EP1716029A1

EP1716029A1 - Method for regulating a brake pressure in case of non-homogeneous coefficients of friction of a roadway

Info

Publication number: EP1716029A1
Application number: EP04804962A
Authority: EP
Inventors: Holger Schmidt; Anthony Dollet; Urs Bauer; Stephan Capellaro
Original assignee: Continental Teves AG and Co OHG
Current assignee: Continental Teves AG and Co OHG
Priority date: 2003-12-23
Filing date: 2004-12-21
Publication date: 2006-11-02
Also published as: JP2007515347A; WO2005063538A1; US20070188020A1; DE112004002250D2; US7775608B2

Abstract

The invention relates to a method for regulating a brake pressure in at least two wheel brakes, which are preferably mounted on a vehicle axle, during a braking action on a roadway having a non-homogeneous coefficient of friction. Said method is characterized in that a side having a low coefficient of friction and/or a side having a high coefficient of friction is/are detected, a stability index representing a driving condition of the vehicle is formed, said stability index is evaluated based on the side having a low coefficient of friction and/or the side having a high coefficient of friction, and the brake pressure is modified in at least one wheel brake in accordance with the stability index value as well as a result of the evaluation of the stability index based on the side having a low coefficient of friction and/or the side having a high coefficient of friction. The invention further relates to a device that is suitable for carrying out said method.

Description

METHOD FOR REGULATING A BRAKE PRESSURE IN INHOMOGENEOUS DRIVE VALUES

Description:

The invention relates to a method for regulating a brake pressure in at least one wheel brake mounted on a vehicle axle, during a braking operation on a roadway with an inhomogeneous coefficient of friction.

The invention also relates to a device for regulating a brake pressure difference between a brake pressure in a wheel brake on a low friction side and a brake pressure in a wheel brake on a high friction side of the vehicle during a braking operation on a road with an inhomogeneous coefficient of friction.

When braking on an inhomogeneous road surface with different coefficients of friction on the left and right side of the vehicle, asymmetric braking forces can occur

Cause yaw moment, which puts the vehicle in a rotational movement about its vertical axis. In order to prevent a spin of the vehicle, the driver must build by appropriate steering movements a compensating yaw moment, which counteracts the moment caused by the asymmetric braking forces. A blocking of the wheels should also be avoided on the vehicle side with the low coefficient of friction, since the associated with the blocking strong Verminde- tion of the transmittable cornering force of a wheel can prevent the construction of the required compensation torque. On vehicles with an anti-lock braking system (ABS), the blocking of the wheels is prevented by a regulator. In situations of the type mentioned a usually applied control strategy pursues the one hand, the goal of delaying the vehicle by a high brake pressure in the wheel brakes on the vehicle side with the higher coefficient of friction with a small braking distance. On the other hand, the driver should not be overwhelmed by a yaw moment caused by different braking forces on the high or low friction side (reaction time of the driver when countersteering).

Therefore, in the situations considered, the control takes place by means of a so-called yaw moment buildup delay (GMA) such that a difference between the brake pressures on the high and the low friction side is built up only slowly at the front axle of the vehicle in order to allow the driver enough time Performing stabilizing steering movements. In the wheel brakes on the rear axle, the brake pressure is limited to the value allowed for the low friction side ("select low") so that sufficient cornering force can be built up on the rear axle to stabilize the vehicle through steering interventions.

Although the measures described allow the driver to control the vehicle more easily, the friction coefficient potential of the high-friction side is not optimally utilized for deceleration of the vehicle. From the German patent application DE 197 07 106 AI it is known to control the brake pressure in the wheel on the wheel located on the Hochreibwertseite, the so-called high wheel, on the basis of a driving condition of the vehicle representing the size, which in dependence Deviation between the yaw rate of the vehicle and a calculated from the set by a vehicle operator steering angle Sollgierrate is formed. Thus, a wheel-specific brake pressure control is performed instead of the above-described select-low control.

However, if the vehicle yaws toward the high friction side, pressure buildup on the high wheel will increase the yawing motion of the vehicle. The known method thus has the disadvantage that it could destabilize the driving condition of the vehicle in possible driving situations.

The invention is therefore based on the object to improve the braking behavior of a vehicle when braking on an inhomogeneous road surface and at the same time to ensure the driving stability of the vehicle reliably

According to the invention this object is achieved by a method according to claim 1.

According to the invention this object is also achieved by a device according to claim 20.

Advantageous developments of the method and the device are the subject of the subclaims.

The invention provides that a method for regulating a brake pressure in at least one vehicle axle mounted on a vehicle axle. brought wheel brake, during a braking operation on a roadway with inhomogeneous coefficient of friction, so performed that

a low-friction-side and / or a high-friction-side of a vehicle is detected, a stability index is formed, which represents the driving state of the vehicle,

- the stability index is valued on the basis of the low-friction-side and / or the high-friction-side, and

the brake pressure in at least one wheel brake is changed on the basis of the low friction side and / or the high friction side in dependence on the value of the stability index "and depending on a result of the assessment of the stability index".

The method has the advantage that it is determined on which vehicle longitudinal side the low or the high frictional value side, and a subsequent evaluation of the stability index ¹ on the basis of the low friction side. This makes it possible to evaluate whether a change in the brake pressure in a wheel brake and in particular an increase in pressure in the wheel brake of the high wheel of the rear axle could lead to a reinforcement of a yaw motion of the vehicle in the direction of Hochreibwertseite and thus to destabilize the driving condition of the vehicle or whether such an adverse effect of changing the brake pressure in a wheel brake is not to be expected.

Thus, it becomes possible in particular to increase the brake pressure in the wheel brake of the high wheel on the rear axle of the vehicle in the driving situations in which this does not lead to impairment of vehicle stability. In such tuations, the braking power can be effectively increased by the method according to the invention.

Preferably, the change in the brake pressure is superimposed on an ABS control. In an advantageous embodiment of the invention, it is therefore provided that an ABS control is performed for a wheel on the low friction side, and that a brake pressure difference between the brake pressures in the wheel brake on the high friction side and in the wheel brake on the low friction side is determined, the Wheel brakes are preferably mounted on a vehicle axle.

Thus, a select-low control can be modified so that the brake pressure at the rear wheel on the high friction side increases in the presence of a stable driving condition and thus the braking distance of the vehicle is shortened.

Advantageously, the stability index is formed as a function of a steering angle at steerable wheels of the vehicle and / or a yaw rate or yaw rate deviation of the vehicle in order to make the vehicle behavior objectively assessable.

Thus, the stability index can take into account quantities which reflect or directly influence the yaw behavior of the vehicle. On the basis of the assessment of the stability index, an immediate assessment of the yaw behavior of the vehicle takes place.

In an advantageous embodiment of the invention, it is provided that the stability index based on a deviation between a current steering angle and at the beginning of a braking process on a road with inhomogeneous Friction value present steering angle is determined.

In this way, it can be determined from the stability index, whether the driver of the vehicle during a braking operation on a non-homogeneous road to generate a yaw moment, which counteracts the Störgiermoment that results from the different braking forces on the high and low friction side and its presence a safe increase in the brake pressure in the wheel of the high-wheel allows.

In a further advantageous embodiment of the invention, the stability index is determined on the basis of a deviation between a current yaw rate of the vehicle and a reference yaw rate determined on the basis of the steering angle present at the beginning of the braking process in a vehicle model.

The reference yaw rate determined in this way represents the yaw rate corresponding to the driver's intention. Based on the difference between the reference yaw rate and the current one

Yaw rate of the vehicle can therefore be recognized again whether the driver has taken appropriate measures to compensate for Störgiermoments.

With particular advantage, the method according to the invention can be carried out in a vehicle in which a desired steering angle can be calculated and set, for example by means of a superposition steering or a steer-by-wire steering, independently of the driver's specification.

In a preferred embodiment of the invention, the stability index thereby becomes dependent on a deviation between a steering angle commanded by the vehicle operator and a set on the steerable wheels of the vehicle target steering angle determined and thus made the driving condition of the vehicle an objective assessment.

The desired steering angle preferably contains a control component which is determined as a function of the disturbance yaw moment in a vehicle model.

The desired steering angle can therefore be determined in particular in such a way that it effects a yaw moment compensating for the disturbance yaw moment.

Advantageously, the desired steering angle also contains a control component which is determined as a function of the yaw rate deviation between the instantaneous yaw rate of the vehicle and a reference yaw rate.

As a result, the vehicle reaction to changes in steering angle in the desired steering angle is taken into account, and this can be adjusted particularly reliably and reliably.

Thus, the heading of the driver can be determined during a braking on inhomogeneous road surface on the basis of the commanded by him steering angle, while at the steerable wheels, a steering angle is set, the driving condition of

Vehicle stabilized quickly and reliably. To adapt the brake pressures, the driving state and in particular the yaw behavior of the vehicle can thus be determined and evaluated, as already described, on the basis of the deviation between the steering angle commanded by the driver and the desired steering angle. In a further preferred embodiment of the invention, which for these reasons is also particularly suitable in connection with the automatic setting of a desired steering angle, it is provided that the stability index is determined as a function of a deviation between a yaw rate of the vehicle and one based on at least one of Vehicle operator predetermined size, preferably the steering angle, formed in a vehicle model Sollgierrate is formed.

It is also advantageous that the stability index in

Dependence of a lateral acceleration of the vehicle is determined.

Furthermore, it is advantageous that the stability index is determined as a function of a float angle and / or a float angle velocity.

In order to evaluate the stability index on the basis of the low-friction-value side and / or on the high-friction-value side, it is provided in a preferred embodiment of the invention that a sign of the stability index is determined as a function of the low-friction-value side and / or as a function of the high-friction-value side.

By selecting the sign of the stability index in this way, no further case distinction with respect to the low-friction-side and / or the high-friction-side is necessary when adjusting the brake pressures based on the stability index.

The brake pressure is here preferably changed as a function of a result of a comparison of the stability index with at least one threshold value. An advantageous embodiment of the invention is characterized in that the brake pressure in the wheel brake on the Hochreibwertseite is increased compared to the brake pressure in the wheel brake on the Niedrigreibwertseite when the stability index exceeds a predetermined threshold.

In order to ensure the driving stability of the vehicle in a particularly reliable manner, the brake pressure difference between the brake pressure in the wheel brake on the low friction side and the brake pressure in the wheel brake on the high friction side are preferably limited.

In an advantageous embodiment of the invention, there is a limitation of the brake pressure difference as a function of a speed of the vehicle.

Due to the inherent dynamics of a vehicle, it has at high speeds a greater tendency to unstable driving behavior and therefore at high speeds preferably only a small or no brake pressure difference is allowed to according to the select-low control a high lateral force potential or a high lateral force reserve at the Holding the rear axle.

In a likewise advantageous embodiment of the invention, in addition, no change in the brake pressure is permitted when the low-friction-side and / or the high-friction-side change because a change of the low or high friction side often leads to unstable driving conditions. The required brake pressure difference equal to zero is maintained until the stability index returns it to an objectively stable state. Biles driving behavior is displayed. Thereafter, depending on the stability index, a renewed pressure build-up is possible again.

In a further advantageous embodiment of the invention, a brake pressure difference at the rear axle is limited to a predetermined proportion of the brake pressure difference and / or brake pressure ratio at the front axle set using an ABS control.

The ABS control takes into account the coefficients of friction existing on the front axle, which can therefore be anticipated - when driving forwards - for the rear axle, which achieves these conditions with a time delay.

The side force potential ensured by means of the known select-low control on the rear axle offers increased safety in stabilizing the vehicle and in the course guidance, especially when cornering.

In an advantageous embodiment of the invention, it is therefore provided that a change in the brake pressure is only made when a straight-ahead driving of the vehicle is determined.

When cornering then the conventional ABS control can be performed, which, although it is associated with a longer braking distance, ensures a particularly high tracking stability of the vehicle. When cornering is recognized, a softening of Select-Low but also conceivable.

The pressure build-ups should, however, be carried out with a lower gradient and the brake pressure difference or the brake pressure ratio between high frictional and low-friction wheel should be more limited in order to provide the vehicle sufficient lateral force reserve for cornering.

The invention also provides an apparatus for carrying out the method. It is a device for regulating a brake pressure difference between a brake pressure in a wheel brake on a low friction side and a wheel brake on a high friction side of a vehicle during a braking operation on a road with an inhomogeneous coefficient of friction

a recognition means for recognizing the low-friction-value side and / or the high-friction-value-side,

a determination means for determining a stability index representing a driving condition of the vehicle, a judging means for evaluating the stability index on the basis of the low friction value side and / or the high friction value side detected in the detection means and

a calculation means for determining the brake pressure difference as a function of a value of the stability index and a result of the assessment of the stability index based on the low-friction-side and / or the high-friction-side.

Further advantages, features and expedient developments of the invention will become apparent from the dependent claims and the following description of preferred embodiments with reference to FIGS.

From the figures shows

1 is a block diagram of a steering angle controller, 2 shows a block diagram of a block of the steering angle regulator illustrated in FIG. 1, in which a control component of an additional steering angle is determined;

3 shows a block diagram of a block of the steering angle controller illustrated in FIG. 1, in which a control component of the additional steering angle is determined;

4 shows a block diagram of a control system for modifying the select-low control,

5 is a flowchart for a method for determining a brake pressure difference and

6 is a flowchart for a method for limiting the brake pressure difference.

By way of example, it is assumed that a two-axle, four-wheeled vehicle has a hydraulic brake system which is designed so that an ABS control can be carried out to control the wheel slip on the wheels of the vehicle. In particular, the vehicle has the necessary, known to those skilled sensors, such as wheel speed and brake pressure sensors, via actuators, such as a controllable pressure supply and controllable valves on the wheel brake cylinders and one or more control devices for controlling the actuators.

However, the invention can also in a simple manner

Vehicles with other brake systems, such as electric or pneumatic braking systems, are transmitted. The vehicle preferably also has a steering system in which the steering angle μ _DRV commanded by the driver can be superposed with an additional steering angle μμ. In this case, the vehicle can be equipped with a so-called overlay steering, in which a planetary gear used in the steering line allows the superposition of the steering movements of the driver with additional steering movements. Likewise, a so-called steer-by-wire steering can be used. It is also possible to use a device for actively influencing the rear wheels (active rear wheel steering systems such as, for example, electromechanical rear wheel steering systems or actively controllable rear axle bearings for generating rear wheel steering angles).

A vehicle equipped in this way makes it possible, during a so-called μ-split braking, ie a braking operation on a roadway with an inhomogeneous coefficient of friction, to set a target steering angle .mu.s, which causes a yawing moment, which transmits the disturbing yaw moment M _Zr the different braking forces on the high friction (high μ) side and the low friction (low μ) side is caused compensated. The vehicle can thus be stabilized quickly and reliably with μ-split braking.

This makes it possible to apply a "more aggressive" brake pressure control in a μ-split situation. In particular, it is provided to set a brake pressure difference μp between the brake pressures in the RadbremsZylindern on the rear axle, in which the brake pressure p _H χg _h in the wheel on the wheel on the Hochreibwertseite (high wheel) against the brake pressure p _low in the wheel at the wheel on the low friction side (low wheel) is increased. This corresponds to a modification of the select-low control explained in the introduction, which causes a faster deceleration of the vehicle in a μ-split braking.

In order to detect a μ-split situation, vehicle-measured and estimated driving dynamics variables as well as brake parameters, which can be provided by a vehicle dynamics control system, are accessed by sensors of the vehicle. This may be a yaw rate control ESP (Electronic Stability Program) and / or an anti-lock braking system (ABS).

It is also checked whether it is a μ-split braking during a straight-ahead driving of the vehicle or during cornering. A straight-ahead or cornering is thereby in particular based on the yaw rate μ of the vehicle, which can be measured for example with a yaw rate sensor, the lateral acceleration a _{y of} the vehicle, which can be measured for example with a lateral acceleration sensor, as well as of the driver on the steerable wheels the vehicle set steering angle μ _DRV determined.

From these signals it is determined whether the vehicle is in a straight line or in cornering. Cornering is detected, for example, when values of the aforementioned signals each exceed a predefined threshold value, it being possible to determine from the signs of these signals whether it is a right-hander or a left-hander. The detection of a straight-ahead travel accordingly takes place when the values of said signals are smaller than predetermined threshold values. However, these signals can also be imaged in the form of a curve index (eg curve index = 1/3 * [Kl * yaw rate + K2 * steering angle + K3 * lateral acceleration]) and an imaginary index. Turning occurs when this curve index exceeds a threshold for cornering. If the curve index does not exceed the threshold value for cornering, this indicates straight-line driving and, accordingly, it is detected. The threshold should have a

Hysteresis be considered for the transition between cornering condition and straight condition.

A braking process on an inhomogeneous road surface becomes in particular based on the speed v of the vehicle and on the basis of wheel speeds v _± and brake pressures p _± in the wheel brakes on the right front wheel (i = vr), on the left front wheel (i = vl), on the right rear wheel (i = hr) and on the left rear wheel (i = hl).

In this case, a longitudinal slip μ of the wheel i can be determined by a comparison of the wheel speed Vi and the vehicle speed v, which indicates to what extent the wheel tends to lock up. An analog recognition of the driving situation and in particular of the longitudinal slippage μ of a wheel is carried out to activate an ABS control, which prevents the locking of a wheel by holding or lowering the brake pressure p _± .

Thus, the rules described below can be used to detect a μ-split braking and to activate the steering angle control and to determine the brake pressure difference μp at the rear axle. These are also based on the ABS control strategy of the yaw torque limit at the front axle and the select-low control on the rear axle already described at the beginning. A start of μ-split braking is recognized during a straight-ahead driving when one of the following conditions is satisfied: a) A front wheel is in ABS control for a predetermined period while the other front wheel is not in ABS control. b) Both front wheels are in the ABS control and a difference of the brake pressures p ± at the front wheels exceeds a predetermined threshold. c) Both front wheels are in an ABS control for a predetermined period of time, an ABS lock pressure on at least one front wheel exceeds a predetermined threshold, and the ABS lock pressure on a front wheel is a predetermined multiple of the lock pressure of the other front wheel.

Termination of μ-split braking is detected during straight ahead driving when one of the following conditions is met: a) It is not a front wheel in ABS control. b) The ABS blocking pressure at both front wheels is less than a predetermined threshold for a predetermined period of time. c) The ABS blocking pressure of a front wheel is less than a predetermined multiple of the ABS locking pressure of the other front wheel.

During cornering, the start of μ-split braking is detected when one of the following conditions is met: a) The outside wheel enters an ABS control before the wheel on the inside of the curve. b) Both front wheels are in the ABS control for a predetermined period of time, and at least one front wheel has an ABS lock pressure exceeding a predetermined threshold, and the ABS lock pressure on the inside front wheel is at least a predetermined multiple of the ABS lock pressure of the outside front wheel.

The termination of μ-split braking is detected during a turn if one of the following conditions is met: a) It is not a front wheel in the ABS control. b) The ABS blocking pressure of both front wheels is less than a predetermined threshold for a predetermined period of time. c) The ABS blocking pressure at the inside front wheel is less than a predetermined multiple of the ABS locking pressure at the outside front wheel.

The activation of the steering angle control takes place on the basis of an activation signal, if this assumes the value 1.

When the ignition is restarted, this activation signal is set to the value 0. A change to the value 1 is carried out in particular if, as described above, a μ-split braking is detected.

Preferably, however, one or more additional conditions must also be fulfilled for the activation signal to assume the value 1. Such conditions are also examined, for example, to activate a particular ABS regimen, such as a yaw element build-up delay on the front axle or a select-low control on the rear axle.

For example, the activation signal is then set to the value 1 if, in addition, a difference of the coefficient of friction μ for right and left-hand wheels, which is estimated in an ABS controller, exceeds a predetermined threshold value. Furthermore, the results of a driving situation detection, which were determined in an ABS and / or ESP system, can be taken into account when activating the steering angle control.

The activation signal is reset from the value 1 to the value 0 when the termination of a μ-split braking is detected and one or more of the other conditions considered are no longer satisfied. In the case of conditions based on a comparison of a variable with a threshold value, it is preferable to use threshold values other than the activation previously, so that the regulation is calmed by a hysteresis.

An advantageous embodiment of a steering angle controller 110 for setting the target steering angle μsoii is shown in the block diagram in FIG. The controller comprises a block 120, in which an additional steering angle μμ _{z is} determined which is determined on the basis of an estimated value M _{z of} the disturbance yaw moment M _z . The setting of Zusatzlenkwinkel request μμ _z corresponds to a StörgrößenaufSaltung based on a control component of the manipulated variable to compensate for Störgiermoments M _z . In addition, a driving state controller 130 is provided, which has a control component μμ _{R of} the additional steering angle μμ, in which further disturbances and in particular the vehicle reaction are taken into account.

The additional steering angle μμ, which is superimposed on the steering angle μ _D Rv set by the driver, results as the sum of the control component μμ _z and the control component μμ _R.

A preferred embodiment of the block 120 for determining the control component μμ _{z of} the additional steering angle μμ is shown in FIG. The input signals of the block 120 are the steering angle μ _WH at the steerable wheels of the vehicle, the brake pressures p _± at the wheel brakes, the angular velocities μ _± the wheels of the vehicle and the reference speed v of the vehicle.

In block 210, a Störgiermoment M _z is estimated that by the different braking forces F _Xr j_ (i = VR, VL, HR, HL) is effected on the wheels of the vehicle in a μ-split situation.

From an equilibrium condition for torques about the vertical axis of the vehicle, the result is: M _z = cos ( _mL ) • [s ₂ • F _KιVl - s _r • - sin (μ _ro • l _v • [F _Xfbl - F _Xrhr ] + s _x • F _Xrhl - s _r • F _Xrhr

Here, si denotes the distance between the center of gravity of the vehicle and the left wheel contact point in the vehicle transverse direction, s _r the distance between the center of gravity of the vehicle and the right wheel contact point in the vehicle transverse direction and l _v the distance between the center of gravity of the vehicle and the front axle in the vehicle longitudinal direction. In an advantageous embodiment of the invention, locking of the wheels of the vehicle is prevented by an ABS control. It can thus be assumed that there is a linear relationship between the braking forces F _Xf i at the wheels and the brake pressures pi in the wheel brakes, so that the

Braking forces F _x , i can be determined by the following relationship: F _{Xr ±} K _pi • p _± (i = vr, vl, h, hl)

The proportionality constants K _{p ±} are determined for example in driving tests, but can also be determined from the brake parameters such as brake friction coefficient, effective brake disk radius and brake piston diameter and are stored in the block 210.

Of course, for determining the braking forces F _{Xf ±} also sensors, such as Seitenwandtorsionssen- sensors or measuring wheels, are used, which directly measure the braking forces F _x .

Based on the estimated value M _z for the Störgiermoment M _Zr which is passed from the block 210 to the block 220, the control proportion μμ _{z of} the additional steering angle μμ is determined in an inverse vehicle model, preferably based on a linear one-track model and the context between Störgiermoment M _z and steering angle for a stationary driving condition is linearized.

The control component μμ _z is determined by a multiplication of the interference yaw moment M _z with a gain factor K _M :

Wz = ^K M ^{• M} z It has been found that this relationship dependencies of the FahrZeuggeschwindigkeit v and the brake pressures p _± has. As a result, the amplification factor K _{M is} determined adaptively as a function of these variables, for example on the basis of characteristic curves which are determined in driving tests and are valid.

It has also been shown that the brake pressures phr and phi have only a small influence in the wheel brakes of the rear axle. Furthermore, the brake pressures p _vr and p _v ι can be summarized in the wheel brakes of the front axle. In a preferred embodiment of the block 220, the additional steering angle component μμ _z can therefore be determined by a relationship of the following form:

W _R = KM ( _Vf * =.) • M _z (4)

As already explained, it is not possible in every driving situation to ideally compensate for the disturbance yaw moment on the basis of the additional steering angle component μμ _z , since this possibly overlaps with other disturbances and inaccuracies in the estimation of the disturbance yaw moment M _z can occur (parameter inaccuracies are precipitated due to the nature of a disturbance compensation as a pure control directly in the control signal as a control error by). These result, for example, from inaccuracies in the determination of the brake pressures p _± in the wheel brakes or from changes in the friction coefficient of the brake linings, which may occur due to changed operating conditions, such as a changed operating temperature or due to an increasing operating time. Therefore, as shown in FIG. 1, the disturbance on the circuit in the block 120 is superimposed on a driving condition control in the block 130. In the block 130, depending on driving state variables, such as the yaw rate μ of the vehicle and optionally in addition from the lateral acceleration a _y or the slip angle μ of the vehicle, a control component μμ _R of Zusatzlenkwinkε μμ determined. A preferred embodiment of block 130 is shown in FIG. 3 as a block diagram.

The control component μμ _R is based in particular on the yaw behavior of the vehicle. To evaluate the yaw behavior, a reference yaw rate μ _{ref is} determined in block 310 in a vehicle model on the basis of the steering angle μ _DRV commanded by the driver and on the basis of vehicle speed v. This is done using a reference model of the vehicle, for example, based on a linear single-track model. In an advantageous embodiment, a vehicle model is selected which takes into account an estimated value μ for the (average) road friction coefficient μ, which can be determined, for example, based on the measured lateral acceleration a _y . Thus, the remaining coefficient of friction _potential is included in the reference _yaw rate μ _xef .

To determine the additional steering angle component μμ _B , an adaptive driving state controller 320 is used, which is preferably designed as a proportional differential controller (PD controller). The additional steering angle component μμ _R results here as the sum of a P component μμ _{R /} p and a D component μμ, o'-μμ _R = μμ _RrP + μμ _RrD . The controlled variable for the P component μμ _R , _R is the yaw rate deviation μμ. The regulatory requirement applies to the proportion of steering demand resulting from the P share

The yaw rate deviation μμ is defined as the difference between the measured yaw rate μ of the vehicle and the reference yaw rate μ _ref : μμ = μ -μ _ref .

The yaw rate of the vehicle μ is measured directly with a yaw rate sensor. The yaw rate sensor is integrated with a transverse acceleration sensor into a sensor cluster in which both the yaw rate μ and also the lateral acceleration a _y are measured with redundant sensor elements.

The amplification _factor K _FBfP (v) for the controller feedback of the

Yaw rate deviation μμ is adapted above the current vehicle speed v. Since the vehicle speed significantly influences the driving behavior of the vehicle, this is taken into account in the controller gain and thus also in the closed loop of the vehicle via the controller.

The controlled variable for the D component μμ _Rr p of the additional steering angle component μμ _R is a yaw acceleration deviation μμ. For the steering requirement component resulting from the D component, the control law W _R , _D = ^K FB, D ( ^V ) ^■ PP The yaw acceleration deviation μμ is determined by differentiation of the yaw rate deviation μμ: d. d ₍ . = - dt PP = - dt ψ ^~ Pref)

The yaw acceleration deviation μμ is thus based on the same signal sources as the yaw rate deviation μμ and is determined therefrom by means of a differentiator.

The amplification factor K _{m D} (v) for the regulator feedback of the yaw acceleration deviation μμ is adapted via the vehicle speed v. Since the vehicle speed v substantially influences the driving behavior of the vehicle, this is taken into account in the controller gain and thus also in the closed loop of the vehicle via the controller.

An analogous to the illustrated yaw rate control can also be made in addition to the lateral acceleration a _y and / or estimated from several driving state variables float angle μ of the vehicle. Controlled variables are then a deviation between the lateral acceleration a _{y of} the vehicle and a reference lateral acceleration a. _y , _r e _f or a deviation between the estimated slip angle μ of the vehicle and a reference float angle μ _re f _r where the reference lateral acceleration a _y , _r e £ and the reference float angle μ _ref, for example, by appropriate

Thresholds can be specified. However, the corresponding reference variables for the lateral acceleration or the slip angle are preferably determined based on the driver's model based on models (eg single-track model). With an additional consideration of the lateral acceleration a _y and / or the float angle μ, preferably by P-controller or by PD controller, corresponding proportions of Zusatzlenkwinkelanteilμμ _{R determined} in the controller 320 and subsequently subjected to arbitration.

The control component μμ _z and the control component μμ _R are added in an adder, and the additional steering angle adjustment request μμ resulting from the sum of the two components is transmitted to a control unit of the actuator used in the steering column, for example to a control unit of a superimposed steering system, and by the actuator set.

The steering angle wπ, at the steerable wheels of the vehicle, is thus obtained as the sum of the steering angle μ _DRV commanded by the driver and the additional steering angle μμ:

The determination of the steering angle required for stabilization μmL and the adjustment of the steering angle in a μ-split braking done much faster than an average driver recognize the corresponding situation and can respond by counter-steering. This rapid response of the control system and the active steering system make it possible to adapt the electronic brake system ABS such that the coefficient of friction potential can be better utilized on the individual wheels (especially on the high friction side).

For this purpose, the control strategies of the ABS are modified with μ-split braking: The yaw moment build-up deceleration on the front axle is greatly attenuated so that a large difference in pressure between the high wheel and the low wheel builds up more quickly at the front axle, ie a high pressure build-up ratio is set on the high wheel.

Almost simultaneously with the build-up of the pressure difference, a yawing moment is created around the vehicle's vertical axis. Due to the estimation of the Störgiermoments M _z from the brake pressure information o- with the help of directly measuring the tire forces systems is immediately countered by the control system, even before the driver can recognize the situation on the yaw behavior of the vehicle.

In particular, the select-low control of the ABS at the rear axle of the vehicle is also modified. The modification corresponds to a "softening" of the select-low control, in which a pressure difference .mu.p between the brake pressure paig _h at the high wheel and the brake pressure p _low at the low wheel is determined based on a stability index 'S.

In a preferred embodiment of the invention, the modification of the select-low control of the selected by the ABS controller select-low control is superimposed.

In this case, the brake pressure P _low in the wheel brake of the low wheel is determined exclusively by the ABS controller, and the brake pressure p _H igh in the wheel brake of the high wheel based on the allowable pressure difference uP determined.

A basic structure of a control system 460 for modifying the select-low control is shown in FIG. 4 as a block diagram. The input signals for this system are provided by the ABS controller and the above-described steering angle controller, which are schematically represented here by block 410. The control system has a block 420 which contains a logic circuit for activating the controller function, a block 430 for determining the pressure difference μps _e iio _w , Req and a block 440 for limiting the pressure difference μpseiiσw, Req-

The limited pressure difference μpseiiow, nm corresponds to the position requirement μp for the brake pressure, which is set by a pressure build-up, a pressure reduction or a holding of the pressure in the wheel brake of the high wheel, for example by an ABS control unit on the rear axle.

The modification of the select-low control is only performed if a Sellow Req bit determined in the block 420 assumes the value 1. If the Sellow Req bit is zero, no pressure difference μp between the brake pressures in the wheel brakes on the rear axle is allowed. This is illustrated schematically in FIG. 4 by means of the multiplication point 450, which only passes on a value, which is different from zero, of the limited pressure difference pseiJio _w , iim when the Sellow Req bit assumes the value 1.

When the ignition is restarted, the sellow req bit in the

Block 420 is set to the value 0. To activate the controller function, the Sellow Req bit is set from the value 0 to the value 1 if at least the following conditions are met: a) μ-split braking is detected. b) A straight-ahead drive is detected. c) The low friction side is detected. The detection of the low coefficient of friction side takes place at the beginning of a μ-split braking in the driving state controller 410 based on the difference between brake pressures in the wheel brakes on the right and the left side of the vehicle. In this case, the vehicle longitudinal side is recognized as a low friction side, to the brake pressures of the wheel brakes are lower by a predetermined threshold than the brake pressures on the other vehicle longitudinal side.

Of course, it may additionally or alternatively also be provided here to recognize the high-friction-value side in an analogous manner.

The Sellow Req bit is reset from the value 1 to the value 0 if the aforementioned conditions are no longer met.

In a preferred embodiment of the invention, in the block 430 for determining the brake pressure difference μpseiiow, Req, the steps illustrated by means of the flowchart illustrated in FIG. 5 are carried out.

The pressure difference μpseiiow, Req indicates the value of the brake pressure by which the brake pressure pHig _h in the wheel brake of the high wheel is increased relative to the brake pressure p _low in the wheel brake of the low wheel. The brake pressure p _Low is determined in a known manner on the basis of the longitudinal slip μ of the low wheel by the ABS controller. A reduction of the brake pressure PHigh below the value p _Low is not provided. First, a variable S is calculated from the additional steering angle μμ determined in the steering angle controller 110 and the yaw rate deviation μμ: k * S = K ^• μ + K ₂ ^■ μμ

The gains Ki and KΣ are positive and are determined, for example, in driving tests.

Based on the size S * can be determined whether the vehicle with positive sense of rotation (ie to the left) or negative rotation (ie to the right) yaw or whether due to a positive or negative additional steering angle μμ a yawing motion with positive or negative direction of rotation is to be expected , On the basis of the size S * can also be recognized whether the driver o- has steered a driver assisting the driver assistance system in the direction of the low friction coefficient.

In further embodiments of the invention, it is additionally provided in addition a factor-weighted deviation between the measured lateral acceleration a _y of

Vehicle and a reference _{lateral acceleration} a _y , _ref and / or a deviation between an estimated slip angle μ of the vehicle and a reference floating angle μ _ref as additional summands in size S * to be considered.

The reference _{lateral acceleration} a _yrΣ e _f and the reference _{floating angle} μ _rθf can be predetermined, for example, based on threshold values. The corresponding reference variables for the lateral acceleration or the slip angle, however, are preferably determined based on the driver's specifications based on the model (eg, single-track model). An evaluation of the size S * on the basis of the low-friction side (low μ side) is based on the query 510 and leads to the determination of the stability index 'S. In particular, it is provided that the sign of the stability index S is determined as a function of whether the low frictional side is right or left with respect to the vehicle longitudinal direction, as determined by the query 510. The following applies: {S ^* , if the low-friction coefficient side is on the left - S ^* , if the low-friction side is on the right

On the basis of the value of the stability index S determined in this way, it can be determined whether the vehicle is yawing in the direction of the low-friction side (with positive / negative direction of rotation, if the low-friction side is left / right), or if the vehicle is yawing in the direction of the high-friction value (with negative positive / positive sense of rotation if the low friction side is left / right).

A positive value of S indicates that the vehicle yaws towards the low-friction side. The driving state of the vehicle in the μ-split situation is stable with a positive value of the stability index 'S.

A negative or very small positive value of S indicates that the vehicle yawed in the direction of the high friction side and thus the driving condition of the vehicle could not yet be stabilized by the steering interventions.

Of course, the evaluation of the size S * can also be made here on the basis of the high frictional value side. In an analogous way to the above-described evaluation on the basis of the low friction value side, it is used to determine the stability index. S set a negative sign if the high fron value page is on the left.

When determining the pressure difference μpseiiow, Req is first - regardless of the value of the stability index 'S - checked by means of the query 520, whether the high wheel on the rear axle has a tendency to block. In this case, this query comprises a comparison of the slip μ of the high wheel with a predetermined threshold value in the block 520. If the wheel slip μ of the high wheel exceeds the threshold value, the brake pressure at this wheel is reduced as a function of the slip μ. However, according to the select-low control of the ABS system, the brake pressure paigh at the high wheel does not drop below the value p _{low of} the brake pressure in the wheel brake of the low wheel.

If the high wheel does not show a tendency to lock, a change in the brake pressure difference .mu.p is determined in each control cycle on the basis of an evaluation of the stability index 'S on the basis of query 530.

If the value of the stability index 'S exceeds a predetermined positive threshold value pinc_thr, then an increase in the pressure difference .mu.p and thus a build-up of pressure in the high-wheel are performed. The pressure build-up in this situation does not lead to a destabilization of the vehicle and serves to shorten the braking distance in a μ-split braking with a stable driving condition of the vehicle.

In this case, the output signal of block 430 is a brake pressure difference PPsslloW _r Req = PP + X which is larger by a predetermined value μx than the instantaneous brake pressure difference μp.

If the value of the stability index S is below a predefined threshold value pdec_thr, the pressure difference μp is reduced and thus a pressure reduction in the high wheel is initiated. An increase in pressure in this case would lead to an increase in the yawing motion of the vehicle in the direction of the high friction coefficient side and thus to a destabilization of the driving state. However, the pressure reduction increases the lateral force potential at the rear axle and allows the driver or the steering angle controller 110 to be able to effectively counteract a possible unstable driving state.

The output signal of the block 430 results here a brake pressure difference PP _S eUow, Req = PP ^~ P ¥ which is smaller by a predetermined value μy than the instantaneous brake pressure difference μp. The value μy can correspond to the value μx.

If the value of the stability index S is between the two threshold values pdec_thr and pinc_thr, the brake pressure difference up is kept constant. In this case, there is a "borderline" driving condition, which is reevaluated in the next control cycle, to possibly perform a pressure reduction on the high-wheel or allow a pressure build-up.

The output signal of the block 430 results in this case, the brake pressure difference PPsellow, Req ~ PP The aforementioned steps are performed once in block 430 in each control cycle, resulting in a pulsed pressure build-up with a gradient resulting from the values μx and μy, respectively, or holding the brake pressure in the high wheel.

Preferably, a driving situation-dependent limitation of the brake pressure difference μpseiiow, Req is provided. A preferred embodiment of the block 440 for limiting the brake pressure difference μpseiiow, Req is shown in the figure 6 with reference to a flow chart.

In this embodiment, a speed-dependent limitation first takes place in step 610, wherein the limitation is made, for example, on the basis of a characteristic curve. Since the vehicle tends to become more unstable at high speeds, only little or no pressure difference μp is admitted at high speeds. This limitation thus provides a high lateral force potential at the rear axle as a stability reserve at high speeds.

If a change of the low frictional value side is determined by means of the query 620, the pressure difference μpseiiow, Req is reduced to the value zero. When changing the low friction side, there is a significant risk that unstable driving conditions occur. In order to assist the driver or the steering angle controller in the stabilization of the vehicle is therefore resorted to in such a change to the select-low control, and as long as the stability index is not again objectively stable driving behavior is signaled. In the query 630, the current pressure difference at the rear axle is determined and compared with the pressure difference and / or the pressure ratio at the front axle. In step 640, the pressure difference and / or the pressure ratio μpseiiow, Req 'at the rear axle is limited to a value of, for example, 50% of the pressure difference and / or the pressure ratio at the front axle, if this is exceeded.

The ABS control sets the brake pressures at the front axle as a function of the coefficients of friction at the front wheels. The pressure difference at the front wheels - during forward travel, which is the starting point here - takes into account friction coefficient conditions that are present for a short time later on the rear axle as well. Thus, the

Brake pressures on the front axle early on for changed friction coefficients. Based on the above limitation of the pressure difference at the rear axle, however, this change can already be anticipated by the control system.

The output signal of the block 440 for limiting the pressure difference μpseiiow, Req is the limited pressure difference μpsei- iow, to _r the adjustment request for the brake pressure difference up and by a pressure build-up, a pressure reduction or holding the brake pressure PHi h in the wheel brake of the high-. Wheel is adjusted to the rear axle. In this case, however, the value of the brake pressure p _{L θ} w in the wheel brake of the low-wheel in the wheel brake of the high-wheel is not _exceeded .

In the embodiment of the invention shown so far with reference to the figures, it is assumed that the steering angle, the yawing moment compensating for the disturbing yaw moment M _z causes and allows the modification of the select-low control is set by the steering angle controller 110

However, it is also possible to make an analogous modification even if the driver counteracts.

In this embodiment of the invention, the size S * is determined in an altered manner. In this case, it is provided to store the steering angle μD _R v to) set by the driver at the beginning of the μ-split braking at time to and the reference yaw rate μ _ref (t ₀ ) calculated on the basis of this steering angle at time to, and the variable S * for example in the form where K ₁ and K _{2 are} predetermined constants. Further modifications are not required.

The steering angle μo _R v to) or the yaw rate μ _ref (t ₀ ) represent the course request of the driver. On the basis of a comparison of these values with instantaneous values μ _{mL of} the steering angle at the steerable wheels and the yaw rate μ of the vehicle, it is determined whether the driver has initiated stabilizing measures, in particular a countersteering, which modifies the select-low control in the allow as described above.

Furthermore, it has been described so far that the modification of the select-low control is only performed when a straight-ahead driving of the vehicle is detected. This is done so as not to jeopardize the lane stability of the vehicle guaranteed by the select-low control. However, it is also possible to modify the above-described modification of the select Low-control in an analogous manner to make a turn. In this case, for example, a more restrictive limitation of the brake pressure difference can be carried out and / or a slower pressure build-up than when driving straight ahead can be provided.

Claims

Claims:

A method for controlling a brake pressure in at least one wheel brake mounted on a vehicle axle during a braking operation on a roadway having an inhomogeneous friction coefficient, characterized in that: a low friction side and / or a high friction side of the vehicle is detected, a stability index is formed, representing a driving state of the vehicle, - the stability index is evaluated on the basis of the low friction side and / or on the high friction side, and - the brake pressure in at least one wheel brake as a function of the value of the stability index 'and depending on a result of the assessment of the stability index based on the low friction side and / or the high frictional side is changed.

2. The method of claim 1, characterized in that an ABS control is performed for a wheel on the low friction side, and a brake pressure difference is applied to the brake pressures in the wheel brake on the high friction side and in the wheel brake on the Low coefficient of friction side is determined, the wheel brakes are preferably mounted on a vehicle axle.

3. Method according to one of claims 1 and 2, characterized in that the stability index is determined as a function of a steering angle. kels is formed on the steerable wheels of the vehicle and / or a yaw rate of the vehicle.

4. The method according to any one of the preceding claims, characterized gekennzei Chne t that the stability index is determined based on a deviation between a current steering angle and a present at the beginning of a braking process on a road with inhomogeneous coefficient of friction steering angle.

5. The method according to claim 1, characterized in that the stability index is determined on the basis of a deviation between a current yaw rate of the vehicle and a reference yaw rate determined on the basis of the steering angle present at the beginning of the braking process in a vehicle model.

6. The method according to any one of the preceding claims, characterized t that the stability index is determined in response to a deviation between a commanded by the vehicle operator steering angle and a set on the steerable wheels of the vehicle target steering angle.

7. The method according to any one of the preceding claims, characterized in that the target steering angle contains a control component which is determined as a function of a Störgiermoments in a vehicle model.

8. The method according to any one of the preceding claims, characterized gekennzei chne t, in that the desired steering angle contains a control component which is determined as a function of the yaw rate deviation between the yaw rate of the vehicle and a reference yaw rate of the vehicle.

9. Method according to one of the preceding claims, characterized in that the stability index is formed as a function of a deviation between a yaw rate of the vehicle and a set yaw rate determined on the basis of at least one variable predefined by a vehicle operator in a vehicle model.

10. The method according to any one of the preceding claims, characterized in that t the stability index is determined as a function of a lateral acceleration of the vehicle.

11. The method according to any one of the preceding claims, characterized zei chne t that the stability index is determined in dependence of a float angle and / or a Schwimmwinkelgeschwindigkeit.

12. Method according to one of the preceding claims, characterized in that a sign of the stability index is determined as a function of the low-friction-value side and / or as a function of the high-friction-value side.

13. Method according to one of the preceding claims, characterized in that the brake pressure depends on a result of a A comparison of the stability index with at least one threshold value is changed.

14. The method according to any one of the preceding claims, characterized in that the brake pressure in the wheel brake on the Hochreibwertseite is increased compared to the brake pressure in the wheel on the low friction side, when the stability index exceeds a predetermined threshold.

15. Method according to one of the preceding claims, characterized in that the brake pressure difference between the brake pressure in the wheel brake on the low friction side and the brake pressure in the wheel brake on the high friction side is limited.

16. The method according to any one of the preceding claims, characterized ike t that a limitation of the brake pressure difference in dependence on a speed of the vehicle is made.

17. Method according to one of the preceding claims, characterized in that no brake pressure difference is permitted when the low-friction-side and / or the high-friction-side change.

18. Method according to one of the preceding claims, characterized in that a brake pressure difference at the rear axle is limited to a predetermined proportion of a brake pressure difference at the front axle. the axis is limited.

19. The method according to any one of the preceding claims, characterized i te chne that the brake pressure ratio at the rear axle is limited to a predetermined proportion of the brake pressure ratio at the front axle.

20. The method according to any one of the preceding claims, characterized ze ichne t, that a change in the brake pressure is then made when a straight-ahead driving of the vehicle is determined.

21. Method according to one of the preceding claims, characterized in that when cornering when cornering a slower pressure build-up and a faster pressure reduction is performed and the limitation of the brake pressure difference and / or the brake pressure ratio of the rear axle is more restrictive.

22. The method according to any one of the preceding claims, characterized gekennzei chne t that cornering by means of a curve index, which results from yaw rate, steering angle and lateral acceleration.

23. A device for regulating a brake pressure difference between a brake pressure in a wheel brake on a low-friction side and a brake pressure in a wheel brake on a high-friction side of a vehicle. a braking process on a road with an inhomogeneous friction coefficient, with - a detection means for detecting the low friction side and / or Hochreibwertseite, - a determination means for determining a stability index 'representing a driving condition of the vehicle, - a means of evaluation for assessing the stability index' based the low friction side and / or high friction side recognized in the detection means; calculation means for determining the brake pressure difference as a function of a value of the stability index and a result of the stability index from the low friction side and / or the high friction side.

24. The device according to claim 20, characterized GE kennze i Chne t that it contains an ABS controller that controls the brake pressure in the wheel on the low friction side.

25. Device according to claim 20, characterized in that it has storage means for storing a steering angle present at the steerable wheels of the vehicle for the start of the braking process.

26. Device according to claim 20, characterized in that it has a first steering angle comparison means for comparing a measurement with a steering angle sensor Has steering angle with the stored in storage means steering angle.

27. The device of claim 20, further comprising: a first yaw rate comparison means for comparing a yaw rate of the vehicle measured with a yaw rate sensor with a reference yaw rate calculated based on the stored steering angle in a vehicle model.

28. Device according to one of claims 23 and 24, characterized in that the determination means for determining the stability index 'accesses the results of the comparison carried out in the first steering angle comparison means and / or of the comparison carried out in the first yaw rate comparison means.

29. The device according to claim 20, further comprising: a steering angle controller for determining a desired steering angle.

30. The device as claimed in claim 26, wherein the steering angle controller comprises a means for determining a control portion of the nominal steering angle as a function of a disturbance yaw moment.

31. Device according to one of claims 26 and 27, characterized gekennzei chnet that the steering angle controller comprises a means for determining a Rule portion of the target steering angle based on a deviation of the measured yaw rate of the vehicle from a reference yaw rate comprises.

32. Device according to one of claims 26 to 28, since it is ch e e s e s e that it comprises a means for setting the target steering angle.

33. The device according to claim 29, characterized in that it is necessary that the means for setting the desired steering angle is a superposition steering system.

34. The device according to claim 29, characterized in that it is necessary that the means for setting the nominal steering angle is a steer-by-wire steering.

35. The device according to claim 26, wherein it has a second steering angle comparing means for comparing the target steering angle with a steering angle commanded by the driver.

36. The device according to claim 26, wherein the second yaw rate comparison means compares the measured yaw rate of the vehicle with a reference yaw rate calculated on the basis of the steering angle commanded by the driver in a vehicle model.

7. Device according to one of claims 32 and 33, characterized in that the determination means for determining the stability index is based on the results of the comparison made in the second steering comparison means and / or the comparison made in the second yaw rate comparison means accesses.