How to get robot velocity in gazebo

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Hello all.

I have built a little vehicle robot which can be controlled using keyboard. Now i am interested to get the robot velocity in order to close control loop on it.

The question is what is the best way to do that ?

Is there a special sensor that I can include in my robot URDF model ?

Or, should I try to get it directly from the /gazebo/model_states topic ?

update :

I tried to subscribing to /gazebo/model_states topic using the following code :

ros::Subscriber vehicle_velocity_sub_;

vehicle_velocity_sub_ = nh_.subscribe<geometry_msgs::Twist>("/gazebo/model_states/twist[2]", 1000, &Dany_Obstacle_Avoidance_Commander::twist_data_processor, this);

But i got the following error :

terminate called after throwing an instance of 'ros::InvalidNameException' what(): Character [[] at element [26] is not valid in Graph Resource Name [/gazebo/model_states/twist[2]]. Valid characters are a-z, A-Z, 0-9, / and _. Aborted (core dumped)

It seems that ROS does not understand the vector operator "[]" used with topics.

nevertheless if I try to use it in rostopic command :

rostopic echo /gazebo/model_states/twist[2]

from the terminal it works fine.

Does anyone know what I doing wrong ?

thanks.