Anti-lock braking systems (ABS) for road vehicles to avoid prema

Details Anti-lock braking systems (ABS) for road vehicles to avoid prema Anti-lock braking systems (ABS) for road vehicles to avoid prema

1995-11-24

1998-01-13

Graham, Matthew C.

Fluid-pressure and analogous brake systems

Speed-controlled

Odd condition or device detection

303154, 303164, B60J 800

Patent

active

057071217

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to ABS systems for road vehicles.
2. Description of the Related Art
Such systems operate by responding to an impending wheel-lock/skid condition at any particular wheel to reduce (dump) the actuating pressure applied to the brake associated with that wheel and to later re-apply the pressure when the tendency of that wheel to lock has reduced. A wheel-lock/skid condition is usually determined in such systems by detecting the occurrence of a predetermined drop (.delta.v) in wheel speed (corresponding to a predetermined level of relative slip between the wheel and the road surface) following the achievement of a predetermined wheel deceleration (often identified as -b).
There is a trend for modern vehicle chassis to allow considerable rearward movement of the axles when bumps are encountered. This helps cushion the reaction felt at the passenger cabin, and thus contributes to improved comfort in the vehicle ride.
Unfortunately, any longitudinal compliance will also allow axle deflection when the tires are reacting to normal braking forces. If the deflection occurs in a short space of time, e.g. due to a sudden brake application, then it will be perceived by the ABS as a rapid fall in wheel speed. This is because rearward movement of the axle, taking place in a finite period of time, corresponds to a velocity in the opposite sense to that of vehicle motion.
Premature ABS activity may be triggered in prior art systems because these systems are unable to distinguish between normal adhesion-generated slip and transient axle deflection described above. This could only be avoided by setting the system's skid-detection thresholds at relatively insensitive levels. However, these may well be quite inappropriate to slower rates of pressure increase under which conditions the axle deflection takes place over a longer time period, such that the effect upon the perceived slip is insignificant.
Thus an awkward compromise must be achieved. Premature detection can delay the establishment of full deceleration levels and may also lead to driver complaints if the ABS intervenes under "normal" operating conditions. On the other hand, late detection (due to a lack of sensitivity) of an impending skid can also cause loss of performance due to excessive pressure overshoot, which precipitates high slip between the tire and road surface, needing considerable pressure reduction before the wheel will show signs of recovery. On a high-.mu. road surface, this recovery will be quite rapid, leading to a period of underbraking until the ABS can re-establish the correct pressure level. The resulting see-saw behaviour of the vehicle is also likely to draw driver criticism.
However, deflection-generated slip has a maximum value determined by the movement allowed and the maximum rate at which the braking force can be developed (determined by the characteristics of the brake actuating system). A typical maximum value for a passenger car is 6 km/h, although this will vary by a small amount (say .+-.1 km/h) according to the torsional stiffness of the tires.


SUMMARY OF THE INVENTION

In accordance with the present invention, the slip sensitivity of the system is arranged to be set to a relatively insensitive level at the first sign of an impending skid, is maintained at this level for a period corresponding to the time needed by the axle to move rapidly through its available deflection, and is arranged to be then restored in one or more stages to a normal level.


BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:
FIG. 1 shows an example of how the wheel speed drop thresholds (Delta-V) might vary with speed in the application of the present invention;
FIG. 2 shows an example of how the minimum speed drop thresholds (minimum Delta-V) might vary progressively with each successive scan-period;
FIG. 3 illustrates, by way of a series of traces, the operationa

REFERENCES:
patent: 4921312 (1990-05-01), Harris
patent: 5324102 (1994-06-01), Roll et al.
patent: 5342118 (1994-08-01), Kampfmann et al.
patent: 5386366 (1995-01-01), Roll et al.
patent: 5425574 (1995-06-01), Sano
patent: 5505532 (1996-04-01), Tozu et al.

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