Actuator

What it means

An actuator is a component of a machine that produces force, torque or displacement when an electrical, pneumatic or hydraulic input is supplied to it. In a humanoid robot, the actuator at a joint sets how much torque that joint can deliver and how quickly it can move.

The actuator is the driving part. The sensors that measure joint position and the controller that decides what each joint should do are separate components.

Actuator types in humanoid robots

Wikipedia's humanoid robot article says that humanoid actuators can be electric, pneumatic or hydraulic, and that electric actuators are the most popular type. It also says hydraulic actuators produce more power than electric or pneumatic ones.

According to Wikipedia, the most popular hydraulic example is the Boston Dynamics Atlas. Boston Dynamics has since announced the retirement of its hydraulic Atlas and unveiled a fully electric Atlas, which it describes as designed for real-world applications.

Unitree's G1 page is an example of the electric designs the article describes as most popular. It specifies permanent-magnet synchronous motors for the robot's joints.

Rotary and linear actuators

Wikipedia distinguishes rotary and linear actuators. An electric rotary actuator uses a rotary motor to turn the target part through an angle, whereas a linear electric actuator produces force along its length. The article notes that rotary motion is more natural for small machines that make large displacements.

A rotating joint can be driven by a rotary actuator directly, or by a linear actuator acting through a linkage. The actuator type is therefore a design choice rather than a fixed rule, and a spec that says 'electric' does not say which kind of motion a joint uses.

Counting and terminology caveats

One joint can have several actuators. Wikipedia's humanoid article says it is common to use multiple electric actuators for a single joint, so the number of motors is not the same as the number of degrees of freedom.

Torque values are peaks for a named joint. A maximum joint torque describes one joint at its peak, not the robot's overall strength, so a high knee figure says little about the hip or the ankle.

'Electric' is a broad label. It covers motors with and without gearboxes and several transmission designs, so the label alone does not tell you how much torque or speed a joint can deliver.

How to read the numbers on humanoidversus.com

The actuation field is one of electric, hydraulic, pneumatic or hybrid, as stated by the maker or a cited source. The max_joint_torque_nm field is the peak torque of one joint, and its note names the joint. A null means the value was not disclosed.

Read actuation together with the robot's status. A discontinued hydraulic platform can still be a useful reference, but it does not describe what is on sale. For background, see Wikipedia's actuator and humanoid robot articles, Boston Dynamics' electric Atlas announcement and Unitree's G1 product page.