Missing Label Detection

[PaulB 0]

We are manufacturing slitter rewinders, which inspect and slitt printed label materials. Roll of material (paper or film) 10"-20" width, 30" diameter, and with multiple streams of labels (4-8) on it, is unwinded, slitted on separate streams and rewinded on the small size rolls. The max speed of the machine is 900 fpm. During this process strobe light is working and printed material is visually inspected by operators. Label size and material varies from customer to customer. We are looking to automate missing label detection. Maybe I am wrong, but I am envisioning this sensor as some sort of "light curtain". Web will pass through this "light curtain". This "light curtain" will be able to detect missing label at the speed that machine is running and provide signal to PLC. I was not able to find any suitable sensor for this application. Is anybody familiar with this type of application and know anything about this type of sensors? Thanks in advance.

[Chris Elston 78]

I am having trouble envisioning this. Are you allowed to show a picture of this? If I could see a picture, I might be able to help suggest. Also at 900fpm, what is the distance between each label? Or are you saying it's one continuous label?

[PaulB 0]

I have attached two pictures. This is material that is one of our customers is using. These are example of film material labels clear on clear. It also may be paper material paper on paper. The distance between labels usually about 0.2". I depends on the label size and die cut size. Labels may be as small as 1.5". It is all depends for what product they are used. For counting these labels we are using capacitive sensor Banner SLC1. This sensor needs to be oriented perpendicular to the direction of the moving web therefore I am not able to assemble array of sensors to "look" at each stream of labels. Another problem with using array of individual sensors that I do not have that many available interrupt inputs on my PLC. I would need to find some sort of device, which will combine the inputs from these sensors and generate signal in any one of the inputs is missing. Thanks. PaulB

[Smoke 0]

From what I see at 900 fpm you are talking about 30 labels in each row each second. I think you can do this with a vision system either DVT or Cognex. I have done something sort of close to this with a DVT. You would leave the camera on internal trigger then program it to report when one is missing. DVT has been really good about helping to setup apps.

[PaulB 0]

I don't think that 30 labels per second is correct rate. 900 fpm is 15 feet per second or 180 inch per second. If label length is 1.5" it makes it 120 labels per second. I am not familiar with vision systems. Will it work at this rate? PaulB.

[Chris Elston 78]

In all your different types of labels, if you have a missing label, is there a "hole" created just like the picture shows? I was trying to thinking of a cheaper way to detect it, but if I go with PaulB, rate at 120 labels per second, that would only leave us 8.3 milli seconds (am I thinking right?) to detect if a label was missing or present. A SLC can't do that with an input without an interrupt. You possibly have to go with a vision system. The fastest Cognex application I have ever done was 200 part per second, but it was almost a blur to see that.

[TimWilborne 108]

Wow. 200 parts a second. Way out of my league

[Smoke 0]

I still cant get the math correct. 1 column 900 fpm, 6" labels = 1800 labels per minute / 60 seconds still equals 30 labels per second to me. Easy camera app. camera (black and white) around $1100.00 + lighting + cables You might want to use 2 or 4 cameras though. I have read the bar-codes at a box printing factory faster than my calculation but haven't tried as fast as chakorules, but I do set my exposure for under 1ms usually and I have not checked the processor time.

[Chris Elston 78]

I don't think everyone is on the same page. The size of the label matters. In Paul's post he says 1.5" labels. You're using 6" labels in your math. Technically, it depends on the size, and so far I think I understand there is a range in size, right Paul? 1.5" being the smallest label? And the webbing runs at the same speed no matter if you are running the 6" label or 1.5" label correct?

[Chris Elston 78]

Well, let me clarify....it ran at 200 part per second, but I could see 10 parts in my FOV, so divide 200 by 10 and you get 20. So the camera took a picture every 10 parts, and inspected 10 of them at a time. They where pins that had UV glue on them. Lit up like a chirstmas tree using a UV light. I looked at the specs of the 5400 Insight Cognex, you can do 60 frames per second with a 1ms shutter. It can do 16 micro-sec shutter, but I bet your going need ALOT OF LIGHT to shove in the camera to get that to work.... DVT has a 10 micro-sec shutter in their high speed, again lighting will be key at those speeds. I recommend doing what I did if your labels are 1.5". See what is going to happen is you need to layout your FOV (field of view for your biggest label, if that is a 6" label, then your FOV is 6 x 6. At 6" your running a rate of 30 labels per second. No problems.... If your running a 1.5" label, just plan on looking for 4 of them at a time, so you only take a picture every fourth label. This keeps the cycle of the camera still at 30 labels per second so long as your FOV remains at 6 inches. Still at 30 parts a second, I would be a bit concerned with lighting, also you said you have different labels, some clear, some white....this would play rough on your vision application. I think a Cognex 5400 can do it using some rules I've stated above. The second task will be triggering every four labels, but you said you have an interrupt input now, the Cognex camera has a high speed input you can wire up directly, though I don't know if you can program or setup the hi speed input to take a picture every four cycles.....hmmmmm but if you had a hispeed output from your PLC, you could drive your trigger to your camera this way. Again, lighting will be KEY FACTOR, two reasons: 1. different parts, clear labels and white labels. 2. 30 parts per second rate, this means about a 2ms electronic shutter. You have 2ms to get some light into the camera. Might need a strobe light to do it...rather than a steady on LED. I always have cognex tell me what's the best based on shutter speed.

[Smoke 0]

I think I got cault up in the pictures he did say 1.5" he also said. It must have a VFD or something.

[PaulB 0]

Machine speed of 900fpm and label size of 1.5" are within capability of this equipment. We are able count labels at that speed. I might follow your advice, and investigate vision system for this application. As you said the proper lighting will be the key factor. Do you think camera needs to look at every label or it might look at the some area equal to the width of the web by some number of label, lets say four or six? What I mean if web width is 17" and camera able to evaluate area of 17" by 6" for empty spot then speed requirments become much easier. Do you think camera can be configured to generate output only on empty spot and do not pay attention to other deffects? From what I've heard you teach camera with the "golden image" and it will react on any deviation from this image, so any difference from the image will generate output signal. PaulB.

[Chris Elston 78]

An FOV (field of view) of 17" by 6" is doable. I would consider pursuing looking at multi labels to limit your camera cycle time. If you do me a favor, stand on a stool and take your digital camera and take a picture of your labels from the top down, try to make the labels lay pretty flat, and if possible no glare, maybe even turn off your camera flash so no flash glare is in the picture make the picture so you see four labels in each row. Then remove one label and take a second picture. I can take those two pictures and put them in my cognex simulator to see what it will look like and post screen shots back to you. I have attached a spreadsheet which is called a lens chart. I normally try and use a 25mm lens for everything I do, to eliminate "fish-eye" if I picked a 25mm lens for your application, my camera would have to be mounted 88.5 inches away from the labels for a 17" FOV. If you don't have room to mount the camera that high you can pick like an 8mm lens. Which now you can move your camera closer around 28.3 inches. An 8mm lens will make alot of fish-eye, but all your doing is absent-present, it won't matter. Another consideration is the camera application itself. It seems to me each label will have to be "taught". This can be done with a wizard in Cognex one camera program, but every time the label is changed, the person has to teach the camera good from bad. Or if you don't have that many different types of labels you can have a program for each label, and recall that program each time you run those parts. It depends on how many labels you have to how cumbersome that will become. Of course if your machine knows what label it is running, and you can interpolate that into a BCD output to the camera, you can have your machine recall the correct label program based on BCD (binary) to load the correct program automatically when the machine changes over. The learning of the camera is generally referred to as "pattern matching". Pattern matching in Cognex can count patterns. So if we have a 17" x 6" FOV, which includes a matrix of 4 x 4 labels, 16 labels total, the pattern match should always count 16 in the pattern. If it's less than that, it fails and fires and output. Cognex_FOV.xls

[PaulB 0]

I've posted this and after that opened your excel file. Indeed mounting height depends on inspection area and it is expressed in tenth of mm. How critical is it? PaulB.

[TConnolly 13]

Check out the Cognex Checker http://www.cognex.com/products/Checker/default.asp

[Chris Elston 78]

I've used the cognex checker before, and although it's a nice camera for certain applications, I don't believe it will do the trick for this missing label application. The application is too fast and the FOV will be too large for this camera. There are not many lens options for a cognex checker as a standard c-mount Insight type camera.

[Chris Elston 78]

Ok I am beginning to see the application now. It's a option to a machine and that machine will be sold to many different types of people. This can still be done with a cognex. There are two ways to program the cognex, either a laptop or through a joy-pad. It looks like a playstation controller. If it was me, I would probablly go with the playstation controller option. Which means you need to purchase the Cognex Insight 3400 series camera. You'll get a complete stand alone system with a joypad. You don't need a computer or laptop to make adjustments to the vision or teach the vision system. The 3400 system can aquire up to 38 frames per second, so if you stick with the 6" rules of 20 parts per second, this 3400 will do ok for the speed. It's slower than the 5400 series camera, but the 5400 series camera can only be programmed with a laptop. Next I would create a single cognex application using the wizard inside cognex, create a step by step for your end user to "teach" labels. He can use the control pad to walk him through the wizard of presenting good and bad labels under the camera. Once the camera is trained, press cycle start and away you go... The FOV is not critical. Depends on what lens you use. That is the top column. 8mm, 16mm, 25mm, 50mm. These are lens sizes. Follow the chart down to the H x V (horizontal or vertical) FOV (field of view). In your case you said 17 inches. I highlighted 17" FOV. Follow that across and you see using a 25mm lens your 80 something inches away. The lens, has a focus ring on it, this gets you in the ball park of your focus. You fine tune your distance by focus.

[IamJon 0]

it was mentioned many times that lighting is key. one way to make the lighting situation easier on the camera for the most difficult situation (clear labels) may be to 'green screen' the backdrop. whatever is behind the labels could be painted a bright color that contrasts the color of the clear labels made (meaning text, pics etc on the label). green might not work for jergens because one of the pics is yellow(ish). anyway, backdropping could possibly make detection more reliable for the camera.

[wborst 0]

How about using a small dedicated PLC for detecting the missing label. For example, use a through-beam or retroreflective photosensor to detect the labels. Use a timed interupt set to interupt every 2 or three milliseconds. at each interupt, do and immediate input of the 4 photosensor signals. If any input is not detecting a label, Increment a counter. If any counter every reaches 2, that indicates that a space between labels that is greater than the normal gap. Use this to set an output that is connected to the main control system. I believe that a Dl06 from Automation Director a Micrologix from AB is capable of the required speed. Just a thought.

[Smoke 0]

We are doing some High Speed counting. occasionally a count was missed. We have just added a high speed counter module and stopping the scan with the input and its working great. This is with CLX.