Electromagnetic actuators exert small amounts of force, but are simple and definitely have their niche. [SeanHodgins] took a design that’s common in flip-dot displays as well as the lightweight RC aircraft world and decided to make his own version. He does a good job of explaining and demonstrating the basic principles behind how one of these actuators works, although the “robotic” application claimed is less clear.
It’s a small, 3D printed lever with an embedded magnet that flips one way or another depending on the direction of current flowing through a nearby coil. Actuators of this design are capable of fast response and have no moving parts beyond the lever itself, meaning that they can be made very small. He has details on an imgur gallery as well as a video, embedded below.
As we mentioned, flip-dot displays are one example of device that use this method. Compared to things like motors, these actuators may not be capable of exerting much force but have to be quite strong relative to their size. Their simplicity also means they can be made very small, which is why they are among some of the serious tricks used by micro RC airplane builders where even the most aggressively miniaturized of servo actuators are simply not an option.