Soft and shape morphing robots driven by twisted-and-coiled actuators

2022 Summer.Includes bibliographical references.Soft robots are a new type of robot with deformable bodies and muscle-like actuation, which are fundamentally different from traditional robots with rigid links and motor-based actuators. Owing to their elasticity, soft robots outperform rigid ones in safety, maneuverability, and adaptability. With their advantages, many soft robots have been developed for manipulation and locomotion in recent years. To enable their unique capabilities, soft robots require a key component—the actuator. Many different actuators have been used, including the conventional pneumatic-driven and cable-driven methods, as well as several novel approaches proposed recently such as combustion, dielectric elastomers, chemical reactions, liquid–vapor transition, liquid crystal elastomer, and shape memory alloy. Besides existing actuation approaches, another promising actuator for soft robots is the twisted-and-coiled actuator (TCA). Compared with existing actuation methods, TCAs exhibit several unique characteristics: like large energy density and being directly actuated by electricity with a small voltage. All of these characteristics will potentially enable small-scale and untethered soft robots that in general are difficult to be accomplished by pneumatic and tendon-driven methods. Further, unlike shape memory alloys, TCAs are intrinsically soft, making it possible to embed them in any shape inside a soft body to generate versatile motion. To better actuate soft robots with TCAs, we introduce a novel fabrication technique of contraction TCAs that will have uniform initial gaps between neighboring coils. In this case, they can contract larger than 48% without a preload, termed free stroke. We also characterize such a TCA and compare it with self-coiled TCAs. Besides the free stroke property, the TCA can also be directly used as a sensor that provides its displacement information. To better design, optimize, and control TCAs for various applications, we developed a physics-based model based on TCAs' physical parameters as opposed to system identification methods, since such physics-based models are expected to be a general model for different types of TCAs (self-coiled, free-stroke, conical) We demonstrate soft robots with programmable motions by placing TCAs in different shapes inside a soft body. Specifically, we embed TCAs in a curved U shape, a helical shape, and straight shapes in parallel to enable three different motions: two-dimensional bending, twisting, and three-dimensional bending. We also combine the three motions to demonstrate a completely soft robotic arm that mimics a human forearm. A model is also developed to simulate the TCA-driven soft robots. The framework can model 1) the complicated routes of multiple TCAs in a soft body and 2) the coupling effect between the soft body and the TCAs during their actuation process. When not actuated, a TCA in the soft body is an antagonistic elastic element that restrains the magnitude of the motion and increases the stiffness of the robot. By stacking several modules together, we simulate the sequential motion of a soft robotics arm with three-dimensional bending, twisting, and grasping motion. The presented modeling and simulation approach will facilitate the design, optimization, and control of soft robots driven by TCAs or other types of artificial muscles. Finally, we design shape morphing robots that can morph the shape of their bodies to adapt to a different environment. These robots can be built with shape-morphing modules. A shape-morphing module has a variable stiffness element that allows it to switch between soft and rigid states. While it is in a soft state, it can morph to different configurations driven by TCAs. We demonstrate robots built with these modules can morph to different shapes that facilitate grasping and locomotion