Phoenix Hexapod

[gfish]

My RC hexapod arrived last week, and I assembled it over the weekend.

The servos still need some fine calibration, but it works quite well even as it is. It's a very nice design, and while maybe not worth what I paid for it in any rational sense, I don't regret the purchase. Most importantly, it will serve its intended purpose as a research platform nicely. Because, see, I want to build another one.

I want to build one I can ride on. And I've almost convinced myself that this is feasible. Stay tuned...

Comments

[solarbird]

You're going to make some Drow child very, very happy.

[nimitzbrood.livejournal.com]

So you'll be competing with this then?

http://www.youtube.com/watch?v=1D2yl6B3Kyg&hd=1 ;-)

[gfish.livejournal.com]

Not really. While the Spider is a beautiful piece of equipment (UBC represent!), it's pretty limited, mechanically. It's a vehicle, not a robot. It only has two degrees of freedom. Each leg of the hexapod has three, giving a whopping 18DoF for the entire device. That's why they're so expressive. That's why there is an annual dance contest for them. (And, yes, it's why this might be beyond my abilities to pull off.)

[nimitzbrood.livejournal.com]

My point was that since you mentioned riding on the hexapod you're looking to build something the same scale.

As for construction of such a beast I suppose you're imagining a hybrid power design - ICE to charge batteries to run giant servos? Or are you looking at just a limited fully electric design?

You do realize that this will cost a bundle in milled aluminum right? Or did you have another material in mind. You could probably design a star-shaped truss style underbelly and use carbon fiber or bamboo in theory then use aluminum for the tips of the "claws".

I so wish battery technology had been allowed to advance at the same rate as the rest of technology. We're still using a lot of the same basic battery structures we've used for years IMHO.

[gfish.livejournal.com]

Yeah, on that general scale. Not small.

While I haven't given up on using industrial servo motors, the actuators will probably be hydraulic, driven from a diesel pump. The solenoid valves for computer control are the main cost worry, but I suspect it's really the only way. I'll have to add position sensors and write my own PID-style control loop to mimic the RC servo functionality, but I'm not too worried about that part.

The legs and frame will be welded tube steel. Cheap, strong, easy, and an excellent excuse to finally get my own mig kit. That part will be fairly forgiving -- a lot of heavy work, but the tolerances really won't be that tight.

[nimitzbrood.livejournal.com]

Actually in theory you could just add scaled relay/servo control to an existing hexapod controller to run the larger systems. Nothing says you have to reinvent the wheel. :-)

If you do want to reinvent the wheel for some reason I'm thinking homebrew CNC controllers hung off the initial control unit would allow you control of all six axis and actuators. In fact that makes it very good for fly-by-wire in that you have a computer that spits out G-Code to the CNC controllers that control the servos and actuators as if they are XYZ+tool changers.

I still think welded tube steel will be too heavy but you're a math person and I'm not. My choice for an initial build would be to use surplus 80/20 (similar to this: http://hackaday.com/2010/12/14/my-reprap-is-bigger-than-yours/ - search E-Bay for "80/20 garage sale") for as much as possible and then use welded aluminum tubing for the rest.

With something this size weight is one of the biggest limiting factors in regards to both speed of movement and power requirements.

Please take all of this with a huge block of salt as I'm in no way formally trained in physics. Just a long-time tinkerer/thinker/dreamer.

[niac.livejournal.com]

For steel construction, I'd suggest looking at 4130 alloy steel -- twice as strong as hot rolled steel, and only 1/3rd more expensive. I'm looking at building the Air Kraken with 4130 tube because I want to keep the weight down, and the strength up.

[zzyzx-xyzzy.livejournal.com]

also, the only place to get this much steel tube is from Dillsburg

[niac.livejournal.com]

Based on what?

I called up Everett Steel in Ballard, and they would sell me as much as I wanted for 1/2 of onlinemetal's price (or, about $15 for a ten foot stick of 1" OD, 0.0625 wall 4130)

[zzyzx-xyzzy.livejournal.com]

That's a pretty good price; not usually a size I'm looking for. I'm usually shopping for .035 and .028 wall

[peteralway.livejournal.com]

I can see one reason for using all those degrees of freedom would be to get it to move while giving the driver a survivably smooth ride. The spider looked fun, but I can see how any speed would shake the driver pretty badly.

[gfish.livejournal.com]

Yeah, I'm curious to see how jarring the ride is when scaled up. Only one way to find out! It. Won't ever be going that fast, though I'll be tempted to see if I can get some obscure land speed record if it works at all.

18DoF will also allow it to do thing like turn in place, have adjustable stride lengths, multiple gauge to choose from, body height adjustment (great for watching the Burn over the crowd!), and a dance mode. Cause, well, why not? It's just a button on the console and some code at that point. Imagine pulling up to a rave out of a duststorm on one, and then the whole contraption starts bouncing and twisting in time with the music. Legendary.

[keystricken.livejournal.com]

It already looks like a dancer on pointe shoes, at least when it lifts itself up to its full height. Could I interest you in a techno remix of Swan Lake?

This is amazing work, by the way. I can't wait to see more.

[happyinmotion.livejournal.com]

How would the hexapod controller cope with including some kind of suspension system into the legs/feet?

[gfish.livejournal.com]

Depends how aggressive the suspension was, I guess. At some point it would start to mess up the inverse-kinematics which determine the gait. Judging by the small RC version, I don't think it will be needed, but it's hard to say once things are scaled up ~10x. That's something I'll just leave to testing and deal with if I have to.