Most of the high-performance robots built to date employ hydraulic actuation. Examples such as BigDog, LS3, HyQ, and Atlas enhance the potential of this actuation technology. Hydraulics has many properties that make it an ideal choice for highly dynamic articulated robot applications:
Higher power-to-weight ratio;
The centralized power source (pump + tank) can provide power for several actuators. This permits to distribute better the actuation weight, keeping the robot base heavier and the links lighter;
The hydraulic transmission, characterized by the fluid, is stiffer than electric drives transmissions (e.g. gear box), resulting in the possibility to have higher closed loop gains, greater accuracy, and better frequency response;
Hydraulic actuators are mechanically very simple and allow for robust design against impacts and overload. This is very important in dynamic applications, where high peak forces on the robot structure cannot be avoided and are even part of the requirements
Hydraulic actuators (e.g. valves) guarantee high enough actuator bandwidth to achieve near perfect torque control in the spectrum of interest for dynamic locomotion of medium to large scale robots. This provides legged robots, such as HyQ, the compliance needed for legged locomotion by using a naturally very stiff actuation system;
The hydraulic fluid serves also as coolant of the actuators.
The HyQ leg prototype consists of two links and it has three actuated degrees of freedom (DOF): two in the hip (abduction/adduction and flexion/extension) and one in the knee (flexion/extension). Both flexion/extension DOF are actuated by small-sized hydraulic components: an asymmetric hydraulic cylinder (Hoerbiger LB6 1610 0080) driven by high-performance electro-hydraulic servovalves (Moog E024).