Add doc for controller plugin interface

Author: novakfi8

docs/45-plugin-interface/02-controllers.md

@@ -0,0 +1,55 @@
+---
+title: Controller plugins
+pagination_label: API for the controller plugins
+description: API for the controller plugins
+---
+
+# The Controller plugin interface
+
+The Controller plugin takes current UAV state and command from the [Reference tracker](https://ctu-mrs.github.io/docs/features/trackers/) and provides the desired control ouptuts, which are sent to the flight controller (Embedded autopilot block in diagram below) via [Hardware API](https://ctu-mrs.github.io/docs/plugin-interface/hardware-api/).
+Diagram below shows the data architecture of the MRS system where the block containing the Controller plugin is marked in red.
+
+![Diagram of MRS system architecture](./fig/diagram_of_system_architecture.jpg)
+
+The controller plugin is compiled as *ROS plugins* ([http://wiki.ros.org/pluginlib](http://wiki.ros.org/pluginlib)) with the [interface](https://github.com/ctu-mrs/mrs_uav_managers/blob/master/include/mrs_uav_managers/controller.h) defined by the [control manager](https://github.com/ctu-mrs/mrs_uav_managers).
+A controller plugin from any ROS package can be loaded dynamically by the [control manager](https://github.com/ctu-mrs/mrs_uav_managers) without it being present during [control manager](https://github.com/ctu-mrs/mrs_uav_managers)'s compile time.
+Loaded controllers can be switched by the [control manager](https://github.com/ctu-mrs/mrs_uav_managers) in mid-flight, which allows safe testing of new controllers and adds flexibility to the [MRS UAV system](https://github.com/ctu-mrs/mrs_uav_system).
+
+An example of a cutom controller plugin can be found at [this link](https://github.com/ctu-mrs/mrs_core_examples/tree/master/cpp/controller_plugin).
+It is highly reccomended to base your controller plugin on this example.
+In the example, there is also a [tmux folder](https://github.com/ctu-mrs/mrs_core_examples/tree/master/cpp/controller_plugin/tmux) containing a script that starts the simulation of a UAV in the [Gazebo simulator](https://github.com/ctu-mrs/mrs_uav_gazebo_simulation), where the UAV uses the [example controller plugin](https://github.com/ctu-mrs/mrs_core_examples/tree/master/cpp/controller_plugin).
+You have to install [MRS UAV system](https://github.com/ctu-mrs/mrs_uav_system) first to run the example.
+To load the controller plugin into the [MRS UAV system](https://github.com/ctu-mrs/mrs_uav_system), you need to provide a custom config file to the [MRS UAV Core](https://github.com/ctu-mrs/mrs_uav_core) containing the parameters for the controller plugin.
+The custom config file should contain the following lines (taken from [example controller plugin](https://github.com/ctu-mrs/mrs_core_examples/tree/master/cpp/controller_plugin)):
+```yaml 
+mrs_uav_managers:
+
+  control_manager:
+
+    ExampleController:
+      address: "example_controller_plugin/ExampleController"
+      namespace: "example_controller"
+      eland_threshold: 20.0 # [m], position error triggering eland
+      failsafe_threshold: 30.0 # [m], position error triggering failsafe land
+      odometry_innovation_threshold: 1.5 # [m], position odometry innovation threshold
+      human_switchable: true
+
+      # which outputs the controller can provide
+      outputs:
+        actuators:             false
+        control_group:         false
+        attitude_rate:         false
+        attitude:              true
+        acceleration_hdg_rate: false
+        acceleration_hdg:      false
+        velocity_hdg_rate:     false
+        velocity_hdg:          false
+        position:              false
+
+    # list of names of dynamically loaded controllers
+    controllers : [
+      "ExampleController",
+    ]
+```
+
+