Anti-lock brake system

Fluid-pressure and analogous brake systems – Speed-controlled – Having a valve system responsive to a wheel lock signal

Patent

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Details

303104, 303106, 303110, 303100, B60T 832

Patent

active

053121708

DESCRIPTION:

BRIEF SUMMARY
PRIOR ART

So-called slip regulators are known in which the wheel slip is formed from the wheel speed and a reference variable which is derived from the wheel speed and is approximated to the characteristic of the vehicle speed. There, valves are actuated and the brake pressure varied when thresholds are exceeded and undershot by the wheel slip.


SUMMARY OF THE INVENTION

The present invention is an anti-lock brake system for a vehicle wheel of the type wherein the wheel speed VR and the brake pressure PB and determined and, with the aid of a reference speed VRef, slip actual values S are formed which are used to regulate the brake pressure PB at the wheel brake. According to the invention, the regulating phases and brake pressure control phases alternate, in that: during the regulating phases, the friction coefficient .mu. is determined continuously from the brake pressure PB and the wheel deceleration BR; during each regulator phase the maximum value of the friction coefficient .mu.max is determined, as is, in addition, the slip value SK when the maximum friction coefficient is reached; the slip value SK is in each case used to form a somewhat smaller slip setpoint value Sdes. which is specified as the slip setpoint vale for a slip regulator in the following regulating phase; if the wheel slip S rises during the regulating phases above a predetermined value SB, the system is switched over to brake pressure control, during which a reduced brake pressure Pmin initially is introduced; after the slip S has fallen below the value SB during the brake pressure control phase, an increased brake pressure PS relative to Pmin is introduced; and the slip regulator is brought into effect with the new setpoint slip Sdes. when the slip S becomes smaller than SK.
In the invention, the wheel slip is used as the controlled variable. The slip setpoint value for regulation is here obtained from the vehicle speed, the brake pressure and the wheel speed. The invention guarantees good utilization of the available friction coefficient potential and automatic and rapid adaptation to changed friction coefficient potentials. The invention is of particular interest for lorries with an ELB system.
In the invention, the driver specifies the brake pressure via the service brake valve during the initial braking phase. In parallel therewith, the primary control enables the acquisition of the maximum friction coefficient and the slip at the maximum friction coefficient .mu..
As soon as a switching threshold is exceeded (instability present), the pressure control is activated. From the .mu.max determined, the pressure control specifies the brake pressure. After stabilisation of the wheel, the regulator is activated and then regulates the pressure to the setpoint slip determined during the initial braking phase.
Due to changed roadway conditions or due to the effect of the adaptation, the wheel passes briefly into the unstable range, resulting in an end to the regulating phase Since the recording of .mu.max and SK is restarted afresh during each regulating phase, current values for .mu.max and SK are available again for the pressure control phase and for the subsequent new regulating phase. The primary control switches the brake pressure over to the driver-specified pressure and initialises the overall system as soon as the brake pressure required by the driver is smaller than the existing brake pressure or a low threshold (minimum value vmin) for the vehicle speed is undershot.
Further details relating to the acquisition of the individual variables and the regulation and control are provided below.
From the wheel speed (hub speed) VN measured at the wheel rim, the "hub slip" SN is obtained:
The dynamics of the slip build-up in the footprint is taken into account by a filter acting as an average former. The term "footprint" is taken to mean the tire contact area (tire-roadway contact area). A filter with a variable "time constant" is given as an illustrative embodiment. The filter time constants are determined with the aid of a brush mode

REFERENCES:
patent: 4794538 (1988-12-01), Cao et al.
patent: 4916619 (1990-04-01), Walenty et al.

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