DETROIT - Electronic stability control programs - ESPs for short - work in conjunction with antilock brakes and the traction control system.
Unlike those two systems, though, ESP requires no driver input. In fact, ESP often works by counteracting the driver's input - for example, by correcting the driver's oversteer or understeer.
'The car is a willing servant. You ask it to do something stupid and it will,' says Ed Heitzman, principal engineer of Automotive Testing Inc., a Pennington, N.J., company that does quality testing for manufacturers.
'With a stability control system, if you ask the car to do something stupid, it says, `Hell no, I won't do it.' '
ESP systems automatically apply the brakes to an individual wheel or wheels - in some cases as many as 150 times per second - to pull the vehicle back on the course intended by the driver.
'It senses understeer and oversteer faster than any living thing can, and it reacts by applying the brake on one wheel, which even the world's best race car driver can't do,' said Fred Heiler, a spokesman for Mercedes-Benz of North America.
The systems are based on algorithms within a microcomputer that evaluate signals from several sensors. Although the software differs among suppliers, the following hardware generally can be found in each stability system:
Steering sensor, which monitors the angle of the steering wheel to help determine if the vehicle is going in the direction intended
Wheel sensors, which monitor the speed of each wheel to determine if the vehicle is sliding
Yaw sensor, which registers the vehicle's movements around its vertical axis to determine if the vehicle is sliding
Lateral acceleration sensor, which reacts to the centrifugal forces that are generated when cornering. This helps determine whether a vehicle is sliding through a curve.
Together these sensors provide the control unit with information concerning the vehicle's behavior at any instant. But stability control can't defy the laws of physics. If a vehicle is traveling too fast for conditions, the driver still may have an accident.
'What we think we have done with stability enhancement,' says Terrence Connolly, director of the General Motors Safety Center, 'is to make the vehicle intuitively perform the way the driver thinks that situation is going to unfold.'