An Innovative Control Logic for a Four Wheel Steer Vehicle – Part 2: Simulation and Road Test

A four wheel steer control logic is described. A first control logic release, obtained during previous research activity, is based only on feed forward (F.F.) but is here upgraded merging closed loop control (C.L.). Integration between F.F. and C.L. is described. Rear steering electromechanical actuator frequency response is analyzed, in order to consider its not ideal behaviour during control logic design. Several simulation are performed to qualitatively evaluate the error committed considering an ideal actuator during the control logic design. Specific manoeuvres are chosen to investigate about active system influence on vehicle handling; a 14 degrees of freedom vehicle model is validated in order to compare simulation results with experimental data. Simulated manoeuvres and analogous road tests are then compared and control logic efficacy for different longitudinal velocities is evaluated, as well as for different vehicle working conditions (weight, tyres change etc.) and different reference target behaviours. Considering acknowledged the relation between test driver's feelings and typical frequency response parameters, control transfer functions are chosen in order to increase “driver's feeling”. Further experimentation leads to the evaluation of more specific control logics. Brief overlook on possible integration between different active systems closes the paper, still considering feed forward based control structures. Particular importance is given to linear reference behaviour uniqueness, which gives origin to several control functions applied to the different active systems.