Design and control of soft truss robots
Keywords: Mechanical Engineering.

The shape-changing ability of truss robots has promising implications for the adaptability of robotic systems to a range of tasks and environments. Truss robots are a class of robots with truss-like structures in which members can change in length to effect global shape change of the robot. Typically, truss robots are networks of interconnected linear actuators joined at universal joints. Simulations of these systems have imagined interesting behaviors such as locomotion through unstructured environments, shrinking in volume to fit in tight spaces, shoring rubble, or conveying information to a human through their form. A number of researchers have been compelled by this truss robotic concept to build physical embodiments. However, these systems have proven difficult to realize. One of the most significant challenges is designing custom linear actuators that have high extension ratios. This is an important characteristic because the extension ratio of the actuators, together with the structural topology, defines the range of shapes that can be assumed. A common approach to high-extension actuation is to cascade multiple low-extension actuators (like a lead screw actuator) together. This has worked at the cost of added complexity but exacerbates the second challenge: resiliency. Due to the rigidity and complexity of many of these actuation solutions, they are often prone to failure. Many of them lack a mechanism to absorb and dissipate energy and will break or jam after exposure to high impact forces. Furthermore, this rigidity can limit truss robots from certain behaviors like safe operation in the presence of people, leveraging mechanical intelligence to reduce control complexity, and direct interaction with objects in the environment. Compliance in truss robots may enable new modes of interaction with the outside world that rigid truss robots are not well suited for. One compelling interaction that can leverage the compliance of a truss robot is the grasping and manipulation of objects. A compliant truss robot can change shape to engulf an object, grasp it between two or more members, and manipulate it within the robot's environment. Similar to soft grippers, the compliance of the structure affords large contact areas with even force distribution, allowing for successful grasping with imprecise open-loop control. In this dissertation, we present methods for high-extension and compliant actuation in truss robots and explore how the compliance can be utilized for unique behaviors. First, we develop a high-extension and compliant pneumatic linear actuator, called the pneumatic reel actuator, that is designed to be used within a truss robot. The actuator consists of an inflated tube that is stored within a reel. As the tubing inflates, the flexible, but mostly inextensible, tubing forms into a cylindrical beam with significantly increased stiffness. As the volume of air inside the actuator rises, more of the tubing is pulled out of the reel to form the beam---lengthening the actuator and storing energy in the springs. Next, the insight gained from the development of that actuator facilitates a shift in our design strategy that enables a full large-scale truss robot with compliant elements capable of operating untethered from a source of compressed air or energy. This robot is composed of inflatable, constant-length tubes that are manipulated by a collective of simple robotic roller modules to form a truss-like structure that can change shape without a pressure source. This shape change can be applied to locomotion, physical interaction with the environment, and the engulfing, grasping, and manipulation of objects. Finally, we explore deeper the grasping and manipulation of objects with these compliant truss robots. The compliance of the members affords large contact areas with even force distribution, allowing for successful grasping even with imprecise open-loop control. We present methods of analyzing and controlling isoperimetric truss robots in the context of grasping and manipulating objects