Robot Actuators

What an actuator is

An actuator turns energy into motion. In most robots it is an electric motor paired with a gearbox, an encoder that measures position and electronics that control it. Common designs include harmonic drives for precise arms, planetary gearboxes for strong compact joints and linear actuators, often built on roller or ball screws, for pushing and lifting in humanoid legs and arms.

Why actuators matter to buyers

A humanoid can have more than twenty actuators, and they make up a large share of its cost. Their torque sets what a robot can lift, their efficiency sets battery life, and their backdrivability and sensing affect how safely it works near people. When comparing robots, ask about joint torque, heat limits under continuous work and how easily actuators can be replaced.

Who makes them

Actuators come from established motion-control and gear specialists as well as robot makers that build their own joints to cut cost and tune performance. Supply chains for precision gears, motors and screws are concentrated in a few countries, so lead times and tariffs can affect robot prices. Many newer humanoid makers design actuators in-house and source components from specialist suppliers.

Sources and further reading

• ISO 8373:2021 Robotics — Vocabulary — International Organization for Standardization (ISO).
  The ISO glossary of robotics terms, giving agreed meanings for words such as actuator, manipulator and service robot.
• Mini cheetah is the first four-legged robot to do a backflip — MIT News.
  Explains the mini cheetah's 12 low-gear-ratio motors, adapted from drone motors and easy to swap out for repairs.
• The MIT Humanoid Robot: Design, Motion Planning, and Control For Acrobatic Behaviors — MIT Biomimetic Robotics Lab (arXiv).
  MIT design paper on two new proprioceptive actuators for a humanoid, tested on a dynamometer and modelled for acrobatics.

Common questions

Why do humanoid robots cost so much?

Much of the cost sits in the actuators, sensors and batteries. Prices are falling as makers design simpler joints and produce them in larger volumes.

What is a quasi-direct-drive actuator?

A motor with a low gear ratio, which keeps the joint easy to move back by hand and responsive to force. It suits legged robots that need to absorb impacts.