MEMS sensors expand active automotive safety

Matthieu Rezé, Freescale

February 1, 2011

Matthieu Rezé, Freescale

Electronic stability control (ESC) is an additional improvement to the anti-lock braking system (ABS) and traction control system (TCS). Its basic function is to stabilize the vehicle when it starts to skid by applying differential braking force to individual wheels and reducing engine torque. This automatic reaction is engineered for improved vehicle stability, especially during severe cornering and on low-friction road surfaces, by helping to reduce over-steer and under-steer.

Additional sensors must be added to the ABS system in order to implement ESP functionality which includes a steering wheel angle sensor, a yaw rate sensor, and a low g acceleration sensor that measure the vehicle dynamic response. This, obviously, creates new opportunities for MEMS sensor manufacturers.

Add to this inherent demand government mandates for ESC systems and a huge demand arises. Thus it is without surprise that Strategy Analytics recently announced that Safety systems will provide one of the highest growth applications over the 2009 to 2014 period. ISuppli estimates that this would represent a market of 47.7 million MEMS accelerometers at that time, 66% being stand alone dual axis low-g sensors.

Indeed, the system requirements have evolved and refined over the years to better take into account various vehicle types (like four-wheel drive cars) and roads under a variety of weather conditions. The use of two axis low-g sensor gives also the possibility to integrate new functionalities such as hill start assist and electric parking brake (EPB) by measuring accurately the tilt of the vehicle while on a slope. The addition of such functions together with the already tight performance required by ESC, is a challenge for the accelerometer.

Sensor requirements
In an ESC system, the various MEMS sensors are usually installed very close to the vehicle's center of gravity and their task is to continuously watch for the vehicle’s chassis movements. Together with a yaw rate sensor which measures the angular acceleration along the vertical axis, a low-g inertial sensor is used to detect the vehicle’s lateral acceleration and thus provide additional information to the system. During a loss of control when the vehicle starts to slide, this acceleration is less than 1g. So, the inertial sensor must have a high sensitivity to sense the low-g motion together with a high accuracy.

This translates into a device’s output need with very low noise and a small zero-g acceleration shift in temperature. Furthermore, the accelerometer needs to be immune to the parasitic high frequency content present in the car at the chassis level. Low energy signals with large frequency bandwidth can be found, from few hundred Hz during normal driving condition to few kHz due to shocks coming from the road. All frequencies above 1kHz must be filtered to avoid corrupting the sensor response.

By definition, an inertial sensor is highly sensitive to acceleration of any origin, since the micromachined sensing element is based on a seismic mass moving relative to a fixed plate. The sensor output signal is typically cleaned of parasitic high frequencies via electronic low pass filtering.  A sensor with an overdamped transducer which can eliminate this unwanted higher frequency acceleration content mechanically provides additional benefit.

Read this complete article here, courtesy of Automotive Designline Europe, and find out about improvements in MEMS fabrication technology, signal conditioning, and condition monitoring that provide the performance required by modern automotive safety sensor systems.