For fixed joints, axis error checking was skipped. (#58)

Author: ft-lab

tests/data/fixed_joint_axis_0.urdf

@@ -0,0 +1,27 @@
+<robot name="fixed_joint_axis_0">
+  <link name="BaseLink">
+    <visual>
+      <origin rpy="0 0 0" xyz="-2 0 0.5"/>
+      <geometry>
+        <box size="1 1 1"/>
+      </geometry>
+    </visual>
+  </link>
+
+  <link name="link2">
+    <visual>
+      <origin rpy="0 0 0" xyz="0 0 0.5"/>
+      <geometry>
+        <box size="0.5 0.5 0.5"/>
+      </geometry>
+    </visual>
+  </link>
+
+  <!-- The axis xyz is (0, 0, 0). For fixed joints, the axis is ignored. -->
+  <joint name="JointA" type="fixed">
+    <parent link="BaseLink"/>
+    <child link="link2" />
+    <axis xyz="0 0 0"/>
+    <limit lower="-20" upper="20"/>
+  </joint>
+</robot>

tests/test_parser.py

@@ -655,3 +655,19 @@ def test_get_undefined_elements(self):
         self.assertEqual(element.undefined_element, False)
         self.assertEqual(element.undefined_attributes, {"data": "custom_data"})
         self.assertEqual(element.line_number, 42)
+
+    def test_fixed_joint_axis_0(self):
+        # In the case of a fixed joint, the axis (0, 0, 0) is skipped without causing an error.
+        model_path = pathlib.Path("tests/data/fixed_joint_axis_0.urdf")
+        parser = URDFParser(model_path)
+        parser.parse()
+
+        root_element = parser.get_root_element()
+        self.assertEqual(len(root_element.joints), 1)
+        joint = root_element.joints[0]
+        self.assertEqual(joint.name, "JointA")
+        self.assertEqual(joint.type, "fixed")
+        self.assertTrue(joint.parent)
+        self.assertEqual(joint.parent.link, "BaseLink")
+        self.assertTrue(joint.child)
+        self.assertEqual(joint.child.link, "link2")

urdf_usd_converter/_impl/urdf_parser/parser.py

@@ -353,7 +353,8 @@ def _parse_xml_elements(self, node: ET.Element, prev_element: ElementBase = None
 
         elif isinstance(element, ElementAxis):
             element.xyz = self._convert_attribute_float3(node, "xyz")
-            if element.xyz == (0, 0, 0):
+            # If the axis xyz is (0, 0, 0) and the joint type is not fixed, an error occurs.
+            if element.xyz == (0, 0, 0) and prev_element_type == ElementJoint and prev_element.type != "fixed":
                 raise ValueError(self._get_error_message("Axis xyz cannot be (0, 0, 0)", node))
 
         elif isinstance(element, ElementVerbose):