Modulating the stiffness of soft actuators is crucial for improving the efficiency of interaction with the environment. However, current stiffness modulation mechanisms are hard to achieve high lateral stiffness and a wide range of bending stiffness simultaneously. Here, we draw inspiration from the anatomical structure of the finger and propose a bi-directional tunable stiffness actuator (BTSA). BTSA is a soft-rigid hybrid structure that combines air-tendon hybrid actuation (ATA) and bone-like structures (BLS). We develop a corresponding fabrication method and a stiffness analysis model to support the design of BLS. The results show that the influence of the BLS on bending deformation is negligible, with a distal point distance error of less than 1.5 mm. Moreover, the bi-directional tunable stiffness is proved to be functional. The bending stiffness can be tuned by ATA from 0.23 N/mm to 0.70 N/mm, with a magnification of 3 times. The addition of BLS improves lateral stiffness up to 4.2 times compared with the one without BLS, and the lateral stiffness can be tuned decoupling within 1.2 to 2.1 times (e.g. from 0.35 N/mm to 0.46 N/mm when the bending angle is 45 deg). Finally, a four-BTSA gripper is developed to conduct horizontal lifting and grasping tasks to demonstrate the advantages of BTSA.

翻译：调节软体执行器的刚度对于提高与环境交互效率至关重要。然而，现有刚度调节机制难以同时实现高横向刚度和宽范围弯曲刚度。本文受手指解剖结构启发，提出了一种双向可调刚度执行器（BTSA）。BTSA是一种软硬混合结构，结合了气动-肌腱混合驱动（ATA）和类骨骼结构（BLS）。我们开发了相应的制造方法和支持BLS设计的刚度分析模型。结果表明，BLS对弯曲变形的影响可忽略不计，远端定位误差小于1.5毫米。此外，双向可调刚度功能得到验证：弯曲刚度可通过ATA从0.23 N/mm调节至0.70 N/mm，调节倍率达3倍；相较于无BLS结构，BLS的加入使横向刚度提升高达4.2倍，且横向刚度可在1.2至2.1倍范围内实现解耦调节（例如，当弯曲角度为45度时，从0.35 N/mm调节至0.46 N/mm）。最后，开发了四BTSA夹爪以执行水平提升和抓取任务，验证了BTSA的优势。