Best way to count glass marbles

Question

I've got a reason to count a number of marbles in a game. Up to about 100. I can feed them down a track one at a time. I can ensure that they each go down separately, although I can't accurately control the distance between them.

I can adapt the track if need be. At the moment I have several variations: two straight pieces of thick wire with "sleepers" soldered to maintain the gauge, wooden channels, pvc pipe. I'm assuming that at the point of detection the track will hold one marble.

Marbles are glass and hence they are both translucent and reflective. They vary in diameter between about 16mm to 18mm, and are not perfectly spherical, although they roll easily down a 2% slope.

I had several ideas in mind and I wonder if anyone has any practical experience on this. Almost certainly, it will involve a uC that counts pulses. To make a pulse, I suppose I could have some kind of beam breaker. Or have each marble land on a piezo. Or even activate a micro switch. I'm sure each of these has it's own advantages and drawbacks, but before I make a prototype I'd like to canvas any suggestions first. Also, I'm not adverse to using two methods and corroborating the results.

I know this is also an engineering question, but I thought this board would be more or less the best.

Answer 1

I think any of your suggestions can be made to work.

A small microswitch would be my first choice (I have a box of tiny ones that take almost no force to close).

But that is mechanical so it will break over time. A beam breaker might be a better solution in the long run. I think even a transparent marble (don't they all have some coloration?) will block the light at least on some of the positions. You might need to suppress quick 'open/close' actions due to parts of the marble passing the beam while other parts don't. If you know a little about the rolling speed and have a distance that is at least a few marble diameters this should not be a problem.

Answer 2

Some ideas:

• At the bottom of the declined track, let the marbles' path be blocked by the edge of a rotating horizontal disk, edge-on to the marbles' approach. Let one or more marble-sized gaps into the edge of the disk such that each gap transports one marble at a time from the feed track to an optical counting device. The mechanism spaces the marbles arrival at the counting section and normalizes their transit time through it.

• Same idea but let the marbles accumulate into a funnel at the end of the declined track. Let a chain of buckets scoop up one marble per bucket and lift it to a chute feeding the counting section. (Think of a roller-coaster's starting tractor).

• Collect them and weigh them.

• Drop them one at a time onto a hard surface. Detect the acoustic signature and count those.

• Drop them into a container of water and collect the water displaced. Weigh the displaced water, measure its height in a collection cylinder, ...).

• Drop them into a tube holding a known number. Detect a full tube by the last marble interrupting a light beam continuously instead of falling through the beam. Open a trap at the bottom to release the N marbles and add N to the count. (This might be useful for an ongoing count, but probably not for counting how many in a collection, since you'd have to count them singly to get the remainder in the last tube-full).