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

TL;DR

This paper introduces a novel wire-wrapped cam mechanism with an optimization design method for static balancing in collaborative robots, enhancing safety and payload capacity while addressing parameter uncertainties and friction effects.

Contribution

The paper presents a new 2-DOF static balancing mechanism and an optimization-based design approach ensuring convexity, spring limits, and robustness to parameter deviations.

Findings

The cam mechanism's torque matches model predictions.

Friction effects are significant in non-circular cams.

The design improves safety and payload capacity tradeoffs.

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.