⁶ CIM drive-train brown out

I have been talking about brownout problems with our drive train design, but no one seems to listen. I have attached the picture of what we were planning. 1 It is a 6 CIM drive train. 2 The CIMs are spaced out (longer wiring) 3 We will sometimes be running 2 mini CIMs or a bag motor along with it.

Screenshot 2016-01-17 at 11.12.59 AM - Edited.png


Screenshot 2016-01-17 at 11.12.59 AM - Edited.png

We had a 6-CIM drivetrain with our 2014 bot. No brownouts. We were running 2 bags and/or minis along with it

But the electronics changed last year…Robo-Rio- Brownout voltage 6.8v. C-Rio-Brownout voltage4.5v

The concern is because the electronics changed significantly between 2014 and 2015. And nobody ran 6 CIM drivetrains in 2015.

I would be concerned about running 6 CIMs in a drivetrain this year if you don’t have plans to make some funky software or something to prevent brownouts. However I haven’t done the math or tested it, so there might be a more educated person to answer.

I also don’t see the need for 6 CIMs in a drivetrain this year. The field is cramped, and defense is limited.

This robot is currently running on a Roborio. and still no Brownouts

Edit: I should add that we had each wheel independently powered by it’s own CIM through Versa planetaries

Not enough information. What is your drivetrain gear ratio? What kind of wheels?

It’s a different ball game now that the roborio has certain brownout conditions in place to prevent mid-match rebooting loss of comms. I have heard several teams say the brownouts were not noticeable to them while driving a 6 CIM drive. However, we do not have a representative sample because last years game did not subject motors to the normal rigor of an FRC game that includes defense or other high torque situations such as field obstacles.

Your set up with driving each motor to a wheel independently is a generally poor design choice. Here’s why:

  1. You will need to have a gearbox for each wheel. This will be heavier.
  2. With a 6 wheel drop center drive, one pair of your corner wheels will not be in contact with the carpet when flat on the ground (or it will at least have substantially less normal force). It will therefore be unable to transmit any force to the ground (or substantially less). If you chain your wheels together and have 1 central gearbox on each side, the torque could be transmitted to another wheel to make use of the extra motor power.
  3. When in situations like a pushing match or obstacle traversal, your wheels may have abnormal weight distribution. Since their normal force on the carpet or otherwise field surface is unequal, different wheels may begin to break traction and slip at a lower torque than others. If you had the wheels chained or belted together you could essentially allow the motors to distrubte the torque to which wheel it is needed on, and avoid this loss of traction issue. In this years game in particular, if you picture a situation where you’re caught strangely on an obstacle and only 2 of your wheels contact a field surface you would only be able to use 2 motors worth of power to drive by your design.

There are plenty of performance related reasons to do a single gearbox per drive side with chains or belts or gears connecting all the wheels and physical justifications behind them. Hope this helps.

Have you gotten in a pushing match with said robot while running it on the roborio while also running multiple other subsystems of the robot after 2 solid minutes of play?

Those are the brownout conditions we’re talking about

Yes, but you are presumingly not getting the normal match strain on it.

+1 to everything KrazyKarl92 said

In addition, in my experience, versa planetaries are not suitable for sustained, high torque applications. We had versa planetaries with bag motors powering our rather heavy intake arm in 2014, and they tend to fail internally. They are fixable, but you don’t want to have one fail.

Brownouts are not likely especially because only four CIMs will be on the ground at any time, I expect the gearboxes to fail before you could get near there anyway.

I really don’t like this topology.

First, VPs are awesome but… …they are not as efficient as straight up spur gears - not usually a problem but the drivetrain is almost always the most power hungry system on the robot, a few % of efficiency on this system matters.

Second, with 6WD you have at most 4 wheels on the ground at any one time. The CIMs on the wheel in the air is doing nothing. WORSE, when you get in a pushing match, you will have most of your weight on the rear wheels as you push against the other robot. In this case, nearly all the power to the ground will be coming from those back 2 CIMs, the front 4 will be providing next to nothing.

In cases like what you have described, I believe it is much better to have the wheels driven by a common drive system (chain, gears, belts, …). In this type of system, all the CIMs have the opportunity to drive whatever wheels are on the ground. If you have a good reason not to do this, fine, but if you can, then you do.

Dr. Joe J.