  Robotic legs have bimodal operations: swing phases when the leg needs to move
quickly in the air (high-speed, low-force) and stance phases when the leg bears
the weight of the system (low-speed, high-force). Sizing a traditional
single-ratio actuation system for such extremum operations leads to oversized
heavy electric motor and poor energy efficiency, which hinder the capability of
legged systems that bear the mass of their actuators and energy source. This
paper explores an actuation concept where a hydrostatic transmission is
dynamically reconfigured using valves to suit the requirements of each phase of
a robotic leg. An analysis of the mass-delay-flow trade-off for the switching
valve is presented. Then, a custom actuation system is built and integrated on
a robotic leg test bench to evaluate the concept. Experimental results show
that 1) small motorized ball valves can make fast transitions between operating
modes when designed for this task, 2) the proposed operating principle and
control schemes allow for seamless transitions, even during an impact with the
ground and 3) the actuator characteristics address the needs of a leg bimodal
operation in terms of force, speed and compliance.
