Drone is a 21kg hexacopter with 24in biblade propellers, motor freq at approx 50Hz (can confirm with future logs that have RPM data). There is a third large peak at around 150Hz frequency and I don’t understand what the source of this is or where this is coming from. My first thought was it’s a motor harmonic but I don’t believe harmonics are typically stronger than the fundamental frequency.
Could it be frame resonance amplifying the motor harmonic or am I overthinking this? I will leave the log file below.
The harmonics can certainly be greater amplitude than the primary, and the 150Hz peak will definitely be a harmonic of the primary motor frequency. As you suspected it’s because of resonances in the frame and possibly the flight controller itself.
The trick is not to select too many options or filter too aggressively or other issues can be introduced. Look at the post-filter output to decide if the extra harmonics even need filtering applied.
From your log the primary and harmonics hardly have any noise getting past the 20 Hz INS_GYRO_FILTER. This is what the gyro filter is doing without any other filters, so you can see why the extra harmonics have decreasing post-filter output despite having substantial input.
2nd harmonic is not actually harmonic but fundamental frequency because you have 2 blade props and blades pass 2 times over arm each rotation, that’s why you get it stronger than just motor RPS.
@xfacta Appreciate the quick response and the filter suggestions. I probably must have mentioned that the drone carries a dynamic payload (it’s an agriculture drone) which is why I did not go with throttle based filtering. But you are right, I probably don’t need any notches at all.
Just out of curiosity, during filter tuning if I can keep my noise below -50dB and get a lower phase delay on my control signals by using notches and dialing back on LPF, I go for it right?
@mike_tk I’m not sure I follow. I believe that the tiny peak at 50Hz is my motor harmonic and the large one next to it is the Blade passage frequency (+motor harmonic?) at 100Hz. The 150Hz can only be motor harmonic coupled with resonance right? Or am I far off here
Motors usually do not produce any major noise at rpm frequency, peaks you see are the air pushed by props that interfere with arms, that’s why it’s 2x rpm frequency because you have 2 blade props, with 3 blades 3xRPM is strongest. 150hz and 200hz are actually a real harmonics for 2 blade props, but it’s high frequency and usually attenuated enough by lpf.
