Measurement related to the COP of AnyForceSurfaceContact

Hi,

i am working on a FDK knee model and I have a question regarding the COP.
My goal is to determine the velocity of the COP on the implant surface.
My idea was to drive a segment with the current position of the COP and do different measurements witch respect of the tibial and femoral coordinate system, but it does not work.
I tried different things, but I can not do a measurement on the COP.
I get Errors like: argument will not be ready for evaluation until moment ‘ForceVar’
or:
ERROR(SYS1) : Model load : Unknown error
Closing model…
Deleting loaded model…

Does anyone have any idea how to solve my problem?

My last option is to make a separate model, load all implant surfaces with their calculated motions and than drive a dummy Segment witch the fully calculated trajectory of the COP. But I want to avoid creating a new model.

Thank you for your help!