Added some docs

Author: destogl

index.rst

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+========================
+Controllers
+========================
+
+`GitHub Repository <https://github.com/ros-controls/ros2_controllers>`_
+
+Nomenclature
+-------------
+The ros2_control framework uses namespaces to sort controller according to the type of command interface they are using.
+The controllers are using `common hardware interface definitions`_.
+The controllers' namespaces are commanding the following command interface types:
+
+  - ``position_controllers``: ``hardware_interface::HW_IF_POSITION``
+  - ``velocity_controller``: ``hardware_interface::HW_IF_VELOCITY``
+  - ``effort_controllers``: ``hardware_interface::HW_IF_EFFORT``
+  - ...
+
+
+Controllers
+--------------
+
+The following standard controllers are implemented:
+
+  - `Joint Trajectory Controller <joint_trajectory_controller/docs/index.rst>`_ - provided a list of waypoints or target point defined with position, velocity and acceleration, the controller interpolates joint trajectories through it.
+  - ... <the list is not complete> ...
+
+
+
+
+.. _common hardware interface definitions: https://github.com/ros-controls/ros2_control/blob/master/hardware_interface/include/hardware_interface/types/hardware_interface_type_values.hpp

joint_trajectory_controller/README.md

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+# joint_trajectory_controllers package
+
+The package implements controllers to interpolate joint's trajectory.
+
+For detailed documentation check the `docs` folder or [ros2_control documentation](https://ros-controls.github.io/control.ros.org/).

joint_trajectory_controller/docs/index.rst

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+=================================
+Joint Trajectory Controller(s)
+=================================
+
+The controller expects at least position feedback from the hardware.
+Joint velocities and accelerations are optional.
+Currently the controller does not internal integrate velocity from acceleration and position from velocity.
+Therefore if the hardware provides only acceleration or velocity states they have to be integrated in the hardware-interface implementation ot velocitiy and position to use this controllers.
+
+The controller types are placed into namespaces according to their command types for the hardware (see `general introduction into controllers <../../index.rst>`_).
+
+The following version of the Joint Trajectory Controller are available mapping the following interfaces:
+
+  - position_controllers::JointTrajectoryController
+    - Input: position, [velocity, [acceleration]]
+    - Output: position
+  - position_velocity_controllers::JointTrajectoryController
+    - Input: position, [velocity, [acceleration]]
+    - Output: position and velocity
+  - position_velocityy_acceleration_controllers::JointTrajectoryController
+    - Input: position, [velocity, [acceleration]]
+    - Output: position, velocity and acceleration
+..   - velocity_controllers::JointTrajectoryController
+..     - Input: position, [velocity, [acceleration]]
+..     - Output: velocity
+.. TODO(anyone): would it be possible to output velocty and acceleration?
+..               (to have an vel_acc_controllers)
+..   - effort_controllers::JointTrajectoryController
+..     - Input: position, [velocity, [acceleration]]
+..     - Output: effort
+
+The controller uses `common hardware interface definitions`_.
+
+(*Not implemented yet*) When using a pure ``velocity`` or ``effort`` controllers a command is generated using the desired state and state error using a velocity feedforward term puls a corrective PID term.
+
+A yaml file for joint trajectory controllers looks something like:
+
+1. Position-control hardware interface:
+
+   .. code-block:: yaml
+
+     head_controller:
+     type: "position_controllers/JointTrajectoryController"
+     joints:
+       - head_1_joint
+       - head_2_joint
+
+     constraints:
+       goal_time: 0.6
+       stopped_velocity_tolerance: 0.02
+       head_1_joint: {trajectory: 0.05, goal: 0.02}
+       head_2_joint: {trajectory: 0.05, goal: 0.02}
+       stop_trajectory_duration: 0.5
+       state_publish_rate:  25
+
+
+2. Position-control hardware interface:
+
+   .. code-block:: yaml