  Industrial manipulators do not collapse under their own weight when powered
off due to the friction in their joints. Although these mechanism are effective
for stiff position control of pick-and-place, they are inappropriate for legged
robots that must rapidly regulate compliant interactions with the environment.
However, no metric exists to quantify the robot's performance degradation due
to mechanical losses in the actuators and transmissions. This paper provides a
fundamental formulation that uses the mechanical efficiency of transmissions to
quantify the effect of power losses in the mechanical transmissions on the
dynamics of a whole robotic system. We quantitatively demonstrate the intuitive
fact that the apparent inertia of the robots increase in the presence of joint
friction. We also show that robots that employ high gear ratio and low
efficiency transmissions can statically sustain more substantial external
loads. We expect that the framework presented here will provide the fundamental
tools for designing the next generation of legged robots that can effectively
interact with the world.
