ironbot:
If I command motor (still not by PID) to keep on a position, say, if potx turn left, the noisy movement is not only coming from the fact that I didn't use a PID, but also from the pot being a <1$, too noisy type.
Then you might want to think about a rotary encoder, but not the type specifically made for motor control. Instead there are the type made for dials, knobs, and other turnable human input devices, you can find those from $0.50 to $5.00 a piece (higher priced ones exist of course, if you could spend that kind of money you might as well get one made to work with a motor). The price will vary with due to a few different things; how many bits in the code (which determines the maximum precision), whether they are optical or mechanical (which probably won't matter in your application, but mechanical are cheaper), and overall quality of construction (i.e. how many tens or hundreds of thousands of rotations before they wear-out). Digikey, for example, has thousands different of models...