  We present a new open-source torque-controlled legged robot system, with a
low-cost and low-complexity actuator module at its core. It consists of a
high-torque brushless DC motor and a low-gear-ratio transmission suitable for
impedance and force control. We also present a novel foot contact sensor
suitable for legged locomotion with hard impacts. A 2.2 kg quadruped robot with
a large range of motion is assembled from eight identical actuator modules and
four lower legs with foot contact sensors. Leveraging standard plastic 3D
printing and off-the-shelf parts results in a lightweight and inexpensive
robot, allowing for rapid distribution and duplication within the research
community. We systematically characterize the achieved impedance at the foot in
both static and dynamic scenarios, and measure a maximum dimensionless leg
stiffness of 10.8 without active damping, which is comparable to the leg
stiffness of a running human. Finally, to demonstrate the capabilities of the
quadruped, we present a novel controller which combines feedforward contact
forces computed from a kino-dynamic optimizer with impedance control of the
center of mass and base orientation. The controller can regulate complex
motions while being robust to environmental uncertainty.
