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1) - I suggest you look into an IMU to get 'odometry' data. It will provide you accel and turn rate info you can integrate into velocity, twist. I think there are IMUs products that will actually do the integration for you and can output relative position data.There are filters that will allow you to combine the IMU data with GPS data to give you more precise location data. The IMU will account for things such as wind and current of water, etc.

2) - Yes, move_base is exactly a controller on top of the planners, for better or worse.

3) - The output of the local planner is the heading, but there is a controller that takes the heading and attempts to drive that way. Now to get the boat to do what move_base is asking, you'll need to implement an additional layer of control that uses IMU in a feedback loop to match the requested velocity and turn rate, and I expect that to get decent performance you'll need to add a layer of feed forward to that additional control loop.

The local planner publishes the trajectory of the highest scoring trajectory from the last planning loop. So it's possible for you subscribe to that if you want and develop a custom control loop fit for your application.

~<name>/global_plan (nav_msgs/Path) The portion of the global plan that the local planner is currently attempting to follow. Used primarily for visualization purposes.

~<name>/local_plan (nav_msgs/Path) The local plan or trajectory that scored the highest on the last cycle. Used primarily for visualization purposes.