METHOD FOR OPERATING AN ANTILOCK BRAKE SYSTEM OF A VEHICLE AND CORRESPONDING ANTILOCK BRAKE SYSTEM

Abstract

A method for operating an antilock brake system of a vehicle, in which a braking torque at at least one wheel of the vehicle is cyclically controlled in at least build-up phases and reduction phases, in order to prevent locking of the wheel. In a build-up phase, the braking torque is increased until a maximum adhesion at the wheel is exceeded, and in a subsequent reduction phase, the braking torque is reduced by a differential braking torque, which is ascertained, using a wheel acceleration value of the wheel measured after the build-up phase and a target acceleration value for the wheel.

Claims

1-9. (canceled)

10. A method for operating an antilock brake system of a vehicle, in which a braking torque at at least one wheel of the vehicle is cyclically controlled in at least build-up phases and reduction phases, to prevent locking of the wheel, the method comprising the following steps: in a build-up phase, increasing the braking torque until a maximum adhesion at the wheel is exceeded; and in a subsequent reduction phase after the build-up phase, reducing the braking torque by a differential braking torque, which is ascertained using a wheel acceleration value of the wheel measured after the build-up phase and a target acceleration value for the wheel.

11. The method as recited in claim 10, wherein a minimum value of a wheel acceleration characteristic of the wheel acquired after the build-up phase is used as the wheel acceleration value.

12. The method as recited in claim 10, wherein the wheel acceleration value is measured a time step after a triggering time of the reduction phase.

13. The method as recited in claim 10, wherein the exceedance of the maximum adhesion is detected when a wheel acceleration gradient of the wheel is less than a limiting gradient for the wheel.

14. The method as recited in claim 10, wherein in a plateau phase following the reduction phase, the braking torque is held constant up to a next build-up phase, and a wheel acceleration characteristic of the wheel is acquired at least during the plateau phase, and wherein a factor for ascertaining a next differential braking torque is increased when a maximum of the wheel acceleration characteristic is less than the target acceleration value.

15. The method as recited in claim 10, wherein in a plateau phase following the reduction phase, the braking torque is held constant up to a next build-up phase, and a wheel acceleration characteristic of the wheel is acquired during the plateau phase , and a factor for ascertaining a next differential braking torque is reduced when a maximum of the wheel acceleration characteristic is greater than the target acceleration value by more than a tolerance range.

16. An antilock brake system for a vehicle, the antilock braking system being configured to cyclically control a braking torque at at least one wheel in at least build-up phases and reduction phases, to prevent locking of the wheel, the antilock braking system being configured to: in a build-up phase, increase the braking torque until a maximum adhesion at the wheel is exceeded; and in a subsequent reduction phase after the build-up phase, reduce the braking torque by a differential braking torque, which is ascertained using a wheel acceleration value of the wheel measured after the build-up phase and a target acceleration value for the wheel.

17. A non-transitory machine-readable storage medium on which is stored a computer program for operating an antilock brake system of a vehicle, in which a braking torque at at least one wheel of the vehicle is cyclically controlled in at least build-up phases and reduction phases, to prevent locking of the wheel, the computer program, when executed by a computer, causing the computer to perform the following steps: in a build-up phase, increasing the braking torque until a maximum adhesion at the wheel is exceeded; and in a subsequent reduction phase after the build-up phase, reducing the braking torque by a differential braking torque, which is ascertained using a wheel acceleration value of the wheel measured after the build-up phase and a target acceleration value for the wheel.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0022] In the following, specific embodiments of the present invention are described with reference to the figures, in which case neither the figures, nor the description are to be interpreted as limiting to the present invention.

[0023] FIG. 1 shows a representation of characteristics of a braking torque and of a wheel acceleration according to an exemplary embodiment of the present invention.

[0024] The FIGURE is merely schematic and is not true to scale. In the FIGURE, identical reference numerals denote like features or features functioning in the same manner.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0025] FIG. 1 shows a braking torque characteristic 100 of a braking torque 102 at a wheel of a vehicle, and a wheel acceleration characteristic 104 of a wheel acceleration 106 of the wheel. Braking torque characteristic 100 and wheel acceleration characteristic 104 are shown during a control action of an antilock brake system according to an exemplary embodiment, for preventing locking of the wheel. Braking torque 102 is cyclically increased in build-up phases 108 and lowered in reduction phases 110. In this context, braking torque 102 is increased in build-up phases 108, until the wheel exceeds a maximum adhesion and starts to lock. In reduction phases 110, braking torque 102 is reduced, in each instance, by a differential braking torque 112, so that the wheel is re-accelerated due to a residual adhesion to the ground.

[0026] The differential braking torque 112 to be controlled is calculated for each subsequent reduction phase 110, using a wheel acceleration value 114 measured after the specific build-up phase 108. To that end, wheel acceleration 106 is measured after build-up phase 108 and reflected in wheel acceleration value 114. Wheel acceleration value 114 and a vehicle-specific target acceleration value 116 are substituted with other fixed vehicle parameters into a processing specification, and differential braking torque 112 is calculated. In this context, target acceleration value 116 represents a desired re-acceleration of the wheel after the start of the locking.

[0027] In one exemplary embodiment, wheel acceleration characteristic 104 is monitored after the end of build-up phase 108. Due to an inertia of the wheel, wheel acceleration 106 reacts to the reduction of braking torque 102 in a delayed manner. In this context, a minimum wheel acceleration value 130 of wheel acceleration characteristic 104 is used as the wheel acceleration value.

[0028] Since the inertia of the wheel is known, wheel acceleration value 114 may alternatively be measured a time step 118 after a triggering time 120 of reduction phase 110. Reduction phase 110 may be triggered, for example, when an acceleration gradient 122 of wheel acceleration 106 is less than a limiting gradient.

[0029] In one exemplary embodiment, braking torque 102 is held constant after reduction phase 110, for a plateau phase 124, until next build-up phase 108 begins. The wheel stabilizes in plateau phase 122. In plateau phase 124, it is checked if wheel acceleration 106 increases sharply enough to reach target acceleration value 116. If target acceleration value 116 is not attained or is exceeded, then, in one exemplary embodiment, a factor in the processing specification is adjusted to calculate differential braking torque 112. If target acceleration value 116 is not reached, that is, the wheel is not accelerated sharply enough, the factor is adjusted in such a manner, that for the next reduction phase 110, a greater differential torque 112 is calculated in the case of a wheel acceleration value 114 measured in the same manner. Conversely, if target acceleration value 116 is exceeded by more than a tolerance, the factor is adjusted in such a manner, that for the next reduction phase 110, a smaller differential braking torque 112 is calculated in the case of a wheel acceleration value 114 measured in the same manner.

[0030] In one exemplary embodiment, wheel acceleration value 114 is compared to attained wheel acceleration value 130 at the end of build-up phase 108 and a time step 118 later. If the difference of wheel acceleration value 114 and wheel acceleration value 130 in plateau phase 124 exceeds or does not reach differential wheel acceleration 126, calculated differential braking torque 112 may additionally be corrected, using a factor.

[0031] In one exemplary embodiment, braking torque 102 is increased very rapidly to an initial value 128 at the end of plateau phase 124, in the beginning of next build-up phase 108. Initial value 128 is selected in such a manner, that the wheel certainly does not yet lock, but a braking action is to be expected already. In this manner, a duration of build-up phase 108 may be shortened, and/or braking torque 102 may be increased, using a lower slope.

[0032] In other words, an algorithm for optimizing braking force with the aid of an instability controller is put forward.

[0033] An ABS controller of today is based on an instability regulation principle utilizing cyclically occurring pressure build-up, pressure-holding and pressure reduction phases. In this context, pressure is built up in the wheel until the maximum of the tire characteristic, that is, the p-slip characteristic, is exceeded and the wheel becomes unstable.

[0034] This ensures that the controller reacts robustly to changes in the coefficient of friction of the road and consequently detects the changes in the maximum transmittable braking force. Subsequently, the wheel is stabilized by a controlled reduction in pressure, before the next pressure build-up may be started.

[0035] The pressure reduction is intended to function at all possible coefficients of friction and to consequently ensure that the wheel does not lock.

[0036] Currently, the pressure reduction variable is applied to the corresponding vehicle in a complex manner. In this context, the challenge is to apply the pressure reduction in such a manner, that it functions reliably and ensures the wheel stabilization both in the build-up phase including sharp change in the normal force, at low vehicle speeds including marked slip dynamics, in response to disturbances including short-term changes in the coefficient of friction, and in the steady-state condition. This is rendered possible by selectively correcting an applied, basic reduction variable over additional application parameters and situation detection.

[0037] The model-based pressure reduction put forward here is adaptive, since it independently adapts to the situations described above. It may increase or decrease exclusively via the input variables. Thus, the new pressure reduction does not require any extensive application of the situations described and therefore includes markedly fewer application parameters.

[0038] If the wheel has been brought past the maximum of the adhesion characteristic, using an arbitrary pressure build-up gradient, then, in the approach put forward here, the pressure is reduced again as rapidly as possible to a level, at which the wheel stabilizes again. Since, in this context, the wheel dynamics are markedly greater than the vehicle dynamics, one may make a simplifying assumption, that substantially no change in the normal force takes place during the pressure reduction. In addition, the simplifying assumption is made, that after the pressure reduction, and consequently, on the pressure level, which is necessary for wheel stabilization, the same adhesion μ2 sets in, which was attained prior to the pressure reduction μ1.

[0039] For the purpose of clarification, the torque balance at the start of the pressure reduction is modeled in (1).

F.sub.x 1=1/R.sub.wheel*(J.sub.wheel*a.sub.x1/R.sub.wheel+Cp*p.sub.x1)   (1)

where F.sub.x1 is the braking force at the start of the pressure reduction;
R.sub.wheel is the rolling radius of the wheel;
J.sub.wheel is the mass moment of inertia of the wheel;
a.sub.x1 is the acceleration at the start of the pressure reduction;
Cp is the braking coefficient (=wheel radius of the brake disk*surface area of the brake piston*coefficient of friction); p.sub.x1 is the brake pressure at the start of the pressure reduction.

[0040] The torque balance at the end of the pressure reduction is modeled in (2).

F.sub.x2=1/R.sub.wheel*(J.sub.wheel*a.sub.x2/R.sub.wheel+Cp*P.sub.x2)   (2)

where F.sub.x2 is the braking force at the end of the pressure reduction;
a.sub.x2 is the acceleration at the end of the pressure reduction; and
p.sub.x1 is the brake pressure at the end of the pressure reduction.

[0041] Given the assumptions just made, μ is constant and F.sub.N is constant, braking force F.sub.x remains constant, from which the following results

F.sub.x1=F.sub.x2   (3)

[0042] If (1) and (2) are substituted into (3), and consequently, subtraction is carried out in accordance with the pressure, then the necessary pressure reduction step results in (4).

Δp Reduction=J.sub.wheel/(R.sub.Wheel*Cp)*(a.sub.x2−a.sub.x1)*K   (4)

[0043] In this context, J.sub.wheel/(R.sub.wheel*Cp ) are vehicle parameters, and target wheel acceleration a.sub.setpoint=a.sub.x2 is the only application parameter. K is a correction factor, which may be increased or decreased as a function of the attainment of the target wheel acceleration.

[0044] A control cycle of an instability controller according to the approach put forward here is represented in FIG. 1. The wheel pressure characteristic is reflected in the upper part of the graph, and the corresponding a.sub.wheel characteristic is reflected in the lower graph.

[0045] In the steady-state condition of the ABS control system, in each control cycle, the re-acceleration of the wheel reaches the target wheel acceleration during the wheel stabilization, after the pressure reduction. The magnitude of the pressure reduction is increased with increasing wheel deceleration and reduced with decreasing wheel deceleration. The pressure reduction step adapts automatically in response to disturbances and changes in the coefficient of friction. There are fewer instances of subsequent pressure reduction. Subsequent reductions do not have a uniform pressure-step magnitude.

[0046] Finally, it should be pointed out that terms, such as “having,” “including,” etc., do not exclude any other elements or steps, and that terms, such as “a” or “an,” do not exclude a plurality. Reference numerals are not to be regarded as a limitation.