Stiffness transition of a soft continuum body is an essential feature for dexterous interaction with an unstructured environment. Softness ensures safe interaction, whereas rigidness generates high force for movement or manipulation. Vacuum-based granular jamming is a widely used technique for on-line stiffness transition because of its high reconfigurability and intuitive driving method. However, vacuum driving method produces limited force levels, and the heavy weight and bulky size are unfavorable for portable applications. In this work, we propose a tendon-driven jamming mechanism for configurable variable stiffness. Compared with a vacuum system, an electric motor-tendon drive system has the benefits of force, bandwidth, size, and weight, but has different force characteristics for distribution, directionality, and transmissibility. In this study, a long snake-like shape is chosen instead of a lump shape for compatibility with tendon-drive characteristics. The snake-like shape is likely to cause buckling under the tendon force as the length increases, making the system extremely unstable. Implanting skeletal disk nodes in the structure is our solution to the buckling phenomenon by maintaining the tendon path in the desired position and for distributing the force evenly, thereby achieving stable stiffness transition capabilities for long free-curved shapes. As a proof of concept, a soft wearable device for wrist support is presented using the proposed variable stiffness mechanism. The weight of the device is 184 g, including the actuators, and it can support 2 kgf. Furthermore, the stiffness transition is completed within 2 s, thus achieving quick responses.
Keywords: continuum structure; granular jamming; tendon-driven.