Archive for August 30th, 2011

Stepper Dynamometer: First Light

As a quick test of the stepper dynamometer, I lashed the larger stepper to that Pololu driver hairball, connected one winding of the smaller stepper to the oscilloscope, and recorded open-circuit voltages as a function of rotational speed:

Output voltage vs rps - open circuit

Output voltage vs rps - open circuit

Now, if that isn’t suspiciously linear, I don’t know what is!

The slope is 0.583 v/(rev/s).

I used the scope’s RMS trace calculator, which smushes out the non-sinusoidal nature of the lower speed waveforms. As expected, there are several nasty mechanical resonances that appear in the output waveform while they’re tormenting my ears:

Stepper Resonance - 4.82 rps

Stepper Resonance - 4.82 rps

Top trace is the winding output voltage, bottom trace is the drive input current, plus a line of junk I forgot to turn off.

Useful conversions:

  • Drive waveform frequency / 50 = rev/s
  • Drive waveform frequency * 6/5 = rev/min

So it works. Now I must figure out how to connect load resistors with something more reliable than crappy alligator clips.