Re: How many motors can you run at once?
 

I also suspect that part of the problem is that you were only running 2 motors on drive. It seems paradoxical, but if you put 4 motors on drive, each motor will have a much easier time accelerating your robot, reducing your overall current draw.

Re: How many motors can you run at once?
 

Also, can you describe your drive setup in more detail? It sounds like you might not have been running your drive at the right ratio. Also, what was the ratio and load on the motor younwere stalling? Like others, this sounds much more like a mechanical issue than an electrical one. If there's no smoke or popping circuit breakers when the problem occurs... you can usually eliminate electronic issues.

Re: How many motors can you run at once?
 

Can you (or someone on your team) show the math for this ?

Re: How many motors can you run at once?
 

Not to steal the fun from anyone who might be working on Ether's challenge above, only to offer a hint: The CIM performance data (available at the FRC website under Technical References not there any more, but can be found here) indicates that at 12V the motor reaches peak efficiency of about 65% at about 20 Ampere current draw. When the current draw reaches 27 Ampere, efficiency is still about 63%. Operating beyond that current draw penalizes efficiency, and of course exceeding about 65 Ampere current draw put the motor behind the power curve (i.e., past peak).

Re: How many motors can you run at once?
 

This year we ran 10 motors (because we were limited to a single digital I/O module in our cRIO):
5 CIM Motors (4 drive and 1 pyramid climbing)
1 Mini CIM (shooter)
4 Banebot 540s (steering)
No problems.

Last year, with two digital I/O modules and two digital sidecars, we ran 13 motors without problems.

High overall current draw, however, can lead to low supply voltage (exacerbated by batteries in poor condition) and this can cause control problems consistent with those you experienced. Do you check battery condition? Also, using just (2) CIMs for drive is light, and CIMs draw an awful lot of current if run close to stall.

Re: How many motors can you run at once?
 

Not to derail the thread, but if more than one of your motors is doing the same thing (drivetrain, for example), you can splice PWM cables together so a single output on the sidecar gives signals to multiple speed controllers. We were planning to do this at one point, but it turned out to be unnecessary.

Re: How many motors can you run at once?
 

Yeah, we considered this for our climber and shooter, but with fully independent swerve drive there aren't many motors doing the same thing. (On the other hand, we were also at max weight.)

We're also really hoping the 2014 rules allow you to change which controller gets the PWM signal by using a spike relay (basically put a switch in the Y splitter), so teams don't have to run two motors together while only using one at a time. The Q&A turned us down, but it would provide for another interesting take on the OP's question if allowed next year.

Re: How many motors can you run at once?
 

Let's say I want to push with 100lbs of force, with a single speed drivetrain with a free speed of 11ft/s on 4in wheels. This means an overall gear reduction of 8.43:1. The wheels exert a total of 200lbs-in of torque, meaning the motors exert a total of 23.7lbs-in, or 380oz-in. With two CIMs, each is drawing 74.8A, for a total of 149.6A. With four CIMs, each is drawing 38.7A, for a total of 154.8A. (Figures obtained from your motor calculator.) These two numbers are nearly identical.


With more pushing force, I suspect that four CIMs will do better than two, but I'm too lazy to do the math right now. As for simple acceleration without pushing, it is unfortunately too difficult to factor in all the forces resisting the acceleration, and probably more useful to test such things out in the field.