"more than one puffer would be active at once" — sounds like a job for prime numbers!
And that's exactly how it was solved. The puffers are now spaced prime distances apart and that took care of that, it would be harder for a longer belt because then you'd start wasting an awful lot of space next to the belt.
Since the pieces fall onto the faster conveyor belt with random spacing, isn't it possible that two consecutive pieces will have the spacing of the puffers they are destined for (within margins), regardless of the puffer placements?
In the end the error rate went down a lot because of a less predictable spacing. But you are right that if the pieces would fall with random spacings that it would not matter what the distance between the puffer stations would be.
The funny thing is that I spent a lot of time measuring out the vertical spacing on the conveyor in the hopper. If I had done that more sloppily it would have worked better :)
At any snapshot, the pieces are lying with a Gaussian distribution around x multiples along the belt i.e having sigma at x, 2x, 3x,...nx....
So for the bins to not overlap:
1) their width/span-along-the-belt should be lesser than x
2) And they can be placed at x, 3x/2, 5x/2, 7x/2 (i.e. prime multiples of x/2)
Wow! Learnt something useful today. Thank you. :)
Edit: I realize after posting that, my solution won't work! If somebody can explain how the prime thing works will be great. I can imagine, though, that the bin placements should be such that, at any given time the piece is only in front of a single bin. Meaning, no pair of bins should have a distance of x-multiple. I can guess, perhaps heuristics which work well, can be devised. But it will be great to know the mathematical solution for this.
12V on the valve -> air streams out of the corresponding tube.
If necessary a lot can be pushed through the machine twice for instance to sort parts by length or to pick out sets (that last bit works in theory but in practice there are a lot of problems to overcome because of the limited number of bins to deposit into).
As for the hardware, there is a nifty little camera with a macro lens that connects to the USB port (noname Asian stuff), it has a 10x magnifying lens, a pololu servo/gpio to USB card to drive the relays and a Sainsmart 16 port relay board to drive the solenoids for the air valves.
The software is all in python with a generous amount of help from the people who wrote numpy, opencv, keras and theano.
The error rate is between 3 and 5% depending on how fast I set the machine, there are a number of sources for the errors, obviously classification errors, also sometimes two parts are too close to each other and even if the classifier got them right the airpuff for one pushes the other of the belt as well. To minimize this effect I keep the airpuff super short, on the order of 10 ms, which is about as fast as the solenoids can open and close reliably, but it does mean that if it misses even by a bit there is nothing to be done about it and that part will land in the 'other' bin.
That error rate is still too high but with every run the classification errors go down and that's the main component.
One nasty little problem was that I spaced the puffers too regular in the first iteration which meant that sometimes the parts would line up just so in the order in which they came under the camera so that more than one puffer would be active at once leading to a reduction on pressure and no parts would be pushed off the belt. That was a tricky one!