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Advice on sensor for plastic sheet

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  • Advice on sensor for plastic sheet

    I am looking for a sensor that can be mounted under a conveyor to detect when plastic sheet is present. I want to avoid using a through beam sensor since it would be impractical to mount something over the rack. The plastic is a continuous that is cut into sheets at the end of the line. As shown in the picture is starting up, where the burlap is pulling the start of the sheet until it reaches the end of the line. The plan is to have a set of sensors far enough apart so the burlap won't trigger it. The color of the plastic can be any color including black.

    Would a diffuse sensor work in this case, mounted under the rollers to see the plastic just above the rollers?

    Mike
    Attached Files


  • #2
    An ultrasonic should work well in this application.
    https://www.automationdirect.com/adc...ximity_sensors
    A diffuse photoelectric with background suppression. This should also work if the plastic can reflect the light. (Not Clear)
    https://www.automationdirect.com/adc...on_(gx_series)

    Regards,
    Garry

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    • #3
      I have never had a diffuse sensor work reliably detecting a black object. Ultrasonic seems to be the best choice.
      Why worry? If you've done the best you can, worrying won't make it any better

      - Walt Disney

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      • #4
        A capacitive proximity sensor may work for this application.

        The sensor CK1-00-2H has a 12mm sensing range.
        I use them to detect plastic pellets.

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        • #5
          If the plastic is always perpendicular to the sensor then an ultrasonic in object sensing mode would work (sensing plastic directly). If sometimes it's not perpendicular, then you would need to reflect the beam off of any perpendicular surface behind the plastic and use the sensor in background mode. Basically teaching a window on the object behind the plastic and then inverting the output like you would a retroreflective or thru beam sensor.

          P.S. Almost all ultrasonics have a dead / instability zone so make sure you mount the sensor at least that distance plus a bit more away from the plastic. It will detect almost any material in background mode with no problems.
          Last edited by Adisharr; 10-04-2019, 11:21 AM.

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          • #6
            I had thought about a capacitive proximity sensor because we do use them for other applications such as detecting the level of pellets in the hopper. The downside in this case would be they would have to be mounted close to the sheet and during startup & shutdown, the edge of the sheet can be very irregular and could damage the sensors. As shown is when the line was shut down to an unexpected problem, the edge of the broken sheet is dragging across the embossing heaters and can easily crack them.

            What I want to do is mount them a few inches below the rollers, between two rollers so the ragged edge of sheet does not strike them.

            We do run all colors of sheet, never clear, but black is a common color, so the idea of the ultrasonic sensors looks to be best solution. I will have to look in the spec sheets and see.

            Mike
            Attached Files

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            • #7
              Originally posted by mikeexplorer View Post
              I want to avoid using a through beam sensor since it would be impractical to mount something over the rack.
              Putting aside the impracticality of the "mount something over the rack". It appears that you recognize that a thru beam would be the best sensor/reliable method. Looking at the photo you supplied We cannot see the entire ceiling above the "rack", but is it possible to use some power from the lighting circuit to energize some emitters right at the ceiling? If the loss of power to the emitters would cause a control issue perhaps an extra emitter receiver pair that will not be blocked by product or people could modify the logic to prevent unwanted action. Obviously this would have to be high power pairs to "see" at this distance.

              Originally posted by mikeexplorer View Post
              The downside in this case would be they would have to be mounted close to the sheet and during startup & shutdown, the edge of the sheet can be very irregular and could damage the sensors. As shown is when the line was shut down to an unexpected problem, the edge of the broken sheet is dragging across the embossing heaters and can easily crack them.

              What I want to do is mount them a few inches below the rollers, between two rollers so the ragged edge of sheet does not strike them.

              We do run all colors of sheet, never clear, but black is a common color, so the idea of the ultrasonic sensors looks to be best solution. I will have to look in the spec sheets and see.

              Mike
              If you still like the capacitive sensor solution, can you mount a "skid plate" just after one of the rollers with a 4" hole in it and mount the sensor in the middle of the hole, but just below the surface of the skid plate?
              Last edited by Anvilsoup; 10-05-2019, 07:45 AM.
              Futti Utu

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              • #8
                This is where I want to mount the sensors, on this support beam below the rollers. Looking at the cost difference between diffuse sensors and ultrasonic sensors is not much. I did originally look at diffuse sensors but a good point was made about sensing black plastic. The different colors would cause a problem and we do run black a lot. It sounds like an ultrasonic sensor would be more stable since it wouldn't care about the color, just that there is an object in the sensing range.

                I don't have a good picture of ragged plastic, but a capacitive sensor would need to be mounted close to the sheet and we can run sheets as thick as 1-1/2" thick which is a lot of weight. We have these capacitive sensor mounted at the end of the line to sense the sheet for the stacker to pick them up to the skid, and a ragged edge literally wiped the top of the sensor right off.

                By mounting the sensors on this support beam, it will be low enough so the rollers can provide the protection in the event of bad plastic

                Mike
                Attached Files

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                • #9
                  Is it required that the sensor be non-contact?

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                  • #10
                    Originally posted by kewakl View Post
                    Is it required that the sensor be non-contact?
                    Yes, because where I plan to mount the sensors, the plastic sheet is still quite hot (about 250 degrees) and somewhat soft so any contact sensor could leave a mark. I also want to keep it below the rollers because when the line is shut down (sometimes expectantly) the edge of the broken sheet would be jagged and when it hardens, could strike the sensor and take it out.

                    Mike

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                    • #11
                      Understood.

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                      • #12
                        I too think that if through-beam is not a possibility, then ultrasonic would probably be the best bet. Capacitive just doesnt have the range you are wanting, and light wont work with clear plastic and need adjustment each time the plastic color changes. Ultrasonic should work the same regardless of plastic color used.


                        This actually got me thinking, I wish someone made a radio frequency sensor. Same principle as ultrasonic where it generates a frequency wave, only at a much higher frequency. The speed could be quite a bit faster (one of the main drawbacks of ultrasonic) and I would imagine you could also produce a tighter frequency beam closer to a light source diffusion pattern (the other main drawback of ultrasonic). There is plenty of frequency available set aside for use in the MF, HF, and UHF bands that could be used for this. And since they would be directional, low power, and meant to be reflected back you could even use the same frequency for all sensors without interference. Someone really should make a product like that.
                        Last edited by MikeN; 10-07-2019, 10:13 AM.

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                        • #13
                          Originally posted by mikeexplorer View Post
                          This is where I want to mount the sensors, on this support beam below the rollers. Looking at the cost difference between diffuse sensors and ultrasonic sensors is not much. I did originally look at diffuse sensors but a good point was made about sensing black plastic. The different colors would cause a problem and we do run black a lot. It sounds like an ultrasonic sensor would be more stable since it wouldn't care about the color, just that there is an object in the sensing range.

                          I don't have a good picture of ragged plastic, but a capacitive sensor would need to be mounted close to the sheet and we can run sheets as thick as 1-1/2" thick which is a lot of weight. We have these capacitive sensor mounted at the end of the line to sense the sheet for the stacker to pick them up to the skid, and a ragged edge literally wiped the top of the sensor right off.

                          By mounting the sensors on this support beam, it will be low enough so the rollers can provide the protection in the event of bad plastic

                          Mike
                          You could easily program a ultrasonic sensor to see the sheet. Just remember there's normally a dead space in front of the sensor, so you need to choose one that can see between the rollers and not get false triggers. I'd go with one that has a sensing distance no further than 400mm, like THESE.
                          Why worry? If you've done the best you can, worrying won't make it any better

                          - Walt Disney

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                          • #14
                            We manufacture plastic film, clear and white. This is what we use to sense the presence of the film. They work very well. Click image for larger version  Name:	web break sensor (2).jpg Views:	0 Size:	103.9 KB ID:	125607
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