Unexpected drop of altitude

Hello, today we experienced unintentional hard landing.

While hovering in altitude around 1.5m drone suddenly dropped on the ground at vertical speed about 2.5 m/s according to logs. The pwm input didnt change the whole time during this event. What could be cause of this ? We looked into logs but cant find the reason for this, everything looks normal to us.

Thanks !

Log

If the RCOUT (PWM outputs) did not change, you must have a ESC and/or motor problem.

sorry my bad, i meant PWM input, pwm output did change on all 4 motors

@rmackay9 could you please look into log if you find something that could be the cause ?

Thank you

Your throttle input is channel 1.

Log shows altitude went down because throttle went down. (Throttle in orange, Alt in Blue)

@manavgandhi17 No thats not the moment i was talking about. I did mark the moment when it happend. The one you mean was done on purpose, i stopped descent at arround 0.5m and then gained some altitude as you can see in logs. Then it dropped without change in throttle as you can see

Probably you need to do this:
https://ardupilot.org/copter/docs/airspeed-estimation.html

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All I could tell thus far is that the autopilot decided to decrease that altitude at a rapid pace. Desired and actual altitude match well and desired and actual climb rates both drop to very large negative values. The rangefinder you have installed does seem to go out a couple of times right before the drop - but it also gets excluded from altitude every time that happens.

Barometer and RC-In don’t seem to be the culprits. Anyone see anything different?

I got a similar problem with no error messages and a copter falling down after 10min of flight… After changing ESC, pixhaw and motors I realized that the problem came from my battery which was dead. Check it with the internal resistance (should be < 7 mOhm/cell)

We dont use battery, it have constant power supply through cable so thats not what caused it