Abstract

Several control strategies can be implemented in road vehicles to avoid roll over, improve ride quality or customize handling performance. Handling performance is one of the crucial areas of research from the safety point of view. Most of the control strategies depend on manipulating the motion of the tires, which are the prime source of the forces acting on the vehicle. Some of the common control strategies for handling explored in recent years include: active control of tractive and/or braking torque; and a wide variations in active steering control. For vehicle stability and handling improvement, Active Front and Rear Steering (AFS, ARS) prove to be excellent control techniques, as active torque control fails to generate required forces and moments in certain situations. In recent years, major research effort has been directed towards active steering control, where the steer angle of the wheels is actively controlled to improve handling performance at high speeds. Such controls, however, have limitations as they do not attempt to utilize tires' force generating potential. The present study proposes a new Active Independent Front Steering (AIFS) technique with independent control for each front wheel. A non-linear 4-wheel vehicle model incorporating tire 'Magic formula' and load shifts in longitudinal and lateral direction is studied. This model agrees well with a simpler bicycle model and CarSim simulation. The 4-wheel vehicle model with proposed AIFS is simulated for step and sinusoidal lane-change inputs. A simple PI control algorithm that differentiates between under and oversteer handling characteristics is developed and utilized for the simulations. The results show that by controlling one wheel only, AIFS can provide the ideal yaw-rate and trajectory responses at any speed, and the performances are as good as those obtained by AFS and significantly better than conventional uncontrolled system. Furthermore, AIFS is shown to equalize the tire workload at the left and right front tires improving the vehicle's ability to generate maximum possible lateral force. Only exception to this is when the vehicle is strongly oversteer. It is also shown that this limitation can be overcome by introducing an AIFS where both wheels are actively controlled. A physical design using tandem planetary gear trains is proposed for the AIFS that can provide the required control and is fail safe. The present is a first investigation of AIFS control that has significant potential for the integrated control of road vehicles and is identified in proposed future studies.