H-015 measurements

[kresty]

I want to start by saying that Savage's plans are brilliant, I'm not suggesting they be changed.  I started looking into this because I couldn't figure out how to get the Lego to fit under the skirt. My figures are as much as 7% larger than the skirt in the plans.

I wish I had better pictures....  Anyhoo, Oliver built a ruler into the droid when he put Lego greebles on the bottom.  Lego studs are 8mm across.  The short flat tiles are 3.2mm tall, and bricks are 3 tiles (plates) tall, or 9.6mm.  (Trivia: they aren't square, so to make a cube you have to have 5 plates (or 1 2/3 bricks) 2 studs wide for a 16mm cube).

When investigating the Lego greebles it seems like they are perhaps 64 studs wide. That is 512 mm, and that won't fit in the skirt, which has an opening of about 506mm - That's 6mm too small even completely ignoring that the Lego is recessed from the edge a bit and raised up under the skirt as well.

Anyway, since my Lego didn't seem to fit in the plans, I started looking at the ruler.  The catch is that the camera has parallax and lens distortions that make using that ruler hard - however there are some clues I could figure out from a few of the photos.

First is that each edge has a "center" (for Alan & Giles that's the same as centre ;-)  Oliver cut the omniwheel ports manually, and they aren't the same, but the variance is small, maybe 1/2 a Lego stud.  Even so, we can find the center on this image:

Center of Sentry Droid bottom greebles
Finding the center of a side of bottom Lego greebles

The row indicated by the cyan horizontal marks are at the same "depth", the Lego from one end would run straight to the other end, though Oliver put a few details in front of that row.

We can see the ends of that row near the omniwheels are 32 studs apart from each other.  So the center is 16 studs from either edge at the turquoise mark.

The center is important because the "straight" photos of the edge exhibit parallax and lens curvature problems.  So we can't just look at the straight photo and see how many bricks we can see.

Instead we have to look near the edges to find a photo where the parallax is small and we're looking as close as possible to straight on at the edge of a brick.  The right side was a total loss as far as I can tell, however I have a couple images on the left side that are pretty close.  And since we know the center, we can just double that distance.

I found two images on the left side where we can see the same brick, however the parallax error is going in different directions.

Sentry Droid Lego ruler edge parallax
We can't tell exactly where the Lego aligns from these two images, but we can tell where it must lie between.

On the left image we can see that the bricks about where the blue line are have an edge perpendicular to the camera lens. Maybe not exactly there, but close. On the pink line we can see the right edge of that brick. The edge of our skirt seems to be about where the light green line is, however since we can see a little bit of the right edge of the nearby brick, we know that line is probably a little far to the left of the actual alignment point.

In the right image we have the opposite. We can see the left side of the brick we previously saw the right side of, and below the green line where our edge would lie based on the left image, we can see the left side of another brick. So we can tell that the purple line in this image is going to be too far to the right of the actual edge we're trying to measure.

You can also see that the same thing happens in the vertical direction as the camera was significantly lower in the right picture. That doesn't impact our measurement, but we can see more of the top of the bricks so it looks a little different.

Sticking that on our Lego model, we can see that those marks are 24.5 to 25 studs. Double that for the full edge and the bottom edge of the skirt must be near 49 or 50 studs long.

Sentry Droid Lego ruler
Measuring distance long bottom edge of skirt

Each Lego stud is 8mm, so using the Lego ruler tells us that the bottom edge of the skirt must be 392-400mm long. Current plans are 373mm for that dimension.