Author Topic: Clear Tape Detection  (Read 648 times)

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Offline PBn

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Clear Tape Detection
« on: October 03, 2019, 09:09:02 pm »
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?
 

Offline james_s

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Re: Clear Tape Detection
« Reply #1 on: October 03, 2019, 10:02:57 pm »
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.
 

Offline PBn

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Re: Clear Tape Detection
« Reply #2 on: October 03, 2019, 11:38:07 pm »
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.
 

Online Marco

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Re: Clear Tape Detection
« Reply #3 on: November 21, 2019, 01:44:46 pm »
The angled reflection seems worth a try, just bodge it up with a pot and a scope to see if it works.
« Last Edit: November 21, 2019, 01:48:40 pm by Marco »
 

Offline grifftech

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Re: Clear Tape Detection
« Reply #4 on: December 12, 2019, 06:38:28 pm »
different tape ;)
 

Offline m98

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Re: Clear Tape Detection
« Reply #5 on: December 30, 2019, 08:59:48 pm »
You could use an arrangement of a soft light
source sharply limited across the whole detection length and a camera, both pointing up at the package. Then you can do some image processing, use a high pass filter somewhere, and you can detect wether there is a sharp line in the right orientation or not.
 

Offline vu2nan

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Re: Clear Tape Detection
« Reply #6 on: January 04, 2020, 05:14:03 pm »
Use auto bottom boxes to avoid the taping and its problems!

Regards,

Nandu.

Online wraper

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Re: Clear Tape Detection
« Reply #7 on: January 04, 2020, 05:24:23 pm »
Does it has to be new? You can get used sensor made specially for the task for about $200. And even new does not cost anywhere close to $2000

https://www.radwell.co.uk/Buy/WENGLOR/WENGLOR/GM04VC2?redirect=true
 

Offline ebastler

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Re: Clear Tape Detection
« Reply #8 on: January 04, 2020, 07:48:27 pm »
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.

I like the "how directed or diffuse is the reflection" approach. To make this robust, you should use one directed source of light (cheap laser pointer?), and at least two detectors (or use a line sensor for multiple detector elements?).

One detector should be arranged to capture the direct reflection (i.e. incident angle of light onto the box surface = exit angle from illuminated spot to the first detector). The other detector should be placed away from the direct reflection angle, but such that it captures enough of the diffuse scatter from a cardboard surface.

Then you calculate the ratio of intensities on detector 1 / detector 2. That ratio will be high for glossy (tape) surfaces, and low for matte (cardboard) surfaces. But it will be largely independent of the degree of overall reflectivity or absorption (i.e. the color) of the substrate.

You could get fancier with a line sensor which covers a range of scattering angles. This might be needed if you deal with multiple different materials and want to distinguish them. But for the simple "glossy tape or cardboard" classification, two detectors should do, I think.
 

Offline Doctorandus_P

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Re: Clear Tape Detection
« Reply #9 on: January 07, 2020, 07:01:07 pm »
I assume the boxes are moved over an conveyor belt of somse sort.
Is the tape parallel to the conveyor belt, or square to it (What's the word?)

I would not rely on a single sensor, but would do at least 2 measurements and compare those.
One measures a base level of reflection on an area there should not be tape, and another measurement which measures the reflection level of the tape.

If the tape is longitudinal (parallel) to the conveyor belt this would need 2 sensors, but if the tape is at a 90degree angle to the conveyor belt, you can measure refelctivity of both and even measure the width of the tape (assume constant speed of the belt ?)

So if you have your sensor output "change" for a time comparable with the widht of the tape, you have a big chance it's really tape you have just measured.

Also, stuff like reflectivity & transparancy is wavelength dependent. Some time ago I saw a few pictures made with an IR camara of a sowing machine, which had a black rubber timingbelt in it. On the IR photograph, the rubber of the timing belt was almost invisible. You could see right through it, but the strengthening wires in the timingbelt were pitch black.

You could make something with fast blinking laser pointer and a photo diode in a tube.
The tube shields from most of the ambient light, and the blinking of the laser pointer (a few kHz) can also be used to distinguish from other light sources.

Some problems with this approach are with non-uniform color of the box (dirty patches, labels, etc)

A completely other approach is not to detect the tape itself, but the tape dispenser. Put some rotation sensor (encoder) on the roll of tape itself (or with a friction wheel, etc). If a box passes the tape dispenser, and no tape is dispenced, you have a box without tape.
More accurate: Measure the length of tape dispensed, so it also detects if a box is only half taped.
 


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