Bendix Commercial Vehicle Systems A-18 TRAILER ABS 5/04 - Manual (Page 5)

5
Speed sensors
monitor wheel
rotation
Speed signal
to ECU
ECU interprets
speed signals
and activates valves
Hold and release solenoids
control air pressure in the
brake chambers
Braking force
remains at
optimum level
Res.
3
4
5
1
2
FIGURE 4 - Overview of Trailer ABS Operation
ABS PERFORMANCE CHARACTERISTICS
Routine Braking
During routine braking operations, there is no indication of
excessive wheel slip. The electronic control unit interprets
this condition as normal and ABS remains inactive.
ABS Controlled Braking
The control unit continuously monitors all available wheel
speed sensors. Data from the sensors is used to calculate
values of wheel speed and wheel slip and to make a best
estimate of the true vehicle speed. This data allows the
control unit to detect impending wheel lock and to hold the
wheel slip at an optimum value to maximize braking
effectiveness. The best possible vehicle stability is assured
while stopping distance is minimized.
Control is accomplished by operation of relay based
modulator valves. The control unit makes a new
assessment of conditions and updates the control signal
to the modulator valves at a rate of approximately 100 times
per second.
Under normal (non-ABS) conditions, trailer ABS relay valves
operate exactly like conventional mechanical relay valves.
(Refer to Figures 11 through 14.) During ABS operation,
the control unit operates the valves to override the supply
of air to the chambers. During an ABS release, supply air
is held off while the chambers are vented to the atmosphere.
In hold mode, supply air is blocked and chamber air is held
constant. When required, air is applied to the chamber at
a controlled rate by modulating the hold side of the valve.
The antilock system does not apply additional braking
power. Rather, it controls air pressure to release and hold
brake torque, thereby increasing a vehicle’s capacity for
quick, straight stops. With ABS installed, vehicle operation
is safer, resulting in improved protection of driver, cargo
and equipment.
ABS Component Function
Figure 4 shows an overview of the operation of the Bendix ®
A-18 ™ trailer ABS system.
Speed sensors (1) monitor wheel rotation and provide
information (2) on wheel rotation to the central electronic
control unit.
The Electronic Control Unit (3) receives the sensor signal,
interprets the pulse information, and constantly calculates
the relationship of speed, acceleration, and deceleration.
A control signal (4) is sent to the ABS relay valve (5), which
then controls the pressure to the air chambers.
System Designs
When operating on high traction surfaces with a loaded
vehicle, there is little difference between types of ABS
control. Performance differences appear when vehicles
are lightly loaded and operating on variable and poor traction
surfaces. Examples of poor traction surfaces are ice and
combinations of ice, snow and asphalt. Operating a vehicle
in a curve highlights differences in stability between various
systems.
ABS system designs provide compromises between stability
and stopping distance while addressing cost, complexity
and reliability issues.