CA_ROTOR{i}_CT Documentation is unhelpful and maybe incorrect

[udalmia94]

In the parameter reference the documentation for the CA_ROTOR{i}_CT or CA_ROTOR0_CT, CA_ROTOR1_CT etc. is as follows:

The thrust coefficient if defined as Thrust = CT * u^2, where u (with value between actuator minimum and maximum) is the output signal sent to the motor controller.

This is unhelpful for the following reasons:

1. Does not mention units - this is not a full non-dimensionalization of thrust, therefore units are important here, an example of potentially valid units here would be N/"PWM"^2.
2. Is not clear what the range of u should be - I think I understand, but an example would go a long way here to clarify.

Also, I found evidence in this PX4 issue report and in the Advanced Tuning Guide that this description is incorrect.

Would be super helpful if someone in the community could clarify how to correctly calculate the CA_ROTOR{i}_CT parameter and update the documentation at the same time.

[hamishwillee]

@udalmia94 The docs are generated from the source here: https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/control_allocator/module.yaml#L200-L202

The result is a normalized value between 0 and 100.

Suggest you post an issue in that repo, since that is where code issues go and cross link to here. Even better, perhaps a PR for a proposal for a fix there. If you cross link here I will see if I can get some eyes on it.

[udalmia94]

Suggest you post an issue in that repo, since that is where code issues go and cross link to here. Even better, perhaps a PR for a proposal for a fix there. If you cross link here I will see if I can get some eyes on it.

Cross post

• [Bug] CA_ROTOR{i}_CT Documentation is unhelpful and maybe incorrect PX4-Autopilot#24388

Once I get a proper answer to what exactly the parameter should be, I will be happy to make a PR.

Thank you.

[udalmia94]

Sorry accidentally marked as closed. @hamishwillee can you please re-open?