Measuring a very fine wire with 10 µm precision

This is a nice diagram explaining the difference between accuracy and precision

JimboZA: This is a nice diagram explaining the difference between accuracy and precision

Yes I understand. I would like to know if there is a technique that would maximize both accuracy and precision when dealing with a wire and target resolution of 0.001 mm.

Any thoughts?

If I wanted to wind a coil with maximum packing density (like for a Tesla coil secondary) I would just have the feed carriage lag a little behind so that the new wire is pressed lightly against the previous turn.

Boots507: Instead of working form the theorical diameter of the wire when making the coil, I am interested in measuring it as it unwinds from it's original spool (more accurate).

I'm skeptical that your measurement accuracy would be better than the manufacturing accuracy. Have you measured the actual thickness, and does it vary (between spools, or within a spool) by enough to matter?

If your goal is just to wind wire onto a coil former then I would have thought you effort would be better spent designing the winding mechanism so that the wire stacked itself correctly. Doesn't a conventional winder sort this out for you? I'm surprised that it's necessary to do anything with any great precision when feeding the wire on, or that there are any benefits in knowing the wire thickness precisely.

If you really do need to know the wire thickness as an academic exercise then I'd use a micrometer. If it's necessary to measure this repeatedly at different places along the wire then I'd pass it between a pair of fixed/floating rollers and use a micrometer to measure the position of the floating roller. But this still feels as if you're solving the wrong problem.

Hi,

Thank you all for the answers.

My idea is to spool the wire exactly from one "lip" of the coil former to the other without any gaps. To solve the gap problem, lag works quite nicely. However, even if I measure the wire at several place and use that measurement any slight variation of the wire creates problems when it comes to calculating the amount of turns required (coil_length / wire_diameter). Over several thousands of turns even a variation of 0.001mm can amount to a gross gap. I was thinking that actively measuring the wire diameter for discrepancies would allow me to set the number of turns as a "moving target" during coiling so that the wire ends up exactly at the lip.

Perhaps I should be looking for a strategy/some kind of sensor that would be able to sense the presence of the wire when it reaches the coil lip instead of trying to measure the error? I want to minimize contact with the wire as it is coated with a fine layer of plastic (that I want to avoid stripping away).

Hi, How much variation is in the wire itself or the thickness of the insulating coating? How would this effect your project? Goodluck, jolphil

Are you trying to determine when a single layer of the coil has reached the far lip (and presumably needs to reverse to form a new layer), or when the layers of the coil have built up to the required outside diameter?

jolphil: Hi, How much variation is in the wire itself or the thickness of the insulating coating? How would this effect your project? Goodluck, jolphil

There is enough variation that taking several 0.001 precision measurements of the wire and using an average to calculate the amount of turns (coil_length / wire_diameter) yields inconsistent results. This can be due to the coating or other imperfection in the wire. This affects my project because I aim to make "multi-layer" coils and so I need to be able to stop exactly at the "lip" of the coil before reversing the direction of coiling.

If there is still a gap left between the wire and the coil lip, the reversal is problematic.

PeterH: Are you trying to determine when a single layer of the coil has reached the far lip (and presumably needs to reverse to form a new layer), or when the layers of the coil have built up to the required outside diameter?

Yes;

I am trying to determine when a single layer of the coil has reached the far lip in order to properly reverse to form a new layer.

Stopping exactly on the far lip is required in order to properly reverse to the new layer and keep a tight packing without overlaps. When using a single average value for the wiere diameter, the accumulation of error over thousands of turns yields inconsistent results (i.e. the wire will stop before or after (spool unto itself) the far lip).

Basically, I am trying to find a way that will guarantee that the wire will always stop exactly at the far lip.

Stopping exactly on the far lip is required in order to properly reverse to the new layer

But unless the error was such that you had accumulated an integral number of wire thicknesses, that's not going to help.

Let's say it nominally holds 100 turns along the length. You measure that the wire is a bit thicker say, and by the time you get to where the 99th row would be, you think Oops, we've actually got 99.5 thicknesses. So what do you do? You can't fit another full turn into 0.5 of turn's gap, so you can't stop exactly on the far lip anyway... not in terms of wire widths.... so you reverse with half a wire width gap.

(That made sense to me in my head- might need a drawing to describe it but I cba doing that right now.)

Point is, I'm pretty sure there are best-practices for doing this commercially, and I don't think this is one of them with all due respect.

Boots507: Stopping exactly on the far lip is required in order to properly reverse to the new layer and keep a tight packing without overlaps.

By measuring the wire thickness you could estimate the number of turns needed but I don't see how you'd ever get it exact.

I don't know whether it would be practical with wire that fine, but I wonder whether it might be more effective to detect the outer diameter change as the wire hits the lip at the end of the former, rather than try to [u]predict[/u] when it will hit? If you used a floating pulley, you'd need to detect a movement equal to the thickness of your wire.

Boots, me thinks you're approaching this problem from the wrong direction.

Also, copper wires are force-drawn/pulled through a die at the factory, so they're pretty much the same diameter. Being able to detect variations of 0.001mm won't do squat to what you're trying to accomplish. It's immaterial.

I would concentrate my efforts on a mechanism for consistent tight winding, and accurate inductance values. If the wire ends in the middle of the coil (to achieve the correct inductance value), then so be it. Tape the wire at that spot, then run a few inches of flying lead wires, solder to the transformer terminals (or solder to a thicker flying lead wire). The wire doesnt necessarily have to end at the lip. -- thats how commercial audio transformer makers do it.

EDIT: The core you use for your transformer will also have variations in their permeability. Even if you got your wire diameter detector correct (which I doubt), the next core you grab may be slightly off, and the next transformer you wind will not end up exactly at the lip (to achieve the correct inductance value)... so now what? You're back to where you started. You cannot predict where the wire should end because you'd have variations in the size and permeability of your core (if you care about getting the right inductance value).

I try to go back to the original question, how can I measure the wire with that precision ? I don't think there is a practical mechanical way to achieve such precision. So what about a digital microscope ? http://www.leica-microsystems.com/products/digital-microscopes/ well I don't know your budget (there are cheaper options), but the idea is to "enlarge" the wire, and measure the variation of the image. I mean, measure the difference of light transmittance ..... Maybe it's just an weird idea but I have no other.

Lets try to design the sensor you need,

design 1: - conductive wire

imagine a V shaped sensor that guides the wire. depending on the thickness of the wire it will be higher or lower in the V shape make the sides of the V of a conducting material with known (relative to wire high) resistance The place of the wire will change the resistance of the V as the level where the wire is short-cuts the current.

place a laser let the wire move in front of it. @10uM a known amount of light will pass above and below the wire make that minimal @12 uM - you need mirrors at exact angles place a TSL235 in the laser bundle - this convert intensity to frequency. when the diameter varies the intensity will vary

laser >-------------------- | - - - - - - [sensor]

now shoot!

Best option is to use an optical microscope. "Seeing" 1um is not hard at all. Get a decent CCD camera from Pixelink or Basler or any other brand with a 1" sensor and a pixel size of 2-3um. You'll also need a microscope and some basic image processing to automatically identify the wire (easy when looking for a straigth line on a uniform backgound) It shouldn't cost more than \$1000.

Hello, I think the weapon of your choice would be a so called 'micrometer' :P http://en.wikipedia.org/wiki/Micrometer

Plus: If you want to measure in such small dimensions, be aware of the fact that even the warmth of your hand and also the atmosphere around the wire has an impact on the extension of the wire. I'm going with vasquo's oppinion.

//edit: With what I want to say: If you're going to measure a wire by applying voltage, it will expand slightly due to warming up slightly (electrons colliding inside the copper), so I assume you can basically forget that...

osterchrisi: Hello, I think the weapon of your choice would be a so called 'micrometer' :P http://en.wikipedia.org/wiki/Micrometer

No I don't think so, resolution and precision are not enough maybe something like this ... http://www.keyence.com/services/download.php?file=im6000_ka.pdf&fs=IM-6000&ws=none&img=oc_cmm_lp.jpg&layout=d2d&lil_id=1309212783&lil_ly=aln-l_tfsz-20_dfsz-16&aw=gagooglekaim899063b&gclid=CLC97ta9-rQCFUFb3god9Q8AvQ but again, the budget is ? I think szangvil and robtillaart have the point.

robtillaart: Lets try to design the sensor you need,

design 1: - conductive wire

imagine a V shaped sensor that guides the wire. depending on the thickness of the wire it will be higher or lower in the V shape make the sides of the V of a conducting material with known (relative to wire high) resistance The place of the wire will change the resistance of the V as the level where the wire is short-cuts the current.

You do know the wire is insulated (and not BARE). It's a "magnet" wire, the kind used to make coils. If you strip the insulation just so you can measure it's resistance, well, you just made your coil useless.

I think the OP needs to visit an audio transformer manufacturer so he can see how it's done. These guys don't measure their wire diameters to micron precision or use lasers or microscopes as they wind. But they do measure their inductance and DC resistance values. They don't care where the loop ends. Overwind a few extra turns, measure, unwind, measure, tap it, then tape it. Done.

Have you guys seen the insides of an audio transformer?

robtillaart:
The place of the wire will change the resistance of the V as the level where the wire is short-cuts the current.

I don’t think that would work, because the wire is insulated. Using a camera and microscope seems like the most likely approach to me, but the other option would be to create a rolling version of a standard wire gauge (using a pair of opposed precision ground rollers with a slight taper, and angled so that they tended to pull the wire away from the narrow end of the V as it rolled between them). The height of the wire in the gauge would tell you the thickness of the wire to whatever precision you wanted, as long as your rollers were sufficiently accurate.

For initial optical testing, there seems to be inexpensive microscope cams up to advertised 800x power.