Wheel Slip Identification and its Use in the Robust Control of Articulated Off-road Vehicles

In this paper, sliding mode controllers (SMC) are designed for a steerable tractor-trailer system, which is operating in the presence of longitudinal and lateral slips at front and rear wheels. Such vehicles intended for autonomous operations usually traverse at low speeds along straight lines. As very low accelerations are involved kinematic models seem sufficiently accurate for developing controllers. However undulating, sloping and very uncertain terrain, besides rough environment, introduce significant disturbances which makes the control design challenging. To take these effects into account, lateral and longitudinal sliding velocities are incorporated into otherwise ideal kinematic model, and an error model is developed. The slip velocities are estimated by an on-line recursive least squares (RLS) algorithm. The sliding mode controllers are updated adaptively and continuously based on the estimated slip velocities. Since there are two sets of steered wheels, two separate SMCs are developed; one for the tractor and the other for the trailer. For the tractor, an adaptive sliding mode controller with disturbance observer (aDOB-SMC) is proposed to cope with uncertainties and residual errors in the slip estimations. The proposed aDOB-SMC is able to guarantee precision path tracking as well as robustness for a typical tractor-steerable trailer pair. We prove that the combination of the on-line estimations and SMCs provide adaptability, stability and accuracy in the presence of significant slips. The simulation results are compared with two other controllers in the literature.