There are many types of systems geared towards enhancement of vehicle stability and control. The most common is the almost ubiquitous Automatic Braking System (ABS). Until very recently, most ABS systems did not use an inertial sensor. They simply read wheel speed and apply pulsed braking if the wheels are thought to be skidding. However most all-wheel-drive systems and some newer high performance ABS systems, look at longitudinal acceleration to determine if the chassis is still moving. This is particularly important for all-wheel-drive equipped vehicles where all four wheels may have lost traction due to the application of drive torque.
The most important performance parameters for accelerometers used for ABS is zero g bias and sensitivity stability. In general it is assumed that the minimum available deceleration force available (even on slippery surfaces) will be about 100mg (0.98m/s2). So the combination of zero g bias drift and sensitivity variation must not vary more than 100mg over the automotive temperature range (see Table 1 for more information regarding performance requirements). The Analog Devices’ ADXL103 iMEMS® accelerometer with a typical zero g bias stability of 16mg and sensitivity drift of 0.3% over the automotive temperature range is ideal for this application.
Electronic Stability Control (variously known as ESC, VSC, VDC – each auto maker seems to have their own acronym) assists the driver regain control of the automobile just as it is starting to skid. A VDC system uses a yaw rate sensor (or gyroscope), a low g accelerometer, wheel speed sensors (which may also be used by the ABS system), and steering wheel angle input. Wheel speed from each wheel is measured, and the predicted yaw (or turn) rate of the car is compared to that measured by the gyroscope and the intentions of the driver (as predicted by the steering wheel angle). A low g accelerometer is also used to determine if the car is sliding laterally. If the measured yaw rate differs from the computed yaw rate, or lateral sliding is detected, single wheel braking or torque reduction can be used to make the car "get back in line."
ESC systems require a yaw rate sensor with fairly low noise (typically less than 0.5 degrees/sec) and low sensitivity to mechanical vibration. Many types of yaw rate sensors actually generate vibration (most MEMS gyros use a vibrating mass to generate the velocity component used to sense angular rate). This is undesirable as the accelerometer is normally place on the same PCB as the gyro. Just as in ABS, the accelerometer must be very stable over temperature, as small amounts of lateral acceleration must be measured.
Roll Stability Control (RSC) changes the roll dynamics of the vehicle in response to road handling demands. This is particularly important for vans, pick-up trucks, and SUVs where the higher center of gravity makes loss of control, and potentially roll-over of the vehicle, more likely during hard maneuvers. Roll stability control systems use a roll rate sensor (a gyro whose axis of sensitivity is in the roll axis), and a lateral accelerometer. Torque adjustment, selective braking, and suspension adjustment can be used to alter the roll performance of the vehicle in response to inertial measurements.
The performance requirements of the inertial sensors used for RSC are similar to that of ESC.