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Slip Sheet Sensing
2

Slip Sheet Sensing

Slip Sheet Sensing

(OP)
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.

RE: Slip Sheet Sensing

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"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

RE: Slip Sheet Sensing

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/>

RE: Slip Sheet Sensing

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.

RE: Slip Sheet Sensing

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.

RE: Slip Sheet Sensing

(OP)
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.



RE: Slip Sheet Sensing

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

RE: Slip Sheet Sensing

Porous?  How porous?  And how transmissive?  

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

TTFN



RE: Slip Sheet Sensing

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
www.gke.org

RE: Slip Sheet Sensing

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

RE: Slip Sheet Sensing

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

RE: Slip Sheet Sensing

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

RE: Slip Sheet Sensing

(OP)
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

RE: Slip Sheet Sensing

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

RE: Slip Sheet Sensing

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/smile what am I missing?

Chuck

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

RE: Slip Sheet Sensing

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.

[img 'your link here']

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Slip Sheet Sensing

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

RE: Slip Sheet Sensing

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

RE: Slip Sheet Sensing

I would put some weight behind Farmboy's vacuum theory.  Ever broken a board using a single sheet of newspaper as a child?  Lay a piece of wood, like a 1x1 over a counter's edge.  With a full 2-section sheet of newspaper, spread it over the counter so it also covers the end of the wood stick, then smooth it out as much as possible.  Take a second stick and give the first a good whack... if set up properly, the first should break near the counter's edge.  Nothing holds the first stick down except the vacuum created under the newspaper... you're trying to lift the paper up so fast when you hit the stick, the air cannot enter quickly enough around the edges, so it acts like a pretty darn good vise.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Slip Sheet Sensing

Hey Dan thanks for explaining that.  I have actually seen that setup somewhere before but never slowed down enough to wonder what it was for.. Now I know.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Slip Sheet Sensing

Hello Dan

Good point! Although this is nothing more then a card
trick there are many other forces at work there. This
example is a little out in left field, but lets use it!
Now imagin just before the 1x1 is hit, three columns
of 22 P.S.I of compressed air is inserted between counter
and newspaper. How much vacuum will be there at the
time the 1x1 is struck? air is still on, now how much
after it is struck.

Heck if you try to lift machined plate steel one straight up
from the other they will stick together, due to vacuum.
The process needs help.

Our process does not allow vacuum. Does this make sense or
have I been in the think tank too long?

Chuck viking

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

RE: Slip Sheet Sensing

Farmboy,
Back to your original question of the sensor; ultrasonics provide a reflection of the transmitted signal at each material boundary.  If you aim the sensor perpendicular to your paper surface, you will see a strong reflection from the top surface of the slip and a weaker reflection from the bottom surface, separated in time.  You will see progressively weaker reflections from subsequent layers of slip which may be stuck to the top slip.  
Although these sensors are primarily designed to measure liquid levels and things like that, you might be able to get with the applications guys from a company like the one below to see if they can talk you through how to macgyver their sensors to work for you.

http://www.sensorsforindustry.com/prod13.htm

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