I discovered a little detail on the, let's call it tension-guitar, that Ranjit Bhatnagar build. With his project Moonmilk Ranjit is if not the leading, at least one of the most interesting people who build and play self-made instruments. I am not sure, if this technique is also used at his artbot that we reported about.
The tone-height is controlled by a mechanical element, that pulls directly at the string. Anyone, who tried to pull a string like this by hand, will know that you need very much strength to do so. The magnetic-driven coils have to have this power. A microcontroller maps the notes into positions of the coil that holds the string. I think also another design would be possible - with indirect manipulation of the tone-height, but that eventually would maybe harder to design, to build or to control.