Implement PlanarJoint (RobotLocomotion#13747)

multibody/tree/BUILD.bazel

@@ -103,6 +103,7 @@ drake_cc_library(
         "multibody_forces.cc",
         "multibody_tree.cc",
         "multibody_tree_system.cc",
+        "planar_joint.cc",
         "planar_mobilizer.cc",
         "prismatic_joint.cc",
         "prismatic_mobilizer.cc",
@@ -140,6 +141,7 @@ drake_cc_library(
         "multibody_tree.h",
         "multibody_tree-inl.h",
         "multibody_tree_system.h",
+        "planar_joint.h",
         "planar_mobilizer.h",
         "prismatic_joint.h",
         "prismatic_mobilizer.h",
@@ -510,6 +512,14 @@ drake_cc_googletest(
     ],
 )
 
+drake_cc_googletest(
+    name = "planar_joint_test",
+    deps = [
+        ":tree",
+        "//common/test_utilities:expect_throws_message",
+    ],
+)
+
 drake_cc_googletest(
     name = "prismatic_joint_test",
     deps = [

multibody/tree/planar_joint.cc

@@ -0,0 +1,69 @@
+#include "drake/multibody/tree/planar_joint.h"
+
+#include <memory>
+
+#include "drake/multibody/tree/multibody_tree.h"
+
+namespace drake {
+namespace multibody {
+
+template <typename T>
+template <typename ToScalar>
+std::unique_ptr<Joint<ToScalar>> PlanarJoint<T>::TemplatedDoCloneToScalar(
+    const internal::MultibodyTree<ToScalar>& tree_clone) const {
+  const Frame<ToScalar>& frame_on_parent_body_clone =
+      tree_clone.get_variant(this->frame_on_parent());
+  const Frame<ToScalar>& frame_on_child_body_clone =
+      tree_clone.get_variant(this->frame_on_child());
+
+  // Make the Joint<T> clone.
+  auto joint_clone = std::make_unique<PlanarJoint<ToScalar>>(
+      this->name(), frame_on_parent_body_clone, frame_on_child_body_clone,
+      this->damping());
+  joint_clone->set_position_limits(this->position_lower_limits(),
+                                   this->position_upper_limits());
+  joint_clone->set_velocity_limits(this->velocity_lower_limits(),
+                                   this->velocity_upper_limits());
+  joint_clone->set_acceleration_limits(this->acceleration_lower_limits(),
+                                       this->acceleration_upper_limits());
+
+  return joint_clone;
+}
+
+template <typename T>
+std::unique_ptr<Joint<double>> PlanarJoint<T>::DoCloneToScalar(
+    const internal::MultibodyTree<double>& tree_clone) const {
+  return TemplatedDoCloneToScalar(tree_clone);
+}
+
+template <typename T>
+std::unique_ptr<Joint<AutoDiffXd>> PlanarJoint<T>::DoCloneToScalar(
+    const internal::MultibodyTree<AutoDiffXd>& tree_clone) const {
+  return TemplatedDoCloneToScalar(tree_clone);
+}
+
+template <typename T>
+std::unique_ptr<Joint<symbolic::Expression>> PlanarJoint<T>::DoCloneToScalar(
+    const internal::MultibodyTree<symbolic::Expression>& tree_clone) const {
+  return TemplatedDoCloneToScalar(tree_clone);
+}
+
+// N.B. Due to esoteric linking errors on Mac (see #9345) involving
+// `MobilizerImpl`, we must place this implementation in the source file, not
+// in the header file.
+template <typename T>
+std::unique_ptr<typename Joint<T>::BluePrint>
+PlanarJoint<T>::MakeImplementationBlueprint() const {
+  auto blue_print = std::make_unique<typename Joint<T>::BluePrint>();
+  auto planar_mobilizer = std::make_unique<internal::PlanarMobilizer<T>>(
+      this->frame_on_parent(), this->frame_on_child());
+  planar_mobilizer->set_default_position(this->default_positions());
+  blue_print->mobilizers_.push_back(std::move(planar_mobilizer));
+  return blue_print;
+}
+
+}  // namespace multibody
+}  // namespace drake
+
+DRAKE_DEFINE_CLASS_TEMPLATE_INSTANTIATIONS_ON_DEFAULT_SCALARS(
+    class ::drake::multibody::PlanarJoint)