Digital Caliper with custom plates? - Page 1
Forum section: electronics / projects

author: q12
author_rank: Contributor
timestamp: November 05, 2018, 03:14:39 am
content: First, I am a hobbyist in electronics for 20 years. I am not professional, but not a rookie either.-Digital Caliper. I google about how it works and they say "it use a capacitive sensor to register the measurement". Here is a quick and dirty explanation of the principle:https://www.wonkeedonkeetools.co.uk/calipers/how-does-a-digital-caliper-work/I want to be able to make the 2 'sensors' as long or wide as I need. And by 'sensors' i mean the 2 sliding grid plates. And connect them to the existing electronics. Practically, I want to put new (customized) plates + corespondent wires. ﻿I want to know if I can build these plates myself. Can someone make a sample or give me an answer on this matter? What should i pay attention to?Also, i may add, i want to make it and put it on my lathe for traveling cariage - wich is pure mechanical at this point. I dream over this little gadget for a couple of years to have it.Thank you and i hope you can answer.

author: ataradov
author_rank: Super Contributor
timestamp: November 05, 2018, 03:48:37 am
content: The problem with making them yourself is holding the accuracy and tolerances over the entire length.There are DROs with sensors of any length you like. And while they may seem to be expensive, it would be much more expensive to actually make this stuff yourself.And also, good calipers use a combination of inductive and capacitive sensing. And the rails for that are a bit more complicated.

author: rhb
author_rank: Super Contributor
timestamp: November 05, 2018, 04:18:13 am
content: The Chinese make DROs using the same technology as digital calipers and the prices are quite nominal. The readouts are separate from the sensor. But they are not glass scales, so the temp co will not be as good as the professional DROs.Here's one seller:http://www.grizzly.com/search/?q=(categoryid%3a240480)And lots of people just modify a cheap DRO. That is especially true on the smaller mini-lathes. Though the readout is not as convenient.

author: q12
author_rank: Contributor
timestamp: November 05, 2018, 11:33:05 am
content: thank you mister ataradov;The basic idea that i think of, (and feel free to correct me if im wrong) these plates are just simple copper prints over the isolator board, and all that they have special, is their shape with specific height/width and probably thickness too. And thats it. If i can replicate their form and position, (let's assume) scratching a blank electronic board, it must do it's job most probably.And only for the 'fixed' part that is the longest. The mobile part that is on the reading device, that I can leave it alone and just use it.Make a smart assumption. Philosophy a bit on the subject. I need ideas, especially from someone who have more experience in electronics in general. Think like you would do it for yourself. The cheapest and dirty you can make it, just to prove it works. The finer calibrations i will have to fight with myself.-thank you mister rhb;your idea with DROs is to be kept as plan B if plan A will fail miserably.--But I am EXTREMELY passionately curious about the functionality of the sensor itself.Can you (guys) make me a cheap and dirty schematic + explanations for a functional electronic meter/scale ? The actual reading device (probably with a PIC16f84) or some demultiplexors or just led scale or a needle ... can be made after I get some sort of reading from the sensor itself. By the way, I will read (mili) -Volts or Amperes from the output sensor? Or (mili/micro) Farads or Resistance? That's why i came with this question and I really need some clarification. Please make a good case. For science and progress.

author: q12
author_rank: Contributor
timestamp: November 05, 2018, 01:32:54 pm
content: I responded alone (with ataradov help actually) to my own question by searching a bit more harder on the subject.I found this relevant page:https://en.wikipedia.org/wiki/Capacitive_sensingIt turns out that  is the contemporary technology that is widely used all around. Measuring and today touch technology. I didnt put too much thought into it. Until todayThe most important information for me from this page is this:"Capacitance is typically measured indirectly, by using it to control the frequency of an oscillator, or to vary the level of coupling (or attenuation) of an AC signal. "ataradov mentioned that: "good calipers use a combination of inductive and capacitive sensing"- wich complicates things indeed. I had no idea about capacitive sensing until today (not seriously i mean). But you specified "good calipers" - i want a cheap /concept proof solution for myself.My next step is to try to imitate as best as i can the [fixed plate] markings. If you have any helpful suggestions, please tell.

author: rhb
author_rank: Super Contributor
timestamp: November 05, 2018, 02:47:14 pm
content: A capacitance bridge will work. Take a look at this thread:https://www.eevblog.com/forum/projects/suggestions-for-high-resolution-tiltmeter-(inclinometer)-sensor/msg1531160/#msg1531160I have the parts for one, but want to anneal the mount I made and just haven't gotten back to it. But JBeale is repeatably measuring micron level displacements of a concrete slab caused by human body weight with a very simple circuit.As for the scale and sensor, those are just PCB strips. Fundamentally it's a vernier scale with electronic sensing of position. You'd likely have a great deal of trouble matching the accuracy of a PCB fab house.I think that the best way to proceed is to buy a cheap 15 cm digital caliper, take it apart and study it.If you make your own plates, you'll need to calibrate it over the full travel just as you would with a dial caliper or micrometer. But you can then store the cal constants. You could get fancy about it and setup a Michelson interferometer and automate the process.

author: ataradov
author_rank: Super Contributor
timestamp: November 05, 2018, 05:22:12 pm
content: Quote from: q12 on November 05, 2018, 11:33:05 amThe problem is precision with which you can manufacture those things. Realistically manufacturing them in the PCB process is the cheapest option. The problem is that very long PCBs are rare and expensive. And connecting multiple shorter PCBs may be a challenge.Quote from: q12 on November 05, 2018, 11:33:05 amYou will probably need a specialized IC for that. You can probably mock something using general purpose MCUs, but you will not get a robust solution. The closest general purpose MCUs that may work are the ones containing integrated capacitive touch sensors with enough channels.In the naive implementation you will be getting capacitance readings, probably in a form of time it took to charge or discharge the capacitor. Or oscillator frequency set by the capacitor.The biggest problem is that you will need to sample many channels (10-20) very often, since values will wrap around periodically, and you will have to keep track of the position by counting the number of overflows. That's where inductive sensing comes in in absolute origin instruments.

author: rhb
author_rank: Super Contributor
timestamp: November 05, 2018, 06:35:36 pm
content: Think femtoFarads (1/1000ths of a pF, 10e-15).I just linked the grizzly.com units because I knew they carried them. They are widely sold. for example a 20" single axis DRO is $56 US from:https://www.aliexpress.com/item/Horizontal-Vertical-Dual-Use-Digital-Display-Scale-Machine-Tool-Equipment-Displacement-Sensor-Optical-Grating-Positioning-Ruler/32890493676.htmlI've never investigated the details, but the programming job to convert the sensor output to distance is not trivial. I've got a 10" x 20" Clausing lathe and a 6" x 24" Clausing mill, along with many years of professional programming experience writing numerical codes for oil exploration. I'd be very reluctant to write my own software for such applications. It's probably 200+ hours of work *if* you already know how to program the MCU of choice. I haven't decided on what DROs to get, but I would not build one from scratch. It would be under $100 to add carriage and cross-slide DRO for a small lathe like mine and about $150 for 3 axes for my mill. That's really cheap for the result. No, they won't calculate the XY values for a bolt circle for you the way a $1000 DRO will. But if you're not trying to make your living as a machinist it's good enough to calculate that by hand.The digital calipers have a data port with a very simple protocol which has been documented. So echoing the LCD display to a separate screen with an Arduino or similar would be pretty easy. But compared to the prices I cited above, I don't think it's worthwhile of personal time.

author: q12
author_rank: Contributor
timestamp: November 06, 2018, 05:02:31 am
content: to mister rhbI failed to mention that i have already a very cheap digital calliper - @3$ from ebay. Problem is it's from plastic and not metal. (I have also a mechanical one from metal - so i know the diference).I purchase it exactly to this reason, to see it and touch it, and to understand its technology. Also i had this customization in mind before i took it. So, I have the components already and I can measure right and scratch on the left.I am very pleased of your explanations ! Both you guys. I understood the problem better here in this forum.Here is the cheap 3$ one that i have:https://www.ebay.com/itm/150mm-6inch-LCD-Digital-Electronic-Carbon-Fiber-Vernier-Caliper-Gauge-Micrometer/392099296347?hash=item5b4af0745b:g:Z0oAAOSwYp9aBSFL:rk:3:pf:1&frcectupt=trueI dont care its from plastic. I care about that fixed plate to make it longer and to measure more distance.Unfortunately i live in romania and i cant buy stuff more than 10-20$ since its too expensive to my pocket. Your suggestion about DRO is the plan B in the future when (or IF) i will have the possibilities. That is the easy solution.You said "The digital calipers have a data port with a very simple protocol which has been documented. " What are you reffering? Put a link please. Thank you.to Mister ataradovI love how you explained ! And I agree with everything (you and mister rhb) said.- "very long PCBs are rare" - yes, good point ! My longest travel is ~35cm (350mm). I just look on ebay - the majority are 20x30cm [PCB Circuit Board Single Sided] -after the longest i looked. You have a good point, but until this stage, i have to prove the concept that is working first... so this problem is for later to attend.- "In the naive implementation you will be getting capacitance readings, probably in a form of time it took to charge or discharge the capacitor. Or oscillator frequency set by the capacitor." - this explanation just made me to step back and reflect. Because it is what it said in wikipedia page about Capacitive_sensing.I understood the overflows capacitive sensing but this "That's where inductive sensing comes in in absolute origin instruments." i dont get it.Awesome explanations and it made me more aware of the technology. I still think it can be 'hacked' somehow, that length. Its a feeling. Ah...

author: ataradov
author_rank: Super Contributor
timestamp: November 06, 2018, 05:10:18 am
content: Quote from: q12 on November 06, 2018, 05:02:31 amWell, I was thinking about ordering them, and 350 mm may not be that long. Keep in mind that you will need longer boards if your full travel is 350 mm.I'm not sure how you will make them at home with required accuracy.Quote from: q12 on November 06, 2018, 05:02:31 amOverflows will happen real fast, every few mm. So if you power your device in the middle of the range, or just skip counts because of very fast movement, then you will be lost. You will not know your absolute position. There are much more expensive calipers that can actually detect their absolute position even if the head was moved when there was no power at all. But their construction is more complicated than $3 plastic stuff. Don't worry about that for now.

author: q12
author_rank: Contributor
timestamp: November 06, 2018, 10:30:43 am
content: (Mention and give credit to"q12" if you are doing this project and post it on internet){This is an image from internet - not mine.On my caliper, I peeled the plastic cover a bit in the margin, (the unused side), and the copper shapes are exactly like this image i put here from internet. Excellent.The copper shapes are T's and I's. No connection between them. Just 'floating' shapes delimited by space/gap.the fixed plate circuit color is the brown one.the mobile plate circuit color is the green one.}The model circuit is like this: copper + space + copper + space +etc.1. My plan, how i see it from my intuition so far, is to unite Not in a straight line, but following the contour of the space between the copper shape. And 'voila', i extended it like that.2. If by some motif i can't cut the board exactly in the contour of the copper shape, I cut it in straight line, and where the gap between fiberglass pcb boards unite, I stick/glue a shaped copper foil between the 2 boards. I will reinforce underneath pcb's with a metal plate the 2 boards anyway.Exactly for this reason, is best to experiment with this juncture first, before using the full scratched length of the plates - to avoid any errors.3. Make the desired length board from a non conductive material. Plastic, wood, fiberglass, glass, cement, molded resin, stone, rubber etc. Glue the cut copper shapes into their positions onto the isolator board. Stick a band of scotch over all this top surface for protection. A couple rows of scotch for being sure. It can be matte scotch, not transparent, either. The capacitance field will not be interrupted.4. Copper erosion with an acid, the old way of etching ! I almost forgot about it. For those who don't know, you paint on top of the finish part you want to remain with a acid resistant paint, but easy to remove after. We used special markers and before them, fingernail paint which we remove it with diluant or benzin/gasoline. Submerge the PCB plate into a bath of acid and check from time to time until you see the white textolite surface underneath copper. Then is etched. Remove the paint to expose the copper trace that remained. (we used sometime fine sand paper and slightly remove the paint).Merge more plates together using method 1 or 2 described here."I'm not sure how you will make them at home with required accuracy."I am an artist by trade (digital and traditional painter). Patience is a requirement in my work. I have plenty. Well, I did some sort of fine resolution modifications in my past and I am confident. Using a simple needle to do the scratching(if using copper plated PCB boards), it will give very good results, i am sure. But it will take a LOT of time!Another idea is to buy some gold thin foil (used in orthodox iconography in this parts), and shape the required elements and glue them.Of course, testing all the time, if it works and how it works. Making 1-2 cm length in the start until getting some results. If the results are ok, proceed to making the full length. Experimenting will take 1 week to 1 month or several months maybe, but actual building the full length will take a couple of days. Is how i predict it so far.

author: q12
author_rank: Contributor
timestamp: November 06, 2018, 10:52:53 am
content: This is the gold foil I am talking about:https://www.ebay.com/itm/100-Sheets-Gold-Sliver-Copper-Leaf-Leaves-Foil-Paper-Edible-Gilding-Craft-Decor/382556673030?hash=item5912279406:g:GZ8AAOSwDYBbjgSf:rk:5:pf:1&var=651312637216&frcectupt=true&autorefresh=true@ $2.25Here is more practical solution : 5mm Copper band. Why 5mm and not 6mm that you find a lot on ebay? My T shape copper element width is 5mm. So 1 side is resolved automatically.https://www.ebay.com/itm/1-Roll-Copper-Foil-Tape-Single-Adhesive-Strip-For-Stained-Glass-5mm-Copper-Faced/323444613013?hash=item4b4ecd1f95:g:dVsAAOSwnTdaHmBa:rk:12:pf:0Now, how to cut such thin material? And precise enough...Practice i suppose.If you have any ideas on this subject, please tell me.1. I have a stone for knife sharpening. I sharpen my chirurgical knife with that stone, it was like a samurai sword cutting efficient. I cut a paper and some surface layer skin to test it. Also some hair from my arm. That can be used as efficient as possible. Also using cutter blades is another solution.2. I also wonder if there are some LASER cutter individuals, who can make the desired shapes from copper foil for me(us), put those shapes in a bag, mail it ... And I just pick and stick them. That will solve the precision cut problem. Mmmm, that will be awesome.

author: ataradov
author_rank: Super Contributor
timestamp: November 06, 2018, 04:57:43 pm
content: There is absolutely no way you will cut so many lines by hand with required accuracy. You may get it to show you something, but how are you going to prove that whatever is shows is accurate?I don't want to stop you from trying, but I don't think any approach that requires cutting something by hand will work. You need to figure out a process to make this work in a more automated way.If I was going this route, I would be exploring ways to accurately join two PCBs like this together, and then order a bunch of shorter boards from cheap manufacturers like AllPCB or JLCPCB. May be put vias on the edge copper panels and join them together on the back side. Not sure what this will do to the readings.How cheap is your labor that you are willing to work for a month, and don't want to spend $50 on a readily available solution. You know you can make much more than $50 in a month doing something else?

author: q12
author_rank: Contributor
timestamp: November 07, 2018, 02:05:53 am
content: Thank you for believing me, mister ataradov. I will be forced by life to change work very soon, but until then, I will endure a bit more. I resisted 6 years already. Not the place here to discuss such things. But I appreciate.Again, you are right about automation as a safety measure against (manual) errors."- how are you going to prove that whatever is shows is accurate?" - I will draw a measured ruler on the table and I will slide the thing against it for comparison. Smart,da?I will have to actually start doing some tests. I am agreeing with all that you said, and all that we discuss here is standing in the back of my mind. But experience will teach me the truth. Probably I will fail as you predict and I am not that a large hope myself... i'm rather curious about the thing. Very ! And that's all that matters. I will update here the results, bad or good, to leave a nice finish.Again, your advises were very direct and to the point. Don't change! Thank you.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 07, 2018, 05:20:43 am
content: 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/

author: ataradov
author_rank: Super Contributor
timestamp: November 07, 2018, 05:28:35 am
content: Quote from: Conrad Hoffman on November 07, 2018, 05:20:43 amThis 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.

author: q12
author_rank: Contributor
timestamp: November 07, 2018, 08:13:23 am
content: 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 GmbHThe 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!

author: ataradov
author_rank: Super Contributor
timestamp: November 07, 2018, 08:19:06 am
content: Quote from: q12 on November 07, 2018, 08:13:23 amMaking 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 amSure, 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.

author: rhb
author_rank: Super Contributor
timestamp: November 07, 2018, 03:01:40 pm
content: 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.htmlIf 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 precisionthe sensor and electronics are easy to buildyou can get all the balls for free from any repair shop that works on equipment with ball bearings or from scrapped electric motorsIf 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.

author: coppercone2
author_rank: Super Contributor
timestamp: November 07, 2018, 05:20:21 pm
content: 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

author: q12
author_rank: Contributor
timestamp: November 08, 2018, 05:07:12 am
content: I read everything you wrote guys. I am thinking on a solution.Here I measured as close as I could possible with my Mechanical Caliper, the copper elements.The copper elements are shown on my end digital caliper.Help from a magnifying glass + light = higher resolution reading.Everything is measured. Even the obvious sizes like that 0.730I just gather data for now. I hope i can use it. I am aware how small everything is.The problem I was confronting with is (surprisingly) not these tiny elements, but the support that they should stay. I realized I don't have too many options to fabricate it as long as I want. I could, but I was thinking on alternatives. And it hit me.I had a brilliant idea while doing all this measurements:- What if I continue with another (same) caliper tail to the existing one?And I actually purchased already from ebay another best cheap one i found. 3$ gone from my life. Off.The plan is to unite 2 (same) tails and make the thing longer."Estimated delivery Dec 13, 2018 - Jan 24, 2019" - i'll have to wait for this possible solution to arrive.In the mean time, I will search for a solution with what i have already and what can be done.Here is the proof i'm serious about it:

author: rhb
author_rank: Super Contributor
timestamp: November 08, 2018, 01:29:00 pm
content: The expansion coefficient of your substrate is a significant obstacle. However, I have little doubt that you have the ability to get the fit between the scale pieces to make it work. But you may run into a snag with the absolute position determination. Short of comparing the pattern of a 12" scale to a 6" scale the only way to find out is to try.As has been commented, buying a longer scale is cheaper unless your time is free and your money is *very* limited.But this is a fine example of why I like this forum so much. It's a bunch of thoroughly demented, but very smart and mostly very pleasant people. There is a steady stream of obnoxious, rude and ignorant fools who imagine themselves the smartest person in the world that wander through, but I've observed that they don't last long.Cheers! I look forward to seeing how things work out.