Koopman operator theory provides a powerful data-driven technique for modeling nonlinear dynamical systems in a linear framework, in comparison to computationally expensive and highly nonlinear physics-based simulations. However, Koopman operator-based models for soft robots are very high dimensional and require considerable amounts of data to properly resolve. Inspired by physics-informed techniques from machine learning, we present a novel physics-informed Koopman operator identification method that improves simulation accuracy for small dataset sizes. Through Strang splitting, the method takes advantage of both continuous and discrete Koopman operator approximation to obtain information both from trajectory and phase space data. The method is validated on a tendon-driven soft robotic arm, showing orders of magnitude improvement over standard methods in terms of the shape error. We envision this method can significantly reduce the data requirement of Koopman operators for systems with partially known physical models, and thus reduce the cost of obtaining data. More info: https://sunrobotics.lab.asu.edu/blog/2024/ristich-icra-2025/

Perching onto objects can allow flying robots to stay at a desired height at low or no cost of energy. This paper presents a novel passive mechanism for aerial perching onto smooth surfaces. This mechanism is made from a bistable mechanism and a soft suction cup. Different from existing designs, it can be easily attached onto and detached from a surface, but it can also hold a large weight when attached to a surface. Further, the mechanism can still work when the suction cup is not precisely aligned with the surface, alleviating the requirement for precise motion control of flying robots. The attachment and detachment are facilitated by the bistable mechanism, while the strong holding is enabled by a locking mechanism that can disable the bistable mechanism. We conduct experiments to characterize the required forces for successful attachments and detachments. We also equip the perching mechanism onto a quadcopter to demonstrate it can be successfully used for perching onto smooth surfaces (e.g., glass).

Twisted and coiled actuators (TCAs) have recently emerged as a promising artificial muscle for various robotic applications because they are strong, low cost, and customizable. To better facilitate the applications, it is critical to establish general and precise models for different types of TCAs (e.g., self-coiled, free-stroke, conical, etc.). Although several modeling methods have been proposed recently, existing models either fail to capture the nonlinearity during large deformations or cannot model TCAs with nonuniform geometries. In this work, we establish a general framework for modeling TCAs using Cosserat rod theory that can capture the nonlinearity of large deformations and simulate TCAs with nonuniform geometries. Furthermore, we show existing methods are special cases of our general model. Comprehensive statics and dynamics experiments are conducted to verify the proposed model, and the results demonstrate that the model is more accurate than existing ones, especially when a TCA is subject to large deformations. Given the wide applications of TCAs, our general model can help to better design, optimize, and control systems/robots/devices driven by different types of TCAs.

Soft robots demonstrate great potential compared with traditional rigid robots owing to their inherently soft body structures. Although researchers have made tremendous progress in recent years, existing soft robots are in general plagued by a main issue: slow speeds and small forces. In this work, we aim to address this issue by actively designing the energy landscape of the soft body: the total strain energy with respect to the robot's deformation. With such a strategy, a soft robot's dynamics can be tuned to have fast and strong motion. We introduce the general design principle using a soft module with two stable states that can rapidly switch from one state to the other under external forces. We characterize the required triggering (switching) force with respect to design parameters (e.g., the initial shape of the module). We then apply the soft bistable module to develop fast and strong soft robots, whose triggering forces are generated by a soft actuator - twisted-and-coiled actuator (TCA). We demonstrate a soft gripper that can hold weights more than 8 times its own weight, and a soft jumping robot that can jump more than 5 times its body height. We envision our strategies will overcome the weakness of soft robots to unleash their potential for diverse applications.

Traditional soft robots require separate sensors and actuators to precisely control their motion. A twisted-and-coiled actuator (TCA) is a new artificial muscle with both actuation and self-sensing capability that can simultaneously serve both as a sensor and an actuator allowing to control the motion of TCAs without external sensors. This paper investigates the integrated sensing and actuation for TCAs, and the self-sensing function is realized by only measuring the TCA's electrical resistance change. The closed-loop control of a single TCA is realized, and an innervated soft finger that can respond to external load without extra sensors is demonstrated. Our results will lay a foundation for integrated sensing and control by directly using the actuator, paving the way for self-contained smart robotic systems (e.g., untethered soft robots).

Small aerial robots usually face a common challenge: they can only fly for a short time due to their limited onboard energy supply. To tackle this issue, one promising solution is to endow flying robots with perching capability so that they can perch or land on walls, trees, or power lines to rest or recharge. Such perching capability is especially useful for monitoring-related tasks since the robot can maintain a desired height for monitoring without flying. One of the major challenges for perching is to design a light-weight and energy-efficient perching mechanism. In this paper, we present a 3D-printed compliant bistable gripper which is easy to close, stable to hold, and easy to adjust for a palm-size quadcopter to perch on cylindrical objects. If installed on the bottom of aerial robots, it can also be used for aerial grasping. The gripper can be directly activated by the impact force during contact to switch from open state to closed state. It can also hold the quadcopter safely since the required force to open the gripper is larger than the robot weight. We analyze the required forces for closing and opening to provide design guidelines for the mechanism. Experimental results show that the designed gripper can successful make the quadcopter perch on cylinders as well as grasp objects.

Shape morphing, meaning a structure can first morph and then lock into another shape, can be applied to robot designs to endow robots with adaptive morphology for increased functionality and adaptivity. In this paper, we introduce a novel shape morphing scheme enabled by a new artificial muscle: twisted and coiled actuators (TCAs). This new actuator is purely soft, low cost, and electrically driven. Embedding a TCA and a thermoplastic material with variable stiffness into soft materials, we create a miniature shape-morphing link. We also establish a general model to predict the steady-state shape of the link given an input power applied to the TCA. Experiments are conducted to characterize parameters and verify the proposed model. Finally, we demonstrate this shape-morphing link can serve as a link in a mechanism to change the trajectory of its foot or endpoint. We envision that such a new shape-morphing scheme can enable robots to leverage the same mechanical design for different functions.