Does anyone have any confirmed CIM RPM data for a real world robot ? Something is going wrong with our spreadsheet re ratios and predicted speeds this year and some real data would be nice.
I’m looking for
Measured full power speed over ground (any units) of the robot and corresponding measured or calculated RPM or RPS of CIM. Number of CIMs in drive train.
Nice to have Robot Data
number of speeds of tranmission
Type of drive (# of wheels driven per side and # chains or track etc would be interesting).
wheel size / width.
Robot weight if materially less than 120lbs + 13lbs battery
De,
I know many people will not look for an electrical cause for under speed motors, but I do. If you are using long wire runs, lots of connectors, loose terminals and under size wire, then you may not be getting full power to the motors. All of these things add up. remember that at the stall current on the CIM motors, even #10 wire will drop 0.1 volt/foot. Double that for #12 and if you have chosen to feed the CIM motors with 30 amp breakers so you could use #14, then quadruple the #10 loss. Loose connections can account for up to 5-10 feet of #10/connection. The same for bad crimps. Multiply that for loose connections on the #6 primary wiring.
Finally, have you calibrated the speed controllers in some way? The LED on the controller needs to come on solid when you are at the extreme of the joystick, Red for one direction and Green for the other.
It sounds like I should provide some more details. For the past 4 years, our spread sheet pretty accurately predicted robot speed if we designed an end to end gear train ratio (factoring in wheel circumference of course) and specified an expected CIM RPM (RPS). This worked acceptably for our needs for our typical 2 cim motors per per two speed transmissions each driving 3 wheels with #35 chain. Of course the higher speed required more energy so the RPM would drop somewhat compared to the low speed. This year we used a new transmission and the calculations were 25% slower than expected.
I just recently created new spreadsheet from scratch based on published ratios guessing I must have made some dumb mistake in the original spreadsheet but again I got the same results. My next step is to take the transmission apart and count teeth (not easy on our robot). Maybe I misread the specs.
Our simple model was that 4 cims at full power would run at ~5100 RPM to drive the robot at the low speed of ~4.5 ft/sec and that that 4 cims at full power at 4800 RPM would would drive the high speed of 10-11 ft/s. (all say -/+ 10%). This year it was slower by 25%.
So I was hoping someone else had done the calculations from actual robots (ie measured the robot speed, know the overall ratio and hence know the CIM RPM).
What we need is the gear ratios and wheel diameter in order to calculate ground speed.
Case in point, our robot this year.
We had a 3 speed (1:3, 1:4, and 1:12 ratios) with a 9:21 ratio output sprocket driven by CIM motors (5500 rpm free spin) into 7 inch diameter tires. Drivetrain efficiency was guesstimated at ~81%.
This gave us a ground speed of:
1st gear: ~4.8 F/S
2nd gear: ~14.5 F/S
3rd gear: ~19.4 F/S
Using your data, my spreadsheet indicates the CIMs must have been turning 4450 RPM for each of those speeds. I would have thought as the speed went up (by 4 times between low and high speed), the RPM would have dropped noticeably due to the extra load. But maybe not.
2 Cims per side ? Full ~130lb robot or just a chassis ?
Just out of interest, how controllable was the robot at 19 fps ?
So, I think your issue is the rpm you’ve chosen to calculate by. A free spinning CIM with NO load spins around 5200rpm. A CIM under load (like on a FIRST drivetrain) only spind sup to about 4500 (sometimes a bit more or less depending on the weight of the drive system). A good place to start calculating from is 4000rpm, as your max speed will end up closer to what you want it to be, and you won’t have to do a crazy amount of math to factor in drag and so on.
As for the 19fps, that’s a little quick, but controllable. The main issue you’ll have with that speed is acceleration. Add another motor to the gearbox, and you’ll find your acceleration increasing a decent bit.
The robot was decently controllable at that speed but to get there we needed to run through the gears as it’s accelleration was poor in 3rd gear. Fortunately our gearboxes were shift-on-the-fly.