Archive for August, 2011
- 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!
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:
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:
Top trace is the winding output voltage, bottom trace is the drive input current, plus a line of junk I forgot to turn off.
- 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.
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!
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.
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.
I did the same thing to the other side as a prophylactic measure…
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.
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.
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:
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.
This simple cylinder connects two NEMA 17 stepper motors together:
- 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.