Erroneous RCOU command in Auto mode

We have been flying this bird for a good amount of time. But in one of the auto mission, it was observed that the RCOU value dipped to 1100 which was not required as the bird was flying straight and level. Subsequently, to counter that RCOU for motor 4 increased, leading to a crash.
As per my understanding, if a motor/ESC is not performing well, the RCOU value should increase in order to counter for less thrust. But it wasn’t the case here.

What could be the reason for this crash?
Following is the log file for the same.

Motor4 stopped, ap tried to compensate.
Bad ESC or connection.

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I hope you noticed. Before Motor4 gave up, RCOU gave a low signal to Motor3. I am not able to understand the reason for that. Under what conditions would that happen?

This is a tricky one.
Usually Andras would be perfectly correct - the motor that hits 100% demand is usually the one that suddenly stopped producing thrust.

In this case I think there’s multiple issues going on.
First it’s like weight shifts (maybe) and something mechanical goes wrong, like an arm coming loose or maybe a motor. Vibrations increase significantly.
The resulting roll and pitch just happen to cause both M3 and M4 outputs to go very low to compensate.
Motor3 seems to be at minimum output for a while, loses sync and doesnt recover for quite some time (in relative terms), thus it is commanded to 100% output since it is now producing no thrust.
Motor 4 recovers sooner (or doesnt actually lose sync) and starts contributing to thrust again.
Vibrations go to crazy levels, there’s definitely things broken.
Motor 3 has recovered sync and all motors are trying to contribute thrust, but with some breakage and increasing ridiculous vibrations there is no hope of stability or control

Current also goes to 4 to 6 times it’s hover value of 20 amps - this is just a side effect and it quickly drops at the end. There’s no indication of a short circuit.

Actually since those Motors 3 and 4 reduce output, lose sync and all the trouble starts then, I’m wondering if wind got under them and caused the desync. That doesnt explain the vibrations though, unless those motors and ESCs never completely recovered sync. A breakage is still the likely root cause, with desync being secondary.

What motors, ESCs and prop mounting method do you have?

There’s actually quite a bit more tuning that could have been done to improve the stability and resistance to disturbances.
If you get it rebuilt and ready for operation, yell out and we will help.

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Thank you for such a detailed explanation :slight_smile:
I am using T motor Air 40 amps esc with MN 4014-9 400 KV motors.

I did check the vibrations. But it seems the vibration came only when the drone flipped and started falling from the sky not before. Same is the case for current consumption. Also, the vibration wasn’t visible on the primary and secondary IMU at the initial stages.
Does the drone use all the IMU for its stability? (How do I check that?)
If it’s using all three then I think something was loose or broken during the flight.
I am just trying to be curious to be able to troubleshoot this issue.

Thank you once again for your reply.

First of all it was all motor3 and after deeper look, it still seems a failed motor3.

There is 1.5 second before motor3 goes max, when the PWM value slowly decreasing, but no change in pitch/roll/yaw and no change in current/voltage either. Looks like that the motor spun up then stopped. (I have no ide what kind of electrical or mechanical error can cause this, but it is definitely a motor/esc failure)

I think the motor did not recovered, when the max output ends at 20:11.829, when the IMU registers an impact, anything after it (including vibration) happened on the ground.

You are probably right Andras - thanks.
I thought I checked the altitude compared to vibrations, but it was getting late so I probably messed things up a bit :frowning:
I would be concerned about the ESCs if they are prone to losing sync and not being able to recover.

You are right. I also noticed the same issue. Couldn’t understand the circumstance under which the motor would spin up without the need for doing it.
Hence I posted it to understand if someone else has noticed such an occurrence before.
More important would be how we avoid such failures.
What could be best practices to increase the reliability of the system?

  • Buy better components.
  • Check for motor/ESC overheating after every flight
  • Use ESCs with telemetry.
  • Add temperature sensors to the motors
  • Control the propellers for defects before every flight

Thank you for the response.
Your points are valid.
I am using T motor ESC and Motor. Do you think it’s not the best?
Did you face issues with them in the past?
What other motor/esc would you recommend.

There are cheap T-Motor components and not so cheap T-Motor components.
You get what you paid for.

BTW can you complain to T-Motor so that they release the “T-Motor Datalink device driver for ArduPilot” to the general public?

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I think their ESCs are based on ST-SPIN. I think if they don’t release it, either we should plan on flashing the ESCs with different firmware or not use the brand.
I think something like APD ESCs should be on recommended list.

I am not asking them to release the firmware that they use on their ESCs.
I`m asking them to release the source code of the device driver that runs on ardupilot to interface with their “Datalink v2” hardware!!

Please ask them to do that. The more people ask the better the chances they do it.

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