Hello all!
Our team participated in a pre-inspection event this year and were shocked to be told that our robot had isolation issues (pun intended). The resistance was very high and the robot was fully functional however it did not pass inspection due to this. We proceeded to tear apart and reassemble our electrical box over and over again to find the error and spent a good day figuring it out. In the CTRE Talon SRX manual they specify that the casing of the device is electrically isolated from internal components, however we found that 3 of our 8 Talons on the robot had their CAN wires shorted to the casing. I have taken apart a couple Talons and they should be isolated but the solder joint for the data wires is a little close to the edge which caught my attention. The Talons still operate completely as expected but are not easily implementable into our robots unless extensive isolation mounting practices are used.
TL;DR Check the CAN wires and the casing of your Talons for continuity if you are having isolation issues.
We aren’t using pneumatic this year but we will definitely look out for that. I’d love if people posted the causes of their isolation faults here as an archive of known issues in case it isn’t a Talon next time.
It’s things like these that drive us to make our electrical board out of polycarb every year. It’s not really an “extensive isolation mounting practice” if it’s just part of your normal process year after year, and it really does help to prevent these sorts of issues.
We verified the issue with several multi-meters. A couple belonging to the official inspector and a few of our own. We have not reached out to CTRE yet but are planning to request service or replacement of our affected devices.
Resistance to chassis really doesn’t have to be that high. Only 3K (r46). The majority of chassis continuity issues I have seen have been custom circuits and cut wires. If you have it down to a motor controller, isolate the motor to sure that is not the issue. The brute force way to find the issue is to start disconnecting both the plus and negatives for each circuit from the PD board until the issue goes away
A frame short to a CAN signal will not cause any problems. Nor will a frame short to battery positive terminal.
But if you ever get both of those at the same time, say goodbye to your CAN bus. Momentary problems with wiring happen with surprisingly high frequency. If something is constantly connected to the frame, it is likely that an otherwise trivial glitch later will result in a nonfunctional robot, if not an outright release of magic smoke.
Caveat: Not really direct application to OP’s issue, but this is what we found works for us:
If you have isolation issues, the first thing (I’m assuming the battery is already disconnected!) to disconnect is power connector to the RIO. This will isolate 30% to 60% of the wiring from the battery terminal on most of the robots I’ve seen, simplifying the rest of the process one way or the other.
If the fault continues after this, note that pulling fuses/breakers is not particularly effective; unless the fault is in the mains or PDP, the short will still lead back to the negative battery terminal. Remove one or two wires from the PDP until the issue goes away, and fix the last thing you removed. Then add things back one or two at a time and re-test as you go, in case you had multiple issues.
Once any remaining fault(s) have been isolated to something passing through the RIO, disconnect one or two sensors, PWM wires, or the CAN bus at a time, and continue analogously to the power situation above.
Edit (obviously):
It was something similar to this (perhaps exactly this) that made us start mounting most of our rangefinders and other electronic sensors with dual-lock, and use rubber washers when that would not be secure enough.
Just a though on a problem we had.
We were using a Sharp IR sensor, and it turned out after much searching, disconnecting, and so on, that the PLASTIC housing of the sensor was conductive! :ahh:
It was screwed into the frame, and thus, we had a leak that was around 1000-3000 ohms.
I measured it off the bot and it was definitely the sensor body.
I just wanted to throw that out since it took us 2 hours to find it, and then mount it on top of electrical tape with nylon screws.
This is surprising, and makes me suspect there is something else going on with your robot. I recommend you contact our support to figure it out. This is not common.
The pads are far enough from the case border to ensure this doesn’t happen. Likely something else is going on. And why did you void your warranty?