Input-Output Decoupling Control by Measurement Feedback in Four-Wheel-Active-Steering Vehicles

The standard second order single track linearized model for four-wheel-steering vehicles is considered: yaw rate and lateral speed are the outputs to be controlled while front and rear steering angles are the control inputs (only additive steering angles with respect to the pilot commands are considered for the front wheels). It is shown that the lateral speed dynamics and the yaw rate dynamics can be decoupled by feeding back longitudinal speed, yaw rate and lateral acceleration measurements: lateral speed measurements are not required. The yaw rate tracking error dynamics follow a second order reference model with arbitrary poles, while the lateral speed dynamics tend exponentially to zero with a vehicle-dependent time constant and lateral acceleration tends to be proportional to the yaw rate. Simulations on a nonlinear third order single track model show significant improvements in the closed loop behaviour: larger stability regions, larger bandwidth, resonances suppression, improved manoeuvrability. A key feature of the input-output decoupling control is the improved comfort since both the lateral speed and the phase lag between lateral acceleration and yaw rate are greatly reduced