Dectection of hole in moving clear transparent plastic film...

Discussion in 'The Projects Forum' started by Msinks, Dec 20, 2014.

  1. Msinks

    Thread Starter New Member

    Mar 3, 2012
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    Am working on a packaging project and part of it, with my current design requires that I find a way to detect a hole (aperture) in the moving clear packaging film after which a signal must be sent to the machine and stop the movement of the film.

    Is this possible? How can it be archived? Or do I need to change my design?
     
  2. GopherT

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    Nov 23, 2012
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    I assume the hole is always in the same position on the width of the film and not randomly scattered on the film.

    You can try using an IR emitter LED and a ir phototransistor each placed at 45 degrees to the film on the same side of the film (reflection design). iR light will pass through any hole and not reflect.

    Because the film is clear, reflection may be very dim so you need a sensitive system to amplify. You may also want to pulse the light and Capacitovely coupled sensor to limit ambient interference unless you can make a box that encloses the edge of the film.
     
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  3. KMoffett

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    Dec 19, 2007
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    Lack of information!
    Is this a school project or a commercial endeavor?
    What's the packaging film made of?
    How fast is it moving?
    How wide is it?
    What size hole is OK and what is not?
    Have you looked for existing systems?
    Do you have a design already? ("...do I need to change my design?")

    Ken
     
  4. jpanhalt

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    Put a small manifold the same width as the film over it. The manifold wouldn't touch the film, but would have a slight positive pressure by a continuous supply. Any hole would result in a drop in pressure, which would be your signal. You could also apply the same approach using light -- either reflected, transmitted, or fluorescent. Instead of monitoring pressure, you would monitor intensity.
    John
     
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  5. djsfantasi

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    If the hole is in the same position on the film, you could use an air pressure supply mounted over the expected hole position, and a pressure switch beneath. This is simply a variation of John's idea.
     
  6. Msinks

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    Mar 3, 2012
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    @GopherT that sounds like a good idea, thanks alot...
    @KMoffet...am sorry for my lack of information...
    its a school project to be completed by late next year. Thus my design is pretty much rough with a few sketches here and there. Most of the staff is mostly in my head hence the reason I was not able to post most of the necessary details.

    All holes are to be of same diameter (3-4cm) and at the same position along the width of the film. The Velocity of the film is to be about 0.2m/s. The material of the film is to be polyethylene and should be about 20cm in width. Most of the material I found on the same involves stationary and mostly translucent film.

    I need to be sure it can work so that I can fully develop the design.
     
  7. Msinks

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    Mar 3, 2012
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    @jpanhalt...@djsfantasi...good ideas i never thought of...thanks...
     
  8. GopherT

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    @Msinks
    You can also use a simple pair of copper contacts on springs or rollers. Once the hole passes, the contact is made. Very reliable, fast response and easy.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Does the film de-polarize light? I had a project once that involved finding tiny holes in a film that would de-polarize light. So I set up a backlighting covered by two polarizing sheets at right angles (so they would pass almost no light) and then ran the film between them. Continuous film was bright white, because the polarized light from the first filter was rotated by the film and passed thru the 2nd filter. Any hole appeared coal black because the light remained polarized and could not pass the upper filter.

    The extreme contrast allowed an easy, visual test method. This replaced an airflow method that was cumbersome.
     
  10. ISB123

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    May 21, 2014
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    Wouldn't that introduce noise to the circuit?
     
  11. #12

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    Nov 30, 2010
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    Yes. Big, sharp edged noises that are easy to detect electronically.
     
  12. JWHassler

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    Sep 25, 2013
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    The film might have a variation in light transmission that you can exploit.
    It might be difficult to find that exact wavelength in a transmitter/receiver, and the film thickness will be factor,
    but if you have most of a year, this investigation merits a few hours
    upload_2014-12-20_8-55-56.png
     
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  13. GopherT

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    It could but appropriate pull-up resistors and filtering, the signal should be pretty easy to identify.
     
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  14. GopherT

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    What type of emitter/detector/filter do you recommend? The spectra you show are mid-Infrared (450 to 4000/cm). Common IR emitter LEDs are near-IR (900 nm or 11111/cm).
     
  15. Msinks

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    Mar 3, 2012
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    Does this mean I can make it in such a way that I can adjust my receiver to any one of the three frequencies of the materials you have listed...?
     
  16. GopherT

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    See post 14. There is no easy way to do this without an infrared spectrometer. Don't get distracted with ideas that are not a standard solution used in industry.
     
  17. jpanhalt

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    If you can at least tell us what the plastic film is made of, or why that seems to be such a secret, and what the minimal "hole" you need to detect is relative to the sheet width (see post #3), we may be able to offer more practical suggestions.

    As for using specific IR absorption bands, GopherT has alluded to some f the problems you might see. I agree. Also, don't forget the absorption of IR by water vapor is not shown in the spectra shown in post #12. Assuming the holes are relatively small (say, <5% of the area), do you want your results to depend on the relative humidity?

    John
     
  18. GopherT

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    John,
    The OP gave some details in post 7
     
  19. Bernard

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    See post # 6, holes 3-4 cm, polyethylene film 20 cm wide traveling .2 m/ sec.
    Re post #4, with slight positive pressure, place low mass thermister under film with enough currert to slightly raise temp, puff af air might cool sensor to give a reading.
     
  20. strantor

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    Oct 3, 2010
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    What's the project, the machine or the sensor? Is a commercial sensor allowed? I have used these sensors to detect panes of clear glass on conveyors. They work well and are simple to set up. The pic on the page shows them sensing clear sheet like you describe. I'm confident they would work for you. If you this route, don't forget the fiberoptic cable that goes with it (there are many options).
     
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