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First of all, I would suggest that you go through the example here under the Detailed Description section because it is a minimum viable example. We got our UR3 to work with ViSP based on this example. If you want to do a PBVS task, you have to change the task type to some EYETOHAND type. In our case, EYETOHAND_L_cVe_eJe is used, so we need to build features from the desired end-effector pose, supply current end effector pose and read current Jacobian in order for ViSP to calculate the velocity command for all the joints (not the end effector velocity).

Then you need to understand that ViSP is a general purpose library that is not designed only for ROS. So it's your work to bridge ViSP and ROS. We used tf to get both the desired and current end-effector pose and converted them to ViSP homogeneous matrix with visp_bridge (BTW, we are not using any ViSP visual tracking algorithm. So in your case, you may not need to get the desired end-effector pose from tf). For the Jacobian, you can get it from the getJacobian function of MoveIt!. Of course, there might be other ways to feed this information. But that's enough for us to get joint velocities from ViSP.

At last, since you are also using ur_modern_driver, to send the velocity command to the robot you may simply publish it to the /ur_driver/joint_speed topic.

First of all, I would suggest that you go through the example here under the Detailed Description section because it is a minimum viable example. We got our UR3 to work with ViSP based on this example. If you want to do a PBVS task, your camera is not on the end effector, you have to change the task type to some EYETOHAND type. Otherwise, you should use EYEINHAND type. In our case, EYETOHAND_L_cVe_eJe is used, so we need to build features from the desired end-effector pose, supply current end effector pose and read current Jacobian in order for ViSP to calculate the velocity command for all the joints (not the end effector velocity).

Then you need to understand that ViSP is a general purpose library that is not designed only for ROS. So it's your work to bridge ViSP and ROS. We used tf to get both the desired and current end-effector pose and converted them to ViSP homogeneous matrix with visp_bridge (BTW, we are not using any ViSP visual tracking algorithm. So in your case, you may not need to get the desired end-effector pose from tf). For the Jacobian, you can get it from the getJacobian function of MoveIt!. Of course, there might be other ways to feed this information. But that's enough for us to get joint velocities from ViSP.

At last, since you are also using ur_modern_driver, to send the velocity command to the robot you may simply publish it to the /ur_driver/joint_speed topic.