Archive for August, 2011

Monthly Aphorism: On Complexity

  • When faced with a problem you don’t understand, do any part of it you do understand, then look at it again.

Heinlein, of course: The Moon is a Harsh Mistress

Yes, this algorithm can stall you at a local maximum, but that’s better than remaining stuck in the starting gate while you’re thinking too much.

In less high-falutin’ terms: Don’t just sit there, do something!

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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.

6 Comments

Antenna Decoration

Dragonfly on antenna

Dragonfly on antenna

This dragonfly decided that the tip of the 2 m / 70 cm antenna on Mary’s bike was the best  place around to survey the area; it periodically zipped off to snag a meal, then returned to stand watch again.

Those wraparound compound eyes don’t miss much!

Dragonfly on antenna - detail

Dragonfly on antenna - detail

A few weeks ago, a much larger dragonfly bounced off my helmet and snagged itself in the delay line coil near the middle of the antenna: the dragonfly’s head slid 1/4 turn around the coil and latched firmly in place. Amid much buzzing of wings and thrashing of legs, I managed to unscrew the poor critter, whereupon it flew off undamaged.

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Bicycle Water Pack Leak Repair

So the hydration pack I’ve been using for a few years started piddling all over the floor, whereupon some debugging revealed a pinhole leak where the large thermally sealed flange meets the bag side. Nothing, but nothing adheres to the polyethylene (or some such) bag material, but a blob of acrylic caulk (armored with a layer of electrical tape, not shown) may suffice for a while.

Hydration pack leak repair blob

Hydration pack leak repair blob

I did the same thing to the other side as a prophylactic measure…

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High Security Access Panel

I was really, really tempted to pocket a key, just in case it might come in handy elsewhere… but I’d have to stand on the toilet and that’s just gross.

Locked access panel - with keys

Locked access panel - with keys

Back in the day, I was third-chair lockpick in my college dorm and those piddly little locks weren’t all that difficult even then.

5 Comments

Stepper Dynamometer Mechanics

Combine two of those mounts with one of those couplers, add two NEMA 17 steppers (the one on the right is that one), slide a baseplate underneath, sprinkle with various screws, and shazam you get a stepper motor dynamometer:

Stepper Dynamometer

Stepper Dynamometer

The baseplate puts the mounts 65 mm apart on the 10-32 screw centers, which is entirely a function of the coupler length, and is easy with manual CNC on the Sherline.

Changing the motors is straightforward: loosen coupler setscrew, remove base screws, slide motor away from coupler, remove mount screws. Won’t happen that often, methinks.

The general idea is to drive one stepper with a known current, apply a known resistive load to the other motor’s windings, and then plot torque vs. speed. It won’t be quite that simple, of course, but should produce some interesting data.

 

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16 Comments

Stepper Motor Shaft Coupler

This simple cylinder connects two NEMA 17 stepper motors together:

Stepper Shaft Coupler

Stepper Shaft Coupler

It’s quick-and-dirty:

  • Cut 2+ inches of 0.375 drill rod, face both ends
  • Drill #8 = 0.199 inch = 5.06 mm (because #9 = 0.196 inch = 4.98 mm is a bit too snug)
  • Cross-drill #41 in the Sherline (because #43 makes for stiff tapping)
  • Tap 4-40 for the setscrews
  • File off rough edges, run #8 drill through the bore to clean out tapping chips &c

Now, you probably don’t want to do this in real life, because you want a coupler with a bit of compliance to soak up the inevitable misalignment and dampen the mechanical resonances.

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7 Comments