The
8 CIM thread got me interested in this again. I had added battery voltage to my calculator earlier in the year without getting it to produce sane results, but today I gave it another run:
Acceleration Spreadsheet Nemo V2.xlsx
Embarrassingly, somehow I missed
Andrew's white paper link in Post #3 when this thread was new. Now that I've read that, it's very helpful to see what he did, and I think I am attempting to do something similar with this spreadsheet, albeit without having voltage data to give me a reality check. Logging some voltages is something I'd really like to do with one of our robots this fall. Knowing that 33's voltage was consistently dipping below 8 V or so is a really handy thing to know.
I'll add a quick link to
Al's post from another thread in which he explains how to calculate resistance losses.
As before, my spreadsheet is based on copying the process from the modeling tab in
JVN's 2004 spreadsheet.
I still don't really trust what the spreadsheet tells me without doing some quantitative tests on actual robots. But the results seem more reasonable than before.
From playing around with this spreadsheet, here are some of the things it tells me based on some of the various inputs I've tried:
- 6 CIM drive can do sprints around 10% faster than 4 CIM drive, assuming both drives have gear ratios optimized for sprint speed
- 6 CIM drive can be traction limited at about 3 ft/s faster gearing than an equivalent 4 CIM drive
- If 4 CIM drive drops voltage to 7.8 V, a 6 CIM drive drops it to 6.8 V and an 8 CIM drive drops it to 5.8 V.
- A trio of similar 18 ft/s drives with 4, 6, and 8 CIM's would initially pull 344, 441, and 514 Amps (it would be academically interesting to see how long one could drive an 8 CIM drive hard without tripping the 120A breaker, wouldn't it?)
- Adding an extra foot of 12 AWG wire (0.5 ft black + 0.5 ft red) causes a loss of about 1% of a robot's pushing force.
- Given the same gearing, 2 CIM drives take a significant hit (10-20%) to their spring speeds and a huge hit (30-40%) to their pushing force. No wonder it's so easy to push 2 CIM robots around.
Those are all pretty interesting to me if they're true.