BlueROV2 Heavy 6-DOF model

[EliotBR (Eliot)]

Took some time to get back to this, as responding meaningfully to this kind of detailed post takes a decent chunk of time.

@krakjack

The comment in Motors Setup in QGroundControl saying “When a slider is moved down, the thruster should push air/water toward the cable entering the housing” also conforms to the convention that positive input corresponds to forward direction for each thruster. Could you confirm?

That depends what you mean by “positive input”. ArduSub operates at the abstracted vehicle level, where the operator/pilot provides vehicle motion commands in the x/y/z/yaw/pitch/roll directions, and ArduSub converts those into relevant commands that are sent to the thruster ESCs. At the vehicle level, “positive input” corresponds to intended vehicle motions, and the signals that get sent to the ESCs depend on the active frame configuration and flight mode.

At the thruster level, the front of our thrusters is the more rounded side, where the cable comes in. ArduSub’s frame configurations are set up such that a thruster has positive contributions in the forwards direction of the thruster. Accordingly, if a thruster is pointing backwards relative to the vehicle then a “positive input” to that thruster will have a backwards effect on the vehicle motion, so the corresponding thrust factor (in the frame configuration) is negative.

At the ESC level, “positive input” just spins the motor in a particular direction. If that does not match the forwards direction for the thruster (which is what ArduSub intends, with our default frame configurations) then it can either be corrected for in hardware (by switching two of the thruster’s phase wires), or in software (by telling ArduSub to invert the signal it sends to that ESC).

@krakjack

Does the physical ROV pitch up or down in water moving forward without any vertical thrust?

Honestly I’m not certain, because

• we rarely operate without some form of feedback-based stabilisation/depth hold mode on,
• I don’t personally have much experience with a plain BlueROV2 Heavy,
• different revisions and variants (e.g. aluminium vs acrylic enclosures) of the vehicle will have slightly different mass and volume distributions, which moves the centers of gravity and buoyancy, and
• we tune to the water conditions and mass/volume distributions by adding and moving ballast weights on the bottom (e.g. salt water is more buoyant than fresh, so may need more ballast)

@krakjack

As the control input increases beyond 30%, ROV basically becomes unstable and just keeps pitching extremely steeply, almost in place. I wonder if this is the expected behavior and you see large pitch angles on the real ROV with only horizontal thruster inputs for forward motion. Or if my thrust configuration is still messed up and if I am still not clear about it.

I expect this is an issue with the modelling, but it’s possible it’s a model simplicity problem rather than an incorrect application of the chosen model.

It’s worth noting that

• the ROV is designed (with buoyancy at the top, and weight at the bottom) to have a self-righting moment if it pitches or rolls unexpectedly
• there is more drag area near the top of the front face, which would contribute to some amount of upwards pitching force when moving forwards
• if the vehicle has forwards momentum but starts pitching then the top/bottom surfaces would increase the drag, which has a stabilising effect (but the most stable angle then depends on the speed)
• tether drag can also play a role
• fluid interactions between the thruster flows may also be relevant, and with asymmetrical thrusters like the T200s it may be non-negligible that the front thrusters are facing backwards, so when trying to move forwards there is more thrust coming from the rear of the vehicle

@krakjack

In order to figure this out I looked at SIM_Submarine code (I have not run the sim yet) and realized that the model used there is substantially different in many details. Is there any documentation about the model used in SIM_Submarine? Does SIM_Submarine exhibit any such pitch behavior as I am seeing?

I’m not sure - it’s not something I’ve worked with. The only simulation-focused documentation I’m aware of is

• ArduSub S.I.T.L. / Gazebo setup page
• general ArduPilot simulation docs
• BlueSim README

I’m not aware of documentation for SIM_Submarine specifically, beyond the comments in the source code files.

@krakjack

I tried to find out where ArduSub does the motor mixing. Could you point me in the correct direction?

Unfortunately we don’t yet have decent documentation on the ArduSub components of the ArduPilot codebase, although that is something I’m planning to work on.

As a (brief) walkthrough of finding this particular information,

1. ArduSub’s manual mode uses motors.set_{motion}() commands
2. searching for set_roll yields that it’s declared in libraries/AP_Motors/AP_Motors_Class.h, and sets an internal variable _roll_in
3. searching for _roll_in we find that it is used within AP_Motors6DOF.cpp, which is where ArduSub’s thrust factors are defined (for frame configuration)
4. within that file it is used in three functions, the most relevant of which is output_armed_stabilizing_vectored_6dof (because the BlueROV2 Heavy is a 6DoF capable vehicle with vectored horizontal thrusters)
5. In that function, we find the motor mixing code
   • note that it’s a bit unintuitive to follow, because ArduPilot was initially written for vehicles with no direct lateral control, so the extra motions have been somewhat shoehorned into the existing variables