So, we've been debating building a walker for the Sportsman Class at Motorama for the last couple of years. Actually, we've dreamed of building a walker since roughly 2001, but better late than never! One of the problems we saw was that it was pretty unlikely that our first iteration would be anything close to competative, but that's the great thing about the Sportsman Class - cool designs take precidence over destruction - so we're going to give it a shot.
We're drawing our design from the Lynxmotion AH2 model, which has two degrees of freedom on the legs:
We've selected this base design because 2DOF is the minimum to be able to articulate a leg properly, and 3DOF would be way too ambitious for a first attempt. Although we haven't shelled out the $500 for an AH2 kit, we're going to have to make do with what we've got. Six legs means for a start we need twelve motors ... and it just so happens that we have a whole mess of 19.2V Craftsman drill motors and gearboxes, although we haven't figured out yet how to get the chucks off them ... maybe we'll give that a try this coming weekend. We're also going to need 12 speed controllers: ouch! Our initial thoughts are to build simple forward-backward H-bridges, with limit switches, rather than get a whole bunch of expensive speed controllers for this first go. 120A relays are only a couple of bucks each.
We also need to figure out how actually articulate the leg joints. Our first thought was to assemble around the drill motors a kind of actuator using plates, threaded rod, and hinged ends:
That seemed like an awful lot of work, and would likely be slow and suck batteries dry quite quickly. You can see towards the top of the picture that our minds then moved into the idea of using bevel gears, with the pinion on the drill output shaft, and the big gear fixed to the joint axle, which would also be fixed to one side of the joint, forcing the other end to move. Remembering how expensive gears were, we looked around to see how many mortgages we'd need for twelve joints, and were pleasantly surprised to see cheap and cheerful Chinese gears on eBay for seven dollars a pair:
Now we decided this might actually be doable - so we opened up Rhino3D and set about designing a leg joint. We're not kidding you here - it took three days to get this simple joint worked out:
Three days! We started with round tubing, but couldn't figure out how to get two pieces to mate up properly, and even with square tubing it took a while to figure out how to design the ends so they would move and not actually need to physically pass through each other! Doh. Anyhow, in the design the drill motor is actually mounted inside the tube, the large bevel gear is fixed to the rod acting as the joint axle, which in turn if fixed to the outside of the joint. Our next step will be to make one, and see if it acts the way we picture it in our heads!
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