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Stepper Fundamentals Slide 26

Previously, we learned that the rotor is a permanent magnet that will attempt to align with the magnetic vector produced by the two coils on the stator. For full step, the two coils are energized with equal currents producing a magnetic vector aligned at 45°. For the half step positions only one coil is energized and the magnetic vector is either horizontal or vertical. These two cases are shown in the diagram as the half and full step positions. For microstepping, the rotor needs to be moved to a position between the full step and half step positions. The vector can be aligned to different positions if current is independently controlled in the two windings. For example, if one phase were energized with a current equal to twice the current in the second coil, then the magnetic vector would be along an angle of 26.6°, as shown by the third vector in the diagram. In the limit, for any angle on the circle, the coils could be driven by a current proportional to the sine and cosine of the desired angle. In this way, it is theoretically possible to position the rotor at any desired angle. Of course there are some mechanical friction issues and positioning of the two coils that will give a lower limit to what is achievable.

PTM Published on: 2011-12-14