Variable stiffness modules add significant robustness to mechanical systems during forceful interactions with uncertain environments. Traditionally, 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 modules have been proposed to reduce weight. While passive, these modules depend on significant internal tension to provide the desired stiffness and their stiffness modulation capability tends to be limited. In this paper, we present a planar 2DOF cable robot formed by three active variable stiffness modules that we developed which decouples tension from stiffness. Controlled changes in structural parameters (independent of cable actuation) now permits independent modulation of the perceived stiffness. By varying each module's stiffness, the overall Cartesian stiffness of the robot can be modulated. We show that this approach is more effective than by increasing internal tension only. It is also easier than varying configuration to achieve variable stiffness. Further, thanks to the added active stiffness adjustment motor, it is able to independently vary stiffness and internal tension. Therefore such active module can achieve same Cartesian stiffness as passive modules but with much lower internal tension, which is more efficient. We present the analysis of the system and verified via both simulation and experimental results for the effectiveness of the Cartesian stiffness varying capability of the planar cable robot.
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