[Hendrikus]

bbp,

Your table of torque versus microstep resolution is somewhat misinterpreted. Holding torque is defined as the torque a motor exerts when its shaft is displaced +/- 1.8 degrees from its rest position.

The relationship between a stopped motor's torque versus its shaft angle is sinusoidal over the span of 1 full step and your table reflects this relationship.

Let's say you have a stopped 300 in-oz (about 2.1 Nm) motor. You apply a torque load that displaces the shaft 1/10 of a step (0.18 degrees) from its unloaded rest position. The applied torque equals 300 in-oz times sine (90 / 10) or 47 in-oz (1/3 Nm).

This is different than holding torque. Remove the 47 in-oz load and the motor returns to its original rest position. Exceed the 300 oz-in holding torque of the motor and it will jump to an adjacent pole location 7.2 degrees (4 full steps) away. It will not return to the original location when the load is removed.

What this means practically is the motor will always be 0.18 degrees behind where you think it is with a continuous 47 in-oz load and it will be 1.8 degrees behind if you apply a continuous 300 in-oz load. Exactly the same numbers apply for a full-step drive as well. Remove the load and the motor will spring to the zero error location whether it's being microstepped or full-stepped.

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Things get more interesting at higher speeds. The motor's inductance introduces a 90 degree phase lag in winding current versus driving voltage. This results in an unloaded motor being 1.8 degrees behind where you think it is at higher speeds and 3.6 degrees behind when it's loaded to just short of stalling. This behavior is drive-independent and the error is "reeled-in" when the motor slows down again.

Mariss

The situation that you described for current vs voltage waveform, is not valid if the motor drive controls directly the current in the windings, by applying any voltage on the windings needed to maintain the wanted current (at any time). Most drivers will try to control current instead of voltage.