Guidelines for Managing Protective Stops on Robotiq Palletizing Solutions

Context

Protective stops (P-Stops) can occur during palletizing operations when the robot detects unexpected forces or collisions. These events are often triggered by factors such as payload changes, unstable box gripping, or sudden robot accelerations. This article outlines proven methods to reduce or eliminate false P-Stops when using Robotiq’s Palletizing Solution (PAL).

Robot Motion & Payload Management

• Reduce acceleration and speed
  Preferred strategy when the cycle time allows it.

  • Reduces strain on the robot and lowers vibration, especially when the vertical axis (AX) is at its highest.

• Manual payload change
  Use manual set_target_payload() commands rather than relying solely on the PAL URCap.

  • Example: set_target_payload(new_payload, new_cog)

• Wait is_steady before and after payload change

  • Add wait_is_steady() before updating the payload to avoid applying force mid-trajectory.

  • Also use wait_is_steady() after setting the new payload to allow vibrations to settle.

• MoveL at low acceleration after grip

  • Insert a short moveL just after gripping the box.

  • Use very low acceleration to control the initial motion and reduce residual forces from the suction.

  • Set target_payload just after vacuum ON.

Box & Grip Quality

• Use new, undamaged boxes
  Deformed boxes reduce suction reliability and may slip, triggering a P-Stop.

• Test with a lighter box
  Helps determine if the P-Stop is caused by the payload weight or other factors.

• Modify suction cup position on the box
  Teach the pick position with cups as far apart as possible.

  • A wider grip reduces sway and vibration.

• Modify the pick position to increase vacuum cup compression
  Adjust the height to create more or less compression depending on your setup.

  • Helps avoid box movement during grip.

Vacuum and Gripper Behavior

• Check pneumatic pressure
  A weak vacuum results in poor grip and increases risk of dropping the box.

• Use wait_is_steady() before gripper release
  Ensures the robot is stationary before releasing the box, which helps prevent unwanted force feedback.

• Set a higher drop position than the actual pallet
  In the PAL node, teach a drop height slightly higher than the physical surface.

  • Prevents contact before the box is fully released.

  • Especially useful when using blow-off, which can apply upward force.

• Release with incremental payload change
  Allows a smoother transition when updating the payload after drop-off.

  • Helps avoid a sudden jerk that can trigger a stop.

URCap Parameters & Safety Settings

• Set safety mode to "Least restricted"
  Within installation settings, select “Least restricted” mode.

  • Makes the robot less sensitive to minor external forces.

• Use find_surface before gripping
  Compresses the suction cup without robot movement when vacuum turns on.

  • Reduces impact forces on the robot during grip activation.

• Activate freedrive before gripping
  Temporarily enable freedrive mode before vacuum ON.

  • For example: freedrive([0,0,1,0,0,0]) allows motion only on Z-axis.

  • Helps absorb vertical shift during grip activation.

Additional Tips

• Adjust pick position to reflect TCP and CoG
  Make sure the taught pick position reflects the actual Center of Gravity.

  • When CoG is far from the flange, it increases the pendulum effect.

• Ensure TCP and payload values are accurate
  Incorrect payload or CoG settings lead to poor motion control.

Conclusion

Reducing false Protective Stops in palletizing requires careful tuning of motion parameters, payload management, and grip quality. The strategies above can be tested and combined based on your application’s constraints. Always validate each change one at a time to isolate the root cause of the issue.