MEMS sensors for acceleration and rotation entered the automotive market about 15 years ago. Acceleration sensors were providing high-g signals of crashes in airbag systems, whereas gyroscopes and low-g sensors were providing signals for roll-over detection and ESP® systems. Typically, systems for active and passive safety demand for a high degree of functional safety (addressed by a redundant system architecture) and for a moderate performance of sensors with respect to offset stability (±100 mg or ±3.5 °/s) and to noise (<15 mgRMS or 0.4 (°/s)RMS).
The ongoing introduction of advanced driver assistance systems and comfort features to modern cars relies on improved sensing capabilities of inertial sensors: The low-g offset stability needs to be narrowed down to ±50 mg (and an impressive 20 mg for short-term drifts) and the noise being as quiet as 6 mgRMS or 0.2 (°/s)RMS, respectively [ref: MM7].
Reflecting the different sets of requirements (summarized in Table 1 ), Bosch developed two product categories for its SMI7 sensor family (see Figure 1 ): SMI700 and SMI710 support advanced comfort and driver assistance functions (ADAS) such as adaptive cruise control, hill-hold control, roll-stability control, active damping systems or vehicle motion observers. Active and passive safety functions are targeted by the second product category, constituted by SMI740 for ESP® and SMI720 for roll-over sensing.
Table 1: Measurement characteristics of the acceleration channels
If a car is equipped with SMI7 sensors, the integrated gyroscopes will notice whether the chassis is rolling or pitching, or whether the car is skidding in a curve (the ESP® system will react). The low-g accelerometers will assist when starting at a steep hill. They will notice whether the chassis is jumping (the damper control will react) or whether the driver is accelerating or braking. Considering all this information, the SMI7 components truly provide the equilibrium sense to the car. They could even be used to improve the performance of a GNSS based navigation system