July 1, 2004
Electronic Stability Control (ESC) systems are still only on a small percentage of U.S. models yet they offer significant improvements in performance by sensing when a vehicle is about to lose control and intervene to keep the vehicle stable. ESC systems, which are known under a host of other acronyms and various trade names, work by using ABS brakes as a foundation and with the addition of sensors measure steering wheel angle, yaw rate and turning force. Software algorithms interpret the sensor data and determine whether the vehicle is travelling the way it should given the driver input. If not, the system automatically activates the brakes on one or more wheels or activates the throttle slightly to bring the car back in line.
Because rollovers are frequently preceded by evasive maneuvers–particularly steering and braking–ESC provides the ability to avoid the loss of control and prevent subsequent rollover. However, ESC systems were not introduced on the most rollover prone vehicles until years after luxury models received the feature. Now ESC suppliers are by-passing their customers, some of whose interest in ESC has been lukewarm, and appealing directly to consumers to request ESC. ESC makers blame auto manufacturers for failing to offer the equipment as standard and packaging the feature with other “luxury” items as a high-cost options. Further erosion of potential ESC applications may occur as manufacturers “decontent” vehicles for cost reasons, removing essential foundation equipment like four-channel ABS brakes and equipping models with systems that will require significant upgrades in order to add ESC controls. Supplier also appear to be exercising caution in their external marketing as their customers are loath to see a system promoted that highlights rollover problems with their vehicles.
The newest ESC systems are more sophisticated than ever and include vehicle roll rate sensors and other anticipatory features to actively prevent scenarios that are likely to lead to a rollover crash.
ESC’s first iteration was essentially traction control that developed in the late 1980s by BMW and used the engine control system to reduce torque in “stability critical” situations. BMW recognized the tendency of drivers to “overreact and lose control” and developed systems that actively worked to prevent these problems. The BMW system was developed with Bosch and Continental-Teves and was fitted to the entire BMW model line for 1992, including its lowest cost 3-Series (a vehicle priced similarly to many popular SUVs). By 1992 Mercedes was steeped in development and testing of ESC with supplier Bosch. At this time published technical papers describe testing and document improved vehicle controllability using ESC systems. Manufacturers like Ford began investigating and developing ESC in the early 1990s as well, referring to their system as Interactive Vehicle Dynamics (IVD).
In 1995 Mercedes began using ESC systems in production. Like BMW, the Mercedes system evolved out of the ABS, electronic traction control (ETS) and acceleration skid control (ASR). In the Fall of 1995 Toyota introduced its Vehicle Stability Control System (VSC), that combined with ABS, and the traction control system worked much along the lines of the system introduced by Mercedes. This system was designed by Toyota and major components supplied by Aisin and Denso. During this time Ford was developing its first production system for the 1999 Lincoln LS with supplier ITT (now Continental-Teves). However, Ford also began testing SUVs with ESC, but these higher rollover risk vehicles took a backseat to their luxury car lines.
By 1996 technical papers continued to describe testing of ESC systems and their development and BMW began talking about its 3rd generation design.
During 1997 Mercedes advertisements included descriptions of its ESC as a feature that “stabilises the vehicle electronically” and “remains on standby to control the car if and when the driver loses control.” That same year GM began equipping its 1997 Cadillac DeVille, Eldorado and Seville with and ESC design called Stabilitrak made by Delphi. Stabilitrak was later adapted to a number of other GM vehicles.
In late September 1997 ESC became much more prominent thanks to a rollover of the Mercedes European-market microcar the A-Class. During tests in Denmark a journalist rolled a fully loaded A-Class at 37 mph in what is commonly referred to as the “Elk” or “Moose” avoidance maneuver. In response Mercedes announced it would retrofit the A-Class with ESC as well as new tires and suspension components to prevent rollover. In addition Mercedes stopped production and changed the design. The recall was free to A-Class owners and was estimated to have cost 50 million DM ($23 million) for the retrofit and another 100 million DM to modify new models. At the time a Daimler spokesperson said “the company had little choice. . . As Daimler-Benz and Mercedes-Benz, we have to go over the top.” It is important to note that the A-Class was an inexpensive, mass-market, microcar priced less than the VW Golf.
Following the A-Class flap, industry trade journals reported in 1998 that both Ford and VW were clamoring to get ESC on their lower priced models and ESC was first added to vehicles with four-wheel-drive. In 1999 nearly every manufacturer had at least one model with ESC–Ford offered the new Lincoln LS with its version of ESC and GM added a Bosch system to the mid-sized Oldsmobile Intrigue. Meanwhile Ford was preparing to offer IVD on their low-cost compact 2000 model year Ford Focus in Europe and on the U.S. model in 2001. One of the vehicle manufacturer concerns that arose at this stage was the supply base was being quickly being consumed as component suppliers began showing signs of difficulty meeting demand–especially for company’s who were late to bring ESC into their models.
Bosch, an early leader in ESC technology published a paper in 2000 describing five years of experience with ESC systems in production. They cited crash data from Germany that 20 – 25 percent of all car crashes with injuries and fatalities resulted from loss of vehicle control (vehicle spinning) and 60 percent of those crashes were single vehicle. The authors claimed this stemmed from the difficulty drivers face when attempting to control their vehicle in limit scenarios and the tendency of drivers to steer too much, worsening the situation. To resolve these known problems their ESC was designed to help the driver control the vehicle, particularly in panic maneuvers as the primary task of the ESC is to limit slip angle and prevent vehicle spin, which helps to prevent the driver from steering too much.
As ESC became more sophisticated and less costly, many of the vehicle manufacturers continued market research and found that when consumers were presented with ESC capabilities they included the feature high on their list of “product wants.”
For 2001 dozens of makes and models offered ESC either as standard or optional equipment and a study, co-authored by a German Ford engineer, evaluating the European Accident Causation survey data found ESC had a positive influence on the total number of loss-of-control crashes. The analysis indicated that approximately 18% of all injury crashes and 34% of fatal crashes ESC would have reduced the likelihood of the crash or prevented it altogether. In crashes in which the causation was identified as “loss of vehicle control” ESC would have a certain benefit in 42% of cases with injury outcome and in 67% of the fatal crashes. This study was followed by several others by Toyota and Mercedes that also found crash data supported the contention that real-world crashes were being reduced in vehicles with ESC. More recently a study from Sweden and simulation results from the U.S. confirmed the efficacy of ESC to reduce crashes. This is a particularly important because of the past history with “smart” technology like ABS brakes.
Following the widespread implementation of ABS, studies showed that these safety systems were not reducing crashes and in some types of conditions vehicles with ABS were experiencing greater numbers of single vehicle crashes. These findings left NHTSA and other safety organizations feeling “burned” by their strong recommendations of the first “smart” technology. The ABS problem also left researchers looking for reasons why a system that performed well under test track conditions was not showing improvements in the real world. While most concluded the problem stemmed from driver interaction and the failure of vehicle manufacturers to properly educate consumers about the need to simply depress brakes and hold (as opposed to pumping), it is likely the on-road problems were due in part to low-cost, low-performance designs that flooded the market. A number of these cheap systems were subject to NHTSA investigations and recalls. Regardless of the cause, the ABS experience had a chilling affect on NHTSA and other safety groups’ support for smart technologies. Concerns now abound within the top-tier ESC suppliers that low cost, low-performance systems are likely to find their way into the market, which will dilute the positive real-world performance data.
High rollover risk vehicles like 15-passenger vans, which began to catch the attention of safety regulators and federal safety officials in 2001, were among the last vehicles fitted with ESC. Even after prodding from the NTSB to use ESC on these models Ford, the market leader in this segment, added more warnings and announced it wouldn’t add the feature until 2006. According to Ford, the technology it will incorporate in its vans is similar to the ESC design that include rollover sensing, known as Roll Stability Control or RSC, developed jointly with Volvo, a Ford subsidiary, and introduced on the 2004 Volvo XC90, Lincoln Aviator and Navigator SUVs. In addition to normal ESC features, RSC measures the degree and rate at which the vehicle’s body is leaning and if it senses a rollover is about to occur, the system is activated. The roll sensor also activates the seat belt pretensioners and triggers the side curtain air bags.
Even as light trucks are increasingly moving to lower, wider designs that appear to be less rollover prone, ESC will likely remain an important active safety feature. Newer designs are even incorporating algorithms to address issues like trailer sway in the event the vehicle is towing.
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