One, motor encoder or linear scale is still a closed loop system.
But you may increase your problems, to implement scales, especially if you have backlash requires a dual feedback system, an encoder on the motor as well as the scale input, and for a system with backlash, the PID LOOP is divided between the scale and the motor encoder, otherwise you will most likely get 'hunting' problems.
So unless you are sure the machine can be set up this way, you may need a rethink.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.