Stepper Motors and Microstepping
A stepper motor moves in fixed steps, usually 200 per turn, each time its coils are switched. You can count steps to know where it is, with no sensor at all, as long as it never misses one. 3D printers and many small robot axes work this way.
Steps and torque
A typical stepper has 200 full steps per turn, so 1.8° each. Microstepping drives the coils with in-between currents, splitting each step into smaller ones:
The torque a stepper can deliver falls as speed rises, because the coil current has less time to build up each step. This limit is called the pull-out torque curve.
What to notice
Push the speed or the load past the curve and the motor slips. The commanded and actual positions separate, and the controller has no idea, since it is only counting the steps it sent. Microstepping makes motion smoother and quieter, shown by the finer staircase, but does not add torque or true accuracy. Robots that cannot afford missed steps add an encoder, which makes a closed-loop stepper.