1726 Electric Problem

[dtengineering]

We had some odd electrical problems in Oregon this year. Fortunately, most of them occurred on Thursday.

The first one was due to the RC main power spade connector ground being loose. This was due to an incident during build where the spade connector was pulled out of the RC. We thought we had it soldered back in well, but apparently we were wrong. We re-soldered it (just from the top this time) and it has held so far. We re-check it regularly now.

The second was due to a poor connection between the battery and the 120A circuit breaker. This resulted in our main breaker getting hot and tripping. The key that it was a poor connection (that we eventually clued in to) was that the stud on one side of the 120A breaker was hotter than the stud on the other side. The challenge in diagnosing this may have been compounded by loose connections in the Rockwell block, as mentioned in a post above, as we found a few of them needed an extra turn of the screwdriver or two.

We are fortunate to have a clamp on ammeter, as that allows us to easily monitor current flow in the motors/circuits. (If anyone is thinking of buying one... make sure you get one that does DC current as well as AC.) Ours has two settings 4A and 80A, so at maximum load we can only monitor the current to one CIM. Using the ammeter we were able to recreate medium to high current draw situations in the pit by having a couple of team members kneel in front of the robot and draw the bumper up tight to their legs. The driver would then gradually add "forward" to the robot until the motors got up to about 40A (in stall) and hold it there for about ten or 20 seconds. Another option would be to drive the robot up against a wall and push for a while. Again, it is nice if you can monitor current draw for at least one motor so you have an idea of how hard you are pushing the motors/electrical system.

We discovered (technically these probably aren't discoveries to the rest of the world, but they were enlightening to us) that with two CIMs hooked up to a toughbox that when the wheels run freely, one CIM would be drawing 5A and another would draw pretty close to zero. Differences in the motor? The Victor? We haven't figured that out yet, but the discrepancy seems disappear as the load is increased.

We also discovered that the eventual solution to our wiring problem, although obvious in retrospect, was the very last thing we checked... even going so far as to pull a gearbox apart to check if anything was wrong in there. (Okay, we just pulled a motor and peeked in... but we were most definitely looking in the wrong place.)

We also had a weird situation where the robot died during a match... but came back to life after hitting "robot reset". This is one of the advantages of experimenting with buggy code... the drive team gets to know the location of the "reset" button very well! If you conk out in the middle of a match, give it a push... you've got nothing to lose and it might just get things going again.

Good luck getting the wiring sorted out, intermittent wiring problems are a real pain and it is a shame they held you back. Mind you, it still looks like you shared a pretty good playoff run with a couple of rookie teams. Well done, and good luck,

Jason