The distance will be defined by the sensors so the motor doesn't need to be that accurate but it does need to be as quiet as possible (some sound is fine). The servo motors I have tried have been quite loud. I was thinking a geared DC motor might be better?
Your diagram of the cables will not work. Looks like the cable on the left side is fastened to the cable on the right side. As soon as the fastened position is moved, the curtain will be twisted.Paul
You need to provide the radius or diameter of the drum, some indication of kind of rope/wire and pulleysyou plan to use - then a better idea of necessary torque can be figured out.How fast does the curtain need to move?
Ultrasonic distance sensors are not particularly reliable and WILL lead to malfunctions. The Arduino might also malfunction.I would use simple electrical limit switches as sensors instead, with a safety backup system that will interrupt the motor current if the curtain is moved beyond additional hard limits.
Do you mean Ultrasonic sensors are unreliable in that they will give some inaccurate readings or in that they will break after time?