Detecting PET bottles using LEGO NXT

Discussion in 'Embedded Systems and Microcontrollers' started by Stick1000, Nov 7, 2015.

  1. Stick1000

    Thread Starter New Member

    Nov 7, 2015
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    Hi! I'm just a novice guy who wants to create a simple reverse vending machine using LEGO Minsdstorms NXT that can accept plastic bottles and can dispense tickets. I managed to make the ticket dispenser but I can't seem to find a way to detect the PET bottle in the first place. Is there any way of detecting PET bottles using LEGO Mindstorms NXT?

    Any help is appreciated. Thanks!
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    How accurate do you have to be? What is the environment (unsupervised public access vs supervised public access vs trusted adult/corporate area)?

    What is the value of a ticket? That will explain what will be the risk of an error and quality of PET bottle detection required.
     
  3. Stick1000

    Thread Starter New Member

    Nov 7, 2015
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    Thanks for your help, what I want is to make it detect PET bottles as accurate as possible because I want to place it in a school. The value of a bottle is 1 ticket as long as the bottle < 500 mL in size. I have placed a hole (where the bottles will be dropped) which has a size limitation so bottles larger than a common 500 mL will not fit. I also want it to reject an object if it was not detected as a plastic bottle.

    Again, thank you for your time,

    Stick1000
     
    Last edited: Nov 7, 2015
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Do you have a general layout of there receiver? Anything more than just a hole?

    Do you have a list of the detectors/sensors available for the nxt?
     
  5. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    At the bottom of this thread is a suggested link to a thread, which someone started four years ago, whose requirements sound very similar to yours. The discussion there may be informative (don't respond there, though). Apparently, there is no simple solution,
     
  6. Stick1000

    Thread Starter New Member

    Nov 7, 2015
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    My LEGO NXT has the following sensors:
    • ultrasonic
    • light
    • touch
    • sound
    I cannot provide a picture of the hole since I'm at home right now and I left it at school. I'll describe it as best as I can.

    The hole is a vertical circular hole which has a diameter of about 2-3 inches. It is located on top of the machine and it is like a cylinder where the bottle will be temporarily placed while the it is detecting the bottle. The bottom part of the cylinder will open if it is a PET bottle, dropping the bottle inside the machine which has a garbage bin.

    Very thanks,

    Stick1000
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I would detect the presence of a something (hopefully a bottle and not a hand or other object) by connect the touch sensor to some sort of a flapper or spring-tensioned roller in the middle of the cylinder-shaped space.

    I would also use the light detector to make sure it detects the neck of the bottle (near the cap).

    Now comes the programming. You have to make sure that a user does not hold the bottle and just keep getting tickets. So, once the door opens on the bottom, you have to make sure that the light sensor at the neck of the bottle goes back to an empty state BEFORE the touch sensor detects that the bottle has fallen through. The ticket is only dispensed after the both sensors return to normal (bottle gone) and the door closes again.

    Requiring that the top sensor detects the bottle is gone first, then the bottom touch sensor indicates the bottle is gone will insure that the bottle leaves through the bottom door rather than being pulled back out (or being tricked with a hand or other object). It will be difficult to measure that a bottle is in the machine and not a rolled up magazine but, how many magazines are in schools these days (after all, playboy changed their photo policy). Anyhow, good luck. I am sure other members will improve on these initial thoughts with mechanics and logic.
     
  8. Stick1000

    Thread Starter New Member

    Nov 7, 2015
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    Great idea, (never thought of that)! I hope this will work, since I'm joining a robotics competition. (do not read)I am technically qualified to join the national level (Feb 2016) but some other city just can't accept the fact that our city was qualified, that is why they arranged an event to compete us this November.(do not read)Anyways, thank you for your time and suggestions. I will post an update if this set up will work.

    Very thanks,

    Stick1000
     
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