An Electronic Stability Program (ESP) system on a Mercedes-Benz S550.

AP Photo/Chris Greenberg

Electronic Stability Control Components

There are lots of safety and regulatory devices in cars these days, and they all work together to keep the wheels on the road and the passengers safe. Electronic stability control, in particular, takes advantage of two other systems, ABS and traction control, plus a few special sensors, to do its job

Before the 1990s, drivers were taught to pump the brake pedal to keep the brakes from locking up and causing a slide. With the invention of anti-lock brakes, driving safely became much easier. ABS electronically pumped the brakes faster than the driver could, which kept them from locking and causing understeer or oversteer. ESC uses this system to correct the problem almost before it can start by activating the ABS for as many wheels as needed, from one individual wheel to all four. The nature of ABS keeps the over- or understeer from getting worse while slowing the car to a controllable speed.

ESC also uses traction control for driving safety. If ESC is in charge of monitoring side-to-side motion around a vertical axis, traction control is in charge of front-to-back motion. If the traction control system is detecting wheel slippage, the electronic stability control sensor will pick up on the direction of the slide. If there's a difference between the angle of the steering wheel and the direction the car is sliding, the ESC will work with the traction control system to engage the ABS at the proper wheel (or wheels) and control the throttle to reduce the speed of the vehicle, too.

ESC information is fed into the car's central computer via three types of sensor:

  • Wheel-speed sensors: One wheel-speed sensor at each wheel measures the speed of the wheel which the computer can then compare to the speed of the engine.
  • Steering-angle sensors:This sensor, in the steering column, measures the direction the driver intends to aim the car. If it's different than the direction the car is actually traveling, the ESC system will kick in.
  • Rotational-speed sensor:This is also known as the yaw sensor. It's the one in the middle of the car that measures the side-to-side motion of the vehicle.

If you want to know what ESC can do for driving safely in the real world, head to the next page.