The compliance and deformability of soft robotics allow human–machine interactions in a safe manner without the need of sophisticated control systems inherent in rigid-body robotic devices. However, these advantages introduce controllability and predictability challenges. In this study, we propose a novel fluidic-driven variable stiffness revolute joint (VSRJ) based on hybrid soft-rigid approach to achieve adjustable compliance while addressing the abovementioned challenges. The VSRJ is composed of a silicone rubber cylinder as a pressure chamber and two identical rigid links. The soft cylinder is positioned in a fully closed compartment created by the assembly of the two rigid links, thus constraining its expansion when pressure is applied. By applying pressure, the stiffness of the joint increases accordingly for the following reasons: (1) increasing the friction force between the cylinder and the compartment walls and (2) creating a locking mechanism through the expansion of the cylinder into space between rigid links in a “bump” formation. Experimental results show that the VSRJ can achieve up to 8-fold rotational stiffness enhancement from 0 to 5 bar input pressure within −30° to +30° rotation angle. The modular design of the rigid link allows the assembly of multiple VSRJs to build a variable stiffness structure in which each VSRJ has an independent stiffness and relative position. The VSRJ was characterized in terms of repeatability, torque, and stiffness. The experimental results were validated by finite element analysis. This approach can provide opportunities for the use of this new variable stiffness concept as an efficient alternative to traditional variable-stiffness linkages.

中文翻译：

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一种新型变刚度压力控制旋转接头

软机器人的柔顺性和可变形性允许人机交互以安全的方式进行，而不需要刚体机器人设备固有的复杂控制系统。然而，这些优势带来了可控性和可预测性挑战。在这项研究中，我们提出了一种基于混合软刚体方法的新型流体驱动可变刚度旋转接头 (VSRJ)，以在解决上述挑战的同时实现可调节柔顺性。VSRJ由一个作为压力室的硅橡胶圆柱体和两个相同的刚性连杆组成。软缸位于由两个刚性连杆组装而成的完全封闭的隔间中，从而在施加压力时限制其膨胀。通过施加压力，关节的刚度会相应增加，原因如下：(1) 增加气缸和隔间壁之间的摩擦力，以及 (2) 通过将气缸膨胀到“凸块”形式的刚性连杆之间的空间中来创建锁定机制。实验结果表明，VSRJ 可以在 -30° 到 +30° 旋转角内实现高达 8 倍的旋转刚度增强，从 0 到 5 bar 输入压力。刚性连杆的模块化设计允许组装多个 VSRJ 以构建可变刚度结构，其中每个 VSRJ 具有独立的刚度和相对位置。VSRJ 的特点是可重复性、扭矩和刚度。实验结果通过有限元分析得到验证。