Add DeformableBody as a new MultibodyElement

Since deformable bodies have discrete states, allow MultibodyElement to
declare discrete states in the tree system.

Move the bookkeeping for deformable bodies from DeformableModel to
DeformableBody. The deformable bodies are stored as an element
collection in DeformableModel.

Remove deformable_ids.h and move the definitions there
into multibody_tree_indexes.h.

Author: xuchenhan-tri

multibody/plant/BUILD.bazel

@@ -19,7 +19,6 @@ drake_cc_package_library(
         ":contact_jacobians",
         ":contact_results",
         ":coulomb_friction",
-        ":deformable_ids",
         ":desired_state_input",
         ":discrete_contact_data",
         ":discrete_contact_pair",
@@ -43,7 +42,6 @@ drake_cc_library(
         "constraint_specs.h",
     ],
     deps = [
-        ":deformable_ids",
         "//multibody/tree:multibody_tree_indexes",
     ],
 )
@@ -105,7 +103,6 @@ drake_cc_library(
     hdrs = [
         "compliant_contact_manager.h",
         "deformable_driver.h",
-        "deformable_ids.h",
         "deformable_model.h",
         "discrete_step_memory.h",
         "discrete_update_manager.h",
@@ -128,7 +125,6 @@ drake_cc_library(
         ":contact_jacobians",
         ":contact_results",
         ":coulomb_friction",
-        ":deformable_ids",
         ":desired_state_input",
         ":discrete_contact_data",
         ":discrete_contact_pair",
@@ -262,16 +258,6 @@ drake_cc_library(
     ],
 )
 
-drake_cc_library(
-    name = "deformable_ids",
-    hdrs = [
-        "deformable_ids.h",
-    ],
-    deps = [
-        "//common:identifier",
-    ],
-)
-
 drake_cc_library(
     name = "desired_state_input",
     srcs = [

multibody/plant/constraint_specs.h

@@ -10,7 +10,6 @@
 #include <vector>
 
 #include "drake/math/rigid_transform.h"
-#include "drake/multibody/plant/deformable_ids.h"
 #include "drake/multibody/tree/multibody_tree_indexes.h"
 
 namespace drake {

multibody/plant/deformable_driver.cc

@@ -631,18 +631,17 @@ void DeformableDriver<T>::AppendDeformableRigidFixedConstraintKinematics(
           .template Eval<geometry::GeometryConfigurationVector<T>>(context);
   for (DeformableBodyIndex index(0); index < deformable_model_->num_bodies();
        ++index) {
-    DeformableBodyId body_id = deformable_model_->GetBodyId(index);
-    if (!deformable_model_->is_enabled(body_id, context) ||
-        !deformable_model_->HasConstraint(body_id)) {
+    const DeformableBody<T>& body = deformable_model_->GetBody(index);
+    if (!body.is_enabled(context) || !body.has_fixed_constraint()) {
       continue;
     }
 
-    const FemModel<T>& fem_model = deformable_model_->GetFemModel(body_id);
+    const FemModel<T>& fem_model = body.fem_model();
     const DirichletBoundaryCondition<T>& bc =
         fem_model.dirichlet_boundary_condition();
 
-    /* Returns true iff for the deformable body with `body_id`, the given vertex
-     index is under boundary condition. */
+    /* Returns true iff for the deformable body, the given vertex index is under
+     boundary condition. */
     auto is_under_bc = [&](int vertex_index) {
       return bc.index_to_boundary_state().count(
                  multibody::fem::FemNodeIndex(vertex_index)) > 0;
@@ -653,15 +652,13 @@ void DeformableDriver<T>::AppendDeformableRigidFixedConstraintKinematics(
     // Each deformable body forms its own clique and are indexed in inceasing
     // DeformableBodyIndex order and placed after all rigid cliques.
     const TreeIndex clique_index_A(tree_topology.num_trees() + index);
-    GeometryId geometry_id = deformable_model_->GetGeometryId(body_id);
+    const GeometryId geometry_id = body.geometry_id();
     const PartialPermutation& vertex_permutation =
         EvalVertexPermutation(context, geometry_id);
     const VectorX<T>& p_WVs = configurations.value(geometry_id);
 
-    for (const MultibodyConstraintId& constraint_id :
-         deformable_model_->fixed_constraint_ids(body_id)) {
-      const DeformableRigidFixedConstraintSpec& spec =
-          deformable_model_->fixed_constraint_spec(constraint_id);
+    for (const DeformableRigidFixedConstraintSpec& spec :
+         body.fixed_constraint_specs()) {
       const int num_vertices_in_constraint = ssize(spec.vertices);
       /* The Jacobian block for the deformable body A. */
       Block3x3SparseMatrix<T> negative_Jv_v_WAp(
@@ -1056,23 +1053,21 @@ void DeformableDriver<T>::CalcDeformableContact(
   /* Complete the result with information on constraints other than contact. */
   for (DeformableBodyIndex body_index(0);
        body_index < deformable_model_->num_bodies(); ++body_index) {
-    DeformableBodyId body_id = deformable_model_->GetBodyId(body_index);
+    const DeformableBody<T>& body = deformable_model_->GetBody(body_index);
     /* Add in constraints. */
-    if (deformable_model_->HasConstraint(body_id)) {
+    if (body.has_fixed_constraint()) {
       std::unordered_set<int> fixed_vertices;
-      for (const MultibodyConstraintId& constraint_id :
-           deformable_model_->fixed_constraint_ids(body_id)) {
-        const DeformableRigidFixedConstraintSpec& spec =
-            deformable_model_->fixed_constraint_spec(constraint_id);
+      for (const DeformableRigidFixedConstraintSpec& spec :
+           body.fixed_constraint_specs()) {
         fixed_vertices.insert(spec.vertices.begin(), spec.vertices.end());
       }
       /* Register the geometry in case it hasn't been registered because it's
        not participating in contact but is participating in other types of
        constraints. */
-      GeometryId geometry_id = deformable_model_->GetGeometryId(body_id);
+      GeometryId geometry_id = body.geometry_id();
       if (!result->IsRegistered(geometry_id)) {
-        result->RegisterDeformableGeometry(
-            geometry_id, deformable_model_->GetFemModel(body_id).num_nodes());
+        result->RegisterDeformableGeometry(geometry_id,
+                                           body.fem_model().num_nodes());
       }
       result->Participate(geometry_id, fixed_vertices);
     }

multibody/plant/deformable_ids.h

@@ -6,13 +6,6 @@
 namespace drake {
 namespace multibody {
 
-/** Uniquely identifies a deformable body. It is valid before and after
- Finalize(). */
-using DeformableBodyId = Identifier<class DeformableBodyTag>;
-
-/** (Internal use only) Indexes deformable bodies, only used after Finalize().
- */
-using DeformableBodyIndex = TypeSafeIndex<class DeformableBodyTag>;
 
 }  // namespace multibody
 }  // namespace drake