How to start the ur modern driver in pos based instead of vel based

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This would be the same as with other ros_control based hardware_interface drivers: by changing the configuration of which controllers are loaded and started vs loaded and stopped.

For the UR3 with ur_modern_driver that can be done with the controllers and stopped_controllers launch file arguments here:

<arg name="controllers" default="joint_state_controller force_torque_sensor_controller vel_based_pos_traj_controller"/>
<arg name="stopped_controllers" default="pos_based_pos_traj_controller joint_group_vel_controller"/>

as you can see, by default the joint_state_controller, force_torque_sensor_controller and the vel_based_pos_traj_controller are loaded and started, and the pos_based_pos_traj_controller and joint_group_vel_controller are loaded but in the stopped state.

You'll want to change that such that pos_based_pos_traj_controller is loaded and started and vel_based_pos_traj_controller is loaded but stopped.