I don't think this is doable. If you hold a DC motor in position with no power to it, it won't hold position. If you supply power to it, it warms up and eventually burn out the coil. They are simply not meant to hold position. Unless you have a very large screw, and turn the screw to change position, you won't be able to hold position. Stepper motors are designed differently to step and hold.
I must go to feedback and correct motor position if arm, and for the head I must use a mechanical solution like:
What I would like to know:1. some google key word to search for the right mechanism,
There's nothing stopping you from putting a feedback loop on your motor control