Does Your Team Use 4 CIMs on Your Drive?

[davidthefat]

I am taking a survey to explain to my mentor that it would be a disadvantage to use only 2 CIMs on the drive, especially if playing as a feeder. His argument is that we used 2 CIMs for the past 3 years, but my argument is that we do not have enough power to overcome opponents, let alone push them around. Last year, we DQed in the semi finals because we were pushed into the scoring zone due to the fact that we only used 2 CIMs. Also, using 4 will draw less current per motor and will not trip the amp breaker. Our robot tripped the amp breaker too much that it was almost undrivable. I want to prevent that this year, but he does not think it is necessary.

[MikeBrock]

I think you are actually correct.

[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.

[Andrew Lawrence]

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]

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]

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 (http://www.youtube.com/watch?v=zGLuxRw_y3A&feature=plcp&context=C309e12bUDOEgsToPDskJ2DJXUP6ybY3aY-S5U7G6U)(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]

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]

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/FRC_DrivetrainRefTables.jpg It clearly shows the advantage to 4 CIMs in a conventional traction drivetrain.

[Taylor Hawkins]

We always use 4 sims in out drive train because we found that with 2 you needed to use more power to get the same result. We also found out last night that with 4 cim's you have so much torque the robot was able to push a mentor across the floor. (with Ease!!)

Definitely suggest using 4 cims

[JamesCH95]

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/FRC_DrivetrainRefTables.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]

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]

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

[GilaMonsterAlex]

Definitely go with 4 CIMs. Like someone already said, we have so many other options that there's no reason not to use 4 CIMs.

Last year we used 4 CIMs, Toughbox(12.5:1), and 8" wheels. We played defense, and were super quick and tons of torque (very technical terms). We never popped a breaker, and we were at max weight.

[Brinks3302]

If you use 4 CIMs, all with separate speed controllers, is it legal to split the PWM feeds so that the right side speed controllers are both receiving input from PWM 1 and the left from PWM 2 (or some variation on that)? Is the meaning of rule R61 to prevent this?

Thanks

[buildmaster5000]

If you use 4 CIMs, all with separate speed controllers, is it legal to split the PWM feeds so that the right side speed controllers are both receiving input from PWM 1 and the left from PWM 2 (or some variation on that)? Is the meaning of rule R61 to prevent this?


If you want to know if you can use a Y-connector like the one in the picture below, the answer is yes. It should be in the KOP according to the checklist. It is listed as "Cable, Servo, Y-splitter."
I would say that this is the best practice for controling multiple motors/speed controllers that need to be run at the same speed because of the minimal failure points.

[ProgrammerMatt]

Our team uses 4 cims and a 2 speed dog shifter

[Cory]

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.

I suppose it depends on your definition of high gearing, but we have had robots in the 18-20 FPS range that accelerate sluggishly.

If you want to run a max weight robot at those kinds of speeds your acceleration will suffer with only 4 CIMs.

[joek]

we have used mechanum every year except '09, requiring 4 cims... we used 4 cims in '09, 2 for shooter, geared up 1:2, 2 for drive w/ 12.75:1.
we will almost certainly use 4 this year

[KennyLives]

Definitely use 4 CIMs.There is NO advantage to using 2 of them. If you need another high-torque motor, there are plenty of banebots motors to suit your purposes.

[JamesCH95]

I suppose it depends on your definition of high gearing, but we have had robots in the 18-20 FPS range that accelerate sluggishly.

If you want to run a max weight robot at those kinds of speeds your acceleration will suffer with only 4 CIMs.

Good point. A 2-spd transmission will really help getting to those high speeds I imagine.