Design Considerations and Robustness to Parameter Uncertainty in Wire-Wrapped Cam Mechanisms

TL;DR

This work tackles the safety-vs-payload tension in collaborative robots by developing a 2-DOF wire-wrapped cam static-balancing mechanism and a novel optimization-based cam-design pipeline. The method enforces cam convexity, bounds spring deflections, and reduces sensitivity to spring-constant uncertainty while incorporating a distributed wire-cam friction model. Experimental validation shows the model predicts cam torques within a few hundred Nmm and reveals friction effects are pronounced for less circular cams. Overall, the approach yields manufacturable, safe actuators with torque profiles that closely approximate desired static-balancing torques in practical scenarios.

Abstract

Collaborative robots must simultaneously be safe enough to operate in close proximity to human operators and powerful enough to assist users in industrial tasks such as lifting heavy equipment. The requirement for safety necessitates that collaborative robots are designed with low-powered actuators. However, some industrial tasks may require the robot to have high payload capacity and/or long reach. For collaborative robot designs to be successful, they must find ways of addressing these conflicting design requirements. One promising strategy for navigating this tradeoff is through the use of static balancing mechanisms to offset the robot's self weight, thus enabling the selection of lower-powered actuators. In this paper, we introduce a novel, 2 degree of freedom static balancing mechanism based on spring-loaded, wire-wrapped cams. We also present an optimization-based cam design method that guarantees the cams stay convex, ensures the springs stay below their extensions limits, and minimizes sensitivity to unmodeled deviations from the nominal spring constant. Additionally, we present a model of the effect of friction between the wire and the cam. Lastly, we show experimentally that the torque generated by the cam mechanism matches the torque predicted in our modeling approach. Our results also suggest that the effects of wire-cam friction are significant for non-circular cams.

Figures (11)

• Figure 1: Cam design concept for a 1 DOF system: (a) $\theta = 0$ and (b) $\theta>0$
• Figure 2: Cam design concept for a two DOF system
• Figure 3: Free body diagram of the wire with distributed loads and internal forces.
• Figure 4: Differential forces due to friction between the cam and the wire.
• Figure 5: (a) Friction characterization experimental setup: benchtop vise, cylindrical portion of cam material, wire-rope, basket for holding mass that creates $f_0$, basket for holding mass that creates $f$. (b) Schematic of experimental setup showing terms used in Eq. \ref{['eq:mu']}