pic: Effective Aperture of Low Bar Defense



Just wanted to add a visual note here for folks to take a look at. Lots of folks here and elsewhere are thinking about designing low bar-capable robots that are ~16" tall. In reality, since you must approach the low bar from an angled ramp, the effective aperture for some of your robot’s transit of the obstacle is nearer to 14". This is especially true of 6- or 8-WD configurations that won’t tip onto the flat top of the platform until much of the robot’s length has climbed the ramp.

Nice catch! This will undoubtedly come up during strategy discussions today…

You can get a tad more room than this, assuming the effective wheelbase of your robot is longer than the ramp. But it’s an excellent point, I’m glad you posted this! I bet there are teams that didn’t catch this.

Glad someone highlighted this! On 1519 we just realized this last night… I hadn’t tried to quantify the minimum (14.15"), but we did notice that a robot 15.9" on the leading edge won’t fit. It needs to be shorter at the front, and as Mr Forbes noted, the longer your wheelbase the taller you can be (but still probably only around 15" or so at the leading edge).

I should add a disclaimer that I made this sketch pretty quickly based on the dimensions provided in the game manual. It may be that the actual construction of the field elements results in slightly different geometry.

Don’t build a robot that’s 14.14" tall because of my picture. :slight_smile:

And for those teams that are kind of new at this…beware that the fields don’t always measure exactly what the drawings say they should. It’s a good idea to add some extra clearance for something like this. Maybe the 14" number is what you should be shooting for, even if you have a rather long robot. Think about what happens when you go down the other side, too.

You also only have to be ~14" tall until the CG of your robot passes over the end of the ramp (if you drive slow), then you could be 16" tall for the length of the horizontal portion of the platform. That might be splitting hairs though - easier to design to the 14" limit.

It’s in the picture, but it’s worth highlighting that the bar is not centered on the ramp. As a result, the maximum height will vary depending on which side you’re approaching it from.

…and also that the neutral zone would be on its left side in this picture.

Glad this was brought up and shared with a picture. This was a factor we were considering with our short bot idea yesterday.

That’s fun. You’ll need to have a CG that’s something like 14" from one end of your robot to make it backwards through the low bar, in that case…

Or keep the ends of the robot a little bit shorter than the middle.

I was thinking about this too. Something where the front end of the robot is less than the ~14" limit for when the robot is on the ramp, but the rest of the robot can go up to 15.9" or so and be fine.

Warning- MS Paint incoming:
http://i.imgur.com/7Xf9F9u.png

something like this would tackle the low goal well, but it would have significant issues with the overhang of the frame. The approach and departure angles make some defenses impassible.

True. My wheel placements weren’t accurate to a real robot, i just kind of drew them on in 10 seconds to show that this was a robot. The wheels would definitely be closer to the outside edge of the frame.

hey, looking at measurements in solidworks, you seem to be off.
in reality the field elements are 13.73" measured the same way:ahh: :ahh:
https://drive.google.com/open?id=0B4wTwk4tzevEbG5rM1Q0YlNCYmc

Thanks for this! It really helped us visualize what’s needed for a low bar robot.

When I measure against the field model provided by Autodesk, I get 13.86". Did you model those elements yourself or were they taken from elsewhere?

In any case, that discrepancy is why I added a disclaimer in this thread saying that y’all shouldn’t take my word for it and should check things out yourselves.

Those slopes would go a lot farther back, depending on how long the robot is and where the CG is. The initial approach would actually have a better aperture, since you’re on your front wheels and rear wheels, and thus at a shallower angle. Then the aperture would shrink down to whatever that minimum amount is, until your CG (or something else) tilts you down to horizontal again.

Coming from the backside is even worse. If you’re not flat before you get to that bar, you’re going to have to be something like 3" shorter yet. so that end of your robot will have to be 11" or maybe even 10".

The real answer to all this, of course, is to make up an inventor sketch of a side view of the low bar and your chassis and layout. And then run your chassis through with wheels tangent to the correct surfaces, and see what happens.

Thanks for posting this. This was a fact some teams were able to catch very early, but unfortunately we all know we are going to get to events and see a 15.9" tall robot with a strategy designed around a nontrivial utilization of the low bar.

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