UR5e extrapolating the trajectory control command send via ROS at 50Hz about universal_robots_externalcontrol_urcap HOT 8 CLOSED

Why would you come to the assumption, that control commands are only sent with 50 Hz?

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I am using a custom IK solution to generate trajectory commands like these:

---
header: 
  seq: 191
  stamp: 
    secs: 1639835958
    nsecs: 765319108
  frame_id: ''
goal_id: 
  stamp: 
    secs: 1639835958
    nsecs: 765327930
  id: "/move_group-4-1639835958.765319108"
goal: 
  trajectory: 
    header: 
      seq: 0
      stamp: 
        secs: 0
        nsecs:         0
      frame_id: "world"
    joint_names: [shoulder_pan_joint, shoulder_lift_joint, elbow_joint, wrist_1_joint, wrist_2_joint,
  wrist_3_joint]
    points: 
      - 
        positions: [7.374982670751816e-05, -1.3939911525172393, 1.4019192847185409, -0.005670634384257868, 0.35640352556791954, 0.0031893071445614055]
        velocities: [7.006015059003896e-07, 1.975990426839047e-06, -8.795930761529291e-07, -1.360765239637879e-07, 6.620829717807197e-08, -6.75472778405295e-08]
        accelerations: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
        effort: []
        time_from_start: 
          secs: 0
          nsecs:  20000000
  path_tolerance: []
  goal_tolerance: []
  goal_time_tolerance: 
    secs: 0
    nsecs:         0
---

These commands are then published to the ROS topic /scaled_pos_joint_traj_controller/follow_joint_trajectory/goal using a custom publisher publishing at 50Hz

from universal_robots_externalcontrol_urcap.

We are using the driver file ur5e_bringup.launch from the repository for establishing the connection to the URCap externalcontrol-1.0.5.urcap.

The Figure 1 below shows the update of /scaled_pos_joint_traj_controller/follow_joint_trajectory/goal.goal.trajectory.points[0].positions[4] and /scaled_pos_joint_traj_controller/follow_joint_trajectory/goal.goal.trajectory.points[0].velocities[4] from the trajectory command generated using our custom IK scripts to node /ur_hardware_interface

Figure 1 : Joint 4 Position and Velocity Graph

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This is the result from the UR Log Viewer. This there a way to make the Velocity, acceleration and currents smoother. The Joint Position as shown in the result is smooth.

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Velocity and Position command for the Joint 0 sent from our package to /ur_hardware_interface for a sine wave motion recorded using rosbag :

from universal_robots_externalcontrol_urcap.

@fmauch
It looks there is obvious added velocity noise from the 'ur_hardware_interface', when it outputs to the robot.
Despite the command input to the 'ur_hardware_interface' was smooth.

Also adjusting the value in the PID gains or velocity_ff in https://github.com/UniversalRobots/Universal_Robots_ROS_Driver/blob/master/ur_robot_driver/config/ur5e_controllers.yaml made no observable difference in performance.

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You are using the scaled trajectory controller to interface the robot. This controller is generating trajectories based on the input given. In the example above, a trajectory with one setpoint is given. The Setpoint should be reached in 20 ms. The controller now creates a trajectory from the current configuration to the target configuration using a spline interpolation (This is basically, how the joint trajectory controller works). This generated motion is then sampled by the hardware interface (through the controller implementation) in each control cycle (with 500Hz) to generate commands sent to the robot.

Thus, the robot is not "simply going there".

The "noise" mentioned in #26 (comment) comes from the fact, that a servoj command is used to execute the commands. The actual behavior of this servoj command can be altered by changing the values for servoj_gain and servoj_lookahead_time.

For real smooth velocities, a velocity-based controller such as the scaled_vel_joint_traj_controller could be used. Please see the controller documentation for details on the available controllers.

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Thank you for the information. @fmauch
We have tested that reducing the servoj_gain reduces the jitters significantly. It seems putting it to 100 (minimal) instead of 2000 (default also maximum) gives the best smoothness in simulation.

Hopefully it will not compromise too much on the control precision?

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