Motor/ Driving issues

Basic overview of our robot:
~12 lbs
-4 motor 393 motor drivetrain @ 160 RPM, geared 1:1 with 4" omni wheels
-Front wheel motors are linked to a power expander
-battery levels above 8.0 volts, charged 9.0 volt backup battery
-cortex microcontroller

Our robot drives fine for around a minute and a half, but our left chassis gradually stops functioning. This has affected our left chassis for the past few months, while the right chassis moves fine, showing that it is not a battery problem. Despite this, our robot drives perfectly straight when both sides are going full power. The back wheels are directly driven by the motor, so not an issue with friction.

Is there any logical explanation to our loss of power on the left wheels? Weight distribution is fairly consistent.

Could this be mechanical, such as a problem with the motor itself, a programming issue (need for a cubic scalar), overheating issue, power expander issues, etc?


Are the motors overheating?

They have a self-resetting thermal fuse inside of the motor to prevent damage.

If you are overheating, you could change the 393’s to run in high strength mode as the motor heats up when it cant move at its theoretical speed (at a given power) due to the load placed upon it.


Friction can still be an issue - check that the shaft is not rubbing against the sides of the square vex hole and that it is properly supported at each end. Just turning the wheel should give you an idea if you have issues, but be wary that when it has a load on it, it could behave differently.

Well, I’m not sure if overheating is a problem, since it only afflicts one side of our chassis. And our robot should definitely be able to run 160RPM with 4 393’s. Robots such as Currahee’s (19.5 lbs!) run at the same speed at 160RPM.

We took the motor off, the wheel turns pretty smoothly.

Our team is going to go for a 6-wheel approach, so that we spread the weight a bit more effectively. Still, it seems as some other teams have had very similar problems, so still up for suggestions, comments.

Hi RampantFang,
I have had this problem countless times in my past 3 years of VEX and that issue sounds exactly like a brown out. My guess is that you are overheating the motors on the left side of your drive-train due to some friction or other obstruction.

When we experience this problem with one of our robots, we keep driving it, and if the entire drive fails after extended driving but the rest of the robot remains functional (arm and intake), we can narrow it down to overheated drive motors :).

Now it has been proven that 4 unmodified 393’s can power a 19 pound robot at that speed so you should try to isolate the issue. Maybe its a collar screw rubbing against the metal frame or some other odd issue. If you are still experiencing problems try switching out motors from one side to the other to see if one is malfunctioning. Good luck troubleshooting!

i also think its friction
and if its chained check if the chain is too tight, thats the #1 problem with chain drives
and i also noticed some disconnect issues at your competition, what happened?

The fact that it’s just one side of your drivetrain makes me think it’s a motor issue, unless you have a really funky weight distribution.

Try switching the sides the motors are mounted to and see if the problem switches sides.

It was one of our most unlucky competitions ever! We had a variety of vexnet disconnect issues, drivetrain failures, battery failures, etc. We even had a match that started without us turning on our robot (noobie mistake). We only had one match where everything worked, and we won that =/

And we will check the friction again.

Btw, congrats on the excellence award, even though you lost in the semis because of your chain breaking. Your robot was still a beast at descoring though.

@ chris is me
Yes, we are considering having a 6 wheel drive to better distribute the weight, though our robot is symmetrical, so it doesn’t really explain the left side not working.

@254pride and chris is me
We will definitely try switching the motors around. Great suggestion!

We had the exact same problem yesterday during the qualifications rounds… and by exact I mean it was our left side too… We fixed the problem before our last match though with programming.

For teams with “left side dies” problems, please post your wheel motor to Cortex motor port connection list.
Each motor has a breaker.
Cortex ports 1-5 have a breaker,
and Cortex ports 6-10 have another breaker.
Try to debug if it is the motor breakers or the Cortex breakers that are browning out.
Maybe don’t put all the left side drive motors on the same Cortex breaker?

Another difference between left and right side is CW vs CCW motor rotation. Usually there is not much difference in the DC motor itself, but gear trains (in the motor gear head, for you 1:1 geared teams) can add some additional friction in one direction. If you have a rotation encoder, try checking free speed of each side in forward direction vs reverse direction.

“Luck favors the prepared team”, especially for those teams with avoidable errors. One of the most photographed robot features at 2009 Texas BEST Robotics contest was a laminated checklist taped to the robot…

btw did you lose the first match?
(the match where a robot got DQ’d for pinning you)

Yes, we lost it because the pinning robot was DQ’d, and since they didn’t score any points, it didn’t affect the overall score. And plus, one of our motors broke that match so we couldn’t just shove the robot out of the way.

I am having the same issue but i am using two 269 motors
What fixed this issue?