Stepper motors, and how a machine loses its place

Open-loop motion, missed steps, and why the machine never finds out.

A stepper motor keeps track of where it is only by counting how many small steps it has been told to take, never by checking where it actually ended up, so if anything stops one of those steps from happening, a jammed belt, a knock, or too much speed asked of it, the motor's own count of its position simply carries on as if nothing happened, wrong from that point onward.

A print nine hours in shifted sideways by four millimetres and carried on regardless, laying the rest of the model neatly onto a wall of plastic that had no business being there, the shift only obvious once the print finished and stood on the table as two mismatched halves stacked on top of each other.

The mechanism behind a stepper motor

Inside a stepper motor, a ring of magnetic poles is energised in a fixed sequence, and each step in that sequence pulls the shaft around by one small, fixed angle before stopping to wait for the next pulse. The motor does not sense its own position with any kind of internal ruler, it only knows how many pulses it has been sent since the machine last knew for certain where it was, and it assumes, because it has no way to check otherwise, that every pulse it was sent actually turned the shaft by the expected amount. If the load on the shaft is briefly too great, if something catches or jams for even a fraction of a turn, the shaft can fail to keep up with a pulse or several of them, and the motor has absolutely no way of noticing that failure, because nothing in the system was ever watching the shaft itself, only counting the instructions sent to it. A motor turning freely and a motor that has just lost several steps send back exactly the same signal to the rest of the machine, which is silence, because there was never a signal coming the other way to begin with.

The dark-room comparison

Walking heel-to-toe across a dark room with the eyes shut, counting each pace to keep track of how far you have gone, works perfectly as long as every pace lands exactly where it was meant to. Catch a foot on the edge of a rug and stumble half a pace sideways, and nothing about the stumble itself tells you your count is now wrong, you simply carry on counting from a position you believe is correct but no longer is, confident all the way to the far wall. A stepper motor that misses a step is in exactly that position: nothing in the sequence of pulses that follows carries any information about the stumble, so the motor's internal count, and everything the machine does based on that count, stays wrong until something outside the system corrects it.

The one number worth remembering

A typical stepper motor divides one full turn of its shaft into around two hundred individual steps, each only slightly less than two degrees of rotation, which is precise enough that a single missed step is often too small to see on its own, but a print running for hours can accumulate many thousands of pulses on each axis, and it only takes one of those pulses failing to land for the machine's belief about its own position to drift permanently away from where it actually is.

What follows from this

Because the motor cannot tell the difference between a step that succeeded and one that did not, the responsibility for avoiding missed steps falls entirely on keeping the load within what the motor can reliably deliver, moving no faster than it can keep up with, and not asking it to fight a jam or a friction spike it cannot overcome. Once a step is lost there is nothing in an open-loop system built to notice, and the error simply becomes a permanent part of everything the machine does from that point on, carried forward as confidently as every step that came before it.