Constraint force for closed loop MultibodyPlant

问题

Since RigidBodyTree is on its way out, I'm trying to use MultibodyPlant to study my closed loop multibody robot.

I'm testing things out using the drake/examples/simple_four_bar/FourBar.sdf

my_plant, scene_graph = AddMultibodyPlantSceneGraph(builder)
Parser(plant=my_plant).AddModelFromFile("FourBar.sdf")

But get the following error

RuntimeError: This mobilizer is creating a closed loop since the outboard body already has an inboard mobilizer connected to it. If a physical loop is really needed, consider using a constraint instead.

I'm guessing I should be using AddForceElement to add the constraint forces as shown under Bilateral Position Constraints, is that correct?

I would further believe that DoCalcAndAddForceContribution should implement the constraint force lambda. However, what would be the CalcPotentialEnergy, CalcConservativePower, CalcNonConservativePower?

来源：https://stackoverflow.com/questions/58559987/constraint-force-for-closed-loop-multibodyplant