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Anti-lock brake systems (ABS) - generally also referred to as anti-lock
systems (ALS) - are designed to prevent the vehicle wheels from locking
as a result of the service brake being applied with too much force, especially
on slippery road surfaces.
The idea is to maintain cornering forces on braked wheels to ensure that
the vehicle or vehicle combination retains its driving stability and manoeuvrability
as far as physically possible. The available power transmission
or grip between tyres and carriageway should also be utilised as
far as possible to minimise the braking distance and maximise vehicle
deceleration.
Why ABS? Although today commercial vehicle brakes are designed to a very high
technical standard, braking on slippery roads often results in potentially
dangerous situations. During full or even partial braking on a slippery
road it may no longer be possible to fully transfer the braking force onto
the road due to the low coefficient of friction (friction coefficient (k)) between
the tyres and the carriageway. The braking force is excessive and
the wheels lock up. Locked wheels no longer provide any grip on the
road and are almost incapable of transferring any cornering forces
(steering and tracking forces). This often has dangerous consequences:
– The vehicle becomes unsteerable
– The vehicle breaks away in spite of countersteering, and starts to
swerve.
– The braking distance is significantly increased
– Tractor-trailer combinations or semitrailer trains may break away or
jackknife.
Load sensing valve influence On dry roads today’s load sensing valves (ALB) alone are often capable
of preventing the wheels from locking if the vehicle is unladen; they also
help the driver to effectively grade the braking process on wet road surfaces,
but they are unable to prevent locking as such (no slip monitoring).
In addition, they are unable to counteract any overreactions on the
part of the driver, or any variances in frictional or adhesion coefficients
which may apply to different sides of the vehicle, or indeed to its different
axles (μ-split road surfaces).
Benefits of ABS: Only the Anti-Lock Brake System (ABS)
– guarantees stable braking characteristics on all road surfaces.
– maintains steerability and generally reduces the braking distance
– prevents vehicle combinations from jackknifing
– reduces tyre wear.
Limits of ABS Although ABS is an effective safety device, it can not suspend the limits
defined by driving physics. Even a vehicle fitted with ABS will become
uncontrollable if driven too fast around a corner.
So ABS is not a licence for a maladjusted style of driving or failure to observe
the correct safety distance.
Increasing the engine output (accelerating) on a slippery road surface
can easily lead to the maximum adhesion on one or all powered wheels
being exceeded causing them to spin, especially if the vehicle is unladen
or partially laden.
Spinning wheels when driving off or accelerating represent a safety risk
just like locked wheel do when braking.
Reasons – Wheels that spin transfer just as little cornering force as locked
wheels.
– They also no longer transfer any tractive power onto the road.
Consequences – Vehicles that do not move or get stuck.
– Vehicles that can no longer be steered, jackknife on uphill uphill
gradients, or swerve in corners.
Benefits of ASR ASR prevents the powered wheel from spinning and provides the following
benefits:
– Tractive power and cornering forces are maintained.
– Stable driving behaviour is ensured when moving off, accelerating
and negotiating corners on slippery roads.
– The indicator lamp (if installed) is used to warn the driver of slippery
road conditions.
– Tyre wear is reduced to a minimum, and the motor vehicle’s drive
train protected
– The risk of accidents is further reduced.
ASR and ABS: ASR represents a worthwhile addition to an ABS-controlled braking system.
All that is required to turn ABS control into full ABS/ASR control is
an ECU with the additional ASR function and a few additional components
for controlling the differential brake and the engine. This why ASR
is only available in combination with ABS.
Even a differential lock for off-road use and ASR do not exclude but complement
each other.
Limits of ASR The traction capacity of an all-wheel driven commercial vehicle can not
be achieved by a motor vehicle with only one driving axle - not even with
optimal ASR.
From the ABS E version onwards, lorries, semitrailer tractors and buses can be
equipped with Roll Stability Control (RSC) in addition to ASR. This function is integrated
in the ABS control unit and can be activated by the vehicle manufacturer.
RSC controls the engine output and applies the service brake to reduce the risk of
overturning in corners. RSC identifies the critical lateral acceleration for this purpose.
When the lateral acceleration exceeds a specific level, RSC reduces the engine
torque, activates the engine brake and brakes, if required, the towing vehicle axles as
well as the trailer, if applicable.
RSC also applies the brakes on the towing vehicle front axle by means of a 3/2 solenoid
valve fitted there.
A lateral acceleration sensor and the software for signal processing, monitoring and
driving dynamics control are integrated in the ABS control unit for RSC.
ESC (Electronic Stability
Control)
Lorries, semitrailer tractors and buses can be equipped with Electronic Stability Control
(ESC) in addition to ASR control. ESC is available from the ABS E version onwards
and requires additional components.
Within physical limits, ESC operates automatically and takes corrective action in
terms of engine output and brake to keep the vehicle on track during extreme driving
situations.
It operates when driving as well as braking and comprises two independent functions:
Control of tracking stability (yaw Control)
This function is activated when the vehicle loses stability in critical situations (e.g. during
a sudden change of track. In such situations ESC uses ABS or EBS to regulate
the braking forces on each wheel, throttles the engine output, thereby reducing the
risk of swerving when cornering and during avoidance manoeuvres.
ESC prevents potential "jackknifing" of a semitrailer train by simultaneous, dosed
braking of the semitrailer even if it is equipped with a conventional braking system.
Driving stability control (ROP – Roll Over Protection)
This function is activated when there is a risk of overturning, e.g. when cornering too
fast. The function is similar to RSC (Roll Stability Control).
The ESC control unit processes the data from the yaw rate, lateral acceleration and
steering angle sensors and communicates with the ABS or EBS control unit via the
braking system data bus.
The ESC function requires the following additional components:
– ABS control unit with ESC functionality
– ESC module
– Valves in accordance with the respective vehicle. The 3/2 solenoid valve on the
front axle is needed to brake the front axle wheels separately. The 3/2 solenoid
valve upstream of the trailer control valve is needed to brake the trailer at the same
time.
– Brake pressure sensor
– Steering angle sensor