I spent the evening working on the mechanicals for an automatic dry cat food dispenser. It consists of a screw-driven slide with a 2 inch hole. The assembly will be placed under a large cat food bucket with a hole in the bottom, with a chute directing dispensed cat food into a bowl.
At the moment the screw is driving the slide simply by threads cut directly into the wood. If necessary I will embed a nut in the slide, but I'm hoping the forces will be low enough that the wood will be sufficient. I've got the motor coupled directly to the screw. This means I'm using a far larger motor than is really necessary for this job. Unfortuantely I didn't have any gear drives that were really suitable. Also unfortunately I didn't have any reasonable way to couple the motor to the screw, so I just bored the end of the screw out to the diameter of the motor shaft and put some Loctight in it. This obviously won't last long, but it was a quick fix. I'll need to come up with something interesting to couple the shafts. Maybe it would be interesting to cast a three-part coupler for this.
The next part of the project will be the electronics for driving the motor. This will consist mostly of some end switches for the slide and an h-bridge driver for the motor. I'll probably put an AVR into it, just because they're cheap and I've got a bunch lying around looking for work. On the other hand, since it just needs to cycle once per input trigger, it might be fun to do it with something more rudimentary with some basic logic.
3 comments:
Very nice build! Does the dispenser tray just slide wood to wood or are there runners of some kind hidden in there?
It's just wood on wood. I sanded it all smooth with 200 grit and left enough play that it shouldn't bind up. I may need to provide a guide for the lead screw at the rear-most position of the slide. That would give the lead screw enough support to eliminate any side-to-side binding.
I have some concerns about jamming when there is food in the dispenser. If I try to run the tray out far enough to completely close the top hole it will probably be scissoring some food pellets.
Part of the reason I went with a lead screw instead of a scotch yoke was to address this scissoring. A lead screw and a gear drive should generate sufficient force to cut through food pellets (though I may need to install blades on the opposing edges, the wood won't last long under that kind of abuse).
This direct-drive setup is temporary. I expect I'll have to replace it by the time I finish with the rest of the system. I found a nice gear-head motor that runs at about 2 RPM. On a scotch yoke setup it should be acceptable (that would give it about a 30 second cycle time, and make it quite, both things that are acceptable for cats).
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