  In this paper, we propose a novel design of a hybrid mobile robot with
controllable stiffness and deformable shape. Compared to conventional mobile
agents, our system can switch between rigid and compliant phases by solidifying
or melting Field's metal in its structure and, thus, alter its shape through
the motion of its active components. In the soft state, the robot's main body
can bend into circular arcs, which enables it to conform to surrounding curved
objects. This variable geometry of the robot creates new motion modes which
cannot be described by standard (i.e., fixed geometry) models. To this end, we
develop a unified mathematical model that captures the differential kinematics
of both rigid and soft states. An optimised control strategy is further
proposed to select the most appropriate phase states and motion modes needed to
reach a target pose-shape configuration. The performance of our new method is
validated with numerical simulations and experiments conducted on a prototype
system. The simulation source code is available at
https://github.com/Louashka/2sr-agent-simulation.git}{GitHub repository.
