Fuse in cRIO is blown.

[mikets]

While we were tuning our PID control constants, the cRIO stopped working (no power light). We finally figured out the 2A surface mount fuse inside the cRIO is blown. Fortunately, we have a backup cRIO, we quickly replaced it and tried several more runs and the 2nd one is blown too. Did anybody experience this? We couldn't figure out why the fuses blew. Visually, I don't see any metal debris in any of the slots. We've been driving the robot hard and nothing blew until we did the PID turning constants calibration. In both times, the robot was turning back and forth vigorously (PID ringing). I would think the cRIO can take this kind of motion. I am going to get some replacement fuses tomorrow but I am a little concern what actually happened. Because it could happen again if we don't figure it out. Did anybody have similar experience and figured out what happened? There are several theories but none make sense. One concern was the back emf from the motors somehow got into the power bus of the cRIO. I can't believe this because I would imagine the digital side car and the cRIO should have voltage regulators, diodes and capacitors etc to guard against power contamination like this. Another theory was the violent left and right turning motion physically breaks the fuse. Can't believe that either. Of course the most make sense theory was something shorted while the robot is shaking, but we can't find anything visually. I will take a much closer look again tomorrow. But I am running out of ideas to look.