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Joints not independent

asked 2017-05-02 09:15:59 -0600

sal14 gravatar image

updated 2017-05-08 06:57:43 -0600

I am trying to set the PID position controller on my 7-DOF robotic arm using ros-control. However, the joints are not behaving independently. For instance when I move joint 2, joint 4 moves a bit as well. This makes the PID tuning complicated. How can I make the joints independent of each other so that I can set each joint's PID separately? Do you think that the problem is because of the inertial values?

Here is a video showing this behavior. I am giving a reference angle (position) to the last joint (the wrist joint). However as can be seen, the joints before are also moving even though I tuned the PID values of each joint. Also, when increasing the damping value, sometimes the link starts vibrating a lot.

Also, when setting one of the joints (which is on a horizontal plane) as revolute, the link starts moving even when the set position is 0, however when changing it to continuous joint it behaves correctly. Why is this?


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It's a bit difficult to imagine what you're talking about without an example or images/videos, do you think you could add any of those to your question?

chapulina gravatar imagechapulina ( 2017-05-02 09:43:17 -0600 )edit

I added a video to show such behavior. Thanks

sal14 gravatar imagesal14 ( 2017-05-08 06:58:10 -0600 )edit

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answered 2017-05-08 09:30:10 -0600

Thank you for the video showing the problem.

If I understood correctly, your problem is that you want to tune the PID of a joint on the base of the robot, but as you're doing it, other joints move too, right? They are moving because they're not "locked" and react to the forces generated by the joint you're controlling, just like they would in real life.

One thing you could do is changing your robot's description to use fixed joints on all joints except the one you're trying to tune.

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Thanks for your reply. I did tune each joint while setting the other joints as fixed. However, once each joint behaved correctly I replaced each joint back to revolute. But still, when moving certain joints, others still oscillate as in the video.

sal14 gravatar imagesal14 ( 2017-05-08 16:12:21 -0600 )edit
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Asked: 2017-05-02 09:15:59 -0600

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Last updated: May 08 '17