Characterization of Lightweight, Low-Force Cable and Hydraulic Transmission Systems

Abstract

In the field of rehabilitation robotics and wearable exoskeletons, a common challenge forsystem designers is how to transmit force from the actuators to the joints. In small-scale applications, for the working range of 50-500 N, cables and hydraulics are the two most common ways to transmit force. This study characterized wire rope, braided synthetic line, Bowden cable, and hydraulic transmission types based on their size, weight, efficiency, and controllability. Analytical and experimental methods were used to evaluate individual aspects of each transmission. Analysis was performed to compare the transmission types. The rate at which cables increase in size and weight is approximately linearly with rated load; however, cable construction had the largest influence on the rate of increase. It was observed that cable stiffness can be fit to a 1/L model in the approximate range of 20 to 50 cm, but not for much longer lengths. Hydraulic stiffness was modeled, and it was shown for small diameter actuators the stiffness is comparable to the cables studied. Cable efficiency was studied using the capstan equation and found to be Coulomb friction dependent decreasing as wrap angle or coefficient of friction increased. Bowden cable efficiency is also friction dependent, however Bowden cables do not follow the capstan equation. Over-constrained Bowden cable paths led to more surface contact and decreased efficiency. Hydraulic transmission efficiency is dependent on hose diameter and flowrate. Optimal designs operate at high working pressures and low flowrates. It was shown in a case study that the optimal transmission type is often application dependent.