Inverse Design of Soft Robotic Actuators using Nonlinear Finite Element Modeling

Abstract

The field of soft robotics has empowered robots to maneuver, traverse, and complete tasks where traditional rigid robots fall short. These robots are able to bend continuously and conform to their environments which makes their designs inherently safe. This makes soft robots a suitable candidate for use in medical devices. This thesis explores an inverse soft robot design algorithm, with possible future applications to a soft catheter robot. Soft robot techniques were used to create a large scale prototype of a hydraulically powered, serial, soft catheter robot. The locomotion section of this robot consisted of three fiber reinforced elastomeric enclosures (FREE) actuators connected by passive valves. When controlled properly, the locomotion section was able to ‘inchworm’ through a tube, thus demonstrating the feasibility of a serially controlled catheter. Although the FREE actuators were able to produce locomotion in the tube, the limitation of realizable actuator shapes severely hampered the robot's performance. This limitation motivated the need for a generalized design tool where the user could dictate the desired actuator shapes. To accomplish the additional design freedom, an inverse problem was explored. First, a mathematical description of cylindrical actuator shapes was developed. Allowing a user to create arbitrary actuator shapes that deformed from an initial state to a final state. Next, a nonlinear inverse Finite Element Modeling optimization algorithm was developed to reconstruct the material properties when the boundary conditions and internal pressure were known. The inverse algorithm was tested on three cylindrical actuator motions. The first was a ballooning actuator which expanded uniformly in every direction. The second was a bending actuator capable of rotation constrained to a single plane. The third was a twisting actuator that rotated along its axis in a nearly pure shear translation, transforming a pressure input into out of plane motion. The material properties of all three actuator motions were successfully reconstructed with the developed inverse algorithm. The reconstructed twisting actuator was then 3D printed with a multi-material polyjet 3D printer and experimentally shown to match the twist of both the ground truth design and simulated results. This provided some initial validation of the inverse algorithm.