Thursday, July 4, 2013

Center Leg Lift Development: An Overview

One of the toughest things to replicate from the STAR WARS movies is what we call "232".  

2 stands for 2-leg mode, when R2-D2 is standing upright.  Then, R2 transitions into 3-leg mode.  The body pivots, the center ankle/foot is extended out of the body and R2 is now in its driving mode.  When R2 gets where it needs to be, it returns to 2-leg mode.  Thus, "232".

Here's a video clip I pieced together that shows this effect in the original STAR WARS movie...


Many of us R2 builders have tried to pull off this effect in a variety of ways.  Some have specifically built their R2s out of much lighter materials, like wood or styrene, to make the movement easier to articulate.  

There are several aspects that make the 232 system.   

1)  The shoulders.  These need to pivot 17 degrees for R2 to be in 3 leg mode.  The mechanism to do this needs to rotate the entire weight of the droid precisely.  If one side doesn't move smoothly with the other, damage could occur.

Shoulder linkage and linear actuator

2)  The center leg lift.  (This is what I am working on.)  As you see in the video clip above, the center ankle and foot need to fit inside the body.  Then, it needs to smoothly extend out and be prepared to handle the load placed on it.  It needs to deploy quickly, as seen in the movie.  This is a challenge since most modern linear actuators that can handle 150 pounds or more have travel speeds of 1/2 an inch per second.  With the ankle and foot size taken into consideration, my setup travels 8.5 inches.  However, when you take into account gravity and the weight of the ankle/foot and wheels inside it, that could be a bit quicker.

Center leg lift setup I'm working on

3)  Locking the ankles.  R2's ankles, where they attach into the foot, are triangular and bolt into a channel in the foot.  Many lighter droids can stay in 2-leg mode simply because they do not put nearly as much force on the joint.  A 200-pound aluminum robot is a different story!

Note how the ankles are triangular in shape with a single hole mount point to pivot

Ankle freely travels in the foot.  This needs to be secured for proper movement in a 232 setup.

Locking the ankles is challenging.  There isn't much room inside the ankle, or the foot, to put much in there.  The foot is already largely off limits since the foot motor/drive system is in there.  And this is probably the toughest piece of the puzzle, since R2 can't go back upright until those ankles rotate.

So you now have the basics of what 232 is all about.  In my next post, I'll show you how the center leg lift is coming along!