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Measurement device

Discussion in 'General Electronics Discussion' started by dsoileau, Dec 17, 2017.

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  1. dsoileau

    dsoileau

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    Dec 17, 2017
    I'm an electronics novice, so please pardon my ignorance in asking this question. I'm wanting to know how to use mechanical contacts to provide feedback (LED) of positions on a continuum (measuring tape). Start is zero, pressing a contact on any point of measurement would report that specific measurement in units. Thanks in advance.
     
  2. kellys_eye

    kellys_eye

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    Jun 25, 2010
    If the tape has regular markings on it you can use opto-devices to record the number of 'marks' that pass under the sensor. Its accuracy would only be as good as the actual markings on the tape though.

    Another method would be to couple the centre of the tape reel to a slotted disk with a much higher resolution of slots - such disks can be found in junk printers, as can the required opto detectors. You then need some form of processing device (micro-controller) to display the reading.

    It's not a 'novice' task I can assure you!

    There will be stuff like that on the market if you need one specifically but making one without the requisite skills will be very difficult.
     
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  3. BobK

    BobK

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    Jan 5, 2010
    I am trying to decode what it is you are trying to do.

    If I understand you, you have a linear measure, marked as usual, and you want to be able to press at various marks and get a output that encodes the position at which it is pressed.

    If that is correct, what you need is a linear membrane potentiometer, like this:

    membrane potentiometer


    This will give a resistance that depends on where you press it. A microcontroller could read it with its analog to digital converter and report the position via LEDs.

    Bob
     
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  4. duke37

    duke37

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    How does a digital caliper work?
     
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  5. dsoileau

    dsoileau

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    Dec 17, 2017
    Your ideas sound worth investigating, and they must be good ideas because you've made me want to learn more! Though this project is beyond my skill level, your DIY suggestion utilizing junk parts is my preferred way to approach all projects.
     
  6. BobK

    BobK

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    Jan 5, 2010
    Would you please clarify what you are trying to do?

    Both Kellys_eye and and Duke37 seem to think you moving something along the tape. My interpretation is that the tape simply sits there and you press it at various spots. The solutions are entirely different.

    Bob
     
  7. kellys_eye

    kellys_eye

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    Jun 25, 2010
  8. dsoileau

    dsoileau

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    Dec 17, 2017
    You are correctly understanding what I'm wanting to accomplish, and the potentiometer would require a lot less work than the way I had imagined doing this. I have no idea of how to connect a potentiometer to a microcontroller, but I plan to learn! I'll have to investigate the available levels of sensitivity (get what you pay for), as I'm wanting to measure at 1/32 inch increments. Thank you!
     
  9. dsoileau

    dsoileau

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    Dec 17, 2017
  10. duke37

    duke37

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    Do you need a measure of difference between two points or an accuracy of distance?
    To measure to an accuracy of 1/32 inch over 10 inch, then you will need an accuracy of about 1 part in 1000 which will be difficult to do with a potentiometer system.
     
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  11. BobK

    BobK

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    I don't think he is measuring anything. He is using a ruler as an input device, where you press the position on the ruler to input data in 1/32s of inches. However, unless you finger is very skinny, I think 1/32 in accuracy is quite impossible.

    Bob
     
  12. dsoileau

    dsoileau

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    Dec 17, 2017
    I'm wanting to locate precise measurements when a tool is repeatedly repositioned along a continuous length, which presently is done with a measuring tape. I've considered using magnification, which would be simple enough, but an LED readout would make identifying precise locations on the tape much easier.
     
  13. Bluejets

    Bluejets

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    Try a dro, as used in mills/ lathes. Available in small sizes these days or as suggested before a digital caliper. Maybe even hack the latter, many have done so in the past as they can be purchased for around $10 on Ebay for a cheapie from China.
     
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  14. BobK

    BobK

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    Jan 5, 2010
    Okay. I don't think the potentiometer will give you that kind of accuracy.

    Bob
     
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  15. dsoileau

    dsoileau

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    Dec 17, 2017
    OK. Read the ABCs of DROs, and because of the cost I'll be taking duke37's suggestion more seriously. Hacking a digital caliper should be easy, right. ;-)

    Thanks for all the great replies. Hopefully I'll learn enough and have some success to share.
     
  16. Externet

    Externet

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    Aug 24, 2009
  17. AnalogKid

    AnalogKid

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    Like post #2. There is either a long thin precision glass strip with alternate dark and light stripes, or a hwel with alternate dark and light pie wedges. These go past two optical sensors spaced so that when one is seeing a transition between stripes, the other is in the center of a stripe. This is 90 degrees phase difference, and the two square waves that come out of the sensors as they move past the stripe or the wheel revolves past them is called quadrature encoding. With a couple of gates you can turn the two waveforms into a clock signal and an up/down flag for a multi-digit up/down counter that drives the display. Other technologies are magnetic, sinewave, absolute encoding, etc. But the basic idea is the same - a very accurate pattern moves past sensors.

    https://en.wikipedia.org/wiki/Linear_encoder
    https://en.wikipedia.org/wiki/Rotary_encoder

    I worked on a tech refresh of an FAA radar system. The horn was mounted on an 8' diameter turntable. The table had a steel band around it with 2048 holes drilled in it. The base had two photo-interrupter assemblies to make the signals.

    ak
     
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