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Microstepping

Oct
28
by Jeff Kordik

In our last post, we learned about step motor holding torque and pullout torque when full stepping. There was a time when full stepping was the only affordable way to drive a step motor, but advances in processing and sensing have made it possible to divide the typical hybrid step motors 1.8 degree full steps into much smaller steps. Why would we want to do that? Smaller steps improve smoothness and accuracy. Let's revisit the torque versus displacement curves of a step motor being driven in full step. Holding torque and pullout torque are noted.

Holding torque pullout torque

Full stepping the motor only allows us to apply full current to our choice of motor windings, so the resulting torque curves are fixed at 90 degrees apart. The equations governing the torque for each phase are:

full step torque formula

This chart shows the just phases A and B with the stable equilibrium noted for each. The distance between full steps is 90 degrees.

full step torque curves

The magic of microstepping is that we can precisely control the current in each winding and thereby more precisely control torque and position. If we want to divide each full step into 10 microsteps, we would use this formula to determine the first microstep past Phase A:

microstep formula

Let's see what that looks like on a chart. Note that the step positions are now just 9 degrees apart. By microstepping, we have improved by position resolution by an order of magnitude. microstep torque curve

But wait, there's more! Remember what the plot of full step dynamic torque looked like in our last post?

pullout_torque

While the motor is turning, the torque varies from 1.00 to 1.414. That's 41% torque ripple, which results in significant speed variation. Now let's plot all 40 torque curves that can be commanded using 1/10 microstepping:

microstep torque curves

That's a lot of curves to choose from, all very close together. As a result, the dynamic torque when microstepping look like this; the torque ripple is reduced to just 0.3%. Microstep torque ripple

Switching from full step to 1/10 microstepping has improved our position resolution by 10X and reduced the torque ripple by 138%. Microstepping drivers are commonly available with resolution as high as 1/256 of a full step. So remember: friends don't let friends full step.

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