Design and applications of a pneumatic-tendon coupled air spring (PTCAS) with variable stiffness and self-sensing properties

Abstract

Abstract Integrating variable stiffness elastic elements (VSEEs) into tendon-driven systems significantly enhances intrinsic compliance. However, conventional VSEEs typically employ rigid springs with complex mechanical structures, leading to increased system complexity, weight, and cost. To overcome these limitations, this paper introduces a novel pneumatic-tendon coupled air spring (PTCAS), weighing only approximately 30 g. The proposed PTCAS consists of a bellow-type pneumatic chamber constrained by a rigid frame and integrated tendon-driven connections, exhibiting two distinct stiffness stages: a high-stiffness preloading stage and a more compliant compression stage. By regulating the pre-charged air pressure, the PTCAS achieves a broad stiffness modulation range (up to 8 times variation in the preloading stage) without altering external dimensions. Moreover, its closed air-chamber structure inherently enables self-sensing capabilities by detecting internal air pressure variations, eliminating the need for extra sensing components. Two robotic applications, a robotic joint, and a soft gripper were developed to validate the PTCAS design. Experimental results demonstrate that the robotic joint exhibits versatile capabilities, including variable stiffness interactions, object manipulation, and collision perception, while the gripper provides adaptive compliant grasping with tunable stiffness, supporting objects up to 3.5 kg in a pneumatically locked state and enabling reliable shape sensing during grasping. These findings highlight the significant potential of PTCAS for compact, adaptive robotic systems requiring adjustable stiffness, compliant interaction, and integrated real-time sensing.