An Innovative Control Logic for a Four Wheel Steer Vehicle - Part 1: Analysis and Design

Through a single track model, correspondence between typical frequency analysis coefficients and test driver's opinion developed after experimental tests has been stated. Benchmark analysis of several vehicles, considered significant, has been carried out as well as a sensitivity analysis of vehicle behavior depending on passive design parameters, such as vehicle sideslip stiffness and tyre relaxation length. It led to the definition of the different transfer function capable of describing passive vehicle linear behavior; vehicle performance limits, due to unbridgeable physical phenomenon, has been also considered. 4WS vehicle chance to overcome these limits has been investigated, depending on rear steering control logic complexity. Vehicle frequency response has been then analyzed for different longitudinal velocity, introducing thus the concept of “natural vehicle”. The design of a four wheel steer system control logic, based only on feed forward, is described. The control logic aim is to improve active vehicle handling and its setting is controlled by a limited number of parameters. Different control logic versions are presented, increasing its complexity and changing target ideal vehicle behavior. Simulation results obtained through a 14 degrees of freedom vehicle model are obtained. They show the difference between active and passive steer vehicle behavior as long as active steer vehicle response in increasing lateral acceleration conditions, when not linear phenomena occur.