[ros-users] IK with KDL

Jeroen Willems jjpa.willems at gmail.com
Thu Nov 11 10:52:06 UTC 2010

On 2010-11-09 09:27, Herman Bruyninckx wrote:
> On Tue, 9 Nov 2010, Jeroen Willems wrote:
> > I have a question regarding the inverse kinematics using KDL.
> >
> > I would like to use carttojnt of the inverse kinematics solver.
> > The problem is I have to use a destination frame as input.
> > I'm using something similar to a elbow manipulator so you can imagine I
only want a reference position.
> > The arm only can have 2 poses to a reference position, to get the
rotational part of the frame I actually have to get the joint positions to
calculate it (correct me if I'm wrong).
> >
> > So
> > 1. Am I right?
> > 2. Could I use KDL to compute the IK or should I use an analytic
> The major issue with your problem is that "IK" does not make too much
> for a 2DOF system: when you don't take precautions the input frame will
> never lie in the span of your 2DOF, so you will have make a "projection"
> from the full 6D space onto your 2D subspace. KDL has algorithms to do
> that, but they make choices about how to project that might not correspond
> to your intuition. But that's inevitable.
> So, please try to formulate exactly what kind of input you would like or
> need to have.
> Herman
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Thanks for the response.

Okay to be more precise to give a better idea of what I'm doing.
I have an hexapod, so a torso with 6 legs consisting of 3 links (without
torso) and 3 joints.
I want to be the tip/foot on an specific point (x,y,z) in respect to the
torso (thorax) so I have to know the three joint positions to get there.

So preferable without giving a rotation. So I'm really curious how to obtain
this or how to use a "projection".

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