Electronic Stability Control
Electronic stability control utilizes onboard computers to 
monitor vehicle direction and stability. If the vehicle is not going in the direction the driver is steering the system uses individual wheel brakes to maintain control.
According to the US Department of Transportation (FMVSS no. 126) this will become the standard in many vehicles produced in 2012. These systems must meet certain minimum requirements.
a. The ESC system must be able to apply all four brakes individually. It must also have a control of the algorithm that utilizes this capability.
b. The ESC system must be capable and operational during all phases of driving (acceleration, coasting, and deceleration).
c. The ESC system must stay operational while the antilock braking system or the traction control system is active.
During driving the control module continuously monitors vehicle steering and direction using input sensors to determine if the vehicle is moving in the intended direction. These sensors include the four wheel speed sensors mentioned earlier, the vehicle speed sensor, and a few other directional and speed inputs. The system estimates the direction of the skid and applies different brakes in order to counter the skid and regain control and stability. It uses several other input sensors including:
Steering Wheel Position Sensor: The steering wheel position sensor is used to inform the control module of the drivers intended direction. It takes this information and compares it to the vehicles actual direction in order to make an informed braking decision based on factory set parameters. They are located on the steering shaft and look similar to a small clock spring.
Yaw Rate Sensor: This sensor is located somewhere in the passengers compartment under the center console or the front seat. It too is used to sense vehicle direction. Its purpose is to sense when a vehicle is beginning to fishtail and spin sideways.
Lateral Acceleration Sensor: This is known as the G-sensor and is capable of sensing the speed and force the vehicle has while entering a turn. The Yaw Rate and Steering Wheel Position sensors are sometimes combined together into one unit.
