Would love some feedback on this automated bench press spotter project

Retroplayer

We thought of creating a variable force depending on how much effort the user was putting in but we didn't really know how to apply a variable force without having something constantly being attached to the bar. The goal is to still provide the user free range of motion, hence why we are using the arms that raise when the user needs help.

How would you apply a variable force like what you speak of without having something touching the bar at all times?