Things from necessity: a linear servo. Used in real life for anything that needs to do heavy lifting. Or, as i need it, for making humanoid robots (aka androids).
The .scad file can be used as a library. function linearServo() requires 2 parameters: modelnr and position.
model numbers are as follows:
0 : 25mm max moving distance
1 : 50mm max moving distance
2 : 100mm max moving distance
3 : 150mm max moving distance
The actual thing can be found here amongst others.