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?

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…

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.

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?


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

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).

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).


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
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!


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.

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…:stuck_out_tongue:

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)

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. :slight_smile: )



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

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.

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.

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.

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… :frowning:

that’d actually be good, u want ur oppenents to knock down ur tetras :slight_smile: :stuck_out_tongue: 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 :smiley: :smiley: :smiley:

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. :stuck_out_tongue:

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 :wink: )

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