Harmonic and Notch Filters

1

Hello,

I am getting into tuning larger Quadcopters and have come across a lot of high vibrations in my logs. I would like to implement a harmonic and a notch filter. However, I just want to make sure my thinking is correct on this.

Here is my log:
https://review.px4.io/plot_app?log=f79f2b01-82fe-4844-a41a-4a6508271105

Here is an image of the frequencies

image
image623×710 82.5 KB

As you can see I have a large spike at around 50Hz (what I assume to be from the skids). I was going to implement a notch filter at 48Hz with a bandwidth of 10 to get rid of that large spike.

Is there any other changes I should consider regarding IMU_GYRO_CUTOFF or IMU_DGYRO_CUTOFF? I am not a controls engineer so I am a bit of a newbie on these things.

Thank you!

2

@bresch @bkueng Is this something you can advise on?

3

For context, this is an acecore Zoe I am tuning for PX4

4

From my understanding of the filtering process, you are already filtering most of that noise. Your IMU_GYRO_CUTOFF and IMU_DGYRO_CUTOFF are lower than those spikes.
Only if you wanted to move IMU_GYRO_CUTOFF and IMU_DGYRO_CUTOFF to higher values, let’s say 80 and 70 Hz, would make sense to notch filter those spikes. But I don’t think that makes much sense for larger quads, as they don’t have to be very agile.
Or if you had a spike at 20Hz, in which case you don’t wanna low pass filter that low, as that would probably make your quad unstable and hard to tune, you could use a Notch filter .

But I might be wrong, in which case I too am following the thread for some better info.

5

I was thinking about using a notch filter, but someone told me it would be risky given the large band of the spike. I am assuming that I am seeing this because the downward wind from the props is hitting the skids of the UAV

6

For vibrations particularly, I feel that they are the prop-arm vibrations. This happens when air crosses the drone arms while props pushes air downwards. Although this is a common behaviour I observed with 20-22inch prop frames. I guess you can proceed with the notch filters and try taking a flight and then you can compare the results. I also have the similar frames and tuned it with the notch filters if that gives you a push :slight_smile:

1 Like
7

That is very helpful, thank you! Yes I am going to add notch filters and see how things turn out while tuning

8

I applied a notch filter at 47 frequency and a band of 8, things are looking much better. although my vibrations are still higher than I would like them to be

9

I will add some documentation regarding this with larger quads

1 Like
10

I can have a look at the log file and maybe can tell you something.

11

All Set Aman, thank you though!