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The Landrover Discovery has ABS
as an option. ABS calibration for dirt
surfaces & constant 4-wheel drive is
quite complicated.
Anti-lock braking
systems: how they work
Now commonplace on family cars,
anti-lock braking systems require fancy
electronic control circuitry to do their job.
Here’s a rundown on how they work.
By JULIAN EDGAR
An anti-lock braking system (commonly known as ABS) prev
ents a
car’s wheels from locking during
panic braking. This has two distinct
advantages: (1) it gives shorter stopping distances; and (2) it allows the
car to be steered during hard braking.
A car with locked wheels cannot be
controlled by steering input and will
also take much longer to stop than one
with the wheels still turning while it
is being braked.
6 Silicon Chip
Anti-lock braking systems have been
used in automotive applications for
around 25 years but have only recently
found widespread use in mass-produced family cars. This has been made
possible by a reduction in the cost
of the electronic circuitry required
and by increased public awareness of
safety issues.
Unlike airbags, which protect the
car’s occupants after the car has hit
something, ABS gives a car greater
primary safety – meaning that it is less
likely to be involved in an accident
in the first place. In the vast majority
of situations, an ABS-equipp
ed car
will have a braking advantage over a
conventionally-braked car, although
it should be noted that in some (rare)
situations, an ABS will actually give
longer stopping distances.
The task
An anti-lock braking system has
an apparently simple role – to stop
individual wheel lockup while still
providing maximum braking efficiency. In stable situations where the
frictional coefficient between the tyres
and the road is constant, where vehicle
mass is unchanging, and where the
road surface is smooth, appropriate
ABS behaviour is relatively easy to
organise. However, in the real world,
CONTROL ZONE
BY ABS
the driver pull the car fully back onto
the bitumen, this lateral difference in
braking effort could result in the car
yawing rapidly.
An optimal anti-lock braking system
would thus give the following charac
teristics during operation:
(1). Driving stability maintained
through the retention of sufficient lateral guiding forces at the rear wheels;
(2). Steering ability retained through
the provision of sufficient lateral guiding forces at the front wheels;
(3). Reduced stopping distances; and
(4). Rapid matching of the braking
force to different adhesion coefficients.
FRICTIONAL COEFFICIENT
BETWE E N T YRE AN D ROAD S URF ACE,
ASPHALT
ROAD
ICE-SNO
W ROAD
0
SLIP RATIO
100%
Fig.1: the maximum braking effort is obtained when there is a certain amount
of wheel slippage. While it depends on the road surface, best braking is usually
obtained with a wheel slippage ratio of between 8% & 30% (Subaru).
a large number of variables means
that anti-lock braking systems need
to be very sophisticated in the way
they operate.
An ABS control system must take
into account:
(1). Variations in the amount of adhesion between the tyres and road
due to changes in the road surface
and in wheel loads (especially during
cornering);
(2). Irregularities in the road surface
which cause the wheels and suspension members to vibrate;
(3). Out-of-round tyres and brake
hysteresis characteristics; and
(4). Different friction coefficients
which might exist between the left
and right-hand wheels, and a possible
subsequent transition to a homogeneous surface.
Taking the last point as an example,
if a car is heavily braked while the
right-hand wheels are on dry bitumen
and the left-hand wheels are on the
dirt verge, then the ABS would obviously reduce the braking effort in the
left-hand wheels. However, should
Braking behaviour
Obtaining the optimal braking
force is more complicated than it first
appears, with brake slippage actually
necessary for best results. The brake
slip ratio is defined as follows:
Slip Ratio = (Vehicle speed - Wheel
speed)/Vehicle speed x 100%
When the slip ratio equals zero the
wheel is travelling at the same speed
as the car (ie, there is no slippage).
Conversely, when the slip ratio is
100%, the wheel is locked and does
not rotate at all. The relationship between the longitudinal frictional force
of the wheel and the slip ratio depends
on the road surface.
Fig.1 shows this relationship for as-
Fig.2: this diagram shows the
main components of a typical
anti-lock braking system, in
this case for the Subaru Liberty
(Subaru).
November 1994 7
TOOTHED WHEEL
+V
FULL SPEED
POLE PIECE
0
S
N
PERMANENT MAGNET
-V
SLOW SPEED
Fig.3: inductive sensors are used to signal wheel speed to the
electronic control unit & this then calculates the vehicle speed
&the slippage for each wheel.
phalt and ice-snow surfaces. It can be
seen that in both cases the maximum
frictional coefficient between the road
and the tyre is achieved when in fact
there is some slip. In other words, allowing the wheel to continue to rotate
at the same forward speed as the car
– that is, not skidding at all – will not
give maximum retardation. The slip
ratio at which the maximum friction
exists is generally 8-30%, depending
on the road surface.
While an 8-30% slip ratio works
well on dry and wet bitu
men, ice
and many other road surfaces, it
does not hold true for fresh snow
and gravel. In Australia, the latter
road surface is especially important
Operation
Fig.4: the toothed wheel (tone wheel)
is located on the inner hub of each
wheel to excite the pick-up sensor. In
some cars, the same sensors are also
used for traction control (Subaru).
Even cheap compact cars like this Holden Barina can now be supplied with
ABS as an optional extra.
8 Silicon Chip
and on gravel a slip ratio of 100%
gives maximum retardation. In other
words, locked wheels stop the car in
a shorter distance on gravel than any
other technique.
This is because a small dam of gravel (or snow) builds up in front of the
locked wheel and helps to slow the
vehicle. The skidding wheel can also
gouge its way down to a firmer surface
beneath the gravel. Of course, while
this is happening there will not be any
steering control available!
Some manufacturers provide a
dash-mounted switch which allows
the driver to switch off an anti-lock
braking system while driving on surfaces for which it is not compatible.
However, most manufacturers avoid
doing this, mostly because of potential
driver confusion and the fact that the
anti-lock braking system might be left
deactivated just when it’s needed.
An anti-lock braking system comprises a series of input sensors which
read the wheel speeds, an electronic
control unit (ECU), and a hydraulic
control unit (HCU). Fig.2 shows the
essential elements of a typical ABS.
The wheel speed input sensors are
typically inductive pickups and these
use a permanent magnet and a coil.
A toothed ring attached to the inner
part of the wheel’s hub rotates past
this sensor. As it does so, the teeth
change the magnetic coupling into the
coil and so the sensor generates an AC
waveform whose frequency depends
on the wheel speed.
Fig.3 shows an example of a sensor
and its output, while Fig.4 shows its
location on the car. Note that in this
Subaru system, the toothed ring is
called a “tone wheel”.
A typical ABS electronic control
unit is shown in Fig.5. As well as
the sensor amplification and shaping
circuitry, it comprises the ABS comparison and control circuitry, plus a
number of output transistors which
control the solenoids and pump within
the HCU (hydraulic control unit). A
self-diagnosis circuit is included to
allow easy fault-finding and self-check
circuits monitor the electrical condition of the input sensors and output
actuators.
If a fault is detected, a dash-mounted
warning light is illuminated and the
brakes then operate conventionally.
In sophisticated 4-channel systems,
Fig.5: block diagram of a Bosch ABS. Note the safety monitoring & the self-diagnosis circuits (Subaru).
the ECU uses the input signals from
two diagonally opposite wheels to
derive a vehicle reference speed. Using
this speed and the individual wheel
speeds, it then calculates the brake
slip for each wheel.
When a wheel’s deceleration exceeds a preset value, the ECU transmits a “hold” signal to the HCU. At
the same time, the ECU computes a
dummy vehicle speed, and – should
the wheel speed drop below this – the
ECU decreases the brake fluid pressure
to prevent lockup. However, with
the decrease in brake fluid pressure
the wheel accelerates. When this
acceleration passes a preset value,
another “hold” signal is transmitted
to the HCU; should wheel acceleration
continue then the brake fluid pressure
is increased. The frequency of this
brake fluid pressure cycling varies
from 4-10Hz.
The HCU consists of solenoid
valves, a hydraulic pump and accumulator chambers. Fig.6 shows an external view of an HCU. Depending upon
the switching state, the brake cylinder
is con
nected to the corresponding
circuit of the brake master cylinder,
the return pump, or is isolated. When
pressure is reduced, the return pump
moves the fluid flowing out of the
wheel brake cylin
ders back to the
master cylinder via the accumulators.
The accumulators are present to absorb
any temporary brake fluid surplus that
may be produced when the pressure
suddenly drops.
Other systems
Fig.6: external view of a Bosch
hydraulic control unit (Subaru).
Not all anti-lock braking systems
use four input sensors and three or
four control channels – indeed not
all anti-lock braking systems are even
electronic. Teves, Lucas Girling and
individual vehicle manufacturers use
variations on the theme. Some, for
example, set the hydraulic pressure
applied to both rear wheels according
to the wheel with the highest slippage
(ie, the same pressure is applied to
both wheels). Others may do the same
for the front.
Some anti-lock braking systems
also use an acceleration sensor which
measures the rate of vehicle slowing.
For example, on the Subaru Liberty,
a G-sensor is used when ABS is installed on manual constant 4-wheel
SC
drive cars.
November 1994 9
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