the reaction force of the joint between two vertebrae is oriented with respect to the orientation of the first node of the spherical joint. This corotates like the adjacent below vertebra.
I would like to have the reaction forces with respect to an orientation which is in the midplane of the two adjacent vertebral endplates, i. e. it is neither the orientation of the caudal nor of the cranial vertebra but in between.
I vainly tried to calculate the desired ARel using current coordinates of some nodes which are - of course - not available before they are calculated.
You can run two analyses - first to find the midplane reference frame that you would like to use for output. And second to output the forces, where you would add a dummy segment, driven by data from the first analysis, and create an AnyForceMomentMeasure to recompute the reaction forces in the reference frame of that segment.
Is to add a dummy segment and drive it to stay in the midplane of the adjacent endplates. And then again use an AnyForceMomentMeasure to recompute the joint reaction forces.
And I guess they are very similar, except that the implementation will differ a little.
i think, idea 2 is more elegant. I would like to create a dummy segment which is driven to be oriented in the midplane and: I would call it disc :o. I could than connect this disc via two coincident sperical joints to the adjacent vertebrae, one for each. The joint reaction forces would then already be in the desired coord frame.
Unfortunately i am not familiar with drivers. How could i drive this dummy (or disc) segment to have the desired orientation? I created already several nodes lying on the vertebral endplates. I guess, i could use them for this purpose, e. g. by using the midpoints of each corresponding pairs of nodes from the adjacent vertebral endplates.
How can nodes be driven to stay in the middle of two other nodes?
Create 4 kinematic measures (2 linear: endplates-dummy segment and 2 rotational: endplates-dummy segment). Then you can drive the distances/angles to be the same by using coefficients {1, -1} for each pair and driving this measure to all zeros. Schematically it would look like:
It is clear what the normal of the midplane would be - a mean of the endplate normals, but you need to be careful about the point, that defines the plane. Depending on where it will be - the force/moment configurations may be different due to different moment arms. But possibly you have an idea on that.
just to avoid misunderstanding:
Do you mean an additional segment driven to have the middle rotation and translation? Or do you refer to my suggestion, creating a disc segment which contains two coincident spherical joints replacing the currently existing ones?
I meant an additional floating in space segment. I was referring to solution #2, because I am not sure how adding a ‘disc segment’ will help you. Spherical joints add linear constraints, that will not allow the output to be on midplane.
now i understand what you mean. But you are right, there is still the problem with the moment lever arms.
Another suggestion: What about using an additional segment that has two sperical joints (that means 3 more DOFs than in original model) which are each connected to the adjacent vertebrae. And than remove these additional 3 DOFs by using your suggestion and equally distribute only the rotations of the adjacent vertebrae to the additional segment with
If i understand it correct - you suggest to fix this segment in the joint node, but allow rotation and then rotate it by a half of the angle. Yes, that would work too, in a way it is the same as solution #2, but you constrain the distance between endplates and this segment. Bear in mind that this segment will not necessary lie on the midplane, since the joint node-endplate distances are most likely not equal for the adjacent vertebra. Although it may be close enough - needs to be checked.
