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Digital Caliper with custom plates?
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ataradov:

--- Quote from: Conrad Hoffman on November 07, 2018, 05:20:43 am ---Here's another way to do it using a tube of precision balls, something that better lends itself to DIY- http://www.newall.com/technology/

--- End quote ---
This is super cool.

Apparently there are so many ways to do this. I have guts from a magnetostrictive linear position sensor (https://www.linearmotiontips.com/how-do-magnetostrictive-sensors-work/), but I can't find time to experiment with it. Electronic-wise  it is trivial, but finding magnetostrictive wire may not be easy.
q12:
from your post mister ataradov:
"
When a current pulse is applied, the magnetic fields created by the current and the position magnet interact and cause torsional deflection in the waveguide. When the current is turned off, the strain relaxes, and a mechanical wave begins to propagate. The wave propagates in both directions. In one direction, it is detected by the signal (pulse) converter. In the other direction, it is eliminated by a damping device to avoid interference.
Image credit: Balluff GmbH

The time between the initial current pulse and the detection of the mechanical pulse indicates the location of the position magnet, and therefore, the position of the moving part being measured.
"
Very nice concept! I like it.
And the balls one too, but the emitter and reader are all on the mobile part and is kind of funky. (im not a professional , and it looks like that to me). Or i really dont get it's concept very clearly.
 ataradov concept is a bit more easy to follow. I'm not saying the balls concept is wrong, but i dont see it as easy.  Thank you Conrad Hoffman.
To complete the circle....
Now... heh, can't this be just a single long variable resistor? It' so much easier to implement.  I don't know how to make a long resistance (35cm long)... Yet!
ataradov:

--- Quote from: q12 on November 07, 2018, 08:13:23 am ---Now... heh, can't this be just a single long variable resistor?
--- End quote ---
Making a long resistor that is linear enough to have even 0.01 mm resolution is next to impossible. And this method is contactless, the wire is completely encapsulated inside the carbon fiber tube and the magnet is moving on the outside. This is sort of important for industrial applications. Your resistor will have a wiper, which will wear down the resistive material over time making the measurements less and less accurate.


--- Quote from: q12 on November 07, 2018, 08:13:23 am --- It' so much easier to implement. 

--- End quote ---
Sure, and all those companies making the devices and selling them for 100s and 1000s of dollars can't figure it out.

Although I've seen equipment from the 70's that used essentially a long wire-wound resistor with a wiper. But the resolution of that is the diameter of the wire, and the accuracy depends on the diametric stability of the wire.

You are imagining things in ideal conditions, ideal materials and ideal manufacturing capabilities. This does not happen in a real world, so the design must be resilient to those factors.
rhb:
There are a large number of articles on the topic.  Here's the one I recalled:

https://hackaday.com/2010/12/15/reading-a-digital-caliper-with-a-microcontroller/

I urge you to give serious thought to Conrad's suggestion about steel balls in a tube.  You can use steel air rifle shot for experiments.  The reason being that small very high precision steel balls are cheap and for your needs you could scavenge them from scrap ball bearings.

For the sensor use a linear variable differential transformer, LVDT. 

https://www.te.com/usa-en/industries/sensor-solutions/insights/lvdt-tutorial.html

If you use two glass tubes and balls of different sizes combined with two sensors it will make absolute position possible over a short distance.  It *might* be possible to get absolute position over a larger range with a 3rd size of ball. I'll leave you to ponder that.

Another possible design option is a single LVDT and a rack and some other means for determining which ball you're at.

Conrad's suggestion has three major advantages:

it is inherently very high precision

the sensor and electronics are easy to build

you can get all the balls for free from any repair shop that works on equipment with ball bearings or from scrapped electric motors

If you want to go ahead with making  a caliper scale you'll need to make a "step and repeat" mask to expose a photosensitive PCB and then normal etching.  If you have any significant machining experience you out to know how hard it is to hold even a 0.1 mm tolerance, much less the 0.01 mm you'd need for a usable scale.  But with patience and a short LVDT to measure the step it would be doable.
coppercone2:
What you can also do is modify commercial calipers.

Robrenz of this forum and youtube actually uses them as a building block for his projects. Its going to be hard to beat the mechanical precision of a mitutoyo but if you know how you can grind it and braze stuff to it etc.

Search for robrenz youtube channel and look for his caliper related videos. He is very detail oriented and precise in his work
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