Add bindings for MultibodyPlant::CalcGeneralizedForce() (#20102)

... and a comment to help find it. This came up in https://stackoverflow.com/questions/77003297/how-to-initialize-a-multibody-force-object-for-calcforceelementscontribution Co-authored-by: Jeremy Nimmer <[email protected]>
RobotLocomotion · Aug 31, 2023 · 09752cc · 09752cc
1 parent ef98b12
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Showing 3 changed files with 18 additions and 2 deletions.
diff --git a/bindings/pydrake/multibody/plant_py.cc b/bindings/pydrake/multibody/plant_py.cc
@@ -637,6 +637,16 @@ void DoScalarDependentDefinitions(py::module m, T) {
         .def("CalcGravityGeneralizedForces",
             &Class::CalcGravityGeneralizedForces, py::arg("context"),
             cls_doc.CalcGravityGeneralizedForces.doc)
+        .def(
+            "CalcGeneralizedForces",
+            [](const Class* self, const Context<T>& context,
+                const MultibodyForces<T>& forces) {
+              VectorX<T> tau(self->num_velocities());
+              self->CalcGeneralizedForces(context, forces, &tau);
+              return tau;
+            },
+            py::arg("context"), py::arg("forces"),
+            cls_doc.CalcGeneralizedForces.doc)
         .def("MakeActuationMatrix", &Class::MakeActuationMatrix,
             cls_doc.MakeActuationMatrix.doc)
         .def(

diff --git a/bindings/pydrake/multibody/test/plant_test.py b/bindings/pydrake/multibody/test/plant_test.py
@@ -2542,6 +2542,8 @@ def test_multibody_dynamics(self, T):
 
         forces = MultibodyForces(plant=plant)
         plant.CalcForceElementsContribution(context=context, forces=forces)
+        tau = plant.CalcGeneralizedForces(context=context, forces=forces)
+        self.assertEqual(tau.shape, (2,))
         copy.copy(forces)
 
         # Test generalized forces.
@@ -2552,6 +2554,8 @@ def test_multibody_dynamics(self, T):
             np.testing.assert_equal(forces.generalized_forces(), 1)
             forces.SetZero()
             np.testing.assert_equal(forces.generalized_forces(), 0)
+            tau = plant.CalcGeneralizedForces(context=context, forces=forces)
+            np.testing.assert_equal(tau, [0, 0])
 
         # Test standalone construction.
         standalone_forces = MultibodyForces(nb=1, nv=2)

diff --git a/multibody/tree/multibody_forces.h b/multibody/tree/multibody_forces.h
@@ -15,8 +15,10 @@ template <typename T> class MultibodyTree;
 template <typename T> class MultibodyTreeSystem;
 }  // namespace internal
 
-/// A class to hold a set of forces applied to a MultibodyTree system.
-/// Forces can include generalized forces as well as body spatial forces.
+/// A class to hold a set of forces applied to a MultibodyTree system. Forces
+/// can include generalized forces as well as body spatial forces.
+/// MultibodyPlant::CalcGeneralizedForces() can be used to compute the _total_
+/// generalized force, combining generalized_forces() and body_forces().
 ///
 /// @tparam_default_scalar
 template <typename T>