I try to animate a pantograph just for fun, but it doesn'nt work. I have try lot of ideas particularly with the observation of this pages :

https://blender.stackexchange.com/questions/7372/how-can-a-mechanical-linkage-with-a-closed-loop-be-rigged
https://blenderartists.org/t/animating-a-rigid-linkage/685592

The best result i obtain is this one : 

[![Pantograph lowered][1]][1]


  [1]: https://i.sstatic.net/twMWn.jpg

But there is still a mistake, the pantograph bow does not keep a vertical position.

I search the solution to give the blend file in attachment, i come back in few minutes

A few moment later : Here is the file : http://www.wfr.zone/w/blender_animation_images/pantographe_pli%C3%A9.blend

Thank's !