unusual Sensor requirement

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 :)

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.

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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.

Or perhaps even just sensing the reflectance of the stock?

The reflectance could easily be the same on either material, it wont work.

M.K.

don't think any method would work, as you're trying to detect paper from plastic, and most likely your paper sticker would have a plastic coating on it, so electronically it won't work, optically it won't work

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.

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.

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.

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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.

"Mark Kelepouris" wrote in news:4356202a$0$11682$ snipped-for-privacy@news.optusnet.com.au:

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.

Try comp.robotics.misc

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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

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.