Stepper motors are motor
s designed so that they may be held in a fixed position as well as being rotated bidirectionally in fixed increments (steps). Most stepping motors can be "stepped" at audio
frequencies, allowing them to spin quickly; with an appropriate controller, they may be started and stopped "on a dime" at controlled orientations. The precision makes them popular in CAD
, and floppy drive
To get them to "step", anywhere from two to five (or more) coils are energized in a specific order.
(1) Known "step" allows precise positioning: multiply # of steps and the stepper's "step angle" (generally 0.9 to 1.8 degrees/step, although the outliers are extreme) and you know how much the shaft has rotated. In theory.
(2) Although on the surface the sequencing of the coils is a big pain, there are many many chips that will do all the heavy lifting for you: just hook up the (often 40+ volt) hefty coil voltage, then give the chip a pulse and direction signal (and a heat sink). If you guess the right coil sequence, away it goes.
(0) Under some conditions (high load, fast stepping, insufficient coil currents), the shaft can miss a few steps, causing consternation in those using the steps for exact positioning. Slipping can quickly demagnetize your motor, killing the specs that you designed around without you being any the wiser: lower torque means more slippage means lower torque.
(1) In addition to "slipping", the repeatability of positioning done with a stepping motor depends on the geometry of the motor rotor. Tolerances are pretty good, but if you are milling something, you quickly notice than some steps are not exactly the same as others, and (like gear backlash) can vary with forces on the shaft.
(1.5) Sure, you can "half-step" or take microsteps by monkeying with the current, but pretty much you have to go expensive (or use gears) if you want really tiny steps.
(2) They are not very power efficient: unless you are clever and cut the power back while you are stock-still, you consume lots of power all the time. This is worsened by the usual low tech approach for more torque, which is to throw some power resistors upstream from the coil. (sounds bizarre, but through the magic of inductance, it works).
Mo' : http://www.cs.uiowa.edu/~jones/step/