Three Mini-CIMs in a gearbox?

[mrnoble]

What do you think the results would be if three minis were used in place of two CIMs in a drive train? I know about the speed/power differences. Has anyone tried it? A couple advantages I can think of would be:

• Reduction in space taken up by motors
• Saving the most powerful motors for other applications

Anyway, just curious what y'all think.

[AdamHeard]

Originally Posted by mrnoble

What do you think the results would be if three minis were used in place of two CIMs in a drive train? I know about the speed/power differences. Has anyone tried it? A couple advantages I can think of would be:

• Reduction in space taken up by motors
• Saving the most powerful motors for other applications

Anyway, just curious what y'all think.

It would work out to being roughly equivalent to two CIMs. Likely slightly lower power/weight in terms of motor thermal mass, so slightly more robust to overheating (which isn't really a concern for most teams in drive anyway).

Requires more speed controllers, wire etc...

[jkelleyrtp]

If one min-icim has around 2/3 the power output of a normal cim, the why not just use 2 cims? I understand that the power output does not directly correlate to speed and torque, but you will start to see limits of the mini-cims before you see it on the cims.

[GeeTwo]

Because of the extra motor controller required, and the increased cost/weight/complexity of a 3 motor gearbox, you would almost have to be in some sort of edge case in order to want to make this trade. That said, I can think of a number of those cases pretty easily:

• Out of CIMs according to rules (most likely useful only if CIMs and Mini-CIMs draw large currents at different times)
• You need a bit of extra torque-to-speed trade space (e.g. for a lifter), but not enough to justify a shifter. This takes advantage of the fact that you can (for example) run 3 miniCIMS at a total of 120A for much longer than you can 2 CIMS without tripping breakers.
• Short depth available behind gearbox

[notmattlythgoe]

I could see using mini-cims in a drive train in a game list Recycle Rush where you might want to use the cims somewhere else and don't need that extra power in your drivetrain.

[sastoller]

We used a single mini CIM on each transmission last year give us more space for totes in our robots. The min CIMs gave us more distance between our transmissions, and saved us a little weight. The two mini CIM drivetrain was still sufficient to push our loaded robot around the carpet.

In addition, Vex has a fun little plot for the CIM and mini CIM motors on their website that shows a 3 minute peak power test. Looking at the two plots side-by-side, you can clearly see that the mini CIM has better peak power output characteristics (especially in the first 20 seconds of the test).
http://motors.vex.com/cim-motor
http://motors.vex.com/mini-cim-motor

[MichaelBick]

Originally Posted by sastoller

In addition, Vex has a fun little plot for the CIM and mini CIM motors on their website that shows a 3 minute peak power test. Looking at the two plots side-by-side, you can clearly see that the mini CIM has better peak power output characteristics (especially in the first 20 seconds of the test).
http://motors.vex.com/cim-motor
http://motors.vex.com/mini-cim-motor

If I'm reading the graph correctly, the CIM produces higher power than the Mini CIM, though I assume it is also pulling more current.

[mrnoble]

Here's my current belief: depending on the requirements of the game, using three minis in place of two CIMs is a viable option. I'd still love to hear from someone who's actually tried it. Thanks for musing on this point, folks

[sastoller]

Originally Posted by MichaelBick

If I'm reading the graph correctly, the CIM produces higher power than the Mini CIM, though I assume it is also pulling more current.

The miniCIM output power decreases less drastically than the CIM output power, especially when you look at the first 20 seconds of the curve, which is likely the area on the curve that most FRC robots operate on.

An interesting plot is to plot efficiency of the CIM vs miniCIM by time. Based on the Vex data, I think you will see the miniCIM is about 5% more efficient in the peak power test. When I ran a quick calculation, I assumed a 10.5V output for the CIM and 11V output for the miniCIM since the CIM pulls more current.

[sanddrag]

We ran zero CIMs on our 2015 robot. We drove our swerve with four Mini-CIMs and it drove just fine for last year's game. In a game like 08, I'd want about 8 Mini-CIMs.

[Chris Fultz]

This paper compares some different CIM - mini-CIM drive combinations.

http://www.chiefdelphi.com/media/papers/3071

[BBray_T1296]

Another noteworthy point is thermal mass
Those power/time graphs are really showing functions of internal temperature. As the motor heats up, its performance diminishes and even after a match it takes a while for recuperation. source and source 2

3 miniCIMs weigh more than 2 CIMs, and thus have greater thermal mass. If both systems were loaded equally (and the motors were equally efficient) the decreased loading/motor and therefore amps/motor of the miniCIMs should result in them running cooler. This is not important on a 1 match basis, but in an eliminations scenario, every degree counts.