t 1  in  FIG.  3   , the motor vehicle  10  has then moved counter to the first corner direction to such an extent that the load distribution changes, and the left-hand front wheel  12   L  which is on the inside of the corner and was previously loaded to a lesser extent then becomes the wheel on the outside of the corner which is loaded more greatly (see section IV in  FIG.  2   ). As  FIG.  3    shows, the brake force request is then increased rapidly to just below the maximum value, which corresponds to the standard request of the rollover protection system  24  for the wheel which is loaded to a lesser extent (see curve section  28  in  FIG.  3   ). 
     Since, however, the brake force F for the left-hand front wheel  12   L  has already risen to a value which is different from zero, the maximum brake force is reached at an earlier time and also assumes a higher value than would be the case if the wheel  12   L  were braked only from time t 1  in accordance with the standard of the rollover protection system  24  (see curve section  30  in  FIG.  3    compared with the dashed line  32  in  FIG.  3   ). 
     From time t 1 , the right-hand front wheel  12   R  which is then on the inside of the corner is no longer braked, since no further counter-steering movement has been detected or predicted in this example (section IV in  FIG.  2   ). 
     Section V in  FIG.  2    represents the situation, in which the motor vehicle  10  again moves during normal operation and a risk of a rollover is no longer predicted. The rollover protection system  24  accordingly at this time deactivates the braking operations on all the wheels  12 , this taking place with a rapidly but constantly decreasing brake force F, as the curve section  34  in  FIG.  3    shows. 
     The continuous line  36  in each case shows the actual profile of the brake force F at the respective wheel  12 . 
     In another variant,  FIG.  2    depicts the planned passing through of an S-shaped corner which is composed of a left-hand corner with a directly following right-hand corner. A driving situation of this type corresponds, for example, to a lane change. 
     In this situation, a steering assistance system can optionally have taken over vehicle steering, which steering assistance system has a suitable image of the surrounding area of the motor vehicle  10  available and which provides a prediction for the entire corner course which already comprises the planned counter-steering and the load change between the from wheels  12   L ,  12   R . 
     The description of the driving situation is possible, for example, in a known way via a finite state machine which depicts all the states which the brake system  16  can assume for a situation of this type, and the possible state changes which lead to the states. A finite state machine of this type can form the basis of the software in the rollover protection system  24 . 
     The upper curve in  FIG.  5    shows the brake force profile for the right-hand front wheel  12   R  which is on the outside of the corner at the beginning, while the lower curve describes the left-hand front wheel  12   L  which is on the inside of the corner at the beginning. 
     In this example, a potential rollover situation is detected at the beginning of the first steering movement into the left-hand corner, and the rollover protection system  24  causes breaking of the right-hand front wheel  12   R  which is on the outside of the corner and, in this example, also of the rear wheel  12  which is on the outside of the corner. 
     To this end, the brake force request which is caused by the rollover protection system  24  is set suddenly to just under the maximum possible value and is then increased steeply but constantly to the maximum value (see curve sections  28 ,  30  in  FIG.  5    at the top). 
     The brake force F which actually prevails at the respective wheel  12   L ,  12   R  follows the specification of the curve sections  28 ,  30  (see in each case curve  36 ) with a corresponding time delay. 
     The left-hand front wheel  12   L  is not yet braked at this time. 
     Since the left-hand corner is followed by a right-hand corner, a reversal of the corner curvature necessarily takes place which is associated with a brief straightahead driving phase (see section III in  FIG.  2   ). During this directional change of the steering movement, in a phase with a low lateral acceleration, a small yaw angle and/or a small roll angle, the rollover protection system  24  causes the two wheels  12   L ,  12   R  on the front axle  14   v  to be braked, that is to say the left-hand front wheel  12   L  which is currently loaded to a lesser extent to also be braked in addition to the right-hand front wheel  12   R  are which is already braked. 
     In  FIG.  5   , this begins at time t 0 , the brake force F being increased with a considerably smaller gradient than in section  28  of the curve to a value F B  below the maximum brake force value (see curve section  26  in  FIG.  5    at the bottom). Since only a slow rise in the brake force F is specified, the actually produced brake force F can follow this specification relatively promptly (see curve  36 ). 
     In this straightahead driving phase, the two front wheels  12   L ,  12   R  are therefore braked, part of the kinetic energy of the motor vehicle  10  being dissipated. It would also be conceivable to increase the brake force F here rapidly to a higher value, in order to dissipate a maximum amount of kinetic energy of the motor vehicle  10  in this phase. The two wheels  12  of the rear axle  14   H  might likewise be braked. 
     At time t 1 , the effects of the counter-steering in the following corner section which is curved in an opposite direction can be seen, and a load change takes place for the left-hand front wheel  12   L  which has up to now been loaded to a lesser extent and now becomes the wheel which is loaded more greatly. For this reason, the standard setting of the rollover protection system  24  also engages again at this time to brake the wheel which is loaded more greatly to a maximum possible extent, for which reason a sudden rise in the brake force F is requested (see curve section  28 ,  30  in  FIG.  5    and section IV in  FIG.  2   ). 
     At the same time, the brake force F at the right-hand front wheel  12   R  which is then loaded to a lesser extent is reduced. 
     Since, however, the motor vehicle  10  is still situated in a (relative) straightahead driving state, the rollover protection system  24  causes the brake force F to be held at a middle level F H  (see curve section  38 ) and therefore to still brake the two wheels  12   L ,  12   R . 
     For example, a reduction and, moreover, an algebraic sign change of the lateral acceleration and/or the roll angle, a detection of a counter-steering movement and/or a detection of the change in the front wheel  12   L ,  12   R  which is loaded more greatly are used to detect the beginning and the end of that phase of straightahead driving. 
     Here, the double-sided braking of the front wheels  12   L ,  12   R  is ended, for example, when a rise in the lateral acceleration after an algebraic sign change or arise in the yaw rate or the roll angle beyond a predefined limit value is detected. 
     The fundamental sequence of this method is shown in  FIG.  6   . 
       FIG.  7    shows an exemplary profile for the request of the brake force F by way of the rollover protection system  24 . 
     At time t 0 , the wheel  12  which is loaded to a lesser extent is braked, the brake force F being requested with a small, constant gradient in the curve section  26 . The brake force F which is requested in this phase is limited to a maximum value F B  which is considerably smaller than the maximum possible brake force value. If no load change of the wheels takes place, the brake force F is held at the level F B . This is shown by the curve section  40 . 
     Here, a load change occurs at time t 1 , with the result that the wheel which has up to this time been loaded to a lesser extent then becomes the wheel which is loaded to a greater extent on the axle  14  under consideration. Accordingly, the rollover protection system  24  requests a sudden increase in the brake force F (curve section  28 ) as far as a considerably higher maximum value than the value F B . 
     As a rule, only a relatively short brake pulse is set by way of this maximum value, with the result that the brake force request is rapidly reduced again. 
     In this case, however, a straightahead driving phase is still detected (the rollover protection system  24  also at the same time detecting a danger position), with the result that the wheel which is now loaded more greatly is braked further with a brake force request F H , in order to continue to brake the two wheels  12  on the axle  14 . Here, the brake force F H  lies between the brake force F B  and the maximum brake force (see curve section  38 ). 
     At the same time, the opposite wheel  12  (not shown here) which is currently loaded to a lesser extent on the axle  14  is braked at most with the brake force F B . 
     If the danger situation is over, the brake force request is reduced constantly (see curve section  34 ). 
     It is generally possible that the rollover protection system  24  in each case only causes the wheels  12  on the front axle  14   v  to be braked. It is also conceivable, however, for the wheels  12  on the rear axle  14   H  to also be braked in addition or as an alternative.