Glossary · Robotics and control
Actuator
Also called motor, actuated joint
Anything that makes a robot or simulated body move, such as a motor at a joint.
An actuator turns a command into motion. In robots it might be an electric motor, a hydraulic piston or an artificial muscle. In simulation it’s a rule for how much force or torque to apply to a joint, given the controller’s command.
A joint with an actuator is actuated. A joint without one is passive: it moves only when pushed, like a hinge.
Kinds of command
| actuator type | command | applies |
|---|---|---|
| motor | torque | the commanded torque |
| position servo | target angle | torque toward the target (see position servo) |
| velocity servo | target speed | torque toward the speed |
| muscle | activation 0–1 | pulling force that depends on length and speed |
Underactuation
A floating body like a human or a humanoid robot is underactuated: it has joints with motors, but nothing directly pushes the pelvis around in the world. It can only move its whole body by pushing on the floor through its feet. That’s exactly what makes balance hard: when the feet can’t push in the needed direction, the body falls.
In this project
The muscular body has 83 actuators (43 of them driven by the policy; the rest hold the fingers in a fist). The simple body v2 has 37. All are position servos.