RuntimeError: std::exception in 'OpenSim::MocoSolution OpenSim::MocoStudy::solve() const': casVector should be 1-dimensional, but has size 0 x 0. Thrown at MocoCasOCProblem.h:108 in convertToSimTKVector().

[Zlin9878]

windowns + anaconda + python3.9+opensim 4.5

I rewrote the official example example2DWalkingMetabolics.cpp into Python, and it runs successfully. Based on that, I modified the model: I removed the left lower leg muscles from the 2D_gait.osim model and added a SpringGeneralizedForce at the left ankle joint.

I want to optimize the stiffness of this spring during the tracking process, so I used addParameter(osim.MocoParameter) to add the spring’s stiffness as an optimization parameter to the problem. However, when running the code with this parameter included, I encountered the following error:
RuntimeError: std::exception in 'OpenSim::MocoSolution OpenSim::MocoStudy::solve() const':
casVector should be 1-dimensional, but has size 0 x 0.
Thrown at MocoCasOCProblem.h:108 in convertToSimTKVector().

Through debugging, I found that if I do not use addParameter() to add the stiffness parameter, the code runs without issues. So I suspect this error is related to how the optimization parameter is defined.

Could anyone advise how to properly include the spring stiffness as an optimizable parameter in the Moco problem? Thank you!

mycode:

stiffness_param = osim.MocoParameter(
    'prosthetic_stiffness',
    'prosthetic',  
    'stiffness',  
    osim.MocoBounds(40.0, 60.0)
)

problem.addParameter(stiffness_param)

Additionally, I am wondering whether this issue might be caused by how I load the model. In the official example exampleOptimizeMass.py, the model is directly passed to the problem using problem.setModel(model), and the parameter optimization works as expected. However, in my code, I use:
modelProcessor = osim.ModelProcessor("my_modified_model.osim")
track.setModel(modelProcessor)

Could this error be due to using ModelProcessor instead of directly passing a Model? If so, how should I correctly define and optimize model parameters like spring stiffness when using a ModelProcessor?

Also, I'm using MocoTrack, not MocoStudy directly. Could it be that MocoTrack does not handle MocoParameter in the same way as MocoStudy does? Is there a recommended way to optimize model parameters (e.g., spring stiffness) when using MocoTrack?

Any help or insight would be greatly appreciated!

[Zlin9878]

I'm trying to explore whether physical parameters like spring stiffness can be optimized using Moco. I modified the exampleOptimizeMass.py script and replaced the line that adds the parameter with the following:

problem.addParameter(osim.MocoParameter('spring_stiffness', 'spring', 'stiffness',
osim.MocoBounds(90, 100)))

However, when I run the script, I get the following error:

[tropter] Failed to find a solution.
[tropter] DirectCollocationSolver did not succeed:
Infeasible problem detected
...
RuntimeError: std::exception in 'SimTK::Real const & OpenSim::MocoTrajectory::getParameter(std::string const &) const':
This trajectory is sealed, to force you to acknowledge the solver failed; call unseal() to gain access.
Thrown at MocoTrajectory.cpp:1473 in ensureUnsealed().
Does this mean that parameters like stiffness cannot be optimized in Moco?
Also, I would like to know if joint position parameters, such as translation/orientation, can be optimized as well.

Thanks in advance for any help!

[nicos1993]

Hey @Zlin9878

Are you confident that you are specifying the correct ComponentPath and PropertyName:
https://simtk.org/api_docs/opensim/api_docs/classOpenSim_1_1MocoParameter.html

You also may have more luck with responses by posting your question on the Moco forum:
https://simtk.org/plugins/phpBB/indexPhpbb.php?group_id=1815&pluginname=phpBB

Thanks,

Nicos

[Zlin9878]

Hi Nicos @nicos1993

Thanks for your reply!

Yes, I believe the ComponentPath and PropertyName are correct—I double-checked them using the following code to print out the component's path and its available properties.

spring = model.getComponent('prosthetic')
print(spring.getAbsolutePathString())
for i in range(spring.getNumProperties()):
    prop = spring.getPropertyByIndex(i)
    print(f"{i:2d} | {prop.getName()} = {prop.toString()}")

the output is:
/prosthetic
0 | components = (No Objects)
1 | appliesForce = true
2 | coordinate = (ankle_angle_l)
3 | stiffness = 50
4 | rest_length = 0
5 | viscosity = 15

I'll also consider posting the question on the Moco forum as you suggested. Thanks again for the help!

Best,
Ziling

[nickbianco]

Thanks for your post @Zlin9878. Closing this to continue discussion on the Moco forum. We can re-open or open a new issue if there indeed is a bug.

[Zlin9878]

@nickbianco Hi, thanks for your response.

I'm not entirely sure whether this is a bug or not. However, I have double-checked that the parameter path I'm using is correct, but the error still persists.

I'd appreciate it if we could continue the discussion to help identify the issue. Please let me know if I should provide more details or test anything specific.

Thanks again!

[nickbianco]

@Zlin9878 please see my response to your post on the Moco forum. We can continue our discussion there.