Electronic Stability Program
Electronic Stability Program (ESP), also known as Vehicle Dynamics Control (VDC), Electronic Skid Protection (ESP) and many other names, are electro-hydraulic systems that help control vehicles as they approach the limits of stability. They are an extension of the technology used in Anti-lock braking systems, and Traction Control.
The system consists of an electronic control unit (ECU), a hydraulic control unit (HCU), an electrically driven hydraulic pump and a set of sensors. These sensors typically are: a steering wheel angle sensor, a wheel speed sensor for each wheel, a yaw rate sensor and a lateral accelaration sensor. These sensors provide information about the driving state of the vehicle. The ESP system then activates the brake on one or more wheels as appropriate to help prevent a skid, and more generally, to improve the feedback to the driver, and make the handling more linear.
In a typical event in which ESP comes into play, the driver decides that the vehicle is slightly too fast and wide in a corner, and so applies more steering lock. The front tyres, which are already heavily loaded laterally, supply a little more lateral force, but not enough for the driver, so he adds more steering lock. The ESP monitors the steering wheel angle and the vehicle's yaw velocity, and when the error between the two exceeds pre-programmed limits, it intervenes, and brakes the inner rear wheel (typically). This tends to cause the rear axle to slide outwards, hence pointing the nose of the car tighter into the corner, which is what the driver wanted to do. The system may also reduce the speed of the vehicle by adjusting the throttle.
