Fluid-pressure and analogous brake systems – Speed-controlled – Pulse frequency or time period controlling pressure rebuild
Patent
1995-11-20
1998-07-07
Graham, Matthew C.
Fluid-pressure and analogous brake systems
Speed-controlled
Pulse frequency or time period controlling pressure rebuild
303174, B60T 800
Patent
active
057757852
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to anti-lock/anti-skid braking systems (ABS) for vehicles.
Anti-skid braking systems for vehicles comprise speed sensors on each of the wheels of the vehicle and a control device responsive to speed signals from the speed sensors to periodically reduce the pressure of actuating fluid to the brake of a wheel which is about to lock in a series of on/off cycles, thereby avoiding or reducing skidding of the wheel.
The control device seeks to combine sensitive control, efficient braking and effective steerability/stability by optimizing the proportion of each cycle in which the wheel is operating close to the tyre's maximum adhesion. As is well known, the magnitude of this braking force, when expressed as a function of the tyre's slip, is characterised by a curve with an asymmetric hump (see curve CB--FIG. 2). The balance of longitudinal and lateral forces which exist at the peak of this curve usually represent an optimum compromise for normal road-going vehicles.
Once the tyre reaches the negatively sloping region of the curve, braking effort begins to diminish--sometimes sharply--and lateral adhesion (curve CS--FIG. 2), which reduces progressively with increasing slip, becomes unable to maintain satisfactory steering or stability. If the wheel strays too far into this region before corrective action is taken, then the control will become coarse, with large angular decelerations and accelerations of the wheel, high slip levels and consequent large-amplitude pressure cycling. To passengers, this will be apparent as a jerky, vibratory sensation.
Efficient braking requires that the wheel should spend as much time as possible at, or very close to, the slip level which corresponds to the peak braking force. This means that the pressure rise rate has to be relatively flat, so that the time taken for the wheel to pass through the region of maximum braking force is prolonged. Control refinement will also benefit through a reduction in "pressure overshoot", i.e. the degree to which the brake pressure exceeds the ideal pressure during the time needed by the system to detect the impending skid and initiate corrective action.
Effective steering/stability is best assured by allowing the wheel to spend part of each cycle in the positively-sloping, i.e. underbraked region, of the curve. This occurs naturally as a consequence of pressure undershoot during the pressure dump phase, but needs to be controlled by providing rapid restoration to the ideal pressure if efficiency is to be maintained.
Thus, there is a need for a rapid pressure reapply rate during a first stage of brake force restoration, but a slower rate thereafter.
It is known to achieve this, in the context of a flow-valve system, by pulsing the pressure dump solenoid at intervals to cause a repeated interruption to the otherwise smooth reapply phase, generating a saw-tooth profile with a flatter overall rise rate. Thus, the emphasis is upon the macro effect of the pulses upon the pressure-rise rate. In the prior art, both the intervals and the timing of the first pulse are predetermined, e.g. by a prescribed relationship with events (e.g. pressure dump and/or reapply duration) from the previous cycle. However, the correlation between these parameters and optimum pulse timing is often non-linear, rendering the predictions reliable only within a narrow range of operating conditions.
In accordance with the present invention, the pulse timing is controlled by the occurrence of wheel angular-deceleration being equal to or in excess of a predetermined threshold value, the magnitude of which is adapted in dependence upon events from the previous cycle.
Thus, the pulse timing is arranged to be dependent upon current data about the ability of the wheel to resist the applied braking force. Each pulse thereby assumes increased importance since only through its action in preventing the wheel from approaching the brink of locking can the wheel's time in the peak adhesion zone be extended.
The main advantage of this principle is that the timing of
REFERENCES:
patent: 4585280 (1986-04-01), Leiber
patent: 4900099 (1990-02-01), Braschel
patent: 4921314 (1990-05-01), Braschel et al.
patent: 5513907 (1996-05-01), Kiencke et al.
Harris Alan Leslie
Phillips Mark Ian
Graham Matthew C.
Lucas Industries PLC
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