07-30-2016 11:33 PM

GeeTwo

Re: paper: 4 CIM versus 6 CIM theoretical calculations

A great overview of the continued advantages of 6 CIMs over 4 - provided that you watch the total current!

Assuming 200A for the drive train is simplistic, but a good way to compare apples to apples. No matter what the battery state is, you will have a certain limited current you can draw from it before beginning the brownout process.

The first (and so far only) technical error was the sentence that spans from the first page to the second. It should read something like:

This does not change any of the high-level conclusions.

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.

07-31-2016 01:21 AM

MichaelBick

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

07-31-2016 10:18 AM

Aboudy Dairi

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

07-31-2016 10:47 AM

ThaddeusMaximus

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

07-31-2016 11:12 AM

thatprogrammer

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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

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!

07-31-2016 12:21 PM

MichaelBick

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

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.

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.

07-31-2016 09:26 PM

GeeTwo

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

08-01-2016 08:14 AM

marshall

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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

08-01-2016 09:53 AM

Chris is me

Re: paper: 4 CIM versus 6 CIM theoretical calculations

The CIM versus mini-CIM case for the 6 motor drive is actually a lot closer of a comparison than you might think. The important thing to consider is that the more load the motors place on the electrical system, the greater the voltage drop, and the lower voltage in the system lowers the output torque / etc. There are diminishing returns on adding additional motors based on this. The performance jump from 2 to 4 CIMs is much greater than 4 to 6 CIMs. I think it was 234 that published some empirical data on this, which showed that 2 CIM + 1 mini-CIM has almost the same characteristics as a 3 CIM drive once you account for the decreased voltage drop on the entire system, and the 2+1 gearbox draws less overall current than a 3 CIM gearbox. Just some food for thought.

08-02-2016 11:56 AM

asid61

Re: paper: 4 CIM versus 6 CIM theoretical calculations

Thank you!
Whoops, I didn't notice that error. Rest assured that the calculations were done the correct way (and not the way stated in the paper!) I'll change that and re-upload soon.
I was considering a 10 775pro drivetrain just before I left for the mountains on Friday, and will do some calcs for those too soon.

You're definitely correct, 200 amps is really conservative estimate. I couldn't dig up any current draw graphs from this year (where we ran 4 CIM and had a few brownouts before adding a center drop), but I hope to get some more concrete data to apply to make a more optimized curve. That being said, the overall gains in efficiency and the output power still exist with a higher current draw, and I think that due to the nature of the 6 CIMs that actually might drag the gap out more.

The main reason to use 200 amps was to allow me to do just a comparison between 4 CIM and 6 CIM given other constant variables.

I'm not too worried about the main breaker. It was a problem in 2014, but main breaker blows these days are extremely rare IMO, just because brownouts happen well before blows. Early in the season we browned out many times in practice, but never faced a breaker blow.

I forgot that the SRX can do that! That makes this way easier. In that case using the function in my paper isn't really necessary because you can just PID the current directly. That being said, coming up with a separate algorithm that takes into account battery voltage drop and max current vs. time would probably be better.

Oh geez, that was supposed to say:
"One final thought: Because a 6 CIM drive hits 200 amps at almost 4,000 rpm, where as a 4 CIM drive hits 200 amps at 3,300 rpm, the 6 CIM drive will be able to utilize all 200 amps for acceleration closer to the free speed of the drivetrain. A 4 CIM drive might only be able to get 200 amps worth of torque up to 12fps in a 16fps free speed drivetrain, whereas the 6 CIM drive can get up to 14fps before dropping under 200 amps current draw."
That makes the gap between 4 CIM and 6 CIM larger.
Thank you!

Gearing is important, definitely. I did address that a bit in my paper, but as a general rule based on my experiences I would just run ~15fps on a 4CIM and ~17fps on a 6 CIM. That lets you take advantage of the extra efficiency of the 6 CIM while still getting a bit more acceleration.
My understanding of the motor curves was that the current and RPM decrease proportionally to the proportion of 12v that you are running at. I.E. if you run a CIM at 6v, it will have a free speed of 2,640rpm and a stall current of 66 amps. What do you mean by "voltage drop in the loads to the motors"? Like losses in the wires?
I was supposed to make a new equation that took into account voltage drop and used kT and kV to make a better comparison while I was in the mountains, but I didn't get around to it then. If you have any current draw vs. brownout graphs or anything like that I would appreciate it.

1. You're right that 200 amps is pretty low. But as a number to use to compare the two drives I would say it's fine. I'm working on a more optimized model, so if you have any current data before brownout I would appreciate it.

2. Michael got it.

3. Current PID (or maximizing current draw without browning out) will let you get the most out of your drivetrain in terms of acceleration and safety. This is important if you want to run a drivetrain at high speeds and still not suffer from decreased acceleration and brownouts.

Shoot, I forgot about the power decreasing with different voltages and the like. That actually screws up a big portion of my paper, so I need to go back and redo a lot of calculations. The basic conclusions should be similar, however.

I'm going through it right now, and due to the mechanical power and such changing I can't make a judgement right now. I'll come back to it once I fix my stuff. That being said, the issue with the 234 paper is that it gears very low and assumes you are going long distances, so it's not a perfect comparison. The plots for voltage and current will come in very handy, come to think of it.

12-02-2016 11:59 AM

InFlight

Re: paper: 4 CIM versus 6 CIM theoretical calculations

The lead acid batteries used by FIRST are about 17 Amp-Hours. They are certainly capable of discharging in excess of 300 amps for a few seconds.

The roboRIO brown out control is based on a voltage threshold of 6.3 Volts. And thus has 5.7 volts of margin with a fully charged battery.

You could reasonably model this situation as a 12 V voltage source with 0.05 Ohms of resistance to the power distribution board; and 0.3 Ohms of resistance through each of motor controls (Talon SRX) and wiring.

A CIM motor has a stated Stall current of 131 amps at 12 volts. For the instantaneous stall current, you could model the equivalent resistance as R=V/I = 12/131=0.09 Ohms

So each CIM motor circuit would have a Series Resistance of 0.09+0.3=0.39 Ohms.

Parallel resistances are totaled as 1/Rtotal = 1/R1 + 1/R2 +…+ 1/Rn

Four CIMs have a combined parallel resistance of 0.0975 Ohms.
Six CIMS have a combined parallel resistance of 0.065 Ohms

Four CIMS
The Four CIM total Resistance is 0.05+0.0975=0.1475 Ohms

Current:
I=V/R=12V/0.1475=81.4 Amps

The voltage drop at the RoboRio is
V=I*R=0.05*81.4= 4.07 Volts
The roboRIO has 1.63 Volts of margin for a brown out.
12-6.3-4.07=1.63

Six CIMS
The Six CIM total Resistance is 0.05+0.065=0.0115 Ohms

Current:
I=V/R=12V/0.115=104.3 Amps

The voltage drop at the RoboRio is
V=I*R=0.05*104.3= 5.21 Volts
The roboRIO has 0.49 Volts of margin for a brown out.
12-6.3-5.21=0.49


What happens later in the match when the Battery voltage has drooped down to 10.5 Volts?

We will leave the CIM resistance unchanged, even though they are now quite hot. Same with the wiring resistances.


Four CIMS
Current:
I=V/R=10.5V/0.1475=71.2 Amps

The voltage drop at the RoboRio is
V=I*R=0.05*71.2= 3.5 Volts
The roboRIO has 0.7 Volts of margin for a brown out.
10.5-6.3-3.5=0.7



Six CIMS
Current:
I=V/R=10.5V/0.115=91.3 Amps

The voltage drop at the RoboRio is
V=I*R=0.05*91.3= 4.57 Volts
The roboRIO is -0.37 Volts below the brownout voltage.
10.5-6.3-4.57=-0.37

Summary:
Standard four CIM Drives consistently maintain a 1 Volt higher brown out margin than six CIM drives. While a Six total CIM Drive can accelerate 150% faster than a Four CIM drive, it does come at considerable more risk of brownout event. Each team will have to balance drivetrain performance vs. robot reliability for the entire match.

12-02-2016 12:11 PM

asid61

Re: paper: 4 CIM versus 6 CIM theoretical calculations

That's some nice math you got there. I'm not sure if your model is perfect however. Are you assuming a stalled CIM? A CIM's resistance becomes less important as it speeds up, although I assume that you are doing the math for starting from a standstill.
A possible solution to this could be just basic speed ramping.

12-02-2016 12:51 PM

InFlight

Re: paper: 4 CIM versus 6 CIM theoretical calculations

Every forum needs a lurking electrical engineer.

This was just at the initial state of powering the motors, with no initial velocity.

My resistance values for the wiring resistances are just reasonable guesses. The actual amperage values could be larger or smaller, but the trends with the brownout margins would be consistent.

The other note is the motors windings only get a fraction of the full stall current; because the battery and wiring system resistance can't deliver that much current.

12-03-2016 12:04 PM

AustinSchuh

Re: paper: 4 CIM versus 6 CIM theoretical calculations

Also, what happens if you go from full forwards to full reverse? Theoretically, you get 2x stall current. Practically speaking, you'll start saturating magnetic fields at some point, so it gets hard to model. Stall is a good starting point.

12-03-2016 06:58 PM

InFlight

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

12-03-2016 07:34 PM

Richard Wallace

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

12-03-2016 07:36 PM

asid61

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

12-03-2016 08:43 PM

GeeTwo

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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).

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.

12-03-2016 08:55 PM

InFlight

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

12-08-2016 10:09 PM

asid61

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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?

12-09-2016 10:06 AM

InFlight

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

12-09-2016 11:21 AM

cbale2000

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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?

12-09-2016 12:23 PM

GeeTwo

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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).

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.

12-09-2016 12:32 PM

Chris is me

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

12-09-2016 01:44 PM

cbale2000

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

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.

12-09-2016 01:55 PM

asid61

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

12-09-2016 02:29 PM

GeeTwo

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

12-09-2016 02:59 PM

InFlight

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

12-09-2016 03:17 PM

GeeTwo

Re: paper: 4 CIM versus 6 CIM theoretical calculations

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.

12-09-2016 03:23 PM

AdamHeard

Re: paper: 4 CIM versus 6 CIM theoretical calculations

This wouldn't cause any issues, you'd just be pumping some current back to the DC bus and everything would be fine.

12-09-2016 05:44 PM

InFlight

Re: paper: 4 CIM versus 6 CIM theoretical calculations

Electrical Power and Mechanical Power are not the same.

The Electrical Power into the a DC Motor is Volts x Amps (Watts)

The Mechanical Power Out is:

Power Out = Torque*(0.112985) * Speed * (2*pi/60) in Watts

Note: 1 lb-in = 0.112985 Nm, and radian/sec 2*pi/60 = rpm

The Motor Efficiency is

η=(Power Out) /( Power in)

At near free speed, the torque and Mechanical Power approach zero.

12-09-2016 06:18 PM

InFlight

Re: paper: 4 CIM versus 6 CIM theoretical calculations

For any of the DC motors used in First Robotics, we can also use the following equations to determine their performance:

Current (Amps) = Torque Load * ((Stall Current- Free Current)/Stall Torque) + Free Current

Torque Load = (Current-Free Current) * Stall Torque/(Stall Current-Free Current)

Speed = Free Speed - (Free Speed / Stall Torque) * Torque Load

12-09-2016 06:59 PM

GeeTwo

Re: paper: 4 CIM versus 6 CIM theoretical calculations

If you didn't have motor controllers, or (illegally) put both motors on the same controller, this would certainly be the case. Are motor controllers designed to handle the reverse bias? - Oh, right. If you decide to change from 100% speed to 50% speed in a short period of time, you'll create the same situation, so they have to be designed that way. Never mind.

12-12-2016 10:17 PM

MrRiedemanJACC

Re: paper: 4 CIM versus 6 CIM theoretical calculations

So I'm a dumb Mechanical Engineer and last year we had a 3 Cim per side shifting drivetrain with tracks. Obviously we could have swapped out a Cim for a Minicim to help out with shooter power and other items. (we didn't and maybe should have). But let me ask this question, How does a Cim plus 2 Minicim drivetrain compare to a 2 cim (per side) in tank drive? Assuming the rules are similar this year with open availability of other motors, would it make sense to keep the 4 cim motors for whatever gamepiece needs there are? (oh and I know I'm asking for everyone to predict the future!)

12-12-2016 10:42 PM

Knufire

Re: paper: 4 CIM versus 6 CIM theoretical calculations

For those debating the merits of the 1:1 gearing, you might want to take a look at this thread.

https://www.chiefdelphi.com/forums/s...d.php?t=123424

To address the benefits of 4 CIM vs 6 CIM vs 4CIM+2 Mini-CIM, 234 has a paper published from a few years back with experimental data.

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

12-13-2016 07:43 AM

GeeTwo

Re: paper: 4 CIM versus 6 CIM theoretical calculations

While 1C+2m would certainly have better performance than a 2 CIM drive train, that performance gain is offset by a greater weight and volume, both of motors and of the gearbox - 3 motor gearboxes usually have a higher center of gravity or more gears than a 2 motor gearbox (or both). There's also the matter of another motor controller. There are certainly cases where this would be better, but I expect they're less common than the configurations already discussed.

12-13-2016 04:57 PM

cbale2000

Re: paper: 4 CIM versus 6 CIM theoretical calculations

I'll second this, it certainly wouldn't be the first choice for motor configurations IMO. In general there's rarely a need to use more than 2 CIMs outside of the drive system anyways, so the number of instances you would actually need to swap out to a 1CIM+2MiniCIM should be minimal (short of maybe saving a little weight if you already have a 2CIM+1MiniCIM drive and desperately needed to loose around 1lb)