The ability to regulate the mechanical stiffness in a large range could be extremely important for soft robots to interact with the environment more effectively. In this article, we propose a novel chain-like granular jamming mechanism to achieve a large range of stiffness variation instantly, based on a method that is totally different from existing vacuum-based granular jamming systems. Theoretical modeling is introduced to find the best combination of granules to form the chain-like structure (CLS) and experiments are conducted to demonstrate it. The experimental results indicate that the novel jamming structure is able to achieve a stiffness variation range as large as 50.7 folds. To further validate the effectiveness of the CLS, a soft-rigid hybrid actuator based on the jamming structure is proposed and an integrated fabrication method is provided. Furthermore, an anthropomorphic hand based on the hybrid actuators is developed and the experimental results show that the hand is not only versatile enough to manipulate various objects with different weights, material properties, shapes, and surface characteristics at the soft state, like existing soft grippers, but also can lift heavy objects (1.5 kg in a cylindrical grasping gesture and 3.52 kg in a hook gesture) at the rigid state, which could be difficult for other soft grippers. Finally, the hand is integrated into our homemade service robot, significantly improving the practicability and safety of the robot when serving humans.
Keywords: anthropomorphic hand; granular jamming; soft gripper; soft–rigid hybrid actuator; variable stiffness.