Advances in soft robotics, materials science, and stretchable electronics have enabled rapid progress in soft grippers. Here, a critical overview of soft robotic grippers is presented, covering different material sets, physical principles, and device architectures. Soft gripping can be categorized into three technologies, enabling grasping by: a) actuation, b) controlled stiffness, and c) controlled adhesion. A comprehensive review of each type is presented. Compared to rigid grippers, end-effectors fabricated from flexible and soft components can often grasp or manipulate a larger variety of objects. Such grippers are an example of morphological computation, where control complexity is greatly reduced by material softness and mechanical compliance. Advanced materials and soft components, in particular silicone elastomers, shape memory materials, and active polymers and gels, are increasingly investigated for the design of lighter, simpler, and more universal grippers, using the inherent functionality of the materials. Embedding stretchable distributed sensors in or on soft grippers greatly enhances the ways in which the grippers interact with objects. Challenges for soft grippers include miniaturization, robustness, speed, integration of sensing, and control. Improved materials, processing methods, and sensing play an important role in future research.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201707035

Soft Robotic Grippers.

Biology is a useful tool when applied to engineering challenges that have been solved in nature. Here, the emulous goal of creating an insect-sized, truly micro air vehicle is addressed by first exploring biological principles. These principles give insights on how to generate sufficient thrust to sustain flight for centimeter-scale vehicles. Here, it is shown how novel manufacturing paradigms enable the creation of the mechanical and aeromechanical subsystems of a microrobotic device that is capable of Diptera-like wing trajectories. The results are a unique microrobot: a 60 mg robotic insect that can produce sufficient thrust to accelerate vertically. Although still externally powered, this micromechanical device represents significant progress toward the creation of autonomous insect-sized micro air vehicles.

The First Takeoff of a Biologically Inspired At-Scale Robotic Insect

Origami can turn a sheet of paper into complex three-dimensional shapes, and similar folding techniques can produce structures and mechanisms. To demonstrate the application of these techniques to the fabrication of machines, we developed a crawling robot that folds itself. The robot starts as a flat sheet with embedded electronics, and transforms autonomously into a functional machine. To accomplish this, we developed shape-memory composites that fold themselves along embedded hinges. We used these composites to recreate fundamental folded patterns, derived from computational origami, that can be extrapolated to a wide range of geometries and mechanisms. This origami-inspired robot can fold itself in 4 minutes and walk away without human intervention, demonstrating the potential both for complex self-folding machines and autonomous, self-controlled assembly.

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A method for building self-folding machines

Thin sheets have long been known to experience an increase in stiffness when they are bent, buckled, or assembled into smaller interlocking structures. We introduce a unique orientation for coupling rigidly foldable origami tubes in a "zipper" fashion that substantially increases the system stiffness and permits only one flexible deformation mode through which the structure can deploy. The flexible deployment of the tubular structures is permitted by localized bending of the origami along prescribed fold lines. All other deformation modes, such as global bending and twisting of the structural system, are substantially stiffer because the tubular assemblages are overconstrained and the thin sheets become engaged in tension and compression. The zipper-coupled tubes yield an unusually large eigenvalue bandgap that represents the unique difference in stiffness between deformation modes. Furthermore, we couple compatible origami tubes into a variety of cellular assemblages that can enhance mechanical characteristics and geometric versatility, leading to a potential design paradigm for structures and metamaterials that can be deployed, stiffened, and tuned. The enhanced mechanical properties, versatility, and adaptivity of these thin sheet systems can provide practical solutions of varying geometric scales in science and engineering.

https://www.pnas.org/content/pnas/112/40/12321.full.pdf

Origami tubes assembled into stiff, yet reconfigurable structures and metamaterials

Origami, the ancient art of paper folding, has inspired the design of engineering devices and structures for decades. The underlying principles of origami are very general, which has led to applications ranging from cardboard containers to deployable space structures. More recently, researchers have become interested in the use of active materials (i.e., those that convert various forms of energy into mechanical work) to effect the desired folding behavior. When used in a suitable geometry, active materials allow engineers to create self-folding structures. Such structures are capable of performing folding and/or unfolding operations without being kinematically manipulated by external forces or moments. This is advantageous for many applications including space systems, underwater robotics, small scale devices, and self-assembling systems. This article is a survey and analysis of prior work on active self-folding structures as well as methods and tools available for the design of folding structures in general and self-folding structures in particular. The goal is to provide researchers and practitioners with a systematic view of the state-of-the-art in this important and evolving area. Unifying structural principles for active self-folding structures are identified and used as a basis for a quantitative and qualitative comparison of numerous classes of active materials. Design considerations specific to folded structures are examined, including the issues of crease pattern identification and fold kinematics. Although few tools have been created with active materials in mind, many of them are useful in the overall design process for active self-folding structures. Finally, the article concludes with a discussion of open questions for the field of origami-inspired engineering.

https://iopscience.iop.org/article/10.1088/0964-1726/23/9/094001/pdf

Origami-inspired active structures: a synthesis and review

The purpose of this work is to create deployment systems with a large ratio of stowed-to-deployed diameter. Deployment from a compact form to a final flat state can be achieved through origami-inspired folding of panels. There are many models capable of this motion when folded in a material with negligible thickness; however, when the application requires the folding of thick, rigid panels, attention must be paid to the effect of material thickness not only on the final folded state, but also during the folding motion (i.e., the panels must not be required to flex to attain the final folded form). The objective is to develop new methods for deployment from a compact folded form to a large circular array (or other final form). This paper describes a mathematical model for modifying the pattern to accommodate material thickness in the context of the design, modeling, and testing of a deployable system inspired by an origami six-sided flasher model. The model is demonstrated in hardware as a 1/20th scale prototype of a deployable solar array for space applications. The resulting prototype has a ratio of stowed-to-deployed diameter of 9.2 (or 1.25 m deployed outer diameter to 0.136 m stowed outer diameter).

Accommodating Thickness in Origami-Based Deployable Arrays

Flexure joints are widely used to approximate the function of traditional mechanical joints, while offering the benefits of high precision, long life, and ease of manufacture. This paper investigates and catalogs the drawbacks of typical flexure connectors and presents several new designs for highly-effective, kinematically-behaved compliant joints. A revolute and a translational compliant joint are proposed (Figure 1), both of which offer great improvements over existing flexures in the qualities of (1) large range of motion, (2) minimal axis drift, (3) increased off-axis stiffness, and (4) reduced stress-concentrations. Analytic stiffness equations are developed for each joint and parametric computer models are used to verify their superior stiffness properties. A catalog of design charts based on the parametric models is also presented, allowing for rapid sizing of the joints for custom performance. Finally, two multi-degree-of-freedom joints are proposed as modifications to the revolute joint. These include a compliant universal joint and a compliant spherical joint, both designed to provide high degrees of compliance in the desired direction of motion and high stiffness in other directions.Copyright © 2002 by ASME

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Design of large-displacement compliant joints

This paper introduces the benefits of exploiting elasticity in the engineering design of surgical tools, in general, and of minimally invasive procedures, in particular. Compliant mechanisms are jointless mechanisms that rely on elastic deformation to transmit forces and motion. The lack of traditional joints in these single-piece flexible structures offers many benefits, including the absence of wear debris, pinch points, crevices, and lubrication. Such systems are particularly amenable to embedded sensing for haptic feedback and embedded actuation with active-material actuators. The paper provides an overview of design synthesis methods developed at the Compliant Systems Design Laboratory and focuses specifically on surgical applications. Compliant systems have potential to integrate. well within the constraints of laparoscopic procedures and telerobotic surgery. A load-path representation is used within a genetic algorithm to solve two gripper example problems. In addition, the paper illustrates the design and construction of an organ (kidney) manipulator for use in minimally invasive procedures.

Design and Application of Compliant Mechanisms for Surgical Tools

Artemis (Adams-based Rover Terramechanics and Mobility Interaction Simulator) is a software tool developed to simulate rigid-wheel planetary rover traverses across natural terrain surfaces. It is based on mechanically realistic rover models and the use of classical terramechanics expressions to model spatially variable wheel-soil and wheel-bedrock properties. Artemis's capabilities and limitations for the Mars Exploration Rovers (Spirit and Opportunity) were explored using single-wheel laboratory-based tests, rover field tests at the Jet Propulsion Laboratory Mars Yard, and tests on bedrock and dune sand surfaces in the Mojave Desert. Artemis was then used to provide physical insight into the high soil sinkage and slippage encountered by Opportunity while crossing an aeolian ripple on the Meridiani plains and high motor currents encountered while driving on a tilted bedrock surface at Cape York on the rim of Endeavour Crater. Artemis will continue to evolve and is intended to be used on a continuing basis as a tool to help evaluate mobility issues over candidate Opportunity and the Mars Science Laboratory Curiosity rover drive paths, in addition to retrieval of terrain properties by the iterative registration of model and actual drive results.

Simulations of Mars Rover Traverses

Compliant bistable mechanisms are monolithic devices with two stable equilibrium positions separated by an unstable equilibrium position. They show promise in space applications as nonexplosive release mechanisms in deployment systems, thereby eliminating friction and improving the reliability and precision of those mechanical devices. This paper presents both analytical and numerical models that are used to predict bistable behavior and can be used to create bistable mechanisms in materials not previously feasible for compliant mechanisms. Materials compatible with space applications are evaluated for use as bistable mechanisms and prototypes are fabricated in three different materials. Pin-puller and cutter release mechanisms are proposed as potential space applications.

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Brian P. Trease

Papers

Accommodating Thickness in Origami-Based Deployable Arrays

Design of large-displacement compliant joints

Design and Application of Compliant Mechanisms for Surgical Tools

Simulations of Mars Rover Traverses

Bistable Mechanisms for Space Applications.