Having closed loop on steppers is really a waste of time even if it could be done, when a stepper loses position it is because it is not capable of producing the torque required at that point and it stalls, trying to make it catch up would just stall it all the more.
It could be argued that it could be slowed so it can recover but then things get very complex as you then have to also slow any other axis that is moving at the time and recalculate the trajectory.
If you want closed loop get servos.
As Terry mentioned encoders/scales can be used to monitor position and report errors and throw an E-Stop if it occurs but all that is telling you on a stepper system is your motors are not up to the job you are asking of them so its kind of pointless I would say.
Where encoders may be usefull is if you disable the drives on a regular basis as when you do that both servos and steppers will move, steppers will jump to the next nearest full step, servos dont have any holding torque when the drives are disable so it will depend on the machine and the forces applied to an axis as to how much it will move when a drive is disabled. If you have encoders then you can monitor the position and compare to machine coords and when enabling the drives you can again update the machine coords from the encoder position without having to rehome. Whether that is useful to you will depend on how often you have the drives disabled in a day.
Hood
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