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Archive for category Science

Monthly Science: Wearable LED vs. Dead CR2032 Lithium Cell

Eight months later, the dead CR2032 cell driving the “ruby” wearable LED has dropped to 2.15 V:

Wearable LED - on window

Wearable LED – on window

It’s not a true red LED with a 1.5-ish V forward drop, but a white / blue LED with red phosphor or a red filter, with a forward drop well over 3 V.

Against the sunlit backdrop from our kitchen window, the LED looks dark:

Wearable LED - daylight

Wearable LED – daylight

Seen in a dim room, it’s still glowing:

Wearable LED - dim light

Wearable LED – dim light

The current is now far below the 1 mA/div of my Tek A6302 Hall effect probe, so I have no way to measure the few microamps lighting the junction.

The coarse grid outside the window is a swatch of deer netting we put up during feeder season to keep the birds from killing themselves on the glass.

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MPCNC: Stepper Motor Back EMF

A plot of the back EMF for an  Automation Technology KL17H248-15-4A stepper motor looks like I’m making stuff up again:

KL17H248-15-4A stepper motor - Back EMF vs RPM - data

KL17H248-15-4A stepper motor – Back EMF vs RPM – data

Maybe the only questions I ask are ones with linear solutions?

Anyhow, the data comes from the Z-axis motor in the lathe:

Stepper back EMF test setup

Stepper back EMF test setup

Scary-looking, but reasonably safe. The chuck holds the motor shaft so it’s not going anywhere, the boring bar prevents any rotation, and the motor bearings do exactly what they’re supposed to. Shorting the motor leads would definitely put a hurt on the PLA frame, so I didn’t do that.

The scope sat on the floor beside the lathe, capturing waveforms and doing calculations:

Motor Back EMF - 500 RPM

Motor Back EMF – 500 RPM

Some waveforms look bent:

Motor Back EMF - 300 RPM

Motor Back EMF – 300 RPM

I asked the scope to measure the RMS voltage, rather than the peak, because it’s less sensitive to distortions.

Each winding produces one electrical cycle across four mechanical full steps, with the windings in quadrature. One shaft revolution thus produces 200 / 4 = 50 electrical cycles, so converting from shaft RPM into electrical cycles/s goes a little something like this:

Electrical cycles/s = (shaft rev/min) * (50 cycles/rev) / 60 (s/min)

Which works out to a tidy 0.833 Hz/RPM, basically spot on the last data point’s 839 Hz at 1000 RPM.

The motivation for this comes from the third column in the scribbles: back EMF = 22.7 mVrms/RPM = 32 mVpk/RPM.

A rapid move at 12 k mm/min = 200 mm/s shows the motor current collapsing to the ragged edge of not working:

G0 X 200 mm-s - 24V 200mA-div

G0 X 200 mm-s – 24V 200mA-div

Converting motor speed to shaft RPM:

RPM = (axis mm/s) / (32 mm/rev) * (60 s/min)
RPM = (axis mm/min) / (32 mm/rev)

So the shaft turns at 375 RPM when the X axis moves at 12 k mm/min, with each motor generating 8.5 Vrms = 12 Vpk of back EMF.

The MPCNC wires the two motors on each axis in series, so the 24 V power supply faces 24 V of back EMF (!) from both motors, leaving exactly nothing to push the winding current around. Because the highest EMF occurs at the zero crossing points of the (normal) winding current, I think the current peaks now occur there, with the driver completely unable to properly shape the current waveform.

What you see in the scope shot is what actually happens: the current stabilizes at a ragged square-ish wave at maybe 300 mA (plus those nasty spikes). More study is needed.

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Egg Size Distribution

We got several cartons of “medium” brown eggs with what seemed like an unusually wide size distribution, so I picked out and weighed an assortment for future reference:

Brown egg size distribution

Brown egg size distribution

Of course, there’s an egg size specification. Actually, there are many egg size specifications.

We occasionally get huge eggs, tiny eggs, eggs with two yolks, no yolks, or blood-spotted yolks, all of which turn out to be no big deal. I admit to not previously encountering the term “fart egg”, however …

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Stainless Steel Water Bottle: FAIL

Although I repaired the spout a while ago, those water bottles were never satisfactory and saw very little use. A recent cabinet cleanout showed the “stainless steel” has passed beyond its best-used-by date:

Stainless steel water bottle - rust

Stainless steel water bottle – rust

With no regard for whether the patient would survive the operation, I peeled off its rubber foot and applied the Lesser Hammer:

Stainless steel water bottle - insulation

Stainless steel water bottle – insulation

The “insulation” seems to be a rigid urethane-like foam disk few millimeters thick on the bottom of the interior flask, with good old air around the sides.

The bottles never worked very well and now we know why.

Gone!

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MPCNC: Z Height Probe vs. Tempered Glass Sheet

Sliding the tempered glass sheet I used for the initial trials and B-size Spirograph plots under the Z height probe eliminated the plywood benchtop’s small-scale irregularities:

MPCNC - Z-probing glass plate

MPCNC – Z-probing glass plate

The first height map looks like a mountain sproinged right up through the glass:

ProbeArray-Glass-50

ProbeArray-Glass-50

More red-ish means increasing height, more blue-ish means increasing depth, although you can only see the negative signs along the left edge.

The Z axis leadscrew produces 400 step/mm for a “resolution” of 0.0025 mm. The bCNC map rounds to three places, which makes perfect sense to me; I doubt the absolute accuracy is any better than 0.1 mm on a good day with fair skies and a tailwind.

The peak of the mountain rises 0.35 mm above the terrain around it, so it barely counts as a minor distortion in the glass sheet. Overall, however, there’s a 0.6 mm difference from peak to valley, which would be enough to mess up a rigidly held pen tip pretty badly if you assumed the glass was perfectly flat and precisely aligned.

Rotating the glass around the X axis shows a matching, albeit shallower, dent on the other side:

ProbeArray-Glass-flip-50-2018-01-05

ProbeArray-Glass-flip-50-2018-01-05

For all its crudity, the probe seems to be returning reasonable results.

The obvious question: does it return consistent results?

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MPCNC: Sakura Micron Pens

After shimming the corner posts, a plot with Sakura Micron pens came out nicely:

Spirograph pattern - Sakura Micron - Z-0.25

Spirograph pattern – Sakura Micron – Z-0.25

They’re all 01 size pens, with a nominal 0.25 mm line.

Just for fun, a plot done with four sizes of black Sakura pens at Z=-1.0 before the Great Leveling:

MPCNC - Sakura Micron black pen widths

MPCNC – Sakura Micron black pen widths

The 005 pen made a nearly rectangular single-pass tour around the perimeter of the plot, so you’ll see it passing through every legend.

The chunky-by-comparison 08 pen = 0.50 mm:

MPCNC - Sakura Micron 08 Black - detail

MPCNC – Sakura Micron 08 Black – detail

The 05 pen = 0.45 mm looks much crisper:

MPCNC - Sakura Micron 05 Black - detail

MPCNC – Sakura Micron 05 Black – detail

The 01 pen = 0.25 mm:

MPCNC - Sakura Micron 01 Black - detail

MPCNC – Sakura Micron 01 Black – detail

The almost-can’t-see-it 005 pen = 0.20 mm:

MPCNC - Sakura Micron 005 Black - detail

MPCNC – Sakura Micron 005 Black – detail

If you were doing this for a living, you’d probably use 05 pens, because plotter pens are hard to find.

Original HP plotter pens produced a 0.3 mm trace (with a hard to find un-worn tip) roughly equal to Sakura 03 pens, but I haven’t seen anything other than black at Amazon. There’s apparently a 003 pen with a 0.15 mm line; that’s just crazy talk.

Jamming Sakura pens into a plotter pen adapter for the MPCNC makes little sense, so I should gimmick up a specialized holder with some thumbscrew action to keep them from crawling upward out of the holder.

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Reading a Vernier Height Gage

The first time around, I simply set both pairs of MPCNC rails to equal heights using my height gage (*) as a reference, rather than as a measurement tool:

MPCNC - Rail height measurement

MPCNC – Rail height measurement

By now, I assume all the plastic bits have shaken themselves down and the rails have settled into their more-or-less permanent locations, so it’d be useful to measure the actual rail heights and adjust as needed. The scale along the vertical bar of the height gage gives the height of the top surface of the projecting arm above the bench:

Brown and Sharpe 585 Height Gage

Brown and Sharpe 585 Height Gage

Normally, the gage base would sit on a surface plate. Building an MPCNC on a big granite slab would certainly cut down on the shakes from overly enthusiastic acceleration settings!

The nicely reshaped and polished lathe bit transfers the top surface of the gage arm to the top of the MPCNC rail, so whatever height shows up on the vernier gives the rail height. The exact value, of course, doesn’t really matter in this situation, but when you need an actual measurement, it’s got you covered.

The two brackets slide along the height gage, with the thumbscrews on the right locking them in position. To measure a height, you loosen both thumbscrews, slide the whole affair to put the arm bracket at about the right height, tighten the top thumbscrew to anchor the adjusting bracket, twirl the knurled wheel to precisely position the arm bracket, then read the height from the scale.

This requires reading a vernier height gage scale:

Vernier Height Gage - 132.20 mm

Vernier Height Gage – 132.20 mm

The other scale on the other side has inches, but nobody uses those any more. Right?

Things I didn’t get quite right the first time around:

  • The numbers along the right side are in centimeters
  • The smallest lines on that scale mark 0.5 mm increments
  • The numbers on the vernier have units of 1/50 mm = 0.02 mm

So, to read the scale:

  • Multiply centimeters by 10 to get millimeters: 130
  • Add the number of whole millimeters below the 0 vernier index: 2
  • Add a half millimeter if needed: 0
  • Find the matching vernier increment: 10
  • Multiply the increment by 2: 20
  • Slap the decimal point two places left and add: 132.20

OK, try this one:

Vernier Height Gage - 159.84 mm

Vernier Height Gage – 159.84 mm

As I see it:

  • Read 15 cm
  • Count 9 ticks
  • Add the 0.5 mm tick
  • Match vernier tick 17, multiply and slap decimal = 0.34 mm
  • Add: 150 + 9 + 0.5 + 0.34 = 159.84 mm

There, now, that wasn’t so hard, was it?

There’s obviously a parallax issue between the edge of the vernier scale and the main scale; it’s easier to get it right in person than in the photograph.

I pronounced the reading as “160 minus point 5 is 159 and a half plus point 34 is point 84”, but I also take eight photographs as I work my way around the MPCNC frame to review any suspicious results.

Obviously, reading a digital height gage would be much easier & faster, but we don’t want to deskill the workforce, do we?

The maker’s mark on my height gage says it’s a Brown & Sharpe 585 with a 19 inch scale; B&S has long since been Borged. Back in the day, this painstakingly applied etching distinguished it from all the other height gages in the shop:

Brown and Sharpe 585 Height Gage - D.E 1-I-3 etching

Brown and Sharpe 585 Height Gage – D.E 1-I-3 etching

We’ll never know the rest of the story.

(*) When Starrett spells it “gage”, it’s good enough for me.

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