Does Your Team Use 4 CIMs on Your Drive?

[EricH]

While you were using 2 CIMs since the year when the number of CIMs to the floor didn't matter, everyone else went back to 4 on the floor. (Or at least some reasonable substitution for the extra CIMs.)

That means that everybody else has 2x the power available in the drivetrain. You can do the math for available acceleration, pushing power, and all that other stuff, but the simple way to look at it is you've got a 1/2 power robot in terms of drive.

[Mk.32]

We are definitely planning on using 4 CIMs in drive this year.
FP/Banebots/etc for shooters and the rest.

While we're not willing to sacrifice our first 4 CIMs from the drivetrain, we are willing to use our pseudo-CIMs for shooters, etc.

Essentially, an RS-775 or FP motor through a Cim-Sim gearbox will be almost exactly like a CIM motor.

[Djur]

2 CIMs are lighter than 4 (and their gearboxes), but they offer so much less power that many robots with 4 CIMs can bully 2-CIM robots very easily. While having 4 CIMs isn't strictly necessary -- you can easily drive a tank style bot with only two -- it removes the power inequality with only 2 CIMs and makes a more responsive/maneuverable/powerful robot.

[davidthefat]

Originally Posted by Djur

2 CIMs are lighter than 4 (and their gearboxes), but they offer so much less power that many robots with 4 CIMs can bully 2-CIM robots very easily. While having 4 CIMs isn't strictly necessary -- you can easily drive a tank style bot with only two -- it removes the power inequality with only 2 CIMs and makes a more responsive/maneuverable/powerful robot.

That is my argument FOR 4 CIMs. Because we will be a feeder, we would be spending our whole time on the opponent's side, we need to resist being pushed.

[rich2202]

Originally Posted by davidthefat

That is my argument FOR 4 CIMs. Because we will be a feeder, we would be spending our whole time on the opponent's side, we need to resist being pushed.

Not only resist, but as a feeder, you are a defender, and have to be able to push them around.

[waialua359]

Of all the years in FIRST, we got the largest selection and amount of motors with lots of COTS options for CIM-ulating.
Its a no-brainer. Use all 4 CIMS in your drive.

[Hawiian Cadder]

Almost all of the Andy-mark Gearboxes are set up to support 2 motors, every gearbox on our robot will have 2 motors in it. 4 cims in the drive-train, and then pairs of 500 series motors elsewhere. Even if you want to avoid the 775 problems, the 500 series motors will more than handle what you need. We are planning on 4 total in our conveyance system.

[Marc S.]

Definitely go with 4 cims. There are plenty of other motors for everything else(especially this year). If your looking into pushing other robots check different tread types. McMaster-Carr sells the same material as the andymark tread in different abrasive qualities.
Also check out this match(and cad below) when 973 pushed 469 with 4 cims in drive.

[Dad1279]

4 CIMs + at least 2 more......

You could demonstrate your idea by using the JVN motor spreadsheet, entering the data for your gearing, it calculates current draw and pushing force.

[JamesCH95]

Originally Posted by Dad1279

4 CIMs + at least 2 more......

You could demonstrate your idea by using the JVN motor spreadsheet, entering the data for your gearing, it calculates current draw and pushing force.

I read a team's post about trying a 6-motor drive train and they found that pushing performance was hurt (vs 4-motor) because the extra two motors dragged down system voltage so much. YMMV though.

We've used 4 CIMs for the past several years. When coupled with an OTS 2-spd transmission from AndyMark and high traction wheels they're very hard to beat.

[Dad1279]

Originally Posted by JamesCH95

I read a team's post about trying a 6-motor drive train and they found that pushing performance was hurt (vs 4-motor) because the extra two motors dragged down system voltage so much. YMMV though.

We've used 4 CIMs for the past several years. When coupled with an OTS 2-spd transmission from AndyMark and high traction wheels they're very hard to beat.

I'm sure that depends on the application, wheels, gearing, etc......JVN spreadsheet is your friend..... we will confirm with testing.

Back to OP, check the following chart (thanks to team 358): http://team358.org/files/mechanical/...nRefTables.jpg It clearly shows the advantage to 4 CIMs in a conventional traction drivetrain.

[JamesCH95]

Originally Posted by Dad1279

I'm sure that depends on the application, wheels, gearing, etc......JVN spreadsheet is your friend..... we will confirm with testing.

Back to OP, check the following chart (thanks to team 358): http://team358.org/files/mechanical/...nRefTables.jpg It clearly shows the advantage to 4 CIMs in a conventional traction drivetrain.

Precisely why I said YMMV.

I really like that spreadsheet, it is what I use quite frequently when picking gear ratios for drivetrains.

This just occurred to me: with 4 CIMs in a drivetrain you will run into two limiting factors in a pushing match. The drive train will be traction-limited, extra motors won't help here because you simply can't get their power/force to the ground. The drive train will be current-limited, i.e. you'll start popping breakers before you lose traction. The issue here is that with 4 CIMs you run the risk of popping the main breaker (120A) before you pop the individual (40A) breakers. Adding two more motors will not improve performance in this situation either because the main breaker will go before any of the individual motors will.

Extra drive motors might help a small amount with acceleration and other dynamic situations, but IMO 4 CIMs can accelerate a full-weight robot very well, even with high gearing.

[Chris is me]

Originally Posted by JamesCH95

The issue here is that with 4 CIMs you run the risk of popping the main breaker (120A) before you pop the individual (40A) breakers. Adding two more motors will not improve performance in this situation either because the main breaker will go before any of the individual motors will.

For what it's worth, the 120A breaker can sustain 160A for about a minute before tripping. The auto-reset breakers are much faster acting.

[JamesCH95]

Originally Posted by Chris is me

For what it's worth, the 120A breaker can sustain 160A for about a minute before tripping. The auto-reset breakers are much faster acting.

Interesting. Do you have a source for that? I'm curious.

Odd that it's dubbed a "120A" breaker if it will allow significantly more current for such a long period of time.

Edit: NVM, found it in the breaker spec sheet here: http://team358.org/files/electrical/120aMainBreaker.pdf