Motor Twitching Issue

VEX V5 Technical Support OFFICIAL Tech Support
1

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

Hi Erik,

Thank you for giving as many details as you did! It really helps when trying to diagnose and troubleshoot issues.

Generally, when people experience motors twitching or cutting out, the PTC located inside the motor or a breaker inside the Cortex is being triggered. The PTC activates after continuous high loads or sudden high current surges to protect the system. After a PTC has been activated, it can take a few seconds for the motor to come back but the PTC will be more likely to trigger immediately afterwards. It can take several minutes for the PTC to fully reset.

Reducing friction or increasing torque on each misbehaving subsystem will reduce load on the motors and should help prevent the twitching. Bearing Flats can be used to help reduce friction along shafts, but should be carefully installed to prevent misalignment. Lubricants can also be helpful, but you should be sure to check the official rules for which lubricants are legal before competition. Check your whole robot for any rubbing or misaligned metal pieces as these can also cause excessive friction.

You can also try splitting your motors across the 4 amp breakers on Ports 1-5 and 6-10. Try wiring your robot so that all of the motors in the drive train are not on a single breaker.

Hope this helps, and good luck at your competition on Friday!