Soft pneumatic actuators (or pneumatic artificial muscles, PAM) have found a wide variety of applications in robotics and adaptive structures.  These actuators typically consist of an elastomeric tube covered by a layer of reinforcing helical fibers.  They are light-weight, robust, and safe to operate close to human operators.   Moreover, one can tailor the performance of these actuators with considerable freedom by carefully designing the orientations of their reinforcing fibers.  Therefore, the fiber-reinforced pneumatic actuators have been increasingly popular. However, these actuators suffer from a critical disadvantage: cumbersome fabrication procedures.  Wrapping long helical fibers around a cylindrical tube would require either expensive equipment (such as mesh braiding or filament winding machine) or excessive manual labor even with the help of 3D printed molds.  Such complexity also increases the uncertainty in the actuator performance.

To address this issue, we propose a new approach towards designing and constructing pneumatic actuators by exploiting the principle of Kirigami, the ancient art of paper cutting.   More specifically, we used "Kirigami skins" — plastic sleeves with carefully arranged slit cuts — to replace the layer of reinforcing helical fibers.  These Kirigami cuts impart anisotropic elasticity to the skin, much like the helical fibers.  Therefore, when the inner tube is pressurized, the Kirigami skin would deform non-uniformly to create the desired actuation motion.  We showed that the fabrication procedure of cutting and wrapping Kirigami skins (using a commercial plotter cutter) is far more efficient and repeatable than winding helical fibers.  Also, one can accurately prescribe the actuation performance — in terms of free stroke, block force, and deformation pattern — by customizing the cut pattern. This customization can show a high accuracy because of the simplicity of cutting.

In this paper, we will discuss our initial investigations on a set of linear extension actuators that have Kirigami skins with a uniform array of cross-shaped, orthogonal cuts.  When under internal pressurization, the rectangular-shaped facets defined by these cuts would rotate and introduce negative Poisson's ratio to the skin.  This auxetic property of the kirigami skin is critical for creating the desired actuation.  That is, when the initially cylindrical-shaped Kirigami skin with negative Poisson's ratio elongates longitudinally, its enclosed volume will increase to accommodate the pressurization.  Based on this insight, we conduct a parametric study through experimentation to establish the correlation among Kirigami design parameters, Poisson's ratio, and the corresponding actuation performance.  THere, the Kirigami design parameters include the density of orthogonal cuts with respect to the actuator size, the length ratio of horizontal cuts over vertical cuts, as well as the length ratio of horizontal cuts and the gaps between cuts. The results of this study can lay down the foundation for constructing more capable Kirigami-skinned soft actuators that can achieve a pre-programmable combination of extension, bending, and twisting motions.


Pneumatic Extension Actuators With Kirigami Skins

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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