It's all about µ

In many cases, the fatal scenario is simple: Skid. Trip. Roll. Crash. Within seconds, the deadly chain of events is irreversible, and another life is lost on the highways. Now, however, that situation may be changing. After more than a decade of experience with ESC (electronic-stability-control) systems, automotive researchers are comprehending the value of this little-used technology. Stop the skid with ESC, they're saying, and you can head off the other events in the gruesome chain.

"When you consider that there are between 14,000 and 17,000 single-vehicle fatalities every year, and we can reduce those fatalities by 50%, you realize we're dealing with a huge number of lives that could be saved," says Rich Golitko, marketing director for ESC at Bosch Automotive.

An ESC system's primary components include a lateral accelerometer, gyroscopic yaw-rate sensor, and steering-angle sensor, as well as an electronics module and wiring harness (Figure). The system works by measuring yaw and lateral acceleration, then comparing that to the driver's desired path, as indicated by the steering-angle sensor. If a microcontroller determines that the difference between the desired path and actual path is too great, the system activates one or more of the wheel brakes by means of the ABS (antilock-braking system).

To fully comprehend the nature of what's happening, most stability-control systems also employ vehicle data, such as vehicle mass, wheel mass, center of gravity, drag coefficient, engine torque, pitch moment of inertia, roll moment of inertia, and more. A stability-control system uses that data to determine whether additional measures, such as pulling back on the throttle or braking another wheel, are necessary.

ESC also uses the sensor data to derive a rough estimate of the coefficient of friction (µ) on the driving surface. Knowing µ, as well as the vehicle's performance characteristics, helps ESC determine how much intervention is necessary and when the time has come for action.

To make ESC affordable, electronics vendors are working on lower cost sensors, as well as on electronic integration methods that would help reduce installation costs. Analog Devices, for example, has developed silicon-based yaw-rate sensors that could ultimately cost substantially less than the quartz-based, piezoelectric tuning fork "gyros" now used in most ESC systems. Unlike the quartz gyros, which use the frequency of the tuning fork to measure yaw, the silicon sensors employ a "vibrating mass," which produces a voltage signal that's proportional to the vehicle's angular rate of change. By employing silicon instead of quartz, Analog Devices engineers believe they can bring down sensor costs from their current levels of $20 to $30 apiece, to less than $10.

Device integration could also be a key way to cut costs, engineers say. Analog Devices, among others, is working on integrating lateral accelerometers onto a single die with yaw-rate sensors. Such efforts go hand in hand with attempts to integrate ESC electronics into ABSs under the hood, instead of in their current location in the passenger compartment. Doing so would eliminate the need for costly wiring harnesses that must pass through firewalls between the passenger and engine compartments. Some automakers are also said to be looking at the possibility of leaving the ESC electronics in the passenger compartment, and instead cutting costs by integrating the ESC module with the air-bag module.