Government regulations keeping this system on vehicles to keep them on the road.
Except, this is 2009 and within three years, all vehicles sold in the U.S. will be required to have just such a system.
From the Start
Electronic Stability Control (ESC) is a further refinement of the original Anti-Lock Brake System (ABS) first co-developed by Bosch and Mercedes-Benz, and introduced on the Mercedes-Benz S-Class in 1978. ESC first appeared on the 1995 Mercedes-Benz S-Class.
After several rollover crashes during magazine testing of the new 1996 Mercedes-Benz A-Class compact car, Mercedes recalled and retrofitted 130,000 A-Class cars with ESC. This produced a significant reduction in crashes and the number of Mercedes vehicles fitted with ESC rose significantly. The remainder of the vehicle manufacturers quickly followed by developing their own versions. To date, there are more than a dozen different systems in use.Fast forward to September 2007. In the wake of governmental and public outcry, the National Highway Traffic Safety Administration (NHTSA) proposed a new rule that requires all new light-duty vehicles sold in the United States to be equipped with ESC by the 2012 model year. This rule became Federal Motor Vehicle Safety Standard (FMVSS) No. 126. As of 2009, close to 74 percent of new vehicles now have ESC standard, with another 13 percent having it as an option.
Operation
Modern ESC systems are completely autonomous units that do not require operator action. The ESC control unit receives inputs from several sensors to determine desired vehicle trajectory and actual vehicle tract.Steering Wheel Angle Sensor is commonly a potentiometer or an optical sensor with two sensing elements positioned at 90 degrees to each other. The potentiometer provides data to the ESC control unit about the direction the driver wants to steer. When replaced in service, the sensor must be calibrated with the wheels in the straight ahead position.
Lateral Acceleration Sensor is a solid state sensor that signals the ESC control unit when the vehicle is being subjected to G-forces in a turn. The signal voltage varies according to the amount of G-force. 4WD/AWD vehicles also incorporate a Longitudinal Acceleration Sensor.
Vehicle Rotation (Yaw) Sensor is a solid-state sensor that mimics a gyroscope to measures the vehicle's angular velocity around its vertical axis. The output is usually in degrees per second. The angle between the vehicle's heading and its actual movement direction is called slip angle, which is related to the yaw rate. The Yaw and Lateral Acceleration sensors are usually combined into one centrally mounted unit.Wheel Speed Sensor is a magnetic sensor and rotor (tone) or in more recent systems, an active wheel speed sensor, that provides individual wheel speed data to the ESC control unit. These sensors are also used for ABS control.
The ESC control unit continuously monitors the wheel speed sensors to determine wheel slippage. At the same time, the lateral acceleration sensor determines acceleration G-force and yaw sensors measure vehicle rotation around its vertical axis. From this data, the control unit calculates the actual movement of the vehicle about all three axes, comparing it multiple times per second with the desired direction.
When the control unit detects loss of steering control — such as when the vehicle understeers, oversteers, skids or hydroplanes and the input values do not correspond to the actual vehicle direction — the system reacts without any action on the part of the driver by reducing engine power and/or applying the brakes on only the appropriate wheel to restore vehicle stability. The resulting rotary movement of the vehicle counteracts the skidding condition within the limits of the vehicle and keeps the vehicle on the desired course.Yet not all ESC systems are identical. Although most systems use similar sensors and hardware, there are variations in the way the systems are programmed to respond once a loss of control is detected. Some ESC systems activate sooner than others or slow a vehicle more quickly when a driver begins to lose control.
Stability control systems can be calibrated to provide different safety and handling characteristics in the same way each car has a unique chassis balance — no two systems are identical. Some stability control systems are safety calibrated and provide an early intervention, while others are calibrated to allow a subtle, more progressive degree of control that only intervenes at high cornering forces.
Evolution
Recently, system co-developer Robert Bosch Corp. started production of a new version of their Electronic Stability Program (ESP), which integrates the lateral acceleration and yaw sensors inside the electronic control unit, eliminating the need for separate wiring harnesses to connect the sensors.Previously, the electronic control unit of the ESP was located in the engine compartment, attached directly to the ABS system's hydraulic modulator. In order to integrate the sensors in the control unit, the sensors were adapted to the considerably higher ambient temperatures and the vibrations that occur when the brake control system intervenes. The result was a vibration-reduced and balance-optimized, three-point installation of the hydraulic unit. This installation also ensures that driving on extremely poor roads will have no influence on system function.
Another challenge was the orientation of the lateral-acceleration sensor. Current sensors installed in the interior of the vehicle are placed precisely at right angles to the direction of travel. By integrating two acceleration sensors that measure precisely at right angles to each other, the hydraulic modulator, which must still be installed horizontally, can be positioned around its vertical axis as desired. Also, the vehicle's acceleration can be calculated precisely, based on the known installation location and the signals from both acceleration sensors. That new information available from the sensors enables measurement of the vehicle's longitudinal acceleration.Servicing
There are unexpected diagnostic procedures needed when servicing vehicles with ESC. Even something simple like a wheel alignment may require special equipment. For example, a scan tool may be necessary to perform the steering angle sensor calibration. If not performed correctly, it can result in trouble codes being set because the steering angle sensor is reading that the vehicle is turning even though the front wheels are pointed straight ahead.While ABS has been around for a long time, technicians should realize that in order to properly perform a brake fluid flush (either as preventive maintenance; or as part of a brake service) the use of a scan tool is required to open and close the solenoid valves in order to flush all of the old fluid. This is particularly important in ESC equipped vehicles, as the ESC solenoids operate independently of brake pedal application.
In a related situation, during routine brake service, wiring harnesses and connectors should be carefully examined to determine condition and proper routing. If a wiring harness is pulled too tight during suspension movement it can result in a damaged wire or connector, thus setting a wheel speed sensor trouble code.For those still using dynamometers to perform IM 240 emissions tests, it should be noted that vehicles equipped with ESC systems will not function on a two-wheel dynamometer. When only the drive wheels spin, the logic circuit in the traction control module interprets this as "slippage" (similar to accelerating on ice). The traction control module will reduce throttle position, making testing impossible.
Jim Marotta is a freelance writer with more than 17 years' experience in the automotive industry. He currently works as a technical editor for ChiltonPRO.com.
