How do I enter a string into a ROS message in Simulink?

I'm attempting to send a path message to a ROS system. However, in order for this to be correctly published to RVIZ it requires a reference frame ID (FrameId in the header). I have attempted to complete this in multiple ways- First I attempted to enter a String Constant into a bus assignment block:

This has come up with the following error:

I then attempted in the shown matlab block to create a 128x1 uint8 array of the ASCII values of "odom", and blank characters (ASCII 32) making up the remaining 124 characters, using the following code:

frameid = zeros(128,1, 'uint8');

string = ['o', 'd', 'o', 'm'];

frameid(1:4) = uint8(string);

frameid(5:128) = 32;

frameid = uint8(frameid);

globalplannermessage.Header.FrameId = frameid;

This, however, simply ended up with RVIZ showing that the reference frame was blank (For frame []: Frame [] does not exist)

What is the recommended method to complete this?

This is my first question on here, so if I have missed any important information please let me know and I'll change whatever needs to be changed!

Answer 1

[Josh Chen]

Hello Harry,

Since ROS strings in Simulink have to be fixed-length uint8 array, normally there are two ways of entering a string into a ROS message in Simulink.

(1) Use a MATLAB Function block to convert a string to uint8. Refer to the following page as an example:

https://www.mathworks.com/help/ros/ug/ros-parameters-in-simulink.html#mw_a8b7626b-c884-45ed-bd6b-7cd56b65faf4

(2) Use the "String to ASCII" block to convert the string signal to uint8 array.

Hope this helps,

-Josh