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Unread 04-01-2013, 10:06
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Re: Drive Train Calcs - Not trusting my results

Quote:
Originally Posted by JesseK View Post
Getting a precise measurement for a specified system is all well and good, yet when considering the end results of controllable decisions, the resistance of the battery & wiring doesn't necessarily effect two outcomes of the same FRC robot differently. Thus can we assume that, given gearing choice A and gearing choice B, the wiring/battery resistance is the same and doesn't need to be modeled?
The voltage drop across a wire (which we can assume is the same resistance, the resistance is a constant) is dependent on current. A different gear ratio would pull a different current over time as it accelerates. So modeling that is good.

The original JVN calc had several battery voltage steps, which gets 'close enough'. The point is that a battery will drop down to at least 8v or lower during launch, the time it stays low is determined by the gearing and current draw. Essentially, torque = acceleration, a faster gear ratio gets you less total motor torque to the wheels, so you will stay at high torque output longer. Torque is proportional to current, also.


It would be hard to use the battery simulation with mechanisms because the current draw is (comparatively) so low that drivetrain motions would screw up your nice voltage simulation. It could be done, but this would be more an RT software kind of challenge (battery voltage compensation for control loops) then a simulation one. Just assuming you are running under a lower voltage and being pleasantly surprised when the voltage is higher is usually good for mechanisms.
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