  We present the design of a new robotic human augmentation system that will
assist the operator in carrying a heavy payload, reaching and maintaining
difficult postures, and ultimately better performing their job. The Extra
Robotic Legs (XRL) system is worn by the operator and consists of two
articulated robotic legs that move with the operator to bear a heavy payload.
The design was driven by a need to increase the effectiveness of hazardous
material emergency response personnel who are encumbered by their personal
protective equipment (PPE). The legs will ultimately walk, climb stairs, crouch
down, and crawl with the operator while eliminating all external PPE loads on
the operator. The forces involved in the most extreme loading cases were
analyzed to find an effective strategy for reducing actuator loads. The
analysis reveals that the maximum torque is exerted during the transition from
the crawling to standing mode of motion. Peak torques are significantly reduced
by leveraging redundancy in force application resulting from a closed-loop
kinematic chain formed by a particular posture of the XRL. The actuators, power
systems, and transmission elements were designed from the results of these
analyses. Using differential mechanisms to combine the inputs of multiple
actuators into a single degree of freedom, the gear reductions needed to bear
the heavy loads could be kept at a minimum, enabling high bandwidth force
control due to the near-direct-drive transmission. A prototype was fabricated
utilizing the insights gained from these analyses and initial tests indicate
the feasibility of the XRL system.
