Method for estimating the locking pressure in the brake system of a multi-axle vehicle

Abstract

In a method for estimating the locking pressure in the brake system of a multi-axle vehicle during a dynamic axle-load transfer, the locking pressure is ascertained during the axle-load transfer and the wheel normal force is ascertained at two points in time during the axle-load transfer and the locking pressure is ascertained therefrom at the later point in time.

Claims

1. A method for estimating a locking pressure in a brake system of a multi-axle vehicle in a dynamic axle-load transfer, the method comprising the following steps: determining, during a braking process, the locking pressure in a wheel brake device of a vehicle wheel at the vehicle wheel, at a first point in time during the axle-load transfer; determining, at the first point in time, a wheel normal force at the vehicle wheel; and ascertaining, at a second point in time subsequent to the first point in time, the locking pressure in the braking process, from the locking pressure determined at the first point in time, and from a relationship of a wheel normal force at the second point in time to the wheel normal force at the first point in time.

2. The method as recited in claim 1, wherein the locking pressure during the dynamic axle-load transfer at the first point in time is ascertained during a brake pressure control process in the brake system.

3. The method as recited in claim 1, wherein the locking pressure at the second point in time (T2) during the axle-load transfer is ascertained from the relationship $p_{\mathrm{bl},T2}=\frac{F_{N,T2}}{F_{N,T1}}.Math.p_{\mathrm{bl},T1}$ wherein: T1 indicates a first point in time during the axle-load transfer; T2 indicates a second point in time during the axle-load transfer; p.sub.b1,T1 indicates the locking pressure at time T1; p.sub.b1,T2 indicates the locking pressure at time T2; F.sub.N,T1 indicates the wheel normal force at time T1; and F.sub.N,T2 indicates the wheel normal force at time T2.

4. The method as recited in claim 3, wherein in an adjustment of the brake pressure, a gradient of the locking pressure is taken into account in accordance with the following: $\frac{{\mathrm{dp}}_{\mathrm{bl}}\left(t\right)}{\mathrm{dt}}=\frac{p_{\mathrm{bl},T1}}{F_{N,T1}}.Math.\frac{{\mathrm{dF}}_N\left(t\right)}{\mathrm{dt}}.$ wherein $\frac{{\mathrm{dp}}_{\mathrm{bl}}\left(t\right)}{\mathrm{dt}}$ is the gradient of the locking pressure at time t, and $\frac{{\mathrm{dF}}_N\left(t\right)}{\mathrm{dt}}$ is a gradient of the wheel normal force at the time t.

5. The method as recited in claim 1, wherein the wheel normal forces are ascertained using a ride-height sensor, which measures body movements between a vehicle axle and a road surface.

6. The method as recited in claim 1, wherein the axle-load transfer is ascertained by a ride-height sensor from a body movement or from the wheel normal forces.

7. The method as recited in claim 1, wherein the method is performed during an ongoing brake pressure control process.

8. A method for improving a braking process, comprising the following steps: estimating a locking pressure in a brake system of a multi-axle vehicle in a dynamic axle-load transfer, by: determining, during the braking process, the locking pressure in a wheel brake device of a vehicle wheel at the vehicle wheel, at a first point in time during the axle-load transfer, determining, at the first point in time, a wheel normal force at the vehicle wheel, and ascertaining, at a second point in time subsequent to the first point in time, the locking pressure in the braking process, from the locking pressure determined at the first point in time, and from a relationship of a wheel normal force at the second point in time to the wheel normal force at the first point in time, and providing the locking pressure ascertained at the second point in time as a locking pressure prediction to a brake pressure control system as an input variable.

9. A control unit for controlling adjustable components of a hydraulic brake system in a vehicle, the control unit configured to: estimate a locking pressure in the brake system of a multi-axle vehicle in a dynamic axle-load transfer, by: determining, during a braking process, the locking pressure in a wheel brake device of a vehicle wheel at the vehicle wheel, at a first point in time during the axle-load transfer, determining, at the first point in time, a wheel normal force at the vehicle wheel, and ascertaining, at a second point in time subsequent to the first point in time, the locking pressure in the braking process, from the locking pressure determined at the first point in time, and from a relationship of a wheel normal force at the second point in time to the wheel normal force at the first point in time, and provide the locking pressure ascertained at the second point in time as a locking pressure prediction to a brake pressure control system as an input variable.

10. An hydraulic brake system in a vehicle, comprising: a control unit for controlling adjustable components of the hydraulic brake system in a vehicle, the control unit configured to: estimate a locking pressure in a brake system of a multi-axle vehicle in a dynamic axle-load transfer, by: determining, during a braking process, the locking pressure in a wheel brake device of a vehicle wheel at the vehicle wheel, at a first point in time during the axle-load transfer, determining, at the first point in time, a wheel normal force at the vehicle wheel, and ascertaining, at a second point in time subsequent to the first point in time, the locking pressure in the braking process, from the locking pressure determined at the first point in time, and from a relationship of a wheel normal force at the second point in time to the wheel normal force at the first point in time, and provide the locking pressure ascertained at the second point in time as a locking pressure prediction to a brake pressure control system as an input variable.

11. A vehicle, comprising a hydraulic brake system including a control unit for controlling adjustable components of the hydraulic brake system in the vehicle, the control unit configured to: estimate a locking pressure in the brake system of a multi-axle vehicle in a dynamic axle-load transfer, by: determining, during a braking process, the locking pressure in a wheel brake device of a vehicle wheel at the vehicle wheel, at a first point in time during the axle-load transfer, determining, at the first point in time, a wheel normal force at the vehicle wheel, and ascertaining, at a second point in time subsequent to the first point in time, the locking pressure in the braking process, from the locking pressure determined at the first point in time, and from a relationship of a wheel normal force at the second point in time to the wheel normal force at the first point in time, and provide the locking pressure ascertained at the second point in time as a locking pressure prediction to a brake pressure control system as an input variable.

12. A non-transitory computer-readable storage medium on which is stored a computer program including program code for improving a braking process, the program code, when executed by a computer, causing the computer to perform the following steps: estimating a locking pressure in a brake system of a multi-axle vehicle in a dynamic axle-load transfer, by: determining, during a braking process, the locking pressure in a wheel brake device of a vehicle wheel at the vehicle wheel, at a first point in time during the axle-load transfer, determining, at the first point in time, a wheel normal force at the vehicle wheel, and ascertaining, at a second point in time subsequent to the first point in time, the locking pressure in the braking process, from the locking pressure determined at the first point in time, and from a relationship of a wheel normal force at the second point in time to the wheel normal force at the first point in time, and providing the locking pressure ascertained at the second point in time as a locking pressure prediction to a brake pressure control system as an input variable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic representation of an hydraulic brake system in a vehicle.

(2) FIG. 2 shows a μ-slip curve showing the characteristic of the coefficient of friction as a function of the longitudinal slip.

(3) FIG. 3 shows a graph showing the time characteristic of the wheel normal force on a vehicle wheel with a dynamic axle-load transfer and the characteristic curve of the brake pressure.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(4) The hydraulic brake system 1 for a vehicle shown in FIG. 1 comprises two brake circuits 2, 3, for example a front-axle brake circuit 2 and a rear-axle brake circuit 3 for supplying and controlling wheel brake devices 9 on each wheel of the vehicle with an hydraulically pressurized brake fluid. The two brake circuits 2, 3 are connected to a common master brake cylinder 4, which is supplied with brake fluid via a brake fluid reservoir 5. Master brake cylinder 4 is operated by the driver via brake pedal 6, the pedal travel implemented by the driver being measured via a pedal travel sensor 7. Located between brake pedal 6 and master brake cylinder 4 is a power brake unit 10, which comprises for example an electric motor, which operates master brake cylinder 4 via a gear unit (iBooster). The actuating movement of brake pedal 6 measured by pedal travel sensor 7 is transmitted as a sensor signal to a control unit 11, in which actuating signals are produced for controlling power brake unit 10. Wheel brake devices 9 are supplied with brake fluid in each brake circuit 2, 3 via different control valves, which together with additional aggregates are part of a brake hydraulic system 8. Brake hydraulic system 8 furthermore comprises a hydraulic pump, which is a component of an anti-lock system (ABS) or an electronic stability program (ESP).

(5) An example method according to the present invention is described below for estimating the locking pressure in the brake system of a multi-axle vehicle during a dynamic axle-load transfer. The locking pressure represents the brake pressure in the hydraulic brake system at which the vehicle wheel is locked. In particular, in the case of a steerable vehicle wheel, the latter becomes unstable when locked and is no longer steerable.

(6) In the μ-slip graph shown in FIG. 2, the maximum of the curve at a longitudinal slip s of approximately 0.2 represents the critical point at which an increasing slip s begins to threaten to lock the vehicle wheel and render it unstable. In the event of a dynamic axle-load transfer, the normal force F.sub.N acting on the wheel changes. Thus, at a uniform coefficient of friction μ, the brake force transferable via the vehicle wheel in the longitudinal direction of the wheel changes as well.

(7) The maximum of the μ-slip curve is assigned a locking pressure p.sub.bl, which must not be exceeded in the wheel brake device of the vehicle wheel in order to avoid an instability of the wheel. The locking pressure p.sub.bl may be ascertained during a braking process with active brake pressure control, in particular when the anti-lock system ABS is active, in which the hydraulic brake pressure moves to the maximum of the μ-slip curve, but not beyond it. It is thus possible to determine the locking pressure p.sub.bl when the anti-lock system ABS intervenes in the brake system of the vehicle in controlling fashion.

(8) The locking pressure p.sub.bl depends on the wheel normal force F.sub.N. The latter may likewise be detected at the time of determining the locking pressure, for example with the aid of a ride-height sensor, which measures body movements between the vehicle axle, on which the vehicle wheel suspended, and the road surface. With this information it is possible to infer a changed locking pressure, which results during the ongoing brake pressure control following a dynamic axle-load transfer. In the event of an intense braking action or when cornering, the axle-load distribution changes between the front axle and the rear axle of a vehicle and thus also the wheel normal force on a vehicle wheel of the front axle or the rear axle. This results in a change of the locking pressure on the vehicle wheel.

(9) The changed locking pressure p.sub.bl,T2 at a point in time T2 may be ascertained, when locking pressure p.sub.bl,T1 at a point in time T1 is known, from the following relationship:

(10) $p_{\mathrm{bl},T2}=\frac{F_{N,T2}}{F_{N,T1}}.Math.p_{\mathrm{bl},T1}$

(11) The wheel normal force F.sub.N,T1 at time T1, at which locking pressure p.sub.bl,T1 was ascertained as well, is known. At time T2, it is also possible to ascertain the wheel normal force F.sub.N,T2 from the sensor information, in particular of a ride-height sensor. Thus all the information required for calculating the locking pressure p.sub.bl,T2 at time T2 is available. Locking pressure p.sub.bl,T2 corresponds to the locking pressure during the axle-load transfer at time T2 and may be utilized for the further control of the brake pressure control system. The knowledge of the locking pressure at time T2 improves the coefficient of friction utilization and allows for a rapid braking force buildup.

(12) Time T2 may in principle be at an arbitrary time interval from time T1, at which locking pressure p.sub.bl,T1 and wheel normal force F.sub.N,T1 were determined, provided that the roadway friction coefficient does not change significantly during this time. This makes it possible to take any axle-load transfer into account and to determine an adapted locking pressure. It is merely necessary to determine the wheel normal force F.sub.N at time T2, which may be done readily however with the aid of the ride-height sensor. It is expedient, however, to determine the new locking pressure (time T2) for every new ABS control cycle at the beginning of the pressure build-up.

(13) Additionally, it is also possible to take the locking pressure gradient into account in accordance with

(14) $\frac{{\mathrm{dp}}_{\mathrm{bl}}\left(t\right)}{\mathrm{dt}}=\frac{p_{\mathrm{bl},T1}}{F_{N,T1}}.Math.\frac{{\mathrm{dF}}_N\left(t\right)}{\mathrm{dt}}.$

(15) At a time t=T2, the locking pressure gradient may be ascertained from the locking pressure and the wheel normal force at time T1 and from the gradient of the wheel normal force at time T2.

(16) In the graph shown in FIG. 3, the upper curve shows in exemplary fashion the normal force characteristic F.sub.N at a varying axle load. At an arbitrarily selected time T1, the normal force characteristic is about to reach a maximum. If the anti-lock system is active in the brake system at this time T1, then it is possible to determine the associated locking pressure p.sub.bl,T1. The associated normal force F.sub.N,T1 may be determined with the aid of a ride-height sensor in the vehicle axle. The associated pressure characteristic of brake pressure p including locking pressure p.sub.bl,T1 is seen in the lower curve in the graph of FIG. 3.

(17) Starting from the locking pressure p.sub.bl,T1 and the normal force F.sub.N,T1 at time T1, it is possible to determine the locking pressure p.sub.bl,T2 at time T2 or at any time, for example at a time T2+Δt. It is merely necessary to ascertain at time T2 or T2+Δt the wheel normal force, whereupon the current locking pressure p.sub.bl,T2 may be determined from the relationship indicated above.