Moving a four wheel robot using a controller

edit

Apart form what @Martin Peris is suggesting in order to comprise with URDF tree structure,

why you are using the diff drive gazebo plugin libgazebo_ros_diff_drive if you already using ROS control? Besides, I think you do not want to load all the controllers, velocity ones for instance, since the controller manager is expecting you to load those 4 controllers in the hardware interface and you have no transmissions associated with them. Can you check those things and also ensuring you are using properly the same ns?

If the thing you want to achieve is controlling the same joint with two different control interfaces I am afraid that is not possible and you will need to provide at least 1 joint for each controller/transmission pair.

With respect the namespace take into account you can launch all your nodes inside a <group ns="/robot"></group> structure.

Martin, I thought Warnings were not crucial in the execution of a code and to be honest I am not sure what I do have to do; Weasfas, I'll try your suggestions and come back with results. Thank you both !

OK, just tried both your suggestion but nothing worked... When I run robot_control.launch I got:

[INFO] [1583137731.242393]: Controller Spawner: Waiting for service controller_manager/load_controller
0x16ee960 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0xcc4470) ): Attempt to set a screen on a child window.
0x16ff280 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0xcc4470) ): Attempt to set a screen on a child window.
0x1700c60 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0xcc4470) ): Attempt to set a screen on a child window.
0x1700380 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0xcc4470) ): Attempt to set a screen on a child window.
[WARN] [1583137761.296283]: Controller Spawner couldn't find the expected controller_manager ROS interface.

Somehow I got a new step forward: I first started the gazebo sim with roslaunch robot gazebo.launch and then the robot_control.launch and I managed to "see" and load my controllers.

That is the normal behavior since the control interfaces resides on the transmission elements of Gazebo.

Dear Weasfas, I am now trying to issue position commands to the wheels via the controller but the robot is not moving in RViz. Only in Gazebo, I see a small movement and then nothing else. Even more, if I issue the same command again the robot is not moving in Gazebo for a second time... Should I change for the velocity controllers?

I tried that and got an error :

[ERROR] [1583144504.715888, 7.266000]: Failed to start controllers: joint_state_controller, FRJ_velocity_controller, FLJ_velocity_controller, BLJ_velocity_controller, BRJ_velocity_controller

Any ideas why?

The position command is:

rostopic pub  /robot/FRJ_position_controller/command std_msgs/Float64 "data: 4"

Hi @John999991,

It will depend on the way you want to control your platform. If you decide you want to use the effort controllers you will need to adjust the PID gains, together with the collision and friction parameters of Gazebo. For me, your current set up has no sense cause you are trying to control the wheels with position. It will be more logic to control by effort or by velocity. In this tutorial you will find all necessary information to follow this solution. Furthermore, you will need to understand friction on Gazebo since you will need to parametrize how the Simulation model this in order to move the model properly in the world.

Yes I agree. I thought having set up the position controller as the tutorial you posted suggests, would be enough to then transition that knowledge to the control of velocity, but it seems it is not straight-forward. For some reason, changing position to velocity produces unexpected errors...