Design and Implementation of Articulated Robotic Tails to Augment the Performance of Reduced Degree-of-Freedom Legged Robots

This dissertation explores the design, and implementation of articulated robotic tail mechanisms onboard reduced degree-of-freedom (DOF) legged robots to augment performance in terms of stability and maneuverability. Fundamentally, this research is motivated by the question of how to improve the stability and maneuverability of legged robots. The conventional approach to address these challenges is to utilize leg mechanisms that are composed of three or more active DOFs that are controlled simultaneously to provide propulsion, maneuvering, and stabilization. However, animals such as lizards and cheetahs have been observed to utilize their tails to aid in these functionalities. It is hypothesized that by using an articulated tail mechanism to aid in these functionalities onboard a legged robot, the burden on the robot's legs to simultaneously maneuver and stabilize the robot may be reduced. This could allow for simplification of the leg's design and control algorithms.

In recent years, significant progress has been accomplished in the field of robotic tail implementation onboard mobile robots. However, the main limitation of this work stems from the proposed tail designs, the majority of which are composed of rigid single-body pendulums that provide a constrained workspace for center-of-mass positioning, an important characteristics for inertial adjustment applications.

Inspired by lizards and cheetahs that adjust their body orientation using flexible tail motions, two novel articulated, cable driven, serpentine-like tail mechanisms are proposed. The first is the Roll-Revolute-Revolute Tail which is a 3-DOF mechanism, designed for implementation onboard a quadruped robot, that is capable of forming two mechanically decoupled tail curvatures via an s-shaped cable routing scheme and gear train system. The second is a the Discrete Modular Serpentine Tail, designed for implementation onboard a biped robot, which is a modular two-DOF mechanism that distributes motion amongst links via a multi-diameter pulley. Both tail designs utilize a cable transmission system where cables are routed about circular contoured links that maintain equal antagonistic cable displacements that can produce controlled articulated tail curvatures using a single active-DOF. Furthermore, analysis and experimental results have been presented to demonstrate the effectiveness of an articulated tail's ability to: 1) increase the manifold for center-of-mass positioning, and 2) generate enhanced inertial loading relative to conventionally implemented pendulum-like tails.

In order to test the tails ability to augment the performance of legged robots, a novel Robotic Modular Leg (RML) is proposed to construct both a reduced-DOF quadrupedal and bipedal experimental platform. The RML is a modular two-DOF leg mechanism composed of two serially connected four-bar mechanisms that utilizes kinematic constraints to maintain a parallel orientation between it's flat foot and body without the use of an actuated ankle. A passive suspension system integrated into the foot enables the dissipation of impact energy and maintains a stable four point-of-contact support polygon on both flat and uneven terrain.

Modeling of the combined legged robotic systems and attached articulated tails has led to the derivation of dynamic formulations that were analyzed to scale articulated tails onboard legged robots to maximize inertial adjustment capabilities resulting from tail motions and design a control scheme for tail-aided maneuvering.

The tail prototypes, in conjunction with virtual simulations of the quadruped and biped robot, were used in experiments and simulations to implement and analyze the methods for maneuvering and stabilizing the proposed legged robots. Results successfully demonstrate the tails' ability to augment the performance of reduced-DOF legged robots by enabling comparable walking criteria with respect to conventional legged robots. This research provides a firm foundation for future work involving design and implementation of articulated tails onboard legged robots for enhanced inertial adjustment applications.