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.

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

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

Don’t give the robot any more Mountain Dew.


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?

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

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

We’re using window motors for the arm.

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

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.


Would bungie cords work?

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.

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.

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.

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.

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…

can you post a picture of the arm?

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

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?

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.

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!)

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…)

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