Automatic Braking System (ABS)
Automatic Braking System (ABS) is in virtually all cars now. Previously, these systems would measure wheel speed to determine if the wheel were slipping and apply controlled braking so the tires can re-grip the road. Recently, accelerometers have been integrated with advanced ABS in order to determine the chassis speed directly. Combining wheel speed and chassis speed from accelerometers yields an ABS system that can more reliably determine if slipping occurs and thus, react more quickly to realign the wheels to the road.
Electronic Stability Control (ESC)
If skidding of the automobile occurs, Electronic Stability Control (ESC) can be used to help the driver regain control of the automobile. Skidding occurs when the tires slip on the surface and results with the automobile both sliding and spinning on the road. As mentioned in ABS systems, accelerometers can be used to measure forward speed. Gyros, on the other hand, can be used to measure the spinning of the automobile. When knowledge of the degree of forward slipping and spinning is known, individual wheel braking strategies can be applied both to realign the automobile and to re-grip the road.
Roll Stability Control (RSC)
For automobiles with a high center of gravity (for example vans, trucks, SUV''''s) the risk of roll-over while making sharp turns is much greater. However roll-overs can be prevented by understanding the dynamics of the turn and applying some sort of active suspension. The system in the car that protects against roll-over is called Roll Stability Control (RSC). Accelerometers and gyros can be used to obtain both speed and inclination. Based on information from these inertial sensors, sharp hard turns could be accompanied by a stiffer suspension while slow gradual turns could have a softer suspension. In the extreme case, automatic breaking could be applied to prevent potential roll-overs.
Roll-Over Detection
For cars or trucks that have relatively high sides, the chance of roll-over is increased. When a roll-over occurs, side curtain air bags can be deployed to protect the occupants. A gyro can be used to detect when a roll-over is occurring and consequently deploy the air bags when necessary.
Tilt Alarm
For antitheft applications, sensors can be used to tell if a car is being stolen. By using an inclination sensor, (a two-axis accelerometer), anti-theft systems can detect lifting of the car when either being towed or jacked up.
In Car Navigation
Car navigation systems are composed of a small touch-screen on the dashboard that indicates the current location and direction of the car. While today''''s GPS systems are used to tell navigation, direction information can be determined by using a set of sensors including magnetometers, accelerometers, and gyros. Sometimes, the car is out of reach of the GPS satellites, and thus, cannot determine its position. In such “black out” scenarios, inertial sensors can be used to determine location using a process known as "dead reckoning". However, the sensitivity requirements for this type of sensor are very strict. Cost, size, and performance trade-off’s are currently prohibiting the introduction of this technology, though in the near future with technology advancements, MEMS inertial sensors will drop in both cost and size. In the meantime, accelerometers and gyros can determine short-term heading and speed with the aid of GPS and magnetometers working together.
Electronic Brake
Electronic brakes can be used to automatically engage the parking brake when it is parked on an incline. Furthermore, brakes can automatically engage to prevent rolling backwards when stopped on a hill when neither the gas or brake pedal is pressed. With electronic braking, a car would no longer need an “emergency brake” which could both free up valuable space around the driver and reduce the number of parts in a car. Like the “Tile Alarm” an inclinometer in the form of a 2-axis accelerometer can be used to detect inclines.
Airbag
For over a decade, collision sensors have been used to detect accidents and deploy front airbags to protect passengers from injury. The collision sensors are simply accelerometers that can detect when a minimum threshold of acceleration is reached. For example, when cars collide, a very large acceleration is detected, causing the accelerometer to fire the airbags.
In Summary
There are a number of applications for inertial sensors in the automobile. Adding all the axes together you can get somewhere between 10 and 20 axes of measurement. Obviously, there is a number of redundant axes of measurement. Having someway to centralize the inertial units could reduce the number of axes. Ideally, just 6 axes (3 axis acceleration, 3 axis angular velocity) should be all you ever need. However, critical fail-safe devices, such as airbags, would likely continue to have their own sensors. On another note, with such a diverse set of applications, the sensitivity and performance of such a six axis sensor would have to be equal to or greater that of the most demanding application.