Modern vehicles incorporate a number of active vehicle control sub-systems that enhance comfort and safety. Two such vehicle control sub-systems include an active front steering (AFS) system and a vehicle stability enhancement (VSE) system.
The AFS system electronically varies the steering ratio based on the intended steering angle, vehicle speed, road conditions and feedback control. Vehicle steering is more direct under normal road conditions at low and medium speeds, reducing operator steering effort. The AFS system also increases vehicle agility in city traffic or for parking maneuvers. Vehicle steering becomes less direct at higher vehicle speeds improving directional stability. Steering stiffens during high speed cornering or sudden maneuvers by monitoring the vehicle yaw rate.
The VSE system assists the vehicle operator in controlling vehicle handling on surfaces such as wet or uneven pavement, ice, snow or gravel. The VSE system also helps the vehicle operator maintain control during rapid or emergency maneuvers. The VSE system recognizes wheel skid based on sensor inputs from wheel speed sensors, steering angle sensors, vehicle speed and a yaw rate sensor. After analyzing the various inputs, the VSE system reduces engine torque and applies braking to maintain vehicle travel along the intended path.
The development of such vehicle control sub-systems reaches a natural limit that presents a compromise in fulfilling contradicting requirements of vehicle comfort, stability, performance and cost. Superimposing the control of each sub-system is not always the most effective means in achieving total vehicle performance.