Weird Base Dying Issue


This past weekend our team just had a competition, and we ran into a very odd and unfortunate issue.
With our robot, we have:
4 motor lift (torque) w/ 1:7 external torque gearing
4 motor base (speed gearing, not turbo)
2 motor claw (torque) w/ 1:15 external torque gearing
2 pistons and 2 solenoids doing minimal load tasks

For two entire weeks before the competition, our robot performed well- never blew any cortex fuses or have overheated motors.
We have a power expander wired to our lift, and we didn’t use any Y-cables.

On competition day, during our 3rd qualifier match, our base stopped working in the middle of the game about 30 seconds in. First our left drivetrain became weak for 3-5 seconds, and then it died off. Trying to move our bot with our right drivetrain, it died about 5 seconds later. Our arms and claws could still both move fine. We know we didn’t blow any fuses because our claw motors are plugged into ports 1 and 10, our base 2-5 and arms 6-9. If the 6-10 port fuse had blown, we wouldn’t have been able to close our claw. So it appeared like overheated motors, but they weren’t hot or even warm at all, and we had never had this issue in practice scrimmages. After letting the robot cool off for 5 minutes, the base could drive fine again at full power, but if we place 3 stars or a cube on the bot, the same issue happens with the drive and it dies off. Still, not heat to the motors at all. We were pretty sure it was a power draw issue since the drive dies out when the robot experiences too much load, so we switched out all the drive motors with brand new ones.
One match in, robot was fine. Second match with the new motors, same issue. We asked around for help and were told that it’s better to split the power expander half and half- to two of your drive motors, and two of the arm motors. We distributed the expander symmetrically between our two systems- to the two back motors in our drive, and one motor on each arm. This however, didn’t help- things got worse. With this change, the base dies about 2-3 seconds after much load is applied to the robot and we try to move after autonomous, the base dies and the arms become slightly weaker. We tried to switch out the drive motors again, but it didn’t help and the issue remained. I think one of the issues may be our back wheels, which are regular ones instead of omnis. Turning with load, we found, killed the base motors immediately.
In between matches, we tested a bit more and found that when the base dies, the motors refuse to turn at all- it doesn’t even make the stalling noises and you can’t feel anything trying to turn in the motor. And before they die, they don’t stall- they just become extremely weak and then feel as though power was cut off.
Even testing with a bare bones joystick control code, we ran into the same issue.

Looking back, we probably should have just used torque motors for the drive, but they were just so slow that we were praying for speed ones to work.

The only thing we haven’t tried is replacing the cortex and motor controllers. We had a number of teachers, advisers, and various other teams look at our bot and our code but to no avail.
I’d like to stress once again that none of these problems arose during our two weeks of testing prior to the competition, which has us very confused and depressed because we were told after the competition that had we made it into quarter finals, we would have won the design journal easily because none of the other team’s journal qualities were even close to ours.

Thanks in advance to any and all feedback.

To me it sounds like friction. If you add load to your robot it increases friction greatly. From what it sounds like to me I would make sure there is no unnecessary rubbing in your drive train( wheels against c-channel) and that you use bearing flats on all axles. Also check to make sure the spacers on the drive train are not pushed together to tightly against the wheel causing unnecessary friction which can be caused by having to wide of spacing for the area. Hope this helps!

How heavy is the robot? Most likely cause is motors tripping the internal PTC. Were you practicing on a competition field? (as in foam field tiles)

Maybe try adding thresholds to the drive ? You could also put some lubricant on the internal gearing also be sure to not make the spacing on your wheels extremely tight try giving them a bit of slack.

Yes, that’s what I thought too, but we were able to drive fine for 2 weeks. We made sure to support every axle with a bearing, that there were no strains on the axles or friction, and that the drive frames weren’t too tight. :frowning: The problem right now probably has to do with friction and power draw, so we’ll try gearing down the speed motors externally, or gearing up torque motors to speed if speed motors don’t work.

Not really sure how heavy the robot is, but we practiced on the same foam tiles as the competition field. Pretty sure the PTC only trips when the motor over heats though, unless there was a fuse to prevent current overflow that I didn’t know about. If that’s the case, that’s probably the issue.

Not really sure how to add thresholds, but we probably do need to lube up the motors some more. Thanks for the reminder.

How old are your batteries? You may try testing the batteries right after this happens.

Each motor has a 1 amp breaker. The cortex has two 4 amp breaker banks–Ports 1-5 and Ports 6-10. My initial thought was that you tripped the 1-5 4 amp breaker except you said that you lost the left side first which would mean that you tripped individual motor breakers. Once a motor breaker trips it takes about 5 secs to reset but when you continued driving they could not reset and it looks like attempting to continue to drive increased the demand on the right side and tripped them as well. The fact that once it cooled down everything was fine would appear to confirm this. The power expander also has a breaker. So it looks like your lift is over protected and your drive under. I’d spread your drive over the two banks and you might consider half right and half left on each bank which will give you a modicum of mobility if you trip a breaker but I’d expect not with the entire lift on the expander. And if you do trip a breaker on the drive, pause everything for five seconds and start back slow.

I see, thanks for that last tip. We did eventually spread our drive and arms over the two banks, as mentioned later in the post, but issues persisted :

maybe try adding a turbo button so you’re not constantly running at full speed.