pic: Sheet Metal with VexPro parts

[Andrew Zeller]

Originally Posted by apalrd

Quick spreadsheet:

2x CIM motors, 150lbs, 100% weight on driven wheels, 0.9 speed loss const
12.5v initial battery voltage, 0.03 ohm battery resistance

Gear ratio resulting in 21.33fps after speed loss:
2.07sec to 20ft
3.18sec to 40ft
1.27sec to 12fps
0.54sec to <160amp total current
3.23sec to 11v battery (battery remains under 11v for 3.23sec)

Gear ratio resulting in 13.29fps after speed loss:
2.00sec to 20ft
3.40sec to 40ft
1.17sec to 12fps
0.18sec to <160amp total current
1.20sec to 11v battery

Looking at the curves, most of the output distance curves for the two gears are relatively close, the velocity is worse than 13.29fps until ~1.5sec, and the higher gear will be under ~1.5x motor load and significantly lower battery voltage the entire time.

Edit:
When you choose a high gear ratio, you don't usually actually care about top speed. You really want to gear for either sprint-distance or time to speed, with the speed and distance adjusted based on game-specific strategy. Being tied to a specific ratio spread will also pull your high gear slightly based on where your low gear wants to be. You usually want low gear to be traction limited, at a current which is determined by your strategy (how much you want to push).

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.