PolynomialFitter detects too many actuated coordinates when model kinematic chain is not standard

[tomvanwouwe1992]

Error:
File "C:\Users\tom_v\miniconda3\envs\BioToHydrow\lib\opensim\actuators.py", line 4802, in run
return _actuators.PolynomialPathFitter_run(self)
RuntimeError: std::exception in 'void OpenSim::PolynomialPathFitter::run()': The path '/forceset/addbrev_l' depends on more than 6 coordinates. This is not supported.
In Object '' of type PolynomialPathFitter.
Thrown at PolynomialPathFitter.cpp:830 in filterSampledData().
See minimal code to replicate error. When I change the model definition, pelvis as floating base and no kinematic constraints, it works well.
testFitting.zip

[nickbianco]

When I change the model definition, pelvis as floating base and no kinematic constraints, it works well.

Did the original model not have a floating base? Though, my guess is that it has to do with the patella constraints, I think I've seen those lead to incorrect moment arms before.

You could try adjusting the moment arm threshold setting in PolynomialPathFitter as a temporary fix to ignore slighly non-zero moment arms that erroneously appear. That might be the best solution for now, fixing moment arm calculations with constraints would be a much deeper issue.

[tomvanwouwe1992]

The model for which it does not work has a slider joint to connect a floating base to the ground. The toes are welded to this base and then the kinematic chain continues to the pelvis. The pelvis is also connected to this base (point on line constraint), closing the kinematic loop.
To perform the fitting process now, I just made a version of the same model with a more standard definition (6D pelvis as floating base and without kinematic constraints). This works well.

[nickbianco]

Thanks for the clarification. I do think this is a deeper issue related to how moment arms are calculated in OpenSim.

By the way, all Moco development happens within the core OpenSim repo (https://github.com/opensim-org/opensim-core), so please post future issues there.