  Impressive animal locomotion capabilities are mediated by the co-evolution of
the skeletal morphology and muscular properties. Legged robot performance would
also likely benefit from the co-optimization of actuators and leg morphology.
However, development of custom actuators for legged robots is often expensive
and time consuming, which discourages roboticists from pursuing performance
gains afforded by application-specific actuator optimization. This paper
presents open-source designs for two quasi-direct-drive actuators with
performance regimes appropriate for an 8--15 kg robot, built completely with
off the shelf and 3D-printed components for less than $200 USD each. The
mechanical, electrical, and thermal properties of each actuator are
characterized and compared to benchmark data. Actuators subjected to 420k
strides of gait data experienced only a 2% reduction in efficiency and 26 mrad
in backlash growth, demonstrating viability for rigorous and sustained research
applications. We present a thermal solution that nearly doubles the
thermally-driven torque limits of our plastic actuator design. The performance
results are comparable to traditional metallic actuators for use in high-speed
legged robots of the same scale. These 3D printed designs demonstrate an
approach for designing and characterizing low-cost, highly customizable, and
highly reproducible actuators, democratizing the field of actuator design and
enabling co-design and optimization of actuators and robot legs.
