Dynamic Rigidity in Aerial Robots

Abstract

As interest grows in incorporating soft compliance structures into aerial robots to enhance safety, resilience, and flexibility, a key challenge arises: maintaining stability and configuration during flight. Traditional aerial robots rely on rigid airframes, but soft robots, by nature, offer lower compliance as well as structural stability. The absence of a rigid framework complicates the precise and predetermined positioning of propulsion systems and sensory components in soft aerial robotic platforms. Moreover, accurately characterizing the underlying dynamics and devising effective control strategies to manage morphing behavior during flight remain difficult.

In this work, we present dynamic rigidity as a novel mechanism for soft aerial robots to obtain stiffness passively through dynamic force induced by motion. With the acquired stiffness, soft robots gain the ability to retain a stable configuration during motion. Within the concept, we present two instances for validating the principle of dynamic rigidity. The first demonstration exploits the internal stress and centrifugal forces generated during rotational motion to create robot's structural integrity in a flight-capable configuration. The dynamics of angular momentum provide a physical foundation for modeling and predicting the system’s passive configuration. Building on this mechanism, we designed an airframe-less bicopter in which cables replace all rigid structural components. During flight, the bicopter rotates while hovering, and its configuration is maintained passively due to the tensile centrifugal force transmitted through cables. With a weight of 21 grams and a footprint of 26 cm, the robot achieves a remarkable revolving speed over 10 revolutions per second. The induced centrifugal acceleration exceeds 9 times the gravitational acceleration dominating structural rigidity in flight.

The second demonstration leverages aerodynamic forces arising from translational motion to stiffen wings of a drone. Based on this principle, we developed an 9-gram fixed-wing drone platform with flexible aerodynamic surfaces. When cruising, the inflow aerodynamic forces acting on the flexible wings provide the necessary force for lift generation against gravity. Concurrently, the wing geometry undergoes controlled morphological transformation along the predetermined creases, maintaining structural integrity against aerodynamic pressure. Theoretically, under inflow conditions of up to 5 m/s, the flexible wing exhibits stiffness variations reaching magnitudes of over 4 times robot's weight.

With dynamic rigidity mechanism, these designs offer significant advantages in shape adaptability, lightweight construction, and portability, while simultaneously providing the necessary stability for controlled aerial maneuverability. Building upon established platforms, our approach enables aerial robots to maintain configuration with passive stability during dynamic operations, while reverting to a compliant state when static, thereby optimizing the balance between structural integrity and versatile functionality.

Date of Award	25 Jul 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Pakpong CHIRARATTANANON (Supervisor)