pic: arm design



Here's a quick sketch of an arm design I was thinking of (yes, ms Paint). Basically a 3 section hinging arm, the first section rotated by a sprocket and chain drive (van door motor?) and the second by a window motor or sprocket mechanism. Could I get some advice on whether this has worked for them in the past, or what kind of gearing ratios are needed? Would pneumatics or scissor lift be better to get up to about 9 feet?- PM me

The pic on the left is a frontal view.

looks good. Cant say the drawing is perfect for manufacturing. Just watch how much weight you have and how far out it is. In response to your question: we looked at pneumatics to raise our arm last year, i didnt pan out for us. But its not to say it wont for you. As for the scissor mech, we made one after the competition for kicks, it worked, but broke quickly because of the force needed to get the initial movement from its compact position. There was a team last year who used a large scissor mech. The only problem is that it took just unger 2 min just to get it extended.

HTH

Marcus

I think that if the right materials are used, the structure would be good. Only problem I can see is possibly center of gravity. Having a second motor up on the second joint and holding a tetra up 9 or 10 feet along with the weight of the arm itself as well will make the robot tippy, so you have to work on problems with your COG (Center Of Gravity). But, if you keep everything else low in the base like you have it you should be ok.

I would definitly make sure you make the arms easily and quickly replacable, because if the robot does fall over it could snap them if not properly made or with other teams hitting into it.

As for the actual mechanisms, do not use pneumatics… with the height you’ll need large pneumatics which waste air like mad, and you will need to wait for the tanks to refill. In 2003 we tried this with our system to knock down boxes… all of the air would be gone after 1 or 2 movements, and for a fast-paced game like this it’ll hurt you. Also, I wouldn’t use scissor lifts, they’re very wabbly and hard to control. Yes, they do compact nicely, but they will not give you control and will not work very well lifting 9 pounds up that high, plus you wouldnt be able to turn it to release the tetra off of your hook making things hard.

http://www.chiefdelphi.com/forums/pictures.php?s=&action=single&picid=6296&direction=DESC&sort=date&perrow=4&trows=3&quiet=Verbose

Our arm last year worked great, and kind of matches your layout, but the lower section pivots from the bottom. I don’t think you could use this wide setup at the height you want within the weight limit though. But you might want to look at the mechanisms used. Particularly the window motors for the upper arm. We used 6:1 for the ball plus the arm weight and it worked easy, but you’d need a little more reduction for a tetra. Unfortunately our lower arm section used 2 globes, but this year they only gave 1. You could possibly combine the Van and globe for the lower arm section. They both operate close the the same rpm (75-80)

Go to our website below if you want to see some video of it in action.

It seems to me you are a little late in the game (only at the mspaint sketch stage) to tackle a design like that.

You’d be surprised what you can do with just a pencil and paper…

Exhibit A:
http://www.wildstang.org/2003/inventor/sketches_hand.html

Also, most things designed on Team #258 before 2003 was done on paper by hand.

I like it, though (as stated before) the center of gravity might be a problem. Maybe move the weight center to the back of the robot and move the frame of the arm back a little more too?

Looks a lot like my first design. I sketched a few designs out even though I am not actively on a team this year as there are no teams near me at college. To solve the CG issue, make two arms. Have them move together but on opposite sides. You could pick up one tetra and than quickly move to the other side of the field or turn around at the auto loading zone and pick up a second tetra and then move out and cap two goals. You would be more efficient and not have the same issues with the CG. I would also make it more like Swamp Things arm but not as wide.

There is nothing wrong with designing something without CAD. Those sketches you link to are more than enough to build a robot out of. The problem is, if you aren’t even sure of the general shape of your design yet, it is probably too late to take on a large, two jointed arm.

I bet that there are many teams still in the design phase, trying to decide what exactly they want to build. I don’t think they are far behind at all.

I could do it in a handful of days, I’m sure, so I don’t see why others cannot do the same. Maybe it’d be better for us to offer advice and knowledge about the design of arms similar to what this student is proposing rather than tell them repeatedly that they cannot do it. What sort of inspiration is that, anyway?

Amen, Maddie!

Thanks for all the thoughts: SwampThing, your design was great, I think we’re leaning towards something simpler like that now (still capable of capping the small goals after dropping the last arm segment). As to the lateness, our school had exams the whole week. :mad:

Alright, you guys are right. What I meant was I, or anyone I know, would not want to tackle this design this late in the game. Maddie Krass, I’m sure you could finish it in a few days, but that in no way means that the best way to “inspire” someone would be to suggest something beyond their capabilities.

And Stephen, I wasn’t criticizing you for your lateness.

Now if you want some serious suggestions:

Get rid of the second joint in the arm. With a nice gripper, it is possible to reach the center goal with a single jointed arm.

Power the main joint in the arm with at least one fisher price or Sliding door+globe. You need the power with the 100+ft-lb of torque you will be looking at.

It looks from your design that you are planning on transferring power via the arm shaft. In most cases, its probably more efficient to bolt your sprockets directly to your arm then having to deal with large sprocket hubs, keyways, etc.

One thing I might give some thought to is what the necessary “standoff” distance is for picking up a tetra (this goes for any design). I foresee a lot of robots that, for instance, won’t be able to pick up a tetra less than 4 or 5 feet a way. Thats a very structurally vulnerable position you want to avoid. The same sort of thinking goes for what position you’ll be able to hold the tetra at while you’re wondering around the field with it.

As far as being behind, we are only slightly ahead of what you have posted so don’t worry too much.

It is not for any other team to say what is beyond a person’s or a team’s capabilities. That’s why they have mentors :slight_smile:

If one week and a day are “late in the game” then we are ALL doomed.

Never underestimate what a group of determined people can accomplish.

A possibility for lowering you CG is to put the motor that moves the “shoulder” lower on your robot and lengthen the chain that runs up to the sprocket. You could do the same thing with the other joint - move the motor closer to the shoulder and run a longer chain (or perhaps a timing belt).

Of course, you could use OUR favorite option… PNEUMATICS!

-Mr. Van
Coach, 599