Balancing an Arm

[Brandon_L]

After my teams horrendous mess of an arm in '11, I've been doing quite a bit of research on arms in FRC. The more I find, the more questions I have about them. I've picked up that you'd want to balance your arm with either surgical tubing or gas springs, so that your motor driving the arm doesn't have to deal with gravity as much. But what is the best way to balance an arm? I'm specifically interested in gas springs and have the following questions (I'm sure I'll have more later):

1) Where do you want your arm to sit when neutral?

-I've read that you'd want it to sit horizontal to the ground. But wouldn't it make more sense to be more neutral where you'll be reaching? (example, closer to the "up" position in 2011 to ease reaching the upper racks). I'm familiar with formulas to get the arm to sit horizontal (weight * distance on one side = weight * distance on the other, or in the case of a gas pring pushing up, weight * distance = distance from center of rotation to where the spring mounts * force of the spring) but I'm not quite sure how to apply that to get the arm to sit more in the 1st quadrant then directly horizontal.


2) Once you have your arm balanced, wherever balanced may be, how can you start a match with the arm down?

-My thought process here: The gas spring or surgical tubing are supplying a constant force to keep your arm up, and I'm using a motor that backdrives. So whats keeping it down with no power to the arm motor? Do some team do quick releases for surgical tubing arms?

The main question I'm currently fighting with: is there some property of gas springs I'm missing?

-I've been told you could manually push a gas spring arm down into your starting config and leave it there, and it won't spring up. So this tells me that the spring isn't just putting out a constant force upwards onto the arm. If its not pushing up, how does it help at all? What am I missing here?