6 drivetrain motors vs 4 motors

As a team that usually has late decisions on drivetrains and decides to move forward this year with a WCD, we struggle to decide whether we want a 4 motors drivetrain or a 6 motors drivetrain. As I can see, the more conventional setup for a WCD is 6 motors, and it certainly gives more power, but we have the concern that the drivetrain takes up 6 out of 8 40amps ports on the one PDP and leave no room for other mechanisms, such as the winch for climbing or motors for shooting. However, with the railing setup, 6 motors are for sure easier to go over them. What are your recommendations?

Seems like the trend is moving away from 6 motors back to 4, especially with all these brushless motors these days.

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Use a drivetrain calculator like JVN or ILITE or a number of others to determine if your chosen strategy or design needs 6 motors.

Consider that 6xmincims < 4 NEO/Falcon in terms of power or torque.

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is there a JVN calculator that can calculate NEOs or Falcons? Or I have to plug in their data sheet myself

An updated one probably exists somewhere, IMO its easier to just plug the values in myself

Aside from space or pushing power, consider how much current you’d be pulling while playing defense or pushing while playing offense. The main 120 amp breaker can hold at overload for a while, but a lot of current spread among six motors can eat that up fast.

I saw a team playing wicked defense last year using a KOP with only 2 CIMs installed.

Easiest to plug the data in yourself.

ILITE calc has all the motor data built in.

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I mean, they definitely have some really good drivers, which means that they can do everything effectively :slight_smile:

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I believe the latest @AriMB calculator spreadsheets have all of the current motors.

If you have the money for 6 drive motrs and two 3-motor gearboxes and enough PDP slots left to allocate six to the drivetrain and the weight allowance for the heavier gearbox, extra motors, and extra controllers to support a 6 motor drive train , a six motor setup spreads the heat generation out farther, and likely puts the motors in a more efficient speed regime, so it may well be worth it*. If any of those three conditions is false, seriously consider your best four motor drive train.

* This was a rule of thumb. Especially if you’re anywhere close, do the math.

The team I was on in high school ran 8 drivetrain motors for a couple years, and enjoyed it. We were running 775s at the time so with NEOs/ Falcons, people are moving away from that, but we didn’t have any issues with motor spots on the robot. We typically filled the PDP up, but we never felt like we were in dire need of another PDP spot, and we liked to overpower mechanisms. 2 motors on an already very light elevator. 2 motors on a climber that realistically, one motor could easily handle. If you have the weight, money, and room, I would go for the additional motor to help with heat dissipation.

First of all it would be good to give the following post by Oblarg a read: paper: Power consumption limits and total motor power in FRC drives

I’m not sure what motors you intend on using for your drivetrain but I modified his script for the Falcons and the Neos and this is the result with a power limit of 1687.5 Watts (the amount of power that will drop a 20 mΩ battery from 12 volts to 7.5 volts):
image
NOTE: The power given to each system of motors is the same

If I understand correctly, more motors means greater efficiency due to the lower resistance of having them in parallel. If the graph above is to be believed, I think a 6 Falcon WCD could operate on 30amp breakers without losses to performance and with added efficiency gains.

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image

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The numerical simulations I have been posting provide the ability to examine:
Robot weight, wheel diameter, gear ratio, motor selection, motor quantity, current limits, cabling limitations, battery Voltage, and battery ESR against game specific goals - generally top speed and acceleration capability.

Latest version is found here:

Would there be any problems with running some motors on 30amps and some on 40?

I think that you are asking … If I run 6 motors can I split the motors between 30 and 40 Amp breakers?
We used 6 NEOs for our mobility solution.
Our calculations indicate that the current limit for this selection should be set at 30 Amps based upon traction limitations.
We also used 2 NEOs for our shooter, and 2 NEOs for our climber (which was actually still in work!). Our choice was to split the mobility motors between 40 and 30 Amp breakers; each side had two from 40 Amps and 1 from 30 Amps. No issue.
I do recommend that you use the same 40/30 Amp split on each side of the robot.
If you don’t otherwise set current limits, make sure the set current limits on the motor drives fed from the 30 Amp breakers.
Make sense?

I was asking more about if it would cause gearbox problems as motors are running at different speeds… And would the answer (both about current and gearboxes) be any different with a 4motor (2 per side) drivetrain?

Can you explain briefly about current limiting? I haven’t dealt with it before, as 2020 was our first year using smart motor controllers, and we barely used any of the smart features.
A bit off topic, but can you also briefly explain how to use onboard voltage compensation?
Linking to a good learning source of these things would be great.

That is kind of obvious.

If you examine the equations of motors, you will find that speed is proportional to Voltage and torque is proportional to current; thus when operated from the same Voltage source, the motors will achieve the same speed (even though the motors potentially produce different torques).
I had hopes of gathering test data to document current limiting behavior, but that has not happened. I prefer to have the plots.
Having said that (in principle) … the motor drive applies Voltage (either on or off) to the motor through a FET switch. As the Voltage is applied to the motor, motor current changes. As the motor current passes the current limit value, the driving Voltage to the motor is removed by opening the FET switch. When closed, the current ramps up; when opened, the current ramps down. That is the general idea for current limiting.
Can you describe what you would like Voltage compensation to provide?