# Finding the center of a wire

#### strantor

Joined Oct 3, 2010
5,324
I used to work in a wire & cable plant and on the wire extrusion lines we had these devices which would measure the offset of the conductor inside the insulation. It did this (I think, 95% certain) by inducing a current via a hollow-core transformer directly followed by a laser micrometer. The laser mic had some circuit to precisely locate the center of the wire by detecting the induced current, and would compare that to its dimensional measurement to present a calculated cross-sectional picture of the wire showing the location of the conductor inside the insulation and how off-center it was.

I am curious about the circuit used to find the center of the wire. I'm not interested in duplicating the entire unit, I don't care about any of that. I'm brainstorming a method of measuring linear accuracy in a CNC machine. I could send a current through the wire with a simple supply, no hollow-core transformer needed. Just need an idea what would be involved in finding the center of the wire. Any ideas? I'm thing maybe hall sensors [4] arranged in an array around the wire? Or maybe a pair of split coils with the wire passing through the center to function sorta like a LVDT? (That last one isn't very well thought out). How would you do it?

EDIT: I said "measuring linear accuracy" when what I meant was "permanent compensating for linear inaccuracy." This is a hair brained idea for making a CNC out of junkyard parts. Instead of paying hundreds or thousands of dollars for quality linear motion components with certified straightness/flatness, make it out of bent crap and force it electronically to move in a straight line.

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#### Alec_t

Joined Sep 17, 2013
11,548
Electrodes both sides of the wire and measure differential capacitance?

#### MisterBill2

Joined Jan 23, 2018
6,784
I regularly see articles on laser measurement systems for all sorts of measurements, some using line-scan cameras. The challenge is that even after you have an exact measurement that still an accurate correction needs to be made, and that requires a feedback arrangement as well as a steady motion system.
Building a machine to be accurate initially requires a means of controlling the build process, and the less expensive measuring processes are slower. But a laser "straight edge" and a line scan camera should allow you to hold to straight. But the really hard part is making a thing that will remain straight and accurate.

#### strantor

Joined Oct 3, 2010
5,324
I regularly see articles on laser measurement systems for all sorts of measurements, some using line-scan cameras. The challenge is that even after you have an exact measurement that still an accurate correction needs to be made, and that requires a feedback arrangement as well as a steady motion system.
Building a machine to be accurate initially requires a means of controlling the build process, and the less expensive measuring processes are slower. But a laser "straight edge" and a line scan camera should allow you to hold to straight. But the really hard part is making a thing that will remain straight and accurate.
Are you referring to lasers because I referred to a laser micrometer? Or because you think that reading the position of a laser beam is more accurate than measuring the position of a taut wire? Because my proposal had nothing to do with lasers. The devices I referred to utilize a combination of laser metrology and inductive sensing, but I was referring only to the inductive sensing portion. Assuming you recognized that, but you think lasers are a better bet, I suppose that's worth looking into.

I realize that building the machine accurate initially is absolutely the tried & true best way to go, but accuracy is expensive. This is/would be a "poor man's" version of an expensive machine. Not a production machine. Production machine speeds are not expected or required. However the faster and more accurate the better.

#### strantor

Joined Oct 3, 2010
5,324
It may not be clear what I'm describing so I've made this toddler quality sketch. I've shown only one sensing wire, installed in the Y-axis, with sensing device installed on the X-gantry which would provide position offset feedback for the X & Z axes. A similar setup would be installed on the other side of the Y-axis, and repeated again on the gantry to provide feedback for the Y & Z axes.

Assuming this system worked, and was installed on a production machine, it would allow the machine to compensate for uneven wear in the rails/dovetails/slides/whatevers. Assuming it were installed on a homebrew machine built from inaccurate materials from the outset, it would allow compensation for inaccuracies from day 1.

The sensing plates would detect the position of the wire passing through the hole (or more accurately the position of the hole passing over the stationary wire) and make small trim adjustments to the axis drives while the machine is in operation.

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#### strantor

Joined Oct 3, 2010
5,324

#### MisterBill2

Joined Jan 23, 2018
6,784
Are you referring to lasers because I referred to a laser micrometer? Or because you think that reading the position of a laser beam is more accurate than measuring the position of a taut wire? Because my proposal had nothing to do with lasers. The devices I referred to utilize a combination of laser metrology and inductive sensing, but I was referring only to the inductive sensing portion. Assuming you recognized that, but you think lasers are a better bet, I suppose that's worth looking into.

I realize that building the machine accurate initially is absolutely the tried & true best way to go, but accuracy is expensive. This is/would be a "poor man's" version of an expensive machine. Not a production machine. Production machine speeds are not expected or required. However the faster and more accurate the better.
Using a laser beam as a straight edge is what I was meaning. Amd then monitoring the position of that beam relative to another part by use of a line-scan camera. This method of measuring relative position is simpler and more stable than using an LVDT or a capacitive scheme, although the KAYMAN products are very good, they are more demanding in the setup. The line scan camera concept does demand a computer able to look at individual pixel values. I am by no means an expert, but I do read the articles.

#### cmartinez

Joined Jan 17, 2007
7,064
Very interesting concept ... but as we say down here, in the end it could turn out that "the soup might be more expensive than the meatballs" ... meaning that the mechanism needed for the compensation might be more expensive than the precision linear parts themselves.

I normally use hardened chrome-coated hydraulic rods as rails for most of my machines. They're very inexpensive compared to what recognized brands such as Thompson and NBC have to offer, for instance. And are almost as accurate. Also, I try to avoid using their recirculating balls linear bearings and use adjustable three-point bearings of my own design. They're not as compact but they do match their accuracy, also for a smaller price.

But back to topic.... I don't think using a taunt wire would be practical, the vibration produced by the machine while in motion would make it vibrate like a guitar string, and God knows what that vibration's amplitude would be. Especially at the very middle of its length.

I think it would be best to use a laser instead. The laser would be pointed at a fixed sensor at the other end of the rail, and the laser would pass through a tiny hole in a plate attached to the traveling bridge of the machine (the laser's diameter would have to be much larger than the hole, of course). The sensor would be a small camera which would report the laser's spot exact XY position in pixels. That would give us all the info needed to accurately compensate for any linear deviation caused by the rail's imperfections. Filtering the laser's light for the camera to distinguish it from ambient light should not be a serious problem.

#### MisterBill2

Joined Jan 23, 2018
6,784
For the rails I suggest looking at the products offered by Bishop Wisecarver. They are different from what you have described, and so you may not be aware of them. BUT it is another approach that may be worth considering. I have not used them, but they are both different and interesting.

#### cmartinez

Joined Jan 17, 2007
7,064
For the rails I suggest looking at the products offered by Bishop Wisecarver. They are different from what you have described, and so you may not be aware of them. BUT it is another approach that may be worth considering. I have not used them, but they are both different and interesting.
I have used them. And I have to say they're pretty good products. Much better suited for mosts tasks than the more fancy brands.

#### shortbus

Joined Sep 30, 2009
7,857
I have to agree with C. Even a laser would still be effected by vibration in the machine. Think you'll just need to bite the bullet on this.

How much accuracy are you expecting? How big is the machine going to be?

#### strantor

Joined Oct 3, 2010
5,324
Very interesting concept ... but as we say down here, in the end it could turn out that "the soup might be more expensive than the meatballs" ... meaning that the mechanism needed for the compensation might be more expensive than the precision linear parts themselves.

I normally use hardened chrome-coated hydraulic rods as rails for most of my machines. They're very inexpensive compared to what recognized brands such as Thompson and NBC have to offer, for instance. And are almost as accurate. Also, I try to avoid using their recirculating balls linear bearings and use adjustable three-point bearings of my own design. They're not as compact but they do match their accuracy, also for a smaller price.

But back to topic.... I don't think using a taunt wire would be practical, the vibration produced by the machine while in motion would make it vibrate like a guitar string, and God knows what that vibration's amplitude would be. Especially at the very middle of its length.

I think it would be best to use a laser instead. The laser would be pointed at a fixed sensor at the other end of the rail, and the laser would pass through a tiny hole in a plate attached to the traveling bridge of the machine (the laser's diameter would have to be much larger than the hole, of course). The sensor would be a small camera which would report the laser's spot exact XY position in pixels. That would give us all the info needed to accurately compensate for any linear deviation caused by the rail's imperfections. Filtering the laser's light for the camera to distinguish it from ambient light should not be a serious problem.
Excellent point, I had not considered vibration. Thank you.

#### strantor

Joined Oct 3, 2010
5,324
I have to agree with C. Even a laser would still be effected by vibration in the machine. Think you'll just need to bite the bullet on this.

How much accuracy are you expecting? How big is the machine going to be?
The machine I plan to build is a CNC plasma table. Capacity of 5ftx10ft. Since everything I make on it will be part of a weldment, I can tolerate some slop in the dimensions. Let's say +/- .030; that would make every piece accurate to within 1/16" which is the same or better than I can do by hand.

I don't want mix expectations of <this kind of scheme> with expectations of the machine I intend to build. I probably won't need much more than ordinary mild steel square tubing but I will try to use the most accurate materials I can afford. I should not "need" anything like what I described but I wanted to try it anyway. See if I can get a hobbyist level machine to achieve production machine accuracy. A CNC plasma table seems like a good test platform. If it worked then higher precision machines like mills could also/maybe be constructed from similar junk.

#### Janis59

Joined Aug 21, 2017
1,205
If I would have such task of measure the excenricality, I would use the USB microscope and certain LabView sub-applet of picture recognition. It may measure accurate, fast, and give out digitized figures how much % is the excenter.

#### shortbus

Joined Sep 30, 2009
7,857
I should have asked what type machine too. A plasma table isn't as demanding as a mill or lathe because there is no contact between the tool and the work. Many of the store bought plasma tables are made from standard stuff like tubes. I've seen years ago one that was made from square tube rails, with the tubes mounted on the diagonal, a corner up instead of one of the sides. Then they used "cam followers' as the rollers. The tubes were fine tuned for straightness and level with jack screws.

For a mill or lathe the measuring the center of a taught wire would take some high speed motor movement and big computer power. To keep the variations between the rails and the tool/work in check.

#### strantor

Joined Oct 3, 2010
5,324
I have been mulling over this pretty hard the past few days and with your input reached the conclusion that it's not a great idea. In order to be effective, the system (whether it be wire sensing or microscope or laser or whatever) would probably require the machine to run at speeds slow enough to disappoint even the hobbyist. But... there may be one application where it makes sense. Suppose you wanted to make your machine from the beginning using very accurate materials, but those materials are not available. Perhaps this method could be used to produce them. A slow & inaccurate (with accuracy compensation system) two-axis machine such as a kludgey surface grinder could (maybe/conceivably?) grind an accurate flat on a piece of stock, one side at a time, to result in good machine rails.

#### strantor

Joined Oct 3, 2010
5,324
I should have asked what type machine too. A plasma table isn't as demanding as a mill or lathe because there is no contact between the tool and the work. Many of the store bought plasma tables are made from standard stuff like tubes. I've seen years ago one that was made from square tube rails, with the tubes mounted on the diagonal, a corner up instead of one of the sides. Then they used "cam followers' as the rollers. The tubes were fine tuned for straightness and level with jack screws.

For a mill or lathe the measuring the center of a taught wire would take some high speed motor movement and big computer power. To keep the variations between the rails and the tool/work in check.
I might also want to do some routing on it so I'll try to build it robust while I'm at it. I know what you're referring to with the "corner up" square tube and it's one of the solutions I'm strongly considering. With hourglass rollers.

#### shortbus

Joined Sep 30, 2009
7,857
I might also want to do some routing on it
I have seen people say that before. But when I see I always cringe. I imagine the fire from all of the wood dust when going back to plasma mode after routing.

#### strantor

Joined Oct 3, 2010
5,324
I have seen people say that before. But when I see I always cringe. I imagine the fire from all of the wood dust when going back to plasma mode after routing.
Funny you should think that way. I was wondering the other day how it is possible for my plasma cutter to shoot out a glowing jet into the air while the ground clamp is still coiled up and hanging from the machine. I wondered what would happen if I took that plasma jet to piece of thin plywood, but with practical tasks looming, stopped at the wonderment phase of discovery. I will attempt to plasma cut wood later this week and report my findings.

#### cmartinez

Joined Jan 17, 2007
7,064
I will attempt to plasma cut wood later this week and report my findings.
How are you planning to accomplish that? Isn't plasma cutting a high frequency electric arc traveling from a torch's electrode to a piece of metal which then melts and is ejected by blasting it with a high velocity gas?