MATLAB: Setup of an /trajectory_msgs/JointTrajectory message in Simulink

busgazeboRobotics System Toolboxrosros messageros publishsimulink

Hello,

i am currently working on interfacing Matlab 2018a with Gazebo on a Virtual Machine. I want to design a robot controller in Simulink and simulate the dynamics of the mobile manipulator(KUKA youBot) in Gazebo.

I have already setup Matlab/Simulink and ROS/Gazebo. I can subscribe and publish messages like /odom or /vel_cmd. But I have problems to setup a /trajectory_msgs/JointTrajectory message to control the movement of the arm. I seem to not be able to setup the message correctly.

For the elements JointNames and Points the error message said that the bus structure doesn't match. And for Points.Positions, Points.Velocities, Points.Accelerations the error states that I cannot acces this data as it is inside a bus element. Can someone help me how to setup this ROS message correctly?

Thank you in advance and if you need further information feel free to ask me…..

Best Answer

Hello Martin,

The message trajectory_msgs/JointTrajectory contains variable-length arrays such as the 'Points' here shown on the screenshot. It is difficult to construct such signal directly through Simulink source blocks directly. The recommended way of assigning these properties in Simulink message is to use a MATLAB Function block. This example shows how to assign elements inside a ROS message in Simulink using MATLAB Function block.

Hope this helps,

Josh

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MATLAB: Trajectory planning for robot manipulator

Robotics System Toolbox provides tools and algorithms for designing, simulating, and testing manipulators, mobile robots, and humanoid robots. Videos and Webinars link will help you understand the technical concepts and software examples for motion trajectory planning with robot manipulators.

Github Submission from MathWorks - robotics consists of educational MATLAB and Simulink examples for trajectory generation and evaluation of robot manipulators. Following through the Blog link of Crash course on Robot manipulators will help in resolving.

MATLAB: Simulink/ROS: Publishing JointTrajectory Messages

I can only guess what's going on...

Are you planning on sticking to desktop simulations from MATLAB or will you be generating code from the model? It's fairlytough to work with string messages with the ROS-Simulink interface, so if you're sticking to desktop simulations I'd suggest writing a MATLAB Function and enveloping all the ROS interface code as interpreted.

For example, here is a 3-joint, 2-point example:

function sendMsg
  coder.extrinsic('sendTrajMsgs'); % Will run the sendTrajMsgs function interpreted
  sendTrajMsgs([1 2 3],[4 5 6]);
end

The following implementation of sendTrajMsgs.m worked fine for me:

function sendTrajMsgs( pos1, pos2 )
      % Create the publisher only on the first function call
      persistent pub
      if isempty(pub)
          pub = rospublisher('/my_joints','trajectory_msgs/JointTrajectory');
      end
      % Create an empty message with the joint names
      msg = rosmessage('trajectory_msgs/JointTrajectory');
      msg.JointNames = {'joint1','joint2','joint3'};
      % Fill in the message positions
      p1 = rosmessage('trajectory_msgs/JointTrajectoryPoint');
      p1.Positions = pos1;
      msg.Points(1) = p1;
      p2 = rosmessage('trajectory_msgs/JointTrajectoryPoint');
      p2.Positions = pos2;
      msg.Points(2) = p2;
      % Send messages
      send(pub,msg)
end

If you have to use the Simulink blocks, you'll notice that you have to treat the strings as you would in C code. The JointNames field of the bus will actually have an array of string buses, each of which has a 128-element uint8 (or char ) array and a supplementary "information" bus that tells you the length of that string -- in other words, how far along the 128 char vector you have to read before it's just empty space.

Going this route, I would follow the tutorial you linked to. What would help is grabbing an "Empty Message" block from the ROS library, filling in the trajectory_msgs/JointTrajectory message type, and then going to Edit > Bus Editor in Simulink. You'll be able to view the structure of all the bus messages that were created for this message.

This should help you add all the necessary "info" metadata to the string messages such as

strLength = length(str);
msg.Name(idx).Data(1:strLength) = uint8(str);
msg.Name(idx).Data_SL_Info.CurrentLength = uint32(strLength);

Hope this helps.

- Sebastian

Best Answer

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