View Full Version : [Question(s)] anudder tripod question

Grand Robot Master
05-12-2009, 10:59 PM
i'm trying to figure out the gait concept for a tripod. I was thinking that each leg has 4DOF, and it shifts on 2 legs and moves one forward, for example, the left one, it shifts on its front, and moves the center forward, it then moves the right in the same way, in a 123 sort of gait. does anybody think that will work? if i could explain it any better i would. i have no means of representing a visual aid. sorry
i have seen other gaits out there, but they seem to require more speed
(note-this bot is still vapor)

05-13-2009, 09:56 AM
Each leg only needs 3DOF if you want to make a simpler bot. I think you have the basis down.
Think of it in 6 steps:
1) Shift the CG over the line that connects the feet of LegA and LegB.
2) Move LegC in the direction you want to go.
3) Shift the CG over the line that connects the feet of LegA and LegC.
4) Move LegB in the direction you want to go.
5) Shift the CG over the line that connects the feet of LegB and LegC.
6) Move LegA in the direction you want to go.
Repeat ad naseum.

The challenge is in steps 1,3,5. If you don't have some way of measuring that the robot is shifting in one direction or other over that line, then you have no way of compensating for it. If you don't have any way of compensating for it, then you have to adjust the steps so that instead of putting the CG of the robot over the line, you put it "forward" of the line ever so slightly. (Forward in this sense being the side of the line that if the robot falls, it will land on the moving leg.) If you watch my videos, thats essentially what my robot is doing. It will shift the CG just in front of the "line". It will then "kick off" with the moving leg (slam it against the table) causing the robot to tilt away. While the robot is standing on two legs, it will quickly move the moving leg into its new position. The robot will "land" shortly after this on three legs again. This is why moving the CG "forward" of the line is crucial, as if you kick off with the CG over the line, or behind it, the robot will topple over when you "kick off". It will take some time and experimentation to find the right force to kick off with that allows just enough time to move the leg to its new position.

Now, if you do have some way to measure the CG of the robot (such as an IMU), then you should be able to balance the robot on the two legs while the third one moves around.

And yes, like you I have seen other tripod gaits, but I also came to the conclusion that they are a lot harder then this one.

05-28-2009, 11:19 PM
Why not make a tripod that has a gait like a gorilla?

The two front legs hold the weight long enough to swing the back leg through the middle, then sequentially reposition the front legs, right and then left.

Use squash balls as feet so that it's got better balance and smooth transitioning between steps.

Grand Robot Master
05-30-2009, 04:23 PM
that's an idea...
you're the bioloid gorilla guy, right? if so i thought your project was pretty cool.