High-Force Soft Robots with Applications in Burrowing

Abstract

Efficient and tube-traversing systems are needed. Hydraulically driven soft robots offer a solution to this problem where the maneuverability of soft robotics and the power density of hydraulic power transmission are both critical. This thesis presents the design, modeling, and manufacturing techniques for developing a multi-segment, hydraulically driven soft robot capable of traversing tubular environments like a burrow. A new force model was developed to address modeling limitations of the traditionally thin-walled, pneumatically-driven McKibben actuator – a common linear soft actuator. Hydraulic contracting and extending actuators were fabricated, and the new model was experimentally validated against commonly used existing models for both actuator versions. In the contracting McKibben experiments, the overall average error for the new model was 9.1% while the overall average error for three commonly used models were 9.9%, 10.5%, and 10.0%. Similar results were reported in the extending McKibben experiments. While the improvement of the new model over the other models is small, it is expected that the new model will be more accurate for high-pressure actuators with thicker walls that are needed in burrowing applications. One contracting actuator was driven at 13.9 MPa – the highest known pressure to date in literature. Further testing showed that extending McKibben actuators follow traditional column buckling theory, and it was demonstrated that extending McKibben actuators can develop extension forces greater than the critical buckling load when operating in a constrained environment such as a burrow. Radially expanding traction actuators were developed to generate the forces needed to anchor the multi-segment robot in the burrow. A new traction force model was developed to predict the generated traction force when actuated in a burrow. Multiple traction actuators were designed, fabricated, and experimentally tested to validate the model. The results of the experiments showed the new model to be a reasonable predictive design tool for the traction actuators. One extending McKibben and two traction segments were combined into a novel multi-segment robot with internal fluid lines allowing for independent actuation of the robot segments - enabling travel in both the forward and reverse directions. Experimental constraints, performance constraints, and performance objectives of the robot design were selected to ensure the robot could generate the forces and motions for efficient travel. A grid-search was used to study the solution space and select an initial geometry for the multi-segment robot within the constraints and objectives of the design. The extending McKibben segment was 76.2 mm long, had a 50.8 mm outside diameter, a 3.2 mm initial wall thickness, and was built with an initial fiber wrap angle of 80 degrees. When pressurized at 207 kPa, the extender could produce up to 206 N of extension force. The two traction segments had the same outside diameter, wall thickness, and operating pressure as the extending segment. The 114.3 mm long segments were able to produce up to 1043 N of traction force. The multi-segment robot was tested in two horizontally oriented tubes of varying diameters to simulate a burrow, and it was able to travel in both directions without issues. The robot was also tested in a vertical tube and was able to travel upwards against gravity. These results demonstrate that the contributions in this thesis provide the framework to develop soft robotic solutions for applications like burrowing where large forces and specific motions are required.