Guest editorial introduction to the focused section on design and control of hydraulic robots

The papers in this special section focus on the design and control of hydraulic robots. Since the 1980s hydraulic robots have been a key technology allowing to push the state of the art in robotics by demonstrating new levels of performance in terms of rough terrain locomotion, balance, speed, dexterity, and robustness. Inspired by these results, an increasing number of academic groups are (re-)discovering hydraulic actuation for applications where high power density, robustness, and high control bandwidth are crucial requirements. The ultimate goal for legged robots is agility comparable to bi-/quadrupedal animals (walking, running, jumping), which requires high-performance control of the hydraulic actuators and sufficient power-to-weight ratio provided by the actuators. In addition to these advanced high-tech hydraulic robots, hydraulic actuation has been used for decades in a variety of mobile (off-highway) heavy-duty machines (e.g., construction, forestry, mining, and agricultural machines) due to their higher robustness and significantly larger power-to-weight ratio compared with electric actuators. It is highly expected that the development in robotics will revolutionize this heavy-duty machine industry just as is currently happening in the car industry.