1 ABSTRACTWe have developed a tri-axial piezoresistive acceleration sensor for automotive applications such as advanced brake control, active suspension control, and air bag systems. Although automobiles are driven on two-axial planes, their movements are affected by tri-axial accelerations, through braking, accelerating, rolling, pitching, and sloping conditions. The tri-axial piezoresistive sensor employs a diaphragm structure, differing from conventional cantilever type sensors, enabling the sensor to detect tri-directional accelerations. Shock resistance properties are also improved.To ensure system dependability, a self test function is incorporated in the sensor by applying electrostatic force to electrodes. By detecting tri-axial accelerations through the use of a single chip, the sensor is expected to allow for the development of low-cost, compact systems.