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Re: Balancing an Arm
Balancing an arm is a bit of an art. There are tons of options for supplying the balancing force (coil springs, constant force springs, gas springs, surgical tubing, even counter weights), tons of different sizes/strengths, and in most rotary arm cases you are dealing with changing the angle of the balancing force during travel.
Without getting into specific designs (others here are far more adept than I at arm design), here are two meta-level thoughts to keep in mind:
1. Does an arm ever really need to be "balanced"? You are using material deformation to supply a force to counteract the effect of gravity in order to reduce the torque/force requirement on some actuator. Even if you aren't really balanced, every bit of torque/force you are relieving is aiding the actuator. Don't lose sleep over making it perfect (that is, F(gravity) = F(counterbalance) for all angles of the arm)
2. All motors stop back driving at some point due to friction. The bigger the gear reduction, the more friction is present in the system. Our 2011 robot's arm was powered by a Fisher Price motor through a multi-stage planetary transmission. There was enough friction in this system to keep our arm stowed at match start even though the arm was balanced in the horizontal position - we had to be careful when doing maintenance on the chains to our arm because once you remove them, the arm springs up.
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