Brushless DC Motors 101 for Robotics Applications

The motor type inside almost every modern robot joint, and why it replaced brushed motors for this job.

A brushless DC (BLDC) motor replaces the mechanical brushes and commutator of an older brushed motor with electronic commutation — a controller switches current through the stator windings in a sequence timed to the rotor's position, usually tracked by Hall-effect sensors or an encoder.

Why robots almost universally use them now

No brushes means no brush wear, less friction, less electrical noise, and higher achievable speeds and efficiency for a given size and weight. The tradeoff is that they need a more sophisticated (and more expensive) motor controller to handle the commutation electronically, but that cost has fallen enough that it's a non-issue for most robotics applications today.

Outrunner vs. inrunner

An outrunner motor spins its outer housing around a stationary inner stator, giving high torque at low speed and a favorable form factor for direct integration into a joint. An inrunner spins an inner rotor inside a fixed outer housing, favoring higher speeds at lower torque — which is why outrunner-style BLDC motors are more common in the low-speed, high-torque joints found in humanoid limbs.