paper: 4 CIM versus 6 CIM theoretical calculations

[MichaelBick]

In practice, my guess is that you can draw way more current than 200 amps. Just based off of my team's 2016 drivetrain, which was 4 CIM with no voltage ramping, we experienced no brownouts even going from full forwards throttle to full backwards throttle. If we assume that our robot was traction limited below 40% of our max speed (a conservative assumption), we were drawing over 300 amps max.

Another cool aspect to current limiting is changing the limits in real-time based on each subsystem's need. For example, in a 4 CIM "hard right" turn -- 0% power on the right drive side, 100% on the left -- you may only be drawing 50% of what the battery is able to provide. If you have 1.5 or 2 times the power and current limit, you can double the current limit on the left side so that you fully utilize the battery. This same idea can be used to change the current limits to better follow a motion profile or prioritize superstructure motions.

One last important item to keep in mind is the main breaker. A high power drivetrain with a current limit pulls the maximum current for more time than an equivalent lower power drivetrain. Because the main breaker is heat triggered, it is sensitive to prolonged periods over the 120 amp breaker limit, and therefore high power drivetrains are more likely to blackout than lower power drivetrains.

Given that the talon SRX now supports current PID, a lot of doors have been opened for teams to find performance improvements with current limiting.

[Aboudy Dairi]

Towards the end of the appendix A is a sentence that starts with "One final thought: ..." Is that supposed to carry on to the last paragraph somehow or is it just incomplete?

Great paper, this was a good introduction to 6 CIM drives for me in terms of the math. The idea of current limiting sounds pretty cool and is definitely something that could be implemented with the Talon SRXs to tune performance and prevent brownouts.

[ThaddeusMaximus]

1. Mechanical power output is not everything. You need to have useful power; proper gearing. However, yes, having more mechanical power output won't hurt.

2. How do you magically draw a hard limit of 200 (or whatever) amps without effecting voltage input to the motors? Though you may be "limited" to 200 amps, when you're drawing this much current, voltage dips due to both surface depletion (?) and voltage drop in the loads to the motors. This shouldn't be ignored. When you drop away from the 12V, the motor curve you use up there is no longer valid.

Look into using kV and kT values to model things and take this voltage drop into account.

[thatprogrammer]

I took a decently quick read and had a few questions.
1. I may be misunderstanding what you are stating here:

4CIM 200/524 = 38%

Are you saying that a 4 CIM drive should be limited to 38% per a motor in order to keep your robot below 200amps of power usage? If so, I think that the 200 Amp number is actually pretty far from the limit of the power system of a modern FRC robot; we have run 4 CIM drives without any voltage ramping and never had any issues with brownouts.*

2. Would limiting the current draw of a motor be similar to limiting voltage? I.e. would limiting max current have a similar effect to limiting a CIM's output to .5 (In terms of limiting the speed at which the CIM will approach maximum acceleration?)

3. Why not simply control the Max RPM of the CIM in code rather than using a current PID?

*I suspect that part of this has been our somewhat low final RPMS from our gearbox reductions. These have both resulted in very short acceleration periods.

Thanks in advance for the answers!

[MichaelBick]

Originally Posted by thatprogrammer

Are you saying that a 4 CIM drive should be limited to 38% per a motor in order to keep your robot below 200amps of power usage?

He is saying that @ 0 RPMs you need to provide 38% of full voltage to draw 200 amps total. At higher RPMs you will need to provide more voltage to draw the same 200 amps. Similarly you need to provide even less voltage if the robot is traveling backwards initially.

Originally Posted by thatprogrammer

2. Would limiting the current draw of a motor be similar to limiting voltage? I.e. would limiting max current have a similar effect to limiting a CIM's output to .5

Halving voltage halves no-load speed, halves torque, and results in 25% mechanical power. On the other hand, limiting current limits torque and thereby limits acceleration. This by itself is very helpful, even without the power increase that is mentioned in the paper, because it causes robots to be much more stable.

Originally Posted by thatprogrammer

3. Why not simply control the Max RPM of the CIM in code rather than using a current PID?

If you only control velocity, you will still draw high amounts of current at stall. For example, a 4 CIM drive will draw 524 amps at stall, and 1048 amps going from full forward to full reverse. This is what causes brownouts.

[GeeTwo]

Originally Posted by MichaelBick

One last important item to keep in mind is the main breaker. A high power drivetrain with a current limit pulls the maximum current for more time than an equivalent lower power drivetrain. Because the main breaker is heat triggered, it is sensitive to prolonged periods over the 120 amp breaker limit, and therefore high power drivetrains are more likely to blackout than lower power drivetrains.

I fully agree with the concern, though I would consider tripping the main breaker a blackout, not a brownout.

Yes, you can draw more than 220A for short durations. This does not change the primary conclusion of the original paper that 3 CIMs operated properly can provide greater mechanical power drawing from a given battery and condition. This is because the only conditions in which three motors run less efficiently than two is on the fast side of the peak of the efficiency curve, when operating near the free speed limit. In this domain, brownouts are not usually of concern. I would not use this paper as the last word in tuning a current limiting system, but I do consider it a valid argument for using a 6 CIM drive train over a 4 CIM drive train when the team's game strategy involves pushing battles or other high-current operation.

Alternately, 6 CIMs would allow you to gear the robot faster for a given application, preserving good startup acceleration while increasing top speed, or leave gearing alone, preserving top speed while increasing startup acceleration (in torque-limited drives). This would not be nearly as much of a difference as a shifting gearbox, but it would be simpler and less expensive.

[marshall]

I'm a little busy to check this but I'd like to know how these calculations line up with the results in this paper:

https://www.chiefdelphi.com/forums/s...d.php?t=131790