Technical Features
Disecting DSTC
By by: Paul Grimes
Jan 23, 2002, 10:16

With the recent announcements from Volvo that an electronic stability program called Dynamic Stability Traction Control (DSTC) will be available on most models, we felt it was a good time to provide some insight as to how this system functions and what its purpose is. The true advantage of this electronic device is its ability to identify and compensate for critical driving situations. DSTC monitors the car's progress continuously and takes action automatically if there is any risk of the car going out of control.

The first application of such a system, the Bosch's ESP, was in the Mercedes Benz S-Class in 1995. Mercedes is now offering ESP as standard equipment on all of its vehicles worldwide. Audi has been equipping all of its top-of-the-range automobiles in most European countries with the ESP package since the beginning of 1999. Interestingly this means that stability programs such as DSTC and ESP have established themselves in the automotive market faster than any other safety equipment, including the air bag.

The DSTC system is a "marriage" of the vehicle's ABS system with a newly designed electronic control unit (ECU). The system's processor takes in a variety of sensor data, switch signals, and valve commands to aid the vehicle in maintaining a consistent and predictable level of control while performing evasive and high speed cornering and braking maneuvers.

The yaw sensor, the "heart" of the DSTC, is produced with high precision silicon micro mechanics. The sensor's design was patented by Bosch in 1998, and is designed to consistently monitor the vehicle's tendency to rotate around its vertical axis.

In practice, the system detects movements that could indicate a "skid condition" for the vehicle, which is detected by the shift of micro-fine comb structures of the spring-mass system causing a change in the event of rotational movements of the vehicle on a vertical axis. The rate of rotation, along with the measure of transverse acceleration is evaluated by the ECU. The car's yaw behavior can vary in three ways:

1. The vehicle can respond to steering wheel movement and make a turn while remaining stable. Depending on road speed, each steering angle is equivalent to a specific yaw rate.

2. The car can begin to oversteer, and its tail run wide. It then rotates more rapidly round the vertical axis than if it were cornering in a stable manner, and the yaw rate is thus too high.

3. The car may understeer severely, in which case it is the front wheels that try to run wide. Steering wheel movements produce too little response, so that the yaw rate around the vertical axis is slower than it should be.

DSTC has only a fraction of a second in which to correct such critical dynamic behavior-patterns. The aim is to maintain stable cornering in all circumstances and to ensure that the car responds in the correct way to the driver's steering-wheel movements. These movements are then processed to deliver the signals that are sent to stabilize the vehicle through the combination of ABS, TCS, and throttle position.



A simple process, though the DSTC systems computer has a great deal of work to perform in a very short time. This is because DSTC not only processes the yaw rate signals, but also those from a steering angle sensor mounted just behind the steering wheel, from a lateral acceleration sensor and from the four wheel speed sensors. From all these data it compares the steering angle and wheel speeds with the yaw and lateral acceleration rates, and detects any departure from the desired values. It then determines where and to what extent the wheel brakes should be applied.

DSTC can modulate the braking forces at the individual wheels to correct vehicle motion patterns. As an example, if the vehicle rear end were to spin towards the left under a panic maneuver, the DSTC system compensates by generating a pulse to the left front wheel to pull the car back into a straight line. DSTC can also "dial in" specific engine power delivery changes to correct the car's diversion from a straight line, either during deceleration or acceleration. In addition, DSTC can help to ensure that stopping distances in corners and on road surfaces that have differing traction levels from left to right side.

Although DSTC monitors the car's movements continuously, the system's action will not be noticed by the driver unless the car should become unstable. Incidentally, the driver can switch off DSTC by pressing a button located on the dashboard. This provision is mainly to enable STC, which is an element of the complete DSTC system, to be put out of action in certain driving situations.

DSTC represents an important milestone in vehicular safety. Product planners have added the system as a stand-alone option to most Volvos available in North America. For those seeking the absolute ultimate in safety or those who drive frequently in extremely slippery conditions, DSTC offers an additional level of security.

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