7 research outputs found
On the Elastic Stability of Folded Rings in Circular and Straight States
Single-loop elastic rings can be folded into multi-loop equilibrium
configurations. In this paper, the stability of several such multi-loop states
which are either circular or straight are investigated analytically and
illustrated by experimental demonstrations. The analysis ascertains stability
by exploring variations of the elastic energy of the rings for admissible
deformations in the vicinity of the equilibrium state. The approach employed is
the conventional stability analysis for elastic conservative systems which
differs from most of the analyses that have been published on this class of
problems, as will be illustrated by reproducing and elaborating on several
problems in the literature. In addition to providing solutions to two basic
problems, the paper analyses and demonstrates the stability of six-sided rings
that fold into straight configurations
Stiffness Change for Reconfiguration of Inflated Beam Robots
Active control of the shape of soft robots is challenging. Despite having an
infinite number of passive degrees of freedom (DOFs), soft robots typically
only have a few actively controllable DOFs, limited by the number of degrees of
actuation (DOAs). The complexity of actuators restricts the number of DOAs that
can be incorporated into soft robots. Active shape control is further
complicated by the buckling of soft robots under compressive forces; this is
particularly challenging for compliant continuum robots due to their long
aspect ratios. In this work, we show how variable stiffness can enable shape
control of soft robots by addressing these challenges. Dynamically changing the
stiffness of sections along a compliant continuum robot can selectively
"activate" discrete joints. By changing which joints are activated, the output
of a single actuator can be reconfigured to actively control many different
joints, thus decoupling the number of controllable DOFs from the number of
DOAs. We demonstrate embedded positive pressure layer jamming as a simple
method for stiffness change in inflated beam robots, its compatibility with
growing robots, and its use as an "activating" technology. We experimentally
characterize the stiffness change in a growing inflated beam robot and present
finite element models which serve as guides for robot design and fabrication.
We fabricate a multi-segment everting inflated beam robot and demonstrate how
stiffness change is compatible with growth through tip eversion, enables an
increase in workspace, and achieves new actuation patterns not possible without
stiffening
Multiple equilibrium states of a curved-sided hexagram:Part I-Stability of states
The stability of the multiple equilibrium states of a hexagram ring with six
curved sides is investigated. Each of the six segments is a rod having the same
length and uniform natural curvature. These rods are bent uniformly in the
plane of the hexagram into equal arcs of 120deg or 240deg and joined at a cusp
where their ends meet to form a 1-loop planar ring. The 1-loop rings formed
from 120deg or 240deg arcs are inversions of one another and they, in turn, can
be folded into a 3-loop straight line configuration or a 3-loop ring with each
loop in an "8" shape. Each of these four equilibrium states has a uniform
bending moment. Two additional intriguing planar shapes, 6-circle hexagrams,
with equilibrium states that are also uniform bending, are identified and
analyzed for stability. Stability is lost when the natural curvature falls
outside the upper and lower limits in the form of a bifurcation mode involving
coupled out-of-plane deflection and torsion of the rod segments. Conditions for
stability, or lack thereof, depend on the geometry of the rod cross-section as
well as its natural curvature. Rods with circular and rectangular
cross-sections will be analyzed using a specialized form of Kirchhoff rod
theory, and properties will be detailed such that all four of the states of
interest are mutually stable. Experimental demonstrations of the various states
and their stability are presented. Part II presents numerical simulations of
transitions between states using both rod theory and a three-dimensional finite
element formulation, includes confirmation of the stability limits established
in Part I, and presents additional experimental demonstrations and
verifications
Multiple equilibrium states of a curved-sided hexagram: Part II-Transitions between states
Curved-sided hexagrams with multiple equilibrium states have great potential
in engineering applications such as foldable architectures, deployable
aerospace structures, and shape-morphing soft robots. In Part I, the classical
stability criterion based on energy variation was used to study the elastic
stability of the curved-sided hexagram and identify the natural curvature range
for stability of each state for circular and rectangular rod cross-sections.
Here, we combine a multi-segment Kirchhoff rod model, finite element
simulations, and experiments to investigate the transitions between four basic
equilibrium states of the curved-sided hexagram. The four equilibrium states,
namely the star hexagram, the daisy hexagram, the 3-loop line, and the 3-loop
"8", carry uniform bending moments in their initial states, and the magnitudes
of these moments depend on the natural curvatures and their initial curvatures.
Transitions between these equilibrium states are triggered by applying bending
loads at their corners or edges. It is found that transitions between the
stable equilibrium states of the curved-sided hexagram are influenced by both
the natural curvature and the loading position. Within a specific natural
curvature range, the star hexagram, the daisy hexagram, and the 3-loop "8" can
transform among one another by bending at different positions. Based on these
findings, we identify the natural curvature range and loading conditions to
achieve transition among these three equilibrium states plus a folded 3-loop
line state for one specific ring having a rectangular cross-section. The
results obtained in this part also validate the elastic stability range of the
four equilibrium states of the curved-sided hexagram in Part I. We envision
that the present work could provide a new perspective for the design of
multi-functional deployable and foldable structures
Physics-aware differentiable design of magnetically actuated kirigami for shape morphing
Abstract Shape morphing that transforms morphologies in response to stimuli is crucial for future multifunctional systems. While kirigami holds great promise in enhancing shape-morphing, existing designs primarily focus on kinematics and overlook the underlying physics. This study introduces a differentiable inverse design framework that considers the physical interplay between geometry, materials, and stimuli of active kirigami, made by soft material embedded with magnetic particles, to realize target shape-morphing upon magnetic excitation. We achieve this by combining differentiable kinematics and energy models into a constrained optimization, simultaneously designing the cuts and magnetization orientations to ensure kinematic and physical feasibility. Complex kirigami designs are obtained automatically with unparalleled efficiency, which can be remotely controlled to morph into intricate target shapes and even multiple states. The proposed framework can be extended to accommodate various active systems, bridging geometry and physics to push the frontiers in shape-morphing applications, like flexible electronics and minimally invasive surgery
Perspective: Machine Learning in Design for 3D/4D Printing
International audienceAbstract 3D/4D printing offers significant flexibility in manufacturing complex structures with a diverse range of mechanical responses, while also posing critical needs in tackling challenging inverse design problems. The rapidly developing machine learning (ML) approach offers new opportunities and has attracted significant interest in the field. In this perspective paper, we highlight recent advancements in utilizing ML for designing printed structures with desired mechanical responses. First, we provide an overview of common forward and inverse problems, relevant types of structures, and design space and responses in 3D/4D printing. Second, we review recent works that have employed a variety of ML approaches for the inverse design of different mechanical responses, ranging from structural properties to active shape changes. Finally, we briefly discuss the main challenges, summarize existing and potential ML approaches, and extend the discussion to broader design problems in the field of 3D/4D printing. This paper is expected to provide foundational guides and insights into the application of ML for 3D/4D printing design
Machine learning and sequential subdomain optimization for ultrafast inverse design of 4D-printed active composite structures
International audienceShape transformations of active composites (ACs) depend on the spatial distribution and active response of constituent materials. Voxel-level complex material distributions offer a vast possibility for attainable shape changes of 4D-printed ACs, while also posing a significant challenge in efficiently designing material distributions to achieve target shape changes. Here, we present an integrated machine learning (ML) and sequential subdomain optimization (SSO) approach for ultrafast inverse designs of 4D-printed AC structures. By leveraging the inherent sequential dependency, a recurrent neural network ML model and SSO are seamlessly integrated. For multiple target shapes of various complexities, ML-SSO demonstrates superior performance in optimization accuracy and speed, delivering results within second(s). When integrated with computer vision, ML-SSO also enables an ultrafast, streamlined design-fabrication paradigm based on hand-drawn targets. Furthermore, ML-SSO empowered with a splicing strategy is capable to design diverse lengthwise voxel configurations, thus showing exceptional adaptability to intricate target shapes with different lengths without compromising the high speed and accuracy. As a comparison, for the benchmark three-period shape, the finite element method and evolutionary algorithm (EA) method was estimated to need 227 days for the inverse design; the ML-EA achieved design in 57 min; the new ML-SSO with splicing strategy requires only 1.97 s. By further leveraging approximate symmetries, the highly efficient ML-SSO is employed to design active shape changes of 4D-printed lattice structures. The new ML-SSO approach thus provides a highly efficient tool for the design of various 4D-printed, shape-morphing AC structures