This letter presents a novel and retractable ring-shaped quadrotor called Ring-Rotor that can adjust the vehicle's length and width simultaneously. Unlike other morphing quadrotors with high platform complexity and poor controllability, Ring-Rotor uses only one servo motor for morphing but reduces the largest dimension of the vehicle by approximately 31.4\%. It can guarantee passibility while flying through small spaces in its compact form and energy saving in its standard form. Meanwhile, the vehicle breaks the cross configuration of general quadrotors with four arms connected to the central body and innovates a ring-shaped mechanical structure with spare central space. Based on this, an ingenious whole-body aerial grasping and transportation scheme is designed to carry various shapes of objects without the external manipulator mechanism. Moreover, we exploit a nonlinear model predictive control (NMPC) strategy that uses a time-variant physical parameter model to adapt to the quadrotor morphology. Above mentioned applications are performed in real-world experiments to demonstrate the system's high versatility.

翻译：本文提出一种名为Ring-Rotor的新型可伸缩环形四旋翼，该飞行器能同时调整自身长度与宽度。区别于其他平台结构复杂、可控性差的变形四旋翼，Ring-Rotor仅使用一个伺服电机实现变形，却能将其最大维度缩减约31.4%。其紧凑形态可保障在狭窄空间中的通过性，标准形态则实现节能效果。同时，该飞行器打破常规四旋翼四臂连接中央机体的十字构型，创新设计出具有中央空腔的环形机械结构。基于此，研究者设计了一种无需外部机械臂的巧妙全身式空中抓取与运输方案，可承载多种形状物体。此外，我们采用基于时变物理参数模型的非线性模型预测控制（NMPC）策略以适应四旋翼形态变化。通过真实环境实验验证了上述应用，展示了该系统的高度多功能性。