既然RigidBodyTree快要出来了，我就试着用MultibodyPlant我的闭环多体机器人来研究。
我正在使用drake/examples/simple_four_bar/FourBar.sdf
my_plant, scene_graph = AddMultibodyPlantSceneGraph(builder)
Parser(plant=my_plant).AddModelFromFile("FourBar.sdf")
但是得到以下错误
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.
我猜我应该使用AddForceElement添加约束力，如Bilateral Position Constraints下所示，对吗？
我会进一步认为DoCalcAndAddForceContribution应该实施约束力lambda。但是， , ,CalcPotentialEnergy是CalcConservativePower什么CalcNonConservativePower？