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unusual Sensor requirement

Discussion in 'Hobby Electronics' started by Mark Kelepouris, Oct 19, 2005.

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  1. Hi group,

    Does anyone know of a way to reliably detect a thin ( 0.1mm ) white
    synthetic plastic label/sticker material, still on its backing substrate of
    silicon surfaced paper, from paper label material ?

    The sensing conditions are that the label material is suspended between
    rollers and moving at a rate of about 250mm sec. from roll to roll before
    being printed.

    Yes this is a rotary printing press.

    A blunder occured last week by an operator who spliced together the two
    different materials without realizing it! It obviously cost us bucks after
    the client rejected the whole labeling run.

    This may not be the most appropriate group, but perhaps someone has heard of
    such a sensor or has an idea that could be R&D'd

    Maybe some sort of static charge and discharge sensing arrangment ? I dont
    know.

    I would be interested in reading any response, including direction to
    another group or web site.

    Thanks to those who reply.

    Mark Kelepouris
     
  2. A vision inspection system would be a solution.
    Quite a few companies around specialise in this sort of stuff, and the
    technology is very advanced and easy to implement these days.

    Dave :)
     
  3. Mark Harriss

    Mark Harriss Guest


    What are the optical differences of the two?. Does the paper label have
    whitener dyes added that make it fluoresce?. It may be as simple as a UV
    LED and an appropriate sensor connected to a microcontroller to
    periodically compare readings and alarm if the values exceed a preset value.
     
  4. value.


    Sorry Dave and Mark,

    I dont think either solution would work. Video inspection or 'black light'
    detection methods cant possibly work here. Essentialy I need to detect the
    difference between plastic and paper, they will both look white and possibly
    contain optical brightners.

    Thanks however,
    M.K.
     
  5. Poxy

    Poxy Guest

    Or perhaps even just sensing the reflectance of the stock?
     
  6. The reflectance could easily be the same on either material, it wont work.

    M.K.
     
  7. Mark Harriss

    Mark Harriss Guest


    Ok then: use capacitance: the dielectric constant will vary between
    paper and plastic, if you use large enough plates, say A4 the difference
    will be pronounced. Use it to set the frequency of a 555 and rectify and
    filter to get a varying voltage depending on the dielectric constant.
     
  8. Mark Harriss

    Mark Harriss Guest

    I'd be extremely surprised if the optical properties are identical
    across the whole optical spectrum, chances are the differences are
    pronounced at the IR or UV end of the spectrum.
     
  9. Poxy

    Poxy Guest

    I'd suggest, assuming the sensor has reasonable resolution, that it's highly
    unlikely that 2 stocks would have the same reflectance, particularly between
    paper and plasitc. I've worked with both paper labels and wine label stock
    (plastic), and while they may, to the naked eye, look similar, with an
    objective sensor, isolated from ambient light with its own reference light
    source, it should be quite feasable to identify a difference in reflectance,
    perhaps using a specific colour.
     
  10. What about capacitance? Run the material between two metal plates and
    measure the capacitance. The two plates might be the capacitance in a
    square wave generator. When the output frequency changes above or below a
    preset level an alarm is sounded.

    R
     
  11. Geoff C

    Geoff C Guest

    Reflection measurements are difficult and not robust. It would be much
    better to use TRANSMISSION of an optical beam through the material. You
    would need first to determine the spectral differences but it is likely
    that paper will allow a small amount of scattered UV through. Plastic
    will allow none through at wavelengths well below 400nm. You would need
    to use a spectrometer to determine this by sampling the paper and
    plastic. From here though it is still a significant challenge to arrange
    UV only light source and probably a detector system using optical
    chopping.
     
  12. Ralph

    Ralph Guest

    Try comp.robotics.misc
     
  13. Thanks to all,
    The capacitance idea sounds plausible indeed but would it actually work with
    new material constantly moving through the press ? Also it may be difficult
    to arrange two large plates especially if they have to be close together.

    The transmission of a UV light source and detecting the amount of UV that
    passes sounds promising especially if a UV LED can be used with an
    apropriate sensor/filter. Are they available ? (I have to do some research)

    Food for thought indeed,

    Thanks again
    MarkKelepouris
     
  14. I have seen this done plenty of times before and in every instance a
    stock standard IR LED and Photodiode assy is used. There is always
    going to be a difference in the materials, and the label is always
    going to be fractionally thicker. Remember that IR is not visible,
    where as the colour you see in the label is. The main problem is that
    unless the label backing is very still, the electronics or software
    gets quite complicated.

    At the end of the day, its not going to be an easy task so I
    reccommend that you engage the services of a professional company that
    has already done this. If that is not an option then the staff need
    better training.
     
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