A composite sliding mode controller with extended disturbance observer for 4WSS agricultural robots in unstructured farmlands

Abstract

Autonomous agricultural robots have gained increasing attention in recent years, as they hold great potential for a wide range of applications in agriculture. However, accurately tracking a specified path for these robots is challenging due to wheel slip disturbances arising from unstructured farmlands characterized by uneven, undulating, and slippery terrain. In this paper, an extended disturbance observer based sliding mode controller (EDO-SMC) is proposed for Four-Wheel Self-Steering (4WSS) agricultural robots subject to lateral and longitudinal wheel slip. First, the novel differential steering structure of the 4WSS robot is introduced. To take slipping effects into account, an improved kinematic model which explicitly integrates the unknown slip disturbances is developed. An extended disturbance observer is then introduced to estimate the slip disturbances and their rates of change, facilitating timely compensation for these time-varying disturbances. To enhance practical applicability in agriculture, an improved sliding surface is designed to mitigate excessive control effort resulting from observer-induced overcompensation under initial conditions. Furthermore, a rigorous Lyapunov stability analysis of the proposed composite control strategy is conducted. Finally, the proposed composite controller is validated through co-simulations and field tests, meeting the control accuracy and robustness requirements of agricultural robot operations in unstructured farmlands.