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I don't know if this is a purely "robot" question or a more DIY/hackish one, but let's give it a try.
I currently have a set of cards that I want to sort based on several criteria.
My setup includes:

  • MeArm A 4 DOF robot arm
  • Raspberry Pi 3 + Shields for controlling the arm
  • A mini vacuum pump, hold in place by the gripper

These are the steps:

  1. Move the arm on top of pile of cards
  2. Turn on the vacuum pump
  3. Pick the first card
  4. Move the arm in the right spot
  5. Turn off the pump and let the card fall
  6. Repeat

Everything is working fine, my only main issue occurs when I'm lifting the arm. It seems that between the cards there are some kind of forces and under the first one, several others come up, attached.
I tried to shake the arm and make them fall, but it's not working.

Any suggestions? Maybe I'm missing some simple/obvious solution.

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  • $\begingroup$ How about not instantly lifting it up vertically, but sliding it off the deck horizontally for a short distance prior to lifting it? It would be even better if you are able to tilt the gripper about a horizontal axis to bend the card prior to lifting it, but it seems that your robot is not able to do so. If this doesn't work, I'm afraid you'll have to prepare your cards in some way or change the set-up alltogether. $\endgroup$ – JJM Driessen May 4 '17 at 11:36
  • $\begingroup$ Is it because the pump force goes through the cards ? Then could you just use a different set of card (plastic ones for example) ? Or why not try to turn on the vacuum pump slightly above the pile ? $\endgroup$ – Malcolm May 4 '17 at 13:36
  • $\begingroup$ The phenomena resulting in the cards adhering together is probably similar to wringing of gauge blocks. It's not well understood, but thought to be a combination of molecular attraction and surface tension: en.wikipedia.org/wiki/Gauge_block#Wringing $\endgroup$ – JDMc May 5 '17 at 21:28
  • $\begingroup$ @JDMc Thank you very much! Now it makes sense: card surface is very smooth, and I think that the pressure of the arm is squeezing them and pack them all together. I think I'll have to change my setup and come up with some "sliding" in place. Do you care to create an answer, so I can accept it? $\endgroup$ – tampe125 May 5 '17 at 22:05
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If your cards are like playing cards, they are likely very smooth. Cards like this can adhere or bond together in a similar way to "wringing" gauge blocks: http://en.wikipedia.org/wiki/Gauge_block#Wringing

The phenomena isn't that well understood, but is thought to be a combination of air pressure, surface tension and/or molecular attraction when two very flat surfaces are brought into contact.

To pick up a single card, you could try a different movement to help break the adhesion between cards, or, if possible, modify the cards to make them less likely to adhere. For example, roughing up the card surfaces may help.

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  • $\begingroup$ That's exactly the case in car industry, for instance. Steel plates are piled in a tower and the whole tower is surrounded by powerful magnets that make the plates separate between them. Aluminium makes this process much more difficult and compressed air and some other stuff is necessary. $\endgroup$ – galtor May 6 '17 at 6:50

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