Flow sensor mounting offset

Discussions about autonomous flight in general, regardless of positioning method
Post Reply
Johannes214
Beginner
Posts: 13
Joined: Tue Nov 03, 2020 12:52 pm

Flow sensor mounting offset

Post by Johannes214 »

Hey,

I am making bigger quadcopter around bolt and I want to use the flow sensor with it.

As stated here in the Mounting/Orientation section, flow sensor should be in the middle of the drone but the battery is there. Can I mount the flow sensor away from the center, measure the amount of offset from the center, and add that offset value to estimator somehow? If this is possible should I take something else in to consideration? Here was stated about alternative mounting, I don´t know what that flip motion means.

Thanks in advance!
kimberly
Bitcraze
Posts: 704
Joined: Fri Jul 06, 2018 11:13 am

Re: Flow sensor mounting offset

Post by kimberly »

Hi!

We had to reverse the detected flow because the motion detector is mounted in the 'wrong' orientation of the flowdeck of the crazyflie. So the x and y motion detection were in reverse according to the coordinate system of the crazyflie. Unfortunately for what you are describing you need more than that.

For the offset, currently we don't have anything in the code that enables an offset since it is already in the middel of the frame. You would need to change the measurement model in the EKF, for the flowdeck that is found here: https://github.com/bitcraze/crazyflie-f ... ore.c#L462

The complication though, is that there is some gyro compensation done for the flow detection. If the flow deck is not perfectly centered, there is also the height change considered as well for which only the gyro compensation might not be enough anymore.
Johannes214
Beginner
Posts: 13
Joined: Tue Nov 03, 2020 12:52 pm

Re: Flow sensor mounting offset

Post by Johannes214 »

Thanks for quick response Kimberly!

That sounds troublesome so I will try to place the flow sensor to the middle :D
there is also the height change considered as well for which only the gyro compensation might not be enough anymore.
What did you mean by this? Does it mean that if the flow sensor is lower than the flight controller (for example 10 cm if flow sensor is under the battery and bolt is on the drone) then more modification is needed? If that is so maybe I should consider then placing the flight controller with the flow sensor to the bottom of the drone and placing the battery above the drone...
kimberly
Bitcraze
Posts: 704
Joined: Fri Jul 06, 2018 11:13 am

Re: Flow sensor mounting offset

Post by kimberly »

Technically the ideal position is to be as close to the rotation point of pitch, yaw, and roll as possible if you want it to not do many changes to the kalman filter's measurement model.

Ofcourse sometimes this will be difficult to do, then you can try it out and see if it still will be okay, maybe 10 cms is still okay.

Or else you can check how you can change the measurement model. There is a small explanation of the flow's measurement model here where you can also find the original thesis which implemented this. It will basically boil down that you will need to change the measurement model of the x and y direction of the flow.
Post Reply