I have been trying to use Stretch robot for grasping tasks but encountered an issue that the Stretch gripper could not be fully opened. After the stretch_robot_home.py command, the robot gripper would not be repositioned into the “wide open” state. Then if I try the keyboard_teleoperate ros2 node to open the gripper more, the gripper would close instead of opening first (see the video), appearing that its joint limit has been identified at a wrong position.
Could it be a hardware malfunctioning or just a bad calibration for the gripper?
The described behavior sounds like a calibration issue for the gripper. To recalibrate the open and closed gripper motor positions, you can use the following command line tool:
REx_gripper_calibrate.py
This tool walks you through manually setting the grasp and fully open gripper positions. Hope that helps!
Thanks! This is exactly what I have been looking for. The gripper works normal now with ros2 control inputs after calibration. The only remaining problem now is that the distance between fully opened fingertips for this gripper (right in the image) is still shorter than the other gripper (left in the image) we have.
The aperture of the gripper has some variability that can depend on the bend accuracy of the spring steel fingers themselves. With care, you can manually bend this part in the direction you like, which may yield an extra cm or two of width at the maximum opening position.
To do this, start with the robot fully powered off, or with the gripper detached from the robot. Gently grasp the fingertips and apply pressure to open the gripper. The joint should backdrive, even past the point where the fingers would naturally stop. Keep going until the gripper is no longer backdriving and you are putting pressure directly on the metal joints. With care, you can put some strain on the middle joint between the fingers as well as the bend joints at the base of each finger. Do this progressively, adding a bit of extra pressure each time, until the gripper aperture is slightly enlarged. I’ve attached a short video of the bending procedure.
Once you have done this, you will want to calibrate the gripper to ensure you can take advantage of the new open position. Run the command line script REx_gripper_calibrate.py and follow the instructions as presented to you.
