Author: Harvey Weinberg

 

While most of us know that cars today are rife with electronics, what we may not appreciate is how many MEMS (Micro Electro-Mechanical Systems) sensors modern cars contain. Today, MEMS sensors have been embraced by the auto industry in their quest to improve performance, reduce cost, and enhance the reliability of the family sedan. In fact hundreds of millions of MEMS sensors have been used in automobiles over the last decade.

The use of MEMS accelerometers to sense rapid deceleration for airbag deployment is well over a decade old and considered a well-known technology. However, MEMS inertial sensors are now being used in many other automobile applications. This article discusses some of the many applications for MEMS inertial sensors and the performance requirements of those sensors that improve the safety, convenience, and reliability of cars and trucks today.

In addition to the ubiquitous frontal and side impact air bags found in most cars today roll-over detection systems are becoming common in vans, pick-up trucks, and SUVs because of their greater tendency to roll over. In the event of roll-over, side curtain air bags can be deployed to protect the occupants. Roll-over detection systems read the roll angle and roll rate of the vehicle to determine if it is in the process of rolling over. If so, the side curtain air bags are fired to protect the occupants.

Roll over detection systems employ a roll rate sensor to read the roll rate. The roll rate is integrated to determine the roll angle of the vehicle. An accelerometer reading vertical acceleration (Z axis) is also required as large roll angles may be encountered in banked curves with no possibility of roll over. Better roll over detection systems also use another accelerometer to measure lateral acceleration as a vehicle striking a curb or other object while sliding sideways is much more likely to roll over.

Gyros used for roll over sensing do not require the same resolution as those used in VDC systems, but they must have excellent rejection of external shock and vibration and have a larger dynamic range. The Analog Devices’ ADXRS300 iMEMS gyroscope is commonly used in this application because of its insensitivity to external shock and vibration. Since roll over events happen over a fairly short period of time, the performance requirements for the accelerometer are not as severe as for ESC or RSC, however better performance can be obtained using a more stable accelerometer. As we will see later, there are other reasons why a roll-over detection system might use an ESC grade accelerometer.

Many automakers (particularly European manufacturers) are currently including anti-theft systems incorporating tilt detection systems as standard equipment. A dual axis (lateral and longitudinal axis) accelerometer is used to detect changes in inclination as would occur if the car were being jacked up or towed. Zero g bias temperature stability of better than 50mg over the automotive temperature range is required to ensure that temperature changes to not result in annoying false alarms.

Large electrolytic fluid tilt sensors have the required sensitivity for this application, but have several disadvantages. Their tilt range is generally quite limited, so it is possible to park a car at an inclination larger than the sensor can tolerate. In addition, the widely varying automotive temperature environment is not well tolerated by fluid tilt sensors. The Analog Devices ADXL213 iMEMS accelerometer is ideal for this application because of its unique combination of stability, high sensitivity, wide range, compact size, and high quality.

Electronic parking brake systems automatically actuate the parking brake sufficiently to prevent slipping when the automobile is parked on an incline. While driving, the system prevents the car from rolling backwards on a hill when neither the gas nor brake pedals are pressed. The system measures the inclination of the vehicle, determines how much braking force is required, and applies it. Aside from safety and convenience advantages, electronic brake systems allow the carmakers to do away with the parking brake lever (or pedal) and associated hardware. Reducing the cost and weight of the vehicle, while improving reliability

As with ABS, a low g longitudinal accelerometer is used in this application. The performance requirements are similar to that for ABS.

In-car navigation systems are becoming an increasingly popular option. More than half of all cars sold in Japan today sport a navigation system. A global positioning system (GPS) is at the heart of a navigation system, but GPS information alone is insufficient for car navigation. The GPS can tell you where you are (position and altitude), but not what direction you are facing. Magnetometers (electronic compass) are highly reliable long term heading sensors, but can give false readings in the short term because ferrous metal objects (like the car next to you) perturb them.

When the system is first started the initial direction of travel is matched up with map data. Once initial direction is established, yaw rate sensor information is used to determine when, and how much, the car has turned until directional data is verified by map matching.

In urban settings it is not unusual to have the GPS signal obscured for short to moderate periods of time by tall buildings or tunnels. At these times the navigation system relies on the yaw rate sensor for heading information and, in some cases, a low g longitudinal accelerometer for position information. The yaw rate information is integrated once to determine heading and acceleration signal is integrated twice to derive position (this technique is called "dead reckoning").

Ideally the noise performance requirements of the yaw rate sensor and accelerometer are much better than what is currently available at moderate cost. So software (in the form of map matching) comes to the rescue and makes certain assumptions about where the vehicle might be.

Many other accessories can (and currently do) use inertial sensors in their operation. But an exhaustive list of them is beyond the scope of this article.