Support reducing a model considering non-zero positions of removed joints

environment.yml

@@ -22,7 +22,7 @@ dependencies:
   - isort
   - pre-commit
   # [testing]
-  - idyntree
+  - idyntree >= 12.2.1
   - pytest
   - pytest-icdiff
   - robot_descriptions

setup.cfg

@@ -73,7 +73,7 @@ style =
     isort
     pre-commit
 testing =
-    idyntree
+    idyntree >= 12.2.1
     pytest >=6.0
     pytest-icdiff
     robot-descriptions

src/jaxsim/api/model.py

@@ -1,9 +1,10 @@
 from __future__ import annotations
 
+import copy
 import dataclasses
 import functools
 import pathlib
-from typing import Any
+from typing import Any, Sequence
 
 import jax
 import jax.numpy as jnp
@@ -55,7 +56,7 @@ def build_from_model_description(
         *,
         terrain: jaxsim.terrain.Terrain | None = None,
         is_urdf: bool | None = None,
-        considered_joints: list[str] | None = None,
+        considered_joints: Sequence[str] | None = None,
     ) -> JaxSimModel:
         """
         Build a Model object from a model description.
@@ -257,24 +258,50 @@ def link_names(self) -> tuple[str, ...]:
 # =====================
 
 
-def reduce(model: JaxSimModel, considered_joints: tuple[str, ...]) -> JaxSimModel:
+def reduce(
+    model: JaxSimModel,
+    considered_joints: tuple[str, ...],
+    locked_joint_positions: dict[str, jtp.Float] | None = None,
+) -> JaxSimModel:
     """
     Reduce the model by lumping together the links connected by removed joints.
 
     Args:
         model: The model to reduce.
         considered_joints: The sequence of joints to consider.
-
-    Note:
-        If considered_joints contains joints not existing in the model, the method
-        will raise an exception. If considered_joints is empty, the method will
-        return a copy of the input model.
+        locked_joint_positions:
+            A dictionary containing the positions of the joints to be considered
+            in the reduction process. The removed joints in the reduced model
+            will have their position locked to their value in this dictionary.
+            If a joint is not part of the dictionary, its position is set to zero.
     """
 
+    locked_joint_positions = (
+        locked_joint_positions if locked_joint_positions is not None else {}
+    )
+
+    # If locked joints are passed, make sure that they are valid.
+    if not set(locked_joint_positions).issubset(model.joint_names()):
+        new_joints = set(model.joint_names()) - set(locked_joint_positions)
+        raise ValueError(f"Passed joints not existing in the model: {new_joints}")
+
+    # Copy the model description with a deep copy of the joints.
+    intermediate_description = dataclasses.replace(
+        model.description.get(), joints=copy.deepcopy(model.description.get().joints)
+    )
+
+    # Update the initial position of the joints.
+    # This is necessary to compute the correct pose of the link pairs connected
+    # to removed joints.
+    for joint_name in set(model.joint_names()) - set(considered_joints):
+        j = intermediate_description.joints_dict[joint_name]
+        with j.mutable_context():
+            j.initial_position = float(locked_joint_positions.get(joint_name, 0.0))
+
     # Reduce the model description.
-    # If considered_joints contains joints not existing in the model, the method
-    # will raise an exception.
-    reduced_intermediate_description = model.description.obj.reduce(
+    # If `considered_joints` contains joints not existing in the model,
+    # the method will raise an exception.
+    reduced_intermediate_description = intermediate_description.reduce(
         considered_joints=list(considered_joints)
     )
 

src/jaxsim/math/joint_model.py

@@ -15,6 +15,7 @@
     JointType,
     ModelDescription,
 )
+from jaxsim.parsers.kinematic_graph import KinematicGraphTransforms
 
 from .rotation import Rotation
 
@@ -87,21 +88,19 @@ def build(description: ModelDescription) -> JointModel:
         # w.r.t. the implicit __model__ SDF frame is not the identity).
         suc_H_i = suc_H_i.at[0].set(ordered_links[0].pose)
 
+        # Create the object to compute forward kinematics.
+        fk = KinematicGraphTransforms(graph=description)
+
         # Compute the parent-to-predecessor and successor-to-child transforms for
         # each joint belonging to the model.
         # Note that the joint indices starts from i=1 given our joint model,
         # therefore the entries at index 0 are not updated.
         for joint in ordered_joints:
             λ_H_pre = λ_H_pre.at[joint.index].set(
-                description.relative_transform(
-                    relative_to=joint.parent.name,
-                    name=joint.name,
-                )
+                fk.relative_transform(relative_to=joint.parent.name, name=joint.name)
             )
             suc_H_i = suc_H_i.at[joint.index].set(
-                description.relative_transform(
-                    relative_to=joint.name, name=joint.child.name
-                )
+                fk.relative_transform(relative_to=joint.name, name=joint.child.name)
             )
 
         # Define the DoFs of the base link.
@@ -243,16 +242,16 @@ def supported_joint_motion(
     """
 
     if isinstance(joint_type, JointType):
-        code = joint_type
+        type_enum = joint_type
     elif isinstance(joint_type, JointDescriptor):
-        code = joint_type.code
+        type_enum = joint_type.joint_type
     else:
         raise ValueError(joint_type)
 
     # Prepare the joint position
     s = jnp.array(joint_position).astype(float)
 
-    match code:
+    match type_enum:
 
         case JointType.R:
             joint_type: JointGenericAxis
@@ -276,58 +275,8 @@ def supported_joint_motion(
             S = jnp.vstack(jnp.hstack([joint_type.axis.squeeze(), jnp.zeros(3)]))
 
         case JointType.F:
-            raise ValueError("Fixed joints shouldn't be here")
-
-        case JointType.Rx:
-
-            pre_H_suc = jaxlie.SE3.from_rotation(
-                rotation=jaxlie.SO3.from_x_radians(theta=s)
-            )
-
-            S = jnp.vstack([0, 0, 0, 1.0, 0, 0])
-
-        case JointType.Ry:
-
-            pre_H_suc = jaxlie.SE3.from_rotation(
-                rotation=jaxlie.SO3.from_y_radians(theta=s)
-            )
-
-            S = jnp.vstack([0, 0, 0, 0, 1.0, 0])
-
-        case JointType.Rz:
-
-            pre_H_suc = jaxlie.SE3.from_rotation(
-                rotation=jaxlie.SO3.from_z_radians(theta=s)
-            )
-
-            S = jnp.vstack([0, 0, 0, 0, 0, 1.0])
-
-        case JointType.Px:
-
-            pre_H_suc = jaxlie.SE3.from_rotation_and_translation(
-                rotation=jaxlie.SO3.identity(),
-                translation=jnp.array([s, 0.0, 0.0]),
-            )
-
-            S = jnp.vstack([1.0, 0, 0, 0, 0, 0])
-
-        case JointType.Py:
-
-            pre_H_suc = jaxlie.SE3.from_rotation_and_translation(
-                rotation=jaxlie.SO3.identity(),
-                translation=jnp.array([0.0, s, 0.0]),
-            )
-
-            S = jnp.vstack([0, 1.0, 0, 0, 0, 0])
-
-        case JointType.Pz:
-
-            pre_H_suc = jaxlie.SE3.from_rotation_and_translation(
-                rotation=jaxlie.SO3.identity(),
-                translation=jnp.array([0.0, 0.0, s]),
-            )
-
-            S = jnp.vstack([0, 0, 1.0, 0, 0, 0])
+            pre_H_suc = jaxlie.SE3.identity()
+            S = jnp.zeros(shape=(6, 1))
 
         case _:
             raise ValueError(joint_type)

src/jaxsim/parsers/descriptions/joint.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import dataclasses
 import enum
 from typing import Tuple, Union
@@ -6,79 +8,60 @@
 import numpy as np
 import numpy.typing as npt
 
+import jaxsim.typing as jtp
 from jaxsim.utils import JaxsimDataclass, Mutability
 
 from .link import LinkDescription
 
 
+@enum.unique
 class JointType(enum.IntEnum):
     """
-    Enumeration of joint types for robot joints.
-
-    Args:
-        F: Fixed joint (no movement).
-        R: Revolute joint (rotation).
-        P: Prismatic joint (translation).
-        Rx: Revolute joint with rotation about the X-axis.
-        Ry: Revolute joint with rotation about the Y-axis.
-        Rz: Revolute joint with rotation about the Z-axis.
-        Px: Prismatic joint with translation along the X-axis.
-        Py: Prismatic joint with translation along the Y-axis.
-        Pz: Prismatic joint with translation along the Z-axis.
+    Type of supported joints.
     """
 
-    F = enum.auto()  # Fixed
-    R = enum.auto()  # Revolute
-    P = enum.auto()  # Prismatic
+    @staticmethod
+    def _generate_next_value_(name, start, count, last_values):
+        # Start auto Enum value from 0 instead of 1
+        return count
+
+    #: Fixed joint.
+    F = enum.auto()
 
-    # Revolute joints, single axis
-    Rx = enum.auto()
-    Ry = enum.auto()
-    Rz = enum.auto()
+    #: Revolute joint (1 DoF around axis).
+    R = enum.auto()
 
-    # Prismatic joints, single axis
-    Px = enum.auto()
-    Py = enum.auto()
-    Pz = enum.auto()
+    #: Prismatic joint (1 DoF along axis).
+    P = enum.auto()
 
 
-@dataclasses.dataclass
+@jax_dataclasses.pytree_dataclass
 class JointDescriptor:
     """
-    Description of a joint type with a specific code.
-
-    Args:
-        code (JointType): The code representing the joint type.
-
+    Base class for joint types requiring to store additional metadata.
     """
 
-    code: JointType
+    #: The joint type.
+    joint_type: JointType
 
-    def __hash__(self) -> int:
-        return hash(self.__repr__())
 
-
-@dataclasses.dataclass
+@jax_dataclasses.pytree_dataclass
 class JointGenericAxis(JointDescriptor):
     """
-    Description of a joint type with a generic axis.
-
-    Attributes:
-        axis (npt.NDArray): The axis of rotation or translation for the joint.
-
+    A joint requiring the specification of a 3D axis.
     """
 
-    axis: npt.NDArray
+    #: The axis of rotation or translation of the joint (must have norm 1).
+    axis: jtp.Vector
 
-    def __post_init__(self):
-        if np.allclose(self.axis, 0.0):
-            raise ValueError(self.axis)
+    def __hash__(self) -> int:
+        return hash((self.joint_type, tuple(np.array(self.axis).tolist())))
 
-    def __eq__(self, other):
-        return super().__eq__(other) and np.allclose(self.axis, other.axis)
+    def __eq__(self, other: JointGenericAxis) -> bool:
+        if not isinstance(other, JointGenericAxis):
+            return False
 
-    def __hash__(self) -> int:
-        return hash(self.__repr__())
+        return hash(self) == hash(other)
 
 
 @jax_dataclasses.pytree_dataclass

src/jaxsim/parsers/descriptions/model.py

@@ -4,7 +4,7 @@
 
 from jaxsim import logging
 
-from ..kinematic_graph import KinematicGraph, RootPose
+from ..kinematic_graph import KinematicGraph, KinematicGraphTransforms, RootPose
 from .collision import CollidablePoint, CollisionShape
 from .joint import JointDescription
 from .link import LinkDescription
@@ -75,6 +75,9 @@ def build_model_from(
                 considered_joints=considered_joints
             )
 
+        # Create the object to compute forward kinematics.
+        fk = KinematicGraphTransforms(graph=kinematic_graph)
+
         # Store here the final model collisions
         final_collisions: List[CollisionShape] = []
 
@@ -121,7 +124,7 @@ def build_model_from(
                 # relative pose
                 moved_cp = cp.change_link(
                     new_link=real_parent_link_of_shape,
-                    new_H_old=kinematic_graph.relative_transform(
+                    new_H_old=fk.relative_transform(
                         relative_to=real_parent_link_of_shape.name,
                         name=cp.parent_link.name,
                     ),
@@ -139,7 +142,9 @@ def build_model_from(
             root=kinematic_graph.root,
             joints=kinematic_graph.joints,
             frames=kinematic_graph.frames,
+            _joints_removed=kinematic_graph._joints_removed,
         )
+
         assert kinematic_graph.root.name == base_link_name, kinematic_graph.root.name
 
         return model
@@ -158,15 +163,12 @@ def reduce(self, considered_joints: List[str]) -> "ModelDescription":
             ValueError: If the specified joints are not part of the model.
         """
 
-        msg = "The model reduction logic assumes that removed joints have zero angles"
-        logging.info(msg=msg)
-
         if len(set(considered_joints) - set(self.joint_names())) != 0:
             extra_joints = set(considered_joints) - set(self.joint_names())
             msg = f"Found joints not part of the model: {extra_joints}"
             raise ValueError(msg)
 
-        return ModelDescription.build_model_from(
+        reduced_model_description = ModelDescription.build_model_from(
             name=self.name,
             links=list(self.links_dict.values()),
             joints=self.joints,
@@ -177,6 +179,12 @@ def reduce(self, considered_joints: List[str]) -> "ModelDescription":
             considered_joints=considered_joints,
         )
 
+        # Include the unconnected/removed joints from the original model.
+        for joint in self._joints_removed:
+            reduced_model_description._joints_removed.append(joint)
+
+        return reduced_model_description
+
     def update_collision_shape_of_link(self, link_name: str, enabled: bool) -> None:
         """
         Enable or disable collision shapes associated with a link.