A Scalable Monolithic 3D Printable Variable Stiffness Mechanism

Variable Stiffness Mechanisms (VSM) are be- coming ubiquitous in mechatronics given the benefit they provide in terms of safety and performance. Despite these assets, VSMs remain fairly complex mechanical devices lacking in compactness, ease of manufacturing and accessibility. In addition, the scarcity of commercially available VSMs requires that such systems are mostly designed in-house. We propose a new type of VSM that improves on the pre-existing Jack Spring concept by making it more compact and robust. The new concept, which we refer to as the Compact Modifier of Active Coils (C-MAC) mechanism, is specifically designed to be manufactured through a monolithic 3D print. This approach enables to modify a minimal set of design features, namely the spring diameter and the coil diameter, to achieve the desired range of stiffness variation. We test the proposed design on six configurations; these show hysteretic energy losses no larger than 35% over the stiffness variation and confirm stiffness to scale according to theory. Stiffness ranging from 0.15 N/mm to 1.02N /mm were measured for an overall device length of 140 mm, including a maximal stroke length of 22 mm. The results confirm excellent scalability and manufacturability of the proposed design, providing a versatile mechanism for fast prototyping and the development of entire 3D printed robotic systems embedding variable stiffness capabilities.