Anisotropic Stiffness and Programmable Actuation for Soft Robots Enabled by an Inflated Rotational Joint

Abstract

Soft robots are known for their ability to perform tasks with great adaptability, enabled by their distributed, non-uniform stiffness and actuation. Bending is the most fundamental motion for soft robot design, but creating robust, and easy-to-fabricate soft bending joint with tunable properties remains an active problem of research. In this work, we demonstrate an inflatable actuation module for soft robots with a defined bending plane enabled by forced partial wrinkling. This lowers the structural stiffness in the bending direction, with the final stiffness easily designed by the ratio of wrinkled and unwrinkled regions. We present models and experimental characterization showing the stiffness properties of the actuation module, as well as its ability to maintain the kinematic constraint over a large range of loading conditions. We demonstrate the potential for complex actuation in a soft continuum robot and for decoupling actuation force and efficiency from load capacity. The module provides a novel method for embedding intelligent actuation into soft pneumatic robots.

Figures (8)

• Figure 1: (a) The proposed inflated rotational joint features a bending plane with low stiffness. Bending motion will occur within the soft plane only unless the load causes buckling along the stiff plane. (b) An example application of the multiple joints, in which the relative stiffness and bending constraint of the joints enables defined kinematics and motion sequence.
• Figure 2: The load and axial tension in (a) isotropic inflated beam and (b) anisotropic inflated beam. (c) Definition o the tensioned and wrinkled regions' locations shown in a section view on the $z-y$ plane.
• Figure 3: (a) The partially wrinkled condition can be enforced by shortening a piece of thin-filmed tubing by folding and using fiber-reinforced tape as a length constraint. (b) Testing setup for restoring moment verification. (c),(d) Restoring moment at various tip displacements, with the shaded region representing one standard deviation uncertainty. (e) Ratio of the restoring moment values along the planes for each axially tensioned region width, $\Delta\theta$. (f) Stiffness values along the stiff and soft bending planes for $\Delta\theta=\pi/4$ as a function of pressure.
• Figure 4: Experimental setup for tendon actuation. Motors control position and tension of the tendon, which is measured by a tension transducer. Eight motion capture markers track position and pressure is kept constant.
• Figure 5: Resulting behavior from tendon actuation of anisotropic and isotropic units. Behaviors are categorized by unit type and primary bending direction. (a) Position of tip throughout the experiment in the XY plane, showing highly clustered bending in the soft plane for the anisotropic joint, compared to widely varied direction in the isotropic joint. (b) Tendon tension as a function of servo motor angle from $0^{\circ}$ to $180^{\circ}$ and bending behavior.