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.
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.
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.