COM_THROW_SPEED - arm or start motors clarification

[hamishwillee]

This is a change to request clarification of the term "arm" as it is used in the slow throw mode.

The process to launch in throw mode is to arm the vehicle (send the arm command/switch), then throw the vehicle. The motors only spin when you exceed a speed and then enter free-fall.

Now normally "arm" means "power the motors and they start spinning", but there it means like a prearm state - motors are powered but they don't spin. The problem is that in this param we use arm to mean start the motors spinning, but at this point we have already sent the arm command so the vehicle is "armed".

I'd like to be precise because the term confused a new user in https://discord.com/channels/1022170275984457759/1022186188255285399/threads/1295828271186972742

@sfuhrer @MaEtUgR How does this work? My guess is that either there is an extra hidden state that we enter like "pending armed" when we get the arm command and then we arm on the freefall trigger. Or it may be that we arm on the command, but there is an extra interlock on the motors.

[bresch]

@hamishwillee The vehicle is really armed but in lockdown state when throw launch is active.https://github.com/PX4/PX4-Autopilot/blob/371a99c3221dd09dce0b218c45df405188d96cfd/src/modules/commander/Commander.cpp#L1894-L1896 When the throw launch state machine passes all the steps (velocity followed by freefall), the lockdown is removes and the motors immediately start. Also, there is a special condition to make sure the drone doesn't auto-disarm when armed, in lockdown and throw launch is active.

[sfuhrer]

@hamishwillee also fixed-wing launch works similarly: you arm the vehicle but only after throwing it the motor ramps up. In general I would define the "arming" state to be "vehicle's motors can start moving anytime without further user inputs".

[MaEtUgR]

If I'd need to implement this I'd probably go with idling the motor(s) before the throw such that's it's very obvious what's going on.

[hamishwillee]

@sfuhrer @MaEtUgR Thanks. I did some very minor clarifications of these points in https://github.com/PX4/PX4-user_guide/pull/3419

f I'd need to implement this I'd probably go with idling the motor(s) before the throw such that's it's very obvious what's going on.

It would be nice to see that on FW too. Can you know exactly what output will start a prop spinning, but still low enough not to be dangerous?

[sfuhrer]

@sfuhrer @MaEtUgR Thanks. I did some very minor clarifications of these points in PX4/PX4-user_guide#3419

Cool, I've dropped a mini-comment.

It would be nice to see that on FW too. Can you know exactly what output will start a prop spinning, but still low enough not to be dangerous?

I think it's rather dangerous to distinguish between low spinning and high spinning props, every motor that could start spinning at and moment should be considered dangerous.