Torque Density

How much torque a motor or joint produces per unit of weight (or volume).

Torque density is usually expressed in N·m/kg (newton-meters per kilogram), sometimes measured by volume instead. It's a core metric for evaluating motors and joint modules: every joint in a robot has to move all the joints downstream of it, so a heavier joint makes limbs harder to move quickly and nimbly, and drains the battery faster. Ways to raise torque density include using large-diameter outrunner motors, axial-flux motors, better cooling, and adding a reducer to amplify torque — but a larger gear ratio also worsens backdrivability and increases reflected inertia, so legged robots often use a low-ratio, quasi-direct-drive approach to balance the two.

ExampleMIT's Mini Cheetah uses a large-diameter, flat motor with roughly a 6:1 planetary reducer, achieving enough torque in a very light joint — a landmark design for quasi-direct-drive joints.

Also called

Torque-to-Weight Ratio

Related

Joint Actuator Module · Peak Torque · Quasi-Direct Drive · Gear Ratio · Power Density (W/kg) · MIT Mini Cheetah Actuator

Sources

Torque density - Wikipedia