The only thing keeping me from buying any of the existing 3D printers or CNC milling machines is the absence of a feedback mechanism. For the same price, I reckon I should be able to make one with a feedback mechanism. For the time being, I am stuck with deciding on a rotary encoder with suitable gear ratio or a linear encoder for feedback. Any pointers anyone?
If all you care about is knowing if your stepper missed a step, ignore all of the following and put a encoder on your stepper;
Assuming you are going to do a true servo control loop on the axis and you care about bidirectional accuracy and repeatbility:
1. Rotary encoder with gearing is the worst. the backlash in the geartrain will add a large amount of position error on top of #2 below. Also will make servo tuning to a fine level next to impossible.
2. Rotary encoder directly on the leadscrew or pulley of a belt system. Still has the play or reversal error of the leadscrew/nut or pulley/belt combination.
3. linear encoder is the best. It measures the position directly and bypasses most all mechanical innacuracies of the axis. (there are still a huge number of things to consider to get the most out of the linear encoder feedback). It does not mean you can have a sloppy mechanical system. You will never get the servo loop tuned if the mechanics are sloppy. You may also need to have a separate motor encoder for closing the velocity loop of your control system independent of the positional control loop that is closed by the linear encoder.