After measuring our chassis isolation to ensure we would pass inspection at our upcoming regional, we discovered a low resistance (~400 \Omega) between the negative PDP terminal and the chassis. Further troubleshooting narrowed down the problem to one of the Neos on our drivetrain. After verifying that the mounting screws weren’t too long and shorting to the armature, we removed the motor from the drivebase. Bench testing indicated a near-short from the black input lead to the case, that varied somewhat when the shaft was turned by hand (the resistance was always in the hundreds of Ohms). The motor will be returned to Rev for an autopsy.
Remember to verify your chassis isolation! Better to deal with it now than at competition.
Thanks for the PSA after finding the root cause!
I’m sure this could be helpful debugging information to many teams as they have their first inspections.
Yes, far better to find it at home than on the road. We had a similar issue with the integrated encoder of a NeveRest that powered the scoosh (gear pusher) in 2017. In solving this, the procedure which found the short was to keep the meter in place, then start pulling breakers and fuses one by one from the PDP until the short clears (don’t put them back), or you run out of breakers and fuses. Then you’ve isolated the problem to perhaps a tenth of your robot wiring. In our case, it was the RIO fuse, indicating the issue was inside the RIO or down a signal line somewhere.
I’d always start with the RIO fuse. Sensors are often the cause of a short to frame, so by pulling the RIO power, you can essentially start by eliminating half your wiring, then go from there.
Unfortunately, the low resistance we discovered was between the negative terminal and chassis, which wouldn’t be affected by pulling fuses.
In that case, I’d pull the wires
But if you pull every breaker and fuse and still have the issue, you know it’s between the battery, SB50s, main breaker, and PDP. That’s still a good start on a win.
Not if it’s on the negative side. The breakers and fuses only interrupt the positive bus. Even with all of them removed, the black outputs of the PDP are still connected to the negative terminal.
OK, start pulling wires out of the PDP in that case.
Yep, that’s what we did. We eventually removed the return for one of our drivetrain Spark Maxes from its PDP terminal, and found that the short disappeared. We then replaced that wire and disconnected the Neo from the Max, and the short disappeared again. That’s how we narrowed the issue down to the Neo.
Combining my experience with @Jon_Stratis and @CarlosGJ’s, here’s my proposed procedure. In every case, if R decreases significantly, check the circuit just disconnected to find a fault. After R is cleared, reconnect in essentially reverse order and find fixes if any of the reconnects result in a significantly reduced resistance.
- Monitor the resistance throughout
- pull the RIO power plug. If R increases, it’s almost certainly a control signal short.
- Pull each of the breakers and fuses (other than the RIO) from the PDP one at a time. When the resistance increases, the issue is likely at least partially in the circuit just disconnected.
- Pull each of the black wires from the PDP, again one at a time. If R increases, check that circuit
- If all breakers and all black wires have been pulled, the problem is somewhere in the battery->main breaker->pdp->battery circuit.
More or less. Since you need to check the resistance from both positive and return terminals to chassis, you can narrow it down a bit. If you see a short from positive to chassis, go to step 2. If you see a short from return to chassis, skip to step 3.
We had a NEO550 go bad in this fashion on us. Another team at granite state had a full size NEO go bad on them in this fashion.
You could pull either motor off and measure ohmage between leads and shaft- sure enough, on the order of 77kOhms.
Has anyone disassembled an affected motor? I’d be curious to see what is shorting
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