  The Extra Robotic Legs (XRL) system is a robotic augmentation worn by a human
operator consisting of two articulated robot legs that walk with the operator
and help bear a heavy backpack payload. It is desirable for the Human-XRL
quadruped system to walk with the rear legs lead the front by 25% of the gait
period, minimizing the energy lost from foot impacts while maximizing balance
stability. Unlike quadrupedal robots, the XRL cannot command the human's limbs
to coordinate quadrupedal locomotion. Using a pair of Rimless Wheel models, it
is shown that the systems coupled with a spring and damper converge to the
desired 25% phase difference. A Poincar\'e return map was generated using
numerical simulation to examine the convergence properties to different coupler
design parameters, and initial conditions. The Dynamically Coupled Double
Rimless Wheel system was physically realized with a spring and dashpot chosen
from the theoretical results, and initial experiments indicate that the desired
synchronization properties may be achieved within several steps using this set
of passive components alone.
