Torque Calculation/Determination-Urgent

[gpetilli]

I should have specified. We are planning RS775-18 with a 20:1 ratio (plus the added torque from the 6:1 MiniCIM).

Since there is enough torque to overcome the CoF (1.2) of our wheels, the motors never stall and the current per motor is 45A when pushing a wall (with the wheels slipping on the carpet). It is hard to find dynamic CoF for FRC wheels. The only one I did find was 85% of the static CoF so that percentage is what I have been using as an estimate.

BTW: you have a similar issue with full size CIMs - if you ever let them stall you draw too much current and pop breakers. You simply have to design to slip the wheels.

[Ether]

Originally Posted by gpetilli

We are planning RS775-18 with a 20:1 ratio (plus the added torque from the 6:1 MiniCIM).

Since there is enough torque to overcome the CoF (1.2) of our wheels, the motors never stall and the current per motor is 45A when pushing a wall (with the wheels slipping on the carpet).

At 12V, a 775-18 motor draws 45 amps at 6385 RPM

At 12V, a MiniCIM motor draws 45 amps at 3008 RPM.

But if you have them geared 20:1 and 6:1, respectively, they can't simultaneously be at those speeds.

6385/20 = 319 RPM
3008/6 = 150 RPM

[gpetilli]

Originally Posted by Ether

At 12V, a 775-18 motor draws 45 amps at 6385 RPM

At 12V, a MiniCIM motor draws 45 amps at 3008 RPM.

But if you have them geared 20:1 and 6:1, respectively, they can't simultaneously be at those speeds.

6385/20 = 319 RPM
3008/6 = 150 RPM

That's an interesting way to look at it. Since I don't yet have a spreadsheet to dynamically calculate operating points, I used JVN table - which does include voltage droop (which looked like about 7.5V).

We did intend for the RS775 to reach max RPM when the MiniCIM was at 1/2 max RPM, so your estimates make sense. Clearly the wheel RPM will be the same for both and currents will split accordingly. The biggest concern I have is what happens after the RS775 exceeds it free RPM. I know it will behave like a generator and load the MiniCIM.

I do have some serious unanswered questions with this configuration. If we put the ESC in coast does that sufficiently break the generator current path such that it does not significantly load the driving motor? I don't think anyone has tried coasting the motor after it exceeds free RPM but instead continue to drive at full voltage. If we back-drive the RS775 to twice it's free RPM (coast or driven) will the generator blow the ESC?

[Ether]

Originally Posted by gpetilli

We did intend for the RS775 to reach max RPM when the MiniCIM was at 1/2 max RPM...

775-18 free RPM is 13000. With 20:1 reduction that's 650 RPM.

MiniCIM free RPM is 6200. Half of that is 3100 RPM. With 6:1 reduction that's 517 RPM. So you're a bit off with your gearing.

That's an interesting way to look at it. Since I don't yet have a spreadsheet to dynamically calculate operating points, I used JVN table - which does include voltage droop (which looked like about 7.5V).

I was just taking the numbers you posted to see where they would lead. The motor calculator I posted here makes this easy to do.

Here are the results using 7.5V instead of 12V:

At 7.5V, a 775-18 motor draws 45 amps at 1407 RPM

At 1407/20*6 = 422 RPM, a MiniCIM draws 48 amps at 7.5V

See example calculation attached.

I do have some serious unanswered questions with this configuration. If we put the ESC in coast does that sufficiently break the generator current path such that it does not significantly load the driving motor? I don't think anyone has tried coasting the motor after it exceeds free RPM but instead continue to drive at full voltage. If we back-drive the RS775 to twice it's free RPM (coast or driven) will the generator blow the ESC?

Good questions. Maybe the engineers from VAX and CTRE can answer for their respective motor controllers.

There's also the question of the 775-18 spinning at 13000*2 = 26000 RPM. Can its rotor, fan, and the bearings handle this? And even if the the motor leads are open-circuit (zero generated current), how much load does the fan put on the MiniCIM at that speed, through a 20:1 gearbox?

[gpetilli]

Good questions. Maybe the engineers from VAX and CTRE can answer for their respective motor controllers.

There's also the question of the 775-18 spinning at 13000*2 = 26000 RPM. Can its rotor, fan, and the bearings handle this? And even if the the motor leads are open-circuit (zero generated current), how much load does the fan put on the MiniCIM at that speed, through a 20:1 gearbox?



[/quote]

Good point on the fan - I had not thought of that. The 26000 RPM is only 33% higher than the 19,500 free RPM at the rated 18V. The robot will not be at this velocity very often or for long so I think there is sufficient margin.

I assume that the ESC have flyback diodes, so it will never truly be open-circuit. I am not sure what load that will impart on the MiniCIM. If the ESC blows, it expect it will be the flyback diodes that smoke.