multiple tetra stacking
 

are there any teams that are planning on stacking more then one tetra on top of eachother before placing on the goal? If so what kind of arm are you using to accomplish this?

Re: multiple tetra stacking
 

I don't think it'll be a commom strategy, besides it's a great way of optimization.

The tetras are heavy. Even if the robot is able to do so, it's a bit dangerous...

Re: multiple tetra stacking
 

We thought about doing it, but decided that it would be way too much weight hanging out five feet out from our robot so we decided against it. Our arm may be able to do it (depending on how large of an area we use to hold the tetra), but we probably won't do it.

Re: multiple tetra stacking
 

I keep on seeing people ask this question, and my only response seems to be:

Can one seriously be competitive and NOT stack more than one at a time?

Maybe I'm making a grave error by over simplifying this game... but I think the winning robots won't be tic-tac-toe experts, but rather just have the largest tetra / minute ratios.

I know the tetras are heavy. I know they need to go high really high in the air. But with a 40 pound drive chassis, you have 80 pounds to do it. This is the sort of things engineers live to do... right?

Am I crazy?

Matt

Re: multiple tetra stacking
 

No, not crazy. Just be VERY VERY careful or you will be watching your robot on its back for many matchs. :eek:

Re: multiple tetra stacking
 

Okay, I'm going to take a shot in the dark and estimate that the average arm length is approximately Eight feet in length. Forget the moment for the arm weight itself and calculate 9 lbs at 8 feet (you do the math).

Re: multiple tetra stacking
 

You only have to consider the force perpendicular to the arm (or the arm length perpendicular to the force). So assuming the average robot will reach outward 4 feet or so to get a Tetra centered on the goal, it is about 36 ft-lbs of torque per tetra (which is still a lot).

Re: multiple tetra stacking
 

Alrighty...

I'll go ahead and just do some math. (And engineering, it's much better than math.)

We're gonna take 3 tetra at 10 lbs each, 30 pounds. We'll be crazy people and make an arm that's 8 feet around a pivot (it telescopes out)

We'll just use a piece of 3 x 3 by 1/8" box aluminum, 6061 with a yield around 36000 PSI.

That arm joint will weigh (3in²-2.75in²) * 96 in * .0975 lbs / in³ = 13.455 lbs

That's 30 lbs * 96 inches = 2880 in-lbs for tetras
and
13.455 lbs * 48 inches = 645.9 in-lbs for the arm weight itself (not really negligible)

bending stress = M * y / I

M = moment
y = distance from center to extreme edge
I = moment of inertia (for a beam = h³*b/12 where b = base width and h = height in the direction of applied force)


(645.9+2880) * 1.5 / ((3^4-2.75^4)/12) = 2665.68 PSI

Factor of safety... about 13.5.

This proves that you can create something that will endure the stress using just simple materials.

The torque is a lot, but only when you start trying to twist around that axle of your pivot. Start thinking pulleys, counter weights, large moment arms on the back end...

I didn't mean to go all formulae crazy... but... you dared me!

Matt

Re: multiple tetra stacking
 

I didn't see anyone worried about the strength of such an arm. The problem is stability of your robot. Try calculating the momments around the farthest edge of your robot. There is really no way you are not going to fall over. And how exactly does stowing a large number of tetras increase your tetras/min? The closest goals to the loading stations are very close. I suspect you will spend significantly more time loading and unloading then traveling, so multiple tetras does not help you that much.

Re: multiple tetra stacking
 

OK Matt...your math is correct the one thing you arn't thinking about is where the center of gravity of your robot is going to be at that point. your arm will definitely be able to support your tetras when your robot it lying on it's side...:p

assuming that the arm is extended 3 ft up and 4 ft out with a relative weight on it of 150 lbs ( perpendicular distance of 5 ft with 3 tetras on it)

even if the previous center of gravity was sitting on the ground the normal force of your robot would be unable to counteract the weight.


now this is assuming no counter actions, it is possible to make counter forces with springs, and shifting weight but i would still be cautious of doing so because of the amount of energy that would be stored. (for examples of this check out team 47's bot from 2003)

Re: multiple tetra stacking
 

I do not like your assumptions.

(edit: YIKES! That sounded a bit harsher than I wanted. I like Greg. I just didn't like the particular assumptions he put on the problem. :) )

Re: multiple tetra stacking
 

i will admit that is a good solution, but that is assuming that you are completely motion less at the time but if you are moving you are going to have a dynamic moment that will also have to be counteracted especially when that arm outstretched starts to oscillate.

and all of your stresses will be on the hinge for that "foot thing"


it defiantly could work though



edit: matt don;t worry about it....this is one of those problems our professors tell us about where there isn't 1 solution and it requires alot of thinking about all the possible solutions and variables involved

Re: multiple tetra stacking
 

Some "rookie" comments, but not giving away anything that our team is thinking about doing.....But maybe food for thought for more-experienced teams (hopefully our ally some day.....ha, ha)

1) I read this thread about "long arms" and "4 feet extension. Just remember that you do NOT need to reach out more than 2.5 feet in the HORIZONTAL direction to be hovered over a goal in order to drop/place a tetra. Don't cantilever more than you have to.

2) To 'pre-load' or not to 'pre-load' with on-board tetras. The trade-off to loading up multiple tetras will be the difference in time to drive back and forth to a goal versus the time to back out of the load station long enough to re-enter the load station to pick up another tetra.

***HINT and concept: I had envisioned a "back flip" robot loader that had an arm on the very FRONT of the robot with a upside down tetra "basket" on the rear of the robot. The loader arm grabs a tetra from the loader station and "back flips" it onto the rear of the robot UPSIDE down in a stack. Each time you grab a tetra, the robot arm will have to lift vertically a bit to compensate for the on-board stack. Then drive to a goal and simply reverse the process.

Re: multiple tetra stacking
 

Our team considered this strategy, but rejected it as impractical for our means. (We're a 2nd year team).

Imagine though, what would happen if 6 tetras were suddenly placed on a goal. At about 6" of height gain per tetra placed, thats 3 more feet that the next robot has to cap on. Depending on the height of the opposing team's arms, this could potentially be an effective strategy.

Re: multiple tetra stacking
 

As CNield said, we though about this, but we could not come up with a effective gripper to be able to pick them up. We also thought about how much power we would need to be able to pick all of the tetras up. It also is unpractical and we think that with the time it takes you to stack them and put them up, you could have done the same amount individually.

Re: multiple tetra stacking
 

Also, what happens if you stack a bunch of tetras and the opponent then comes and knocks over the stack. That wouldn't be very fun. All that effort wasted... :(

Re: multiple tetra stacking
 

that'd actually be good, u want ur oppenents to knock down ur tetras :) :P in the rules it says that the oppenents cant knock down the tetra's and if they do u own the goal for the rest of the match and i think there were some penalty points :D :D :D

Re: multiple tetra stacking
 

What I meant was if you were stacking them to place a bunch on a goal at one time. If your robot were in, say, the loading zone, and mine knocked over your stack, then you'd have a problem. :p

Re: multiple tetra stacking
 

Well, I think our arm may be able to pull off stacking more than one tetra (after adjusting our regulator), but we don't plan to do it.
It makes more sense to focus on effectively capping individual tetras to be more precise in forming and maintaining rows. I think that is our key strategy this year (oops!, well, not entirely ;) )

Re: multiple tetra stacking
 

yet again, if the opposing team messes with you while u are loading tetras or on the loading zone :P the opposing team gets penalty.

Re: multiple tetra stacking
 

Yet.. if you stack a stack of tetras on one goal quickly enough, you might be able to make it a hell of a lot harder for the opponent to stack a tetra of theirs on top of that specific goal. it'd be even better if it's the center goal, considering how many potential rows you can get out of that :)

Re: multiple tetra stacking
 

You can put a tetra on top of the goal with out having to go 2 1/2 feet out

Have you not thought about going over the bottom bar then dropping the tetra unto the goal. So as long as up the human can place the tetra far enough on the robot arm and the arm is in the center of your robot you could put many tetra on at one time and place them all on the top at once without flipping. 5 tetra is not un heard of.

To give you what we had in the past

http://www.valleytech.k12.ma.us/robo...1Robot2001.jpg

this arm went up 14' to place that ball on. It is all about keeping the CG low enough

Re: multiple tetra stacking
 

Only if they interfere with the actual loading of the tetra. As long as the robot in the LZ retrieves the tetra successfully, there is no penalty. You have to put the tetras somewhere, which would be a stack, and it is not against the rules to knock over a stack as long as you don't interfere with the robot itself.

Re: multiple tetra stacking
 

one quick question, in autonomous mode if your in loading zone getting a tetra and an opposing team's robot hits u, thats a penalty right?

Re: multiple tetra stacking
 

I think the only arm that could effectively do this would be an arm that pokes through the tetras. i.e. does not grab the tetras at the top or on the sides. So maybe a forklift kind of arm design?

Re: multiple tetra stacking
 

Strategically... I feel that the quantity of tetras placed will be only slightly less important than strategic placement of tetras on the goals. Although if two members of an alliance work on that strategy while another just works on getting large quantities of tetras to rack up points, then it would be a good idea.

-Alex

Re: multiple tetra stacking
 

we are thinking of multiple stacking. We are currently making prototypes.

Re: multiple tetra stacking
 

I think what will happen is that the multiple tetra stackers won't be stacking a bunch and then capping the whole bunch. They'll probably have multiple "lances"(tetra manipulator term for a stick) coming out of their lifter. For two reasons, one to make it harder to miss a tetra you're trying to grab and two, if you need to you'll have an other available lance to grab another tetra.

Re: multiple tetra stacking
 

The rules jut say that if you interfere it's a 50-point penalty. They don't specify if it doesn't apply at any point in the match.

EDIT: Because I can't post again, I'll just edit the post. Thanks to whoever pointed it out, but it is, in fact, a 30-point penalty. Maybe I need to reread the updates.

Re: multiple tetra stacking
 

I believe it is a 30-pt penalty....

oh and for the discussion earlier of being to tall and too easy to tip.....why not just make your footprint larger....you just have to fit in the envelope at the START of the match. Use some outriggers like on a crane....

Re: multiple tetra stacking
 

If you want to get the bonus you'll have to fit at the end, too.

Oh, yeah, and about the penalty, it's 30 points but you'll pretty much lose anyway, so...

Re: multiple tetra stacking
 

You still might not be able to fit in, however, if something is in the way...

Re: multiple tetra stacking
 

That is something that we have been looking at. from apex of the center tetra in the team zone to the closest corner of the tetras to the right or left is 13 fit 6 inches that is alot of room to fit 2 robots. We took a piece of cardboard and cut it to the robots max dimensions and placed it in that spot and there was enough room for another bot to fit, also they can go under the goals to fit if needed also.

Re: multiple tetra stacking
 

Sounds like a good idea! I wonder how many robots will try and do this.

-SlimBoJones...

Re: multiple tetra stacking
 

Yeah, if a tetra was there blocking a robot, then it would have to lift it out of the way and then go in. It probably wouldn't have enough time to do that, though, because if it did have a lot of time, it wouldn't be trying to fit in in the first place. It's a vicious cycle.

Re: multiple tetra stacking
 

just retract your outriggers then after they are no longer useful....i guess i don't understand the prob? o well....

Re: multiple tetra stacking
 

No problem, just that if an opposing robot tips your robot while the outriggers are reatracting/being retracted, then you'd have a problem. But there's nothing not to understand; you just make your robot bigger at the beginning, then make it smaller to fit in, so there's nothing you were missing.

Re: multiple tetra stacking
 

Matt, as a veteran of FIRST I have seen this happen in 2002, 2003, and 2004, and I see it happening again. you are making a grave mistake.

mathematically, yes, you can build a robot capable of lifting multiple tetras at the same time. what you fail to take into account, however, is the competition environment. teams will play defense, and if they see a robot trying to cap multiple tetras they are going to push it. it doesnt take much of a push to send a robot holding that much weight that high toppling over. it won't even be counted for a flag by the judges, because they were preventing a score with a perfectly legal maneuver. even if you don't fall over, that much weight will make the arm sway when you get nudged. that will make it very hard to cap, and tetras may even come out of your grip with all that movement. in all three seasons that i have seen, one thing that remains constant is the failure of top-heavy robots. the more you carry up high at one time, the more top-heavy you are. if you do choose to make a robot capable of such a task, please be advised to use this strategy sparingly. any aggressive opponant will take immediate advantage if they see you lugging around a stack of tetras.

Re: multiple tetra stacking
 

It's like what one of our mentors said, "What happens to the robots that are designed to score all the points? They don't."

Re: multiple tetra stacking
 

All this talk or reaching out and such....who says you can't drive underneath the spot your putting the tetra and place it on top...much like a forklift. It is legal to drive into the goals as long as damage is not done after all.

Maybe a forklift like this: http://northamerica.yale.com/lift_tr...sle/ndr-cb.asp

Just some brain food for you all ;)

-Pat

Re: multiple tetra stacking
 

we can stack up to 5 at a time... we use a finger and we are designing a structure that can hold two stacks at a time.

Re: multiple tetra stacking
 

Sounds neat. Have any pictures? Good luck getting them out of the loading stations fast enough. :D