Electronic Driver
Aid Systems
Electronic
driver aids is such a vast topic that this website has no hope of covering all
of them. From your ABS to your EBD
covering TCS and CBC, what are they, what do they do and should you have
them? More and more cars are having many
complicated-sounding electronic driver aids installed as standard, indeed antilock
brakes have been standard for the majority of vehicles since the summer of
2004. These are covered elsewhere.
Electronic Brake Force Distribution
The first
driver aid to consider is electronic brake force distribution (EBD), standard on many machines. EBD uses sensors to detect the loading of a
car combined with the grip from an individual wheel to adjust the braking
pressure. This means that the brakes
work differently if the car has people in the back, luggage in the boot or is
towing a caravan – tick all that apply.
Conventional
braking systems have a natural bias towards the front such that the front
brakes do more of the braking effort compared with the rears. This is because as the car is braked, the
centre of gravity shifts forward, reducing the pressure on the road from the
rear tyres and ultimately the grip. At
low speed under moderate braking, all four wheels could have the same braking
effort applied – but from high speeds with firm braking, the rear wheels would
lock up (or trigger the ABS)
resulting in a Sketchleys Moment for the driver and passengers.
EBD may also
compensate for brake fade as a result of overheating, depending on
conditions. One possible side effect of EBD
is more balanced brake wear because the rear brakes are used more,
traditionally front brakes wear out much quicker than the rears.
EBD and ABS
share the same parts and sensors. Under
maximum braking, all cars with ABS have an automatic EBD system anyway since
ABS will keep the wheels at the maximum possible braking effort. Cars with ABS but without EBD (such as our Kermy) tend to trigger the
ABS on one wheel before another and by increasing pedal pressure, this will not
increase the braking effort to the wheel already at its grip limit.
No electronic
brake force distribution system I am aware of has a tell tale warning light for
the driver.
The second
system to consider is electronic traction control, sometimes referred to as TCS
for traction control system. The system
is designed to prevent a wheel from spinning under load. It is similar to ABS in
that it is designed to prevent a wheelspin
skid, it even uses the same wheel speed
sensors as the ABS system - except some traction control systems do allow an
element of wheelspin.
When the
traction control system determines that one wheel is spinning quicker than the
other(s) it can do one of three
things. It can either use the brakes, it
can reduce engine power or it can adjust an electronically adjustable
differential to transmit power elsewhere.
On a front wheel drive machine, earlier generation traction control
systems could result in lift off oversteer
by cutting engine power and braking one or both front wheels.
In wet or
slippery conditions, traction control systems can help prevent a loss of
control on all vehicles but especially with a rear wheel drive vehicle. In snow or icy conditions, traction control
can prevent a car from driving off since an element of wheelspin may be useful. For this reason some cars have the ability to
disable the traction control system.
Some TCS
installations also have a launch control element, whereby the system can be
used for maximum acceleration from rest by allowing a degree of wheelspin.
Many traction
control systems have a warning light that illuminates to tell the driver that
the system is in use.
Electronically Controlled Differential
Linked with the
above is the electronically adjustable differential, which is a system whereby
the car is able to determine where the power is aimed. All wheel drive cars may have three
differentials, one for each axle and one to determine the power split between
front and rear.
Electronic Stability
Control System
The final
electronic system to discuss is stability control (also known as the “Yaw Control System,” “Dynamic Stability System” or
to some people, “Killjoy”).
Stability control systems use similar sensors as the ABS
or traction control sensors and systems.
The system is able to brake an individual wheel so as maintain a neutral cornering aspect. Applying an individual brake when a vehicle
is changing direction results in a yaw moment, that is, the vehicle rotated
about the vertical axis of its centre of mass.
If the car
starts to oversteer in a right hand
corner the nominal calculated yaw rate is exceeded. The system brakes the front left wheel. This brake application subjects the vehicle
to an anticlockwise rotation until the actual yaw rate approaches the
calculated value. This calculation
typically occurs fifty times a second.
Skid control
systems represent an evolution of anti-lock brakes and traction control
systems. As far as I know, it was BMW
that first introduced dynamic stability control (DSC) system in the mid-1990s; it used the wheel-speed information
from the anti-lock brake sensors to determine a vehicle’s cornering
attitude. Since then, this system has
been superseded (some argue that BMW’s
approach really wasn’t sophisticated enough, and subsequent dynamic stability
control systems from the company use more sensors to help the onboard computer
interpret the driver’s intention and the vehicle’s reaction).
To determine
the driver’s intent, the system needs several things. It must have information
about the angle of the steering wheel, the brake-pedal and the accelerator
pedal (often determined by the engine
management system).
Several sensors
are used to track the actual vehicle response to the driver’s inputs. Vehicle speed is available from the anti-lock
brake wheel-speed sensors and cornering force comes from a lateral-acceleration
sensor.
Whilst the
majority of systems are from the one supplier, the vehicle manufacturer may
adjust the settings to suit a particular car.
Some systems allow some oversteer (such
as BMW, Porsche) whereas others are set up to aggressively quash any
oversteer (Lexus and Mercedes-Benz).
As with
traction control systems, many stability control systems have a tell tale
light, usually orange or yellow, that illuminates to tell the driver that the
system is in use.
Disclaimer
As a caveat to
the above, all electronic driver aids (including
ABS) should not be
relied upon in everyday conditions. These
systems can prevent the driver from losing control but this is not the same as
avoiding a crash. The tell tale warning
lights are to indicate to the driver that he or she is attempting to do things
that the car, tyres or road conditions do not permit.