How to retrieve the desired.velocities from JointTrajectoryController

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In addition to what Mike writes here, I noticed this:

My actuators need velocities

if that's the case, then I'm wondering why any position interfaces have been registered -- unless the actuators support both position and velocity control modes. If the latter, the type of controller would indeed determine which type of actuation signals your RobotHW will receive.

Note that there is also support for PosVelJointInterface, which would be used for joints which require both a position and a velocity.

And to clarify:

My controller is of type position_controllers/JointTrajectoryController.

this is a JointTrajectoryController which outputs position commands.

The velocity_controllers/JointTrajectoryController is the same JointTrajectoryController, but this one only outputs velocity commands.

(and for completeness: there is also a pos_vel_controllers variant, and some closed-loop versions)

Yes I also think I misunderstand how hardware_interface works. I think the documentation is not really beginner friendly. Anyways.

I also thought I was just going to get position commands since I was just using position_controllers. What confuses me the most is the values I get from "desired.velocities" in the controller state topic. ros_control seems to be calculating some desired velocities. Where are these velocities coming from? And how do I access these?

I updated my question a little bit to provide better context.

@gvdhoorn I thought MoveIt was only meant to be used with position_controllers/JointTrajectoryController. I tried getting velocity controllers to work with MoveIt but I was unable to.

I also thought I was just going to get position commands since I was just using position_controllers.

well, you are, aren't you? You write yourself:

If I read the position_cmd array registered with the PositionJointInterface I see that it corresponds to the "desired.positions" value in the "/joint_controller/state" topic

The values you find in the state topic are for debugging only. They provide insight into internal state of the JointTrajectoryController. The 'type' of the controller determines what it will output to your hw iface.

if you register both position and velocity interfaces, you can run either position or velocity based controllers on top of your hw iface. But position controllers are not going to give you velocity commands (as you already observed).

If in fact, you only require / support velocity commands, remove the position interface from your hw iface, and load the velocity_controllers/JointTrajectoryController.

I thought MoveIt was only meant to be used with position_controllers/JointTrajectoryController. I tried getting velocity controllers to work with MoveIt but I was unable to.

MoveIt is not involved here.

MoveIt controllers != ros_control controllers.

All variants of the JointTrajectoryController expose a FollowJointTrajectoryAction interface, which is what MoveIt can use.

Changing your ros_control configuration does not need updates on the MoveIt side (in this case).

I tried getting velocity controllers to work with MoveIt but I was unable to.

you may be confusing input with output.

MoveIt cannot generate "velocity trajectories" for you, so you cannot use a ros_control controller which only takes velocity input.

MoveIt can generate trajectories with positions for you, so you pass those to a controller which accepts position inputs.

MoveIt doesn't care whether that position controller then outputs positions, velocities or something else.

Thanks. I managed to get it working. My head was all over the place with Gazebo, ros_control, MoveIt. I think I finally understand how everything comes together (kind off).

So what was the solution in the end?

I switched to velocity_controllers/JointTrajectoryController. I had to make sure to add pid gain values for the controller configuration which the gazebo controller didn't previously need. Embarassingly simple to fix haha. It just confused me why, despite using position_controllers, I could still see desired velocities being calculated. But now I think I understand.