Hi guys, my APM has recently started to reboot mid air for no apparent reason. This first happened on the 9th March when I was flying in stabilize mode and all the controls just froze (no response to RC tx, servos all locked, telemetry reporting no data) for a couple of seconds while the plane slowly spiral downwards and eventually it started responding again but it was too late. The plane ended up being stranded on the tree and this was when I caught a glimpse on my laptop saying it's in "Initializing".
Later that evening when I downloaded the logs via USB, the 2 separate log file confirms a mid air reboot and the starting of a new file. I couldn't figure out the problem and for some reason, have a gut feeling it might be the 900Mhz 3DR radio so I swapped it out with a spare. I'd like to attach this tlog or log file but couldn't find it for some reason.
2 days ago after flying both manual and auto without any problems for about 20 mins I decided to take the plane down as it was getting a little dark. I switched to stabilize and slowly brought the plane down and that's when APM rebooted again. This time it was close to the ground and controls freezing only caused a rougher landing. Attached is the tlog for this particular flight. and it's evident the flight ended abruptly (probably because the radio stopped transmitting when APM resets itself).
I've read through brownout theories causing the reset but I have a 5A UBEC and Vcc plot confirms there is no dip in apm supply voltage. Not sure what else could cause this...
Can someone please have a look at the attached tlog and shed some light on this?
*******Post updated with new tlog I can't find from my first flight********
I did this zoom in on the trace for Vcc and the servo outputs (Log 18/3/2013). As soon as you switch to stabilize and all the servos start working together Vcc starts going up and down like crazy. Your BEC just doesn't have enough power to hold Vcc with all those servo changes at once. This is good example why having the supplies separate makes so much sense. Are you using the APM2.5, if so buy an APM Power Module and get Vbatt and Ibatt measuring as well :-)
According to the data sheet of the power regulator on APM, it's about 3.3v + 90mV = 3.39v. That's how low you can get theoretically before cuts out. Of course you want to keep it between 5-6v though :)
After extensive testing and discussion with a fellow FPVer/EE engineer, we've decided the quickest/most efficient/no-waiting-for-delivery and presumably safest modification is just to split the power rails via the jumper and run all the servos off that UBEC. APM and RX will run off the ESC linear regulator since they only consume minimal current at a constant rate. Linear regulators are also more tolerant to capacitor banks that I will install on that 5v rail.
I'm also really surprised that the current consumption peaked at approx 7A just by moving all the servos really quickly. I always thought it's less than 2A as tested by the multimeter. Seems like I'm running the UBEC close to its limits too...
Yes I made this decision years ago on all my APMS, its just an overall easier way to deal with this situation. And the added weight of an extra battery is moot in most aircraft if you decide to utilize more then one, which I do to keep things separate.
Multimeters are usually not able to show current spikes. If you have one that is a bit fancy, it might have RMS measurement and a min/max feature.
If you read through the linked threads, you'll find the details. I don't know off the top of my head.
yeah John I suppose I've just learnt it the hard way :) it shows only the averaged current