paper: Cyber Blue 234 Drive System Test - Performance and Power
 

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Cyber Blue 234 Drive System Test - Performance and Power by Chris Fultz

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

This fall Cyber Blue completed a series of drive system tests to compare the acceleration, top speed, voltage drop and amperage for 4 different motor combinations and two robot operations.

We used a special built, kitbot chassis, 4 WD and tested with 2, 4 and 6 CIM and 4 CIM + 2 Mini-CIM motors. We tested with the robot "alone" for a 50 foot test run, and also pushing a 130 pound, unpowered, robot for a 25 foot test. The unpowered robot had a 4 CIM drive, with the Victors in "brake" mode.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

Thanks a lot for performing these tests! This is extremely useful data and will definitely be a significant factor in our drivetrain design next season.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

YES!
Thank you so much for releasing this data on CD. We will for certain use this to determine out drive train next year.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

Your data is enlightening. Thank you!

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

This is great!

I find it really interesting that on paper, the 4 + 2 CIMs are comparable, and sometimes beats the 6 CIM option.

Quick question, am I correct in assuming that the "Peak Amps - 1 Motor" for the 4 + 2 CIMS was a Mini-CIM? (as "Peak Total Amps" for 4 + 2 is higher than 6, but "Peak Amps - 1 Motor" is lower)

I'm not sure hard it would be to organise, but from that data I'd be really interested in seeing how more combinations of Mini-CIMs (say 2CIMs + 4 Mini-CIMs) stack up.

It seems that even though the CIMs are higher power, by operating the Mini-CIMs closer to their MPP they perform noticeably better. This implies that in some drive trains, Mini-CIMs would actually perform better than standard CIMs. However it should be noted that (assuming the same gearing form CIMs and Mini-CIMs) the closer you get towards the CIMs MPP, the closer you get to the Mini-CIMs stall condition.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

Very insightful. Thank you for putting together such a detailed paper and making it available to us all.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

Yes, in all of the test runs (3 trials) the mini-CIM was the highest amperage, but they were very close (44-47 Amps for at least one of the CIMs in each test). So while slightly higher, i think i would say "almost the same" given the accuracy of our data capture.

We plan to save the test chassis, and may do more testing next summer / fall to do more comparisons. With the CIM, mini-CIMs and "CIM adapters" for other motors there might be some interesting information to be found.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

This will certainly come in useful in just a few days when selecting our drivetrain. Thanks a bunch for saving us some major time.

Do you have any idea why the 4 CIM + 2 Mini-CIM configuration draws a similar current when pushing as the 6 CIM configuration? I seem to recall that Mini-CIMs are about 2/3 as powerful as a CIM, so it would seem to make sense if replacing 2 CIMs with 2 Mini-CIMs in this configuration, the total current drawn (at least in theory) would be reduced by about 11%.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

Thanks for sharing this very useful data.

Knowing that the 4+2 combination is so close to the 6 CIM opens up options for motor allocation. Your report also provides good objective data for CIM speed under "real world" load conditions.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

This is very interesting and useful. Now I'm really interested and seeing how this data changes as the gear reduction gets reduced and top speed rises. I know for many teams that use 6 CIM or similar drive trains it's to be able to get a higher top end speed without losing to much low end torque on a single speed gear train.

Our final ratio last year was 6:1 and I predict a 4 CIM drive struggling to handle that.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

Very interesting study and thanks for sharing. I am a little surprised that the actual top speed is only about 75% of the theoretical top speed. I had assumed somewhere between 80 to 90% depending on gearbox and gear reduction.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

What are you using to get the theoretical top speed? I ran the numbers quickly and it seems right to me. Theoretical top speed is around 11.65 fps (CIM at 5300 RPM and 4in wheels) and recorded for the 4 CIM is 10.76 fps which is 92%.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

i agree with Allen's math -

The total reduction is 7.95:1, counting the gearbox and the belt pulleys.

Theoretical = 11.63 FPS
4 CIM = 10.76 = 93%
6 CIM = 11.06 = 97%

To be more accurate, we would need the true free speeds of the CIMs and the actual diameter of the "4 inch" wheels with tread, but these are close.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

Sorry, I misunderstood what 37-42 Step Up mean. I also did not notice you published the final drive ratio which would have told me I did it the opposite way.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

Thanks for sharing. Another good check would be how much current it takes to spin the tires without moving such as pushing against an unmoveable robot. This is a key high current consumption situation to watch.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

A couple things I think would also be interesting. A graph of distance / time, you can see some of this in the FPS vs. time but the actual graph could be enlightening to see how the acceleration changes time to different distances (25' and 50' are often to long of distances in many games).

Also the graphs of each motor current. It may just be how it's measured but it's interesting to me that the 6 CIM has the highest individual peak motor current yet the 4+2 had a higher total current.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

there is another paper we posted a few weeks ago that does some of this. we were pushing a wall. we went from 6 CIM to 4, so we wouldn't spin, and that is where we learned about the 7V brownout impacts.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

That will be pretty easy to do. We have the encoder counts and can convert them to inches (or feet) and plot distance v time.

Also easy to do. In both cases, though, there is not a lot of difference in the 6 CIM or 4+2 CIM.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

This is a great paper and I completely agree with what Wayne stated (and I'm sure others were thinking) that it definitely opens up options of what motors to use elsewhere if we can go with a 4+2 combo on the drivetrain. The game and our strategy on handling it will drive that decision, but it is nice to have options!

Thanks again for posting this information, can't wait to share it with my team on Saturday when we meet.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

Thanks for posting the data. This information is very handy to know.

Just a quick thought regarding the 4 CIM + 2 Mini CIM test outperforming the 6 CIM test:
When I look at your battery voltage plots on pages 8 (single robot) and 14 (pushing), I see that your battery voltage on the 4+2 starts out higher than the 6, and the 4+2 voltage does not droop as low as the 6 initially, when the motors are in a stall situation. This is happening despite the higher measured current draw of the 4+2 system than the 6 system. I do see that you listed battery voltage as a variable that changed with each test. Do you think it is possible that the battery could have been in a higher charge state (and thus able to supply more power to the system) in the 4+2 test than in the 6 test? Is this difference significant enough that it could cause the 4+2 test to outperform the 6 test?

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

I would like to see the code you used for the data logging. Were looking into doing similar testing and sample code would save us a lot of time.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

We captured the data from the control, which is output as a .csv file, then opened it with Excel. All of the analysis and plots are Excel.

I can get more details on the data logging if that is helpful.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

I'd like to see some 775 Pro testing. We are intrigued by their capabilities and would consider using them in our drive train.

Re: paper: Cyber Blue 234 Drive System Test - Performance and Power
 

http://motors.vex.com/775pro

http://motors.vex.com/cim-motor

Comparing the peak power and stall torque graphs will give some insight into why CIMs are preferred over high-speed-fan-cooled motors for drivetrain.