IEEE Robotics & Automation Magazine - September 2023 - 20

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Lightweight Untethered Morphing Robots
By Autumn Pratt, Patrick Wilcox,
Caroline Hanson, and Robert F. Shepherd
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ORIGAMI IMAGE ©SHUTTERSTOCK.COM/RUNRUN2
Flexible and shape-shifting structures are common in nature.
Creating shape-shifting machines using rigid components
requires complex design work. Soft robotic attempts are closer to
the muscular hydrostats they mimic but are slow and inefficient
untethered and do not generally allow large-scale shape change
between forms. Here we present the idea of " tensegristat " robots
in which 1) we use tendons drawn by electric motors as active
tensile elements, and fluid as a passive compressive element; 2)
we rely primarily on buckling for motion and shape change; and
(3) the total mass of fluid in the robot remains constant. This
structure is analogous to that of muscular hydrostats and allows
large-scale shape changes, separate control of stiffness and
motion, and fast and efficient untethered operation. We demonstrate
this idea with a morphing amphibious robot.
INTRODUCTION
The world is full of shape-shifting organisms. Consider the
tongue: our ability to speak relies on the many different
shapes it can take. Other shape shifters navigate through small
spaces (like the octopus, which can pass through any hole that
Digital Object Identifier 10.1109/MRA.2022.3204234
Date of current version: 28 September 2022
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IEEE ROBOTICS & AUTOMATION MAGAZINE SEPTEMBER 2023
its beak can fit through), dynamically update their 3D camouflage
(like the cuttlefish), and protect vulnerable components
(as the snail does by retracting its eyestalks inside its body).
Theoretically, we should be able to build machines with similar
abilities.
In this article, we use the definition for shape-changing
robots given by Shah et al.: " robots that actively change their
shape to adapt to their environment or gain new functionalities "
[1]. Despite the prevalence of deployable structures (e.g., umbrellas
and inflatable airplane escape slides), there are few robots
that change between separately functional forms. The scarcity
of shape-changing robots is likely because it is difficult to reuse
material effectively. A " Swiss army knife " robot would always
be burdened by significant unused mass. Most currently existing
shape-shifting robots change shape by rearranging rigid components.
In origami robots, all the pieces are flat, which simplifies
the geometrical problem of reconfiguration [2]. Other robots are
made of separable mobile modules that can attach themselves
into numerous forms [3]. The drawback with rearranging generally
rigid parts for shape change is that reusing the materials
effectively between forms requires complex geometrical design.
An alternative to rigid components is using fluid to support
compressive loads, and membranes to support tensile loads.
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IEEE Robotics & Automation Magazine - September 2023

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