[Robotics] Computing IK Joint Angles Pt.3

[Séverin Lemaignan]

Hello Florian,

While I can't really help you on your joint angles issue, I've quickly
checked your blog and the video you made: As soon as everything works well
enough, it would be great to add a tutorial on the Blender wiki
(http://wiki.blender.org/index.php/Robotics:Index) on "robotic arms &
Blender". I know quite a few people that would be very interested in it!

Séverin

> Hey all and especially Benoit,
>
> i've implemented Benoits approach for computing joint angles (in degrees)
>
> To recapitulate the problem:
>
> I've modelled a robot arm with 5 joints (all joints are revolute
joints). The arm is
> driven by an "empty" target to an IK solver on top of the bone chain. I
need to
> get the joint angles in degrees or rad. Those joint angles are in range
of the X,Y,Z
> limits of the bones which form the chain.
>
> The approach:
>
> *relativePose[n] = inverse(poseMatrixTail[n-1])*poseMatrix[n]
>
> "where n is the bone for which you compute the joint values and
> poseMatrixTail is simply poseMatrix where the 4th column is replaced by
tail. For the root bone, poseMatrixTail is the identity matrix."
>
> *As far as I've tested the setup all angles are computed correctly. The
only problem is, that
> I need to convert the angles from poseMatrix -> toEuler(). That's the
last pitfall. Euler angles
> are a bit tricky because the represent [yaw, pitch, bank]. By rotating
some (or one) of the
> parent bone/s the YAW, PITCH, BANK locally changes for the "target bone"
[n] (for which I want to
> compute the joint angle) so the resulting angle is either positive or
negative - what's not THAT bad.
> The biggest problem is - an euler angle is represented like [x (yaw),z
(pitch),y (bank)].
> Let's assume a bone rotates about the x axis (its' DoF). If all bones
are in rest position the output when
> rotation the bone is like: [100, x, x] after rotation one of it's
parents the result is maybe: [x,x,100].
> To me there's so predictable coherence whether the result can be found
in x y or z.
>
> *How can someone solve this problem?*
>
> I've digged deep in the blender source code especially in armature.c and
the IK_Solver sources.
> The angle I need is represented as a float value in there :/.
>
> [blender source from svn]
>
> blender/intern/iksolver/intern/IK_QSegment.cpp [line 602]
>
> BUT wrapping this code to Python, making the code invokable and finally
extending the Python API
> seems a little to exaggerated to me ...
>
> So having the feature to extract several angles from an IK chain would
be great in case of using blender
> for robotics.
>
> It would be great if someone can help me out - or if we can solve this
problem in a more general manner
> like really extending the python API ? I'll attach the appropriate blend
file so everybody can play around
> or visualize the problem. Most of the code is documented.
>
> You can find the blend file here:
> *http://www.icram.de/BpyJointAngles.blend*
>
> Cheers, Florian
>
>
> P.S. Sorry for crossposting this, but this is / maybe / not only a
robotics issue...
>
>
>
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