Design, Dynamics, and Control of Extra Robotic Legs for Human Payload and Positioning Augmentation

The Extra Robotic Legs (XRL) system is a robotic augmentation worn by a human operator that consists of two articulated robot legs that help bear a heavy backpack payload and a portion of the operator’s own weight. The design was driven by a need to increase the effectiveness of Department of Energy hazardous material emergency response personnel who are encumbered by their Personal Protective Equipment.

Essentially a backpack-with-legs, the XRL system must bear large loads during operation, but also requires a proprioceptive transmission to allow for close physical interaction with the human operator. The linkage and actuator design minimizes the maximum required actuator torque by exploiting torque redistribution using a closed kinematic chain. A prototype was fabricated utilizing insights gained from force analyses and human-robot interaction safety requirements.

A seamless hybrid control architecture was developed to allow the operator command over the pace of the XRL stand-to-squat transition. A failsafe Hybrid Open-Loop/Closed-Loop Control Architecture splits the Cartesian space into a closed-loop subspace in which the robot controls its balance and stability, and an open-loop subspace in which the human operator may move the robot at will through only a force interaction. Distributing the control computation to the joint level wherever possible makes the system robust to disconnections from the central computer. Initial tests of balance control while performing squatting transitions indicate the feasibility of this control scheme for the XRL system.

It is desirable for the Human-XRL quadruped system to walk with an ambling gait in which the rear legs lead the front legs by 25% of the gait period, which minimizes the energy lost from foot impacts while maximizing the margin of balance stability. Unlike quadrupedal robots, the XRL system cannot command the human’s limbs to coordinate quadrupedal locomotion. By modeling the human-robot system during steady state walking as a coupled pair of simple nonlinear limit cycle oscillators, it can be shown that, using only a coupling made of passive mechanical components, a stable limit cycle that synchronizes the gaits while maximizing stability between the human and robot during walking may be achieved. By exploiting these inherently stable passive dynamics, the margin of stability and rate of synchronization may be supplemented with active control.

By using these key design, control, and gait synchronization techniques, the XRL System will ultimately walk, climb stairs, crouch down, and crawl with the operator while eliminating all equipment loads acting on them.



This work was sponsored by the National Science Foundation and the United States Department of Energy.