Steer-by-Wire: Implications for Vehicle Handling and Safety
Abstract
Recent advances toward steer-by-wire technology have promised significant improvements in vehicle handling performance and safety. While the complete separation of the steering wheel from the road wheels provides exciting opportunities for vehicle dynamics control, it also presents practical problems for steering control. This thesis begins by addressing some of the issues associated with control of a steer-by-wire system. Of critical importance is understanding how the tire self-aligning moment acts as a disturbance on the steering system. A general steering control strategy has been developed to emphasize the advantages of feedforward when dealing with these known disturbances. The controller is implemented on a test vehicle that has been converted to steer-by-wire.
One of the most attractive benefits of steer-by-wire is active steering capability. When supplied with continuous knowledge of a vehicle's dynamic behavior, active steering can be used to modify the vehicle's handling dynamics. One example presented and demonstrated in the thesis is the application of full vehicle state feedback to augment the driver's steering input. The overall effect is equivalent to changing a vehicle's front tire cornering stiffness. In doing so, it allows the driver to adjust a vehicle's fundamental handling characteristics and therefore precisely shift the balance between responsiveness and safety.
Another benefit of steer-by-wire is the availability of steering torque information from the actuator current. Because part of the steering effort goes toward overcoming the tire self-aligning moment, which is related to the tire forces and, in turn, the vehicle motion, knowledge of steering torque indirectly leads to a determination of the vehicle states, the essential element of any vehicle dynamics control system. This relationship forms the basis of two distinct observer structures for estimating vehicle states; both observers are implemented and evaluated on the test vehicle. The results compare favorably to a baseline sideslip estimation method using a combination of Global Positioning System (GPS) and inertial navigation system (INS) sensors.