US Patent 9731416 Legged robot passive fluid-based ankles with spring centering

An example robot includes a first hydraulic actuator cylinder connecting a first member to a second member, where the first hydraulic actuator cylinder comprises a first piston and a first chamber. A second hydraulic actuator cylinder connects the first member to the second member, where the second hydraulic actuator cylinder comprises a second piston and a second chamber. A valve system controls hydraulic fluid flow between a hydraulic supply line of pressurized hydraulic fluid, the first and second chambers, and a return line. A controller is configured to determine a gait state of the robot, and based on the determined gait state, provide a signal to the valve system.