Robust lateral–longitudinal control for cooperative unloading of a tracked harvester–grain transporter system

Coordinated operation between a combine harvester and a grain transporter can markedly enhance harvesting efficiency. The crux of such dual-machine collaboration lies in maintaining precise lateral and longitudinal positioning so that grain discharged from the harvester can be accurately deposited into the transporter. To achieve high-precision cooperative unloading control, we establish a leader–follower geometric model of relative positioning based on the harvester–grain transporter kinematic model and then derive the corresponding lateral and longitudinal error dynamics. Building upon an improved power–exponential reaching law, we design a robust, disturbance-resilient sliding-mode controller (RPSMC) for dual-machine lateral and longitudinal position regulation. To address external disturbances such as slip and skidding inherent in cooperative field operations, an improved nonlinear disturbance observer (NDOB) is developed to estimate environmental perturbations; real-time estimation with feedforward compensation further strengthens disturbance rejection throughout the unloading process. A Simulink–RecurDyn co-simulation platform is constructed, and comparative tests under varying soil conditions and disturbance scenarios demonstrate that, relative to baseline control, the proposed method significantly improves response speed and steady-state accuracy. Repeated field experiments further confirm the effectiveness of the proposed method at a cooperative unloading speed of approximately 1 m/s. After convergence, the maximum longitudinal and lateral errors remained within 0.1 m, while the mean absolute errors were below 0.06 m. The maximum steady-state standard deviations were 0.086 m in the longitudinal direction and 0.079 m in the lateral direction, indicating small fluctuation and good repeatability. Compared with the baseline controllers, the proposed method achieved faster convergence, lower peak and average errors, and stronger steady-state disturbance suppression. These results demonstrate that the proposed approach can reliably maintain the unloading alignment between the harvester outlet and the transporter bin center, providing a practical cooperative unloading solution for automated harvesting.