CNC Pickup Winder

[Steve Conner]

Salvarsan: I see your sock full of quarters and raise you a handbag with a brick in it. Real-time programming is difficult. If development seems blissfully easy, it's because someone has already buggered up the real-time issues for you. Read the bug history for the AccelStepper library to get an idea of what I'm talking about. (how did they end up asking an 8-bit micro to compute square roots in real time?)

One of my early real-time experiences involved a diesel engine running away and trashing a bunch of stuff because the code controlling the fuel rack got interrupted by some other routine.

BTW, an Instron what? I've used one of their tensile testing machines, but I struggle to see the relevance to pickup winding.

[Hugh Evans]

An Instron universal tester with a pair of grips for tensile testing. They keep wire manufacturers honest if they know batch tests will be conducted to verify their specs. It wouldn't surprise me in the least if they dumped the wire that doesn't quite meet spec for big customers off on small pickup winding operations. For that matter, I worked with a string manufacturer once that had variation all over the map until the words "we got an Instron" were uttered to the supplier. Verifying the performance of wire prior to winding means that any variation in tension resulting in strain/necking can be accurately interpolated based on thickness.

Allow me to remind everyone again that this system will go through two design iterations. The first will be controlled using the same computer system that drives the gantry of our router and has already proven itself. The second is more of a science fair project that we anticipate will be successful.

The exact tensioning method has not yet been decided.

[salvarsan]

Salvarsan: I see your sock full of quarters and raise you a handbag with a brick in it. Real-time programming is difficult.

Really? No shit?! Damn, and there I was doing it for a coupla decades.

If development seems blissfully easy, it's because someone has already buggered up the real-time issues for you. Read the bug history for the AccelStepper library to get an idea of what I'm talking about. (how did they end up asking an 8-bit micro to compute square roots in real time?)

Okay, they screwed the pooch there by using the default avr-gnu mathlib.
Back in 16-bit device driver land, I did all my non-math coprocessor sqrt() code as a pre-computed lookup table.
There are a few adequate fixed point mathlibs for AVR.
If you stick with 8.8 fixed point, the difference for sqrt() is <32 vs. ~100 microsec on an Arduino.

One of my early real-time experiences involved a diesel engine running away and trashing a bunch of stuff because the code controlling the fuel rack got interrupted by some other routine.

BTDT. Coding for hardware interrupts requires a special mindset. The most useful one was "I am God of all I survey and work in ways subtle, mysterious, and uninterruptable."

Diesel controller. Don't tell me. Let me guess.

Nothing sez "not enough CPU horsepower" like a cyclic executive with unprioritized hardware interrupts.

Back in the day, there was a wonderful old ad for Modula-2 compiler in Dr. Dobbs Journal: "Volkswagens are not meant to carry cows."

[salvarsan]

Having access to an Instron is great

Do I misinterpret Instron's site as one providing materials testing devices?

You'll expect nominal tensions ranging 0-50 grams in pickup winding, depending on how often you use heavier gauges.
For AWG 42, you limit tension to 25 grams (if you can at all) to avoid wire stretch.

Winding a pickup bobbin at 600 rpm, that tension oscillates at 20 Hz (once for each side of the pickup),
so the static tension setup is necessarily empirically determined in the
absence of a good tension regulator.

/edit: Nobody here knows how much the tension varies using only a felt pad
tensioner and a couple feet of wire slack to distribute the stretching.

So, the questions become:
Does the Instron do 0-100 gram ranges well enough?
Does the Instron record fast time-varying tension profiles?

Yup, it gets down to @#$%&\! dynamic range and slew rate.

[Hugh Evans]

They use precise calibrated load cells for virtually any range needed, and depending on the model used are capable of high sampling frequencies. It will not be used for measurements during the winding process.

[salvarsan]

Measuring wire tension

One way to get a wire tension measurement is run the wire over a roller attached to a load sensor.

Industrially, the sensor is either a parallel beam or a bending beam load cell.

In the good ole US of A, they are $120+ from futek.com, rather "F.U. Tek"

Pulling apart a $20 Chinese 100 gram digital scale, I note a parallel beam load cell rated for 250g max.

I'm wondering if a DIY digital wire tension gauge couldn't eventually become the heart of a DIY servo wire tensioner.

-hizself

[RedHouse]

Oh but it sounds cool to say "we have access to an Instron"

Reminds me of the old days when one might make a similar boast referencing a Huntron Tracker.

...Back in the day...in Dr. Dobbs Journal: "Volkswagens are not meant to carry cows."

Dobbs, wow, you're showing your age with that one Sal

[salvarsan]

Dobbs, wow, you're showing your age with that one Sal

What will folks think when I start quoting Richard III?

"Now is the winter of our discontent
Made glorious spring by this son of York;"

aYup, I must be more than, uhm... nearly as old as John McCain.

[RedHouse]

...I must be more than, uhm... nearly as old as John McCain.

Oh crap, sorry I mentioned it! must be time to go

[big_teee]

aYup, I must be more than, uhm... nearly as old as John McCain.

Yes, Your right, I goooogled it!
T

[Hugh Evans]

That just seems like a big can of worms to propose, especially without data to suggest that a closed loop control system should be necessary to control the tension of a pickup winder. Hand winding definitely won't create perfectly even tension over the coil. Friction would be assumed to not vary by a huge amount. My best guess at an easier solution than using a servo involves a simple cam system to synchronize with the bobbin position while keeping machine setup relatively simple. I can tell you right now that I'm interested in studying a wide variety of variables in pickup winding, including tension... But it's not at the top of the list right now.

Rather than working with expensive load cells. How about slapping a strain gauge onto a bending beam? As long as you know the properties of the beam, tension could easily be calculated from an otherwise cheap, dirty, yet effective setup. I think this particular discussion is a perfect example of why making this design open will make such tinkering and testing possible. Hopefully information will be shared when significant findings are made.

[RedHouse]

Yes, Your right, I goooogled it!
T

Well, if Google says it, ...then it must be true! (LOL)

[salvarsan]

That just seems like a big can of worms to propose, especially without data to suggest that a closed loop control system should be necessary to control the tension of a pickup winder.

Servo wire tensioners are out there NOW and as overpriced as ever. In winding an oblong (pickup) bobbin, the wire tension variability should mandate some kind of tension control. Even a DIY mechanical system with a long elastic dancer arm would be an improvement over a felt pad.

Rather than working with expensive load cells.

Yes. I suggested working with cheep ones gutted from cheap <$20 digital scales since domestic load cell suppliers don't bother to compete in that market.

How about slapping a strain gauge onto a bending beam? As long as you know the properties of the beam, tension could easily be calculated from an otherwise cheap, dirty, yet effective setup.

Serious question: What kind of strain gauge would that be? What would we look for online?

Crufting a bending beam sensor from a surplus foil gauge and a thin strip of polycarbonate has a lot of appeal to the SuperGlue and duct tape crowd here.

Hopefully information will be shared when significant findings are made.

Warms my heart to see that in print, thank you.

[ReWind]

I like the idea of the tensioner with gauge feedback, but won't something be needed to calculate/average the tension as it fluctuates with the bobbin shape spinning, and what will keep this from oscillating into resonation?

[salvarsan]

I like the idea of the tensioner with gauge feedback, but won't something be needed to calculate/average the tension as it fluctuates with the bobbin shape spinning, and what will keep this from oscillating into resonation?

A good solution is usually a mix of methods.
The Tanac patent US457087 shows you the tensioner concept in beautiful simplicity.

A mechanical tensioner sets the initial tension with a brake (felt pad, pinch roller, magnetic brake, something?).
A spring-loaded dancer arm often handles the fast changes.
The time constant of the spring+arm and damping from the brake determine how bad the oscillation gets at resonance.

My first guess would be to apply magnetic or friction braking to control the tension.