Harmonic, cycloidal or planetary: choosing a reducer for a robot joint

The reducer sets much of a joint’s behaviour: precision, stiffness, size and shock resistance. Three technologies are most common in robotics. None is better in absolute terms; the right choice depends on the joint and its duty cycle.

The three families

• Harmonic (strain wave): compact and light, with very low backlash and a high ratio in a single stage. Common in arms and wrists.
• Cycloidal: very stiff and shock tolerant, often considered for loaded joints such as hips and knees.
• Planetary: simple, backdrivable and economical, with a lower ratio per stage. Frequent in so-called quasi-direct-drive actuators.

What to compare

• Rated torque, peak torque and permitted emergency-stop torque.
• Angular backlash and how it is measured.
• Torsional stiffness and tilting stiffness.
• Backdrivability, if the joint must be moved by hand or absorb an impact.
• Stated service life and the load conditions it was calculated under.
• Diameter, length, mass and the hollow shaft for cables.

Questions to ask the manufacturer

• Are datasheet values measured on your test bench or calculated?
• Can you provide a backlash and stiffness curve for the exact reference?
• Which lubricant is used, and what maintenance interval do you recommend?
• What is the minimum order quantity and the lead time for samples?

Describe the joint (load, speed, duty cycle, available space) rather than a part number: a manufacturer can then propose the right technology, and the offers you receive stay comparable.