Hino 600, Workshop Manual, Page: 408 / 451

ABS (ABS : FULL AIR) BR03–5
BASIC PRINCIPLE
Based on its relationship with the slip ratio, which is determined from the
wheels' rotational speed and the vehicle's speed, ABS controls the brake
force so that it will be most effective.
When the driver applies the brakes, the rotation of the wheels is controlled
and the vehicle speed drops. However, the momentum of the vehicle
attempts to push it forward further even though the rotation of the wheels is
being halted. At this point, slipping will occur if there is a gap between the
wheels' rotational speed and the vehicle's speed. The slip ratio is a value
that indicates the rate of slippage.
The graph shown at left shows the relationship between the friction coeffi-
cient of the tires and of the road surface and the slip ratio.
In an ABS-equipped vehicle, the brake force is controlled to ensure that it is
within the range where the friction coefficient is high (shaded portion of the
graph) without locking the wheels. This ensures efficient braking perfor-
mance.
OPERATION
The ABS control characteristics line graph at left illustrates how the vehi-
cle's speed, the wheels' rotational speed, the wheel acceleration/decelera-
tion, and the air pressure at the ABS control valve outlet change over time
after the brakes are applied.
When the brakes are applied, the vehicle's speed and the wheels' rotational
speed drop and at the same time, the wheel acceleration/deceleration
speed also drops. At point T1, a gap begins to open between the wheels'
rotational speed and the vehicle's speed. Passing on point T2, the ABS
ECU detects that the wheels are beginning to lock and is lowering the outlet
air pressure of the ABS control valve to prevent wheel locking from occur-
ring.
At point T3, the wheel acceleration/deceleration speed is starting to return
to normal one, and the ABS ECU stops lowering the air pressure at the
ABS control valve outlet and maintains it at a constant level.
At point T5, the wheels' rotational speed and the vehicle's speed are about
the same. The ABS ECU detects that the wheels are no longer likely to be
locked and increases the air pressure at the ABS control valve outlet.
The above processes is repeated over and over until the vehicle comes to a
complete stop.
Slip ratio 0 % : No slipping between the wheels and the
road surface
100 % : Wheels locked
Slip ratio =
Vehicle's speed - Wheel's rotational speed
Vehicle's speed
100 %
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