Slip Sheet Sensing 2

[Farmboy71]

I am working on a project where I need to pick a fiber(or corrugated) paper slip sheet with a robot. Does anyone know of a sensor that would detect if multiple sheets were picked? It would be best if it would be non-contact and only need to see the sheet from one side. Any ideas? Thanks in advance.

 

[analogkid2digitalman]

What popped into my head:

You may want to look at Keyence's laser displacement sensors and micrometers. Single sided, non-contact, fairly high resolution. Gang up (2) for a differential measurement?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's the questions that drive us"
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[nbucska]

a fiber ?
What is slip-sheet?
Is the object you want to pick porous? May it be sticky?
Is the thickness constant?


Plesae read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[logbook]

If multiple sheets were picked up wouldn't the load on the pickup head be twice as great (or more. A strain gauge in the arm should measure that.

 

[VEBill]

If the robot does have two or more sheets, what are you going to do about it?

Well, just do that (whatever you're planning to do) each and every cycle at least once, repeating until your 'extra sheet removal system' comes away empty. Empty versus not-empty should be very easy to detect.

This is a similar algorithm to what many humans do. Pick up a sheet of paper and then 'pick at' the edge until we're slowly convinced that there's only one sheet.

 

[Farmboy71]

Thanks to y'all for the advice. A slip-sheet is placed on a pallet to prevent damage to product during shipment. It's usually very inexpensive because it is a throw away. They are of a consistent thickness but usually porous, causing multiple sheets to be picked on occasion. If multiple sheets are picked, sheets that ride along can be dropped of midway to the pallet and block other photoeyes and make a mess. We currently sense the presence of the sheet, but the multiple sheets are the ones causing us problems. If we miss a sheet pick we just re-try until we suceed.

The Keyence sensor may work, but I would need to have it right at the edge of the sheet and guarantee that any cling-ons would be extending out past the edge. I'm not sure how to do this.

The sheets are usually very light so the sensitivity of a load cell would need to be tight. This could be difficult becuase the robot also has to have a payload of 300+ lbs. I like the idea and may look into it further.

If multiple sheets could be detected, we would probably stop and call the operator to the cell. If we had the robot drop the sheets back on the stack and repick them, it could work. However it would consume robot cycle time which is a premium resource. We could look at doing the sheet seperation independent of the robot to minimize the lost time of the robot's primary function.

Thanks again for all the advice. And if anyone has other ideas please post them.

 

[itsmoked]

Can you not change to a vacuum pick up which would only act on the top surface of the top sheet. Another words never even pick up two sheets.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[IRstuff]

Porous? How porous? And how transmissive?

Can you set up a photocell that compares the transmission of a single sheet vs. multiple sheets?

TTFN

 

[Skogsgurra]

They are porous...

Seen that problem. They used a camera and some intelligent vision system to see if there were more than one sheet picked up. The operator was alerted and he adjusted the vacuum. It didn't happen very often so it didn't pay to automate that part. The supervision was the main thing.

Gunnar Englund

http://www.gke.org

 

[MacGyverS2000]

Can you use an air blower to force any loose sheets to shake off and stay in the pick-up bin? How about using a robotic pickup on both the top and bottom... top picks up a sheet(s), bottom one tries to pick off any clingers, if bottom fails you have a single sheet.

I'd like to see an example of the machine before trying to make more suggestions.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[machmech]

hello folks

I would like to add in a process of separating paper there
are variations in size of sheets, how well the sheets are
jogged together on a stack, and static electricity is often
a problem even if sheets are separated good, sometimes the
top sheet will have static build up and bring the next
sheet with it. Coated papers are even more difficult.

With this in mind would it be possible to mount a light
source below sheet being pulled and measure the amount
of light visible through paper ? Photo cell maybe?

Chuck

A government which robs Peter to pay Paul
can always depend on the support of Paul.
- George Bernard Shaw

 

[machmech]

Hello Farmboy71

Our equipment use a rubber sucker with vacuum to pick
up one sheet of paper, Then a separator operated by a
cam moves in keeping the chosen sheet separate from
the others, The cylinder revolves and meets rubber
segment to pull sheet. This happens at 800 sheets a
minute.

Your problem is very common for us we call Doubles.
Normally on an Envelope machine or Printer this will
cause a jam or skip, Photo cells will see reflector
tape on cylinder which a piece of paper is supposed
to cover and will stop machine, Or feelers will sense
jam do to thickness of paper and stop machine.

Other then this the best thing to do is fix the
problem, Adjust the machine, Change to a better
quality paper, Improve the process so this won't happen.

Chuck

A government which robs Peter to pay Paul
can always depend on the support of Paul.
- George Bernard Shaw

 

[Farmboy71]

Thanks for the replies Chuck. I think often people contribute the sheet sticking effect to static electricity when it's actually caused by the vacuum that is created when multiple sheets are pulled apart quickly. This is especially true with large(40"x48") slip sheets. For example, if (2) sheets are pulled apart(assuming they are perpendicular) to a distance of 1 ft within a second, the instantaneous air demand to fill the space between sheets is about 800 SCFM. Static electricity may have an effect(especially with plastic sheets), but I think it's mostly due to the vacuum creation. Let me know if you agree. Also paper fibers along cut edges can have an effect.

I have seen cam driven label pickers do what you described, but I don't recall a seperator mechanism. Can you give me more details or perhaps a picture? I'm curious how the seperator mechanism works if the cups pick multiple sheets.
The principle may work for what we are attempting to do. Also, what kind of thickness "feelers" do you use?

Thanks,

Farmboy

 

[machmech]

Hello Farmboy71

I disagree with vacuum theory from my perspective
I fail to see this logic however you have an
interesting point. Friction from paper fibers is
very probable, under extreme magnification paper
is very fuzzy on the surface. also if the paper is
straight knifed with a dull blade the process will
weld the sheets together on the edges, to remedy
this an operator can do what we call breaking the
paper this can be done by fanning the edges of
paper on all sides of skid.

Another way is to use compressed air and blow into
stack to break edges apart however this will
disorganize the sheets on skid and someone will
need to jog paper even again.

The following link will display a separator arm and
some of the components the way the arm operates is
after the paper is picked away from stack the pointy
tip with air channels moves into space made in sheets
from vacuum sucker picking up paper, at the end of
separator arm stroke 22 P.S.I. of compressed air is
applied to facilitate separation of sheets during
pull process, now with your theory this compressed
air would fill any void left from vacuum.
(This is why I find your point interesting!)

http://tinypic.com/view/?pic=1z5hhdz

I did try to follow itsmoked instructions for posting pics. but had problems I will need to figure it out later.

If you can tour an envelope or printing company it may
purge some good ideas. If by chance you are in Minnesota
I could help.

Chuck

A government which robs Peter to pay Paul
can always depend on the support of Paul.
- George Bernard Shaw

 

[itsmoked]

RE: machmech's previous post.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[machmech]

Hot dog Keith

You are the Man!

I placed [square] brackets and it would not work!
I have Process TGML checked!
Enlighten me Please

I have no problem w/ what am I missing?

Chuck

A government which robs Peter to pay Paul
can always depend on the support of Paul.
- George Bernard Shaw

 

[itsmoked]

Hi machmech.

I think you had two booboos.

That particular site provides several "links" you need to use the one called specifically "Link" others can take you to their site which my be blocked by eng-tips or some other mechanism.

Second you need more than just the [ and ] you need 'img' too.

[ignore]

[/ignore]

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[machmech]

HA! I got it!!

Keith you made me think, kinda like skoggs only threw me a
bone.

I checked the properties of the pic and got this

http://i6.tinypic.com/1z5n0pj.jpg

I was missing (.jpg)
at the end and wala!

Thanks Keith

A government which robs Peter to pay Paul
can always depend on the support of Paul.
- George Bernard Shaw

 

[itsmoked]

Excellent you saw that! I missed it too.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[machmech]

More thoughts Farmboy71

You said your cups pick multiple sheets, sometimes the
cause is due to paper porosity and amount of vacuum.
With porous paper the vacuum will penetrate and pick
up the next sheet as well, here you need to lower vac.
or bleed it off.

Our feelers are nothing more then basic micro switches
the resolution of switches is not critical as a paper
jam at 800 epm is somewhat dramatic.

Chuck

A government which robs Peter to pay Paul
can always depend on the support of Paul.
- George Bernard Shaw