Search for: All records

Abstract Collaborative robots, or cobots, have been developed as a solution to the growing need for robots that can work alongside humans safely and effectively. One emerging technology in robotics is the use of Discrete Variable Stiffness Actuators (DVSAs), which enable robots to adjust their stiffness in a fast-discrete manner. This enables cobots to work in both low and high stiffness modes, allowing for safe collaboration with human workers or operation behind safety barriers. However, achieving good performance with different stiffness modes of DVSAs is a challenging problem. This paper proposes a method to provide force control of a DVSA by exploiting the dynamic model and the discrete stiffness levels. The two-mass dynamic model, a widely accepted model of flexible systems, is used to model and analyze the DVSA. The proposed method involves using Gain-scheduling and Deterministic Robust Control (DRC) controllers as modelbased control algorithms for the DVSA to achieve high-precision force control. We also conducted a comparison with the commonly used proportional integral derivative (PID) control algorithms. The paper presents a detailed analysis of the dynamic behavior of the DVSA and demonstrates the effectiveness of the proposed control algorithms through simulation with different scenario comparisons, even in the presence of external disturbances. 

Abstract A safety-critical measure of legged locomotion performance is a robot's ability to track its desired time-varying position trajectory in an environment, which is herein termed as “global-position tracking.” This paper introduces a nonlinear control approach that achieves asymptotic global-position tracking for three-dimensional (3D) bipedal robots. Designing a global-position tracking controller presents a challenging problem due to the complex hybrid robot model and the time-varying desired global-position trajectory. Toward tackling this problem, the first main contribution is the construction of impact invariance to ensure all desired trajectories respect the foot-landing impact dynamics, which is a necessary condition for realizing asymptotic tracking of hybrid walking systems. Thanks to their independence of the desired global position, these conditions can be exploited to decouple the higher-level planning of the global position and the lower-level planning of the remaining trajectories, thereby greatly alleviating the computational burden of motion planning. The second main contribution is the Lyapunov-based stability analysis of the hybrid closed-loop system, which produces sufficient conditions to guide the controller design for achieving asymptotic global-position tracking during fully actuated walking. Simulations and experiments on a 3D bipedal robot with twenty revolute joints confirm the validity of the proposed control approach in guaranteeing accurate tracking. 

Abstract Current stretchable conductors, often composed of elastomeric composites infused with rigid conductive fillers, suffer from limited stretchability and durability, and declined conductivity with stretching. These limitations hinder their potential applications as essential components such as interconnects, sensors, and actuators in stretchable electronics and soft machines. In this context, an innovative elastomeric composite that incorporates a 3D network of liquid metal (LM), offering exceptional stretchability, durability, and conductivity, is introduced. The mechanics model elucidates how the interconnected 3DLM architecture imparts softness and stretchability to the composites, allowing them to withstand tensile strains of up to 500% without rupture. The relatively low surface‐to‐volume ratio of the 3DLM network limits the reforming of the oxide layer during cyclic stretch, thereby contributing to low permanent strain and enhanced durability. Additionally, the 3D architecture facilitates crack blunting and stress delocalization, elevating fracture resistance, while simultaneously establishing continuous conductive pathways that result in high conductivity. Notably, the conductivity of the 3DLM composite increases with strain during substantial stretching, highlighting its strain‐enhanced conductivity. In comparison to other LM‐based composites featuring 0D LM droplets, the 3DLM composite stands out with superior properties.