Motor Twitching Issues

VEX V5 Technical Support UNOFFICIAL Tech Support
1

Hi All,

My team’s VEX-U robot is having an issue where if you put an electrical load on certain subsystems, the motors will begin twitching. As we have a competition on Friday, we would like your input. Here are some details about our robot:

  • Four motor shooter (ports 2, 3, 4 and 5 not on a power expander), sensed by one rotary encoder
  • Four mechanically and electrically separated parts of an intake plugged into ports 1, 8, 9 and 10, no sensors and not off of the power expander
  • A four motor flywheel, signal from two ports on the cortex, Y’ed off to four ports in the power expander, ports 6 and 7, sensed by a rotary encoder. The shooter is currently controlled through a proportional controller with a feed forward term, it takes longer for the shooter to twitch if the proportional coefficient is reduced.
  • A pneumatically actuated ramp, solenoid in port 7.
  • When the drive starts twitching, we lose the entire drive and the intake motor plugged into port one, indicating that it is probably a problem with the breaker in the cortex.
  • When we lose the shooter, only the shooter goes out.
  • The issue is only evident when mechanical load is put on the system (the shooter turning on, changing directions quickly on the drivetrain)
  • Our wiring is a bit of a mess but I insulated everything I could and the problem persisted, we will clean it up and post and update when that happens.
  • The effected motors do not return to normal after a power cycle, again, indicating that a breaker is tripping.
  • We have replaced the cortex and this did not help, we have not, but probably will replace the power expander.
  • We are programming in PROS.
  • Joystick and cortex lights do not change while the motors are twitching.

Thank you for any help you can provide, if you would like some more details, I would be happy to provide them.

Erik Orlowski
UWM Panthers VEX-U

2

So which ports is the drive on? you said
port 2,3,4 &5 Shooter
port 1,8,9&10 intake
port 6&7 flywheel.

It really does just sound like you are overloading one of the PTCs. Make sure to remove all power from those motors for several seconds to allow it to reset. Then again it may just be the programming, you are probably going to need to post the code.

3

Sorry, drive is 2,3,4,5 shooter is 6&7.

4

Ok, so it looks like you trip the PTC controlling ports 1 through 5. When you loose the shooter that’s probably the PTC in the power expander. Things like changing direction on the drive without decelerating and accelerating will draw lots of current, that’s what heats up the PTC and causes it to trip. Replacing the cortex or power expander will not help, you need to reduce the motor load by, for example, moving two of the drive motors to the second (ports 6 through 10) circuit on the cortex or changing the gearing so motors are not working so hard. How heavy is the robot?

5

The robot is pretty heavy so that could be part of the issue as well. We’ll move the motors around and post once we do that.

6

That’s a large factor that could be causing it to trip. Keep in mind that having your robot apply force to other heavy loads is practically the same thing. If you try and push a heavy object with a light robot, it can still trip, because the motors are being forced to push a large load.

7

Just a quick update, we were able to move the motors around so that the load would be distributed between subsystems and while we still are on the edge of tripping breakers, it wasn’t an issue at our competition. Thanks for the help guys!