Friday, October 23, 2015

L3-G0 and Terrain

Artoo is a terrible mechanical design.  You can see that in some of Lucasfilm's R2-D2 Extras where they talk about building him and stuff.

For starters, the front foot appears to merely be there as something for him to trip over.  In the replica droid business this leads what is known as a "faceplant." Yes, L3-G0 has faceplanted (fortunately without the dome).

Lego R2-D2 doing the "Faceplant"
Basically, there's not enough distance in the front foot from the center of gravity to provide good leverage when you run into an obstacle.  In this case a sidewalk crack as I was trying to get in out of a light drizzle.

In order for the front wheel to turn, it's on casters, and those can't be very big because they'll hit the foot shell.  So the small diameter also makes it hard to cross cracks like the one at the bottom of the picture.  (I thought all of the cracks would be as small as the earlier cracks). 

L3-G0 is "worse" in this regard because the Lego bricks make his front foot shell pretty thick on the bottom, reducing the available wheel diameter to about 2" (about 5cm).  Another problem is that his front foot is hinged at the ankle (thinking about 2-3-2 mode), which means that if the front wheel drops in, then it stays.


Types of terrain that gives L3-G0 trouble

Any kind of bump is potentially disastrous.  So I really look out for those.  If the caster drops in or has to climb over, it leads to tripping.  If its "small enough" I try ramming it gently and seeing if he'll make it.  If not, then I either back over the obstacle or lift the front over it.

The gap in an elevator is usually "OK" and about the maximum that he can travel over.  Small door sills like on a front door are iffy, sometimes they work sometimes they don't.  Something like an extension cable at an event may be traversable, though high bumps can cause me to drag on the foot shell (he has about 1cm or 3/8th inch clearance)

If the terrain is too soft, then his wheels dig in.  This happens on padded carpet.  He either can't climb out or the foot shells can even ride on whatever it is.

Another problem is "rough" terrain.  The drive train can handle a sidewalk or pavement, but most exterior surfaces are pebbly and cause a lot of vibration, which wants to shake him (literally) to pieces.

There's also a limit to the incline L3-G0 can safely traverse.  Especially ramps going down he wants to coast and we have to use power to brake him.

So is there anything L3-G0 CAN drive on?

Fortunately most convention halls and public spaces have big flat smooth concrete floors.  I just have to watch for the expansion gaps between slabs and cable runs.  Most houses have similarly smooth slabs in their garages, but watch for the crack!  Driveways or streets are often too rough or steep.  (For Halloween we put out mats for him on our driveway).

He can usually climb the 1/4" onto the carpet some folks put down for their booths (like when he visited the Lego booth at Celebration).  The wood floor in our house is fine, though we have a 1/4" rise between rooms I have to watch.

Tile is another scary area because it has lots of little cracks, and can be rough on top depending on the style.  Close fitted tile or linoleum tiles are fine.

Industrial carpeted areas are usually OK, though they can drain the battery a bit more than concrete.  However the carpet in the lobby at Rose City (Oregon Convention Center) seems padded more than normal.  A typical house carpet is too soft to really be passable.

Wheelchair ramps are often a good path for him, not being too steep. 

What can be done?

As a practical thing, if I have complicated terrain, then I drive over it backwards.  The load is better distributed on the back feet that way, and he can drag the problematic front foot over whatever it is.  For the foot shell clearance issue, I raised the shell a tiny amount, which helped a lot.  (Some of the "real" Artoos have a gaping 1" or more clearance, but I think that looks funny)

Tracks (like a tank) could help some, but you still have steering issues.  Some builders have created a system that can navigate a grass yard (though that would shake L3-G0 apart).

Some of the fanciest systems have a single large diameter rotating wheel up front, sometimes even powered, but that typically requires complicated mechanical systems.

L3-G0 could also have a more rigid front ankle, however that can lead to other issues.  I've limited it's play a little, but he was designed to have it be flexible so stiffening it is tough.

Oliver and Lee came up with a system using a hub motor for Star Wars VII: The Force Awakens, which seems to help a lot.  Some of the other builders are trying that. (I confess that it's on L3-G0's to-do list).

In the meantime I watch out for rough surfaces.

And cracks.