Category: Electronics Workbench

Electrical & Electronic gadgets

Micropositioner Rehabilitation: Z-axis Slide

With the planetary reducer off, I could remove all the bits and pieces holding the Z-axis slide to the rest of the positioner.

Rack drive casing

Note carefully the three spacing washers near each mounting screw. They hold the slide off the casting body by a very precise amount: they’re each 4 mils thick and prone to vanish in a light breeze. I discovered that each screw had three washers when I flicked one off the workbench and discovered two on the floor.

The metal plate holding the pinion in place has two flat-head screws to the left and two ramps to the right. The conical points of the two long setscrews on the right bear on the ramps, providing a convenient, if obscure from the outside, way to adjust the slide friction by clamping the pinion shaft. One of the setscrews was partially removed, so a previous owner had evidently tried to reduce the overwhelming stickiness.

With the washers in a safe place, the pinion cover comes off with only slight encouragement.

Pinion parts

Plenty of congealed lubricating oil to be found.

Even without the pinion gear, it was exceedingly difficult to urge the two parts of the slide apart: more congealed oil. Much to my surprise, the slide does not have adjustable gibs: it’s one of those precision-ground gadgets that Just Works. This picture shows all the parts in their gunky glory. Note the random dirt in the rack teeth, along with the goo on the pinion shaft.

Rack and pinion – disassembled

With everything apart, removing the gunk was a simple matter of toxic solvents and mechanical poking with wood picks and splints

I filed off the burrs on the shafts, thought briefly about grinding some flats for the setscrews, and decided to leave well enough alone.

A few dabs of clock oil here & there, reassemble everything in reverse order, and the Z-axis moves gracefully with minimal knob torque. It’s very sensitive to the clamping force of those pointed setscrews, though.

It’s now easy to discover that the planetary reducer has a 5:1 ratio and the Z-axis moves about 6 mm per turn. Because the reducer uses balls, it slips when the slide jams against something, rather than strip its gears.

I should clean the other two slides, but a dot of clock oil on each cheered them up enough to let me punt that for a while…

I like it!

Micropositioner

2010-01-04
Micropositioner Rehabilitation: Planetary Reducer

Micropositioner

An old 3-axis micropositioner recently found itself on my electronics workbench, where it should come in handy for SMD soldering, microscopic examination, and similar projects requiring the ability to move something in tiny, precise increments. This picture gives you the general idea; it’s mounted on a magnetic base stuck to a random chunk of sheet steel.

The knob on the front drives the vertical (Z) axis, with the other two controlling the front-to-back (Y) and left-to-right (X) axes. A rotary joint between the X and Y axes, plus another at the tip of the arm, mean you’re not restricted to orthogonal axes; that may be either a blessing or a curse, depending on what you’re trying to accomplish.

Unfortunately, the Z axis was essentially immovable: that big knurled knob took a remarkable amount of force to drive the slide. Some Quality Shop Time was in order.

Planetary reducer – cover

The thing is a chunk of old-school German engineering: nary a gratuitous plastic part to be seen. The planetary reducer has a cast metal cover secured to the torque arm with an acorn nut, which had obviously been removed several times before, as the cover was somewhat chewed beneath the nut.

I loosened the two setscrews holding the knob in place, gave it a pull, and … nothing. After a protracted struggle and considerable sub-vocal muttering, the knob came off to reveal a thoroughly scarred shaft. Contrary to what I expected, the shaft did not have flats below the setscrews, so the inevitable screw burrs locked the shaft to the knob.

Planetary reducer – torque arm

The picture to the left shows the planetary drive and torque arm after I filed off the burrs. Two plastic washers (the top one sits on the spring; it’s not shown here) provide smooth bearing surfaces that hold the knob under firm spring pressure, which prevents the Z axis from descending unless you turn the knob manually.

Planetary drive output shaft screws

Two more setscrews secure the planetary drive’s output bushing to the Z axis pinion shaft. The picture to the right shows that they’re pretty much inaccessible; one was directly behind a tab holding the drive together, the other was aimed at the shoulder of the casting holding the Z axis slide.

And, of course, even with the knob in place, I can’t turn the mumble shaft, which is why I’m doing this in the first place. The planetary drive uses balls, rather than gears, and the lubricating oil had long since turned into gummy varnish. I slobbered enough light oil into the drive to loosen the gunk enough to make the drive turn-able, albeit with considerable effort. I urged the input shaft barely enough this-a-way and that-a-way to get access to both of the screws.

Pinion shaft

As you’d expect, removing the drive required even more muttering and the application of dangerous tools. The pinion shaft was badly scarred in several places, so this poor thing has been dismantled several times before.

That was entirely enough for one day. Tomorrow, disassembling the Z-axis slide and cleaning things up…

2010-01-03
WWVB Groundwave Signal Is Vertically Polarized

I suppose this shouldn’t be surprising, but the nature of groundwave propagation pretty much requires vertical polarization: the E-field is perpendicular to the ground.

Perforce, that means the H-field is parallel to the ground, which means that ferrite bar antennas must be horizontal in order to work properly.

A simple experiment with the Alpha-Geek Clock conclusively demonstrates this. Turning it vertically is just as bad as aiming a bar end directly at Colorado: the signal drops right into the noise.

Horizontal and broadside to Colorado, it’s fine.

Alas, I’d been hoping to tuck the bar antenna inside the Totally Featureless Clock I’ve been building. The ideal location, mounted vertically behind the right-hand digit at the end of the circuit board, as far from the Arduino Pro as possible, just isn’t going to work. Good location, wrong orientation!

I want to avoid an external antenna or a tall case. Drat!

2010-01-02

HP54602 Oscilloscope Trace Conversion Tweakage

The script (writeups there and there) I use to convert the HPGL screen dumps from my HP54602 into PNG images produced a transparent background. I put the files into an OpenOffice mockup of my Circuit Cellar columns and the background turns white, so I figured it worked OK.

Turns out that the workflow at Circuit Cellar Galactic HQ turns the background black. A bit of digging showed that the ImageMagick convert program produced an alpha channel that selected only the traces and left everything else unselected. Why that produces white here and black there is a mystery, but there’s no point in putting up with such nonsense.

Another wrestling match produced this revision (the two changed lines are highlighted), which has no alpha channel and a white background. That ought to simplify things: an image shouldn’t depend on where it’s dropped to look right.

#!/usr/bin/kermit +
# Fetches screen shot from HP54602B oscilloscope
# Presumes it's set up for plotter output...
# Converts HPGL to PNG image

set modem none
set line /dev/ttyUSB0
set speed 19200
set flow rts/cts
set carrier-watch off

# Make sure we have a param
if not defined \%1 ask \%1 {File name? }

set input echo off
set input buffer-length 200000

# Wait for PRINT button to send the plot
echo Set HP54602B for HP Plotter, FACTORS ON, 19200, DTR
echo Press PRINT SCREEN button on HP54602B...

log session "\%1.hgl"

# Factors On
input 480 \x03

close session
close

echo Converting HPGL in
echo --\%1.hgl
echo to PNG in
echo --\%1.png

# Factors Off
#run hp2xx -q -m png -a 1.762 -h 91 -c 14 "\%1.hgl"
#run mogrify -density 300 -resize 200% "\%1.png"

# Factors On
run sed '/lb/!d' "\%1.hgl" > "\%1-1.hgl"
run hp2xx -q -m eps -r 270 -a 0.447 -c 14 -f "\%1.eps" "\%1-1.hgl"
run rm "\%1-1.hgl"
run convert "\%1.eps" -alpha off -resize 675x452 "\%1.png"

echo Finished!

exit 0

2009-12-31

Failed Switch

Switch Innards

When I flipped this switch on, it started fizzing and emitting ozone-scented smoke while the lights it controlled flickered. This is not a nominal outcome. I toggled the switch a few times, but it continued to misbehave, so I installed a replacement switch and laid the old one out on the desk for an autopsy.

It’s an old-school mechanism, as suits the 1930-vintage structure it came from. The lyre-shaped arch with the spring swings back and forth on its tabs, which rest in the small recesses near the middle of the switch body. The peg on the toggle handle engages the spring, thus providing the over-center snap action.

The switch action takes place at the bottom of the arch, where those two very small tabs stick out. They wipe on the grubby-looking bottom tabs of the oddly shaped flat-brass doodads, the U-shaped ends of which surround the screws that clamp the copper wire to the switch.

I expected to find a scorched contact or perhaps an insect in the mechanism, but nothing seemed out of the ordinary. Apart, that is, from the layer of congealed grease covering everything inside. I suspect the grease was applied in the factory to help prevent contact corrosion, but the volatiles are long gone.

Switch Contacts

A closeup of the switch contacts shows (what I think is) the problem.

All the contact points are covered in grease, but the lyre-shaped gizmo looks like it’s been painted: its contact points were black and resisted cleaning by fingernail scraping.

As nearly as I can tell, all the current passed through a very few high spots that were wiped somewhat clean as the contacts closed. As those spots heated up, the grease melted and flowed over them, increasing the resistance and the heat.

The switch had been working for many decades, as the BX armored cable in the box had fabric-covered rubber (stiff rubber) insulation. I managed to install the replacement switch without breaking the insulation, but it was ugly in there.

2009-12-27
What’s Inside This Box?

Digital Media Player box

Got a package from halfway around the world that, I thought, corresponded to a recent eBay order. Opened the envelope and pulled out a box containing … a Digital Media Player?

That’s odd. I don’t recall ordering one of those.

At this point, anybody who’s read Frank Herbert’s The White Plague should get the chills. Do you or don’t you open a mysterious package from far away that seems to offer something interesting?

Pandora might have something to say, too.

Digital Media Player box – contents

Well, open it I did, and found exactly what I’d ordered: a stash of female headers pins. Of course, one can’t tell what else might have come in the package, but so it goes.

Now I can hand Eks half a lifetime supply of the strips to replace the ones I mooched.

One other mildly surprising part of the package: it seems we’ve gotten to the point where magnetic closures are cheap enough to replace everything else, including intricate origami tucks. There’s a small steel plate pasted under the flap. Who knew?

Digital Media Player box – magnetic closure

2009-12-26
Failed LED

Dead LED

Doesn’t look like much, does it? It’s an ordinary blue LED that I used for the upper colon dot in a clock. Worked fine for a few dozen power-on hours, then it turned off a bit after 6:00 pm one day. Back on an hour later, more or less, then off again by the next morning, back on again, off again.

Might be a software error, as each colon LED is a separate TLC5916 display driver output. Might be a soldering problem, as my board doesn’t have plated-through holes. Might be (shudder) a burned-out transistor inside the TLC5916.

When it’s off, VCC appears on both sides, within a few tens of millivolts.

Resoldered the joints, after which it worked for a while. When it’s on, voltage measurements look normal: about 3.5 V drop across the diode and 1.5 V across the driver transistor.

No obvious code problems, but, then, code problems are never obvious.

Finally the thing stopped working for a few hours. I unsoldered it and there’s no continuity: it failed open. Peering deeply inside with a microscope shows nothing unusual: the flying gold wires look OK, the bonds look flat, and the chip has no burn marks.

Just a bad LED, I suppose. It’s surplus, of course, but that doesn’t mean much these days; there’s a lot of surplus going around.

Soldered in a replacement from the same batch and it’s all good.

So far, anyway.

2009-12-24