  This paper presents a robotic system where the gear-ratio of an actuator is
dynamically changed to either leverage or attenuate the natural load dynamics.
Based on this principle, lightweight robotic systems can be made fast and
strong; exploiting the natural load dynamics for moving at higher speeds (small
reduction ratio), while also able to bear a large load through the attenuation
of the load dynamics (large reduction ratio). A model-based control algorithm
to automatically select the optimal gear-ratios that minimize the total
actuator torques for an arbitrary dynamic state and expected uncertainty level
is proposed. Also, a novel 3-DoF robot arm using custom actuators with two
discrete gear-ratios is presented. The advantages of gear-shifting dynamically
are demonstrated through experiments and simulations. Results show that
actively changing the gear-ratio using the proposed control algorithms can lead
to an order-of-magnitude reduction of necessary actuator torque and power, and
also increase robustness to disturbances.
