WO1991005685A1

WO1991005685A1 - Process for controlling the braking pressure in an antilock braking system

Info

Publication number: WO1991005685A1
Application number: PCT/EP1990/001732
Authority: WO
Inventors: Dieter Seitz
Original assignee: Lucas Industries Public Limited Company
Priority date: 1989-10-13
Filing date: 1990-10-12
Publication date: 1991-05-02
Also published as: DE3934307A1; JPH05501233A; BR9007745A; DE3934307C2; DE3934307C3; EP0495838A1

Abstract

In a process for controlling the braking pressure in an antilock braking system of a vehicle with front and rear wheels, the rotational speeds of braked wheels are measured and, if threshold values of the rotational speed or rotational acceleration are exceeded, the braking pressure in the correponding wheel is reduced or at least maintained constant. If there is no danger of any rear wheel being locked, but the braking pressure on at least one front wheel is maintained constant, the braking pressure on the rear wheels is also maintained constant, at least temporarily. If the braking pressure in at least one front wheel is reduced and there is no danger of any rear wheel locking, the braking pressure in the rear wheels is built-up very slowly, in particular in a pulsed manner.

Description

Method for controlling the brake pressure in an anti-lock brake system

The invention relates to a method for controlling the brake pressure in an anti-lock brake system of a vehicle having front and rear wheels, in which the rotational speeds of braked wheels are measured and given as a function of the ratio of the rotational speed and / or rotational acceleration of a braked wheel Threshold values of the brake pressure of the wheel are kept constant or lowered when there is a risk of locking.

A large number of algorithms for controlling the brake pressure in an anti-lock vehicle brake system are known. The measuring devices, valves and control devices required to carry out an ABS brake control are also known in a variety of configurations. New methods for controlling the brake pressure in ABS systems are implemented using known measuring and control devices (speed sensors, valves, etc.) by appropriate programming of a processor.

The goal of achieving the shortest possible braking distance with good controllability of the vehicle with an ABS control is made more difficult by the fact that the optimization of an ABS control depends essentially on the road and vehicle conditions. The regulation must be adapted to the prevailing conditions, which can change continuously and about which the computer controlling the ABS braking receives only very indirect and restricted information.

The lateral guidance of the rear wheels is particularly important for the good steerability of the vehicle. If the rear wheels slip too much when braking, the steerability of the vehicle is at risk due to a reduced cornering force. It is known that the ABS pressure can result in a so-called "overshoot" of the brake pressure, for example because the threshold values for the ABS control are set relatively insensitive for other reasons or because of the inertia of the system. Exceeding the brake pressure is particularly undesirable on the brakes of the rear wheels. If the rear wheels of a (two-axle, four-wheeled) vehicle slip too much, that is to say a so-called instability range, a highly undesirable torque occurs about the vertical axis of the vehicle, the so-called yaw moment.

It is already known in the state of the art to avoid such yawing moments caused by overshoot of the pressure above optimum values in that the brake pressure, in particular on the rear wheels, has a relatively small gradient (ie with a low rate or speed) at the critical ones Maximum pressure is brought up. In order to avoid excess pressure, the brake pressure is built up more slowly, at least when the maximum value to be reached is reached. The slower pressure build-up can e.g. by throttling or pulsing the flow of the hydraulic fluid.

ABS control is particularly difficult when braking in curves and also in so-called split situations. In a "split situation", the wheels are found on one side of the vehicle a different braking force coefficient than the wheels on the other side of the vehicle. Such situations can occur, for example, when driving over partially icy surfaces.

The aim of the invention is to provide a method for controlling the brake pressure in an anti-lock brake system which, even when braking in curves and when braking with different braking force coefficients on the individual wheels of the vehicle, enables the shortest possible braking distance with good maneuverability of the vehicle.

Solutions to this object according to the invention are characterized in the independent claims. The dependent claims describe advantageous refinements of the method according to the invention.

To achieve the above-mentioned goal, the invention modifies a conventional ABS control method in which the rotational speeds of braked wheels are measured and, when the rotational speed and / or the rotational acceleration of a braked wheel exceed predetermined threshold values, that is to say a danger of locking If the brake pressure on the affected wheel is either kept constant or reduced, such that if there is still no risk of locking in the case of a rear wheel, but the brake pressure is kept constant in at least one front wheel, the brake pressure on the rear wheels also at least secondly is kept constant.

The invention is therefore based on the knowledge that it is advantageous to avoid an overshoot of the brake pressure on the rear wheels and thus to achieve good stability (steerability) of the vehicle, if at the rear wheels still no ABS control takes place and also no front wheel experiences a pressure reduction as part of an ABS control, but the brake pressure on at least one front wheel is kept constant by actuating a corresponding holding valve will also keep the braking pressures on the rear wheels constant for at least a certain period of time. Under the conditions mentioned, the holding valve controls of the front wheels are copied onto the rear wheels.

In this way, good steerability of the vehicle is achieved, particularly in the critical braking situations mentioned in curves or with different coefficients of adhesion. If the front wheels run in an even higher slip and, after the brake pressure on the front wheels has been kept constant, there is a reduction in pressure so that the front wheel or the front wheels return to a stable range, according to another Embodiment of the invention, the brake pressure on the rear wheels is increased with a relatively low gradient. Of course, this presupposes that the measurement of the turning behavior of the rear wheels and their evaluation does not yet indicate that there is a risk of locking on the rear wheels.

In certain types of vehicles, due to the design of the brakes (large wheel brake cylinders on the front wheels, small wheel brake cylinders on the rear wheels), only the front wheels can get into the unstable area during braking and therefore the brake pressure on them can be reduced without the rear wheels get into an unstable area. So that a good braking effect can also be achieved on the rear wheels in such a situation, the pulsation or throttling of the pressure build-up on the rear wheels is interrupted according to a preferred embodiment of the invention when a predetermined number of control cycles has expired on both front wheels , for example three control cycles. Then the pressure builds up on the rear wheels unthrottled or non-pulsed, ie at a speed determined by the pressure in the master brake cylinder. In the case of the pulsed pressure build-up described above, in a preferred embodiment of the invention, the speed (rate) at which the pressure at the rear wheels increases depends on the conditions of the Front wheels set. If pressure is reduced in the course of an ABS control on a single front wheel (of two front wheels), then the brake pressure on the rear wheels (provided that these do not themselves show any risk of locking) is increased with a relatively flat gradient. With a pulsating pressure build-up, the pulse series thus has a relatively low pulse rate. If, on the other hand, the second front wheel also reaches an unstable area, pressure is reduced on this wheel, or if the brake pressures are reduced on both front wheels at the same time as part of an ABS control, the pressure build-up on the rear wheels takes place with a relatively steep gradient , ie with pulsating pressure build-up, the pulse row has a higher duty cycle in comparison with the case where only one front wheel is unstable. The individual duty cycles are experimentally optimized for a given braking system and vehicle.

The ABS control methods described above also ensure good lateral guidance on the rear wheels of the vehicle. For example, if the pressure-holding valve of the front wheel running at a low coefficient of adhesion is actuated when braking to different adhesion coefficients, the pressure on the rear wheels is also kept constant according to the invention, so that the risk of the brake pressure overshooting the rear wheels is largely eliminated is. Even if the front wheel, which has a low coefficient of adhesion, becomes so unstable that the brake pressure in its brake is reduced, the pulsed pressure build-up that then begins on the rear wheels or the rear axle (which reduces the pressure build-up on the rear wheels or the rear axle compared to one) unthrottled pressure build-up means that an excess pressure in the brakes of the rear wheels is avoided.

When braking in a curve, the wheel contact force of the front wheel on the inside of the curve is reduced and the front wheel on the outside of the curve is subjected to greater stress. The method according to the invention therefore yields corresponding advantages for the ABS control like braking on different coefficients of adhesion. If, however, pressure is reduced when braking on both front wheels in the course of an ABS control, the brake pressure is built up relatively quickly on the rear wheels (provided that the rear wheels themselves do not show a tendency to lock), so that the vehicle is not understeered .

The invention is explained in more detail below with reference to the drawing. It shows:

1A and 1B a cornering of a braked vehicle and

Fig. 2 typical courses of the brake pressure on the two

Front wheels and the rear wheels in an ABS control according to the invention.

To understand the invention, it is necessary to take a closer look at the steering ability and the stability of the vehicle when braking a vehicle in a curve.

In the usual designation, a vehicle wheel is said to be "stable" if it runs in a region of the known coefficient of friction / slip curve that lies before a maximum of the coefficient of friction.

A vehicle is considered stable and steerable when all wheels are stable.

When braking in a curve, a vehicle can enter a state in which it is stable but understeering if one or both front wheels are unstable. In this case, due to the large wheel slip, the front axle of the vehicle cannot provide as much lateral guidance as the still stable rear axle, so that the vehicle deviates from the ideal curve in the sense of so-called "understeering". This is in Fig. 1A shown. There the circle K is assumed as the ideal curve. However, the curve actually carried out by the vehicle does not follow the ideal curve K; instead, there is an understeer.

On the other hand, when braking in a curve, a vehicle can get into a state in which it is unstable and oversteering. This is the case when one rear wheel or both rear wheels are unstable and both front wheels are stable. This is shown in Fig. 1B. Due to a lack of cornering force, the curve actually described by the vehicle deviates from the ideal curve K in the sense of an oversteer.

Finally, there is the interesting phenomenon that a vehicle is stable when cornering when both the front and rear axles of the vehicle are in a certain brake slip. The vehicle then behaves "neutral", i.e. neither oversteering nor understeering.

The invention makes use of this knowledge for an ABS control.

This is explained by way of example in FIG. 2. There, brake pressures P and P are plotted on the two front wheels and the brake pressure P on the rear axle above the time axis over a common time axis t.

Before time t, that is to say at time t _Q , braking begins, assuming that the vehicle is cornering. It is already provided in the time period t _n to t that the brake pressure P _τ V _τ - _τ ü (front left) and P "V_R. (front right) increases on the two front wheels with a slightly larger gradient than the brake pressure P on the two wheels of the rear axle. The front wheels come into an almost unstable area in which, according to FIG. 1A, a deviation from the ideal curve K occurs in the sense of understeer. At time t, one of the two front wheels, namely the right front wheel, comes into a state in which the ABS control requires the brake pressure P _{VR of} the right front wheel to be kept constant. This is in Fig. 2 after the time t. represents. At time t, the brake pressure P on the rear axle is therefore increased with a reduced gradient. In section A of the course of the brake pressure P ^ on the rear axle (ie before the time t,) the brake pressure on the rear axle rises unpulsed with a relatively strong gradient. In contrast, in section A * of the curve of the brake pressure P " _A on the rear axle there is a pulsating build-up of the brake pressure P _HA , ie a build-up with a gradient that is reduced compared to the time period before the time t. The rise gradient of the pressure in the curve section A 'between the times t and t ₂ is kept relatively flat in that the pulsation takes place with relatively long holding phases. This is also shown graphically in FIG. 2.

At time t ₂ , both front wheels become unstable (which is determined in a known manner by evaluating the signals from corresponding speed sensors). At time t_, the brake pressure P " _A is therefore built up with a further enlarged gradient. This corresponds to the curve section A ". In the curve section A" after the time t ₂ , the brake pressure on the wheels of the rear axle is built up in a pulsating manner with relatively short stopping phases, so that there is an increased gradient for the brake pressure P compared to the time t until t- . In this way, the vehicle comes into the "neutral" state described above, in which both the wheels of the front and rear axles are in brake slip and the vehicle as a whole can be regarded as stable. Thus, both the front wheels closer to the unstable region, as at about the time t ₂ is the case, then the brake pressure is built ^"9 at the wheels of the rear axle with rela¬ tively short holding phases pulsing to achieve on all wheels a desired brake slip and thus to improve the braking and steering behavior of the vehicle. In addition to the steering behavior, the braking distance is also optimized. In a modification of the curves of the brake pressures on the individual wheels of the vehicle, which are shown by way of example in FIG. 2, the ABS control algorithm can also provide that if the rear wheels are still stable and at least one front wheel is beginning to become unstable, the brake pressure on the wheels of the rear axle remains provisionally constant is held, i.e. is not increased further. This is used to identify the condition of the road surface by measuring the turning behavior of the wheels on the front axle. The rear axle of the vehicle maintains sufficient lateral guidance. If pressure reduction is initiated on at least one front wheel, in this exemplary embodiment there is a flatter pressure build-up on the wheels of the rear axle. In this second stage of the ABS control cycle, at least one wheel on the front axle becomes unstable due to excessive brake slip. In order to prevent the vehicle path from deviating too much from the ideal curve K (FIG. 1), the brake pressure on the wheels of the rear axle is therefore increased slightly, that is, a brake slip is also generated, so that the so-called "neutral driving behavior" mentioned above is generated "occurs and the vehicle has improved steerability.

Claims

Patent claims

1. A method for controlling the brake pressure in an anti-lock brake system of a vehicle having front and rear wheels, in which the rotational speeds of braked wheels are measured and as a function of the ratio of the rotational speed and / or rotational acceleration of a braked wheel to predetermined threshold values Brake pressure of the wheel is kept constant or reduced when there is a risk of locking, characterized in that if there is still no risk of locking at any rear wheel, but at least one front wheel maintains a constant brake pressure, the brake pressure at least also on the rear wheels is kept constant at times.

2. Method for controlling the brake pressure in an anti-lock brake system of a vehicle having front and rear wheels, in which the rotational speeds of braked wheels are measured and as a function of the ratio of the rotational speed and / or rotational acceleration of a braked wheel to predetermined threshold values Brake pressure of the wheel is kept constant or reduced when there is a risk of locking, characterized in that when at least one front wheel the brake pressure is reduced. The brake pressure build-up on the rear wheels is carried out at a rear wheel with no risk of jamming, and the speed is reduced compared to the build-up of brake pressure at the start of braking.

3. A method for controlling the brake pressure in a brake system of a vehicle with anti-lock braking system, in which the rotational speeds of braked wheels are measured and as a function of the ratio of the rotational speed and / or rotational acceleration of a braked wheel to predetermined threshold values of the brake pressure of the The wheel is kept constant or lowered when there is a risk of locking, characterized in that if there is still no risk of locking at any rear wheel, but the brake pressure is constant in at least one front wheel, the brake pressure build-up on the rear wheels also compared with the brake pressure build-up is carried out at the start of braking at a reduced speed.

4. A method for controlling the brake pressure in a brake system of a vehicle with anti-lock braking system, in which the rotational speeds of braked wheels are measured and as a function of the ratio of the rotational speed and / or rotational acceleration of a braked wheel to predetermined threshold values of the brake pressure Wheel is kept constant or lowered when there is a risk of locking, characterized in that when the braking pressure is reduced in at least one front wheel and no risk of locking is found on any rear wheel, the braking pressure on the rear wheels is kept constant at least at times.

5. The method according to any one of claims 2 or 3, characterized in that the brake pressure build-up carried out at a reduced speed is throttled or pulsed.

6. The method according to any one of claims 2, 3 or 5, characterized in that the speed of the pressure build-up on the brakes of the rear wheels when a pressure reduction occurs on only one front wheel is lower than when the brakes Both front wheels are depressurized.

7. The method according to any one of claims 2, 3, 5 or 6, characterized in that the brake pressure build-up is ended at a reduced speed on the brakes of the rear wheels after a predetermined number of anti-lock control cycles on the front wheels.