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Clear Tape Detection

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PBn:
I'm doing some industrial work and I am looking for some feedback and ideas.
 
The issue is some boxes are getting through without tape on the bottom, someone goes to lift them up and all the stuff falls out.

I am looking for a sensing solution that can detect clear packing tape on a variety of colored boxes.

It looks like there are some sensors for "gloss" detection
keyence CZ-H72 And the Sick OPR20G seem to be good options.

The Keyence seems to do some kind of fancy polarization measurement to determine how much of the light was reflected and how much was defused.

The Sick one simply illuminates the surface and a sensor determines how "Tight" the reflection is

However the painted craft paper boxes we use are very matte.

Looking over the details of how some of the fancy gloss sensors work they just reflect light off the surface at an angle


The steeper the angle the less defuse light will reflect to the photo-sensor.
It seems like one could just angle a tx and rx sensors so they bounce off the bottom of a box at 90° and that would produce a strong signal to noise ratio, perhaps some simple polarization.
It will take some tweaking to get the level right. But I suspect it would work well enough.


It may not be quite as nice as the 2000$ sensor solutions but we can deal with some false negatives.

Anyone have thoughts or other ideas?

james_s:
You could try viewing the tape and boxes under various wavelengths and see if the tape is dramatically more opaque or more/less reflective at certain wavelengths, particularly in the UV and IR range. You also might be able to detect gloss fairly easily by using a simple reflective sensor with an LED or laser diode and photodiode angled to one another. Or maybe you can try to mechanically sense the gap between the flaps, something like a piece of piano wire on a microswitch? The theory being that if the tape is applied the wire won't be able to spring up between the flaps.

PBn:
I thought of the light thing, the tape does faintly glow under uv but not substantially more than the dyes they use in the packaging.
It is kind of shocking to me they don't manufacture tape with UV dye in it for this very reason.

Unfortunately the tape webs across and can dip down and stick to the minor flap pretty easy.
Also the boxes are moving on rollers so it would be very hard to measure.
I don't think mechanical techniques will work very well.
The difference in the coefficient of friction between the tape and the cardboard is pretty high, but I can't think of a reliable way to measure that ether.

I'll try to work out a way to test if the tape looks different in near, ir maybe an old night vision camera or something.

Marco:
The angled reflection seems worth a try, just bodge it up with a pot and a scope to see if it works.

grifftech:
different tape ;)

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