Haptic Localization with a Soft Whisker from Moment Readings at the Base.

This article focuses on haptic localization of very lightweight and delicate objects while applying a contact force >5000 times lower than the weight of the object. A soft whisker integrated with a Force/Moment (F/M) sensor at the base, and a novel reconstruction algorithm have been proposed for this purpose. Initially, the mathematical relationships between the deformations of the whisker and the F/M sensor outputs were used to reconstruct the shape of the whisker and the position of the touched object. The Cosserat rod theory was used under the assumption that only one contact point occurs during the exploration, and friction effects are negligible. A new methodology we called moment only reading (MOR) has been tested, verified, and compared with previous methods that employed Force and Moment Readings (FMR). Experimental investigations revealed that the spatial position estimation error of the MOR method was confined within 13 mm, when the force applied ranged between 0.001 and 0.01 N. Moreover, the comparison with FMR demonstrated that MOR is capable of retrieving the position of objects even when the force readings drop below the force resolution of the sensor. Eventually, the MOR method has been applied to demonstrate the localization and grasping, with a soft gripper, of delicate crops like tomatoes and strawberries.

Interference Morphology of Free-Growing Tendrils and Application of Self-Locking Structures.

Organisms can adapt to various complex environments by obtaining optimal morphologies. Plant tendrils evolve an extraordinary and stable spiral morphology in the free-growing stage. By combining apical and asymmetrical growth strategies, the tendrils can adjust their morphology to wrap around and grab different supports. This phenomenon of changing tendril morphology through the movement of growth inspires a thoughtful consideration of the laws of growth that underlie it. In this study, tendril growth is modeled based on the Kirchhoff rod theory to obtain the exact morphological equations. Based on this, the movement patterns of the tendrils are investigated under different growth strategies. It is shown that the self-interference phenomenon appears as the tendril grows, allowing it to hold onto its support more firmly. In addition, a finite element model is constructed using continuum media mechanics and following the finite growth theory to simulate tendril growth. The growth morphology and self-interference phenomenon of tendrils are observed visually. Furthermore, an innovative class of fluid elastic actuators is designed to verify the growth phenomena of tendrils, which can realize the wrapping and locking functions. Several experiments are conducted to measure the end output force and the smallest size that can be clamped, and the output efficiency of the elastic actuator and the optimal working pressure are verified. The results presented in this study could reveal the formation law of free tendril spiral morphology and provide an inspiring idea for the programmability and motion control of bionic soft robots, with promising applications in the fields of underwater rescue and underwater picking.

A Recurrent Neural-Network-Based Real-Time Dynamic Model for Soft Continuum Manipulators.

This paper introduces and validates a real-time dynamic predictive model based on a neural network approach for soft continuum manipulators. The presented model provides a real-time prediction framework using neural-network-based strategies and continuum mechanics principles. A time-space integration scheme is employed to discretize the continuous dynamics and decouple the dynamic equations for translation and rotation for each node of a soft continuum manipulator. Then the resulting architecture is used to develop distributed prediction algorithms using recurrent neural networks. The proposed RNN-based parallel predictive scheme does not rely on computationally intensive algorithms; therefore, it is useful in real-time applications. Furthermore, simulations are shown to illustrate the approach performance on soft continuum elastica, and the approach is also validated through an experiment on a magnetically-actuated soft continuum manipulator. The results demonstrate that the presented model can outperform classical modeling approaches such as the Cosserat rod model while also shows possibilities for being used in practice.

Continuous body 3-D reconstruction of limbless animals.

Limbless animals such as snakes, limbless lizards, worms, eels and lampreys move their slender, long bodies in three dimensions to traverse diverse environments. Accurately quantifying their continuous body's 3-D shape and motion is important for understanding body-environment interactions in complex terrain, but this is difficult to achieve (especially for local orientation and rotation). Here, we describe an interpolation method to quantify continuous body 3-D position and orientation. We simplify the body as an elastic rod and apply a backbone optimization method to interpolate continuous body shape between end constraints imposed by tracked markers. Despite over-simplifying the biomechanics, our method achieves a higher interpolation accuracy (∼50% error) in both 3-D position and orientation compared with the widely used cubic B-spline interpolation method. Beyond snakes traversing large obstacles as demonstrated, our method applies to other long, slender, limbless animals and continuum robots. We provide codes and demo files for easy application of our method.

Computational modeling of braided-stent deployment for interpreting the mechanism of stent flattening.

This study develops a computational model of the braided stent for interpreting the mechanism of stent flattening during deployment into curved arteries. Stent wires are expressed using Kirchhoff's rod theory and their mechanical behavior is treated using a corotational beam formulation. The equation of motion of the braided stent is solved in a step-by-step manner using the resultant elastic force and mechanical interactions of wires with friction. Examples of braided-stent deployment into idealized arteries with various curvatures are numerically simulated. In cases of low curvature, the braided stent expands from a catheter by releasing the bending energy stored in stent wires, while incomplete expansion is found at both stent ends (ie, the fish-mouth phenomenon), where relatively little bending energy is stored. In cases of high curvature, much torsional energy is stored in stent wires locally in the midsection of the curvature and the bending energy for stent self-expansion is not fully released even after deployment, leading to stent flattening. These findings suggest that the mechanical state of the braided stent and its transition during deployment play an important role in the underlying mechanism of stent flattening. NOVELTY STATEMENT: This study developed a computational mechanical model of the braided stent for interpreting stent flattening, which is a critical issue observed during deployment into highly curved arteries. Mechanical behaviors of the stent wires are appropriately treated by corotational beam element formulation with considering multiple contacts. We conducted numerical examples of the stent deployment into curved arteries and found that the mechanical state of the braided stent during deployment associated with occurrences of the stent flattening. We believe this finding gives new insight into the mechanism of stent flattening and would advance the design of the stent and its deployment protocol.

Analytical Modeling and Design of Generalized Pneu-Net Soft Actuators with Three-Dimensional Deformations.

Pneu-net soft actuators, consisting of pneumatic networks of small chambers embedded in elastomeric structures, are particularly promising candidates in the society of soft robotics. However, there are few studies on the analytical modeling of pneu-net soft actuators, especially in the three-dimensional space. In this article, based on the minimum potential energy method and the continuum rod theory, we propose an analytical model and corresponding design approach for a class of generalized pneu-net soft actuators (gPNSAs) with both bending and twisting deformations by combining the geometric complexity and material elasticity. We experimentally verify our modeling approach and finally investigate the effects of geometric parameters, material properties, and external force on the deformations of gPNSAs, which can be used as a tool for the design of gPNSAs. We further demonstrate that our developed model can predict the deformations of gPNSAs made of multiple materials.

Multiscale integration of environmental stimuli in plant tropism produces complex behaviors.

Plant tropism refers to the directed movement of an organ or organism in response to external stimuli. Typically, these stimuli induce hormone transport that triggers cell growth or deformation. In turn, these local cellular changes create mechanical forces on the plant tissue that are balanced by an overall deformation of the organ, hence changing its orientation with respect to the stimuli. This complex feedback mechanism takes place in a three-dimensional growing plant with varying stimuli depending on the environment. We model this multiscale process in filamentary organs for an arbitrary stimulus by explicitly linking hormone transport to local tissue deformation leading to the generation of mechanical forces and the deformation of the organ in three dimensions. We show, as examples, that the gravitropic, phototropic, nutational, and thigmotropic dynamic responses can be easily captured by this framework. Further, the integration of evolving stimuli and/or multiple contradictory stimuli can lead to complex behavior such as sun following, canopy escape, and plant twining.

Enhanced models for the nonlinear bending of planar rods: localization phenomena and multistability.

We deduce a one-dimensional model of elastic planar rods starting from the Föppl-von Kármán model of thin shells. Such model is enhanced by additional kinematical descriptors that keep explicit track of the compatibility condition requested in the two-dimensional parent continuum, that in the standard rods models are identically satisfied after the dimensional reduction. An inextensible model is also proposed, starting from the nonlinear Koiter model of inextensible shells. These enhanced models describe the nonlinear planar bending of rods and allow to account for some phenomena of preeminent importance even in one-dimensional bodies, such as formation of singularities and localization (d-cones), otherwise inaccessible by the classical one-dimensional models. Moreover, the effects of the compatibility translate into the possibility to obtain multiple stable equilibrium configurations.

Mechanical Model of Dexterous Continuum Manipulators with Compliant Joints and Tendon/External Force Interactions.

Dexterous continuum manipulators (DCMs) have been widely adopted for minimally- and less-invasive surgery. During the operation, these DCMs interact with surrounding anatomy actively or passively. The interaction force will inevitably affect the tip position and shape of DCMs, leading to potentially inaccurate control near critical anatomy. In this paper, we demonstrated a 2D mechanical model for a tendon actuated, notched DCM with compliant joints. The model predicted deformation of the DCM accurately in the presence of tendon force, friction force, and external force. A partition approach was proposed to describe the DCM as a series of interconnected rigid and flexible links. Beam mechanics, taking into consideration tendon interaction and external force on the tip and the body, was applied to obtain the deformation of each flexible link of the DCM. The model results were compared with experiments for free bending as well as bending in the presence of external forces acting at either the tip or body of the DCM. The overall mean error of tip position between model predictions and all of the experimental results was 0.62±0.41mm. The results suggest that the proposed model can effectively predict the shape of the DCM.