Archive for September 11th, 2011

Stepper Dynamometer: Microstepping Mode vs. Torque

Conventional wisdom has it that stepper motor torque decreases as the number of microsteps per full step increases. One reasonably careful measurement trumps a kilo-opinion, so here’s a chart (clicky for bigger) of measurements to mull over:

Microstepping Mode vs Speed - 17PM-J034

Microstepping Mode vs Speed - 17PM-J034

Each group of like-colored dots marks the results for full, 1/2, 1/4, 1/8, and 1/16 microstepping with the same load resistance. The caret marks the full-step data point within each group. The load resistances range from a dead short (about 1 Ω due to winding resistance) on the left to 50 Ω on the right.

While I’ll grant the existence of the occasional data-collection error, it’s pretty obvious that:

  • Torque is reasonably constant regardless of microstepping mode
  • Full-step mode doesn’t produce more torque and, indeed, produces considerably less under heavy loads

Now, one can argue that the A4988 doesn’t operate in real full-step mode, because it energizes both windings at 1/√2 of the maximum current setting for each full step rather than energizing a single winding at the maximum current. That may be true, but conventional wisdom seems to not bother with such details when opining about torque, either…

As nearly as I can tell, 1/8 microstepping gives as much torque as you’re likely to get from the motor, while having reasonably smooth motion that avoids exciting mechanical resonances.

That chart (or one remarkably like it) will appear in an upcoming Circuit Cellar column. The tonnage of data supporting those dots suggests building an automated dynamometer would be a Good Idea …