Question about ComputeCartesianPath

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Hello,

I have a robot which is defined as follow:

<group name="arm">
    <chain base_link="arm_base_link" tip_link="arm_link_tool0" />
</group>
<group name="arm_on_rail">
    <chain base_link="base_rail_link" tip_link="arm_link_tool0" />
</group>
<group name="base">
    <joint name="base_joint_b1" />
</group>
<group name="station">
    <joint name="station_joint_s1" />
</group>
<group name="mh6">
    <group name="arm_on_rail" />
    <group name="station" />
</group>

I am trying to use computeCartesianPath function from a set of waypoints. If I define move_group_interface::MoveGroup group("arm_on_rail"); the robot moves properly through the trajectory, but the following error is shown:

[joint_trajectory_action-4] process has died [pid 6491, exit code -11, cmd /home/juliocesar/catkin_ws/devel/lib/motoman_driver/motoman_driver_joint_trajectory_action __name:=joint_trajectory_action __log:=/home/juliocesar/.ros/log/4cdad31a-08f9-11e7-ba80-e811328efeb3/joint_trajectory_action-4.log].
log file: /home/juliocesar/.ros/log/4cdad31a-08f9-11e7-ba80-e811328efeb3/joint_trajectory_action-4*.log

The error doesn't appear if I use move_group_interface::MoveGroup group("mh6"); with group.setJointValueTarget(group_variables_values); where group_variables_values includes data for 8 axes (arm_on_rail: 7 + station: 1).

Then, is it possible to calculate the cartesian path for arm_on_rail group and execute it for mh6 with a fixed value for station position (f.i. 0 rad)?

If I set move_group_interface::MoveGroup group("mh6"); to compute the cartesian path then I get [ERROR] [1489525223.511244021]: No kinematics solver instantiated for group 'mh6'. My kinematics.yaml is:

arm:
  kinematics_solver: trac_ik_kinematics_plugin/TRAC_IKKinematicsPlugin
  kinematics_solver_search_resolution: 0.005
  kinematics_solver_timeout: 0.005
  kinematics_solver_attempts: 3
arm_on_rail:
  kinematics_solver: trac_ik_kinematics_plugin/TRAC_IKKinematicsPlugin
  kinematics_solver_search_resolution: 0.005
  kinematics_solver_timeout: 0.005
  kinematics_solver_attempts: 3

If I add mh6 group, then it is shown:

[ERROR] [1489527221.395979635]: Group 'mh6' is not a chain
[ERROR] [1489527221.396073189]: Kinematics solver of type 'kdl_kinematics_plugin/KDLKinematicsPlugin' could not be initialized for group 'mh6'
[ERROR] [1489527221.396460816]: Kinematics solver could not be instantiated for joint group mh6.

Thank you.

-Julio César