This was one tough cookie but I figured it out. The key was actually in the PR2 example code. If you read it carefully and compare it to the data types in the jointMsg below you can figure this out.
[jointClient,jointMsg] = ...
rosactionclient('/robot/limb/right/follow_joint_trajectory');
Take a look at jointMsg
jointMsg =
ROS FollowJointTrajectoryGoal message with properties:
MessageType: 'control_msgs/FollowJointTrajectoryGoal'
Trajectory: [1×1 JointTrajectory]
GoalTimeTolerance: [1×1 Duration]
PathTolerance: [0×1 JointTolerance]
GoalTolerance: [0×1 JointTolerance]
We look closer to find an object called jointMsg.Trajectory.Points.Position but when we do this :
>> jointMsg.Trajectory.Points.Positions = [0;0;0;0;0;0;0]
Property assignment is not allowed when the object is empty.
Use subscripted assignment to create an array element.
We dig a little deeper an find
>> jointMsg.Trajectory.Points
ans =
0×1 ROS JointTrajectoryPoint message array with properties:
MessageType
TimeFromStart
Positions
Velocities
Accelerations
Effort
Ok, so we have a JointTrajectoryPoint object. So, we will make a JointTractoryPoint message:
joint_send = rosmessage('trajectory_msgs/JointTrajectoryPoint')
joint_send =
ROS JointTrajectoryPoint message with properties:
MessageType: 'trajectory_msgs/JointTrajectoryPoint'
TimeFromStart: [1×1 Duration]
Positions: [0×1 double]
Velocities: [0×1 double]
Accelerations: [0×1 double]
Effort: [0×1 double]
Use showdetails to show the contents of the message
Now we plug in Positions and velocities
>> joint_send.Positions = zeros(7,1);
>> joint_send.Velocities = zeros(7,1);
Now we set our original message
jointMsg.Trajectory.Points = joint_send;
Lets take a look
jointMsg.Trajectory.Points
ans =
ROS JointTrajectoryPoint message with properties:
MessageType: 'trajectory_msgs/JointTrajectoryPoint'
TimeFromStart: [1×1 Duration]
Positions: [7×1 double]
Velocities: [7×1 double]
Accelerations: [0×1 double]
Effort: [0×1 double]
Use showdetails to show the contents of the message
Voila
Now that your object is no longer empty you can work with it directly.
>> jointMsg.Trajectory.Points.Positions = [1;1;1;1;1;1;1]
>> jointMsg.Trajectory.Points.Positions
ans =
1
1
1
1
1
1
1
