Adaptive Control for Outward-Folding Landing Legs of Reusable Rockets: Achieving Synchronized Touchdown Under Tilted Conditions

In this paper, we propose an adaptive four-leg landing system for a quadcopter to enable stable landing on inclined surfaces or rough terrain while maintaining its attitude. A rotorcraft has a disadvantage in that it must have a flat ground for landing because the existing landing gear system is fixed and cannot be changed according to ground conditions. The proposed adaptive landing system makes it possible for a quadcopter to operate in various environments with minimal sensors. For this purpose, a four-leg landing system with two degrees of freedom having a two-link structure was manufactured. Through kinematic analysis of the landing leg, the coordinates of the joint and end-effector were identified, and an equation for horizontal control was established. When landing on the slope, the ground surface was detected using force sensors, attached at the end of each leg. Adaptive landing control was performed according to the roll and pitch values of the inertial measurement unit sensor mounted on the quadcopter. The proposed system was pre-validated in the simulation environment of Matlab Simulink. The superiority of the proposed system was verified experimentally in various environments. In an indoor environment, the effectiveness of the system was verified in a situation where it landed at a constant speed using a testbed. Finally, the system stability was verified through flying and landing of the quadcopter in an outdoor environment.