Hi Jerome,
Thank you for writing to the group.
- Aligning (P and Q) markers:
Right now you have the grey P markers from the motion capture and the
green Q markers belonging to the body. First you have to proceed to
an approximate match of the P and Q markers by aligning the body. Use
the Mannequin,any file for this, this file controls the initial
position of the body at load time. Here you can set all the joint
angles of the body, so you can start by modifying the thorax
flexion/extention to have the Q thoracic markers close to the P one.
Do not modify the drivers yet. Once the thorax is in a good position,
do the same for the arms: modify the joints angles in the Mannequin
file until you have the markers pretty well matching. Notice that the
markers do not need to match perfectly a difference of 2 centimetres
between them for example is usually acceptable, you adjust again the
initial position later if needed to have a closer match. You can also
change slightly the position of the markers on the body using the
MarkerPlacement.any file for a more accurate matching.
Now you can start to remove the joint angle drivers and replace them
by linear drivers between the markers in the JointsAndDriver.any
file. You should remove and replace the drivers on by one and make
sure to keep the model running each time. Start by the body parts
close to the pelvis (thorax) and then the extremities (arms).
When you scale the model you also scale the body markers while the
markers from the MoCap remain at the same place. So the foot markers
that was matching before the scaling are not matching anymore after
it. If you don’t make the necessary changes to make them match again
then the kinematic will end up in some incorrect results.
I also saw a wrong value in your initial position. You set the value
of the PelvisThoraxExtension to -14.9959 in the Mannequin file. This
is bending forward whereas the markers are bending backward at load
time, this will cause troubles in setting the initial position. Just
remove the – sign and set the value to 14.9959.
- Muscles over-stretched error:
It is important to run the CalibrationSequence each time you load the
model, if not you will get this error message you described. It
appeared when I ran the CalibrationSequence in your model that the
calibration stopped due to a wrapping error in some muscle. So the
calibration is not fully completed and some muscles are not
calibrated and then give you the over-stretched error.
The wrapping error can be easily fixed by increasing the StringMesh
of the biceps_brachii_caput_breve muscle in the file
BRep/Aalborg/Arm3D/Muscle.any.
After fixing this the inverse dynamic analysis is running without
problems.
- How do I know the correct strength parameter to use? They can’t
be the same for every person I assume.
It is right, nobody has the exact same strength. We are using
strength from the literature, the references are available in the
Muscle-parameters-shoulder file. You can modify those data if you
have precise data about the strength of the person you are modelling.
4)…………
The body markers do not have to be exactly on the bone. They are
linked to it but can be some centimetres away. Actually the real
markers are on the surface of the skin and not on the bones so it is
normal that the body markers are slightly distant from the bones.
Then the left foot is not constrained to the vicon marker because the
two feet are link together by the plate. Both feet are actually fixed
to the plate, by driving the right foot you also drive the plate and
the left foot.
- I’m still not sure how to choose the directions to constrain the
different markers
The drivers used in your model are driving linear measure. So when
you drive a marker on the X direction that means that the body marker
is going to follow the Vicon marker regarding to the translation on
the X direction (in the local frame of the segment).
The model should work nice after making those two changes (initial
position and muscle wrapping).
Best regards,
Sylvain, AnyBody Support.
— In anyscript@yahoogroups.com, “Jerome” <jromeo_sc_forever@…>
wrote:
>
> Hi Sylvian,
>
> Thanks again for the help prior to this in helping me get started.
I
> am posting these questions as per your request. Also, I’ve uploaded
> the model I’ve been working on so that you can take a look and help
> me where you can. Most especially with the ‘muscles over-stretched’
> issue. For me this is the most crucial to get solved.
>
> The file name is Ank001. Please run the ‘Ank001.LowerBody.Main.any’
> file.
>
> Here are my questions as they pertain to my model:
>
> 1) Aligning (P and Q) markers:
>
> I still do not understand the process of aligning the markers.What
> are the steps I need to take (starting just after converting the
c3d
> files to ‘.any’ files) and which files do I need to modify.
>
> I noticed that some of the misalignment in the model had to do with
> scaling issues, so I tried re-scaling the model. I made the torso
> shorter (from 0.6 to 0.535) by using the measured value we have
here.
> This brought the markers a bit closer. However when I tried to
change
> the foot and shank length these brought about incorrect
orientations
> of the foot and platform, so I left them with the
> values you had used initially. The foot length we measured was
0.265m
> and the shank was 0.45m. Why is it that changing these
> lengths make the foot motion incorrect ?
>
> *2) Muscles over-stretched error:
> I plan on comparing some leg muscle activation with experimental
emg
> data and so I need to use the hill type muscles in the model.
> However, when I run the “SetInitiatialCondition”
> or “KinematicAnalysis” I get a list of messages saying that each
> muscle is over-stretched. I got this error regardless of the fact
> that I ran (what I think is) the calibration sequence in the
> model.
>
> Why is this happening and how can I fix it?
>
> 3) How do I know the correct strength parameter to use? They can’t
> be the same for every person I assume.
>
>
> 4) I noticed that when I’m able to run the “SetInitialCondition”
> process(currently in the no muscle version of the model) the heel
> marker from the motion capture system seem to be quite far from the
> actual heel of the skeleton. Do you have an idea why this may be ?
> Also, only the right foot marker are constrained to the vicon
(gray)
> motion markers. When I try to constrain them for the left foot I
get
> an error saying the model is over-constrained. Why is this ? I
would
> also like the left foot to have the correct motion associated with
> it. The model also becomes over constrained if I try to constrain
the
> P(vicon) and Q (anybody) markers of the heels.
>
> 6) I’m still not sure how to choose the directions to constrain the
> different markers. By contrainin the knee to X
> does this mean it will only move in the x direction or does this
mean
> it will only rotate about the x-axis ?
>
> Thanks in advance.
>
> Jerome
>