Tech Briefs
Interactive vehicle dynamics

2000 Ford Focus.

The Ford Focus' interactive vehicle-dynamics system was designed to help increase safety margins and decrease instability during a wide variety of driving maneuvers. The system uses seven different vehicle sensors and two independent microprocessors to check, 150 times per second, whether the behavior of the car matches the steering input. When it detects signs of any deviation, it intervenes to help restore stability, reducing the risk that the driver might lose control.

The system evolved from ABS and traction control systems (TCS). Like TCS, interactive vehicle dynamics uses the engine-management controls to reduce power if there are signs that traction might be lost when entering a turn. It also uses the individual wheel-speed sensors of the ABS system in combination with three additional sensors: one fitted around the steering column to monitor the driver's inputs, and yaw-rate and lateral-acceleration sensors mounted under the front passenger seat near the vehicle's center of gravity. Whenever the behavior of the car fails to match calibration data for the vehicle speed stored on a dynamic-handling map in the onboard computer, stabilizing countermeasures are initiated.

To provide a fast-acting hydraulic pulse, a new brake booster with an electrical solenoid added to its operating rod is also included in the system. It allows line pressures of 13-18 MPa (1900-2600 psi) to be generated in a few milliseconds. Pressure transducers are also added to the output from the brake master cylinder.

The yaw-rate sensor was originally developed for navigational gyrocompasses and space rocket stabilization systems. It uses a physics concept known as the Coriolis effect, similar to centrifugal force but acting only within a rotating object. It detects and measures minute changes in angular vehicle velocity about its vertical axis when negotiating a turn. It then converts the data to an electronic signal that is monitored by two independent microprocessors every seven milliseconds—equivalent to once every 100 mm (4 in) of road covered at 48 km/h (30 mph).

In the early stages of dynamic stabilization, interactive vehicle dynamics uses the normal TCS system to reduce power via the engine-management module and help prevent loss of traction as the car enters a turn. If it detects that the front of the car will drift toward the outside of the turn (understeer), and the limit of adhesion is approaching, the system applies a braking impulse to the inside rear wheel, inducing a self-stabilizing yaw moment that helps straighten the car back on line. If it detects a tendency for the tail to swing out (oversteer), a braking impulse is applied to the outside front wheel, with the same self-stabilizing effect.

In both instances, the system is designed to act before destabilizing forces take effect, in much the same way that ABS operates before the wheels lock under braking and TCS operates before the driver senses there is any loss of traction. Unlike these other systems, Focus' interactive vehicle dynamics monitors the vehicle characteristics continuously and can operate when the car is rolling freely without braking or driving torque. The system is currently available as an option in Europe and will be offered to North American customers in future model years