Passive Resistance and Strength of a BAG Motor

My team is refurbishing a practice bot from last season so the electronics/pnuematics and programming teams can have a test bot during build season. One thing we want is a PID tester, which we are going to make a simple arm on a shaft.

The shaft is geared on a 1:46.6667 gear ratio from a BAG motor and according to Jvn that should be 6.41 pounds of stall torque. the arm itself is 18 inches long (about 17 past the pivot point) and weighs itself about 0.8 pounds We want to add weight onto it so that the motor needs a PID to hold it up and the passive Resistance can’t hold it up.

How can we calculate how much weight will there need to be to have the motor need a PID instead of the passive Resistance doing all of the work?
Thanks

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If I understand you right, the arm is currently not self-backdriving (i.e. the mechanical resistance of the motor is enough to hold up the weight of the arm) Why would you want to have to constantly stall the motor in order to hold position rather than let the motor hold itself up? Usually people try to do the opposite: take an arm that falls under its own weight and counter-balance it so that it holds position when no voltage is applied.

Anyway, if that is what you want then I would check to make sure the motor controller is on “coast” mode instead of “break” mode. That greatly reduces the mechanical resistance of the motor when no voltage is applied. If that doesn’t work then you can add some weight at the end of the arm to overcome the resistance of the motor.

Because they’re building a PID tester rig. If the arm doesn’t fall on its own, and their code isn’t running the motor, it will look like their PID is performing perfectly…

Ah missed that part. Then yes, I would say the solution is add more weight on the arm, preferably as far out as possible. You should also double check the brake/coast mode, and check for points of excess friction.

To my knowledge, we don’t really have numbers for the mechanical resistance of the FRC legal motors, so calculating the right weight will be difficult. My suggestion would be to add weights incrementally until you get to the point you want.

Put enough weight so that JVN says that your working current draw is 10% of motor stall. We do this to make our arm as controllable as possible. If you want to test PID, you can add more weight, but in-season 10% is very easy to tune.