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1- the yaml file is basically describe the used controllers. It always contains the joint_state_controller which is in charge of providing constant information about the state of the joint, and the description of the other controllers. The /joint_states topic is published by an instance of the JointStateController. This is a read-only controller that does not command any joint, but rather publishes the current joint states at a configurable frequency (publish_rate).

2- the inputs to ros controllers are always in SI units, so a joint velocity controller will accept radians per second.

3- joint_state_controller: it continuously provides the state of the joint.

4- for the question in the comment, I think that it is a bug with velocity_controllers which prevent the topic my_joint_position_controller/state to be published. I changed the controller to effort_controllers/JointVelocityController and I get this topic.

5- a nice tutorial explaining ros_control can be found here.