External weight has no effect on joint torques (iiwa)

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javl0p
Posts: 16
Joined: 05 Apr 2017, 08:53

External weight has no effect on joint torques (iiwa)

Postby javl0p » 15 May 2017, 10:58

Good Morning!

I am currently working on the joint torque limitations in a KUKA iiwa 14. In my VREP scene I added a graph showing the joint torques exerted at every joint. I attached a gripper (generic gripper, just a box) to the end-effector. In order to do so I used a force sensor as linking element.

I programmed a simple motion in order to easily analize what is going on in joints 2 and 4. I tested my scene by changing the gripper weight from 0.01 kg to 14 kg and then watched what happened in the joint torques. It turns out, joint torques are almost the same, as if the weight of the gripper had no effect at all. Then I tried the same thing but changing the weight of the 8th link. Here I got very different result! that added weight had its corresponding effect on joints 2 and 4, resulting on much bigger joint torques, as expected.

So, what am I doing wrong? Is it because the way the gripper is attached to the robot?

I post the scene here: https://www.dropbox.com/s/qku4zprj8awyy ... t.ttt?dl=0


Thank you!!

coppelia
Site Admin
Posts: 5743
Joined: 14 Dec 2012, 00:25

Re: External weight has no effect on joint torques (iiwa)

Postby coppelia » 15 May 2017, 15:50

Hello,

you have two problems in your scene:

  • the weight at the tip of the manipulator is slightly colliding with the manipulator. You can either move the weight a little bit further away, or set the weight as non-respondable.
  • the weight is attached to a static shape, itself attached on a non-static shape. So we have: dynamic --> static --> dynamic, which doesn't make sense. You should attach the force sensor of the weight to object LBR_iiwa_14_R820_link8_resp.

Cheers

javl0p
Posts: 16
Joined: 05 Apr 2017, 08:53

Re: External weight has no effect on joint torques (iiwa)

Postby javl0p » 16 May 2017, 08:53

It works fine now. Thank you!


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