How to Shake Trees With Aerial Manipulators? A Theoretical and Experimental Study

Aerial manipulators are advancing beyond traditional inspection roles to enable complex interactions with flexible structures. Applications such as structural health monitoring, and especially agricultural tasks like fruit harvesting or environmental monitoring, require inducing controlled vibrations into flexible elements. However, current solutions for controlled shaking of trees with aerial manipulators are limited to push and pull forces applied through translational movements, without exploiting the fully-capabilities of aerial platforms. This letter introduces a controlled shaking strategy that enables interaction with trees using both linear movements generated by forces (translation strategy) and rotational movements generated by torques (rotation strategy) thus exploiting the different interaction capabilities of the platform. These two strategies open a previously unexplored question: which strategy is more effective given a specific interaction point? To address this, the two interaction strategies are integrated with the Rayligh-Ritz model of the tree, obtaining the closed-loop dynamics of the system during the vibration. These closed-loop dynamics are then analyzed for the two shaking strategies, deriving which one is better for achieving higher oscillation amplitudes or frequencies. This analysis shows that, for a given interaction point of the tree trunk, this decision depends only on the platform's physical characteristics, such as mass and inertia. Finally, the theoretical analysis is experimentally validated with a hand-made bamboo tree and a fully-actuated platform through indoors flights.