Vehicle Steering Stability System Explained (Stabilitrak)
The vehicle stability enhancement system, VSES or Stabilitrak, provides added vehicle stability during aggressive driving maneuvers. Yaw rate is the rate of rotation around the vehicle's vertical axis. The system is activated when the ABS brake module determines that the desired yaw rate does not match the actual yaw rate as measured by the sensor.
The desired rate is calculated by the ABS module the following inputs...
- The position of the steering wheel
- The speed of the vehicle
- The lateral, or sideways acceleration of the vehicle
The difference between the desired yaw rate and the actual rate is called a rate error, which is a measurement of over steer or under steer. When an error is detected, the computer attempts to correct the vehicle's motion by applying brake pressure to one or more of the wheels. The amount of brake pressure which is applied varies, depending on the correction required. The engine torque may be reduced also, if it is necessary to slow the vehicle while maintaining stability.
VSES or Stabilitrak activations generally occur in turns during aggressive driving. When braking during system activation, the pedal may pulsate. The brake pedal pulsates at a higher frequency during VSES activation than during ABS activation.
The vehicle stability enhancement system sensors values may vary slightly due to differences in temperature, sensor mounting, connector resistances, manufacturing, etc. Since the system is a very sensitive and precise control system, it is imperative that the electronic brake control module (EBCM) be able to accurately equate a given sensor voltage with an actual unit of measurement. For example, the yaw rate signal of one vehicle may be 2.64 volts at +18.0 deg/sec yaw rate while the yaw rate signal of another vehicle may be 2.64 volts at +17.5 deg/sec yaw rate. Therefore, at the beginning of each ignition cycle, the EBCM must perform an initialization procedure to observe how the sensors are correlated with each other and also to determine what each sensor value is when the applicable unit of measurement equals 0. This voltage is referred to as the sensor bias voltage. Although some activation of the system may occur if required to prior to full initialization, the system does not give optimum performance until the sensors are fully initialized.
The following VSES sensors require initialization.
- The yaw rate sensor
- The lateral accelerometer
- The longitudinal accelerometer
- The master cylinder pressure sensor
- The steering wheel position sensor
When the vehicle speed is greater than 25 km/h (15 mph), full sensor initialization must occur during 3 km (1.8 mi) of driving or 1 km (0.6 mi) of straight and stable driving, whichever occurs first. Although an attempt at initialization may fail due to driving conditions, such as driving on a very winding road, failed initialization is usually caused by a sensor bias voltage which is not within an acceptable range. Often, a DTC sets soon after a failed initialization attempt. The stability system (Stablilitrak) not ready indicator illuminates when sensor initialization fails.