Power Density (W/kg)

功率密度Advanced

How much power a motor or joint delivers per kilogram, a measure of being light yet strong.

Power density is the power a system can output per unit of mass (sometimes per unit of volume), usually expressed in W/kg. Power equals torque times rotational speed, so power density differs from torque density (N·m/kg, which only measures how much force something can produce) by also accounting for how fast it can spin. Legged robots need short bursts of high power for jumping, running, and getting up after a fall, while their legs must stay light, so joint motors are designed to maximize power density. Ways to raise it include high-performance neodymium-iron-boron magnets, better cooling (such as liquid-cooled joints), and a higher bus voltage. Power density is also used to evaluate batteries, where it should be kept distinct from energy density (Wh/kg), which determines battery life instead.

Also called: Specific Power, Power-to-Weight Ratio
Related: Torque Density · Peak Torque · Joint Actuator Module · Liquid-Cooled Joint Actuators (Active Thermal Management) · Rare-Earth Permanent Magnet (NdFeB) · Torque-Speed Curve
Sources: Power-to-weight ratio - Wikipedia