How to send ROS messages with string URI designator from Simulink [closed]

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I was trying to implement Kuka Youbot control using the new simulink blocks from Robotics System Toolbox in Matlab 2015a. Mostly it works, but as it turns out I could not control the youbot arm from simulink, because simulink does not support char datatype and I would need to set the URI of the ROS message so that ROS can identify for each joint I am sending the command. For the arm control it is necessary to write the joint designation into the JointValue.

In matlab command window this is done simplify by writing

pubArm1Pos = rospublisher('/arm_1/arm_controller/position_command','brics_actuator/JointPositions');
msgArm1Pos = rosmessage(pubArm1Pos);

msgArm1Pos.Positions(1).Unit = 'rad';
msgArm1Pos.Positions(1).JointUri = 'arm_joint_1';
msgArm1Pos.Positions(1).Value = 3.0;

% similar code for the rest 4 joints

send(pubArm1Pos, msgArm1Pos);

However in simulink I was not able to move the arm. I tried to cast the string 'arm_joint_1' into uint8 type, but it does not work. Is there a solution how to include proper designations for joints, so that the ROS would properly recognize the message for all the joints.

Best regards

Edit: I have written an s-function instead of m-function, but the problem is that the bus cannot store the string format. And even though I send the string in uint8, the ROS listener on youbot does not recognize it as a char. Would it be possible to write a custom publisher for simulink based on the publisher availible in Robotics toolbox?