Rethinking the Low bar

[Legator91]

Yes. The low bar is one of the easiest ways to get through the defenses. By designing for it you also build a robot with a low CG. This then makes going over the other obsticles without tipping over much easier. Its worked well for us so far. It did take some extra time to design, but it was worth it.

[Trevor1523]

Right after we watched the reveal video on the 9th we went to 179's field to see what it looked like. When I saw the height of the low bar I looked at one of my team mates and said "There's no way we're fitting under that". I would probably still have the same reaction.

For me, the low bar is an obstacle that I personally knew most teams would design their robot for because "OOOOHHH WE GOTTA DO EVERTHIN'". So we decided not to. I think we would make that decision again. Even with our #tallbot we still had trouble packaging everything into what we built so kudos to all those low bar robots.

EDIT: I've been wanting to say this for a while and I feel like this is an appropriate place to say it:
The low bar is a defence, yes? Yes. So I looked at the problem as would we design our robot to just be able to damage one defence, say the Portcullis. That Portcullis design is essentially your constraint and you can't remove it, you can't work around it. It has to stay there. So we decided to not let 1 defence constrain us when there are 8 other ones that are available to take down. (I think that makes sense)

[MechEng83]

We would definitely take the low bar approach again. We looked at it from a strategy standpoint from district events through worlds. Add to that the design and engineering challenge of getting everything to fit in a tight package. It's a really good lesson in minimalist design. We opted to not have pneumatics this year to save space, and aren't regretting that decision at all.

We typically are crossing the other defenses in teleop, but we still wanted to be able to handle that one, as we might be paired with 2 robots who couldn't do the low bar. From our week 2 event, there were a few times we had to cross the low bar because an alliance partner wasn't able to complete it for one reason or another.

When we are in tower attack mode, it provides a quick path to go back and forth from the neutral zone and/or secret passage. We used it often (though not always) to make that trip.

Low bar isn't for everyone, but it provides a fun challenge.

[BotDesigner]

My team totally ditched the low-bar Saturday after bag and tag. We are using our withholding allowance to build a mechanism for Colorado that I believe should be able to change the way defense is played in this game.

[engunneer]

Originally Posted by Trevor1523

Right after we watched the reveal video on the 9th we went to 179's field to see what it looked like. When I saw the height of the low bar I looked at one of my team mates and said "There's no way we're fitting under that". I would probably still have the same reaction.

For me, the low bar is an obstacle that I personally knew most teams would design their robot for because "OOOOHHH WE GOTTA DO EVERTHIN'". So we decided not to. I think we would make that decision again. Even with our #tallbot we still had trouble packaging everything into what we built so kudos to all those low bar robots.

EDIT: I've been wanting to say this for a while and I feel like this is an appropriate place to say it:
The low bar is a defence, yes? Yes. So I looked at the problem as would we design our robot to just be able to damage one defence, say the Portcullis. That Portcullis design is essentially your constraint and you can't remove it, you can't work around it. It has to stay there. So we decided to not let 1 defence constrain us when there are 8 other ones that are available to take down. (I think that makes sense)

Counter argument: our design goal was to be able to breach alone, and we picked low bar instead of c class defenses. It means we I ly have to be capable of 7/9 of the defenses instead of 8/9 which would be needed for a solo breaching #tallbot.

I told my students on day two that the game isn't hard per se, but it is a difficult packaging problem, which is very common in engineering.

[GeeTwo]

Confirmed - we'd still be low-bar capable. The low bar was definitely the easiest defense to breach. The mistakes that held us down at Bayou were mechanical implementation, not strategic decisions. No more #25 chain, at least not for dead-axle drive wheels. We achieved our highest regional seeding yet (#18 of 56), and were the highest seeded 5-5 team due to several breaching RP and a capture RP (we scored a few boulders and carried more into the courtyard). Our post season mods will focus on the drive train and tweaking the pickup and launcher.

Originally Posted by GeeTwo

Caveat: this is the eve of our first competition.

No way - I think we made all the right to-level choices. Things really came together tonight - we're driving and (manually) aiming with a camera now, and We have the driving (low bar, B, or D) if not aiming and shooting parts of auto working. We can make high goals more than half the time, with a fairly high arch, and low goals more consistently. The only real concern about making a solid run at the banner is the beating that the robots (and field) are taking this year, and building a tall robot would only have made those worse.

I'll try to remember to post a follow-up on Sunday.

[Joe Johnson]

I think this is an interesting question. I go back and forth on the answer. One thing that I have come to believe is that the low bar saved this game.

I'm serious. I shudder to think of the bedlam that would have ensued had it not been the case that 90% of the teams decided to try to be a low bar robot (forget if they actually CAN limbo, they TRIED, and that saved a lot of them).

I have seen more upside down and tipped over robots this year than in any year I can remember*. Had 90% of teams built 4ft tall robots the highlight reel would have been filled with tipped robots.

Again, the GDC saved this game by making the low bar be the one permanent defense.

Dr. Joe J.

*well, excepting 1997 when when Naval Undersea Warfare went after the other two robots on the field at the opening trumpet blast, tipped them with a spatula type thingy and scored at their leisure once they were the only upright robot left but let us never speak of such things. The horror, the horror...

[themccannman]

Originally Posted by marshall

Actually, a low COM is not the only key to not flipping over. As crazy as it may sound, our originally tall robot (1 inch shy of max height) that we had in Palmetto could tip over crossing defenses despite having a COM that was only 5~6 inches above the ground.

The second key is not the COM in the static case but the overall location of weight in the dynamic case which can be aided by distributing weight out to the edges of the robot.

Granted, I'm not a physics whiz and I'd love to have someone like Ether comment on this but I believe it to be true based on our experience with adding ankle weights to our robot in Palmetto and seeing the same thing work for another team here at the last NC District event.

The only thing that determines when an object tips over is when it's COM passes outside of it's footprint projected in the direction of gravity (downward).

[marshall]

Originally Posted by themccannman

The only thing that determines when an object tips over is when it's COM passes outside of it's footprint projected in the direction of gravity (downward).

Yes but I believe the static case for that occurring and the dynamic case are different. When stationary we could pull our robot back to quite a degree without it tipping over but when we attempted to cross defenses the robot turned into a pendulum and it tipped. Adding ankle weights at the four corners prevented it from happening again. Our COM was already low so it wasn't just the COM that needed to be adjusted to fix the tipping issue.

[mateoland]

Originally Posted by themccannman

The only thing that determines when an object tips over is when it's COM passes outside of it's footprint projected in the direction of gravity (downward).

The problem I saw with the higher robots was not necessarily that they would tip, but that so much time was spent waiting for the teetering to dampen out before being able to drive again. It plagued a couple of teams in Los Angeles. If they hadn't waited a moment to let the back and forth movement settle, they would have most likely driven "under themselves" and tipped backwards. Precious seconds lost multiple times in a match.

[themccannman]

Originally Posted by Kevin Sevcik

That's only true in the static case. It's possible to save a situation that would statically be a tip by accelerating or decelerating, which is the equivalent of pushing the CoM one way or another. You can also cause a tip in the same fashion.

Acceleration is a force and therefore would affect the direction of your footprint projection. The footprint is projected in the direction of all forces acting on the robot, I just said gravity because that is always present.

A heavier robot with more rotational inertia is going to tip less than a lighter one because your motors stay there same. I guarantee you I could build a chassis that flips due to its own wheel force, and fix it by adding weight without changing CoM.

This would prevent the tipping because it would reduce the robot acceleration, and rotational acceleration (which means it would have to undergo lateral acceleration for longer before reaching its tipping point) therefore changing the footprint projection, that's the real reason it wouldn't tip.

Originally Posted by marshall

Yes but I believe the static case for that occurring and the dynamic case are different. When stationary we could pull our robot back to quite a degree without it tipping over but when we attempted to cross defenses the robot turned into a pendulum and it tipped. Adding ankle weights at the four corners prevented it from happening again. Our COM was already low so it wasn't just the COM that needed to be adjusted to fix the tipping issue.

As mentioned above, the cases are the same, you just add another vector for robot acceleration.

Originally Posted by mateoland

The problem I saw with the higher robots was not necessarily that they would tip, but that so much time was spent waiting for the teetering to dampen out before being able to drive again. It plagued a couple of teams in Los Angeles. If they hadn't waited a moment to let the back and forth movement settle, they would have most likely driven "under themselves" and tipped backwards. Precious seconds lost multiple times in a match.

Very true, the reactionary force when the robot stops makes it like a pendulum, you want to accelerate out of phase with the robot tipping period to not magnify it.