Sub: Sudden dive on transition from manual to alt hold

[amarburg]

Bug report

See also this thread at the BlueRobotics community forums.

We are working to confirm this behavior with our other BlueROVs.

Testing on a BlueROV2 Heavy running BlueOS-1.4.2 with Ardupilot 4.5.7 (also tested with 4.5.5).

Under a very specific set of circumstances:

1. Vehicle in the water, armed, running in alt hold
2. Disarm vehicle
3. Set to manual mode
4. Arm vehicle (now in manual mode)
5. Set to alt hold mode

The vehicle will descend suddenly by ~1m, then stabilize at a depth slightly below its original depth. The second and third steps are interchangeable (manual then disarm or disarm then manual).

Expected behavior

On all transitions from manual mode to alt hold mode, the vehicle should hold the current vehicle depth.

Issue details

If the sub is disarmed in manual mode, the output "throttle" (vertical control) is set to 0.0, corresponding to full downward thrust. While this value is not used in manual control, it is used as an initial value on subsequent mode changes (e.g. to ALT_HOLD).

Set to 0.5 (neutral, no vertical thrust) instead.

Version

BlueOS 1.4.2, Ardupilot 4.5.7 (also observed with 4.5.5)

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ X] Submarine

Airframe and hardware type

BlueROV Heavy with Pi5 and Navigator5 board

Logs

Logfile attached, see interpretation in the BR Forum post linked above.

00000039.BIN.zip

[Suresh-Krishna-P]

Hello @amarburg ,

I’d like to work on this issue and have outlined a proposed implementation strategy below. I’d appreciate your feedback.

• Root cause: when disarmed in Manual/Acro the code sets the internal attitude controller throttle output to 0.0 (full-down). That stale 0.0 value can be picked up later when switching into ALT_HOLD after arming, causing a sudden dive.

• Fix: set the internal throttle output to neutral (0.5) while disarmed in ArduSub Manual/Acro so ALT_HOLD never starts from a full-down command.

What I can change

• Make the disarmed branch in ArduSub modes set attitude_control->set_throttle_out(0.5f, ...) instead of set_throttle_out(0, ...).

• Files touched (minimal):

  • ArduSub/mode_manual.cpp
  • ArduSub/mode_acro.cpp

Why this is safe

• ModeAlthold already uses 0.5 for the disarmed path; this makes Manual/Acro consistent with that behavior for subs.
• This only changes the internal controller’s stored throttle output while disarmed; it does not alter RC mapping while the vehicle is armed.
• It prevents ALT_HOLD mode from initializing from a false full-down throttle command.

If you’re okay with this approach, I’m happy to open a PR.

[amarburg]

This matches my PR but I forgot to consider other modes. If Acro mode is also affected, then yes, we should change that as well!

[Suresh-Krishna-P]

Thanks for your reply! Feel free to reach out if you need any help or assistance. Looking forward to collaborating with you.

[amarburg]

@Suresh-Krishna-P I've updated my #32057 to include all of the other modes (Auto, etc).

[Suresh-Krishna-P]

Yeah, that feels great.