  In this study, we present the Differential Spiral Joint (DSJ) mechanism for
variable stiffness actuation in tendon-driven robots. The DSJ mechanism
semi-decouples the modulation of position and mechanical stiffness, allowing
independent trajectory tracking in different parameter space. Past studies show
that increasing the mechanical stiffness achieves the wider range of renderable
stiffness, whereas decreasing the mechanical stiffness improves the quality of
actuator decoupling and shock absorbance. Therefore, it is often useful to
modulate the mechanical stiffness to balance the required level of stiffness
and safety. In addition, the DSJ mechanism offers a compact form factor, which
is suitable for applications where the size and weight are important. The
performance of the DSJ mechanism in various areas is validated through a set of
experiments.
