Freescale Introduces Sensor for Electronic Stability Control

By Francoise von Trapp, contributing editor

According to Francois Gilly, product manager of the Sensor Products Division, Freescale has a long history in the airbag sensor market, which include the inertial sensors located around the perimeter of the vehicle to detect a crash is imminent, and safing sensors located inside the vehicle that confirm the crash and cause airbags to deploy on impact. But, Gilly explained, the sensor is only one part of an entire MEMS system including an ASIC, microcontroller, and software to make it all communicate. The airbag manufacturer works with the auto manufacturer to develop algorithms that detect the severity of crash. It can be very sophisticated, indicating, for instance how much to deploy the airbag.

Airbag sensors are categorized as passive safety, intended to protect the driver and passengers when a crash occurs. This latest focus the company has on active safety is different; active safety tries to prevent crashes. Gilly said that it has been demonstrated through insurance studies that active safety saves more lives than passive safety.

Freescale has been hard at work developing a product to cover an active safety application — an electronic stability control system (ESC) — and has recently introduced a high-performance XY axis low g accelerometer, the Xtrinsic MMA6900Q, based on the concept of intelligent contextual sensing, which involves more than translating a signal, adding increased levels of modular integration combined with multiple sensor inputs, logic and other building blocks to bring greater value and decision making to the overall sensing solution.

"There has been a change in the market in terms of our own customers," noted Gilly. "We need to cost reduce and merge applications together, the airbag and ESC systems used to be stand alone, but are starting to merge together. Freescale is so present in the airbag industry it can use the same technology to develop new sensors."

ESC works like this. The system is ABS based, and includes wheel speed sensors that detect when the wheel experiences zero friction and stops braking. In the steering column, a sensor detects how much you want to turn the front wheels. This is the basis of the system that compares information to differing sensing in the car. A gyroscope sensor measures how much the car rotates. A low g inertial sensor measures how much push there is to the left or right perpendicular to the length of the car. The system calculates that. If the sensors detect that the driver isn't making the turn as intended, it applies the brakes and can correct the trajectory of the vehicle to recover. Pressure sensors measure how much pressure to apply to the brake. Feed back is provided based on inertia of the vehicle. It's an autonomous system. For 4WD, it can detect if the vehicle is lifting from the road and applies power to the wheels. Then, the wheel speed sensor system will be able to calculate the speed of the car and compensate for that due to the inertial sensor that is in both x and y directions, and recalculate the speed of the vehicle.

According to Gilly, the high-performance XY axis low g accelerometer has better high offset accuracy life and temperature than previous generations of available solutions. Features include a high aspect ratio MEMS transducer which allows for better signal/noise ratio. Additionally, the sensor is over-damped. This is significant on a bumpy road, for example, when everything resonates. When a vehicle crashes, high frequency vibrations can reach the transducer and risks output of the sensor, and may be perterbated by 15 Khz. An over-damped transducer operates on a different curve with no signal amplification. Therefore, the sensor is improved in terms of robustness to high frequency vibrations.