Method for adjusting brake pressures, brake system of a motor vehicle for carrying out such method, and motor vehicle comprising such a brake system

A method for adjusting brake pressures on wheel brakes of a motor vehicle includes adjusting, in a normal braking mode as a function of a driver's braking demand determined by a driver of the motor vehicle, the brake pressures on the wheel brakes. The method further includes carrying out, by a brake control unit in a pressure control mode, adjustment of the brake pressures on the wheel brakes to implement at least a drive stability function and/or external braking demands. Furthermore, the method includes determining, by the brake control unit, the brake pressures on the wheel brakes at least when implementing external braking demands while controlling an ideal pressure distribution ratio of the brake pressures at wheel brakes of the front axle to the brake pressures at wheel brakes of a rear axle of the motor vehicle, wherein the control function takes into account a pressure distribution ratio.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application under 35 U.S.C. § 371 of International Application No. PCT/EP2017/000711 filed on Jun. 20, 2017, and claims benefit to German Patent Application No. DE 10 2016 009 997.6 filed on Aug. 17, 2016. The International Application was published in German on Feb. 22, 2018, as WO 2018/033229 A1 under PCT Article 21(2).

FIELD

The invention concerns a method for adjusting brake pressures on wheel brakes of a motor vehicle, to a brake system for carrying out such a method, and to a motor vehicle having such a brake system.

BACKGROUND

The wheels of a motor vehicle are braked to decelerate the motor vehicle. For this purpose, wheel brakes of the wheels each comprise brake cylinders that are actuated by means of a working medium, i.e. hydraulically or pneumatically. In the case of utility vehicles, the desired brake pressure in the brake cylinders is produced pneumatically as a rule.

With modern brake systems with brake control units, the brake pressure is adjusted directly by the driver of the motor vehicle in a normal braking mode. As a rule, the driver transmits his braking demand by operating a brake pedal. Typically, a service brake valve is actuated by means of the brake pedal and as a result the brake cylinders are supplied from a pressure reservoir.

Alternatively to the normal braking mode, the brake control unit carries out the adjustment of the brake pressures in a pressure control mode, wherein the brake pressure is adjusted according to the specification of the brake control unit when determining corresponding braking requirements or control requirements. Such braking requirements can for example be the implementation of drive stability functions that access a pressure medium reservoir, such as for example those of an anti-lock braking system (ABS), if the control unit detects the presence of a tendency to locking of certain wheels.

An anti-lock braking system continuously monitors the revolution rates of each wheel by means of measurement signals of revolution rate sensors and determines therefrom the respective wheel slip. This can for example be carried out by comparing the wheel speed determined from the wheel revolution rate with a vehicle reference speed. If a tendency to locking of the wheel is detected from the wheel slip determined in this way, meaning that a specified slip limit is reached or exceeded, the brake control unit takes over control of the adjustment of the brake pressure.

DE 10 2009 058 154 A1 discloses such a brake system, the brake control unit of which carries out the adjustment of the brake pressure in the pressure control mode. In this case, the brake control unit also carries out the adjustment of the brake pressure in the pressure control mode to implement further external braking demands. External braking demands are independent of the driver's braking demand and are specified by external driver assistance systems of the brake control unit, for example. As systems implemented separately from the brake control unit, driver assistance systems output braking demand signals, i.e. XBR requests (“eXternal Brake Requests”), to the brake control unit of the brake system according to the desired braking performance, for example by means of a data bus.

The brake control unit must aim for driving behavior that is as stable as possible during adjustment of the brake pressures at the individual wheel brakes of the motor vehicle. In doing so, the brake pressure at the front wheels must have a pressure distribution ratio to the brake pressure at the rear wheels such that the front axle locks before the rear axle at all times, i.e. in particular for different loads and load states of the motor vehicle.

SUMMARY

In an embodiment, the present invention provides a method for adjusting brake pressures on wheel brakes of a motor vehicle. The method includes adjusting, in a normal braking mode as a function of a driver's braking demand determined by a driver of the motor vehicle, the brake pressures on the wheel brakes. The method further includes carrying out, by a brake control unit in a pressure control mode, adjustment of the brake pressures on the wheel brakes to implement at least a drive stability function and/or external braking demands. Furthermore, the method includes determining, by the brake control unit, the brake pressures on the wheel brakes at least when implementing external braking demands while controlling an ideal pressure distribution ratio of the brake pressures at wheel brakes of the front axle to the brake pressures at wheel brakes of a rear axle of the motor vehicle, wherein the control function takes into account a pressure distribution ratio starting value that is determined in the normal braking mode and that is stored and maintained to be subsequently taken into account in the pressure control mode.

DETAILED DESCRIPTION

It has been proposed to adjust the brake pressures in the pressure control mode while controlling an ideal pressure distribution ratio. In doing so, the pressure distribution between the axles is implemented while taking into account a differential slip between said axles such that there is a differential slip that is as small as possible. In this case, the differential slip shall be kept as small as possible, so that for an ideal pressure distribution or for an ideal pressure distribution ratio all wheels of the vehicle lock up at the same time or almost at the same time. In this way, the desired deceleration is achieved faster as a rule. In addition, the reduction of the differential slip or the adjustment of an ideal pressure ratio prevents an unwanted intervention by the anti-lock function. An unwanted intervention by the anti-lock function is excluded or is carried out as late as is actually possible owing to the specified physical conditions. A specification of the pressure distribution ratio is adjusted as a function of the differential slip in this case, which requires a certain time in the context of control. Unfortunately, an ideal pressure distribution is unknown in the prevailing driving situations of a motor vehicle and varies over a large range as a function of the load on the motor vehicle. The probability of one or more of the wheels tending to lock is therefore high in practical driving operations.

Embodiments of the invention enable very rapid adjustment of an ideal pressure distribution ratio by a brake control unit in a pressure control mode.

According to embodiments of the invention, in the normal braking mode a starting value for the pressure distribution ratio (pressure distribution ratio starting value) is determined for a controller and is stored and maintained to subsequently be taken into account in the pressure control mode. The influence of the load state of the utility vehicle on the ideal pressure distribution ratio is already taken into account by determining the pressure distribution ratio starting value in the normal braking mode, so that the control is based on a pressure distribution ratio that is already close to the ideal value to be controlled. That is, the pressure distribution ratio starting value provides an accurate estimate of the ideal brake pressure distribution ratio, which can be specified as the pressure distribution ratio starting value for the control function. As a rule, only quantitatively small and mainly few control interventions are necessary to adjust the ideal pressure distribution ratio starting from the pressure distribution ratio starting value that is determined according to the invention. The control of the ideal pressure distribution ratio is preferably carried out by controlling the differential slip between the axles. With the invention, pressure distribution ratio starting values are also determined for motor vehicles with more than two axles, wherein preferably pairs of axles each of two axles are considered or the pressure distribution ratio thereof is controlled with a pressure distribution ratio starting value determined according to the invention in the normal braking mode.

The determination of the pressure distribution ratio starting value in the normal braking mode is possible with little Information. In a preferred embodiment of the invention, the revolution rate measurement values for the wheels that are available to the brake control unit are used for determining the pressure distribution ratio starting value. The pressure distribution ratio starting value is determined in this case by detecting a first revolution rate measurement value for the front axle and a second revolution rate measurement value for the rear axle and determining a reference value of a motion variable of the motor vehicle at the same measurement time. In this case, the reference value can be detected by a separate motion sensor of the motor vehicle or can be derived by analyzing the revolution rate measurement values of the axles. The revolution rate measurement values are detected by revolution rate sensors of the respective wheels, which have a signal transmission connection to the brake control unit. The first revolution rate measurement value of the front axle and the second revolution rate measurement value of the rear axle are each combined with the reference value in a manner representing the wheel slip at the respective axle. In this way, slip values are determined for each axle, which is understood to include both the wheel slip and also other physical motion variables that determine the wheel slip, for example the speed or the acceleration.

The first slip value formed in this way with respect to the front axle and the second slip value with respect to the rear axle are set in a ratio to each other and the distribution index determined in this way is combined with a specified pressure distribution ratio. The specified pressure distribution ratio that is combined, for example multiplied, with the distribution index for the pressure distribution ratio starting value according to the invention, is fixedly specified to the brake control unit or is dynamically determined in the case of an electronic brake system (EBS).

In accordance with the assumption of the ideal pressure distribution ratio, i.e. that there should be no differential slip between the axles being considered, the first slip values and the second slip values are taken into account reciprocally in the distribution index with reference to the axles in the pressure distribution ratio. The following equation for determining the pressure distribution ratio starting value results from this by taking into account the respective wheel slip as slip values to be processed of the respective axles:
(PFA/PRA)=(λRA/λFA)*(PFA/PRA)nonideal

In the equation, the brake pressures are indicated by the symbol “P”. First slip values for the front axle and second slip values for the rear axle are determined as respective wheel slip and represented in the equation by the symbol “λ”. The index “FA” stands for a variable associated with the front axle in the equation. Accordingly, variables related to the rear axle are denoted by the Index “RA”. The specified pressure distribution ratio, which is treated as non-ideal and according to the invention is linked to the distribution index, is denoted by the Index “nonideal”.

Advantageously, for determining the slip values for the respective axles from the first revolution rate value of the front axle and the second revolution rate value of the rear axle, a translational motion variable of the motor vehicle is determined and combined with the reference value in the same physical variable. If in an advantageous embodiment the speed is taken into account as a motion variable, then speeds are derived from the revolution rate measurement values of the axles and the reference value is also determined as the reference speed of the motor vehicle. The following equation for the determination of the pressure distribution ratio starting value results therefrom:
(PFA/PRA)=((vRA−vVeh)/(vFA−vVeh))*(PFA/PRA)nonideal

In the above equation, the speed is denoted by the symbol “v”. In this case, the index “Veh” denotes a reference variable related to the motor vehicle, i.e. the reference speed of the motor vehicle in this case.

In a further advantageous embodiment of the invention, the acceleration is taken into account as a motion variable for the determination of the pressure distribution ratio starting value. The following equation, in which the acceleration is denoted by symbol “a”, results therefrom:
(PFA/PRA)=((aRA−aVeh)/(aFA−aVeh))*(PFA/PRA)nonideal
If the brake control unit has no signal related to the speed of the vehicle available, the measurement value of a longitudinal acceleration sensor is used for the determination of the pressure distribution ratio starting value. If information about the longitudinal acceleration is also unavailable, the speed of the vehicle is derived as a reference value from the individual revolution rate measurement values of the wheels, for example the highest speed value of the detected wheels during a braking process is referred to as a reference value for the determination of slip values.

In one advantageous embodiment of a brake system, the pressure control valves are disposed in one or more brake circuits, which can be connected via respective activation valves to a pressure medium reservoir, wherein each activation valve is electrically connected to the brake control unit and can be switched by the brake control unit. To change the braking mode, i.e. a change from the normal braking mode to the pressure control mode and vice-versa, the brake control unit changes the valve setting of the activation valve. In the pressure control mode, the brake control unit enables a fluidic pneumatic connection between the pressure control valve and the pressure medium reservoir by means of the activation valve of the relevant brake circuit, so that the brake pressure of the wheel brakes can be adjusted by means of the respective pressure control valve. Advantageously, the pressure control valves of the wheels of an axle of the motor vehicle are connected to a pressure medium reservoir via a common brake circuit with an activation valve of said brake circuit.

The activation valves are advantageously embodied as 3/2-way valves, for example solenoid valves, wherein the pressure control valve is connected to the working connection of the activation valve and in a first valve setting is connected to the service brake valve. Said first valve setting is provided for the normal braking mode. For activating the pressure control mode, i.e. for implementing a drive stability function and/or an external braking demand, the activation valve is switched into the second valve setting and the braking medium reservoir is switched through directly to the pressure control valve.

FIG.1shows an electrical-pneumatic schema of a pneumatic brake system1of a motor vehicle2, namely a utility vehicle. Electrical lines are represented with solid lines and pneumatic lines with dotted lines. The motor vehicle2comprises two axles in the exemplary embodiment shown, namely a front axle3lying forwards in the direction of travel and a rear axle4, on each of which wheels5are disposed on both sides. For decelerating the motor vehicle2, a wheel brake6is associated with each wheel5. The wheel brakes6are pneumatically operated and each comprises a brake cylinder7. The wheel brakes6exert a braking force on the rotating wheel5according to the respective pneumatic brake pressure acting in the brake cylinder7.

A brake pedal8that is disposed in the driver's cab of the motor vehicle2is coupled to a service brake valve9. The driver of the motor vehicle2switches pneumatic pressure through to the brake cylinders7by operating the brake pedal8and thereby actuates the wheel brakes6. For this purpose, the service brake valve9controls pneumatic brake lines10,11between the pressure medium reservoirs12,13and the brake cylinders7.

In the exemplary embodiment shown, the brake system1comprises two brake circuits, wherein a first brake circuit14is associated with the front axle3and a second brake circuit15is associated with the rear axle4. The brake cylinders7of the wheels5of the front axle3are therefore connected via the brake line10to the pressure medium reservoir12of the first brake circuit14. The brake cylinders7of the wheel brakes6of the rear axle4are connected via the brake line11to the pressure medium reservoir13of the second brake circuit15. The brake pressure in the brake cylinders7can therefore be adjusted in a normal braking mode (reference character29inFIG.2) as a function of the position of the brake pedal8, i.e. as a function of a driver's braking demand32.

A pressure control valve16that is electrically actuated by a brake control unit17in a pressure control mode (reference character18inFIG.2) is associated with each wheel brake6of the brake system1. The pressure control valves16have a signal transmission connection to the brake control unit17for receiving control signals19. The pressure control valves16are each a combination of two solenoid valves, namely an inlet valve20and an outlet valve21. In this case, in principle the inlet valve20is used to increase pressure or to maintain the pressure in the brake cylinders7, whereas the outlet valve21is opened to reduce the brake pressure and vents the respective connected brake cylinder7.

In the pressure control mode18, the brake control unit17carries out the adjustment of the brake pressure of the respective wheel brakes6by correspondingly actuating the pressure control valves16. An electrically actuated activation valve47,48that can be actuated by the brake control unit17, is associated with each brake circuit14,15. Each activation valve47,48is embodied as a 3/2-way solenoid valve, i.e. it comprises three connectors49,50,51and two switch positions. In this case, the supply connector49of the respective activation valve47,48is associated respectively with the brake line10,11, which is alternatively connected to one of the two supply connections50,51according to the switch position of the activation valve47,48.

The first activation valve47of the first brake circuit14thus controls the supply of the brake line10and the second activation valve48of the second brake circuit15controls the supply of the brake line11. A pressure line52to the service brake valve9is connected to a first supply connector50of each of the activation valves47,48. The second supply connector51is connected to the pressure medium reservoir12,13of the respective brake circuit14,15. In the normal braking mode, the activation valve47,48connects the brake line10,11to the service brake valve9, so that the brake pressure can be adjusted by means of the service brake valve9. In the pressure control mode18, the brake control unit17can connect the brake line10,11directly to the respective pressure medium reservoir12,13by changing the switch position of the activation valve47,48.

The brake control unit17is embodied and configured to implement drive stability functions and/or external braking demands to carry out the adjustment of the brake pressures in the pressure control mode18. The brake control unit17continuously receives revolution rate measurement values22,33from revolution rate sensors23of the wheels5of the motor vehicle2. For this, one revolution rate sensor23each is associated with each wheel5of the motor vehicle2, so that the brake control unit17always has a revolution rate measurement value22,33for each wheel5available. The brake control unit17is, in addition to the revolution rate sensors23and the pressure control valves16, an essential component of an anti-lock braking system of the brake system1and implements the drive stability function45thereof, namely an anti-lock function. The brake control unit17monitors the tendency to locking of the respective wheels5by means of the revolution rate measurement values22,33.

By analyzing the revolution rate measurement values22,33, the brake control unit17infers the tendency to locking of the respective wheel5. I.e., if the controlled braking force exceeds the maximum transferable braking force at one or more wheels, said wheel starts to lock up, whereby the motor vehicle2can become unstable. By means of the revolution rate sensors23, the anti-lock function of the brake control unit17monitors the revolution rate of each wheel5. In doing so, a motion variable (reference character38inFIG.2) determined from the revolution rate measurement values22,33, for example the wheel speed, is compared with a calculated or measured reference value24for the corresponding motion variable of the motor vehicle2at the same respective measurement time. If a tendency to locking of the wheel5is detected by means of the wheel slip determined in this way, i.e. if a specified slip limit is reached or exceeded, the brake control unit17carries out the control over the adjustment of the brake pressure in the pressure control mode18. In this case, a reduction of the brake pressure is carried out in a first step in order to then control the brake pressure of the relevant wheel5along the slip limit. Should a suitable motion variable of the motor vehicle2be known by other modules of the brake system1, then the corresponding information is specified to the brake control unit17as a reference value24for determining slip. In the exemplary embodiment shown, a longitudinal acceleration sensor25is disposed on the motor vehicle2, from the measurement signals of which the brake control unit17derives the desired reference values24.

The brake control unit17is embodied to receive external braking demands26from driver assistance systems of the motor vehicle2. If there are external braking demands26present, then the brake control unit17carries out the adjustment of the brake pressures. For this purpose, the brake control unit17brings the activation valves47,48into the switch position provided for the pressure control mode18(i.e. direct switch through to the pressure medium reservoir) and feeds control signals19to the pressure control valves16of the respective wheel brakes6to implement the external braking demand26.

In the pressure control mode18(FIG.2), at least when implementing external braking demands26, the brake control unit17adjusts the brake pressures while controlling an ideal pressure distribution ratio of the brake pressures at the wheel brakes6of the front axle3to the brake pressures at the wheel brakes6of the rear axle4of the motor vehicle2. In this case, the ratio of the brake pressures of the front axle3to the brake pressures of the rear axle4at which the same wheel slip is determined at the wheels5is adopted as an ideal pressure distribution ratio. This is carried out advantageously by monitoring a differential slip as the difference of the wheel slip of the two axles that are being considered.

By controlling the pressure distribution ratio, an unwanted intervention by the anti-lock function is prevented, which is excluded as long as, or is carried out as late as, is actually possible under the given physical conditions. All wheels5of the motor vehicle2lock up at the same time or almost at the same time.

The control function27(FIG.2), the control variable of which is the brake pressure by means of varying the control signals19for the pressure control valves16, takes into account a pressure distribution ratio starting value28of the pressure distribution ratio, which is determined in the normal braking mode29(FIG.2) and which is maintained to be taken into account subsequently in the pressure control mode18. The determination of said pressure distribution ratio starting value28for the control function27is described in detail below usingFIG.2. A memory element30is associated with the brake control unit17for storing31the pressure distribution ratio determined in the normal braking mode29, which is read out as the pressure distribution ratio starting value28for the control function27at the start of the pressure control mode18.

FIG.2shows in a flow chart an exemplary embodiment for the determination of a pressure distribution ratio starting value28for a control function27of the pressure distribution ratio54of the brake pressures P between the front axle (reference character3inFIG.1) and the rear axle (reference character4inFIG.1). In the pressure control mode18, the brake control unit produces control signals19for the pressure control valves (reference character16inFIG.1) while controlling27an ideal pressure distribution ratio54of the brake pressures P at the wheel brakes of the front axle to the brake pressures P at the wheel brakes of the rear axle. In this case, the brake pressure P is influenced by the control function27by suitable actuation of the pressure control valves with control signals19. Alternatively to the pressure control mode28, the brake pressures P at the wheel brakes in the normal braking mode29are adjusted as a function of a driver's braking demand (reference character32inFIG.1) that is determined by the driver of the motor vehicle. In the normal braking mode29, the driver of the motor vehicle thus has full control over the adjustment of the brake pressure, wherein the brake control unit does not actively intervene in the adjustment of the brake pressure, but passively monitors the prerequisites for intervention to implement drive stability functions45.

A drive stability function45of this type is that of the anti-lock braking system (FIG.1), wherein the tendency to locking of the wheels is monitored. The revolution rate measurement values22,33of the revolution rate sensors23on the wheels are combined with a reference value24of a motion variable of the motor vehicle at the same measurement time in a manner representing the wheel slip, and slip values are determined for each wheel, namely the wheel slip λ in the exemplary embodiment according toFIG.2. For the combination36with the reference value24, motion variables38that correspond to the motion variable of the motor vehicle analyzed as a reference value24are determined in an analysis37of the revolution rate measurement values22,33.

In the exemplary embodiment according toFIG.2, the speed v is taken into account as a motion variable38, wherein the determination of the wheel slip λ is carried out according to the following formula:
λ=(v−vVeh)/vVeh

Instead of the speed v, alternatively the acceleration a can be taken into account as a motion variable38.

In the exemplary embodiment according toFIG.2, the brake control unit17adjusts the brake pressure to implement external braking demands26while controlling27the brake pressure ratio54. In the pressure control mode18, to implement the drive stability function45of the anti-lock braking system, control53of the brake pressure P is carried out by means of the pressure control valves16without taking into account the brake pressure ratio. In further exemplary embodiments that are not illustrated, taking into account the pressure distribution ratio is also provided in the event of anti-lock interventions.

The wheel slip λ of the respective wheel is compared with a specified threshold value40in a comparison step39. In doing so, exceeding the threshold value40signals a tendency to locking of the relevant wheel. If the wheel slip λ of one or more wheels exceeds the specified threshold value40, then a change of braking mode is carried out and the brake control unit17carries out the adjustment of the brake pressures in the pressure control mode18. As a result, the drive stability function45of the anti-lock braking system is implemented. If none of the determined values of the wheel slip λ exceeds the specified threshold value40, then the brake control unit17remains in the normal braking mode29and in passive monitoring of the ride stability by comparing the wheel slip λ with the threshold value40.

In a detection step41, the brake control unit detects the presence of an external braking demand26. If there is an external braking demand26present, then a change of braking mode is carried out and the brake system changes to the pressure control mode18, in which the brake control unit17carries out the adjustment of the brake pressures P.

If the brake control unit17changes to the pressure control mode18to implement an external braking demand26, then the brake control unit17actuates the activation valves47,48(FIG.1) and thereby initiates the active change of the brake pressures P and control27of the ideal pressure distribution ratio54in the pressure control mode18. By the changeover of the activation valves47,48, the pressure control valves16are connected to the pressure medium reservoirs12,13, so that the brake pressure P at the wheel brakes6can be adjusted by means of the pressure control valves16by means of the inlet valve20or outlet valves21thereof.

In the normal braking mode29, a pressure distribution ratio starting value28for the control function27is determined, which is maintained by storing to be subsequently taken into account in the pressure control mode18. From this, a first slip value34for the front axle and a second slip value35for the rear axle are used from the values of the wheel slip λ that are determined during the monitoring of the tendency to locking, for example by the ride stability function45of the anti-lock braking system. The first slip value34for the front axle can be one of the two available values of the wheel slip λ or an average value of the two slip values for the wheels of the front axle. Accordingly, the second slip value35for the rear axle is one of the two wheel slips λ determined for the anti-lock function or an average value of the two values.

The first slip value34and the second slip value35for the pair of the front axle and rear axle under consideration are set46into a ratio relative to each other and thus a distribution index42is determined. The distribution index42is a factor that, in combination43with a specified pressure distribution ratio44, yields the pressure distribution ratio starting value28for the control function27of the pressure distribution ratio in the pressure control mode18. The specified pressure distribution ratio44be can a fixed specification value or is determined dynamically in the case of an EBS.

The distribution index42corresponds to the load state of the motor vehicle, so that by combination43with the specified value of the pressure distribution ratio44, an actual pressure distribution ratio can be determined that is considerably closer to the ideal pressure distribution ratio54of the motor vehicle2in the current load situation.

For the determination of the distribution index42, the first slip values34for the front axle and the second slip values35for the rear axle are taken into account reciprocally in relation to the respective pressure ratios. This corresponds to the assumption that in the ideal pressure distribution ratio the slip values34,35of the axles of the motor vehicle2being considered are equal, or no differential slip exists.

The determination of the pressure distribution ratio starting value28, which forms the basis for the start of the control function27after the change to the pressure control mode18, is advantageously determined during the normal braking mode29during comparatively weak vehicle decelerations, i.e. negative accelerations of for example −1 m/s2to −1.5 m/s2, in order to reduce dynamic influences.

The adjustment of the brake pressure P in a pressure control mode18with a control function27while taking into account a pressure distribution ratio starting value28determined in the normal braking mode29can also be used for motor vehicles with a plurality of axles. Moreover it can also be used for conventional brake systems with an anti-lock braking system (ABS) and for electronic brake systems (EBS).

LIST OF REFERENCE CHARACTERS