Glossary · Hardware
Harmonic drive
In brief
A harmonic drive is a high-ratio, zero-backlash gear reduction commonly used in robot joints. It uses a flexible gear that meshes against a rigid one as it deforms, achieving 50:1 to 200:1 ratios in a compact package. Trade-off: rigidity (which kills back-drivability) and high cost.
Harmonic drives have been the default high-precision robot reducer for decades. They are compact, lightweight relative to their reduction ratio, and have effectively zero backlash. The downsides are non-trivial: they are expensive, they have meaningful internal friction, and crucially they are not back-drivable — you cannot push the joint and have the motor turn easily.
For humanoids, that lack of back-drivability is increasingly a liability. Compliant, force-controlled motion is hard with a harmonic-drive joint. Many recent designs (Tesla Optimus, several Chinese platforms) have moved to planetary-roller-screw or quasi-direct-drive solutions for the most heavily loaded joints.
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