Exploiting Textile Mechanical Anisotropy for Fabric-Based Pneumatic Actuators
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SOFT ROBOTICS Volume 00, Number 00, 2018 ª Mary Ann Liebert, Inc. DOI: 10.1089/soro.2017.0076 Exploiting Textile Mechanical Anisotropy for Fabric-Based Pneumatic Actuators Leonardo Cappello,1,2 Kevin C. Galloway,1,2 Siddharth Sanan,1,2 Diana A. Wagner,1,2 Rachael Granberry,1,2 Sven Engelhardt,1,2 Florian L. Haufe,1,2 Jeffrey D. Peisner,1,2 and Conor J. Walsh1,2 Abstract Knit, woven, and nonwoven fabrics offer a diverse range of stretch and strain limiting mechanical properties that can be leveraged to produce tailored, whole-body deformation mechanics of soft robotic systems. This work presents new insights and methods for combining heterogeneous fabric material layers to create soft fabric-based actuators. This work demonstrates that a range of multi-degree-of-freedom motions can be gen- erated by varying fabrics and their layered arrangements when a thin airtight bladder is inserted between them and inflated. Specifically, we present bending and straightening fabric-based actuators that are simple to manufacture, lightweight, require low operating pressures, display a high torque-to-weight ratio, and occupy a low volume in their unpressurized state. Their utility is demonstrated through their integration into a glove that actively assists hand opening and closing. Keywords: fabric-based actuators, mechanical programming, textile layers, soft robotic glove Objective properties into the body of these structures through geometry and material choice, elastomeric structures can expand in oft actuators produce motion by deforming inher- regions that are less stiff to produce a nearly infinite combi- Sently compliant structures (e.g., elastomers) through power nation of motions.4–6,8,21–24 sources, such as compressed fluids,1–10 shape memory al- However, these capabilities are not unique to elastomeric loys,11–13 cables,14–16 voltage potential drop,17 combustion,18 materials. Textiles also offer a wide range of stretch and strain catalytic decomposition,19 and various combination of these properties that, when strategically utilized, can produce com- methods.20 A common feature of many of these approaches is parably complex motions desired in the design of soft actuators. the reliance on elastomeric materials, such as silicon and ure- In this article, we aim to provide a detailed record of how Downloaded by SCUOLA SUPERIRE SANT' ANNA from www.liebertpub.com at 07/19/18. For personal use only. thane, for the actuator body. These materials offer many ad- three-dimensional (3D) actuators can be constructed from vantages, including heat and chemical resistance, the ability to the two-dimensional (2D), cut-and-sew production methods co-mold multiple materials, and the ability to survive large commonly found in apparel manufacturing. Specifically, we deformations while performing complex ranges of motion. show how bending and straightening motions can be sup- While properties such as material density, stiffness, and ported in a single actuator by varying the arrangement of strength make elastomeric materials ideal for many applica- textile layers that are strained when sewn together and a tions, elastomer-based actuators designed for on-body ap- thin airtight bladder is inserted between them and inflated plications can restrict range of motion due to stiffness and (Fig. 1A). Moreover, we demonstrate how this fabrication material weight. In addition, the relationship between weight method enables material combinations that produce com- and strength can limit size scaling of soft actuator designs. plex, multi-degree-of-freedom actuators. Inflatable soft actuators2–10,21,22 are commonly constructed These fabric-based actuators were empirically character- from elastomeric materials either as monolithic or layered ized to understand how output force and torque depend on structures with embedded chambers. By encoding anisotropic actuator pressure and what range of motion can be achieved 1John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts. 2Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, Massachusetts. 1 2 CAPPELLO ET AL. when unconstrained. In addition, we present a qualitative terials with specific stretch and strain-limiting properties, description of the surface mechanics of the material layers there are uncountable off-the-shelf knit and woven materials that generate bending motion to provide guiding design that exhibit desirable stretch and strain-limiting mechanics. principles for material selection. The material chosen for the bottom layer of the fabric-based Finally, we demonstrate the utility of fabric-based actua- actuator, defined as strain limiting layer, has minimal strain tors by integrating them into an assistive glove that is capable under load. Woven textiles are highly suitable for this layer of actively opening and closing a hand while being nearly because their structures enable minimal mechanical compli- mechanically transparent to the wearer in its unpowered state. ance. Parachutes, seat belts, and air bags are technical appli- cations that utilize wovens for their bidirectional stiffness. From 2D Manufacturing to 3D Actuators We chose a plain weave polyamide textile (Nylon Pack Cloth; Trident) for its high elastic modulus, low weight, Textiles are 2D conformable materials that can be used to drapability, and sewability properties.25 We experimentally produce 3D structures with simple, 2D manufacturing determined the load-strain characteristics of specimens of the methods. Textiles find applications in many fields, including 25 selected woven textile and report the results in Figure 1B apparel, industrial, medical, and civil engineering. This (warp and weft directions) and Supplementary Figure S1 research focuses on knit and woven textiles, schematized in (Supplementary Data are available online at www.liebertpub Figure 1A, which are, for the purpose of this research, con- .com/soro) (bias directions), as well as Supplementary sidered contrasting structures in terms of their mechanical Table S1. The elongation-until-failure in the warp direction compliance when load is applied on grain (i.e., in the x and y was 76.8% at 2009.4 N and 72.2% at 1533.6 N in the weft axes). A woven textile consists of vertical warp yarns inter- direction. Lower elastic moduli were observed along direc- laced with a continuous perpendicular fill, or weft yarn. Al- tions not aligned with the warp and weft, as reported in ternatively, knitted textiles are produced by interlocking Supplementary Figure S1, due to the off-grain mechanical loops composed of a single yarn (i.e., weft knits) or multiple 26 compliance of woven structures. yarns (warp knits). For the top layer, we desire a textile with preferential strain While the strain properties of woven fabrics are primarily along a certain direction, in this case longitudinally, but with dependent on the strain properties of the yarns that compose limited lateral stretch. We chose a commercially available them, the strain properties of knitted fabrics are dependent on warp-knitted raschel polyamide-elastane textile (24710; both yarn and structural compliance. Consequently, a fabric- Darlington), rendered in Figure 1A, for its unidirectional based actuator can achieve complex kinematics when knit and stretch, durability, and stability.25 woven fabrics are combined and layered in such a way that 7,27,28 Knitted materials can exhibit more pronounced anisotropic differential deformations occur within the 3D structure. behavior than their woven counterparts because their looped The wide variety of commercially available textiles with construction creates inherent structural deformation biases different load-strain characteristics makes them a suitable that can be exploited along with the choice of yarn type.25 source material for building anisotropy into the body of the Warp knits are particularly suited for applications that desire actuator. Figure 1A depicts this concept where a thin-film anisotropy because their structures are relatively stiff in one bladder is placed between two layers of textile with different direction and compliant in the other. In addition, warp knits strain properties. Upon pressurization of the bladder, the top are generally balanced knits, meaning their loop structures layer (i.e., high-strain knit) preferentially strains in longitu- are characterized by front-back symmetry.29 While some dinal direction, while the bottom layer (i.e., low-strain wo- weft knits are balanced structures (e.g., garter stitch and rib ven) limits strain in all directions to generate the bending stitch), other weft knits produce asymmetrical bending (e.g., motion rendered in Figure 1C. stockinette stitch). Asymmetry can result in edge curling of The function of the bladder is to hold air while not im- the fabrics, which is challenging for small scale manufacturing peding the actuator’s range of motion. It is purposefully sized of our actuators and makes balanced knit structures prefer- larger than the textile shell such that, under pressurization, able for this application.30 the circumferential and longitudinal stresses carried by the We experimentally tested load-strain characteristics of the bladder material are negligible compared to the textile. Downloaded by SCUOLA SUPERIRE SANT' ANNA from www.liebertpub.com at 07/19/18. For personal use only. chosen textiles accordingly to ASTM D5034-09 standard: we Furthermore, textiles enable the introduction of other geo- cut 76 by 152 mm specimens and proceeded to test