paper: 4 CIM versus 6 CIM theoretical calculations

[InFlight]

Originally Posted by GeeTwo

Another thing to consider if planning a drive train with current limiting is whether using some other motor will be more effective to your strategy than CIMs. A cursory review of the mini-CIM finds it to be inferior in most cases, as it has lower efficiency than the CIM at any given current draw, but there are probably cases where it is the proper choice, due to weight considerations or specific needs. If you're going to do proper current/speed and/or thermal monitoring, you may want to add the 775 pro or other low thermal mass motor to your considerations.

I would caution against using the 775 Pro motor in a drivetrain application. This is an air cooled motor with an internal fan. There is no effective cooling at low speed near stall current conditions. One defensive pushing match will let the smoke out of these motors. The much higher speed output would require additional gear stages as well.

The Mini-CIMs is really the equivalent to 1/2 a CIM in terms of torque and current. If a team wanted a competive advantage of running a three motor gearbox per side; the combination of two CIMs and one Mini CIM would be a better choice. It would provide 125% the performance of a 2 CIM drive, with more brownout margin than a 3 CIM drive.

[Richard Wallace]

Originally Posted by InFlight

The Mini-CIMs is really the equivalent to 1/2 a CIM in terms of torque and current. ...

This is not correct. Check the data, which I can vouch for. It was measured correctly by people who know their motor physics, and their FRC design.

The Mini CIM has about 2/3 the active material (armature core length, permanent magnets) compared to the CIM, and it has the same commutator.

This is why the Mini CIM performs well during prolonged heavy loading -- it does not heat up as fast internally as a CIM under the same load proportional to its size. Look at the test results provided by VexPro; after 60 seconds at peak load, the Mini CIM is still providing 200 Watts shaft output (87% of what it developed starting out with room-temperature innards), while the CIM is down to 230 Watts shaft output, only 70% of what it developed cold. Pound for pound in the heat of combat, the Mini CIM outperforms its big brother.

[asid61]

Originally Posted by InFlight

I would caution against using the 775 Pro motor in a drivetrain application. This is an air cooled motor with an internal fan. There is no effective cooling at low speed near stall current conditions. One defensive pushing match will let the smoke out of these motors. The much higher speed output would require additional gear stages as well.

The Mini-CIMs is really the equivalent to 1/2 a CIM in terms of torque and current. If a team wanted a competive advantage of running a three motor gearbox per side; the combination of two CIMs and one Mini CIM would be a better choice. It would provide 125% the performance of a 2 CIM drive, with more brownout margin than a 3 CIM drive.

Actually, I know of a few teams that did 775pro drivetrains without problems this year (and a few that did have problems). 3310 is one example of a 775pro drivetrain done well, I believe.
1296 had a super light and compact robot, but ran only 4 775pros in their drivetrain which caused numerous burnt motors. They upgraded to 4 CIMs for champs.

[GeeTwo]

Originally Posted by InFlight

I would caution against using the 775 Pro motor in a drivetrain application. This is an air cooled motor with an internal fan. There is no effective cooling at low speed near stall current conditions. One defensive pushing match will let the smoke out of these motors. The much higher speed output would require additional gear stages as well.

I explicitly said that this was worth considering if you're going to do proper speed and thermal monitoring. As I recall, the 775 Pro has better efficiency and far lower weight than the CIM, though by the time you gear down it will be reduced or possibly dissapear. My point here was simply that WITH MONITORING, it is worth looking at these motors. Oh - I also seem to recall an inverse differential based gearbox a few months ago that might make air cooled motors more viable for drive trains (though I was never convinced).

Originally Posted by InFlight

The Mini-CIMs is really the equivalent to 1/2 a CIM in terms of torque and current. If a team wanted a competive advantage of running a three motor gearbox per side; the combination of two CIMs and one Mini CIM would be a better choice. It would provide 125% the performance of a 2 CIM drive, with more brownout margin than a 3 CIM drive.

Looking at the numbers a year or two ago, I came up with 2/3, based (IIRC) on the peak power and stall torque. The free speed is also a bit faster.

Edit: Wow - sniped on both points, by different posters.

[InFlight]

Originally Posted by Richard Wallace

This is not correct. Check the data, which I can vouch for. It was measured correctly by people who know their motor physics, and their FRC design.

The Mini CIM has about 2/3 the active material (armature core length, permanent magnets) compared to the CIM, and it has the same commutator.

This is why the Mini CIM performs well during prolonged heavy loading -- it does not heat up as fast internally as a CIM under the same load proportional to its size. Look at the test results provided by VexPro; after 60 seconds at peak load, the Mini CIM is still providing 200 Watts shaft output (87% of what it developed starting at with room-temperature innards), while the CIM is down to 230 Watts shaft output, only 70% of what it developed cold. Pound for pound in the heat of combat, the Mini CIM outperforms its big brother.

The Mini-CIM has 58% of the rated stall torque at 68% Amps of the CIM. A tank drive with 3 Mini-CIMs provides 87% of the initial performance compared to 2 CIMS.

When hot if we just use the 87%/70% = 1.24 performance ratio. The three Mini-CIM drive would now be 1.08% of the performance of a two CIM drive. The three CIM drive would be heavier, and need extra motor controllers.

[asid61]

I was just doing calculations for the resistance limiting the current again, and I got different numbers than Jim/InFlight.

I modeled it as a 0.05 ohm resistor (battery -> PDP) and then 4 or 6 parallel CIM/wiring resistors. That got me a system resistance of 0.087 ohm for a 4-CIM drive and 0.062 ohm for a 6-CIM drive, which leads to overall current draws of only 137 and 193 amps for a 4-CIM or 6-CIM drive respectively. That seems startlingly low for an entire drivetrain's maximum current. Is 0.3ohms too much to count for a motor + wires + motor controller, or have I done something wrong?

[InFlight]

I was using 0.39 ohms for the motor branches circuits, as I computed the CIM resistance separately. But your results are close enough, and just as valid as my assumptions. You come to the same conclusion that the actual system can't deliver the full stall torque to each motor in either the four or six CIM drives.

Once you get moving the six CIM drive will deliver 150% of the torque; and the acceleration, and time to speed will be much better. The brownout margin is much less, so there's no free lunch.

[cbale2000]

Out of curiosity, how would a 2CIM + 2 MiniCIM drive compare? I've often wondered if this was a practical weight saving option or if the performance drop would make it not worth the trouble.

Also, what are the effects of leaving the gearing on an xCIM + xMiniCIM drive identical between all motors? We've always just geared MiniCIMs the same as CIMs hoping to get a few extra RPM out of the drive. Is this practical or is there some downside I'm not seeing?

[GeeTwo]

Originally Posted by cbale2000

Out of curiosity, how would a 2CIM + 2 MiniCIM drive compare? I've often wondered if this was a practical weight saving option or if the performance drop would make it not worth the trouble.

Do you mean 1+1 on each side compared to 2+0, or 2+2 on each side compared to 3+0?

Edit: I'm going to presume the first, as you're discussing a performance drop. I'll get back to you, but I seem to recall that it was a performance hit, but much closer to 2 CIMs than 1 CIM.

Edit2: By assuming a budget of 100A on one side of the drive train, the 2 CIM can deliver 627W at 3365 rpm, the 1+1 494W at 3638 RPM, and a 0+2 can deliver 425W at 3192 rpm. The output power loss is about 21% for 1+1 and 32% for 0+2. A definite hit, but if you're looking to save weight, it's a viable way to do it without dropping all the way to 1 CIM (247W at 1287 rpm).
- math error!

Edit3: By assuming a budget of 100A on one side of the drive train, the 2 CIM can deliver 627W at 3365 rpm, the 1+1 542W at 3093 rpm, and a 0+2 can deliver 425W at 2648 rpm. The output power loss is about 14% for 1+1 and 32% for 0+2. A definite hit, but if you're looking to save weight, it's a viable way to do it without dropping all the way to 1 CIM (247W at 1287 rpm).

Originally Posted by cbale2000

Also, what are the effects of leaving the gearing on an xCIM + xMiniCIM drive identical between all motors? We've always just geared MiniCIMs the same as CIMs hoping to get a few extra RPM out of the drive. Is this practical or is there some downside I'm not seeing?

The free speeds are only about 10% different, so gearing the same shouldn't be an issue. It's not like you're going to be able to backdrive the CIM at a higher speed than its bushings were designed for, especially after gearbox losses. If you did want to match them even better, you could just use a pinion with one fewer tooth on the mini-CIM.

[Chris is me]

Originally Posted by cbale2000

Also, what are the effects of leaving the gearing on an xCIM + xMiniCIM drive identical between all motors? We've always just geared MiniCIMs the same as CIMs hoping to get a few extra RPM out of the drive. Is this practical or is there some downside I'm not seeing?

Matching free speed exactly isn't nearly as important as is made out to be on Chief, and the motors are designed to be 1:1 drop in replacements for each other without any adjustment in gearing to compensate. The main thing to avoid is one motor forcing the other one to drive faster than its free speed, which doesn't happen (under load, things even out with these motors nicely). Totally fine to do this.

[cbale2000]

Originally Posted by GeeTwo

Do you mean 1+1 on each side compared to 2+0, or 2+2 on each side compared to 3+0?

Edit: I'm going to presume the first, as you're discussing a performance drop. I'll get back to you, but I seem to recall that it was a performance hit, but much closer to 2 CIMs than 1 CIM.

Edit2: By assuming a budget of 100A on one side of the drive train, the 2 CIM can deliver 627W at 3365 rpm, the 1+1 494W at 3638 RPM, and a 0+2 can deliver 425W at 3192 rpm. The output power loss is about 21% for 1+1 and 32% for 0+2. A definite hit, but if you're looking to save weight, it's a viable way to do it without dropping all the way to 1 CIM (247W at 1287 rpm).

I meant the first option 1 CIM + 1 MiniCIM on each gearbox. So if I'm reading right, a 1+1 config would be about 79% of the power of a standard 2 CIM setup? If so, not a bad option depending on the game. In 2015 we went this route on our H-Drive since we didn't need much pushing power and it was all omni wheels anyways.

Originally Posted by GeeTwo

The free speeds are only about 10% different, so gearing the same shouldn't be an issue. It's not like you're going to be able to backdrive the CIM at a higher speed than its bushings were designed for, especially after gearbox losses. If you did want to match them even better, you could just use a pinion with one fewer tooth on the mini-CIM.

Originally Posted by Chris is me

Matching free speed exactly isn't nearly as important as is made out to be on Chief, and the motors are designed to be 1:1 drop in replacements for each other without any adjustment in gearing to compensate. The main thing to avoid is one motor forcing the other one to drive faster than its free speed, which doesn't happen (under load, things even out with these motors nicely). Totally fine to do this.

I'm aware that mechanically it's not critical, I was just curious as to how it affects the efficiency of the overall system. Presumably the MiniCIMs would be working harder since they would be trying to pull the other motors to a higher RPM, but I'm not sure if this would make enough of a difference in motor current to really matter.

[asid61]

Originally Posted by cbale2000

I'm aware that mechanically it's not critical, I was just curious as to how it affects the efficiency of the overall system. Presumably the MiniCIMs would be working harder since they would be trying to pull the other motors to a higher RPM, but I'm not sure if this would make enough of a difference in motor current to really matter.

I think a mix of MiniCIM + CIM at a 1:1 ratio could work fine. The MiniCIM having a free speed about 10% higher and having a lower stall current (and therefore a higher resistance) than the CIM means that when they're running at the same RPM (say, 1,500 rpm) the CIM draws 95 amps and the miniCIM draws only 66 amps. That approximately matches up with their weight proportions, which is analogous to heat capacity, so both of the motors will heat up at the same rate. That means no dead motors.

The closer you are to stall, the more the CIM:MiniCIM heat ratio goes down (which is bad), but not by a large amount. The ratio gets closer to 1:1 as you go nearer to free speed as well, but by the time you are running that fast the heat generated isn't too much anyway.
It's possible to gear the MiniCIM such that it's always running at a higher RPM than the CIM, perhaps by using 11t and 12t pinions, but it probably isn't necessary. Maybe a team that has done CIM + MiniCIM combos can chime in here?


I wonder if using higher-resistance cables to increase your resistance would be a valid strategy to help prevent brownout of 6+ CIM drivetrains.

EDIT: The numbers for a 2 CIM + 2 MiniCIM drivetrain are as follows:
Approximated as 3.33 CIMs
2,931 theoretical rpm
333 W/CIM
1,108w total, compared to 1,280w for a 4-CIM drive, or about 86% as efficient. Not too great of a drop if you're running a significantly lighter robot and don't need the power.

[GeeTwo]

Originally Posted by cbale2000

I'm aware that mechanically it's not critical, I was just curious as to how it affects the efficiency of the overall system. Presumably the MiniCIMs would be working harder since they would be trying to pull the other motors to a higher RPM, but I'm not sure if this would make enough of a difference in motor current to really matter.

I came up with a peak efficiency of an all-CIM drivetrain as 65%, an all-min-CIM as 57%, and a 1+1 as 61%, so no, not unless I'm missing something. The only possible issue I can think of is that when the speed is greater than the free speed of the CIM, it will be generating current rather than consuming it - I'm not sure what effect that would have on the motor controller.

[InFlight]

Originally Posted by GeeTwo

I came up with a peak efficiency of an all-CIM drivetrain as 65%, an all-min-CIM as 57%, and a 1+1 as 61%, so no, not unless I'm missing something. The only possible issue I can think of is that when the speed is greater than the free speed of the CIM, it will be generating current rather than consuming it - I'm not sure what effect that would have on the motor controller.

Motors at or near free speed provide no useful torque. In a real world drive train you have torque losses such as gearbox losses, bearing drag, and chain or belt drive system losses. Thus it's really not possible, particurily in a FRC limited space to accelerate anywhere near to free speed.

Typically you design your drive system around 80% of free speed which is more realistic. AndyMark always used CIMS at 4455 rpms when providing gearbox performance data in Feet per Second.

A CIM + Mini CIM per side would take about 125% of the time to speed, and distance traveled compared to a two CIM drive.

[GeeTwo]

Originally Posted by InFlight

Motors at or near free speed provide no useful torque. In a real world drive train you have torque losses such as gearbox losses, bearing drag, and chain or belt drive system losses. Thus it's really not possible, particurily in a FRC limited space to accelerate anywhere near to free speed.

Typically you design your drive system around 80% of free speed which is more realistic. AndyMark always used CIMS at 4455 rpms when providing gearbox performance data in Feet per Second.

I agree that there would not be a problem while driving the robot, but there might be an issue when on blocks. I figure (using Vex's numbers and a bit of linear extrapolation) that putting a CIM and mini-CIM nose to nose at 12V would reach a free speed of 5507 rpm, at which point the mini-CIM would be drawing 7.9A, 46W of mechanical power would be transferred, and the CIM would be generating 1.6A. The CIM would switch from consuming to generating current at about 5442 rpm, so probably even a gearbox on blocks would provide enough drag. Of course, both the CIM and mini CIM free speeds have a +/- 10% variation, so if you put a fast mini and a slow CIM together, you might have some issues when running the motors with the robot on blocks.