How ABS works
Virtually all modern cars are fitted with microprocessor-controlled anti-lock braking systems (ABS). These can react very quickly to the wheels locking, interrupting and reapplying the brakes up to 25 times a second to ensure the vehicle doesn’t skid.
The best way to prevent skidding is to apply a form of braking called
cadence braking. A driver who is skilled at this can usually avoid wheel
lockup, but an anti-lock braking system does the job automatically and usually
more efficiently. More and more cars are now being fitted with such a
system.
Cadence braking
This is a way of maintaining control in very slippery conditions. The
technique requires that the driver quickly and repeatedly releases and
reapplies the brakes. The brakes should be released just before the wheels
lock up but it is almost as effective to release them just after lock
up.
The technique of cadence braking has to be done in perfect timing with
the car’s natural pitching motion, otherwise it may not be of any
benefit.
How it works
An anti-lock system automatically applies a form of cadence braking by
detecting when a wheel is about to lock, releasing the brake at that wheel and
then immediately reapplying it. The system, therefore, needs three main parts:
a means of telling when a wheel is about to lock; a means of releasing its
brake; and a means of restoring the pressure to the brake line after
release.
The third feature is necessary because the anti-lock system has to work
without the driver releasing and reapplying pressure on the brake pedal, and
without the pedal sinking to the floor.
Skid detection
A car tyre provides its best grip just before it gives up altogether and
slides. Any method of detecting a wheel about to lock must therefore allow for
its speed falling slightly below the free-running speed – the system must not
react too eagerly, but must still work quickly once the point of best grip has
been passed.
In practice, there are two ways of detecting that a wheel is about to lock.
Its speed can be compared with that of the other wheels, or the rate at which
it is slowing down can be measured. In either case the hydraulic pressure at
the brake can be released if the deceleration is too great.
Computer-based electronic systems work by speed-checking the wheels against
each other, but usually run a double-check by keeping track of the rate of
deceleration too. These systems are complex and relatively expensive. Some
anti-lock systems use mechanical sensors that detect when a wheel is slowing
too quickly.
Bosch ABS (Anti-blocking system)
In the Bosch system, wheel speeds are read by toothed discs running Hydraulic pressure to the brakes is cut by electronic solenoid valves. When the driver applies pressure to the brake pedal, the solenoid valve allows pressure through to the caliper so that the pads can grip and slow the disc. If the sensor detects that the wheel is about to lock, the valve’s plunger moves up to cut off the hydraulic supply and maintain pressure at the brake. If the wheel is still in danger of locking, the computer moves the plunger still further to ‘dump’ pressure to the reservoir, while the pump builds up pressure for reapplication. When the wheel has speeded up sufficiently the valve is dropped to its lowest position to allow the accumulated pressure through to reapply the brake. All anti-lock systems use some sort of valve to release the hydraulic This is done by shuttle valves in which an internal body moves so that it Any anti-skid system must include a pump, or pumps, to provide the pressure Cars with fully powered hydraulics usually have a pump driven from the The mechanical system uses a purely mechanical pump close to, and driven by, The first car to go into production with an anti-lock system was the Although the FF proved that anti-lock braking was a workable The system was further handicapped by its cycling rate (the speed at These systems work at the rate of 15-25 on-off cycles a second. They detect When the microprocessor decides it is time to release a wheel, it sends a The SCS is entirely mechanical and only costs about a third of the price of ABS. The hydraulic control units are driven by belts from the front drive shafts and use inertia sensors to calculate deceleration rates. The control units incorporate simple plunger pumps to enable brake pressure to be reapplied once the wheel has been prevented from locking. The SCS is entirely mechanical and only costs about a third of the price The hydraulic control units are driven by belts from the front drive The control units incorporate simple plunger pumps to enable brake When the wheel is in no danger of locking, the modulator unit’s dump valve is closed and brake fluid is allowed through to the caliper at full pressure. If the wheel decelerates too quickly, the unit’s flywheel over-runs it, moves sideways and allows the dump valve to open, relieving brake pressure. The wheel speed increases and ‘catches up’ with the decelerating flywheel, which moves back to close the dump valve. The pump restores pressure. The most popular approach, seen in cars such as the Citroen CX with the ATE Honda adopt a slightly different approach for their ALB system, which works The Lucas Girling SCS was recently introduced as an anti-lock option on Ford’s front-wheel-drive Escort and Orion, although the system is suitable for most light, front-wheeldrive cars. The system only senses when the front wheels are about to lock, although a reduction in pressure at a front wheel is coupled with a proportionally smaller reduction at the diagonally opposite rear one to keep the car straight. The Lucas Girling SCS uses an inertia-type locking sensor, which operates
through sensors. The microprocessor compares wheel speeds and deceleration
rates.
The hydraulic unit also contains a pump and accumulator to maintain
pressure to reapply the brakes.
Brake valves
pressure at the wheel cylinder to prevent the wheel from locking. The valve
diverts the pressure back to the brake fluid reservoir. A simple release valve
is not enough, however, because it would allow all the fluid from the master
cylinder to flow back to the reservoir and the brake pedal would sink to the
floor. Any release valve must have a means of shutting off the pressure supply
from the master cylinder at the same time that it releases pressure from the
wheel cylinder.
simultaneously closes the pressure-supply port and opens a pressure-relief
port. The valve may be moved by hydraulic control pressure or by an
electromagnetic coil. Where the skid detector is a simple mechanical device,
mechanical valves are usually used; with microprocessor systems the valves
receive electrical control instructions.Feed pumps
to reapply the brakes after they have been released without the driver needing
to release and reapply the brake pedal. The pressure in an electronic system is
stored in a tank called a hydraulic accumulator and controlled by a pressure
regulator. In most systems the pump is electrically driven. It keeps the
accumulator at sufficient pressure to ensure that the system can reapply the
brakes often enough to stop in any circumstances.
engine. These types of system are rare, though they have been successfully used
on larger Citroen models for the past 30 years. The new Jaguar XJ6 also has
such a system.
each wheel. Each time the anti-lock system releases a wheel, the action of its
running back up to ‘free’ speed generates enough pressure for the brake to be
reapplied once more.The Maxaret system
Jensen FF. The system was Dunlop’s Maxaret, originally developed for
aircraft wheel brakes.
proposition, the Maxaret suffered in two ways from its aircraft origins. It
was so large and expensive that even on the big Jensen only one unit could
be installed, controlling the pressure to all four wheels together.
which it could release and reapply the brakes), which was too slow to
achieve the best results.Electronic systems
the speed of each wheel by reading the speed of a smaller toothed disc attached
to it. The disc runs through electromagnetic sensors that count the teeth to
find out how fast the wheel is turning. The signals from the sensor are fed to
a microprocessor which works out if any wheel is running more slowly than the
others, or if any wheel is decelerating too quickly.
signal to the electromagnetic valve to close off the pressure supply and
momentarily release the existing hydraulic pressure. Then, when it senses that
the wheel has speeded up enough, braking pressure is reapplied. The rise and
fall in pressure is felt as a pulsing at the brake pedal — the only indication
the system is in operation.Lucas Girling SCS (stop control system)
of ABS.
shafts and use inertia sensors to calculate deceleration rates.
pressure to be reapplied once the wheel has been prevented from
locking.
Anti-lock layouts
system or the Ford Granada and Jaguar XJS with Bosch ABS, is to control the
braking of each front wheel but to handle the two rear wheels together, the
rear brake pressure being governed by whichever wheel is closer to locking.
This ‘three channel’ approach reduces costs while sacrificing little in the way
of effectiveness compared with systems that control all four brakes separately,
such as that in the S-class Mercedes.
in much the same way but assumes that the most important wheel to prevent from
locking is the most heavily loaded front one.Mechanical systems
the brake valves directly, and wheel-driven pumps. With this system, over-rapid
deceleration of the wheel moves a weight against a spring to open a hydraulic
control valve. Although SCS has a slower cycling rate than electronic systems
such as ABS, it gives satisfactory results, especially in light, front-driven
cars such as the Ford Escort on which it was introduced, and it is also much
cheaper.