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Ros + Gazebo SITL : rc/override vs cmd_vel?


(Olaya) #1

Hi all,

I want to develop a controller in ROS for the Bluerov2, and I decided to first start developing the code using SITL.

I did the following procedure to start controlling the ROV:

  1. Run sim_vehicle.py from ~/…/ardupilot/ArduSub
  2. Launched gazebo with roslaunch bluerov_ros_playground gazebo_sitl.launch
  3. Launched QGroundControl (because I found it more convenient to toggle between navigation modes)

After that, the next step is to send the ROV the control commands. AFAIK, mavros/rc/override and /mavros/setpoint (I specifically want to use cmd_vel) can be used to do that.

According to ROS wiki:

Send RC override […] Not recommended to use in automatic control because lack of safety mechanisms. Use one of setpoint plugins and OFFBOARD mode.

However, there was a little discussion about this question in this topic , where you stated that:

rc/override -> (STABILIZE/MANUAL) MODE
cmd_vel -> GUIDED MODE

However, GUIDED MODE requires to have a GPS, because as Patrick said:

You are not able to set speed reference without a position system, the software derivative the position to use it as a velocity reference for the control loop.

I guess that cmd_vel works in the SITL but will not work in the real ROV since I don’t have GPS, but, since cmd_vel is more recommended for automated systems, is there any workaround I can do to be able to use GUIDED MODE and thus this topic? Maybe publishing the Bluerov2 Pose in some topic?

Thank you!!

Edit: cmd_vel was not working at first because I had the joystick connected and I guess there were conflict in the commands. However, the same question for the real ROV remains!!


(Patrick José Pereira) #2

Hi Olaya,

You should be able to control the ROV in guided mode with SITL, the model does have a “GPS” integrated, you can check the position information in /mavros/global_position/ and /mavros/local_position/, can you share your code or tell how are you publishing to the cmd_vel topic ?

Just to make it clear, do you want to use guided mode with the real vehicle to control attitude or position/velocity ? Because it’s not possible to control velocity/position without a GPS or a velocity sensor with accuracy. However, it’s possible to control the attitude without a GPS in althode mode.


(Olaya) #3

Hi Patrick,
That was a real quick answer! I edited the post because cmd_vel finally worked, but the question about controlling the real rov still remains!

I want to do an autonomous controller from the computer (without using the joystick), in all the coordinates, not only the attitude.
I know it will not be accurate with the onboard sensors, but first I want to do a naive controller with all the elements I have, and then incorporate extra measurements (i.e. a fixed camera) to improve the accuracy.
Is it possible to use setpoint without the GPS, or do I have to use rc/override by force?

Thank you,


(Patrick José Pereira) #4

Hi Olaya,

Sorry for answering to quickly, hehe.

If you have a camera or any sensor that can provide a position information for the ROV, you can create your own position system for the guided mode, check GPS_INPUT message.
Note: cmd_vel will only work with GPS modes (like GUIDED).


(Olaya) #5

Nothing to be sorry about :wink:

So, to be clear, if I want to use GPS modes without GPS, I can/have send a fake GPS message according to that documentation.
Otherwise, I have to use rc/override.

Where can I see in my ROV the code with the mapping of the rc message to the rov commands? (not sure if this deserves a new topic!)