Cycloidal Drives vs. Harmonic Drives in Robot Joints

Two different ways to get a big reduction ratio out of a small package, with different failure modes.

Cycloidal drives use an eccentric cam to roll a disc with lobes around an outer ring of pins, producing high reduction with high shock tolerance — a mechanically simpler idea than a harmonic drive's flexing spline, and generally more robust to sudden overload.

Where each wins

Harmonic drives tend to win on backlash (near zero) and compactness for a given reduction ratio, which matters in tightly packed joints. Cycloidal drives tend to win on shock tolerance and cost at larger sizes, which is part of why you'll see them more in heavier-duty industrial and some legged-robot applications.

Neither is strictly better — the choice usually comes down to which failure mode a designer would rather engineer around: backlash and precision, or shock loading and cost.