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!

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.

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.

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…

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.

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.


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.