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Velcro tread
I was thinking about hi-traction treads, and was thinking, how about (hook) Velcro? Roughtop top treads are a very mild form of Velcro (they certainly pick up fluff), and the strong shear strength and (relatively) low peel strength would make them work on wheels (albeit probably very inefficiently) and have amazing traction. I understand no-one is the GDC and can't comment on 2015 rules, but for this discussion let's consider the 2014 manual.
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[G10] is more interesting. I think you could possibly get around the "damaging" part if you chose your strength of Velcro wisely, but "attaching to" is not as clear cut. Potentially you could argue that it's just a stronger tread, or use something like cheaper velcro or dual-lock that has a different (and maybe slightly weaker) adheasion method. But I'm doubtful if you could confidently get an inspector/referee to allow it. My team certainly won't be using Velcro, the 2014 rules isn't clear cut enough and it's doubtful that the 2015 ones will be. However I was hoping to start a discussion as to alternative treads and what benefits/properties affect performance, ie is there anything other than CoF that you look for in a tread? |
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I doubt Velcro would make a good tread even if it were legal, for 3 reasons:
1. Its durability and stickiness decrease drastically with use. 2. It would generate a ridiculous amount of rolling resistance. 3. It might make turning a lot more difficult (not sure about this one). |
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We actually have two drop down traction wheels on our robot that are treaded with Velcro (we ran mecanum until offseason so we had them for pushing). We had no problems all year with inspectors based on the rules you provided. I wouldn't say they were better than any other tread when it came to driving, but once we had them down on the floor it was pretty hard to move us.
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I'm going to comment here based on past history. I think some folks will find this interesting.
Prior to 2003, traction was "anything goes". Metal treads, metal studs, you name it. In 2002, with a high-traction pushing war, many teams had some form of metal cleats on their drivetrain or lock-down devices. The most famous? 71, with their file-card drivetrain. But after every event, the carpet was just about trashed. (Especially where 71 had been driving if they'd hit resistance.) Fast forward to 2003. That year, a rule came into effect that no (traction system) metal could contact the carpet. It's evolved a bit, and in 2014 showed up as R6 and G10 together. Here is the intent of the rule(s): Do NOT damage the carpet by robot design or by gameplay if you can avoid it. Metal (and the other items listed) can be very damaging to carpet. Now, Jay, the inspector would certainly pass you. There's no part of that rule (R6) specifically prohibiting hook-side Velcro. That's a correct ruling. The referees probably should not have under G10D. And that's because Velcro doth most definitely attach to cloth/looped stuff/other items of that nature (including carpet). It's Velcro. That's what it does. The field is held down by industrial-strength Velcro--if you've ever been around for field setup or teardown, the fields come with HDPE sheets for moving items around on the carpet without getting the Velcro to attach (or for sliding in to get the Velcro detached!). And it's always fun getting the side rails down, something on the order of "OK, everybody pull!" with a large group standing next to the rail. If any team were to use Velcro on their wheels to increase traction, I would have the following recommendation: Q&A as soon as you decide to do that. Then... if it's a definite "no", take the Velcro off. If it's a definite "yes", bring a copy of the Q&A with you just in case somebody says something. If it's a "We cannot comment" or similar, print it out, grab the LRI and Head Referee and a spot on the practice field as soon as possible on Thursday, and ask them, demonstrating the use of said wheels on carpet. That way, you know early, and they're aware in case someone else complains later that it's illegal (and they can say "We already checked, they're OK, we'll pass that concern on for next year"). |
If the express design purpose of Velcro is NOT "grasping" or "attaching to" or "becoming entangled", then what is it?
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Edit: we also asked our inspectors about it and showed them the wheels at UNH and Northeastern. Both of our inspectors said they were ok with it but we didn't end up putting them on until NECMP |
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Personally, if a robot showed up with velcro on the wheels, I would probably grab the FTA and head ref and get their opinions - they're the ones who would have to deal with the consequences should the velcro cause issues on the field. I don't think the ruling on this is as obvious or certain as you may think. Definitely one for the Q&A if you're thinking of using it! |
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Which is why I suggested the team grabbing the LRI and Head Ref (forgot about FTA) and some practice carpet if Q&A gave an unclear answer. If the Head Ref points out that the robot is going to be disabled every single match unless something changes (G10), the LRI's take suddenly becomes even more valuable. Conversely, if the team is able to prove that they meet all the rules, everybody is on the same page about it from the get-go, and no further discussion needs to happen (unless they start damaging the field). |
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Now, there are some items where a team does have the burden of proof that they are legal, should an inspector challenge them on it. Namely, electric solenoid actuators (for power rating), servos (same), and pneumatic components (pressure rating, compressor specs, and the like). If an inspector challenges, you need to have the documentation, or be able to produce it before your next official match, or the inspector may assume the device is illegal and require it to be disabled. But the inspector does need to say something... (or you could just show him the item and the paperwork). |
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In ALL cases of disagreement over robot legality, the burden of proof rests with the team. Not just some. Questionable situations are what the Q&A is for, and they are why I always make myself available to teams going to my competitions (any anyone on CD, of course) as much as possible - I'd rather they ask me to look at something before they bag the robot, than run into a problem after they get to the competition. Even if I can't give a clear-cut answer and have to direct them to the Q&A, I can often help them understand what rule(s) the issue really points to, and how best to word the question to get a good answer (one better than "see R15" or "we cannot comment on specific design"). At an event, the LRI will do everything possible to get the team to pass inspection, and that includes bringing the issue to others for opinions in order to get his mind changed. Whenever I find a problem with a team, the very first thing I ask myself is "how do we make this legal?" |
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Hence getting the Head Ref involved early. (This will also serve notice to the Head Ref and FTA to watch that robot for field damage.) Of course, there's the obvious result of this thread to consider--I'm guessing the GDC makes a note to include Velcro in the R6 equivalent next year, making the discussion moot. (To some extent, I'm playing devil's advocate here. I agree that the LRI would generally be able to enforce it. Just looking for all the loopholes a team might possibly try to use to justify this use of Velcro, and ways to block them.) |
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To clear up the facts about 1058's drop-down velcro drivetrain. (you can see it in this video at 1:23 and 1:43):
It is two piston-actuated, drop-down wheels driven off the same axle that drives the rear mecanum wheels. They pivot on swing arms and lift the rear mecanums off the ground. The wheels are 6" Andymark Plaction wheels treaded with commercial-grade (I don't know which product exactly) Velcro. We planned to have the drop-drive on for our first competition, the UNH District Event, but we only ended up bringing a whole bunch of pieces that never made it on the machine. It is an auxiliary drive, and by no means has to be on for the robot to drive well. At UNH, we talked to one of the inspectors about the legality of the wheels and showed him our Velcro-treaded wheels. He got a good laugh and said he was OK with putting it on if we had time at the competition. The same thing happened at Northeastern University District Event, with the same results. We then had the system on for the New England Championship event, and made it a point to show the inspectors that we had the Velcro-treaded wheels. They were ruled legal. The inspectors even called us back to re-measure our bumpers, as there was a question regarding to when the wheels were activated. When the robot tilted forward, the inspectors made sure that the front of the bumper did not dip below 2" from the floor. They passed without modification, and we continued competing. I hope this cleared up any questions you may have had regarding 1058's use of Velcro in 2014. |
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I have no definitive opinion either way about using Velcro wheels for FRC, but as soon as I saw the question I was thinking of a game (with much lighter robots) that involved a carpeted floor curving up a wall that the robots would have to drive up and around. :cool:
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The LRI is the final judge of what the rules say & if a robot is legal as for as the teams are concerned. (OK it might really be the head referee, FTA or some combination of that). You can search CD & find examples of this. You can find examples of robots legal in one competition & not legal in another. So if I thought velcro was legal & wanted to use them in competition, I would have a plan B ready just in case.
Please note I am not criticizing or judging the LRIs or the system. You have to have a final judge & there are times where reasonable people can reach different decisions. |
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Team 230 started out this season with 2 feet covered with velcro that we could push down on pistons to prevent being pushed. We used them at the Suffield Scrimmage but they did not pass inspection at our first district event in Groton. In fact, we replaced the velcro with rubber and they were still not allowed. It was believed that they would damage the carpet so based on both R6 and G10 the feet had to be removed. (In the end it was all good... as we won the event so I guess we didn't really need them. :yikes:)
I recognize that feet are not the same as wheels, but when being pushed in a direction orthogonal to the way the wheels spin, they provide essentially the same traction and forces on/risk of damage to the carpet. My point is be careful in what you believe to be legal based on one event or one inspector. I definitely agree that it is probably better to try to get a ruling from GDC on the Q&A... and if they respond with a "cannot comment on a specific design" then definitely be prepared to remove or replace the possibly offending wheels (or feet in our case). |
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A question for those condemning velcro treads (from someone who's never used them): Are you sure velcro will damage the carpet? I am under the impression that velcro releases by uncoiling the plastic hooks not tearing the loops (if it tore the loops, it wouldn't be very reusable would it?) Even if the velcro tore away some of the loose fuzz, I can't imagine it would be much worse than the reams of carpet remnants we dug out of our (KOP-wheeled) bot this year!
MamaSpoldi - that's incredible! I don't understand how any thoughtful judge could rule that a rubber pad were any different than a stationary rubber wheel. |
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No, I'm not sure it would, but the concern is that it would. I had an old rain jacket with velcro straps that let you tighten the wrists, and over time the velcro really tore up the wrist part where there wasn't any loop side.
The point isn't whether it's legal or not... no one here can say definitively either way. It's questionable enough that a team could get different rulings at different events if they don't ask on the Q&A. |
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Sure, the loops won't necessarily give way right away (same for surrounding fabric. But it doesn't take long (say, a few years for a jacket)--call it a few hundred attach/detach cycles--for wear to show up, even under normal use like that. Now consider a robot with industrial-strength Velcro as its tread material. It's "attaching" for at least a few cycles a match, and if it's slipping in a pushing match, you're dealing with a very, very fast cycle--might hit several hundred in one spot in a couple of matches if you push in the same area. There goes the carpet. (For other similar items, see "HiGrip in powerful drivetrain slams wall in auto and keeps going". That automode ended with a hole clear through the carpet!) |
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Perhaps I can suggest, any team wanting to experiment with velcro treads should do a controlled experiment. Find a way to measure carpet damage, and measure the difference between velcro and HiGrip (or other treads) under identical circumstances. Bring the results to competition to show the judges. Use the power of science! |
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How about we look at this in a practical light. The velcro has a distinct adhesive property based on matching a particular type of "male" and "female" material. The strength of that adhesion is proportional to the mated area of the material. When mated with material other than that for which it is designed, bad things occur for both materials. Additionally, when it does produce friction with the floor, the force needed to pull it along the carpet is also very high. While everyone has admitted that the "male" velcro does pick up fluff, remember that it does not give up that fluff readily. At some point, (within minutes I suspect) the contaminant will have reached a point where the material is filled and all friction is lost.
In the case of wheels, the mated surface area is quite small, less than 1 in^2. in most cases. I would suspect that the coefficient of friction to be quite small. What would determine carpet damage in my mind is watching the robot drive across the carpet. If it leaves a visible trail, I would say it is damaging the carpet. If it picks up a considerable amount of fluff in a short driving session (say one match), it is also damaging the carpet. In this case you will have a harder time with the FTA and Field Supervisor than with the LRI. They can prevent you from playing until you correct the problem. |
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For the rest, see Al's post. I can see where damage would be coming from, having helped with field teardown (and thus the field anchor velcro) on more than one occasion. |
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I'm confused, so have teams used it successfully? If so, and it didn't cause damage tithe field, why is this even an issue. Please stop speculating on whether damage will occur or not. Let teams try it, and if it causes damage, then it's a problem, but don't stifle innovation in the name of it potentially causing damage. It could work perfectly fine, but without testing, we won't know.
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Remember: It's not all that uncommon for something to slip past the officials and only be caught later, when another set of eyes looks at the problem. Should it be highly unusual? Yep. Is it? Not really. |
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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. |
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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...). |
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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. |
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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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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. |
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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. |
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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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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. |
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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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Allow me to suggest a test as to whether something is attaching to the carpet. Take a large chunk of wood and attach an eyebolt to the "top" (and, just for good measure, to one side--I'll explain that one in a moment). Set it on a section of playing field carpet, then pick it up with a fish scale or similar device. You will get a force needed to lift it. Now coat the bottom with your choice of traction material. Pick it up with the fish scale again. (Note: You may want to use the side eyebolt to get the new weight of the block first.) If you use more force to pick up the block with the material on it than you do the un-coated block, you're probably attaching to the carpet. There may be some cases where you're not--"other factors" come to mind including statistical variation--but I'd be willing to bet that you'll be ruled as attaching if spotted at competition. Now here's the real reason for that side eyebolt. Put something heavy onto the block and drag it, again using the scale. Change the traction material and repeat. Which one takes more force to pull (and thus has a higher coefficient of friction, and thus more traction)? Maybe that's the one you ought to use...or not, if your strategy doesn't need more traction from that source. |
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The vacuum device must use legal motors or pneumatic vacuum generators. Teams have used certain items to gain better friction with the carpet, but these do come at a cost. |
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