A wrist-inspired suspended tubercle-type tensegrity joint with variable stiffness capacity.

In complex and unpredictable environments or in situations of human-robot interaction, a soft and flexible robot performs more safely and is more impact resistant compared to a traditional rigid robot. To enable robots to have bionic features (flexibility, compliance and variable stiffness) similar to human joints, structures involving suspended tubercle tensegrity are researched. The suspended tubercle gives the joint compliance and flexibility by isolating two moving parts. The variable stiffness capacity is achieved by changing the internal stress of tensegrity through the simultaneous contraction or relaxation of the driving tendons. A wrist-inspired tensegrity-based bionic joint is proposed as a case study. It has variable stiffness and two rotations with a total of three degrees of freedom. Through theoretical derivation and simulation calculation in the NASA Tensegrity RobotToolkit (NTRT) simulator, the range of motion, stiffness adjustable capacity, and their interaction are studied. A prototype is built and tested under a motion capture system. The experimental result agrees well with the theoretical simulation. Our experiments show that the suspended tubercle-type tensegrity is flexible, the stiffness is adjustable and easy to control, and it has great potential for bionic joints.