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Traction
Control Systems
Using electronics to make
your car corner better!
Many of the world’s car manufacturers are
now adopting traction control systems for
their vehicles. These systems, often fitted in
conjunction with all-wheel drive, reduce the
likelihood of a car leaving the road during
cornering.
By JULIAN EDGAR
Many car manufacturers now have
traction control systems and these
come under a variety of names. Lexus
use the acronym “VSC” for “Vehicle
Stability Control”. Mitsubishi call it
either “Active Yaw Control” or “Active Stability Control”, depending on
which technical strategy is followed.
Delphi (GM’s electronic arm) tag their
system “Traxxar” for some incomprehensible reason, while Nissan uses
Understeer
Fig.1: understeer occurs when
the front of the car slides first.
An understeering car will tend
to head straight on, rather than
following the corner.
18 Silicon Chip
the ghastly acronym “ATTESA ET-S”
for their 4-wheel drive system which
incorporates stability control.
Finally, Mercedes Benz call such
systems “ESP”, for “Electronic Stability Program”. Such is the sophistication of the system, it could stand
for “Extra Sensory Perception” – and
that’s as good a reason as any for sticking with the term “ESP” throughout
this article.
Oversteer
Fig.2: oversteer occurs when
the rear of the car slides first.
An oversteering car will spin
if no correction is made.
Whew; that’s got the nomenclature
out of the way!
Good drivers, bad drivers
A vehicle transmits all its cornering and acceleration forces through
the contact areas of its tyres. Each of
these contact “patches” is only about
the area of a large shoe print and
four of these must control a vehicle
with a mass of perhaps 1.5 tonnes
and travelling at speeds of 30m/s or
more. Viewed in this light, it can be
seen that hard braking, cornering and
acceleration can be very much a balancing act – exceed the levels of grip
provided by the tyres and regaining
control could require a very skilled
driver indeed.
However, most of us aren’t skilled
drivers, especially in an emergency
situation where a combination of hard
cornering and braking may be needed.
This type of swerve, brake, recover
situation often results in a complete
loss of control, unless the driver is
skilled at such manoeuvres.
But what if an electronic system was
constantly measuring and evaluating
individual wheel speeds, steering
input angle, vehicle yaw and vehicle
acceleration? Such a system could
react far faster than a human driver
and, using algorithms developed
through extensive testing, take the
appropriate action to ensure vehicle
stability. In short, it would eliminate
those heart-stopping moments when
the back of the car attempts to overtake the front – a boon for those who
drive in icy conditions!
It would also prevent loss of control
if the road condition changes suddenly or if the driver makes an error, such
as entering a corner too quickly.
But do such systems work? Early
in its ESP development, Mercedes
Benz placed 80 of its vehicle owners
in the Mercedes driving simulator in
Berlin. At 100km/h, an icy situation
was suddenly simulated on four road
bends, the vehicle’s grip on the road
decreasing by more than 70% within a
few metres. Without any form of ESP,
78% of the drivers left the road. By
contrast, when the ESP system was
activated, all drivers safely negotiated
the bends.
Data collected by the General
Motors Safety Center indicates that
29% of severe accidents in the USA
are caused by loss of vehicle control.
This means that ESP systems can play
an important role in vehicle safety –
both by negating the effects of driver
behaviour and by allowing the driver
to retain control in changing road
conditions.
Cornering behaviour
Routine driving behaviour occurs
well within the limits of tyre adhesion. This means that the cornering
forces developed between the road
and the tyres remain proportional
to the tyre slip angles. It also means
that, at a given speed, the yaw rate of
the vehicle remains approximately
proportional to the steering angle.
However, if the vehicle speed or
steering angle continues to increase, a
point is reached where the cornering
forces no longer increase. When this
occurs, small changes in lateral forces
can produce large changes in the slip
angles of the front or rear tyres. Conversely, large changes in slip angles
can result in little or no change in
lateral forces.
When the limits of adhesion are
reached, a cornering vehicle behaves
in two distinct ways. If the front tyres
are the first to lose grip, the car is said
to understeer. The behaviour of an
understeering car is shown in Fig.1.
The car leaves the road on the outside
of the corner, because the front wheels
are “under” steering; ie, not steering
enough!
Conversely, if the rear tyres lose
grip first, the car oversteers. Fig.2
shows the path that an oversteering
car takes. As can be seen, if no correction is undertaken, oversteer can
result in a spin.
It’s important to realise that the
amount of lateral grip that a tyre can
develop depends on both the cornering and acceleration loads placed on
it (among other things). A powerful
Fig.3: speed sensors are integrated into the hub of the car. Here the cable going
to the sensor can be seen just to the left of the drive shaft.
rear-wheel drive car may be prone
to “power oversteer”, where lateral
traction is lost because the rear tyres’
grip is overcome by the magnitude
of the torque being applied. Under
a combination of heavy braking and
strong cornering, a loss of lateral grip
will occur at much lower cornering
accelerations than if a steady speed
was being maintained. These factors
influence the ESP control strategy,
which is most effective in active
4-wheel drive cars.
any control corrections, it must know
how the vehicle is currently behaving. It does this by using a number of
sensors, which are distributed around
the car.
All cars fitted with ESP have an
anti-lock braking system (ABS) fitted.
This means that individual wheelspeed sensors are already present.
It also makes it relatively easy to
implement a system that controls the
vehicle by separately braking individual wheels.
In most vehicles, the speed sensors
Signal inputs
typically use a toothed wheel rotating
Before an ESP system can perform
past an inductive sensor. Fig.3 shows
a Lexus speed sensor, as
seen in its normal (installed) state. The cable
going to the sensor can
be seen just to the left of
the driveshaft.
In addition to speed
sensing, ESP systems
also require a means of
detecting the steering
angle, vehicle yaw rate
and vehicle acceleration.
The steering angle sensor
detects the amount and
direction of steering lock
being applied.
Lexus vehicles use an
optical sensor to perform
this function (see Fig.4).
This particular device
Fig.4: the Lexus steering angle sensor uses
uses three photo interan optical design. Three sensors are used
rupters, which work in
in conjunction with a slotted disc.
conjunction with a slotFebruary 1999 19
Coriolis
Force
Straightline
Movement
Side-to-Side
Movement
Fig.6: the Lexus GS300 yaw
sensor. It is normally located
beneath the centre console in
the cabin.
Detection
Portion
ω
ω=0
ω
Vibration
Portion
Coriolis Force
Output
Voltage
The Lexus yaw rate sensor uses a piezoelectric
vibration type rate gyro.
The resonator is shaped
like a tuning fork, with
a vibrating portion and a
Yaw Rate
detecting portion mount
Right Turn
Left Turn
ed at 90° to each other
Fig.5: the Lexus yaw rate sensor uses a
and located on each arm
piezoelectric vibration type rate gyro. The
of the fork – see Fig.5. To
resonator is shaped like a tuning fork, with
detect the yaw rate, an AC
a vibrating portion and a detecting portion
voltage is applied to the
mounted at 90° to each other and located on
vibrating portion, exciteach arm of the fork. To detect the yaw rate,
ing it. During yaw motion,
an AC voltage is applied to the vibrating
the detecting portion of
portion, exciting it. The detecting portion of
the assembly is distorted
the assembly is then distorted by a certain
amount and direction by the Earth’s Coriolis
by the Earth’s Coriolis
force acting on the arms of the fork.
force, which acts on the
arms of the fork.
The result is an output
voltage from the sensor,
ted disc. Two of the sensors detect which is proportional to the direction
steering angle and direction, while and magnitude of the yaw rate. Fig.6 is
the third is used to determine the
a photograph of one of these sensors.
neutral position of the steering wheel.
As indicated earlier, the magnitude
Self-checking mechanisms are built of acceleration (braking, acceleration
into the sensor.
or cornering) also influences the ESP
The vehicle yaw rate is a critical control strategy that is selected. Veinput for ESP systems. The yaw rate hicles use an accelerometer to detect
is the speed at which the vehicle is this characteristic. The Lexus accelturning around a vertical axis passing erometer is located in close proximity
through the centre of the car. Yaw rate
to the yaw sensor and consists of two
sensors are usually positioned in the
weighted semiconductor elements.
middle of the car – directly behind
These are mounted at 90° to one
the gearshift lever in the case of the
another, with each at 45° to the lonLexus models. However, the Delphi gitudinal axis of the car – see Fig.7.
Traxxarä system locates this sensor The outputs from the two sensors are
under the rear parcel shelf.
fed to the ESP control unit, which
20 Silicon Chip
calculates the horizontal acceleration
in all directions.
Depending on how the ESP system
is integrated with other electronic
systems in the car, additional sensors
may be fitted to detect brake fluid
pressure and throttle opening. In most
cars, these sensors are already present
and so they can be included in an ESP
system for very little additional cost.
Signal outputs
The outputs of most ESP systems
are used to actuate individual wheel
brakes and reduce drivetrain torque
to selected wheels. In no system is the
steering angle automatically changed,
so the wheel isn’t suddenly wrenched
from your grip as the computer takes
over! In 4-wheel drive cars, an ESP
system changes the front/rear torque
distribution, while one Mitsubishi
model can even change the side-toside torque distribution!
Many ESP systems use braking as
their primary control mechanism. The
Lexus GS300, for example, integrates
the hydraulic aspects of the ESP, ABS
and conventional braking systems
into one package. Instead of having a
separate hydraulic master cylinder,
vacuum booster and ABS hydraulic
control unit, these systems are all incorporated into one firewall-mounted
assembly.
An impressive array of hardware
is built into this compact unit, as
follows: (1) a pump and pump motor;
Fig.7: the Lexus
accelerometer uses
two sensing elements
mounted at 90° to
each other, with the
assembly at 45° to the
longitudinal axis of
the car.
(2) a nitrogen-charged pressure accumulator; (3) three pressure switches;
(4) a relief valve; (5) the brake fluid
reservoir; (6) the master cylinder; (7)
the brake booster, which applies accumulator pressure; (8) four switching
solenoid valves, to direct fluid pressure to any or all of the wheels; and (9)
four pressure control solenoid valves
that regulate the hydraulic pressure
applied to each wheel’s brake.
A photograph of this marvel is
shown in Fig.8. Note the small lifting
hooks positioned on the assembly (we
can only conclude that it’s installed
using a small block and tackle)! Other
vehicles in the Lexus range retain a
more traditional approach but this integrated hydraulic unit clearly shows
the way of the future.
The engine torque is reduced by
reducing the throttle opening. The
Lexus models use electronically-controlled throttle bodies, so this is easily achieved. Other systems retard
camshaft timing (when variable cam
timing system is used), reduce the
ignition advance or even bypass individual fuel injectors.
Fig.9 shows a block diagram of the
complete stability control system
used in the Lexus GS300.
Mitsubishi uses a Torque Transfer
Differential in their Automatic Yaw
Control system. This differential is
able to regulate the amount of torque
being transferred to each wheel on
the one axle. Currently, only the rear
axle can be controlled in this manner.
The system works by using an
electrically-controlled hydraulic
unit which engages wet multi-plate
clutches by varying amounts, to give
the active torque split. Fig.10 shows
the system, which is being used in
4-wheel drive performance cars and
is said to be especially effective in
sharp corners.
Nissan’s ATTESA ET-S 4-wheel
drive system has a similar wet multi-plate clutch system. It is used to
distribute torque to the front wheels as
required, to give maximum stability.
Other outputs of an ESP system
include self-diagnostic codes, a dash
Fig.8 (below): the Lexus GS300
hydraulic assembly. It integrates the
ABS hydraulic control unit, the brake
booster and the control valves for the
stability control system.
February 1999 21
Fig.9: the Lexus GS300 stability control system. Inputs include wheel speeds,
steering angle, deceleration and yaw rate. As indicated on the diagram, the
same system is used for anti-lock brakes, traction control and vehicle stability
control purposes.
light (or gauge) to warn the driver
when the system has activated, and
another warning light to indicate that
the system is inoperative.
Control strategies
Designing input sensors and output
actuators for an ESP system is relatively straightforward but that doesn’t
apply when it comes to writing the
software. Developing ESP control
algorithms that work effectively in
all situations is apparently quite
difficult. In fact, some systems have
quite different software, depending
on the market that the car is aimed at.
Delphi, for example, use a different approach in the rear-wheel drive
Chevrolet Corvette sports car to that
used on several front-wheel drive
Cadillac models. As with suspension
22 Silicon Chip
tuning, what is best for one market
sector is not necessarily best for another. That also implies another thing:
when ESP systems become common,
look out for “hot” programs that will
be available on the aftermarket!
When a vehicle is understeering,
braking of the inside rear wheel
substantially reduces the amount of
understeer that occurs. This can be
easily understood if you again look
at Fig.1. The vehicle is attempting to
negotiate a righthand bend but the
front of the car is sliding wide.
If the righthand rear wheel was
slowed while the other wheels continued to turn at their normal rate, the
car would attempt to pivot around this
wheel to the right. This would allow
the car to successfully negotiate the
bend in the road, instead of under-
steering off the road to the left.
In the rear-wheel drive Lexus cars,
both rear wheels are braked and the
engine torque output is reduced – see
Fig.11. Toyota presumably adopted
this approach because the car is designed to initially understeer if the
cornering speed is too great. Simply
slowing the car thus provides the required reduced understeer. Research
from Delphi has shown that braking
the inside front wheel can also significantly correct understeer but this
applies only at small slip angles.
When a vehicle is oversteering,
the most powerful corrective braking
mechanism that can be employed is
to brake the outside front wheel to
near lock-up. In Fig.2, this would be
the front lefthand wheel. If this wheel
is braked but the others continue at
normal speed, the car would attempt
to pivot around to the left, thereby
reducing the amount of oversteer.
The Lexus system does just this
but it’s not always quite that simple.
At times, the Lexus also brakes the
rear wheels during oversteer. This is
likely to occur (in conjunction with a
reduction in engine torque) when too
much throttle is being applied.
While the yaw change that occurs
with the slowing of a single wheel
is the major corrective mechanism,
another factor also has a significant
affect. Earlier, it was stated that the
grip of a tyre depends on both the
cornering and the longitudinal loads
placed on it. When an ESP system
is activated, the car is at the limits
of adhesion and then one wheel is
suddenly braked! The braked tyre
will thus slide sideways more easily
than it did before the braking loads
were imposed.
Let’s now take another look at the
oversteering vehicle in Fig.12. When
the front lefthand wheel is braked,
its lateral grip is also reduced. This
means that the car will have less
front-end grip and so the front of the
car will start to move to the left – ie,
in the same direction that the back
is heading! So this effect also acts to
decrease oversteer.
In an active 4-wheel drive car, the
control strategy is based on reducing
the amount of torque that’s transferred
to the end of the car that’s sliding. For
example, the Nissan Skyline GT-R is
a rear-wheel drive car for most of the
time. However, if power oversteer
occurs during cornering, torque is
transferred to the front wheels, thereby reducing the torque load on the
rear tyres and also pulling the car in
the steered direction. Some forms of
the Nissan system do not use a yaw
sensor, the torque split control being
based only on the inputs received
from accelerometers, wheel-speed
sensors and the throttle position.
With 2-wheel drive cars, a typical
control algorithm consists of the following steps:
(1) Calculate the desired values of
vehicle yaw rate and slip angle, using
the steering angle and vehicle speed;
(2) Using the difference between the
desired and measured yaw rates and
between the desired and estimated
slip angles, determine the desired
change in yaw that should be applied
to the vehicle;
(3) Select the wheel(s) to which
the brakes should be applied and
determine the desired magnitude of
braking pressure or brake slip.
Fig.10: Mitsubishi’s Active Yaw Control allows the amount of torque being
channelled through each rear wheel to be varied by means of a Torque Transfer
Differential.
Understeering
Control Moment
Oversteering
Control Moment
Braking
Force
Braking
Force
Fig.11: the Lexus system brakes
both rear wheels to control
understeer. Other systems brake
just the inside rear wheel, creating
a correcting yaw moment.
Closed loop control can be used
during braking so that maximum
retardation of the chosen wheel occurs. This prevents the need for an
estimation of the surface coefficient
of friction.
The major parts suppliers to vehi-
Fig.12: oversteer in the Lexus
is controlled by braking the
outside front wheel (car shown
here making a right turn).
cle manufacturers have stated quite
clearly that adding an ESP system to a
car already equipped with ABS can be
done quite cheaply. That makes it very
likely that stability control technology
will find its way into a wide range of
cars in the near future.
SC
February 1999 23
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