  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.
