A Cable Based Active Variable Stiffness Module With Decoupled Tension

The ability to modulate the physical energetic interactions of a robot with external entities (such as humans, other robots or even the environment) is very important in many application arenas such as cooperative payload transport, haptics, and dynamic walking, etc.. A common approach to handle the modulation of these energetic interactions is to selectively introduce compliance to accommodate the stiffness (or more generally the impedance) mismatch at the physical-interaction interface, i.e. variable stiffness. Variable stiffness modules add significant robustness to mechanical systems during forceful interactions with uncertain environments. Most existing variable stiffness modules tend to be bulky – by virtue of their use of solid components – making them less suitable for mobile applications. In recent times, pretensioned cable-based variable-stiffness modules have been proposed to reduce weight. While passive, these modules depend on significant internal tension to provide the desired stiffness – as a consequence, their stiffness modulation capability tends to be limited. In this paper, we present design, analysis and testing of a cable-based active variable-stiffness module which can achieve large stiffness modulation range with low tension.