Inverse kinematics issue

[LatvianStudent]

So I've got my KUKA robot model with 6 DoF, I have adjusted it to follow the target in IK mode as in the IK tutorial, it works fine mostly, but the problem is, when it idles in it's default (home) position, the 4th DoF sort of twitches for about +/- 20 degrees.

I've set the IK calculation modules to:

1) Pseudo Inverse, max it. 9 (restore if.. both checkmarks checked)
2) Pseudi Inverse, max it. 3 (restore if.. first checkmark checked)
3) DLS, max it. 99, damping 1.0 (restore if.. zero checkmarks checked)

[coppelia]

Hello,

your robot is very probably in a singular configuration (i.e. a configuration close to the workspace limit, or a configuration among an infinite for a given tooltip pose). Try chosing a different home position since IK cannot nicely handle those ambiguous situations. Also, 3 iterations are most of the time more than enough for a non-damped resolution method.

Cheers

[LatvianStudent]

choosing a different home position didn't help, scaling down the iteration count limited the twiching part to only the first 4 seconds of the simulation, after that is idled smoothly, but it's still not a sufficient solution. what do you mean with close to workspace limit? can I try some more possible fixes?

[coppelia]

Can you share your scene, so that we can have a closer look at it?

[LatvianStudent]

Yes, of course. Thanks!

http://files.fm/u/srkviil

[coppelia]

In your scene, you have joints joint6 and joint4 that are aligned in the home position: there is an infinity of configurations that satisfy the tooltip position and orientation. That's why you see that instability. If you set joint joint5 to 90 degrees as the home position, then you won't have that problem anymore. You really have to keep away from singularities when using IK. Or, you can disable the 2 undamped IK groups, and for the damped IK group use following settings:

• Damping: 0.001
• Max. iterations: 10

Another difficulty is that you have joint limits, and the IK module will not do motion planning: it will try to reach the target position/orientation on a straight path. Next release (V3.0.5) should also have a motion planning module ready for those situations.

Cheers

[LatvianStudent]

Thank you very much!