Design and Application Demonstrations of Variable-Stiffness Dual-Mode Movement Robotic Actuator Based on Low-Melting-Point Alloy and Pneumatic Actuation

Incorporating variable-stiffness components into soft robotic actuators improves environmental adaptability. However, most existing variable-stiffness components are implemented through a single actuation method, allowing only single-mode movement such as telescoping or bending. Drawing inspiration from the biomechanics of sea cucumbers, we design a Variable-Stiffness Dual-Mode Movement Robotic Actuator (VDMRA) based on Low-Melting-Point Alloys (LMPAs) and pneumatic actuation. The VDMRA consists of a pneumatic bellows actuator and multiple variable-stiffness modules integrated on its outer surface. The VDMRA can generate axial contraction in response to input air pressure, while generating bending movement by selectively modulating the stiffness of the variable-stiffness modules. Moreover, the VDMRA can achieve the appropriate stiffness in any movement state through the collaboration of the bellows actuator and variable-stiffness modules, which could further enhance its adaptability to various task requirements. We apply VDMRA to two typical robotic systems: a robotic crawler capable of navigating complex planar terrains and a variable-stiffness robotic arm that enhances stability through stiffness modulation, and experimentally validate its flexible motion capabilities and adaptability to complex environments. This work presents a unified actuator structure which can simultaneously achieve stiffness variation independent of geometric deformation and dual-mode movement, offering a scalable design framework for soft robots.