Prevent TECS throttle from exceeding FW_THR_MAX in underspeed mode

[supergra]

When TECS goes into underspeed mode, it sets the throttle to 100%, instead of respecting FW_THR_MAX.

This is obviously quite dangerous if people rely on FW_THR_MAX to keep their motor in a safe operating level.

We've been flying this one-line fix for over a year now.

Fixes #12716

[supergra]

More SITL segfaults:

[2020-07-14T01:11:35.318Z] PX4 Exiting...
[2020-07-14T01:11:35.318Z] Exiting NOW.
[2020-07-14T01:11:35.585Z] terminate called after throwing an instance of 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::lock_error> >'
[2020-07-14T01:11:35.586Z]   what():  boost: mutex lock failed in pthread_mutex_lock: Invalid argument
[2020-07-14T01:11:35.843Z] 6714, 3.328000000]: VER: 1.1: UID:                 4954414c44494e4f
[2020-07-14T01:11:35.843Z] [ INFO] [1594689027.230065982, 17.324000000]: WP: mission received
[2020-07-14T01:11:35.843Z] [ INFO] [1594689030.589104682, 20.552000000]: WP: mission sended
[2020-07-14T01:11:35.843Z] [ INFO] [1594689032.706607878, 22.612000000]: FCU: ARMED by Arm/Disarm component command
[2020-07-14T01:11:35.843Z] [ INFO] [1594689032.758154316, 22.664000000]: FCU: Takeoff to 18.0 meters above home.
[2020-07-14T01:11:35.843Z] [ INFO] [1594689033.734267853, 23.632000000]: FCU: Takeoff detected
[2020-07-14T01:11:35.843Z] Segmentation fault (core dumped)

Compare #15300, not sure if it's the same

[dagar]

This has been proposed before, but @priseborough was opposed to it. PX4/PX4-ECL#563 (comment)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Thank you for your contributions.

[MaEtUgR]

Is this problem still present on main? @sfuhrer @tstastny There were plenty of TECS changes recently.

I'm asking because it again came up here: https://discord.com/channels/1022170275984457759/1031677385625583627

The commit would need to be rebased to somewhere close to

PX4-Autopilot/src/lib/tecs/TECS.cpp

Line 262 in ccd90ed

void TECS::_update_throttle_setpoint()

but I'm not familiar enough with TECS (yet) to tell if it was resolved or where the same part is now.

[tstastny]

@MaEtUgR @supergra no longer an issue. underspeed logic was changed in #19869

and throttle is now ramped to throttle max here:

PX4-Autopilot/src/lib/tecs/TECS.cpp

Line 349 in 870229e

throttle_setpoint = _percent_undersped * _throttle_setpoint_max + (1.0f - _percent_undersped) * throttle_setpoint;