3 motor gearbox

[A_Reed]

Originally Posted by 548swimmer

I've got the math behind how it will work down, I'm interested more in the construction aspect.

What motor characteristics need to align to pull off a gearbox with different motors?

If that's the case then all you have to worry about is matching speeds with the CIMs (whether COTS or custom) and mounting it with proper alignment to gear central to your other two CIMs. That comes down to simple geometry in CAD.

The one thing you are going to have to double check is the added torque and bending stresses to your shafts from the extra motors. What may have been overkill for 1-2 CIMs may be verging on 'just enough' for the added power. Diameters may have to be increased to maintain a reliable factor of safety on par with the standards set up by common designs and COTS items. Your stresses may check out and look very safe but for alignment and efficiency in spur gears it is better to design for deflections.

[548swimmer]

The rs-775 is about 3x faster than CIMS, so should it's gear be about three times bigger? I'll do the math to get it exact if this is the case, but it seems like the CIMulator is the way to go.

[Chris is me]

Originally Posted by 548swimmer

The rs-775 is about 3x faster than CIMS, so should it's gear be about three times bigger? I'll do the math to get it exact if this is the case, but it seems like the CIMulator is the way to go.

The opposite. Should be three times smaller.

of course, you can't go three times smaller than 12 teeth, hence why CIMulators or other banebots gearboxes are used for an initial reduction.

[548swimmer]

Originally Posted by Chris is me

The opposite. Should be three times smaller.

of course, you can't go three times smaller than 12 teeth, hence why CIMulators or other banebots gearboxes are used for an initial reduction.

Right, that one. I've been studying for my physics AP tests and my brain is fried.

[Jared Russell]

Just be careful running 4 CIMs and 2 RS-775s in one drivetrain. They draw a LOT of electrical power at stall (enough to brown out a cRIO from time to time - hopefully the cRIO-FRC2 will fix this with its 9V minimum supply voltage). Design your gear ratio appropriately.

[A_Reed]

Originally Posted by 548swimmer

The rs-775 is about 3x faster than CIMS, so should it's gear be about three times bigger? I'll do the math to get it exact if this is the case, but it seems like the CIMulator is the way to go.

The CIMulator should work well enough to match speeds.

One thing I have found is that a 775 maxed out @40 amps has the same power output as a CIM @40 amps, although I don't know how many 40A breakers one can use. So the 775 may not be fully understood as far as reliability quite yet but looks as if it is on par with a CIM in power.

[548swimmer]

Originally Posted by A_Reed

The CIMulator should work well enough to match speeds.

One thing I have found is that a 775 maxed out @40 amps has the same power output as a CIM @40 amps, although I don't know how many 40A breakers one can use. So the 775 may not be fully understood as far as reliability quite yet but looks as if it is on par with a CIM in power.

What do the green boxes in the excel document mean? Is that the recommended gear ratio?

[A_Reed]

Sorry I didn't make a distinction between inputs and outputs, both are green. The min ratio was found for the time to cover 54 ft, the max ratio is based on 40A traction limited specs including CoF and the last three were found by finding where the ratio/max torque and ratio/max speed curves are equal. My math my be a little off on some of the numbers and the intersections are based on those numbers. All numbers have efficiencies accounted for, that is what the 'gear stages' input is for.

[A_Reed]

Originally Posted by 548swimmer

Wow, that's awesome! Is everything dependent such that I can change variables?

Yes, your inputs on each page are Speed (ft/s) (don't need to modify that one as you are matching to that speed anyways), Torque (ft-lb) (Pulled from motor spec pages, taken for one wheel), Coefficient of friction, Wheel Dia. (in) and Gear stages. The Min Time is pulled from the 'T54 ft.' column and you have to follow that to look up the min ratio.

*The speed conversion isn't mathematically linked to the wheel dia. cell and you may want to modify the equation in that cell to reflect that change*[edit]

[548swimmer]

Increasing the wheel diameter increases time. Is this right, because at first glance it seems backward?

[JesseK]

Originally Posted by 548swimmer

Increasing the wheel diameter increases time. Is this right, because at first glance it seems backward?

It's a balancing act of more acceleration time vs. more time at max speed. In some cases going to a larger wheel, with no other changes, will indeed cause a higher time.

It appears to be simulating each side as if the 1 side powered the entire robot going forward, and only 1 CIM is ever used on the 3 tabs. It this intentional? Other than that it looks pretty good.

Though I would caution -- if you replay your matches this year, even on an 'open field' like this year, how far did your robot ever sprint in a straight line? Might be best to optimize it for a distance based upon your strategy (20-40ft for us)

[A_Reed]

Originally Posted by JesseK

It's a balancing act of more acceleration time vs. more time at max speed. In some cases going to a larger wheel, with no other changes, will indeed cause a higher time.

It appears to be simulating each side as if the 1 side powered the entire robot going forward, and only 1 CIM is ever used on the 3 tabs. It this intentional? Other than that it looks pretty good.

Though I would caution -- if you replay your matches this year, even on an 'open field' like this year, how far did your robot ever sprint in a straight line? Might be best to optimize it for a distance based upon your strategy (20-40ft for us)

Ah! good point. I do have all other power calculations assumed to be using 1/4 of the robot weigh on one wheel (4 wheels evenly bearing the weight). My mistake is that I assumed the same one wheel for acceleration in that it would be accelerating the entire mass of the robot by itself when it would only be accelerating 1/4th the mass as well, right?

[JesseK]

Originally Posted by A_Reed

Ah! good point. I do have all other power calculations assumed to be using 1/4 of the robot weigh on one wheel (4 wheels evenly bearing the weight). My mistake is that I assumed the same one wheel for acceleration in that it would be accelerating the entire mass of the robot by itself when it would only be accelerating 1/4th the mass as well, right?

Quite honestly, I'm not sure about the 1/4th mass thing. I've only ever analyzed it as an overall robot system of wheels to the floor so weight transfer during acceleration is somewhat negligible; I'd never thought about it that way.

Attached is what I come up with for a SuperShifter with a 42:34 chain connection to 4" wheels @148lbs total, (our drive train this year). Min time to 27ft is 2.6s; your #'s give me 3.3s or so. I think I use 2.43Nm as stall, whereas you use 1.78 @ the 40A breaker; I honestly don't know which is correct. In the end, the times on field roughly match either calculation since there are a number of other factors such as driver control & software tuning.

The only really important thing to know is how any proposed design changes (4"-6" wheel size, 17:48 gearing vs 20:45 on the output of the SS, etc) effect things relative each other so decisions can be made and bad situations (unable to turn, constantly-popping breakers, etc) are avoided.

As a quick aside -- upping the torque allotted for my spreadsheet in order to account for a properly-matched RS-775 saves ~0.5 seconds for getting to 27feet. Yet it also gives you a narrow sliver of extra power curve to push through a road block during your acceleration. I say "narrow" because eventually (as Jared eludes to) you pull too much current for your 120A breaker.

[Ether]

Originally Posted by A_Reed

One thing I have found is that a 775 maxed out @40 amps has the same power output as a CIM @40 amps, although I don't know how many 40A breakers one can use. So the 775 may not be fully understood as far as reliability quite yet but looks as if it is on par with a CIM in power.

Anybody have any data on how long a 775 will withstand 40 amps before smoking?