The two-degree-of-freedom Cable Pulley (2DCP) transmission system: An under-actuated and motion decoupled transmission for robotic applications

Transmission systems with the ability to decouple the motion of several degrees of freedom (DoFs) can provide a solution for the coupling problem that arises between the joints of a kinematic chain when following a remote actuation approach. Additionally, they can be especially interesting in wearable robotic applications, such as exoskeletons, which benefit from the implementation of kinematic redundancy to compensate for joint misalignment and guarantee full human-robot kinematic compatibility. This paper presents the conceptual development and experimental validation of a novel cable-pulley system, the two-degree-of-freedom Cable-Pulley (2DCP) transmission, that, in contrast to common systems, allows to change the transmission angle between its input and output ports. The 2DCP includes one coupled DoF: the input and output port rotations, kinematically linked by the implemented transmission ratio, and a passive, decoupled, DoF that allows to change their relative orientations, i.e. the transmission angle. The concept, working principle, design equations, main mechanisms and prototype construction of the 2DCP are first presented in this paper. Experiments are finally conducted to characterize and model the mechanical properties of the transmission, confirming both experimentally and statistically its ability to transmit and re-direct the input power to the output port in a direction that can change over time without power loss, effectively decoupling both parameters.