Quick note

I ran a little over 100 condoms through the machine last night, to get some quick estimates of where it's at:


 


I'm going to collect three or four more data sets today. Right now, it's an 88% success rate, with 9% multi-vend and 3% non-vend. I think that can be greatly improved on. The left chute seems to be misbehaving a lot, it's also the slightly smaller one -- there may be a connection.

Stuff's Actually Working?

As I mentioned last time, I'd returned to the original Soaring Dragon design, in the hopes of fixing the issues with the inventory system. I think it actually worked. I redesigned the bottom plate which forms the interface between the two cams and the two inventory stacks, ending it 3/8" from the front edge of the condoms, instead of flush. This effectively changed my 1/4" high inventory vending "slot" into a  (3/8"i,1/4"j) open area.

It's kinda hard to see everything at this angle, the gap between the two pieces of identical metal fell behind the third identical metal rod. But you can figure out how things work. Note that the cams are in their normal positions post- or pre-vend, and that one's actually engaging the condoms at Top Dead Center. This is early kinematic design actually working, incredibly. If you look wayyyyy back to the beginning of last semester, I was running kinematic simulations with different cam profiles, and settled on this one because it "lifts" the stack of condoms out of the way, ensuring only one is likely to be vended. What you see here is that exact design actually working. I'm as surprised as you.


Check it, that's what things look like in slo-mo. The wooden blocks you see being moved are to provide a little bit of weight, keeping the condoms nice and orderly. They also could be notched to catch the cams if they came around "dry", making it effectively impossible to "lose" a coin in the system if it's out of inventory. I'll try this on the next prototype, definitely.

If you were paying close close attention, you saw that once the coin has been fully chomped by the system, you're given 10-20 degrees of wiggle. This is incredibly useful -- I aligned the cams so that they "barely" kick the condom out, so sometimes it seems like the machine hasn't vended -- Frustrated, you wiggle the handle, and... ta-da! everything works after all. The reason for this "tight" alignment is that, if I align the cams slightly further forward-phased, the device frequently over-vends and spits two condoms at once. It seems that there's a happy sweet spot of about 10 degrees surrounding Top Dead Center that is the best way to orient the cams -- so, if you're ever trying to build one of these, that little tidbit will save you weeks of problem-finding.

I also mentioned that I'd moved to more rudimentary materials for the new inventory system -- I used a scrap piece of sheet aluminum for the new interface plate, but everything else (new) is wood, plexiglass, or screws. It's not too precise, but precision isn't important for everything but the interface plate -- plus, it's much easier to assemble with simpler tools, not too mention far cheaper -- scrap wood is practically free.

FrankenDragon, with Nalgene for scale.

As you can see, the new single-piece inventory rack dominates the inside of the device. But there's also an incredible amount of wasted space still -- now that things are working, I can probably reduce the volume of the system by 40% or so.

Oh, did I say things were working? Right! This new inventory chute solved most of the problems with Soaring Dragon. Check it out:

Heck yeah.

Here's another vend.

Here's a shot of seven or eight consecutive vends. Everything's working great...

I have about 50 condoms here for the project. I loaded up each tray with half (incidentally, in the current dimensions, the device can hold about 2*100 condoms) and cranked them all through. After repeating this twice, I've yet to have the device not vend, and I've had about four "double vend" incidents. Here's what that looks like:

One condom is spat out the usual down and out direction, but a second was caught by the cam, and slips out above that condom and below the plexiglass lip. I think I can reduce the height of that gap by perhaps .050", maybe that will improve double vending statistics. As it is, I vended roughly 100 condoms tonight for perhaps 95 quarters. I'll spend tomorrow taking ten or twenty "data sets" of as many condoms as I can, and have some meaningful statistics from that to baseline against for improvement.


I'm really excited that I've finally got a "working device." From here, I can start tweaking individual variables, and improving upon current baseline usage data. I'm also looking forward to a second model, smaller, and maybe even higher capacity than the current.

More to follow, it should be an exciting week!

"Soaring Dragon" design, revisited, again

While building the wooden prototype in the post below, I learned some valuable lessons about this whole project. First off, it turns out that condoms are pretty good at self-organizing, but only if there's a dimension that's only slightly larger than their width -- Too wide, and they get caught diagonally. Too narrow, and they jam or don't slide smoothly. Second, I realized that I'd solved most of the issues of inventory storage with this prototype, but vending was still problematic at times. Looking for a simpler approach, that only involved one user input, I decided to apply methods from this design to a new, unified inventory system for the original sheet metal "soaring dragon" design. By replacing the expensive and complex inventory chutes with a single plywood-and-plexiglass design, I reduced the sheet metal parts count by 6, the hardware count by over 25, and the cost of sheet metal manufacture by about 50%. This is significant, I feel, now there's only one major sheet metal component left in the system -- the external shell, which, while the largest, is also the simplest in terms of cuts and dimensions.

I'm adjusting the height of the slot at the bottom -- I think this is the major contributing factor to the persistent trying to vend 1 1/2 condoms problem I'm encountering with every design I've worked on.

If this doesn't iron things out, I'll re-evaluate the geometry of the contact between the condoms and the cam gears -- apart from trying them the other way, I perhaps can move things around to give a better engagement angle.

Pictures to come shortly.