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Re: Velcro tread
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Re: Velcro tread
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Re: Velcro tread
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Suppose two higher-CG robots are in a pushing match. Neither has Velcro tread--but in a previous match, a robot with Velcro tread got into a pushing match and shredded the carpet near that area--but, due to the nature of Velcro damage, it wasn't spotted and covered. Now, suppose that one robot is winning already... and then the other hits the slick area left by the Velcro-bot and loses traction. The pushing robot now takes a huge power advantage, slams the pushee right through the slick area and to the wall, inadvertently getting under their bumpers and sending them over the wall into the nearest volunteer who doesn't have a chance to get out of the way. Is it still not a safety issue? Part of any competition environment has to be that the playing area has to be kept as consistent as possible (unless the inconsistencies are specifically noted in the rules, etc., or otherwise accounted for). If the field (generic) is constantly getting extra wear due to one player, all matches after that player are getting an inconsistent playing area unless the grounds crew got in. And you still haven't answered this: Velcro attaches to the field, which is specifically disallowed in the game rules. The head ref just sent the robot back to the LRI to get that fixed (even with NO damage to the field whatsoever, as determined by the Field Supervisor, the FTA, and the Head Referee). Can the LRI send the robot back out with no changes? Yes. Can the head ref immediately disable the robot prior to the match? Yes, and he most likely will. |
Re: Velcro tread
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Re: Velcro tread
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Re: Velcro tread
Michael -
First off, the question of damage to the field is one of intent of the rules, not letter of the rules. The intent of R6 and G10 combined is to prevent damage through a specific set of actions, and velcro falls into that category of actions. The LRI's are there to make sure that every team has a successful event. In part this means we make sure everyone follows the rules so no one has an unfair advantage over anyone else. In another part, this means we make sure the robots and environment are as safe as possible. In yet another part, this means we work with teams to ensure they can compete on the field as much as possible. The way the rules are written, R6 is, in my opinion, unclear with regards to velcro. As such, it's entirely possible for a team to get different rulings at different events - one inspector may interpret it as legal, while another sees it as illegal. However, G10 is very clear - velcro treads cause you to attach yourself to the field, which is illegal. You may very well have an LRI pass you based on the robot rules, only to be disabled on the field. Personally, knowing the game rules almost as well as I know the robot rules, I see it as part of my job to help such a team fix a problem that will cause them to be disabled, even if it's not technically illegal from a robot rules standpoint, before it actually causes them to have a problem on the field. At the end of such a discussion with a team, for any issue, there are four possible outcomes. First, the team agrees and makes a change. Second, I have to tell the team I can't pass them because of a specific robot rule and they have to make a change. Third, the team convinces me what they did is OK and we all walk away happy. And fourth (my least favorite), the team won't make a change, I don't have any robot rule to point to, and I have to go tell the head ref what to watch out for from the team (This brings back memories of robots that could extend past the legal distance from the frame perimeter, or ones that had trouble getting in the starting configuration while loaded with game pieces...). |
Re: Velcro tread
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Here's an example of the difference between traction by friction and traction by attaching. Go outside to your back yard (if you have one--if not, try the front yard). Push your hand down flat on the grass and try to slide it--it'll probably move (though not necessarily easily). Now, instead of a flat hand, use a hand in a hook shape, digging in (or just grab a handful of grass). Doesn't move, does it? Now, just for good measure... grab that handful of grass and yank hard on it. Now, that's a bit simplistic. But it's kind of what we're getting at here. Most robots will be using friction (yes, there's microscale interactions with the carpet and all, but for the most part that's counted as a slightly higher mu value). Maybe a robot uses attaching--see what happens when you grab and pull grass. That's what the difference is. |
Re: Velcro tread
Still confused as to how friction can happen without any level of attachment. I am also reasonably sure that carpet will neatly fit between the ridges of wheels like this which is hardly a "microscale" interaction with the carpet.
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Re: Velcro tread
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Re: Velcro tread
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Re: Velcro tread
You can consume multiple bowls of popcorn discussing the difference between attaching & friction. You will find the LRI, Head Ref, & FTA don't have a lot of time for popcorn during a competition. :). Their concern will be does it meet their understanding of the rules? Will it potently damage the field.
Anyway in simplistic terms, Friction can be thought of as little ridges on one piece interacting with little ridges on the other piece. With a tangent force, the ridges try to push the pieces away from each other. This counteracted by the normal force pushing the pieces together. At some percentage of the normal force, COF, the pieces will begin to move. Whether or not there is damage to the material depends on the strength of the material, geometry of the rigdes, etc. With Velcro, the tangent force is largely independent of the normal force so clearly something is going on besides friction. All of this is done in a theoretical frictionless physics lab of course. |
Re: Velcro tread
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Being that the burden of proof rests with the team, not the judge, we had to do what we were told to do despite no sound argument (IMO) that what we were doing was any different than other teams, or any evidence of damage to the field. I don't fault the LRI for taking a very conservative stance, but it was quite frustrating. Perhaps once there is more precedent with teams using brakes (or at least we have a longer history with them) people will feel more comfortable with it. |
Re: Velcro tread
regarding pop rivets and tread material... at a recent off season in Minnesota a team's tread wore down enough that, when they got into a pushing match situation the pop rivets that held the material in place actually ripped not only through the carpet but also into the gym floor underneath! It's probably a good thing when inspectors lean toward the conservative side when looking at those!
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Re: Velcro tread
Since this thread has the ear of the inspectors, can I ask if there's any history of teams using low pressure (a vacuum cleaner) to increase the normal force of their wheels? It would do a service to the event by cleaning the carpet! It could be in the form of a wide, non-contacting nozzle that essentially sucks up the carpet (it would need a pretty small gap all around to hold any pressure difference), or in the form of smaller feet that actually contact and (dare I say it?) attach. Think of air-bearing pucks on CMMs: with a positive pressure they are bearings; with a negative pressure they are brakes. I know in a very early game (2003?) some teams used suction devices to attach to a HDPE platform in the endgame.
I'm kinda with Max Boord here in that the nature of the game is to attach to the carpet. Would a single 24" diameter "foot" coated with roughtop be considered illegal - it probably won't damage the carpet, but the other robots trying to push you around probably will. |
Re: Velcro tread
You need to be very careful with any high traction wheels.
As we discovered this year with some grippy wheels, we are responsible for damage done to the carpet by teams trying to push us. |
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