Active Elastic Skins for Soft Robotics

Robots are typically designed to perform a finite collection of tasks in a known context. This approach produces efficient solutions when the environment is structured and predictable. However, in many situations---such as space exploration---knowledge of the task to be performed, or the context in which it is to be performed, cannot be known a priori. In this project, we made decisive progress towards soft robots that can be adapted to changing tasks and environments via robotic skins that can wrap around arbitrary soft bodies to induce the desired motions and deformations. Robotic skins may be leveraged to create a multitude of controllable soft robots with different functions or gaits to accommodate the demands of different environments. Robotic skins are modular, conformable 2D sheets with embedded sensing and actuation, which may be applied to, removed from, transferred between, and reoriented on the surface of soft bodies (e.g., inflatables, foams, and limbs) to impart motion onto those bodies. This surface-based approach allows any passive soft object to be turned into an active soft robot. Three principles enable multifunctional robot design with robotic skins: First, distinct motions may be achieved by reorienting a robotic skin on the surface of a soft body. Second, distinct motions may be achieved by wrapping a robotic skin around bodies with different properties and/or morphologies. Third, multiple robotic skins may be used in combination and reconfigured to perform different tasks. We demonstrated sensor-enabled closed-loop control of the robotic skins independent of specific actuator and substrate material choices. We further demonstrated transferability of the robotic skins between soft bodies to accomplish a wide variety of tasks, including an inchworm robot that was controlled either remotely by an operator or with onboard light sensors, a continuum manipulator that grasped and moved objects, an upper-body wearable garment that communicated posture information to a user, and a tensegrity structure that was surface-actuated using the robotic skins. This project included two extensions. Under the first extension, we developed a six-bar tensegrity robot that uses robotic skins to provide actuation and strain sensing capabilities between its nodes (shape estimation), and contact sensors placed at the ends of each constituent rod to detect direct contact with the environment (orientation estimation). In contrast to prior pneumatic tensegrity structures, we placed the pneumatic control hardware onboard the robotic skin-driven tensegrity, proving the viability of robotic skins for on-the-fly design and construction of tensegrity robots that can be untethered and controlled entirely from their surface. Under the second extension, we explored the translation of lessons learned from robotic skins to robotic fabrics. Technical fabrics offer a variety of potential benefits in use cases such as deployable habitats, parachutes, and wearables. Further, a robot that uses fabrics as its core body material can be lightweight, compact, and highly flexible. Ideally, the robot’s actuation, sensing, and structural support are provided by fiber-based components, designed to integrate seamlessly with the fabric’s soft and conformable nature. Variable-stiffness mechanisms are often used for the structural elements, functioning as “bones” that can be turned on and off as needed. However, most variable-stiffness mechanisms are passively rigid, only allowing the fabric to become soft when powered, and many require bulky external air or power supplies, making them untenable for untethered robotics. In the final phase of this ECF, we developed an electrically driven variable-stiffness fiber that can provide a rigid load-bearing structure when powered but remains flexible otherwise. We formalized design principles for pairing this structural component with compatible actuators and characterized their performance in different configurations. The variable-stiffness component can be arranged into sturdy legs, stable enough for a robotic fabric to lift and hold its own battery pack and onboard electronics. We demonstrated this capability by showing off the first-of-its-kind fully untethered locomoting robotic fabric using two different quadruped gaits. Results from this ECF project have transitioned to NASA and industry through 1) A collaborative NASA STTR with the San Francisco-based company Otherlab, which focused on transitioning the elastomer capacitive strain sensors from developed in this project into commercial soft robots; and 2) A collaborative NASA CIF project focused on the idea of using robotic skins to collect and transport in-situ regolith in future Lunar missions.