pic: Sheet Metal with VexPro parts

[apalrd]

Originally Posted by boarder3512

Thanks for the explanation. I was obviously oversimplifying the idea of robot speed and I missed the relationship between gear ratio and motor load on the CIM motors. Better to learn now. So from your numbers I can conclude that the gear ratio resulting in 21.3 fps has very similar acceleration to a gear ratio resulting in 13 fps while the higher top speed causes voltage to drop quicker.

Is there a specific equation you are using to relate output speed of CIMs to current draw or voltage drop?

Would you be able to PM me the spreadsheet you made?

Thanks for the help.

Basically, at 21fps, you're running with roughly the same final speed as 13fps (after ~1.5sec, before which the 21fps speed is worse), but running in the 'bad side' of the power curve, drawing significantly more current.

I use a spreadsheet originally created by John V-Neun to calculate most of this.
2008 version
2004 version

In general, everything can be modeled with basic physics equations, calculated iteratively. For example, you can use your current speed and voltage to calculate the motor output torque, which you can use to calculate your acceleration, which you can integrate to get velocity.

JVN's spreadsheet is very good. I usually use the 2008 version to model mechanisms (arms, elevators) and the 2004 version to model drivetrains. The 2004 version includes acceleration and sprint-distance graphs, which are very nice.

As to your original question, usually low gear is designed to be traction-limited at 40 amps per motor. Getting this right is usually slightly lower priority than getting high gear correct, usually low gear is what it is (especially since most gearboxes only come in one or two ratio spread options). For FRC robots with ~1.2cof and 4 CIMs, this is somewhere around 5.5fps.