Sewing Machine RPM Sensing: Gun Bluing FTW!

A quick-and-dirty bracket (made from a leftover strip in the pile of chassis clips) affixed an IR reflective sensor (based on the ubiquitous TCRT5000 module) to the sewing machine motor:

TCRT5000 sensor on motor

TCRT5000 sensor on motor

That’s scribbling black Sharpie around the retroreflective tape for the laser tachometer, which worked just about as poorly as you’d expect. Retroreflective tape, by definition, reflects the light directly back at the LED, but in this case you want it bounced to the photosensor.

An IR view shows the geometry and highlights the LED:

TCRT5000 sensor - IR view

TCRT5000 sensor – IR view

The TCRT5000 datasheet suggests that the peak operating distance is 2.5 mm, roughly attained by tinkering with the bracket. The datasheet graph shows that anything between 1 and 5 mm should be just fine:

IR Reflective Sensor module - TCRT5000 - response vs distance

IR Reflective Sensor module – TCRT5000 – response vs distance

Soooo, a bit of contrast improvement is in order:

  • Scrape off the tape
  • Remove adhesive and Sharpie with xylene
  • Scuff with sandpaper
  • Apply Brownell’s Oxypho-Blue gun bluing with a cotton swab
  • Buff with 0000 steel wool
  • Repeat
  • Apply stainless steel tape around half the circumference
  • Burnish flat

Which looks pretty good:

Kenmore 158 motor pulley - black-silver

Kenmore 158 motor pulley – black-silver

The stainless tape butts up against the setscrew:

Kenmore 158 motor pulley - black-silver at setscrew

Kenmore 158 motor pulley – black-silver at setscrew

Adjusting the sensitivity midway between the point where the output is low (OFF) over the black and high (ON) over the tape seems reasonable.

Running at the slowest possible speed produces this pulse train:

Motor sense - min speed

Motor sense – min speed

The motor at 19 rev/s = 1140 RPM corresponds to about 2 rev/s of the sewing machine shaft= 2 stitch/s. Slower than, that, the pedal won’t go in simple open-loop mode.

The setscrew causes those “glitches” on the rising edge. They look like this at a faster sweep:

Motor sense - min speed - setscrew

Motor sense – min speed – setscrew

At maximum speed, the setscrew doesn’t show up:

Motor sense - max speed

Motor sense – max speed

The motor at 174 rev/s = 10440 RPM would do 1000 stitch/s, but that’s just crazy talk: it runs at that speed with the handwheel clutch disengaged and the motor driving only the bobbin winder. I was holding the machine down with the shaft engaged and all the gimcrackery flailing around during that shot.

The sensor board may have an internal glitch filter, but it’s hard to say: the eBay description has broken links to the circuit documentation.

I could grind the setscrew flush with the pulley OD and cover it with tape, but that seems unreasonable. Fixing the glitch in firmware shouldn’t be too difficult: ignore a rising edge that occurs less than, say, 1/4 of the previous period following the previous edge.

Perhaps buffing half the pulley’s circumference to a reasonable shine (minus the bluing) would eliminate the need for the stainless steel tape.

Iterating the bluing operation / scrubbing with steel wool should produce a darker black, although two passes yields a nice flat black.

About these ads


Leave a comment

Search Engine Optimization: Replacement Shelf Bracket Whirlpool Freezer

If I were selling those brackets, I’d be rich:

Search Engine Optimization - Freezer Shelf Bracket

Search Engine Optimization – Freezer Shelf Bracket

Now, that looks like Search Engine Optimization it is to die for! Google will give you a different set of pictures, but I own that all-important top row.

Alas, anybody can just print their own…



Patient Sign-In App: Human Factors FAIL

It used to be we “signed in” at the dentist by exchanging pleasantries with the folks behind the desk, but that was so 20th Century. Now we’re confronted with an iPad sporting a form:

Patient Sign-in Tablet Form

Patient Sign-in Tablet Form

Pop Quiz: Assuming you filled in your birthdate and remembered how their files have recorded your name, where do you tap to proceed onward?

Reasoning by analogy from my Kindle Fire’s keyboard, I assumed the conspicuous bright blue Go button would do the trick.

Nope. That’s not it.

After a bit of fumbling around, it turns out to be the dark blue Next button (on the non-contrasting light gray title bar) at the right edge of the title bar.

I betcha I could have fun with some of those little icons…

In fact, the next time we showed up, the iDingus sported a popup asking if I wanted to update the firmware (or some such). Of course, I gave the receptionist an evil grin and tapped “Hit me!”

Word: this app nonsense isn’t ready for prime time.


Crysknife Modification

For whatever reason, the handle of the ceramic knife extended a few millimeters below the blade heel:

Farberware ceramic knife

Farberware ceramic knife

Now it doesn’t:

Farberware ceramic knife - trimmed handle

Farberware ceramic knife – trimmed handle

Which makes it much more usable for the kind of chopping I do around here: the blade hits the cutting board squarely, producing chunks of veggies along its entire length.

A coarse file removed most of the stub, followed with a fine file and a little sandpaper action to round the edges.

Amazingly enough, none of that fussing around touched the blade, nor did I gash myself!

Leave a comment

LV Interface Board: +7 V Regulator

This takes most of the load off the Arduino Pro Mini’s teeny SMD regulator by knocking the +12 V ATX supply down to +7 V:

LV Power Interface - 7 V Regulator

LV Power Interface – 7 V Regulator

It’s on the heatsink beyond the ATX connector at the right edge of the board:

Low Voltage Interface Board - detail

Low Voltage Interface Board – detail

It also provides a (more) stable voltage for the current sense amp than you can reasonably expect directly from the ATX power supply:

Current Sense Amp - schematic

Current Sense Amp – schematic

Not much to it: the thing Just Works…


Leave a comment

AC Interface Board: Line Voltage Interlock

The relay at the top connects the AC hot line to the rest of the circuitry, with a feeble red LED to show when it’s live:

AC Power Interface

AC Power Interface

The driver lives on the Low Voltage Interface board:

LV Power Interface - AC Relay driver

LV Power Interface – AC Relay driver

The GX270’s front-panel hard drive LED now serves to indicate when the AC power goes live.

I’d originally intended to turn the AC on when the Arduino gains control, but after seeing those pictures, I think it’ll remain disabled unless there’s a call for motor motion.

The interlock switch closes when the case opens, grounding the transistor base and disconnecting the AC power.

Of course, you can cheat by simply unplugging the switch, so it’s not failsafe. If you want failsafe, you need a normally closed switch in series with the collector; that’s not what Dell used as a chassis intrusion switch. That’s my story and I’m sticking with it.


Monthly Image: Red Oaks Mill Dam — Low Water in 2014

Not much rain fell around here during September, lowering the Mighty Wappingers Creek and exposing the rubble of the dam at Red Oaks Mill:

Red Oaks Mill dam - stonework - 2014-09-06

Red Oaks Mill dam – stonework – 2014-09-06

We never noticed the stonework along the far bank; it’s usually underwater.

Some smooth water-worn wood and stone:

Red Oaks Mill dam - 2014-09-06

Red Oaks Mill dam – 2014-09-06

I’ve always wanted to live in the powerhouse of a small dam. If somebody ever rebuilds this poor thing for low-head hydropower, they’ve got a live-in generator tender…

Searching for “red oaks mill” dam will produce some backstory.