Arm goes down too fast?
 

Hello all,

Our team is running into a problem with our arm for the robot. As of now the arm raises up slowly, which is what we want. However when we try to lower the arm, the movement is very fast, violent and erratic. For the moment we have an old bumper hooked up to the supports to stop the arm from crashing down into the vitals, but we really just need to make the arm lower more slowly. Any advice would be much appreciated.

Re: Arm goes down too fast?
 

Could you add surgical tubing or a gas shock, say as a counterbalance?

Re: Arm goes down too fast?
 

We have bungie cords on the back end of the arm but its too bouncy and doesn't really do much. They were super tensioned

Re: Arm goes down too fast?
 

Don't give the robot any more Mountain Dew.

Blake

PS: Are you actively trying to control the arm when it goes down, or is it just settling under the influence of gravity?

If you are trying to actively control it, how does the arm behave when it is disconnected from the active devices?

How do the active devices behave when they are diconnected from the arm?

Is some individual part of the machine acting erractically, or are several parts combining in some odd non-linear way to create the behavior?

Re: Arm goes down too fast?
 

Check if your Jaguar/Victors have the brake jumpers installed. It can help.

Re: Arm goes down too fast?
 

It does it when were powering it in a downwards direction. Without power it stays up fine, and with power up it goes up fine. Its only about .3 power in the programming and it just slams down

Re: Arm goes down too fast?
 

We're using window motors for the arm.

Edit: Does it matter? The window motors don't backdrive.

Re: Arm goes down too fast?
 

Add more surgical tubing -- lots more. The arm isn't neutrally balanced yet. This is one way to make it much more controllable.

It is easier to use software to limit the upward speed than the downward speed. Something about gravity... Use surgical tubing to take gravity out of the equation.

-John

Re: Arm goes down too fast?
 

Would bungie cords work?

Re: Arm goes down too fast?
 

Yes... make sure you get ones that will work in a way that you can apply force for the entire arm travel. Surgical tubing is great because of how much is stretches, in a manageable way. We tried bungies... learned to hate them.

Re: Arm goes down too fast?
 

We have the same problem. Our arm currently falls really fast once it starts going downwards.

What is the best and most efficient way to attach surgical tubing to the arm? We added surgical tubing to it but the arm is still falling out of control.

Re: Arm goes down too fast?
 

We have the same problem. Our arm currently falls really fast once it starts going downwards.

What is the best and most efficient way to attach surgical tubing to the arm? We added surgical tubing to it but the arm is still falling out of control.

Re: Arm goes down too fast?
 

I can't answer your question with any sort of specificity without seeing your design.

In general, attach the surgical tubing in between a stationary point on your robot, and a torque lever which affects the rotation of your arm. The longer your torque lever, the more effective the surgical tubing will be, the less you need to use.

Re: Arm goes down too fast?
 

Ok so, we no longer have the arm going down superfast but when we stop it from moving in the downward direction, its still super bouncy. Is there anything at all we can do to fix this? We bent up some of our arm parts from the bouncing..

Re: Arm goes down too fast?
 

can you post a picture of the arm?

need more details.
Usually
-increase gear ratio
-balance
-reverse powered lowering(requires some programming knowledge
-increase the strength of the joints and materials used.

Re: Arm goes down too fast?
 

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

What do you mean by this?

Re: Arm goes down too fast?
 

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.

Re: Arm goes down too fast?
 

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!)

Re: Arm goes down too fast?
 

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..)

Re: Arm goes down too fast?
 

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

Re: Arm goes down too fast?
 

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.)

Re: Arm goes down too fast?
 

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

Re: Arm goes down too fast?
 

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! :D

Re: Arm goes down too fast?
 

Oh your no fun haha

We'll play with it

Re: Arm goes down too fast?
 

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.

Re: Arm goes down too fast?
 

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.

Re: Arm goes down too fast?
 

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

Re: Arm goes down too fast?
 

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.

Re: Arm goes down too fast?
 

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

Re: Arm goes down too fast?
 

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. ;)

Re: Arm goes down too fast?
 

We played with it and boy are we happy now.

Other then some bouncing from other factors were working on, its now wonderful.

Will look for surgical tubing, because our balance point is pretty much about 6" above the floor with the bungee cords.

Thanks for everything guys!

Re: Arm goes down too fast?
 

Happy to help. New video?

Re: Arm goes down too fast?
 

Soon, its still bouncy but we know why we just didn't get to fix it yet. So the video will look similar to the last one. I'm waiting for our camera guy to send it to me :P

This helped alot though. Were getting surgical tubing tomorrow. How would you suggest tying it?

Re: Arm goes down too fast?
 

In knots...

Re: Arm goes down too fast?
 

Oh, I thought there was some fancy way. I thought that if you did that, when you tension it the knots will just slip through whatever its tied to..oh well I'll have to try it tomorrow.

Re: Arm goes down too fast?
 

We tie it in a knot. Take the two ends, hold them next to each other, ends pointed the same direction, and tie a single normal overhand knot with both ends at once. That makes a loop. Then anchor the loop at both ends over something round, such as a 1/2" od nylon spacer with a 1/4" bolt thru it holding it to the robot or arm or whatever that end needs to attach to. If you make the anchor end adjustable, then you can adjust the tension as needed.



for example

Re: Arm goes down too fast?
 

We found a spring, about 1" in diameter and about 6" long. When we stuck that on and gave the cables slack, the arm floated almost exactly horizontal. That works, then!