Steppers are usually run open-loop and they give excellent accuracy that way. Servomotors are run closed loop because they simply cannot run open loop.
Closed loop operation doesn't confer super-human powers to a motor. Overload a stepper, it stalls; overload a servo, the drive "faults" and shuts down the motor. In both cases the result is the same: a stopped motor when you wanted a running one.
An encoder on a stepper is at best a stall indicator. Your ears or eyes pretty much do the same.
There are a few, expensive drives that do run steppers in true closed loop mode, meaning the encoder commutates the motor. Driven that way, the stepper effectively becomes a 50-pole brushless DC servo motor. The problem is it is at a disadvantage compared to standard 6-pole brushless motors, that being the effects of inductance and iron losses as a result of the high pole-count.
Mariss |