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Unread 04-12-2012, 10:00
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Re: Balancing an Arm

Quote:
Originally Posted by JVN View Post
...
In my experience, it is better not to overthink it, just add some surgical tubing and tune it to taste...

-John
This is important for many things that invovle tuning. I am a huge proponent of "doing the math", but often in development cycles, you plan to get close, and then tune for optimum performance.
When doing any sprung element development on a car (springs, shocks, bushings, engine mounts...), you typically will do analysis and design to pick a nominal, and then have a tuning set of +/-10% to +/-50% to see how changing the rate might help performance. The reason for this is usually, your analysis is based off of some assumptions. Without tuning, the quality of the output is reliant on the quality of the assumptions (which sometimes are poor). Planning for some tuning can vastly improve this.

With things like balancing an arm, you can use the:
"JVN says balance at horizontal" as a rule of thumb, but then try a little extra and a little less and see what works best. If you vex robot works best with it tuned to balance when feeding in the trough... then that is the ri9ght answer for you and your robot. If your robot works best with just a little bit of assistance to keep the motors in the friendly half of the power curve... so be it.

You can get pretty fancy with the way you do the counterbalance as well. Using "over center" principles, you can get some nice variation in forces. The gas shocks on a minivan liftgate are a great example of this. At almost closed, the forces in the shock are at their highest, yet they offer very little lifting force due to the hinge and the push point and the reaction point nearly being in line. This makes it easy for the gate to stay at or near closed. Yet, when fully extended, the shock is at its weakest from a force in the shock, but it is able to hold the liftgate up all by itself. Pretty neat when you think about it.

You can also get this behaviour using bungee and "cams" to change the lever arm length that the bungee has. This can have some very neat and dramatic effects.