Electronic Stability Control

Jan. 1, 2020
A proposed new rule would require ESC to be equipped on all light-duty vehicles sold in the United States by the 2012 model-year. Are you ready to service these vehicles?
Electronic Stability ControlDriving is believing.

Last September, the National Highway Traffic and Safety Administration (NHTSA) proposed a new rule that will require all new light-duty vehicles sold in the United States to be equipped with Electronic Stability Control (ESC) by the 2012 model year. This will become Federal Motor Vehicle Safety Standard (FMVSS) No. 126, and it is expected to be enacted some time in 2007.

What’s It Take?
Wheel speed is detected with four Hall-effect sensors that can read speed and direction down to less than 1 mph. The steering wheel sensor is also a Hall-effect device. The black box contains acceleration sensors and a yaw sensor. The latest generation four-channel ABS pump and valve body assembly is smaller than ever.
(Photo: Robert Bosch Corp.)
NHTSA figures indicate that about 29 percent of all 2006 light-duty vehicles (under 10,000 pounds GVW) left the factory with ESC. While this includes everything from the $14,000 Scion xB to the $250,000 Ferrari 612 Scaglietti, most of them were SUVs. Auto manufacturers were already planning to install ESC on 71 percent of their fleet by 2011.Why does NHTSA want ESC? ESC is the generic name for a hardware/software package that operates the vehicle’s anti-lock brake system (ABS) and electronic throttle to help the driver maintain control of the vehicle in certain situations. Sensors report the vehicle’s movements and the driver’s intentions to the control unit.

If the vehicle rolls or yaws (rotates about a vertical axis) beyond set limits, the control unit will command the ABS to apply the brakes on individual wheels to maintain directional control. If necessary, it can also reduce power output from the engine. These control actions can significantly reduce the likelihood of single-vehicle accidents, particularly on full-size vans and SUVs that have a high center of gravity.

ESC was first developed in Germany as a result of the government’s high priority on reducing traffic fatality rates. Rather than specifying vehicle technology, the German government (and later the European Union) has been willing to leave it up to the manufacturers to develop safer cars, as long as the results can be statistically demonstrated.

Over the years, this has resulted in vehicle structures and restraint systems that greatly improve the occupants’ chances of surviving a crash, and OEMs around the world have developed similar technologies. But along with improving survival rates, the Germans also have taken the lead in developing technology that helps the driver avoid an accident in the first place. One result of those efforts is ESC developed by Robert Bosch GmbH, which became standard equipment on the 1995 Mercedes-Benz (M-B) S-Class and on all M-B models in 1999. Statistical evidence gathered since that time, plus data acquired with track testing and simulator studies, have convinced NHTSA that if all vehicles were equipped with ESC, there would be a 34 percent reduction in single-vehicle crashes, and single-vehicle SUV crashes would be reduced by 59 percent.

Although NHTSA has specified technology in the past, it’s more common for them to set performance goals and allow the industry to define the technology that will meet those goals, as in Europe. However, in response to reports from transportation industry experts and to statements made by NHTSA Administrator Nicole Nason during Congressional hearings, Congress ordered the agency to develop performance goals that are “consistent with stability enhancing technologies [using] automatic computer-controlled braking of individual wheels to assist the driver in maintaining control in critical driving situations…”

Because an electronically controlled throttle (e-throttle) and ABS are required for ESC, all vehicles also will need to be equipped with these technologies.

A 2007 Chevrolet Tahoe navigates an obstacle course to demonstrate the benefits of General Motors StabiliTrak electronic stability technology.
(Photo: GM)
E-throttle provides emissions and fuel mileage advantages, and most manufacturers have already adopted it for all models. However, because ABS is optional on many models, only about 80 percent of new vehicles are built with ABS. While NHTSA estimates the cost of ESC at $111 per vehicle, that doesn’t include the cost of ABS. Even so, both Ford and General Motors (GM) have announced plans to equip all of their vehicles with ESC by 2009 and 2010 respectively, and price is not expected to be an issue.What it takes Two major technologies have made ESC possible. The first is the modern ABS, also developed by Bosch and introduced on the 1978 M-B S-Class. It was a passive system, activated only after the driver pressed the brake pedal.

As development continued, later systems had fewer and smaller components, greater capabilities and most importantly, lower prices. By the early 1990s, ABS from several different companies had become available on almost every model from every manufacturer, and today it’s standard equipment on some of the lowest-priced vehicles on the market.

At first there were three basic systems in production: three-channel, four-channel and rear-wheel anti-lock (RWAL). RWAL was used on pickup trucks to prevent rear-wheel lock-up when there is no weight in the bed, but it has been almost completely phased out in favor of four-wheel ABS.

ABS is being tested on many different road surfaces to make sure that it will always function to the maximum.
(Photo: Robert Bosch Corp.)
Three-channel ABS operates on all four wheels, but the rear brakes are on the same hydraulic circuit (channel) and operated together. At one time it was used extensively on front-wheel-drive vehicles because of its lower cost, but three-channel ABS is also being phased out gradually. Today, almost all vehicles with ABS have four-channel ABS.

With the right software in the control unit, the brakes at each wheel of a four-channel ABS can be applied individually. So essentially, ESC is a software package that’s added to the ABS control unit, transforming it from a passive system that can only react to the driver’s command into an active system that can increase fluid pressure in each brake line based on its own decisions. But making those decisions requires a lot more than wheel speed data.

The other major technology needed for ESC is micro-electromechanical systems (MEMS). MEMS technology is relatively new and covers a broad spectrum of devices, but in this case it refers to solid-state sensors built on a microchip that is only 1/4-inch (6mm) square.
The control unit needs to know the vehicle’s acceleration in three-dimensional space, and that acceleration is typically measured by two MEMS sensors. One is an accelerometer that measures the vehicle’s longitudinal and lateral acceleration. The other is a solid-state coriolis-effect gyroscope that measures roll and yaw.

Both of these sensors are usually packaged in the same “black box,” and in order to measure yaw accurately, the box is typically mounted as close as possible to the vehicle’s center of mass. These same sensors are commonly used in vehicles with multiple airbags, so the technology is well established and already onboard in many models.

Finally, the ESC control program also needs to know how the driver is manipulating the vehicle’s controls. Brake, throttle and transmission data is available from the powertrain control module (PCM), so the only additional information needed comes from a steering wheel sensor that reports the wheel’s position and the speed of its movement.

On the output side, some of the ABS components must be modified or upgraded for ESC duty. Wheel speed measurement requires greater accuracy than is possible with standard reluctor-type wheel speed sensors, so Hall-effect sensors are used. The toothed toner ring is replaced with a magnetic sensor wheel that must be completely shielded from magnetic debris, so it’s typically sealed inside the wheel bearing assembly. Also, because the hydraulic pump and high-pressure valves may be used more often, they must be upgraded to withstand longer periods of on time and more totals hours of on time.

How it works As with any interaction between computers and humans, the computer’s ability to accurately determine the driver’s intent is limited, and the mathematical algorithms in the program are extremely complex and sophisticated. Fortunately, its required responses also are limited. If the ESC decides the car is understeering (turning less than the driver intended), it will apply the inside rear brake so the vehicle “pivots” on that wheel into the turn. In an oversteer condition, it will brake the outside front wheel. In either situation, it may decide a reduction in engine output also is necessary. These actions are momentary and, as we discovered, difficult to detect through the seat of the pants.

Last fall, Motor Age editors were invited to a demonstration of the system’s capabilities at Robert Bosch Corp.’s test track in Flat Rock, MI. One demo used a Chevrolet Tahoe equipped with special outriggers to physically prevent the vehicle from rolling over. The vehicle comes standard with StabiliTrak ESC.

With the truck traveling at 45 mph with the ESC disengaged, the test driver executed a sudden extreme turn of the steering wheel, and the truck immediately rolled hard, and only the outriggers prevented it from rolling over completely.

When the same maneuver was repeated with the ESC engaged, all four wheels remained on the ground. Such dramatic and impressive demonstrations were no doubt part of what convinced NHTSA to propose the ESC requirement rule. However, there’s a world of difference between witnessing a demonstration and experiencing it firsthand.

We drove a new Buick LaCrosse equipped with StabiliTrak‚ through a set of cones designed to simulate a sudden lane change maneuver. We were told to enter the first gate at 50 mph and execute a flat-out panic turn of the steering wheel to enter the second gate. We were also told not to expect to successfully complete the maneuver.

In fact, we never did, but that big heavy front-wheel-drive car never skidded or lifted a tire from the pavement, and we were always in complete control of the vehicle. We also never detected a reduction in engine power.

This dramatic first-hand demonstration made it clear that avoiding a traffic accident is still the driver’s responsibility. However, in an emergency situation, ESC greatly extends vehicle’s normal handling capabilities, allowing the driver to maintain control well beyond all expectations.

Although ESC is only part of a whole suite of “driver assistance” features that are available now or will be soon, a decade of statistics has proven that a few milliseconds of one-wheel braking can make all the difference in the world.

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