Stability control of a wheel-legged mobile platform used in hilly orchards

This study proposes a dynamic control strategy based on hierarchical control to improve the driving stability of a compact skid-steering quadrupedal wheel-legged mobile platform in hilly orchards. The upper layer of the control strategy includes the velocity control submodule based on Proportional-Integral-Derivative (PID) control, a state observer based on the Extended Kalman Filter (EKF) algorithm and an expected yaw torque controller. The lower layer of the control strategy is a torque distribution controller based on the improved axle load proportional distribution algorithm, which redistributes the total driving torque and the yaw torque. The mathematical model of the mobile platform using the dynamic control strategy was established and simulation tests under different working conditions in hilly orchards were carried out. The results showed that the control strategy can overcome the driving stability problem caused by the swing of the wheel legs. The side slip angle was controlled within ±0.2°, the expected yaw rate can be accurately tracked with a maximum error of 0.0075 rad s⁻¹, the maximum relative error of velocity during stable driving was 1.2 %, and the maximum relative error of the driving trajectory was 3 %. A field experiment was carried out in the orchard. The results indicated that the maximum relative deviations of the experimental trajectory and driving velocity of the mobile platform, which occur during steering conditions, were <1.5 % and 10.2 %, respectively, when compared to simulated and expected values. This confirmed the effectiveness of the driving stability control strategy.