Locking Force and Stiffness Oriented Design for an SMA-Actuated Miniaturized Lockable Prismatic Joint

Lockable mechanisms offer significant advantages for robotic systems, such as enabling effective energy management, motion reconfiguration, and stiffness adjustment. Crucially, when unlocked, these mechanisms allow the robot’s intended motion to proceed unimpeded. Upon locking, however, they enable motion reconfiguration and provide substantially enhanced load-bearing capacity (with increased stiffness). This capability allows them to be seamlessly integrated into existing robotic systems. In this study, we propose a novel lockable prismatic (P) joint that is modular, miniaturized, and capable of high load-bearing with high stiffness, based on compliant mechanisms and shape memory alloy (SMA) actuators. We first detail the joint's working principle and identify critical design parameters governing its locking performance and stiffness. Subsequently, we present an optimized design framework, illustrated with two design cases. Experimental validation confirms the joint's functionality, achieving a locking force of up to 180 N and a locked-state axial stiffness of 1400 N/mm. Furthermore, we demonstrate the joint’s practical utility through its application in a motion-reconfigurable, snake-like robotic arm with a compact design space and multiple motion modes. The arm can navigate into confined spaces like wing boxes using diverse motion modes and can lock into a high-stiffness configuration for stable end-loaded operations. Collectively, this research illuminates a pathway towards utilizing smart materials and compliant mechanisms to create high-performance lockable P joints, providing a locking and motion reconfiguration solution that is easy to design and use for robots of different sizes and load-carrying capabilities.