change node execution order in bringup launch file

To fix issues in #30 launch the encoder scrip on the
microcontroller before launching the hardware interface
control loop. This will make sure to send actual tick values
from the encoders to the hardware interface before the control
loop is running.

diffbot_bringup/launch/minimal.launch

@@ -1,22 +1,27 @@
 <launch>
+  <!-- Minimal launch file to bringup the robot hardware -->
+  <!-- This launch file will launch the package that communicates with the encoders
+       before running the ROS Controls hardware_interface control loop.
+       Launching the nodes in this order will make sure that the hardware_interface
+       receives meaningful tick values from the wheel encoders. -->
 
-    <!-- Including the following launch file from diffbot_base package will -->
-    <!-- Load the robot description onto the parameter server -->
-    <!-- Run the controller manager with DiffBot's hardware interface -->
-    <!-- Load the controller config onto the parameter server -->
-    <include file="$(find diffbot_base)/launch/diffbot.launch">
-        <!-- arg name="model" value="$(arg model)" /-->
-    </include>
+  <!-- Encoders -->
+  <!-- Run rosserial to connect with the Teensy 3.2 board connected to the motor encoders -->
+  <node name="rosserial_teensy" pkg="rosserial_python" type="serial_node.py" respawn="false"
+      output="screen" ns="diffbot" args="_port:=/dev/ttyACM0
+                                         _baud:=115200"/>
+
+  <!-- Motors -->
+  <!-- -->
+  <node name="motor_driver" pkg="grove_motor_driver" type="motor_driver.py" respawn="false"
+      output="screen" ns="diffbot" />
+
+  <!-- Including the following launch file from diffbot_base package will -->
+  <!-- Load the robot description onto the parameter server -->
+  <!-- Run the controller manager with DiffBot's hardware interface -->
+  <!-- Load the controller config onto the parameter server -->
+  <include file="$(find diffbot_base)/launch/diffbot.launch">
+      <!-- arg name="model" value="$(arg model)" /-->
+  </include>
 
-
-    <!-- Motors -->
-    <!-- -->
-    <node name="motor_driver" pkg="grove_motor_driver" type="motor_driver.py" respawn="false"
-        output="screen" ns="diffbot" />
-
-    <!-- Encoders -->
-    <!-- Run rosserial to connect with the Teensy 3.2 board connected to the motor encoders -->
-    <node name="rosserial_teensy" pkg="rosserial_python" type="serial_node.py" respawn="false"
-        output="screen" ns="diffbot" args="_port:=/dev/ttyACM0
-                                            _baud:=115200"/>
-</launch>
+</launch>