Architecture-based comparison of hydraulic power units in quadrupeds, hexapods, and monopods: A systematic review

Robots have increasingly become integral in enhancing human life, with growing demands for robots with high payload-to-weight ratios and dynamic capabilities like running and jumping. Hydraulic actuation, renowned for its high power-to-weight ratio, is a critical enabler of these capabilities. However, its effectiveness depends significantly on the specific architecture employed. This review systematically examines centralized hydraulic power units (cHPUs) and electro-hydraulic actuators (EHAs) in legged robots, specifically quadrupeds, hexapods, and monopods, focusing on power density, weight, joint power distribution, and application suitability. Following PRISMA guidelines, we analyzed over 35 hydraulic-legged robots from 70+ articles, finding that while cHPUs offer greater power, their efficiency is reduced by the weight and leakage of components like valves and hoses. In contrast, EHAs demonstrated a superior power-to-weight ratio due to their self-contained, hoseless design. Our analysis suggests that cHPUs are better suited for a few closely positioned joints, while EHAs for powering multiple joints over larger distances. Based on empirical data from manufactured robots, this research highlights EHAs’ advantages in power density optimization. It recommends future studies explore additional metrics like power-to-volume ratios and cost-effectiveness to refine hydraulic architecture selection for robotic systems.