Electronic stability control is known by quite a number of
names including electronic stability program, dynamic stability control,
Stabilitrak, Advance Trak, and more. No matter what name it has, this feature
has one purpose, to aid the driver in recovering from an uncontrolled skid or
wheel spin.
As early as
1987, BMW and Mercedes Benz were working with and perfecting their first
traction control systems for introduction. While traction control does not aid
the driver in steering, it is an integral and very necessary part of the
system. Applying brakes to an individual
wheel as needed and also modulating the throttle can control wheel spin. By
1992, BMW had made this feature standard across its entire model line. The same year, Mercedes, with the help of
Robert Bosch, co-developed a system that they called Elektronisches
Stabilitatsprogrammen or Electronic Stability program, trademarked as ESP,
designed to control lateral slippage. Mercedes introduced the system on its S
Class models in 1995. BMW introduced their Robert Bosch manufactured version
later that same year on their 7 Series models. At the same time, other
manufacturers were working on their own systems; GM introduced Stabilitrak, a
version of ESC, on certain Cadillac models in1997. By 2007 it was standard equipment on all
models except certain commercial vehicles. Meanwhile, Ford introduced Advance
Trak on certain models in 2000, and by 2008 it was standard on most models.
In 1997, during a simulated moose
avoidance test in Germany, a test driver rolled a small A Class Mercedes at a
speed of only 47 mph. Mercedes, with their commitment to safety, recalled and
retrofitted 130,000 A Class cars with ESP at no cost to their owners. This
resulted in a significant reduction in the number of crashes and showed the
importance of the feature. As a result, the European Union made ESC mandatory
on all new cars built by January 1, 2011, and by January 1, 2014; any older
model not equipped or retrofitted with ESC may not be re-sold by a licensed
dealer in any European country.
In North America, Ford and GM
announced all vehicles would be equipped with AdvanceTrak and Stabilitrak
respectively by 2010 while Toyota offers it standard on all models in 2009. As
of the 2012 model year the National Highway Traffic Safety Administration
requires that all vehicles be equipped with ESC. They are investigating placing the
requirement on truck tractors and certain busses, but that is not finalized.
ESC combines a yaw rate control
with anti-lock braking and traction control. Yaw is the rotation of an object
around a vertical axis, in other words spinning to the right or left. To enable
the system to recognize the problem, in other words the skid, requires a number
of sophisticated sensors. The first
sensor is a steering wheel angle sensor. This sensor determines the rotation of
the wheel and determines the direction that the driver was intending for the
vehicle to travel. The yaw rate sensor
determines the rotation rate of the vehicle, or what direction the vehicle is
actually turning. A lateral acceleration
sensor or accelerometer measures the speed of the skid. The ABS wheel speed
sensors measure the speed of each individual wheel. A hydraulic modulator measures and regulates
the amount of braking pressure to an individual wheel, which differs from the
ABS braking. This system can generate additional hydraulic and/or vacuum
pressure as needed for just one or more wheels.
The electronic control unit (ECU)
receives all the inputs from the sensors and systems and then analyzes and
computes a course of action to counteract the uncontrolled state of the
vehicle. Often this is incorporated into the same unit that controls the ABS
and traction control, through a Controller Area Network. This is interfaced
into the ABS, Traction Control and the engine ECU’s so as to not give any
contradictory signals.
Many ECS systems have an override
switch so that the driver can disable the ESC under certain conditions such as
deep snow, driving on a beach or if a smaller spare tire had to be installed.
Some systems also had a limited mode which raises thresholds so that drivers
can performance drive their vehicle at or near its limits. Also, systems are
designed to reset to turn on the next time the vehicle is started. ESC only intervenes when it senses a possible
loss of steering control, when the vehicle is not traveling in the direction
that the driver is steering it. Examples of this are situations such as during
an emergency evasive maneuver, hydroplaning, or an understeer or oversteer
during poorly judged turns on slippery roads. The system could also activate in
an unwanted way during high performance driving because the steering input may
contradict the direction of travel such as during drifting.
The system can work on an surface
from dry pavement to a frozen lake, often correcting a skid much faster and
more effectively than the typical driver, sometimes even before the driver
senses loss of control. This has led to some concerns about drivers becoming
overly confident of their vehicle’s ability or their driving skills. Because of
this, when the system activates, an indicator light illuminates on the dash to
let the driver know that the vehicle’s handling limits have been approached.
Some even offer an audible warning tone if the driver puts the vehicle into a
dangerous situation.
ESC manufacturers emphasize that
the system is not designed as a performance enhancement or a safe driving
enhancement, but rather to help a driver recover from a bad situation. ESC does
not increase traction or enable faster cornering, but it will enable better
cornering. A reckless maneuver such as severe hydroplaning due to excess speed
in deep water or trying to negotiate a turn at a speed the car is not capable
of safely negotiating.
When the steering sensor and the
yaw sensor send contradictory signals, the ECU pulls up the individual wheel
speeds, the gradient and if equipped, the roll angle signals. The ECU quickly
analyzes the signals and sends out the corrective signals, whether it being a
separate braking application, throttle retarding or other corrective action.
This is accomplished by the computer inputting the signals into a preloaded
mathematical control algorithm and compensation signals being sent out
depending on the severity of the departure from the baseline numbers.
As far as cost and effectiveness,
it is estimated that the cost of the ESC technology over and above a car
equipped with ABS and TCS is about $125-250. This is a small price to pay for a
system that can reduce crashes by about 35 percent and by 67 percent in SUVs. An
IIHS study concluded that ESC reduces the likelihood of fatal crashes by 56
percent and single vehicle rollovers by 75-80 percent. This could translate
into 3500 to 5500 lives saved each year.
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