Combining Tail and Reaction Wheel for Underactuated Spatial Reorientation in Robot Falling With Quadratic Programming

Inertial appendages (e.g., tails and reaction wheels) have shown their reorientation capability to enhance robots' mobility while airborne or improve robots' safety in falling. The tail, especially with two Degrees of Freedom (DoFs), is normally subject to its limited Range of Motion (RoM). Although the reaction wheel circumvents this limitation, its efficiency has been shown lower than the tail in terms of inducing Moment of Inertia (MoI). In literature, only one type of inertial appendages has been used on terrestrial robots in the air, e.g., either using a tail on the hexapedal robot RHex or using a reaction wheel on the jumping quadruped robot SpaceBok. In this letter, to benefit from both unlimited RoM and efficient MoI-inducing, we propose combining a 1-DoF tail and a reaction wheel together for spatial reorientation (regulating the robot body's 3D orientation). Inspired by this, a hybrid tail-wheel robot is built, i.e., the tail that creates roll motion is attached to a wheel-equipped robot whose wheels act like a reaction wheel and generate pitch rotation; however, the robot is underactuated on the yaw rotation. To achieve its real-time spatial reorientation, we propose a novel quadratic programming algorithm based on a geometric metric for the underactuated hybrid tail-wheel robot. Within the proposed algorithm, the physical limitations on tail and wheel velocities are automatically accommodated. Numerical comparisons among wheel-wheel, tail-wheel, and 2-DoF tail robots showed the strength of the hybrid tail-wheel appendage on reorientation convergence and free of collision. Experimental results further demonstrated the capability of real-time spatial reorientation with underactuation and velocity constraints by using the combined tail-wheel inertial appendage.