In robotics, rotary hydraulic servo-drives are often used as hydraulic fluid cannot be used for energy storage. As compared to air, fluids are almost incompressible thus giving hydraulic actuators different properties from pneumatic actuators. Also, hydraulic oil cannot be vented into the atmosphere that is why a return path must be provided for the used oil.

A well designed actuator includes oil paths through the actuator shafts. This enables the oil to flow through the actuators from one link to the next, eliminating external hydraulic cables. Servo-drives are controlled by a servo-valve but the output is rotational and requires a different drive mechanism from linear actuators and rotary valve transducer.