CoordinateCouplerConstraint does not handle multiple independent coordinates properly

[nickbianco]

CoordinateCouplerConstraint should support multiple independent coordinates since the property independent_coordinate_names is a list property. However, the function that computes the coordinate coupling error only takes into account the first two coordinate in the total list of coordinate, whether or not they are independent or dependent:

opensim-core/OpenSim/Simulation/SimbodyEngine/CoordinateCouplerConstraint.cpp

Line 47 in 22af32b

double calcValue(const SimTK::Vector& x) const override {

[bfregly]

We would like to be able to have two (or more!) independent coordinates as inputs to a coordinate coupler constraint as well. Please let us know when this capability is available. You will probably need to add the ability to handle a <coupled_coordinates_function> that is defined analytically, like sqrt(x^2/2+y^2/4) where dependent coordinate z is an analytical function of independent coordinates x and y.

[nickbianco]

Hi @bfregly! CoordinateCouplerConstraint already has the property coupled_coordinates_function, which accepts any OpenSim::Function. There's no class to support generic expressions for coordinate functions, but perhaps we could add a class similar to ExpressionBasedBushingForce.

The fix we're working on should support any number of independent coordinates, as long as the Function supports those arguments and is defined up to the second derivative.