VEX motor or drive issues?

In VRC, teams will often find that their robot operates just fine for a few minutes or matches, but after those few minutes, the motors will stop responding to driver control.  In most cases, this is either a result of stalling motors, or of drawing too much current on a breaker in the microcontroller or power expander.  In this post, I hope to describe the symptoms of each, and some ways to fix these problems.

Stalling Motors:
Basically, this occurs when the motor isn’t strong enough to lift the load to which it has been attached.  When a motor stalls, it begins to heat up.  Inside each motor, there is a thermal breaker which will cut power to the motor when the motor has reached a certain temperature.  A symptom you’re stalling your motors is when just one (or two linked) motors “die” after driving for a long time (or after pushing another robot).
Here are a few ways to fix this problem:
1) Add more motors to whatever you’re trying to lift/turn.  This isn’t always an option, but is effective.
2) Change the gear ratio.  You want to “gear down” the motor – it will spin slower but with more torque (turning force).
3) Add something like elastic or rubber bands to counter-act the weight of an arm.

Breakers in microcontroller/power expander are tripping:
This occurs when too much current is being pulled from a single breaker on the microcontroller or power expander.  The Cortex microcontroller has two 4-amp breakers: one for ports 1-5, and a second for ports 6-10.  The PIC (old controller) has a single 4-amp limit for all motors.  The power expander has a single breaker for all four ports.
This has the same symptoms as a motor stalling, except all motors plugged in to the breaker will turn off – not just one or two.  This is often seen on robots with all four drive motors plugged into the power expander.
Potential fix:
Spread out the “high load” motors, and use a power expander.  I would suggest putting drive motors on ports 1, 2, 9, and 10, and other motors in a power expander plugged into the appropriate ports.  If this doesn’t fix the issue, try some of the solutions for a stalled motor.

Test your robot before the competition!  Drive your robot around for about 10 minutes, in a heavy practice environment.  If nothing breaks, your motors will probably hold up under the strain of elimination matches at a tournament.

Posted in Tutorial, VEX