Convert racer joystick rates to PX4 acro rates

[MaEtUgR]

I think it would be useful to support racers who come from a different FC to test PX4 and let them convert their existing rates in cleanflight betaflight fork parameters described here https://oscarliang.com/rc-roll-pitch-yaw-rate-cleanflight/ to our better tunable solution.

Like I promised here
#9358 (comment)
#9358 (comment)
I provide the calculation script and it would probably be easiest to just run it on a website but best to include the functionality in QGC.

FYI @bkueng

[dagar]

That's a great idea.

[MaEtUgR]

I realized existing FCs also have differences and I found an existing easy to use open source tool which already converts between them and visualizes the result: https://apocolipse.github.io/RotorPirates/

I could add PX4 rates and we can link to it in the documentation as soon as I understand where all the calculations are done: apocolipse/RotorPirates#4 That's more future proof because it can directly be maintained if anything changes for any FC in the future.

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[MaEtUgR]

The maintainer of the value converter repo finally answered me now and I'll add the PX4 stick mapping to https://apocolipse.github.io/RotorPirates/ in a pr.

[MaEtUgR]

PR for this now here: apocolipse/RotorPirates#7

[LorenzMeier]

Very cool!

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[MaEtUgR]

Recently I looked into the rate calculation of betaflight and it's in theory slightly different because they use an x^4 term as expo instead of x^3 like we do. But it's in practise totally different because they still use these totally unintuitive configuration parameters that all depend on each other...

I think a first step is to write both formulas down clearly and then figure out an approximate coefficient transformation.

[MaEtUgR]

PX4
https://github.com/PX4/Firmware/blob/6d9bf18ca6e1ef4c3a721832398cae72385bf360/src/lib/mathlib/math/Functions.hpp#L61-L95
y = RateMax * (1 - Expo) * x + Expo * x * x * x * (1 - Superexpo) / (1 - abs(x) * Superexpo)

Betaflight
Here's the betaflight rate formula that we should be compatible to in some documented way: https://github.com/betaflight/betaflight/blob/ecb67fcc097d284db706149bdeefaa40b17ce921/src/main/fc/rc.c#L120-L138

The extracted formula without corner case handling is:
y = 200 * rcRate * (x * x^3 * Expo + x * (1 - Expo)) * 1 / (1 - (abs(x) * (SuperRate / 100.0f)));

So we need to convert the scaling and the expo to approximate x^4.

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[junwoo091400]

This is quite cool! And maybe worth keeping it open 🙌