Arm goes down too fast?

[Brandon_L]

http://www.youtube.com/watch?v=a_yD4oc4j2g

two window motors driving a 22 or 24 tooth sprocket (forget which one) with chain to a 60 tooth sprocket with the arm bolted directly to the sprocket. on the back of the arm is the bungee cord running down the back. This test was from a few days ago so it still shows it slamming down into the frame (why the mentor is holding on to it, to keep it from doing that)

Both window motors are being drivin from victors in brake mode (Had jags, but they locked up)

Let me know if you need any more info

-reverse powered lowering(requires some programming knowledge

What do you mean by this?

[WileyB-J]

I suggest a weighted counterbalance for the backside of the arm (meaning you would have to add an extension to your current arm) like industrial cranes have. you have a great deal of leverage and weight on your business end and you need to be able to have the arm naturally balance horizontally on its own, or close to it.

[pfreivald]

Your arm seems pretty rickety, especially for the weight it is trying to support. Forgive the paint-CAD, but you can add a lot of rigidity to a thin arm with some supports and steel cable, while adding almost negligible weight.

(Also, I agree with the neutral balance suggestions. We're using a four-start screw and a FP motor on a 64:1 BB for our lift, and we can stop it anywhere we want and it just stays there... It helps that we can lift the arm from behind the pivot with one finger!)

[Brandon_L]

Originally Posted by WileyB-J

I suggest a weighted counterbalance for the backside of the arm (meaning you would have to add an extension to your current arm) like industrial cranes have. you have a great deal of leverage and weight on your business end and you need to be able to have the arm naturally balance horizontally on its own, or close to it.

We can't go any farther out the back side of the arm or it will be outside the frame at the start of the match. We can't find our surgical tubing, but we have a ton of bungee cords. problem with it right now is that the cords are currently out of the frame perimeter (this IS illegal, right? Its still within the space limits..)

[Brandon_L]

Originally Posted by pfreivald

Your arm seems pretty rickety, especially for the weight it is trying to support. Forgive the paint-CAD, but you can add a lot of rigidity to a thin arm with some supports and steel cable, while adding almost negligible weight.

(Also, I agree with the neutral balance suggestions. We're using a four-start screw and a FP motor on a 64:1 BB for our lift, and we can stop it anywhere we want and it just stays there... It helps that we can lift the arm from behind the pivot with one finger!)

Sorry for the double, but I do like that alottttt.

The only problem is that I can't really think of a way to do it, we only have about a half inch from the top of the sprocket to the height limit, and the same with to the back of the robot

[pfreivald]

You could do it the other way around -- drill two (small) holes in your sprocket, mount the cable through them, and then have a wider bar as far out on the arm as you can to attach them to for stability. (I'm sure that will give you some issues as far as space and whatnot, too, but it's something to consider. Lightweight reinforcement = good.)

[Brandon_L]

would doing it the other way around have any negative effects on it other then more weight somewhere mid-arm?

Doing that mid arm is perfectly possible, we have plenty of space

[pfreivald]

I'm not going to tell you everything...

Go play. Have fun with the idea. See if you can learn something, and drag some other kids along with you!

[Brandon_L]

Oh your no fun haha

We'll play with it

[Chris Fultz]

We are using a gas shock (McMaster Carr) on the arm. You will need to do some math work to determine how much moment (torque) your arm has, and then determine where you can mount a shock and what size (force) is needed to balance the arm.

McMaster has these in multiple lengths and spring forces. They are pretty light and easy to use.

If you cannot find a way to make the surgical tubing work, this is another option.

[BornaE]

What size aluminum is the arm made of?

the arm is bouncy just because of the thickness of that piece mainly.

Also, I am worried that some other robot will run into your arm, and when that happens, my bet is that you will need to remake your arm.

suggestions: It is late in the build season and im not sure how much manufacturing ability you have.

Replace the ARM piece with a piece of 1.5" by 1.5" aluminum square. with 1/8" wall thickness.

balance the arm so without the motor it can stay in the middle position on it's own.

use a more powerful motor such as a cim or 775 motors. gear ratio need to be around 800 to 1 for 775 and about 200 to 1 for the cim.

bring the bearing holding your main arm shaft as close to each other as possible.

[pfreivald]

Robustness will certainly be an issue. Make spare parts. Lots of them! (...but less than 30 lbs of them...)

[Ether]

Originally Posted by JVN

The longer your torque lever, the more effective the surgical tubing will be, the less you need to use.

True, but the shorter the torque lever, the more constant the force exerted by the tubing (because it changes length less as the arm move). So it's a compromise.

[MrForbes]

Originally Posted by Brandon_L

We can't find our surgical tubing

They sell it at hardware stores and building supplies, in the plumbing section with the other tubing.

[JVN]

Originally Posted by Ether

True, but the shorter the torque lever, the more constant the force exerted by the tubing (because it changes length less as the arm move). So it's a compromise.

The longer the torque arm, the less force is applied to the arm to generate the same torque. This means your surgical tubing attachment point doesn't need to be as robust.
Compromise is the key word.