[EricH]

Why do you need gas springs or other means of balancing an arm other than its power source?

This is not an idle or ignorant question. I have seen multiple arms with no balancing (other than the power source) that did quite well. Single-joint, multi-joint, even with a large-ish mass at the far end like a game object.

As for your second question: Your motor does not backdrive? So why are you worrying about keeping the arm in a specific place? For that matter, why are you looking at things other than the motor to keep the arm up? Motors that backdrive (may) need these things, but motors that do not backdrive almost certainly do not.

For the first question... You're actually asking two. Where you want "neutral" to be is game dependent--ask again sometime in January. But the math behind sizing/placement at neutral...

You're actually pretty close. You would use an equation something like, (weight of arm side 1)*(horizontal distance from pivot point to arm CG)=(weight of arm side 2)*(horizontal distance from pivot point to arm CG)+(vertical force component from balancer)*(horizontal distance from pivot point to balancer attach point).

This would actually fall into the Statics category of problems, which many engineers study early in their college careers--the basic method of balancing out forces is to set up an X-equation, X-unknown system of equations where the sum of all the forces and moments in a given direction or about a given axis of rotation is zero. Then you can solve for your unknowns.


Just to bring something up... I have yet to see one of 330's single-joint arms use any sort of balancer that wasn't the drive motors. Neutral for some was all the way down; for others it was wherever the arm was left--one at least needed a little bit of power to maintain a position. Typical drive, 1-2 FP motors, stock gearbox, heavy reduction afterwards (exception: 2x Globe motors mounted on the arm itself--2004). No braking capability.

Oh, and pneumatics are also an option, if you don't mind having rather limited numbers of postitions.