Hardware

Torque Density

Torque density is the torque an actuator produces per unit mass or volume, a headline figure of merit for robot joint design. High torque density traditionally comes from high gear ratios, which add friction, backlash, and reflected inertia, whereas quasi-direct-drive designs pair large-diameter, high-pole-count motors with low gear ratios to retain backdrivability and impact robustness, an approach popularized by the MIT Mini Cheetah and now standard in quadrupeds and emerging humanoids.

Why it matters for physical AI

Actuator torque density sets the payload, speed, and dynamism achievable at a given robot weight, and progress here is a primary hardware bottleneck for affordable, capable humanoid platforms.