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- Mark Kelepouris
October 19, 2005, 10:28 am
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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
Re: unusual Sensor requirement
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.
Re: unusual Sensor requirement
of
before
after
heard of
dont
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.
Re: unusual Sensor requirement
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.
Re: unusual Sensor requirement
substrate
between
two
light'
possibly
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
Re: unusual Sensor requirement
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.
Re: unusual Sensor requirement
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.
Re: unusual Sensor requirement
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
Re: unusual Sensor requirement
On Thu, 20 Oct 2005 17:28:28 +1000, "Mark Kelepouris"
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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