  While much work has been done recently in the realm of model-based control of
soft robots and soft-rigid hybrids, most works examine robots that have an
inherently serial structure. While these systems have been prevalent in the
literature, there is an increasing trend toward designing soft-rigid hybrids
with intrinsically coupled elasticity between various degrees of freedom. In
this work, we seek to address the issues of modeling and controlling such
structures, particularly when underactuated. We introduce several simple models
for elastic coupling, typical of those seen in these systems. We then propose a
controller that compensates for the elasticity, and we prove its stability with
Lyapunov methods without relying on the elastic dominance assumption. This
controller is applicable to the general class of underactuated soft robots.
After evaluating the controller in simulated cases, we then develop a simple
hardware platform to evaluate both the models and the controller. Finally,
using the hardware, we demonstrate a novel use case for underactuated,
elastically coupled systems in "sensorless" force control.
