You can look at the model DocumentationExamples.Control.CameraStabilizer for help here, specially if, and I'm guessing here, but you seem to be interested in the forces all being aligned along the z axis, so introducing them through prismatic constraints would help, as is done in the model I mention above. This model also has input forces, similarly constrained, and it does some initialization of the system, that I think may be affecting your model. And it forces some state selection through stateSelect = StateSelect.always that may be helpful. If this does not work, consider making a smaller minimal model indicating precisely what you expect in the linearization.