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=== Driving the robot mobile platform ===
Driving the robot is done simply by publishing a [[http://www.ros.org/doc/api/geometry_msgs/html/msg/Twist.html|geometry_msgs/Twist]] message to the '''cmd_vel''' topic.

By default, all robots use the [[twist_mux||twist_mux]], which provides a multiplexer for geometry_msgs:Twist messages. It takes 3 input twist topics and outputs the messages to a single one. This available input topics are:

'''joy_vel''' - This topic is used for teleoperation using a joystick and have the highest priority (priority: 100) so you can use the joystick to takeover control while the robot is in autonomous mode.

'''cmd_vel''' - The main user interface topic (priority: 90).

'''nav_vel''' - This topic is used is used by the [[move_base||move_base]] to send navigation commands (priority: 80).

The output topic is '''mobile_base_controller/cmd_vel''', it's not recommended to publish Twist messages directly to this topic.

For more configuration details see the ''robotican/robotican_common/config/twist_mux.yaml''.

The geometry_msgs/Twist expanded definition looks like:

{{{
Vector3 linear
    float64 x
    float64 y
    float64 z
Vector3 angular
    float64 x
    float64 y
    float64 z
}}}

However, our robot can't drive sideways (linear.y), or rotate about the x and y axes! Therefore, we will only be using linear.x and angular.z to control our robot.

One option is to use the rostopic command line tool to publish a driving message, for example:
{{{
$ rostopic pub /cmd_vel geometry_msgs/Twist -r 10 -- '[0.3, 0.0, 0.0]' '[0.0, 0.0, -0.9]'
}}}
The above command will command the robot to drive with a forward velocity of 0.3 m/s and turn right (positive values will cause the robot to turn left) with angular velocity of 0.9 rad/s.
The "-r 10" argument will cause this message to be sent in a rate of 10 Hz, This is necessary because the robot controller have a 0.25 seconds safety stop timeout watch. After a period of 0.25 seconds without any coming message, the robot will stop.

If you want to use a graphical tool for publishing this command you can use the [[rqt_robot_steering|rqt Robot Steering]] tool or the [[rqt_publisher| rqt Message Publisher]] tool.

Lets, give it a try together. First, lets open a new terminal and launch the armadillo robot in the gazebo simulation:
{{{
$ roslaunch armadillo2 armadillo2.launch gazebo:=true
}}}
Now, in a new terminal, type the following command for driving the robot:
{{{
$ rostopic pub /cmd_vel geometry_msgs/Twist -r 10 -- '[0.2, 0.0, 0.0]' '[0.0, 0.0, 0.6]'
}}}
Open the Gazebo window and watch the armadillo robot drive.

To stop the robot from driving click Ctrl+C in order to stop publishing.

Now, Lets use the Robot Steering tool. In your second terminal launch rqt by typing:
{{{
$ rqt
}}}
From the upper toolbar, select Plugins->Robot Tools->Robot Steering.

Enter the '''cmd_vel''' topic at the top text box. 

Your window should look like this:

{{attachment:robot_steering.png|Robot Steering|width=600}}

Now, play with the two sliders to steer the armadillo robot.


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